UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Quantitative studies of stream drift with particular reference to the McLay model McKone, Warren Douglas 1975

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Notice for Google Chrome users:
If you are having trouble viewing or searching the PDF with Google Chrome, please download it here instead.

Item Metadata

Download

Media
831-UBC_1975_A1 M33_8.pdf [ 7.66MB ]
Metadata
JSON: 831-1.0100064.json
JSON-LD: 831-1.0100064-ld.json
RDF/XML (Pretty): 831-1.0100064-rdf.xml
RDF/JSON: 831-1.0100064-rdf.json
Turtle: 831-1.0100064-turtle.txt
N-Triples: 831-1.0100064-rdf-ntriples.txt
Original Record: 831-1.0100064-source.json
Full Text
831-1.0100064-fulltext.txt
Citation
831-1.0100064.ris

Full Text

QUANTITATIVE STUDIES OF STREAM DRIFT WITH PARTICULAR REFERENCE TO THE McLAY MODEL by Warren Douglas McKone .Sc. (Honours), University of British Columbia, 1 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1975 In presenting th i s thes i s in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. 1 further agree that permission for extensive copying of th is thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t i on of th is thes is for f i nanc ia l gain sha l l not be allowed without my writ ten pe rm i ss i on . W. Douglas McKone Department of Zoo l o gy  The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1WS Date 3 October 1975 ABSTRACT This study was concerned with problems of measuring stream d r i f t and in particular attempts to evaluate the McLay (1970) model of the distance dri fted by stream invertebrates. During 1971-1973 observations and experi-ments were conducted at two spawning channels (Jones and Gates Creek) and at the Abbotsford Trout Hatchery, a l l located in southwestern Br i t i sh Columbia in the Fraser River drainage. Most species of invertebrates dr i fted in increasing numbers shortly after sunset. Variations occurred in the numbers of various species along the length of Gates Creek Channel, although water flow, depth, temperature, gravel depth and stream cover were similar throughout the channel. The distribution of organisms was related to detr ita l content which was high in the upper reaches but was replaced by algae in the lower end of the channel. Daily variations in the d r i f t rate occurred for various species although no changes were observed in physical conditions along the channel. The McLay model thus makes assumptions of uniformity of distribution which may not be met in f i e l d conditions. Laboratory studies suggested that Baetis d r i f t at a low constant rate unti l the carrying capacity of the gravel is reached. Carrying capacity is higher during the day than at night, ref lect ing higher act iv i ty at night. When density exceeded carrying capacity, a higher constant d r i f t rate occurred. Some species introduced into a r t i f i c i a l l y induced laminar flow actively moved toward the substrate by swimming, and drifted a shorter distance than that predicted by the McLay model, while by contrast those that were passive drifted farther than predicted. With turbulence, both types obtained sites on the bottom within a short distance. Increasing substrate density of a particular species caused added animals to d r i f t farther before they could find sites for sett l ing. The time required for an introduced pulse to pass downstream was longer than predicted considering stream velocity. Addit ional ly, a pulse of d r i f t causes animals to leave areas downstream. For several species the distance dr i f ted at dif ferent velocit ies was found to be best described by a power function suggested by E l l i o t t (1971b),but modified by the use of mean rather than modal velocity. The rate at which dead Epeorus l e f t the d r i f t was l inear ly related to distance. Thus, leaving the d r i f t was not a random process for 'dead animals. Various combinations of disturbance of the substrate and the complete f i l t r a t i on (blockage) of d r i f t , including the maintenance of a blockage for-ten days, were carried out under d i f fer ing conditions of l ight intensity and water velocity. The assumption of the McLay model of addit iv ity of pulses of animals was validated but the quantity introduced into the d r i f t at each disturbance varied between and within species. Below a f i l t r a t i o n of dr i f t ing species, numbers of animals which "put up" into the d r i f t natural ly, conformed to the predictions of the model, with each species exhibiting a characterist ic rate of leaving the d r i f t for a given water velocity and l ight intensity. When a blockage was maintained for several days, the observed d r i f t on successive days was a satisfactory f i t to that predicted; but on successive days larger numbers entered the d r i f t just below the blockage than was pre-dicted, possibly indicating support for the hypothesis of intergravel upstream movement. The numbers of most species dropped to low levels in the dr i f t by four days, suggesting that there was a density dependent behavioral response whose magnitude was related to the carrying capacity of the substrate for each species. When stream organisms were simultaneously disturbed from the substrate downstream from where they were f i l t e red out of the d r i f t , the numbers at various distances should be constant i f the correct width of disturbed area is selected. This tendency was confirmed in f i e l d t r i a l s . iv Estimates of the substrate density of organisms can be based on predictions of the McLay model, but only for species and age groups that are actively engaged in dr i f t ing . It is concluded that the McLay model provides a simple, ho l i s t i c conception of the process of stream d r i f t . However, variations in the substrate density, flow conditions, behavior of the various kinds of organisms including d r i f t behavior and upstream migration combine to provide many circumstances in which the model is inadequate. V TABLE OF CONTENTS Page TITLE PAGE 1 ABSTRACT i i j TABLE OF CONTENTS v LIST OF FIGURES v i i i LIST OF TABLES ix ACKNOWLEDGMENTS x i i 1. GENERAL INTRODUCTION 1 1.0 OBJECT OF WORK 1 1.1 THE PHENOMENON OF STREAM DRIFT 4 1.2 THE McLAY MODEL 10 1.3 THE STUDY AND GENERAL METHODOLOGY 14 1.4 LOCATION OF STUDY SITES 18 2. DRIFT BEHAVIOR OF INVERTEBRATES 24 2.0 DIEL DRIFT RATE OF VARIOUS SPECIES AT JONES AND GATES CREEK CHANNEL.. 24 Introduction 24 Methods 24 Results and Discussion 24 2.1 SPATIAL AND TEMPORAL VARIATIONS IN DRIFT RATE OF VARIOUS SPECIES OF INSECTS 32 Introduction 32 Methods 32 Results 33 Discussion 37 (a) Spatial distribution of drifting species 37 (b) Temporal drift rate of the various species at Gates Creek 39 2.2 DISTANCE DRIFTED BY DEAD EPEORUS NYMPHS -40 Introduction 40 Methods 40 Results and Discussion 40 vi Page 3. LABORATORY STUDIES 43 3.0 THE DISTANCE DRIFTED BY VARIOUS SPECIES IN AN ARTIFICIAL STREAM WITH LAMINAR FLOW 43 Introduction 43 Methods 43 Results and Discussion 44 3.1 EFFECTS OF SUBSTRATE SURFACE AREA ON CARRYING CAPACITY 48 Introduction 48 Methods 48 Results 50 Discussion • • • • 52 4. METHODOLOGICAL STUDIES 54 4.0 REPEATED DISTURBANCE OF THE SAME AREA OF SUBSTRATE 54 Introduction 54 Methods 55 Results and Discussion 55 4.1 CHANGES IN CATCHING RATE AFTER A DISTURBANCE 58 Introduction 58 Methods 58 Results and Discussion 59 5. FIELD EXPERIMENTS 62 '' 5.0 DISTANCE DRIFTED AFTER A DISTURBANCE 62 Introduction 62 Methods 62 Results • • • • 63 Discussion 72 5.1 ADDITIVITY OF DRIFT FROM TWO SIMULTANEOUS DISTURBANCES 74 Introduction 74 Methods 74 Results 75 Discussion 75 5.2 BLOCKAGE OF STREAM DRIFT 79 Introduction 79 Methods 79 Results 81 Discussion 85 5.3 THE RESPONSE OF THE DRIFT TO SUBSTRATE DENSITY 87 Introduction 87 Methods 91 Results 91 Discussion 92 5.4 SIMULTANEOUS DISTURBANCE OF THE SUBSTRATE AND BLOCKAGE OF THE DRIFT.. Introduotion  Methods  Results  Diseussion  5.5 ESTIMATES OF THE RELATIVE DENSITY OF INVFRTEBRATES AT GATES CREEK.... 6. GENERAL DISCUSSION LITERATURE CITED VI11 LIST OF FIGURES P-wt^^ w- u> Ws^y 26 ,2^ S ^ ' j GURE n ' ' / ^ -./ ' U - i „ . c<  "?!> 7 ^ ^ 91 com gUM <*t ^ . J . Pa9< 1 The organization of the different experiments in relation to svJa%\:w*» the central theme of testing the McLay model ^ 3 ^ i S t s ' 2 (Top) Theoretically the number of animals remaining in the d r i f t at increasing distances below a s i te of disturbance of the stream bottom 11 2 (Bottom) The number of animals expected to enter the d r i f t from natural causes below a blockage.. 11 3 A typical layout of the nets for blockage and disturbance type experiments 15 4 The Fraser River watershed with location of: (1) Jones Creek, (2) Gates Creek, (3) Abbotsford laboratory, and (4) Vancouver. (Modified from MacKinnon, Edgeworth and McLaren, 1961) 20 5 (Top) A schematic representation of Jones Creek Channel (MacKinnon, Edgeworth and McLaren, 1961) 22 5 (Bottom) A schematic representation of the Gates Creek Channel.. 22 6 The marginal vegetation at Jones Creek Spawning Channel 23 7 The marginal vegetation at Gates Creek Spawning Channel 23 8 Temperature (°C) , l ight intensity at the water surface (ft-c) and diel d r i f t of a variety of species of insects nymphs and pupae at Jones and Gates Creek 26 9 The total numbers of various species of aquatic insects caught in one hour d r i f t samples in the Gates Creek Channel, from May 25 to May 31 and on June 2, 1973 29 10 The numbers of various species of aquatic insects in the d r i f t between May 25 and June 2 at each of seven stations in the Gates Creek Channel 35 11 The l inear least squares regression of the average number of dead Epeorus caught from release points at increasing distances up-stream from a net 41 12 The body shape of some species of mayfly and stonefly nymphs observed in the a r t i f i c i a l stream. The average lengths were: Cinygmula 9.5 mm; Baetis 6 mm; Chloroperla 9 mm; Ephemerella 13 mm 46 13 An a r t i f i c i a l area of "stream bed" made from uniform rocks collected at sites in a stream where test animals were known to occur 49 ix FIGURE Page 14a The number of Baetis remaining in an a r t i f i c i a l stream bed of 18 rocks at the end of 12 hours of daylight (X) and darkness (0) periods, for various i n i t i a l densities of organisms. Two least squares lines of best f i t were generated through the daylight data points and two through the darkness data points ^ 51 14b The same for a stream bed of 36 rocks 51 15 The l ine of best f i t , for a l l species summed, of mean number caught at various distances downstream from a disturbance of the stream bottom. Each t r i a l was carried out under differing l ight intensit ies and water velocit ies (see Tables V and VI) 65 16 The log-log regression of the average distance travelled against velocity for (a) summed species, (b) Epeorus, (c) Baetis 71 17 The lines of best f i t through the average number (X) of a l l the species taken together (summed species) caught at various distances downstream of two simultaneous disturbances of organisms off the stream bed (top) at 0 and 5 meters, (bottom) 0 and 10 meters 76 18 F i l ter ing nets placed across the stream to remove dr i f t ing organisms 80 19 The l ine of best f i t for the regression on distance of the observed average number of organisms in the d r i f t downstream from complete f i l t r a t i on of the d r i f t . Each t r i a l was carried out under differing l ight intensities and velocit ies (see text) . . . 82 20 Theoretically, the proportion of animals expected to "put up" into the d r i f t from natural causes on successive days below a s i te of continuous complete f i l t r a t i on of the d r i f t 90 21 The proportion of summed species and various individual species which entered the d r i f t on four successive days downstream of a continuous blockage of the d r i f t . The lines of best f i t are marked 1 to 4 one l ine for each day. The observed data points are marked by: • = f i rs t -day, 0 = second day, X = third day, A = fourth day 94 22 Linear least squares l ine of best f i t through the average of the sum of a l l the species taken together and various individual species at increasing distances below a disturbance and blockage. Where a 22 cm width was disturbed from one side of the stream to the other and in other cases a 11 cm width was disturbed 99 X LIST OF TABLES TABLE p a g e I The distance drifted by individuals of two species of mayfly in an a r t i f i c i a l stream with laminar flov: (45 cm/sec) when (a) the the individuals released were removed from the stream after they attached to the bottom, and (b) the individuals were not removed 45 II The average rate of departure ( (1-b) where b is the slope of the regression) of Baetis leaving boxes which contained 18 rocks and 36 rocks during periods of daylight and darkness 52 I l i a . The percentage of the total number of disturbed organisms displaced from the stream gravel bed by the f i r s t of a series of disturbances. The middle figure is the mean, the others are lower and upper 95% confidence limits 56 IIlb. The proportion (A) of invertebrates which l e f t the substrate in successive disturbances of the substrate 56 IVa. The average catching rate (per hour) for the summed species in nets placed at various distances downstream of a disturbance...... 59 IVb. The average catching rate (per hour) of Epeorus in nets placed at various distances downstream of a disturbance 60 IVc. The average catching rate (per hour) for Baetis in nets placed at various distances downstream of a disturbance 60 IVd. The average catching rate (per hour) for Chironomidae (pupae) in nets placed at various distances downstream of a disturbance... 60 V The estimated number of invertebrates introduced into the dr i f t from a disturbance, the rate of return to the substrate, the average distance travelled and the estimated and observed natural d r i f t of t r i a l s at various l ight intensit ies and velocit ies at Jones and Gates Creek. Average observed natural d r i f t was ob-tained from net samples taken upstream of the disturbance 66 VI The average velocity and l ight intensity in the experimental area during the various t r i a l s 68 The estimated number of invertebrates introduced into the d r i f t simultaneously from two differing sites of disturbance, the rate of return to the substrate, the average distance travelled and the estimated and average observed natural d r i f t at Jones Creek. Average observed d r i f t was obtained from net samples taken up-stream of the disturbances xi TABLE Page VIIT The average velocity at each t r i a l during the simultaneous disturbance of two areas of stream bottom 7£ IX The recovery of the d r i f t through natural causes below the complete f i l t r a t i o n of the d r i f t , the estimated rate of return to the sub-strate, the average distance travel!ea, the estimated and average , observed natural d r i f t at various velocit ies and l ight intensit ies at Jones and Gates Creek. Average observed d r i f t was obtained from net samples taken upstream of the disturbance 83 X Average velocity and l ight intensity in the various blockage experiments 84 XI The values of the rate of leaving and putting up into the dr i f t per unit time and the instantaneous rate of leaving the dr i f t as estimated from the recovery of the d r i f t on successive days below the continuous blockage of the d r i f t 95 XII The rate of return to the substrate with simultaneous disturbance of the substrate and blockage of stream invertebrates when the total area disturbed was varied 100 XIII Estimates of the relative number per meter squared as determined from the estimated disturbed animals and the effectiveness of the disturbance at Gates Creek Channel 103 XIV The estimated average distance travelled by various species down-stream of a disturbance (velocity 0.766 and 0.758) and downstream of a blockage (velocity 0.616) at Jones Creek during periods of darkness 107 ACKNOWLEDGMENTS I would l ike to thank the Fisheries and Marine Service, Vancouver, B.C., and the International Paci f ic Salmon Fisheries Commission for providing Jones and Gates Creek Salmon Spawning Channels for this research. A special debt of appreciation is due Dr. P.A. Larkin who supported the presentation of this thesis, guided the analysis, and gave valuable cr i t ic i sm. Drs. G.G.E. Scudder and T.G. Northcote, as members of my committee, gave valuable c r i t i -cism and help when necessary. Mr. Neil Gi lbert, and Mr. E.J. Watson (Depart-ment of Mathematics, University of Manchester) gave guidance with the mathe-matical analysis of d r i f t in the circumstances of a continuous f i l t r a t i on experiment. Scott Akenhead gave helpful suggestions and computer programming assistance; Dolores Lauriente assisted in computer programming; and Helen Hahn and Ingrid Pine! helped put i t a l l together. 1 1. GENERAL INTRODUCTION 1.0 OBJECT OF WORK Although the phenomenon of stream d r i f t is well documented, surpris-ingly there have been few attempts to describe the process quantitatively. The model of McLay (1970) was a f i r s t step in developing a mathematical description, and this model has been tested in only a preliminary way. It was the object of this study to evaluate the McLay model. As a bonus in the process of doing these studies, there was gained a considerable amount of information on d r i f t in Br i t i sh Columbian coastal streams for which to date there has been a paucity of information. The investigation began in 1971 with the intention of testing the McLay model and at the same time col lecting d r i f t information throughout the whole of the Similkameen River drainage. This soon proved to be too ambitious and beset with log i s t i c problems. Thus, the study was centered on the Jones Creek Spawning Channel and on laboratory studies at Abbotsford in 1971-1972, so that many physical stream characterist ics could be controlled. Subsequently, observations were made at Gates Creek spawning channel in 1973. The research had three components: (1) Studies of the dr i f t ing behavior of stream invertebrates (2) Studies of the methodologies in d r i f t experiments (3) Field experiments to test the McLay model. The f i r s t group of studies was aimed at elucidating some of the characteristics of stream d r i f t organisms that influence the d r i f t process, and included observations on the effects on d r i f t for various species, of substrate density, stream velocity and l ight intensity. The second group of studies looked at the effectiveness of the shuffling technique in disturbing organisms in the substrate and at the catching rate of nets in streams. . The third and major group of studies was a series of f i e l d experiments designed to put the McLay model to various tests in natural conditions, and included experiments involving single and multiple stream disturbances, f i l t e r i n g or blockage, and combinations of disturbance and blockage. The interrelations between the various parts of the work are indicated in Figure 1, which summarizes the total study. DRIFTING BEHAVIOR OF INVERTEBRATES McLAY MODEL METHODOLOGICAL Field Studies DIEL DRIFT CHANGES IN DAILY PATTERN OF DRIFT SPATIAL DRIFT DISTANCE DRIFTED BY DEAD EPEORUS TWO SIMULTANEOUS DISTURBANCES BLOCKAGES WITH CHANGES IN LIGHT INTENSITY BLOCKAGE FOR A NUMBER OF DAYS SIMULTANEOUS DISTURBANCE AND BLOCKAGE DISTURBANCES WITH 'CHANGES IN LIGHT INTENSITY AND WATER VELOCITY' EFFECTIVENESS OF SHUFFLING METHOD OF DISTURBANCE M CHANGES IN CATCHING RATE OF NETS WITH INCREASING TIME DRIFTING BEHAVIOR OF INVERTEBRATES Laboratory Studies DISTANCE DRIFTED IN LAMINAR FLOW CARRYING CAPACITY Figure 1. The organization of the different experiments in relation to the central theme of testing the McLay model. 4 1.1 THE PHENOMENON OF STREAM DRIFT Benthic invertebrates are known to d r i f t downstream in large numbers. Running water species do not frequent streams necessarily because of the current, but rather because they are adapted to many of the nutr i t iona l , chemical and physical conditions that exist only in flowing water (Hubault, 1927). The l o t i c biotope enforces a conditioning of the fauna which has led to morphological and behavioral adaptations peculiar to the physical limitations of the flowing water habitat. In the absence of extraneous physical forces, d r i f t is a normal feature of the lo t i c system (Ide, 1942; Dendy, 1944; Maciolek and Needham, 1951; Miiller, 1954b; Macan and Mackereth, 1957; Reimers, 1957; Tanaka, 1960; Horton, 1961; Waters, 1961 , 1962a; Warren et al_., 1964; E l l i o t t , 1967a; Bishop and Hynes, 1969b). Dr i f t exhibits a die! pattern which was f i r s t documented by Tanaka (1960). Many others have reported the die! pattern and some species have shown to be active in the d r i f t during the night and others during day (Waters, 1962a,b, 1965, 1968, T969a,b; Miiller, 1963a,b,c, 1966a,b; Levanidova and Levanidov, 1965; E l l i o t t , 1965a,b, 1967a,b; Madsen, 1966; Anderson, 1967; Tobias and Thomas, 1967 and Bishop and Hynes, 1969b). Mechanical factors, such as gravel movement, have been found to result in a continuous low level of d r i f t , which is to be expected, but high nocturnal d r i f t is largely the result of greater act iv i ty by the benthic invertebrates (Moon, 1940). As the level of movement of the animals r i ses , the l ikelihood heightens of detachment and displacement downstream by the current. Some species show a negative phototaxis (Wodsedalek, 1912; Chapman and Demory, 1963; Hughes, 1966; E l l i o t t , 1967a; Holt and Waters, 1967; Bishop, 1969) while others demonstrate a positive phototaxis (Hughes, 1966; E l l i o t t , 1967a). The former tend to attach themselves under rocks during daylight hours. Hynes (1941, 1970b), Brinck (1949), Chapman and Demory (1963) and E l l i o t t (1967a) observed the movement of nymphs onto the upper surface of rocks at night to search for food. Once exposed, animals which jost le with one another cause some dislodgement and, hence, "recruitment" into the d r i f t . 5 Light intensity has proved c r i t i c a l to the act iv i ty level of aquatic insects. For example, moonlight depresses the ac t iv i ty , recruitment (Casper, 1951; Waters, 1962a; Anderson, 1966), and emergence (Hartland-Rowe, 1955) of dr i f t ing species. Moreover, a r t i f i c i a l l ight above a c r i t i c a l level induces a s ignif icant response in d r i f t patterns of insects whose act iv i ty level increases with low l ight level ( E l l i o t t , 1965a, 1967a; Muller, 1965 , 1966b; Holt and Waters, 1967; Bishop, 1969). Bishop (1969) found that various wave-lengths of l ight had l i t t l e apparent effect on act iv i ty level . F ina l ly, a few species are light-independent; natural act iv i ty cycles continue regardless of l ight and dark periods (Waters, 1962b). Otherwise, l ight intensity appears to be a major external factor controll ing the diel pattern of act iv i ty . The developmental stage of both nymphs and larvae is reflected in their level of act iv i ty in the d r i f t . Older nymphs are more active than young, and hence occur in the d r i f t in greater abundance (Moon, 1935; Macan, 1957). Some species have endogenous act iv i ty rhythms (Harker, 1953a; Muller, 1965 ). Morphological adaptations have been suggested as a means of preventing displacement (e.g., dorso-ventral f lattening, (Steinmann, 1913); streamlining of body and leg structures so as to reduce resistance to water flow (Dodd and Hisaw, 1923, 1924). Spines and hairs to prevent slippage, reduction in the total size of the organism, and various special attachments (e.g., heavy case of caddisfly) have been suggested as an advantage to current adapted species (Neave, 1930; Hora, 1930). Neave (1930) further pointed out that dead nymphs and exuviae orient to current flow showing the importance of body shape in influencing the force of the current on the animal. Moreover, the distribution of many species with respect to flow has been documented in relation to their morphology (Scott, 1958; Cummins, 1964; Edington, 1965), but Neilson (1950, 1951) found there are many exceptions. Ulfstrand (1967) observed many species spent most of their time in areas of s l ight turbulence. Additionally, Lehmkuhl and Anderson (1972) found high seasonal fluctuations in flow caused the dis-placement of many species to areas with gentle current or no current. Ambiihl (1959, 1961) and Jaag and Ambiihl (1964) maintained that many modifications are of l i t t l e consequence because there are dead water spaces at the boundary layer of water on rocks and behind obstructions, which act as refuges. 6 Various workers have studied the l i f e history of aquatic stream nymphs and larvae with particular reference to their presence in the d r i f t (Ide, 1935; Harker, 1953; Macan, 1957; Hynes, 1961; E l l i o t t , 1967b; Minshall and Keuhne, 1969). Some species produce several generations per year, while others only one generation every year or two. For particular species, differences in the number of generations per year, occur from place to place, (Ide, 1935; Macan, 1957, 1961). The period of egg incubation may be short (Berner, 1959) although very small nymphs of many species have been observed at a l l times of the year, indicating delayed or prolonged hatching (Macan, 1957; Hynes, 1961). Generally, the frequency of occurrence of a species in the d r i f t is dependent to a large degree on local environmental conditions. Changes in water flow, part icularly a spate, can displace a large pro-portion of the stream benthos (Needham, 1928; Beauchamp, 1932; Moffett, 1936; Surber, 1937; Anderson and Lehmkuhl, 1968). A reduction in flow may also increase d r i f t (Minshall and Winger, 1968). Insecticides have been found to in i t i a te a high d r i f t rate (Hoffman and Surber, 1948; Scott, 1961; Coutant, 1964; Dimond, 1967) and ice, part icularly during break-up, removes benthic organisms by scouring the stream bottom (Maciolek and Needham, 1951; O'Donnell and Churchi l l , 1954). Other workers (Lennon, 1941; Muller, 1963b; Waters, 1968) have found d r i f t to show a temperature dependence, but E l l i o t t (1967a) could not substantiate the dependence. Wojtalik and Waters (1970) found that a change in the temperature regime in enclosed channels in a natural stream had l i t t l e effect on determining the length of the period of d r i f t , the start and f in ish of high nocturnal d r i f t and the general shape of the diel pattern. They did f ind, however, that the amplitude of d r i f t within the diel period was affected for some species but not for others. Hence, the physical and chemical state of running water causes di f fer ing responses peculiar to various species. Aquatic insects arriving by wing usually ensure a speedy colonization of new channels and eroded areas (Surber, 1937; Leonard, 1942; Muller, 1954a, b; Patrick, 1959). Ten to fourteen days was found suff ic ient to recolonize depopulated areas (Surber, 1937; Muller, 1954b; Waters, 1964, 1969a). Several authors suggest that intermittent streams are colonized by winged adults which 7 f ly i n , or by drought-resistant states in the subsurface (Hynes, 1958; Harrison, 1966; C l i f f o rd , 1966, 1972). Hence, the "normal" numbers are in a constant state of flux depending on the physical conditions of the stream and the l i f e cycles of various species. Upstream movement by f ly ing adults and positive rheotaxis of the nymph and larval stages act as mechanisms compensating for depopulation, which d r i f t may cause, of upper reaches of a stream (Muller, 1954b; Roose, 1957; Minckley, 1964; Hughes, 1966; Bishop and Hynes, 1969a; Hultin, Svensson and Ulfstrand, 1969; E l l i o t t , 1971a). Stehr and Branson (1938) observed the recolonization of previously dry areas of an intermittent stream by the upstream movement of Plecoptera, and Harker (1953b), Macan (1957) and Leonard and Leonard (1962) suggest that Ephemeroptera move upstream as a response to the ins tab i l i ty of the substrate. Ba l l , Wojtalik and Hooper (1963) followed the upstream dis-persal of radioactive phosphorus in a stream and concluded that benthic invertebrates were the carr iers. Others (Ambiihl, 1961; Muller, 1966a; Nielsen, 1950, 1951) found upstream movement was due to a constant rheotaxis and Hughes (1966) observed that Trichoptera larvae and Ephemeroptera numphs migrated as a response to current. E l l i o t t (1971a) noted higher numbers moved upstream during the night than during the day, and upstream movement compensated for 30 percent of the numbers dr i f t ing . Roose (1957) indicated that mature females of several species flew mainly upstream before laying their eggs. E l l i o t t (1967a), on the other hand, found several species flew in the direction of the wind regardless of the direction of the stream. Loss of dr i f t ing insects to f i sh has been reported in numerous papers (Muller, 1954a,b; Kawai, 1959; Hunt, 1965; Maitland, 1965; Chapman, 1966; Jenkins, et a]_., 1970; Metz, 1972; Cadwallader, 1973). Chapman (1966) and Chapman and Demory (1963) point out that the proportions of invertebrate species change with the season, thereby affecting their ava i lab i l i ty to f i sh . Night feeding during peak d r i f t has been reported (Brett, 1957; E l l i o t t , 1969; Jenkins, 1969), but Allen (1951) found no diel difference in the amount of food present in the stomachs of f i sh . As d r i f t increases at night, the amount of benthos lost to f ish is proportionally increased (Allen, 1941, 1942; 8 Maciolek and Needham, 1951; Jenkins, 1969; Chaston, 1969; Metz, 1972). Selective use of larger dr i f t ing invertebrates (McCormack, 1962; Madsen, 1966) and the individual f i sh preference for certain food types (Bryan and Larkin, 1972) reflects the inaccess ib i l i ty of young instars and the dis-proportionate us? of some food types by stream fishes. The impact of pre-ference, se lect iv i ty and quantity eaten by f ish should a l l be considered in evaluating predation by f ish on stream d r i f t . This extensive background of l i terature indicates the d i f f i cu l t y of developing quantitative models for the stream dr i f t process, and perhaps explains why so few investigations have been concerned with the relationship of the rate of d r i f t to population density of benthic invertebrates. Muller (1954a) was the f i r s t to document a numerical relation between d r i f t and benthos density. Waters (1961, 1962a,b, 1965) followed Muller's work with several investigations and postulated that d r i f t was related in some way to "excess in production" and the benthos-carrying capacity. Waters (1962a, 1966) additionally presented some methods to compute production ( i .e. total tissue elaboration of a population per unit time per unit area regard-less of the fate of the tissue but independent of maintenance requirements) of the fauna of streams. Recently, Pearson and Kramer (1972) attempted to c la r i fy some relationships among population density, d r i f t rate and production, and several pertinent environmental factors (e.g. water temperature, discharge and density). Some doubt as to the usefulness of d r i f t measurement for estimating production has been expressed (Bailey, 1966; E l l i o t t , 1967b; Hynes and Coleman, 1968; Hynes, 1970a). These objections have been based on the inabi l i ty of some authors to f ind a correlation between quantity of d r i f t and benthos density. Hynes and Coleman (1968) offered a method for estimating production based on benthos samples which were collected from a series of cylinders set into the substrate for various lengths of time. (The inner cylinder constituted the sample.) The problem with the cyl indr ical samples is that the gap between the two parts allows vertical migration. 9 Fager (1969) maintained that the technique of estimating production from the data used by Hynes and Coleman (1968) underestimates production, while Hamilton (1969) disagreed with the Fager attempt to modify the Hynes and Coleman technique, because i t dealt only with discrete cohorts. Hynes and Coleman were trying to deal with a situation v;here the cohorts were ind i s t in -quishable. Hamilton, on the other hand, suggested modifications to the Hynes and Coleman technique which were more consistent with their underlying assumptions. Sampling techniques, such as corers have recently been developed to look at vertical s t ra t i f i ca t ion of benthic organisms. Hopkins (1964), Maitland (1969), Williams and Hynes (1974), Stocker and Williams (1972) and Efford (1960) used freezing corers. More recently, Williams and Hynes (1974) used a series of pipes which were f i r s t f i l l e d with gravel and then driven into the substrate to confirm that invertebrates frequent the deep substrate. Brinkhurst (1967) points out, however, that the small volumes encompassed by corers make them d i f f i c u l t to use in coarser sediments and that they are subject to error re-lated to the patchy distr ibution of the animals. Bishop (1973) designed a rectangular box with two sides which were l e f t open, one on the upstream side and one on the downstream side which could be closed without disturbing the box, in some respects, a f i e l d experimental technique for measuring coloniza-tion. A l l attempts to quantify the vertical distribution of organisms in benthic habitat, have been inadequate, but they indicate that a large number of benthic organisms are present to considerable depths in the substrate. While there is an extensive descriptive and qualitative l i terature on stream d r i f t , estimates of the total number of animals occupying the substrate under various conditions is sadly lacking. Estimates of benthic production, derived either by measuring d r i f t or sampling the benthos, have not been successful. Moreover, in studies in which d r i f t has been used as a measure of production, l i t t l e or no consideration has been given to the source of the d r i f t when col lecting samples. Dri ft and benthos samples have generally been taken at the same place in the stream, although the dr i f t ing animals evidently came from increasing distances upstream of the sample net. Deter-mination of the mean distance travelled by individual species would be of obvious value in understanding the relationship between d r i f t and benthos productivity. 1.2 THE McLAY MODEL McLay (1970) developed a model for estimating the mean distance travelled by animals in the d r i f t , and demonstrated i ts appl icabi l i ty to some previously published data. McLay's model *s simply i l lustrated in Figure 1. If the substrate is disturbed, then a number of animals is introduced into the d r i f t . They return to the substrate at a constant relat ive rate, and hence the number that remain in the d r i f t f a l l s off exponentially with distance (Figure 2 top), i . e . : N D-Noe- R D <" where N D is the number in the d r i f t at distance D from the s i te of disturbance No is the number of organisms that enter into the flow from disturbance * R is the instantaneous rate of return to the substrate D is the distance from the s ite of the disturbance Thus, when the stream bottom is disturbed, the number of organisms that enter the d r i f t is N^  and at an in f in i te distance downstream they are a l l returned to the substrate. ; If the flow of the stream is entirely screened, then a l l of the d r i f t is "blocked". Downstream from the block, the d r i f t increases, until at an in f in i te distance the d r i f t is the same as the natural d r i f t (Figure 2 bottom). The number of organisms in the natural d r i f t (N m a x ) is the result of the natural entering of animals into the d r i f t at a l l points upstream, at each of which the number entering the d r i f t is No. Hence, "max 1N01 e N 0 2 e + • • • • ' % e 11 i I Figure 2. (Top) Theoretically the number of animals remaining in the d r i f t at increasing distances below a s i te of disturbance of the stream bottom. Figure 2. (Bottom) The number of animals expected to enter the d r i f t from natural causes below a blockage. Below a blockage, the number in the drift (Nn) is thus: N n = No ( l -e" K U ) (3) R At an infinite distance Nn approaches N and max N, (4) Hence: (5) Natural drift at any collection point is the result of a number of animals entering the drift from differing distances downstream. The number of animals collected depends on the rate at which the animals return to the benthos, the number init ia l ly entering the drift and the distance the organisms travel to the collection point. The major contribution of the McLay model is a method for estimating the mean distance that organisms drift. The model does not take into account the effect of changes in the physical environment, or behavior of the animals. The model does, however, suggest that individual species have differing rates of return to the benthos given the same experimental constraints. The mean distance travelled (d), which is inversely proportional to the rate of return, is given by the sum of the distances travelled by the individuals, divided by the total number of organisms, i.e.: The experimental methods presented by McLay (1970) to collect the data in support of the model are not sufficiently rigorous to come to the conclusion that the model actually represents distance drifted. Samples were taken by a single net placed in a fixed midstream location. The procedure used was to f irst take a half-hour control drift sample. Then a pulse of animals was introduced into the current by disturbing an area of the stream bottom oo RD (6) 13 approximately 20 cm wide across the width of the stream, and a second half-hour sample was collected. The introduced pulse was repeated at increasing distances upstream of the stationary net (6, 12, 18, 24, 30, 36 meters). The time al lotted for the experiment was seven hours (0900 to 1600 hours), of which three hours were needed for controls end three hours for experiment?!. Hence, i t must be assumed that half an hour is suf f ic ient for a pulse to clear the experimental area, that the introduced pulses upstream do not cause density dependent interactions between individuals of the same or other species present between the net and the introduced pulse, and that the density and species composition of each species occupying the benthos is the same at each point of disturbance. The curvi l inear regressions in McLay's data are each based on one series of samples with no repl icat ion. The general form of the model has lat ter ly been found to f i t a few sets of data ( E l l i o t t , 1971b; Ogilvie, 1971 unpubl.), but there is a clear absence of information about many of the re-lationships and quantities. Thus, the model has not yet been exploited for i ts potential for giving estimates of the abundance of organisms in the d r i f t . ( 14 1.3 THE STUDY AND GENERAL METHODOLOGY The work was carried out mainly in the f ie ld, but a few experiments were done in the laboratory. The field work was mainly concentrated on a variety of experiments in which the stream bottom was disturbed or drift was blocked from downstream movement. Two types of perturbation experiments (with variations) were carried out at the differing locations in the f ield. First, mechanical disturbance of a transverse strip of stream bed introduced benthic invertebrates which were sampled at increasing distances downstream. Second, drift was removed from the water by fi ltering nets and the drift was sampled at increasing distances downstream. The design of the individual disruption experiments was similar through-out. A series of net frames with an opening 10 cm by 30 cm was constructed of 0.953 cm round stock steel. Each frame was covered with 471u nitex shaped in a right cone with a height of 45 cm. Glued to the nitrex was a 9 cm diameter plastic bottle with the bottom cut away. The top faced outwards so i t could be removed to collect samples without disturbing the net. Four f lat washers (.64 cm), two to each long side of the net, were welded so that the nets could be slipped over two stakes driven into the stream substrate. The washers were placed so that the nets would interlock, i f needed. Further nets used for the fi ltration of drift were similarly constructed but had open-ings of 30 cm by 30 cm and in the shape of a right cone of height 95 cm. Three control nets were placed two meters upstream at the site of disruption. The substrate was mechanically disturbed by "shuffling" across the stream. Morgan and Eggishaw (1965) found that kicking up the substrate is a good method for removing most of the invertebrates. For blocking experi-ments, the drift was filtered by placing interlocking nets from one side of the stream to the other. Downstream sampling nets were placed at random locations across the width of the stream at various measured distances below the disruption (Figure 3). Additionally, each net was placed with the restriction that no net was subject to "interference" from nets placed up-stream. Each net was held in place by stakes driven into the substrate 24 hours before a t r i a l . igure 3. A typical layout of the nets for blockage and disturbance type experiments. 16 The data from each t r i a l were corrected for differences in velocity at each net opening, by dividing each net velocity by the average net velocity. The data for the disruptive experiments was f i t ted using a non-linear function optimization routine, which f i t ted the unconstrained function so as to minimize the sums of squares of deviations of observed from theoretical values. No assumptions were made about the functions, except that there was an unique minimum in the area of search. The mathematical analysis included consideration of the effects of sampling. When invertebrates are introduced into the d r i f t by shuffling across the width of the stream, the disturbed animals l e f t in the d r i f t (Nd-|) can be given by: Nd] = Ho^e'RD (7) which is a specif ic case of equation (1). Because the nets remove some of the d r i f t at each distance downstream of the disturbance, the number at distance " i " is given by: Nc. = Nc._.,e~R D (8) Thus, the number caught (Na-j) at a particular distance can be given by: Na] = Nd] + Nc^ (9) For experiments in which two areas of the stream bed were disturbed and samples were taken which included animals from each of the disturbances, the number introduced by the second disturbance can be given by: Nd2 = No 2 e " R D (10) and provided the contributions are additive: Na2 = Nd] + Nd2 + Nc. (11) where the number caught at a particular distance (Na2) below the second dis-turbance is the sum of those remaining from each of the disturbances and the natural d r i f t . For blockage experiments, below the point where the total d r i f t is ••-f i l te red out, two phenomena occur. F i r s t , dr i f t ing animals are leaving the d r i f t according to equation (1). Second, those that enter the d r i f t naturally follow the form of equation (5). Thus, as the two relationships are additive: Nc, • NCi.1e-RD • « K I ( W f f l ) (12) where the number caught at a particular distance (Nc^) is a function of the number caught at a previous set of nets (Nc^-j) times the rate of set t l ing, plus the number caught of those which enter the stream from natural causes. One of the blockage experiments included f i l t e r i n g out the dr i f t ing organisms over several days. A modified version of the non-linear f i t t i n g routine was used which found the optimum of an unconstrained function (equation 29) by finding a jo int local minimum for a series of curves. 1.4 LOCATION OF STUDY SITES The data for this study were chief ly obtained during May and June of 1971-2-3. The laboratory work was done at the Provincial Government Trout Hatchery near Abbotsford and the f i e l d work was done at Jones Creek and Gates Creek Spawning Channels. Jones Creek Spawning Channel is located 100 miles east of Vancouver and Gates Creek Spawning Channel, 150 miles northeast (Figure 4). Both Creeks are tr ibutaries of the Fraser River system and both channels parallel their respective streams (Figure 5). Jones Creek Channel is 610 meters long and 4.25 meters wide at the water surface. Baffles are insta l led at 30 meter intervals, each baffle having a drop of between 10-30 cm. Thus, each interval has a controlled depth and velocity. The gravel is graded, ranging from 0.64 cm to 3.18 cm and is deposited to an average depth of 40 cm throughout the channel. Additionally, the gravel is f irmly planted throughout most of the channel because of s i l ta t ion resulting from controlled flow. A canopy of deciduous trees covers about 90% of the channel (Figure 6). Gates Creek Channel is 1891.28 meters long and 6.09 meters wide at the water surface. The gravel is graded to a range of 1.27 cm to 10.16 cm. The depth of the gravel averages 40.64 cm throughout the channel. Because of the porous subsoil, the channel is l ined with plast ic sheet to prevent leakage. The gravel is loosely packed and clean for the whole length. There is not a canopy of trees, but the stream has small trees, less than 4 meters high, along i ts banks (Figure 7). 19 Figure 4. The Fraser River Watershed with the location of: (1) Jones Creek, (2) Gates Creek, (3) Abbotsford laboratory, and (4) Vancouver. (Modified from MacKinnon, Edgeworth and McLaren, 1961) ' D r i f twood R Ankwlll Cr. ^ T A K L A L A K E <j 5ak«nich« R. M i d d l « T R E M B L E U R # * ^ y * u j | , L E G E N D S o c k i y e Produc ing L a l i « i - C o n t e n t i o n Waters S T U A R T " ^ . * <5N F R A N C O I S L *o N2i? j N a d i n o R ft L I L L O O E T « 9 * N -4 8 - N -iITTV-- °< 5 s i - > * \ Q ' 1 3 ' Cri...;»<ock R. C A N A D A BRIT ISH C O L U M B I A -W A S H I N G T O N U.S.A. T H E F R A S E R R I V E R W A T E R S H E D 21 Figure 5. (Top) A schematic representation of Jones Creek Channel (MacKinnon, Edgeworth and McLaren, 1961). Figure 5. (Bottom) A schematic representation of the Gates Creek Channel. 23 Figure 7. The marginal vegetation at Gates Creek Spawning Channel. 2. DRIFT BEHAVIOR OF INVERTEBRATES 2.0 DIEL DRIFT RATE OF VARIOUS SPECIES AT JONES AND GATES CREEK CHANNELS , Introduction Die! d r i f t is a common occurrence in streams in the north temperate regions. Some species d r i f t in large numbers during the night (Waters, 1962b; E l l i o t t , 1969), while others are mainly active in the d r i f t during the day (Anderson, 1967; Waters, 1968; E l l i o t t , 1970). The purpose of this part of the work was to separate the night-active from the day-active species, a separation that might lead to better understanding of goodness of f i t of the differing species to the McLay model. Methods A series of three nets (30 cm by 30 cm opening) was randomly placed on l ine across Jones Creek Channel on June 9-10, 1971. At Gates Creek, two nets were used on June 6-7, 1973. Every hour for 24 hours,samples were removed from the nets and fixed in formalin for later evaluation. Light intensity at the water surface was recorded from measurements by a Tri-Lux footcandle meter and the water tem-perature in degrees Celcius on an hourly basis. Results and Discussion The l ight intensity at Jones Creek measurably increased from 0400 hrs to a peak of 204 f t - c at 1300 hrs, with cloudy skies over the fu l l 24-hour period (Figure 8). At Gates Creek, with clear skies, the rise of l ight intensity was more pronounced remaining high between 1200 and 1500 hrs and reaching a maximum of 7913 f t - c at 1300 hrs. The dip at 1400 hrs was caused 25 Figure 8. Temperature (°C), light intensity at the water surface (ft-c) and die! drift of a variety of species of insects nymphs and pupae at Jones and Gates Creek. 27 by a small cloud passing across the sun. The night sky was moonlit. Water temperature at both channels showed l i t t l e variation over the 24-hours. At Jones Creek, minimum was 8°C at 0700 hrs and maximum 10.5°C between 1700 and 2100 hrs (Figure 8). Gates Creek remained relat ively constant at 7°C dipping to 6°C between 0700 and 0800 hrs. The mayfly, Ephemerella sp., was the most common species throughout Jones Creek. At Gates Creek, Ephemerella sp. was poorly represented. En-trance into the d r i f t occurred rather suddenly peaking at 2400 hrs at Jones Creek (Figure 8) and at 0100 hrs at Gates Creek. The maximum d r i f t rate at Jones Creek was 1212 per hour through a cross-section of the channel, and at Gates Creek Channel, 30 per hour. Few individuals were caught during the day at either spawning channel. The Cinygmula sp. were evident in the d r i f t at Jones Creek Channel by 2200 hrs and reached a f i r s t peak at 2400 hrs and a second at 0300 hrs. At Gates Creek there was a sharp increase between 2400 and 0100 hrs and a sharp decrease at 0300 hrs. Neither of the sites produced many Cinygmula sp. by days in the d r i f t . In both channels the d r i f t showed a pattern of rapid increase and rapid decrease over a short time interval. At both s i tes , Paraleptophlebia sp. became active in the d r i f t at 2300 hrs, reached a peak near midnight, and gradually declined to negligible numbers by 0500 hrs. At Jones Creek the maximum was 204 nymphs per hour; at Gates Creek 950 per hour. The high d r i f t rate at Gates Creek did not re f lec t the conditions in the rest of the channel, as can by seen in Figure 9. Appar-ently, nets were set downstream from a local concentration of Paraleptophlebia sp. L i t t l e d r i f t was evident by day. Epeorus sp. was the most common species in the series of experiments at Gates Creek although i t is not evident from Figure 8. Nets were placed halfway between the top of the channel and station 1, and the number caught reached a peak at station 2 (see Figure 4 bottom for station positions). Epeorus did not frequent the upper part of the channel in as large numbers as further downstream. Dr i f t rate increased from 2200 to 0300 hrs, when a sudden drop occurred from the maximum of 760 nymphs per hour. A few continued to 28 Figure 9. The t o t a l numbers of various species of aquat ic in sec t s caught in one hour d r i f t samples in the Gates Creek Channel, from May 25 to May 31 and on June 2, 1973. 30 d r i f t throughout the day. In contrast, at Jones Creek, Epeorus sp. rapidly reached a maximum of 344 per hour at 2400 hrs, dropped of f , reached a second •peak at 0300 to 0400 hrs, then dropped off again. During the day, very few were caught. The dr i f t ing act iv i ty of Baetis sp. at Jones Creek quickly peaked at 2400 hrs, with 24 per hour, then slowly dropped off to become negligible by 0500 hrs. During the day few were found to be in the d r i f t . At Gates Creek, the Baetis sp. gradually bui l t up after 2300 hrs, to reach a peak of 270 per hour at 0300 hrs. By 0500 hrs. the d r i f t had dropped to a low level which persisted throughout the day. Parameletus sp. occurred only at Jones Creek where i t increased after 2200 hrs. to reach a maximum of 84 per hour at 2300 hrs, and then gradually dropped off to 0 by 0400 hrs. During the day, Parameletus sp. was not found in the d r i f t . At both Jones and Gates Creek Channels, the stonefly, Chloroperla sp., sharply increased after 2300 hrs reaching a maximum of 972 per hour at Jones Creek and 180 per hour at Gates Creek at 2400 hrs. Numbers gradually dropped off to 0 by 0600 hrs. During the day no d r i f t of any consequence was evident. The caddisf ly, Lepidostoma sp., was evident in large numbers in 1971 at Jones Creek. It did not occur, however, the second year, and at Gates Creek very few were collected over the whole series of experiments'. The d r i f t pattern of this species differed from that of other species.. Every sample during the 24-hour period contained Lepidostoma sp., and during the day the d r i f t rate was higher (84 per hour) than at night (44 per hour). The maximum dr i f t occurred at 1300 hrs and the minimum at 2400 hrs. Chironomidae pupae were evident in large numbers at Gates Creek Channel with the peak d r i f t of 1060 per hour between 2300 and 2400 hrs. At 0600 hrs the d r i f t rate was reduced to very low numbers. During the day a second but rather small peak occurred at 1300 hrs. 31 For the remaining species, comprising mostly Ephemeroptera and Trichoptera species, at Jones Creek a peak d r i f t rate of 43 per hour occurred at 2400 hrs, but some animals (only one of which was Trichoptera) were caught during the day. Gates Creek showed a similar night time peak at 2400 hrs (80 per hour), but l i t t l e or no d r i f t was evident during the day. For most species, increases in d r i f t rate were mainly related to sun-set and sunrise. Invertebrates entered the d r i f t and were swept downstream in increasing numbers starting one to two hours after sunset. The pattern of evening increase in number of d r i f t organisms did not seem to be affected by weather conditions, as Jones Creek was overcast with periodic ra in , while at Gates Creek i t was clear. The maximum d r i f t rate for some species was shifted an hour later at Gates Creek, perhaps reflecting that the moon disappeared behind the mountains at 2200 hrs. In almost a l l cases, decrease in d r i f t from a maximum was sudden and occurred just before sunrise, suggesting that animals had returned to their daytime practice of concealment in unlighted areas ( E l l i o t t , 1967a, 1968). Temperature change did not seem to have any effect on the increase in numbers of insects into the d r i f t . The peak temperature occurred between 1800 and 2200 hrs at Jones Creek, which was an hour at least before the d r i f t rate began to r ise. Temperature may have affected the amplitude of the nocturnal d r i f t rate. Lennon (1941), wojtalik and Waters (1970) report an increase in d r i f t rate with a r ise in temperature. In summary, the d r i f t rate of most of the individual species showed a signif icant increase after dark. These changes were l i ke ly the result of higher act iv i ty which is known to occur one to two hours after sunset. There was a sharp decrease to daytime lows just at sunrise. A l l the species observed at the two loca l i t ies were night-active except the caddisf ly, Lepidostoma sp., which dr i f ted mainly by day. 32 2.1 SPATIAL AND TEMPORAL VARIATIONS IN DRIFT RATE OF VARIOUS SPECIES OF INSECTS Introduction Percival and Whitehead (1929) were among the f i r s t to note differences in the stream benthos with changes in substrate. Latterly, others have found that the distr ibution was related to food ava i lab i l i ty and to various physical characteristics of the streams. Egglishaw (1964) found a correlation between some species and the detr ita l content, and Minshall (1967), Albrecht (1968) and Thorup (1966) found that certain species feed extensively on algae which are most abundant in unshaded reaches of a stream. Others (Ambiihl, 1959; Bournauld, 1963) looked at the effect of water velocity, under laboratory conditions, on the distribution of certain species of bottom organisms. Madsen (1968) discovered that some species of Plecoptera are highly sensitive to oxygen concentration but other species have been found to show temperature preference in habitat selection ( E l l i o t t , 1972). Few works have attempted to use d r i f t as an indicator of variations in the density of stream bottom dwelling organisms in a homogeneous environ-ment (such as a spawning channel). This part of the work was carried out to look at changes in daily d r i f t patterns and the underlying assumption of homogeneity of density of the benthos impl ic it in the McLay model. Methods Between May 25 and June 8, 1973, at Gates Creek Channel, at seven equidistant stations, along the length of the channel, two nets (30 cm by 30 cm opening) were placed at random by selected locations across the width of the channel. The nets were lowered in sequence into the water at 2300 hrs and raised at 2400 hrs. The depth at each net varied from a maximum of 30 cm to a minimum of 24 cm. The mean depth was 28.3 cm. The mean velocity at each station varied from a maximum of 55 cm/sec to a minimum of 48 cm/sec. The data are presented 33 as col lected: the small changes in depth and velocity at each station were checked and found not to s igni f icant ly influence the volume of water f i l tered at each station. The samples were a l l collected between 2300 and 2400 hrs when the d r i f t rate was on the r ise from the typical diel pattern prevalent in stream benthic insects. Results When the samples were being analyzed the relative mix of detritus to algae was markedly different from the upper to the lower end of the channel. The detritus content dropped off to a negligible level by station 4, but the algal content increased beyond station 4, both in the d r i f t and in growth on the rocky substrate. The daily temperature regime varied between 6°C and 9°C. Additionally, within the channel on a given day, no more than one degree difference was noted between the upper and lower ends of the channel. The number of Ephemerella sp. caught at each of the seven stations (Figure 4) along the Gates Creek Spawning Channel varied, with the greatest number at the upper end of the channel at station 1 where nine animals were caught from the series of one-hour samples (Figure 9). No Ephemerella were caught at stations 2 and 7 and very few at stations 3 to 6. The daily d r i f t varied from a high of f ive Ephemerella per hour from a l l stations simulta-neously on May 30 to none from May 26 to 29 (Figure 10). A highest d r i f t rate of Cinygmula was at the upper end of the spawn-ing channel (39 per hour) and was negligible at station 6. The total daily catch varied from 30 per hour from a l l stations on May 25 and dropped off to one per hour by June 2 (Figure 10). Paraleptophlebia primarily frequented the upper reaches of the channel where 252 were taken from the eight one-hour samples. At the other stations, by comparison, less than six were caught. The daily d r i f t rate varied from 34 Figure 10. The numbers of various species of aquat ic in sec t s in the d r i f t between May 25 and June 2 at each of seven s tat ions in the Gates Creek Channel. PARAMELETUS BAETIS EPEORUS PARALEPTOPHLEBIA CINYGMULA EPHEMERELLA o o o o o o o o o i r I I I ! I I I I I I I I I I- I I I I o v f „ Iv I I I I HI o o o o no OJ o o o o OTHERS TAKEN TOGETHER SIMULIUM I i I i i i o o o o "o o o O l "o CHIRONOMIDAE PUPAE 1 I r o o o o o o o CHLOROPERLA NEMOURA 36 a peak of 53 per hour on May 30 to a low of 8 per hour on May 25, the numbers increasing for the f i r s t f ive days and then stabi l iz ing between 40 and 60 animals per hour. Maximum numbers of Epeorus (4456) were caught at station 2 and dropped off to less than 255 by station 4. The daily d r i f t rate fluctuated from 720 per hour on May 29 to 2849 per hour on May 31. Baetis increased in numbers from the top of the channel to stations 2 (2923 per hour) and 3 (2558 per hour), but downstream there was a sharp decrease. The d r i f t rate showed a decreasing trend during the sampling period, from 1809 per hour May 25 to 499 per hour by June 2. Parameletus was most abundant upstream of station 1. A maximum of 38 animals was caught at station 1, and only one or two at each of the stations be-low station 3. There was a sharp drop from 30 animals per hour on May 25 to be-tween one and six per hour for the rest of the period of observation. Nemoura predominantly occupied the lower reaches of the spawning channel, with a peak of 29 at station 6. The d r i f t rate reached a peak of 25 per hour on May 27 and declined to a low of four per hour on May 29. Chi proper!a was most abundant above station 1 where 363 were caught. Downstream of station 3 the catches were less than 10. The daily d r i f t rate reached a high of 150 per hour on May 25 and sharply decreased to a low of 17 per hour by June 2. Chironomidae pupae were evident in the d r i f t in large numbers at the lower end of the channel. Beyond station 4, between 10,000 and 11,000 were caught at each station. At the upper end of the channel not as many were present, but Chironomidae pupae were by far the most common of a l l the organisms sampled in the d r i f t . The d r i f t rate tended to have l i t t l e f luctu-ation relative to the total number dr i f t ing with the exception of May 28 with a high of 10,595 per hour. 37 Simulium was found in a l l areas of the channel but the largest number (204) was caught at station 4. The d r i f t rate decreased from a high of 183 per hour on May 25 to a low of 55 per hour on June 2. The least abundant species, summed, mainly occupied the upper half of the channel. The largest numbers dr i f t ing were at station 2 and 3 where 178 and 179 were caught, and the lowest was at station .7 (57). The d r i f t rate was extremely high on May 25 relat ive to the other days of observation (362 per hour), but declined sharply on succeeding days to reach a low of 47 per hour on June 2. Discussion (a) Spatial distribution of drifting species The relative abundance of the species at Gates Creek Channel thus showed a marked change along the length, with a greater diversity of species at the upper end. R i f f le habitats are characterized by having species complements which are reflected in the d r i f t . The micro-distribution and the d r i f t rate are interrelated (Lehmkuhl and Anderson, 1972). The pattern of the benthic distribution is partly the result of the d r i f t , and conversely, the benthic distribution modifies the spatial differences in the d r i f t rate of various species. Many species were found to reach their maximum d r i f t rate at d i f fer ing stations along the spawning channel eventhough the common physical character-i s t ics of temperature, water and gravel, depth, gravel size and compactness, velocity and incidental sunlight were relat ively constant throughout the channel. Moreover, because of the similar physical characteristics along the channel length and i ts proximity to the river (Figure 6) one would perhaps expect that f ly ing oviposting females would lay eggs equally throughout the channel length. Addit ional ly, no immigration of catchable nymphs and larvae was evident at the water intake at the top of the channel! Thus, the d r i f t as 38 an on-going phenomenon should have fac i l i t a ted dispersal throughout the channel, but despite the observed d r i f t , the distribution of the species remained re-lat ively constant during the period of investigation. Thus, either a small fraction of the animals d r i f t each day or once they have dri fted they move back upstream, as was observed by Bishop (1969) and E l l i o t t (1971a). Many authors have looked at the feeding relationships of the different running water species (Slack, 1936; Chapman and Demory, 1963; Egglishaw, 1964; Thorup, 1966; Cummins, 1973). Chapman, et a l . (1963) studies the gut contents of several species related to those of Gates Creek. For example, Cinygmula gut contents contained 35% algae and 65% detr itus, Paraleptophlebia was found to consume 5% algae and 95% detritus and Epeorus consumed between 25-60% algae and 40-75% detritus. Ephemerella consumed 60% algae and 40% detritus and Baetis between 55 and 70% algae with the rest detritus. Similarly, Minckley (1963) found Baetis vargans consumed 41% algae and 59% detritus, and Coffman (1967) that nine species of Chironomidae consumed on an average, 78.3% algae and 21.7% detritus, Simulium consumed 89.9% algae and 10.2% detritus, and 98.6% of the gut contents of Nemouridae was animal tissue. The feeding patterns suggested by these workers tend to be reflected by the distr ibution of various species in Gates Creek. Species which consumed a large percentage of detritus were found near the top of the channel where f loating detritus passed through the sett l ing ponds to become broken up by water action and lodged on the stream bed. Algae consumers were found to frequent the lower stations in the channel where algae growth was evident on the substrate and in the d r i f t . Other species, for example: Baetis and Epeorus, which tended to feed equally on algae and detritus, were found to frequent areas where the detritus content was beginning to diminish and the algae content to increase. Although the evidence is circumstantial, the distribution of the Gates Creek species seems to ref lect the food distribution along the channel. The distribution of the different benthic organisms in streams is apparently affected by a complexity of factors, and benthic distribution must be taken into consideration when evaluating stream d r i f t . To assume homogeneity of distribution of benthic organisms seems unreal ist ic when applying the McLay model. 39 (b) Temporal drift rate of the various species at Gates Creek Daily d r i f t rate is dependent on the source of the individuals that enter the d r i f t and their behavior. Waters (1965) suggested that d r i f t could be the result of behavioral characterist ics, catastrophic events and the constant d r i f t of occasional individuals of a l l species which, for one reason or another, acc i -dentally lost their hold on the bottom and dri fted in low numbers without regard to any diel periodicity. In Gates Creek Channel, some species showed a gradual decline of increase in the d r i f t rate during the eight days of observation period, while others were relat ively constant except for sudden peaks. As the physical conditions were v irtual ly constant, i t must be assumed that there were behavioral reasons for the day to day variations. Addit ional ly, the different species travelled differing distances downstream given similar conditions. Bishop and Hynes (1969a) and E l l i o t t (1971b) and others, have found that dr i f t ing species may move back up-stream during the non-drifting hours. One can thus imagine changes in the micro-distribution and density of the individual species which would cause the daily patterns observed. The problem is similar to walking down a busy street where people are walking at different speeds. If the people were classed in groups re-lated to their speed of movement, one would observe, at any particular place, an in f in i te number of patterns of density and group mixes as time progressed. The daily d r i f t pattern is the result of the sum of the physical and behavioral char-acterist ics which are affecting the species mix at the time of observation. In l ight of the above discussion, the McLay model might better be restat-ed, that where there is a mix of species and they have different densities in various parts of a stream, the average density of a l l species in the substrate (No) is a random variable. If organisms of each species are displaced a characteristic distance downstream in the d r i f t , then the rate of leaving the d r i f t (R) may also be considered to be a random variable. The relation described in equation (1) in real i ty may only describe an aggregate phenomenon, related to various abundances of the different species on the substrate, their spatial distribution and the various rates at which they leave and enter the d r i f t . For any particular species observed along any particular reach of a stream, the variations in spatial abun-dance may make measurement of the rate of leaving a risky proposition. 2.2 DISTANCE DRIFTED BY DEAD EPEORUS NYMPHS Introduction Dead nymphs released into the d r i f t cannot actively return to the stream substrate and are removed from the d r i f t by chance effects. For example, they may sett le behind rocks in s t i l l waters by quirks of the current. Needless to say, i f dead animals settle at the same rate as l ive animals, the McLay model is generated through purely chance effects. It was to this chance effect that this part of the work was addressed. Methods Marked dead nymphs were released at increasing distances from fixed nets at Gates Creek Channel. Six nets (30 cm by 30 cm opening) were placed in a l ine across the middle of the stream in an area where turbulence was minimal. Two hundred Epeorus sp. nymphs were spray painted to enable separa-tion of experimental from non-experimental animals. One hundred of the sprayed nymphs were put into a large mouthed glass container and released in the centre of the stream in the middle of the water column at increasing distances from the nets (2, 5, 10, 15, 20, 25, 30 m). Sufficient time was allowed for the animals to be carried to the nets. Lost animals were replaced from the stock so that one hundred was the number released at each distance. Results and discussion Dead marked Epeorus were released at increasing distances upstream of a series of nets. A signif icant portion of the variance associated with the dr i f t ing dead Epeorus was explained by the regression of the numbers on the distance from the release point (Figure 11). The number dropped off l inearly with distance and the rate of loss to the substrate was - 1.057 (p (b = 0) = 0.3014 x 10~ 3). igure 11. The l inear least squares regression of the average number of dead marked Epeorus caught from release points at increasing distances upstream from a net. 42 The dead Epeorus gave a good f i t for a l inear regression of numbers with distance, in an area where l i t t l e turbulence was observed. It can be envisioned, however, that the form of the loss from the d r i f t of passively dr i f t ing animals is at the whim of the current (there were no f i sh in the channel). If the flow is laminar the distance dr i f ted is a function of sinking rate and velocity. E l l i o t t (1971b) observed that dead invertebrates are lost from the d r i f t at a slower rate than l ive off the same species. If the water in a stream is extremely turbulent the probability of a dead orga-nism leaving the d r i f t may increase because the number of dead spots in the current increase. More formally, i f the probability of sett l ing is a random variable, then the animals should set t le , according to the McLay model, in a logarithmic fashion. This could happen in situations where opportunities for sett l ing were unlimited, or where i t was irrelevant where an animal sett led. However, i f the number sett l ing is constant, as was the case for dead Epeorus in the linear f i t , then either the number of opportunities is constant or the number of physical events that fortuitously affect set t l ing, is a constant. Thus, dead dr i f t ing organisms may drop out of the water in a fashion similar to the relationship predicted by the McLay model, only i f current conditions are pro-pitious. If the current is not "just r ight", the relationship would be other, such as l inear or possibly l inear with slope of zero when the current flow is laminar. 43 3. LABORATORY STUDIES 3.0 THE DISTANCE DRIFTED BY VARIOUS SPECIES IN AN ARTIFICIAL STREAM WITH LAMINAR FLOW Intvoduction The behavior of organisms entering the d r i f t can greatly affect the distance dr i f ted. Additionally, density dependent interactions between in -dividuals of the same and different species which are on the substrate affect the dr i ft ing pattern of individuals of various species. It was the intent of this part of the work to determine the response of various dr i f t ing species to laminar flow and changes in the benthic density. Methods An a r t i f i c i a l stream was constructed at Abbotsford Trout Hatchery. The stream was 7.62 cm wide, 15.24 cm deep and 731.52 cm long. One side and the bottom were constructed of 1.90 cm plywood, the second side was 0.64 cm clear luc i te. A laminar water velocity of 45 cm/sec was in i t iated and the stream le f t for 2% months, when i t was checked for organisms which were removed. Insects were gathered at Jones Creek and transported to Abbotsford and tested in the experimental stream on the same day. The procedure was f i r s t to release an individual of a particular species from a large pipette into the middle of the water column and then to record where i t attached to the substrate. The clear luc i te allowed observation of responses while the animal was in the water column. The animal was then removed and a new organism of the same species was used. Each animal was allowed to leave the pipette and enter the moving water on i t s own in i t i a t i ve . In a second series of observations, the same procedure was used but animals were not removed once they attached to the substrate. 44 The data was expressed as the percentage of the total number released that attached to the substrate in each 20 cm interval of the length of the a r t i f i c i a l stream. Results and Discussion Individuals of Cinygmula and Baetis actively moved to the substrate by quickly orienting and swimming against the current, descending gradually to the substrate while slowly being displaced downstream. Presumably, they were search-ing for a suitable s i te for attachment. Cinygmula reached and attached to the substrate mainly between 160-220 cm from the release point, while Baetis obtained sites between 0-80 cm (Table-I). The differences in distance can be explained by the differences in morph-ology and behavior. Cinygmula (Figure 12) has a short dorso-ventrally flattened body with short t a i l and cerc i . The swimming motion is re lat ively slow compared to the Baetis sp. Baetis (Figure 12), has a long tubular shaped abdomen which is f lexible and a long t a i l and cerci to push the animal through the water. Thus, The Cinygmula are not as good swimmers as the Baetis sp. A species of Chloroperla (Figure 12), swam by dorso-ventral flexion and movement of the legs, and occupied attachment sites on the substrate more than 300 cm from the release point. Nemoura and Ephemerella grandis ingens'McD. (Figure 12) assumed a fixed body position and did not orient to the current. They drifted through the a r t i f i c i a l channel with legs extended latera l ly from the body and bent ventrally at the joint of the femur and t i b i a . Their cerci were held at an upward angle of about 30°from the horizontal and spread latera l ly forming an angle of about 90° with respect to each other. For the Ephemerella sp. the attitude of the body resulted in a high resistance with respect.to the surrounding mass of moving.water. The animal was swirled about by the sl ightest change in water pressure caused by turbulence. Species that are able to swim effect ively in the fast moving water of streams orientated and moved to the bottom, while those that were motionless moved at the whim of the water current and i t s turbulence. I. The distance drifted by individuals of two species of mayfly in an a r t i f i c i a l stream with laminar flow (45 cm/sec) when (a) the individuals released were removed from the stream after they attached to the bottom, and (b) the individuals were not removed Cinygmula Baetis Distance Travelled Before Attachment (a) (b) (a) (b) Lcm) Percent Percent Percent Percent 20 1 1 8 12 40 3 1 34 6 60 6 3 19 6 80 2 3 10 13 100 2 4 5 11 120 5 2 6 13 140 4 5 4 7 160 9 7 6 5 180 18 9 3 2 200 10 6 _ 1 220 17 7 1 1 240 3 7 1 2 260 2 3 — 4 280 1 4 1 300 3 3 320 2 _ 340 1 - 1 1 360 3 _ _ 380 2 2 _ 400 1 1 1 3 1 1 1 * 700 I 5 I 18 1 1 1 Did not attach 12 16 Figure 12. The body shape of some species of mayfly and stonefly nymphs observed in the a r t i f i c i a l stream. The average lengths w e r e : Cinygmula 8.5mm; Baetis 6 mm; Chloroperla 9 mm; Ephemerella 13 mm. 47 Current patterns greatly influence the distance drifted. Turbulence can return any of the species almost immediately to the substrate. An important factor in obtaining a s i te on the substrate, is the ab i l i t y to attach to the substrate. When turbulence was introduced into the a r t i f i c i a l stream a l l the species were able to attach where a downward current created an oppor-tunity. A l l of the Baetis sp. and Cinygmula sp. found sites at increasing distances just below the area of turbulence, which was much closer to their respective sites of attachment in laminar flow. To a lesser degree, other species that were found not to attach also found sites below the area of tur-bulence. Seventy per cent of Ephemerella found s i tes , but the remaining 30 per cent drifted through the channel. Similarly, 50 per cent of Nemoura sp. found sites and 80 per cent of the Chloroperla. Thus, a l l species took ad-vantage of the current structure in finding sites on the substrate. Density affected the ab i l i t y of individuals to f ind a position to occupy on the substrate. Cinygmula and Baetis f i r s t occupied the preferred area of the substrate. Additional animals oriented and were poised to sett le, but, where several sites had been occupied, the animals continued to dr i f t downstream before selecting a s i te to attach. Thus, as the available sites f i l l e d , the animals drifted farther before attachment. Some of the individuals of the two species oriented and landed in areas already occupied, but either the occupying animal or in some cases, the arriving animal, almost immediately departed. In either case, a l l surrounding animals nearby moved s l ight ly in response to the intruder. The density of organisms in the substrate of the same species as the dr i f t ing organisms, greatly affects the distance the d r i f t -ing animals travel. Whether the arrival of other species causes the same response was not tested, but i t seems l i ke ly that for some combinations of species, this might also occur. 48 3.1 EFFECTS OF SUBSTRATE SURFACE AREA ON CARRYING CAPACITY Introduction The causes of d r i f t must be sought before a better understanding of the relationship between d r i f t and benthos numbers wi l l be forthcoming. Dimond (1967) plotted the benthic density of organisms against the d r i f t and showed a sharp inf lect ion at a density of about 450 insects per 0.9m , suggesting that the data might best be f i t ted by two straight lines which converged at an in -f lect ion point. The causal factors of d r i f t are thus perhaps twofold, one reflecting the effect of physical factors, the other, behavioral factors related to density. It was the intent of this part of the study to look at the effect of change in available space of carrying capacity of stream substrate for stream organisms. Methods Four boxes, 30 cm long by 15 cm wide by 7 cm deep, were constructed of clear lucite and covered on the sides and bottom with black polyproplene (Figure 13). The boxes were set up in a laboratory at the Abbotsford Trout Hatchery so that the l ight period and water flow could be controlled. The l ight period was from 0600-1800 hrs and the dark from 1800-0600 hrs. Each box was fed with water from a header box and drained into an aquarium. Nets were placed at the outfal l to catch nymphs which l e f t the box. Rocks were collected from a stream near the laboratory where Baetis sp. was known to occur. Each box contained 36 rocks of similar size on the f i r s t t r i a l and 18 rocks on the second. Each day at 0600 and. 1800 hrs, the nymphs which had l e f t the boxes were counted, tabulated and reintroduced into the boxes. A number of new nymphs was collected from the stream and added to those already in each box. Figure 13. An a r t i f i c i a l area of "stream bed" made from uniform rocks collected at sites in a stream where test animals were known to occur. 50 The re-introductions were considered to be successful when half an hour passed without a nymph leaving the box. The experiments with 18 rocks per box were continued for 14 days, and those for 36 rocks for 17 days, each day using a different number of organisms in the range of 10 to 700. A l l the nymphs were counted at the end of each experiment to determine losses. The data was treated by l inear regression. Two lines of best f i t were calculated for each set of data. Data points near the point of intersection were used alternatively to calculate both regression l ines , and the two lines eventually chosen were those that in combination yielded the smallest aggregate sums of squares of deviations. Results The relationship between d r i f t numbers and substrate numbers was best described by the two intersecting regression l ines (Figures 14a,b), the inter-section representing an estimate of the carrying capacity of the boxes. The regression l i ne , to the l e f t of the intersection, can be considered as repre-senting constant d r i f t due to a variety of circumstances, but the regression l ine, to the right of the intersection, represents a behavioral response. A behavioral d r i f t response occurred at a s l i ght ly higher density by day than by night. In the 18 rock s i tuat ion, the point of intersection of the regression lines occurred at approximately 400 organisms, and for the 36 rock situation, at approximately 300. The proportion of animals leaving was lower by day than by night and lower for 18 than for 36 rocks (Table II). The difference in the behavioral and constant departure rates,which represents the behavioral response beyond the background leve l , was in a l l cases of about the same order of magnitude (18 rocks by day .359, by night .325, 36 rocks by day .384, by night .295). 600 500 400 300 o CO 5 200 S ce o •ct cc UJ 100 300 200 100 O h 0 100 200 300 400 500 600 700 AVERAGE NUMBER RELEASEO IN BOX Figure 14a. The number of Baetis remaining in an a r t i f i c i a l stream bed of 18 rocks at the end of 12 hours of daylight (X) and darkness (0) periods, for various i n i t i a l densities of organisms. Two least squares lines of best f i t were generated through the daylight data points and two through the darkness data points. Figure 14b. The same for a stream bed of 36 rocks. 52 TABLE II. The average rate of departure ( (1-b) where b is the slope of the regression) of Baetis leaving boxes which contained 18 rocks and 36 rocks during periods of daylight and darkness. 13 Rocks Per Box 36 Rocks Per Box Constant Behavioral Constant Behavioral Dr i f t Dr i f t Dr i f t Dr i f t Day 0.165 0.524 0.405 0.789 Night 0.509 0.834 0.570 0.865 Discussion The relationship between the d r i f t and the benthos density can fluctuate greatly within an area on a day to day basis, but the "carrying capacity" of the gravel is evidently greater during the day when the insects are less active than they are at night. The density of animals which occupies an area prior to the start of the l ight or dark period thus determines whether the d r i f t is caused by a constant or both constant and behavioral responses. Regardless of the number of rocks, the density at which the behavioral response was in i t ia ted was only s l ight ly higher for day t r i a l s than at night, but both constant and behavioral responses were higher at night. The difference between the two responses, which presumably reflects the effects of density was similar throughout the whole experiment. Perhaps this indicates that the density dependent behavior of the animals is directly related to the act iv i ty patterns of the organisms. The movement of Baetis during the night is greater than by day, thus the probability of one animal jost l ing with another is greater than during the day and could cause a higher d r i f t rate. More formally, i f act iv i ty were visualized as random movement, then the number of contacts between animals, and hence the d r i f t , would be direct ly proportional to the act iv i ty and to the square of the density. In this case, the relation between number leaving would be parabolic, which is evidently not the case in the experimental boxes. It seems more reasonable to assume that the constant component is related to act iv i ty that 53 is unrelated to density, in consequence of which, the regression lines to the l e f t of the intersection are in a l l cases straight l ines. Beyond the inter-section i t could be argued that the points would as well f i t a parabola, and might ref lect behavioral effects that are proportional to the square of the density which tel'e place when a certain threshold density is exceeded. This explanation has the merit of accounting for the relat ively fixed difference between the constant and behavioral responses. The organisms are more active by night than by day and hence have a higher constant rate of departure, but this act iv i ty is not relevant to the behavioral factors that increase the rate of departure at higher densit ies. When fewer rocks were available there were lower departure rates which seems somewhat surprising. However, current apparently plays a role in l imit in the area that Baetis can occupy. The same rocks were used in both 18 and 36 rock boxes, but the flow characteristics around each individual rock were obvi-ously di f ferent, there being much less vigorous flow with 36 rocks. The change in flow presumably increased the area available for occupancy. One would thus expect this species of Baetis to occur in greater numbers when flow was at some optimum rate which would create the greatest number of good s i tes. Perhaps current is one of the major factors in the patchy distr ibution of Baetis and many aquatic invertebrates which frequent r i f f l e areas. Addit ional ly, for Baetis, the importance of the current could influence the vert ical distribution in the substrate. 4. METHODOLOGICAL STUDIFS 4.0 REPEATED DISTURBANCE OF THE SAME AREA OF SUBSTRATE Intvoduation Many methods of disturbing invertebrates of the stream substrate can be devised, - dragging a garden rake over an area of stream bottom or forcing high pressure a i r through a pipe into the substrate would suf f ice. It was not the intent of this work to determine a "perfect" method for getting invertebrates to d r i f t . Hence, a simple method of kicking up the substrate by shuffling across the stream was used. Morgan and Egglishaw (1965) suggested that kicking up the substrate was "most effect ive" in removing "just about a l l " the invertebrates. This was also the method used by McLay (1970) when his model was developed. It was, however, useful to estimate the effectiveness of the method to determine the relation between how many were disturbed and the numbers occupying the substrate. If the substrate is disturbed over the same area a number of times, one would expect that with each disturbance the same proportion of animals would leave the substrate. The number caught (N^) immediately downstream from succes-sive disturbances would be: N E = No e " A t (13) where No is the total number of animals which occupy the substrate A is the proportion disturbed t is the number of disturbances. Thus, i f "A" is known for each of the species observed, estimates of the total number occupying the substrate can be calculated from single disturbance experiments. 55 Methods Two 30 cm by 30 cm by 95 cm high right cone nets were placed at random chosen points on a l ine across the channel. For each t r i a l an area 22 cm wide, from one side of the stream to the other, was disturbed by "shuff l ing" across direct ly in front of the nets six successive times at two minute intervals. The samples from the four t r i a l s were removed from the nets between shuffles and fixed in 7% formalin for later evaluation. At the end of the six t r i a l s , bottom samples were taken from the disturbed area. Catches were expressed as percentages of the eventual total number caught and 95% confidence l imits were calculated for those samples which contained 10 animals or more. The data was f i t ted by regression of the natural logarithm of the average number caught, on the number of disturbances. Results and Discussion The percentage leaving the substrate from the f i r s t disturbance (of the total that were caught from six disturbances) varied from t r i a l to t r i a l (Table I l i a ) , but was close to 50% for the organisms that were suf f ic ient ly abundant for good estimates. The 95% confidence intervals consistently overlapped. Hence, the method used was consistent in removing, in the f i r s t disturbance, a relat ively constant percentage of those common animals that were "disturbable". The less abundant species showed a greater apparent variation which would be expected because of the small sample s ize. Nevertheless, the range of the average number of three mayfly species (Ephemerella, Cinygmula and Paraleptophlebia) coincided with that of the more common species ( i .e. between 40 and 60% of the total displaced were caught at the f i r s t disturbance). Simulium sp., was found not to be easi ly disturbed into the d r i f t eventhough i t was very abundant on the disturbed rocks. Perhaps this reflects the tenacity with which Simulium can hold to the rocky substrate. For the other species, the number remaining in the stream bed, after a l l the disturbances, was an average of 32%, with a range of 28-36%. TABLE I l i a . The percentage of the total number of disturbed organisms displaced from the stream gravel bed by the f i r s t of a series of disturbances. The middle figure is the mean, the others are lower and upper 95% confidence l imits . Percentage Displaced Species Tr ia l 1 Tr ia l 2 Tr ia l 3 Trai l 4 Mean Summed 47-51-55 38-42-46 48-51-54 46-51-55 49% Ephemerella * 50 * 67 - - -Cinygmula * 38 - - * 44 -Paraleptophlebia * 67 * 63 31-59-69 - -Epeorus 47-51-55 41-45-49 45-48-51 48-52-56 49% Baetis 41-51-61 41-48-56 48-57-66 40-48-58 51% Simulium * 25 - - - -Others 43-61-81 39-54-68 48-63-76 44-59-72 59% * Samples contained a total of less than 10 organsims. In log-arithmetic regression analysis of the numbers taken from succes sive disturbances, a s ignif icant proportion of the var iab i l i t y was explained for a l l species, except for the category Other Species. There were no s ignif -icant difference among the slopes for different t r i a l s for the Summed Species, Epeorus and Other Species, but for Baetis the slopes were s igni f icant ly hetero geneous {a = 0.05)(Table IIlb). TABLE 11 lb. The proportion (A) of invertebrates which l e f t the substrate in successive disturbances of the substrate. Species Date Tr ia l Summed Epeorus Baetis Others June 9 1 -.4152 -.4165 -.3871 -.3870 2 -.5654 -.4984 -.7475 -.7326 3 -.5518 -.5054 -.7443 -.6535 4 -.4397 -.4663 -.4518 -.3422 A simple negative exponential relationship was a good f i t in a l l case but the "Others" category. "Others" comprise the various less abundant speci and therefore, s l ight modifications in the species mix affect the analysis. The rate of departure from the substrate due to disturbance was thus found to be relat ively constant between t r i a l s for most species and the value of the proportion departing (A) could be used to calculate the number occupying the disturbed area of substrate from the number taken in the d r i f t . This wi l l be considered in another section of the work. 58 4.1 CHANGES IN CATCHING RATE AFTER A DISTURBANCE Intro duct-ion The length of time that nets are maintained in a stream may be impor-tant in determining what proportion are caught of the organisms that have been a r t i f i c i a l l y introduced into the d r i f t . If the animals d r i f t passively then the velocity wi l l determine how quickly the introduced pulse of animals wi l l reach the catching nets. If the movement is intermittent, nets should be. in place for a longer period. This part of the work was carried out to determine when an introduced pulse of animals returned to the substrate. Methods Three nets were placed at each of four stations located one, two, three and f ive meters below the s ite of a disturbance, and three immediately above the s ite of the disturbance to measure the natural d r i f t . At each station the three nets were placed at randomly chosen points within the right, middle and le f t thirds of the width of the stream (right, middle and l e f t were denoted, by facing downstream). At each time of sampling, two of the three nets were sampled. For example, at distances of one and three meters below the disturbance, the middle and the right net were sampled; at two meters, the middle and the l e f t nets, and at five meters, the le f t and right nets. The samples were taken at 7.5, 15, 30, 60 and 120 minutes after a disturbance. The three control nets were sampled at the same times as the experimental. At the end of two hours a l l the nets were raised and a l l the samples fixed in 7% formalin for later evaluation. The data were adjusted for variations of velocity which occurred at each net, by dividing the average velocity by the net velocity. The number caught was divided by the time the net was catching, to calculate the catching rate. Results and Discussion Al l species took longer to d r i f t f ive meters from a disturbance than would be expected i f velocity were the only governing factor. The average velocity was 47.3 cm/sec. Thus, the dr i f t ing animals would have passed the f ive meter mark by approximately 11 sec i f the distance travelled was only velocity dependent. At 0.125 hrs, for a l l species summed, about 10 times the average control d r i f t rate was caught at each of the distances at which nets were placed (Table IVa). Thus, by 0.125 hrs, by far the largest proportion of the disturbed animals had passed the f ive meter mark. During the second seven minutes about f ive times the control d r i f t was caught. On the third and succeeding samples the experimental d r i f t had dropped to a lower level than the control, and this was also true for the dominant species (Tables IVb,c,d). The experimental catching rate was higher than the control during the f i r s t 15 minutes of the t r i a l , but lower for the succeeding samples. TABLE IVa. The average catching rate (per hour) for the summed species in nets placed at various distances downstream of a disturbance. Sample Time Distance from Disturbance  (Hours) Control 1 meter 2 meters 3 meters 5 meters 0. -0.125 269 3192 2277 1554 2485 0.125-0.25 254 946 1262 862 663 0.25 -0.50 276 76 100 116 184 0.50 -1.00 344 144 156 192 242 1.00 -2.00 214 142 170 140 79 60 TABLE IVb. The average catching rate (per hour) of Epeorus in nets placed at various distances downstream of a disturbance. Sample Time (Hours) Control Distance from Disturbance 1 meter 2 meters 3 meters 5 meters 0. -0.125 146 2869 1808 1154 1815 0.125-0.25 153 800 938 669 515 0.25 -0.50 160 48 44 72 152 0.50 -1.00 170 90 96 146 204 1.00 -2.00 144 108 126 105 59 TABLE IVc. The average catching rate (per hour) for Baetis in nets placed at various distances downstream of a disturbance. Sample Time Distance from Disturbance (Hours) Control 1 meter 2 meters 3 meters 5 meters 0. -0.125 31 269 315 223 431 0.125-0.25 69 85 192 162 138 0.25 -0.50 56 16 44 32 28 0.50 -1.00 48 30 34 20 34 1.00 -2.00 45 30 37 31 20 TABLE IVd. The average catching rate (per hour) for Chironomidae (pupae in nets placed at various distances downstream of a disturbai Sample Time (Hours) Distance from Disturbance Control 1 meter 2 meters 3 meters 5 meters 0. -0.125 46 31 131 162 246 0.125-0.25 15 69 46 31 15 0.25 -0.50 56 12 16 16 4 0.50 -1.00 40 24 24 16 4 1.00 -2.00 25 2 4 5 1 The high catching rate during the f i r s t 15 minutes was the result of the introduced pulse of animals, but, the time required for the pulse to pass was longer than expected, i f velocity were the only controll ing factor. The disturbed animals appear to enter the d r i f t , then settle and remain on the o bottom for a period of time, then enter the d r i f t again. Thus, the movement was intermittent, the animals entering and leaving the d r i f t in some kind of a damped osc i l lat ion pattern until the d r i f t returned to natural (control) levels. The lower subsequent d r i f t rate was perhaps a reflection of the dis-tribution of the animals on the substrate prior to the disturbance. More l i ke ly , however, was that the density changes which occurred on the bottom of the stream as the introduced pulse passed downstream, caused more animals to enter the d r i f t than would naturally enter. Thus, a reduction in the bottom density would have occurred and a lower d r i f t rate would be expected. 62 5. FIELD EXPERIMENTS 5.0 DISTANCE DRIFTED AFTER A DISTURBANCE Introduction The McLay model states that the number expected in the d r i f t below an introduction is function of the number introduced, the negative exponential of the rate of return to the stream bed and the distance downstream from the intro-duction (equation 1). Hypothetically, the d r i f t rate at any point is a sum of the individual events which cause individuals to enter the d r i f t . The intention of this part of the work was to test the McLay model by repetitive collections of d r i f t in two differing streams under di f fer ing con-ditions of l ight and velocity, variables which might be expected to influence d r i f t rate. Methods A series of three nets was placed in the stream at chosen distances from a disturbance. The width of the disturbance from one side of the channel to the other was 22 cm giving a total area of 0.935 sq meters at Jones Creek and 1.34 sq meters at Gates Creek. The procedure was to lower the nets, shuffle across the stream, and raise the nets h or 1 hour after the disturbance and f ix the samples in 1% formalin for later evaluation. Ten night t r i a l s were carried out between 2200 hrs and 2400 hrs. One day t r i a l was at 102 f t - c and the other at 4090 f t - c . Two of the night t r i a l s and both of the day t r i a l s were at Jones Creek, while the other eight night t r i a l s were at Gates Creek. Disturbances carried out at Gates Creek were done at different velocit ies. Deliberately, no regular pattern was followed when I / 63 increasing or decreasing the velocity between t r i a l s . Velocity was regulated by adjusting a gate valve at the head end of the channel. At Gates Creek each morning, 0800 hrs on successive days, the valve was changed and between 2200 and 2400 hrs in che evening, a t r i a l was performed. In a l l of the experiments the velocity was measured at each net opening by a Tsurumi-Seiki Kosakusho flow meter. The temperature was not controlled and varied between 8°C and 10°C over the period of a l l of the experiments. Occasionally the non-linear f i t t i ng routine for f i t t i ng the data e s t i -mated negative values for the natural d r i f t . Where this occurred, the d r i f t was recorded as zero. Results Under differing conditions of l ight intensity and water velocity the results of disturbance experiments conformed to the McLay model. The observed numbers caught at various distances were adequately explained by the theoretical exponential decay model (Figure 15), but the value of the estimated natural d r i f t by f i t t i n g was in general a poor estimator of that which occurred in the control nets above the disturbance. The rate of return to the substrate and thus, the average distance travelled by individuals,differed with conditions (Table V). At Jones Creek, at night, rates of return to the substrate for two t r i a l s were similar, as were the velocit ies (Tables V and VI). The day t r i a l s were not as consistent. For the t r i a l of June 1, the unconstrained f i t t i n g routine estimated a negative natural d r i f t . The scatter of the points suggested that a l inear re-lationship would possibly f i t better (Figure 15). The mix of species was es-sentia l ly the same as for other t r i a l s and no reasons for this departure were noted. The June 7 day disturbance resulted in a quick return to the substrate by the summed species. When compared to the night t r i a l s the rate of return was faster. The average water velocity was slower during the day than at night so that the velocity could account in part for the faster return rate. The 64 Figure 15. The l ine of best f i t , for a l l species summed, of mean number caught at various distances downstream from a disturbance of the stream bottom. Each t r i a l was carried out under differing l ight intensit ies and water velocit ies (see Tables V and VI). 65 150 r 125 -100 -75 -50 -25 • 0 • 125 • 100 • 75 50 • 25 • 0 50 • . 40 • ] 30 '. 20 '• 10 > 0 • i 50 • ; 40 [ 3 0 c 20 • 10 0 200 • 150 • 100 • 50 -0 -700 -600 -500 -400 -300 -200 -100 -0 -JONES CREEK, MAY 12, 1971 JONES CREEK, MAY 13, 1971 JONES CREEK, JUNE 1, 1971 JONES CREEK, JUNE 8. 1971 GATES CREEK, JUNE 2, 1973 GATES CREEK, JUNE 8, 1973 300 250 200 150 100 50 0 175 150 125 100 75 50 25 0 250 <E 200 U J X t 150 o 2 100 U J £ 50 co LU G 0 U J o_ CO ri 2 0 0 < u. ° 150 lu o < § 100 50 \ ^ GATES CREEK, JUNE 9, 1973 x 10 15 0 300 250 200 150 100 50 ' 0 175 150 125 100 75 50 25 h 0 N.. A V I* GATES CREEK, JUNE II, 1973 GATES CREEK, JUNE 12, 1973 . GATES CREEK, JUNE 13, 1973 GATES CREEK, JUNE 14, 1973 GATES CREEK, JUNE 15, 1973 2 0 DISTANCE (METERS) 10 15 20 66 TABLE V . The e s t i m a t e d number o f I n v e r t e b r a t e s i n t r o d u c e d i n t o t h e d r i f t f r o m a d i s t u r b a n c e , t h e r a t e o f r e t u r n t o t h e s u b s t r a t e , t h e a v e r a g e d i s t a n c e t r a v e l l e d and t h e e s t i m a t e d a n d o b s e r v e d n a t u r a l d r i f t o f t r i a l s a t v a r i o u s l i g h t i n t e n s i t i e s and v e l o c i t i e s a t J o n e s a n d G a t e s C r e e k . A v e r a g e o b s e r v e d n a t u r a l d r i f t was o b t a i n e d f r o m n e t s a m p l e s t a k e n u p s t r e a m o f t h e d i s t u r b a n c e . S P E C I E S SPAWNING CHANNEL TIME OF DAY DATE ESTIMATED RATE OF RETURN TO THE SUBSTRATE ESTIMATED DISTURBED ANIMALS CAUGHT AVERAGE OBSERVED NATURAL DRIFT CAUGHT ESTIMATED NATURAL DRIFT CAUGHT AVERAGE DISTANCE TRAVELLED IN DRIFT ( m e t e r s ) Summed J o n e s N i g h t May 1 2 , 1971 - 0 . 4 1 7 0 109 19 19 2 . 4 0 J o n e s N i g h t May 1 3 , 1971 - 0 . 4 2 8 9 84 2 8 41 2 . 3 3 J o n e s Day J u n e 1 , 1971 - 0 . 0 3 0 8 54 7 - 1 9 3 2 . 4 7 J o n e s Day J u n e 8 , 1971 - 1 . 9 8 9 9 189 1 12 0 . 5 0 G a t e s N i g h t J u n e 2 , 1973 - 0 . 1 1 9 6 1 8 3 96 6 3 8 . 3 6 G a t e s N i g h t J u n e " 8 , 1973 - 0 . 3 0 2 4 272 4 7 5 5 4 S 3 . 3 1 G a t e s N i g h t J u n e 9 , 1973 - 0 . 3 6 1 1 1 6 8 101 101 2 . 7 7 G a t e s N i g h t J u n e 1 1 , 1973 - 0 . 2 1 1 3 • 132 8 0 49 4 . 7 3 G a t e s N i g h t J u n e 1 2 , 1973 - 1 . 7 1 8 5 8 9 4 5 3 44 0 . 5 8 G a t e s N i g h t J u n e 1 3 , 1 9 7 3 - 0 . 2 7 6 6 129 9 5 64 3 . 6 2 G a t e s N i g h t J u n e 1 4 , 1973 - 0 . 0 7 2 2 254 114 6 9 1 3 . 8 5 G a t e s N i g h t J u n e 1 5 , 1973 - 0 . 0 7 3 7 102 67 67 1 3 . 5 7 E p h e m e r e l l a J o n e s • Day J u n e 1 , 1971 - 0 . 2 5 4 8 7 - 1 3 . 9 2 J o n e s Day J u n e 8 , 1971 - 0 . 2 6 3 0 6 - 1 3 . 8 0 C i n y g m u l a J o n e s N i g h t May 1 2 , 1971 - 0 . 6 3 0 5 19 - 1 1 . 5 9 J o n e s N i g h t May 1 3 , 1971 - 0 . 0 7 0 5 13 1 - 2 1 4 . 1 8 J o n e s Day J u n e 1 , 1971 - 0 . 1 3 4 2 7 - 1 7 . 4 5 J o n e s Day J u n e 8 , 1971 - 0 . 8 5 0 4 2 3 - 1 1 . 1 8 ' P a r a m e l e t u s J o n e s N i g h t May 1 2 , 1971 - 0 . 6 1 7 1 13 7 2 1 . 6 2 J o n e s N i g h t May 1 3 , 1971 - 1 . 6 2 2 0 102 2 2 2 0 0 . 6 2 P a r a l e p t o p h l e b i a J o n e s N i g h t May 1 2 , 1971 - 0 . 1 1 6 7 35 4 - 8 . 5 7 J o n e s N i g h t May 1 3 , 1971 - 0 . 0 5 6 5 30 5 - 4 1 7 . 7 0 J o n e s Day J u n e 1 , 1971 - 0 . 1 6 9 8 7 - 1 5 . 8 9 J o n e s Day J u n e 8 , 1971 - 0 . 2 4 7 5 4 - 1 4 . 0 4 G a t e s N i g h t J u n e 9 , 1973 - 0 . 2 5 2 2 4 2 1 3 . 9 7 G a t e s N i g h t J u n e 1 3 , 1973 - 1 . 0 7 8 5 15 2 1 0 . 9 3 J o n e s N i g h t May 1 2 , 1971 - 2 . 3 1 9 2 151 1 2 0 . 4 3 J o n e s N i g h t May 1 3 , 1971 - 0 . 9 3 2 0 11 1 . 1 1 . 0 7 G a t e s N i g h t J u n e 2 , 1973 - 0 . 1 5 2 6 129 4 9 33 6 . 5 5 G a t e s N i g h t J u n e 8 , 1973 - 0 . 3 2 9 7 184 3 0 8 454 3 . 0 3 G a t e s N i g h t J u n e 9 , 1973 - 0 . 2 6 8 2 127 71 77 3 . 7 3 G a t e s N i g h t J u n e 1 1 , 1973 - 0 . 3 2 7 7 121 4 5 4 2 3 . 0 5 G a t e s N i g h t J u n e 1 2 , 1973 - 1 . 7 8 0 4 874 4 3 38 0 . 5 6 G a t e s N i g h t J u n e 1 3 , 1973 - 0 . 2 8 1 0 109 8 0 56 3 . 5 6 G a t e s N i g h t J u n e 1 4 , 1973 - 0 . 0 6 8 1 2 2 3 114 50 1 4 . 6 8 G a t e s N i g h t J u n e 1 5 , 1 9 7 3 - 0 . 2 1 3 5 60 • 51 76 4 . 6 8 67 TABLE V. (CONTINUED) SPECIES SPAWNING CHANNEL TIME OF DAY DATE ESTIMATED RATE OF RETURN TO THE SUBSTRATE ESTIMATED DISTURBED ANIMALS CAUGHT AVERAGE OBSERVED NATURAL DRIFT CAUGHT ESTIMATED NATURAL DRIFT CAUGHT AVERAGE DISTANCE TRAVELLED IN DRIFT (meters) Jones Night May 12, 1971 -1.8023 Gates Night June 2, 1973 -0.0285 Gates Night June 8, 1973 -0.0461 Gates Night June 9, 1973 -0.1366 Gates Night June 11, 1973 -0.1263 Gates Night June 12, 1973 -0.9281 Gates Night June 13, 1973 -0.2130 Gates Night June 14, 1973 -0.0376 Gates Night June 15, 1973 -0.0123 47 1 2 0.55 74 27 -19 35.09 92 94 31 21.69 15 19 12 7.32 22 18 , 8 7.92 31 9 4 1.08 11 11 7 4.69 29 11 6 26.60 63 10 -31 81.43 Chi proper! a Jones Night May 12, 1971 -0.2974 1 3 2 1 3 36 Jones Night May 13, 1971 -0.4392 9 - 3 2*28 Jones Day June 1, 1971 -0.2037 4 - 1 4^ 91 Lepidostoma Jones Night May 12, 1971 -0.1960 3 4 6 5 10 Jones Day June 1, 1971 -0.3024 1 5 6 3!o7 Chironomidae Gates Night June 2, 1973 -0.2237 12 15 11 4.47 Gates Night June 8, 1973 -0.0208 -23 66 51 48.08 Gates Night June 9, 1973 -0.6873 12 7 2 1.45 Gates Night June 14, 1973 -4.3477 1179 5 21 0.23 Others Jones Night May 12, 1971 -0.2391 6 2 2 4.18 Jones Night May 13, 1971 -1.2546 15 1 1 0.64 Jones Day June 1, 1971 -0.3251 5 1 1 3.08 Jones Day June 8, 1971 -0.4054 13 - 2 2.47 Gates Night June 2, 1973 -3.6344 94 3 2 0.28 Gates Night June 8, 1973 -0.3515 10 2 2 2.84 Gates Night June 9, 1973 -1.5661 14 2 2 0.64 Gates Night June 13, 1973 -1.1179 17 1 2 0.89 Gates Night June 15 -0.1241 18 4 6 8.06 TABLE VI. The average velocity and l ight intensity in the experimental area'during the various t r i a l s . JONES CREEK CHANNEL • Date  May 12 May 13 June 1 June 8 Conditions 1971 1971 1971 1971 Velocity (m/sec) 0.766 0.758 0.664 0.791 Light Intensity (ft-c) 102 4080 GATES CREEK CHANNEL Date  June 2 June 8 June 9 June 11 June 12 June 13 June 14 June 15 Conditions 1973 1973 1973 1973 1973 1973 1973 1973 Velocity (m/sec) 0.456 0.473 0.370 0.330 0.260 0.340 0.490 0.650 Light Intensity ( ft-c) 69 return rate during the night of the summed species at Jones Creek was much faster than at Gates Creek and was presumably related to the different species mix at the two loca l i t ies . The disturbance t r ia l s at Gates Creek were at various veloc i t ies. E l l i o t t (1971b) found that a relationship between rate of return to the sub-strate and modal velocity could be given by the equation: log R = log a-j-b log V where a-j and b are constants and V is the velocity. E l l i o t t unnecessarily subscripted "b" , and i t has been eliminated here for s implicity. The mean distance travelled i s given by d = 1_ and is proportional to D modal velocity, hence: R = a,V ,-b (14) or d = 1 (15) anV ,-b Setting (16) The equation relating distance travelled to modal velocity i s : d = (17) and log d = log a 9 + b log V (18) Modal velocity, i f f a i r l y regularly distr ibuted, along the stream length, could be visualized as causing a l inear relationship of the log log plot, but mean velocity seemed more appropriate for the series of t r i a l s here reported 70 where the velocity was relat ively uniform, but the samples came from across the width of the stream. For the Jones Creek and Gates Creek data the linear re-gression of equation (14) was a good explanation of the relationship between the average distance travelled and mean velocity for the summed species (Figure 16). The correlation between the average distance travelled and mean velocity was 0.8422. The various individual species conformed adequately to the relationship of the McLay model. Additionally, changes in the distance travelled occurred with changes in velocity and l ight intensity for the various species. In some of the individual t r ia l s numbers declined nearly l inearly and the f i t t i n g routine estimated negative values for the natural d r i f t (as though the disturbance had caused the animals not to d r i f t as much as they would natu-ra l l y ) . For Chironomidae on June 8 at Gates Creek, the best theoretical l ine was v i r tua l ly horizontal, and the estimated number of animals disturbed was negative (as though the disturbance had added animals to the disturbed area). During the day, Ephemerella, Paraleptophlebia and "Others" drifted a greater distance at the lower velocity and l ight intensity (Tables V and VI), while Cinygmula drifted farther at the lower velocity and l ight intensity. During the night, Cinygmula and Paraleptophlebia travelled a consider-ably longer distance in the d r i f t at a s l ight ly lower velocity (Tables V and VI), and Epeorus travelled only about two and one half times as far at the lower velocity. At Jones Creek, Parameletus, Chloroperla and "Others" dr i fted farther at the s l ight ly higher velocity. Similarly, Chironomidae at Gates Creek tra-velled farther at a higher velocity while "Others" show the same trend, except for the t r i a l of June 2. Several species were disturbed into both the day and night d r i f t . Cinygmula and "Others" did not apparently d r i f t different distances in the presence or absence of l ight. The average velocity between the two night t r ia l s at Jones Creek was 0.762 m/sec and during the day, 0.728 m/sec. Paraleptophlebia drifted a shorter distance during the day than at night at a higher velocity, while Chloroperla and Lepidostoma did the reverse. 100 ce o z 10 U J cc z C O ce L U 5 ( 1 I 1 ! 1 1 1 ! 1 1 1 1 1 1 1 1 1—1—1—1 1 1 1 BAETIS « \ 1 ; SUMMED SPECIES EPEORUS : / / / X " / j X 1 / / / / -V / 4 z X / A -/ xl / x x . • " - 7 x x y / A . 7 / IX - f x / : 1 1 J 1 1 1 1 1 1 1 1 — i i i i ' i •30 -60 •30 -GO VELOCITY (M/SEC) •30 -60 Figure 16. The log-log regression of the average distance travel led against velocity for (a) summed species, (b) Epeorus, (c) Baetis. 72 The distance travelled varied between the two loca l i t ies during the dark period. Paraleptophlebia dr i f ted farther at a higher velocity at Jones Creek than at Gates Creek. Epeorus and Baetis, however, drifted a much shorter distance at a higher velocity, and "Others" did not show any loca l i ty dependency. For Epeorus and Baetis at Gates Creek, the rate of return increased with a decrease in velocity. Thus, both species travelled farther as the velocity was increased. In regression, the correlation between the distance travelled and the velocity was 0.7005 for Epeorus and 0.982 for Baetis (Figure 16b and c). The rate of change in the distance travelled with increasing velocity was 2.248 for Epeorus and 4.583 for Baetis. The estimated number of disturbed animals fluctuated for each of the species from s ite to s ite and between loca l i t ies (Table V). Similarly, variations occurred in the estimated natural d r i f t . Moreover, a s ignif icant difference occurred between the observed and the estimated natural d r i f t for Epeorus, but did not for Baetis. For the rest of the species the observed and estimated natural d r i f t was at low levels. Discussion Under a variety of conditions of l ight and velocity, the summed species and most of the individual species at Jones and Gates Creeks conformed to the theoretical expectations of the McLay model. For those cases that did not con-form, a l inear relationship would have f i t ted better. The various species responded to the changes in velocity in various ways during a few t r i a l s . Some species apparently d r i f t farther at a lower velocity than at a high velocity, and some do the reverse. For a large number of t r i a l s , a power function, such as that of E l l i o t t , expresses the relationship between the average distance travelled and the average water velocity. The summed species, Epeorus and Baetis conform to the expectations of the power function with the log of the average distance travelled l inearly related to the log of the average velocity. 16 The distance travelled during the night is greater than that during the day for some species, but the reverse is true for others. From these data i t would be unreal ist ic to draw any firm conclusions except that the circumstances of l ight intensity and velocity evidently differently influence the dr i f t ing behavior of various species of stream organisms. More repl ication over a range of dif fering l ight intensit ies should be undertaken. Comparisons of the distance travelled at each of the two loca l i t ies in -dicate that some species d r i f t farther at a higher velocity at Jones Creek than at Gates Creek. For other species, the reverse situation occurs. Thus, i t appears that species react differently at the two l oca l i t i e s , and that velocity is not the only parameter controll ing the distance travel led. The results for the individual species demonstrate that dr i f t ing inver-tebrates vary in their ab i l i ty to return to the substrate. Some species are known to actively leave the d r i f t , others do not. The differences in the e s t i -mated number of disturbed animals probably reflects differences in abundance along the channel. 74 5.1 ADDITIVITY OF DRIFT FROM TWO SIMULTANEOUS DISTURBANCES Introduction Animals introduced by a disturbance are caught in nets placed at i n -creasing distances downstream according to equation (1) providing of course that the f i t t i ng procedure takes account of the effects of the catches on the nets further downstream. If two or more disturbances are done simultaneously, the total number of organisms caught (N D), at a distance D downstream, would be given by: N D = N o 1 e - R D + No 2 e- R D 2 + . . .No r ,e- , ! D n ( « ) provided the contributions were additive, and the organisms were equally abun-dant and disturbable along the stream bed. It was the intention of this part of the work to test the underlying assumption of addit iv ity of the contributions and uniform abundance implied in the model. Methods The design and procedure was similar to that for single disturbance experiments. Nets were placed at increasing distances from two sites of d i s -turbances. On the f i r s t t r i a l , the disturbances were f ive meters apart and the nets were set at 1, 2, 4, 6, 7, 8, 10, 15 and 20 meters below the f i r s t disturbance. In the second t r i a l , the disturbance nets were 10 meters apart and nets were placed at 1, 4, 9, 11, 12, 13, 15, 20 and 30 meters. In each t r i a l , three control nets were placed two meters above the f i r s t disturbance. The procedure was to, f i r s t , lower the nets sequentially and then to disturb simultaneously two 22 cm wide strips across the stream. The nets re-mained lowered for half an hour, some time between 2200 and 2400 hrs. At the end of each t r i a l , the nets were raised and the samples fixed in formalin for later evaluation. 75 Results For two simultaneous disturbances, the contributions from each distur-bance were found to vary greatly (Figure 17). For the summed species, there were s ignif icant differences between the number disturbed at each of the two s i tes , varying from 27 times as many at the f i r s t s ite than at the second, to five times more at the second s i te (Table VII). On one occasion (June 27), the estimated contribution from two sites was within three animals of being equal. The distance dri fted did not follow the usual pattern of decreasing with increasing velocity (Tables VII and VIII). A mixture of increasing and decreas-ing rates occurred for the summed species with velocity changes, but the velocity changes were s l ight between t r i a l s . A good f i t was achieved between the theoretical and observed by the non-l inear f i t t i n g routine, for each of the individual species. For each species, the estimated number disturbed at the upstream disturbance was s ignif icant ly different from that at the downstream disturbance (Table VII), sometimes being higher and other times being lower. The differences between observed and estimated natural d r i f t were sl ight for each species. For most species, l i t t l e natural d r i f t occurred during the sampling interval. In one t r i a l , Cinygmula gave a negative estimate for the natural d r i f t . The distance dr i f ted by Ephemerella and Epeorus was farther when the velocity was high, than when the velocity was low (Tables VII and VIII). The reverse occurred for Cinygmula, Paraleptophlebia and "Others". Baetis showed no clear relationship between distance dri fted and velocity. Discussion The addit iv ity implied in the model was found to be supported by the experiments. In each of the separate t r i a l s the individual taxa and a l l the animals summed together conformed to the predicted relationship, with the ex-ception of one case of Cinygmula. 76 50 40 30 20 10 i r JUNE 23 20 10 0 40 30 20 10 0 JUNE 24 - I — • 1 L _ L JUNE 27 -i 1 1 i i_ 10 15 20 25 DISTANCE (METERS) i _ J _ 30 Figure 17. The l i ne s of best f i t through the average number (X) of a l l the species taken together (summed species) caught at various distances downstream of two simultaneous disturbances of organisms o f f the stream bed (top) at 0 and 5 meters, (bottom) 0 and 10 meters. TABLE V I I . The estimated number of Invertebrates Introduced Into the drift simultaneously from two "dif fer ing sites of disturbance the n f S S S L ? H t K i . S U b S t l i S ? s t he /verage distance t r a v e l l e d and the estimated and average observed natSraldJIft It JonSs Creek Averaa. observed drift was obtained from net samples taken upstream of the disturbances. Averag( Taxon Distance Between Date Disturbances 1971 (meters) Estimated Rate of Return Estimated Animals Disturbed Caught Summed Summed Surrmed Summed June 23 June 24 June 26 June 27 Ephemerella  Ephemerella -June 26 June 27 Cinygmula  Cinygmula  Cinygmula June 23 June 26 June 27 Parameletus June 26 Paraleptophlebia June 23 Paraleptophlebia June 24 Paraleptophlebia June 26 Epeorus  Epeorus  Epeorus  Epeorus June 23 June 24 June 26 June 27 Baetis  Baetis Baetis June 24 June 26 June 27 Chloroperla June 27 Others Others Others Others June 23 June 24 June 26 June 27 0/5 0/5 0/10 0/10 0/10 0/10 0/5 0/10 0/10 0/10 0/5 0/5 0/10 0/5 0/5 0/10 0/10 0/5 0/10 0/10 0/10 0/5 0/5 0/10 0/10 -0.1185 -0.1200 -0.5354 -0.1572 -0.2961 -0.2723 -0.3137 -0.2718 -0.0586 -1.3760 -0.4451 -0.1430 -0.0909 -0.4673 -0.1934 -0.6360 -0.2835 -0.5771 -0.3042 -0.8265 •0.2568 -0.2864 -0.1787 -0.3079 -0.4481 27 15 33 35 2 2 11 38 18 2 6 4 3 6 Average Observed Natural D r i f t Caught  0 53 165 38 8 11 15' 17 7 15 13 6 6 22 13 11 77 11 11 10 16 31 15 23 11 18 7 5 2 Estimated Natural D r i f t Caught Average Distance Tr a v e l l e d i n D r i f t 23 10 27 7 5 3 0 13 1 9 2 3 3 1 4 8.44 8.33 1.87 6.36 3.38 3.67 3.19 3.68 17.06 0.73 2.25 6.99 11.0 2.14 5.17 1.57 3.53 1.73 3.29 1.21 3.89 3.49 5.60 3.25 2.23 78 The number contributed by each of the disturbances was not equal. The species were thus not equally dense on the substrate in the contributing areas. Moreover, each of the species collected was not present in a l l of the t r i a l s indicating a patchy distribution along the channel length. Thus, recruitment to the d r i f t was dependent on the abundance of species found along the length of the stream bed. In most cases observed, the average distance travelled with changes in velocity, was the reverse of that predicted by the relationship f i r s t presented by E l l i o t t (1971b). Ephemerella and Epeorus were found to comply with the distance-velocity relationship, and Cinygmula, Paraleptophlebia, Baetis and Others, not. Table VIII. The average velocity at each t r i a l during the simultaneous disturbance of two areas of stream bottom. Date Velocity June 23, 1971 0.625 June 24, 1971 0.590 June 26, 1971 0.619 June 27, 1971 0.688 / 79 5.2 BLOCKAGE OF STREAM DRIFT Introduction A stretch of stream was blocked to d r i f t by f i l t e r i ng a l l of the water in the stream through a series of interconnecting nets. The natural d r i f t below the blockage should increase with distance downstream in the form of equation (5) (Figure 1 bottom). The i n i t i a l increase in the d r i f t should be rapid because the number of organisms in the d r i f t should be small relative to the number on the stream bottom. As the numbers in the d r i f t increase downstream, the rate of sett l ing from the d r i f t (rate of return) should eventually be equal to the rate of entering the d r i f t . The intention of this part of the work was to test the McLay model by blocking the d r i f t . The various t r i a l s were done in different areas at various conditions of l ight intensity and velocity. Methods A series of interlocking nets (30 cm by 30 cm opening) was placed on a l ine across the spawning channel to prevent the downstream movement of d r i f t (Figure 18). Downstream of the blockage, a series of nets was placed at i n -creasing distances (1, 2, 3, 5, 10, 20 meters), one net in each of three sections of stream, as outlined in Section 1.3. Three control nets were placed two meters upstream of the blockage nets. At night, two t r i a l s were conducted between 2200 and 2400 hrs, while during the day a t r i a l was carr ied out at 1960 f t - c . The procedure was to lower the blocking nets, and then lower the sampling nets in sequence. The nets were raised in the same sequence after one hour and the samples fixed in formalin for later evaluation. 80 81 Results The various blockages resulted in di f fer ing responses of the summed species, but a l l cases were found to f i t the model remarkably well. The l ine of best f i t was evidently a satisfactory description of the data (Figure 19). Rates of return to the substrate from the d r i f t and the numbers in the dr i f t varied with l ight intensity and on the species mix (Table IX). The mean dis-tance travel led, the reciprocal of the absolute value of the rate of return, increased as the rate of return decreased. Velocity was more or less constant and did not seem to have a measurable effect on the rate of return of the summed species (Table X). Comparing the night t r i a l s (May 25,31) for two successive years is risky (see Description Section 2.1), but at Jones Creek a s l ight ly faster rate of return occurred at a s l ight ly lower velocity (Tables IX and X). A much lower rate of return occurred at Gates Creek where the velocity was much lower. The species mix was quite different in the two loca l i t i e s . The dominant species at Jones Creek were Cinygmula, Parameletus, Paraleptophlebia and Lepidostoma, but at Gates Creek, Epeorus and Baetis dominated. Thus the lower rate of return, which occurred at a higher velocity at Gates Creek, was l ike ly the result of the dif fer ing species mix in the two loca l i t ie s . Daytime blockage resulted in a low rate of return compared to the night d r i f t , but the velocity was higher. The dominant species during the day was Lepidostoma and this species substantially affected the rate of return for the summed species. Nets placed above the blockage caught s ignif icant ly less than that e s t i -mated from catches below the blockage. Thus, either the blockage by nets caused a higher number to enter the d r i f t downstream, or the density of organisms in the stream bottom above the blockage was less than in the area downstream. The lat ter case has some support (Figure 10) because variations occur in the dr i f t along a channel which appears to be physically homogeneous. Rates of return to the substrate differed from species to species with l ight and dr i f t density, but in a l l cases, the model was a good f i t (Table IX). Velocity affects were not apparently important because they were fa i r l y constant (Table X). 60 50 40 30 20 10 0 300 250 a: 200 LU E 150 o j. 100 UJ 5 50 <J~> LU UJ v a. to d 50 ° 40 £ 30 20 10 0 125 100 75 50 25 0 JONES CREEK, MAY 25, 1971 NIGHT JONES CREEK, MAY 31, 1972 NIGHT JONES CREEK, JUNE 2, 1971 DAY GATES CREEK, JUNE 16, 1973 NIGHT 0 5 10 15 DISTANCE (METERS! 20 Figure 19. The l ine of best . f i t for the regression on distance of the observed average number of organisms in the d r i f t downstream from complete f i l t r a t i o n of the d r i f t . Each t r i a l was carried out under d i f fer ing l ight intensit ies and velocit ies (see text). TABLE I X . T h e r e c o v e r y o f t h e d r i f t t h r o u g h n a t u r a l c a u s e s b e l o w t h e c o m p l e t e f i l t r a t i o n o f t h e d r i f t , t h e e s t i m a t e d r a t e o f r e t u r n t o t h e s u b s t r a t e t h e a v e r a g e d i s t a n c e t r a v e l l e d , t h e e s t i m a t e d a n d a v e r a g e o b s e r v e d n a t u r a l d r i f t a t v a r i o u s v e l o c i t i e s a n d l i g h t I n t e n s i t i e s a t J o n e s a n d G a t e s C r e e k . A v e r a g e o b s e r v e d d r i f t was o b t a i n e d f r o m n e t s a m p l e s t a k e n u p s t r e a m o f t h e d i s t u r b a n c e , T a x o n S p a w n i n g C h a n n e l L i g h t P e r i o d D a t e E s t i m a t e d R a t e o f R e t u r n t o S u b s t r a t e A v e r a g e O b s e r v e d N a t u r a l D r i f t C a u q h t E s t i m a t e d N a t u r a l D r i f t C a u q h t X 2 a.OS 0 1 f f e r e n c e s B e t w e e n O b s e r v e d and E s t i m a t e d N a t u r a l D r i f t E s t i m a t e d A v e r a g e D i s t a n c e . T r a v e l l e d ( m e t e r s ) F i t t i n g R o u t i n e C o n v e r g e n c e R e m a r k s Summed Summed Summed Summed J o n e s J o n e s J o n e s G a t e s N i g h t N i g h t Day N i g h t May 25/71 May 31/72 J u n e 2/71 J u n e 1 6 / 7 3 - 0 . 2 5 6 0 - 0 . 2 6 6 1 - 0 . 1 0 7 9 - 0 . 2 1 3 9 53 146 40 118 46 324 53 137 • S i g n i f i c a n t 3 . 9 1 3 . 7 6 9 . 2 7 9 . 6 9 g o o d g o o d g o o d g o o d E v i d e n c e f o r g o o d c o n v e r g e n c e 1s p r e s e n t e d F i g u r e 1 9 . E p h e m e r e l l a J o n e s N i g h t May 31/72 - 0 . 2 1 3 9 12 21 4 . 6 8 g o o d d n y c m u l a C i n y q m u l a J o n e s J o n e s N i g h t N i g h t May 25/71 May 3 1 / 7 2 - 0 . 4 4 4 5 - 0 . 2 7 4 6 7 5 3 7 9 3 N o t S i g n i f i c a n t 2 . 2 5 3 . 6 4 g o o d g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t t h e l o w e r v e l o c i t y d u r i n g d i f f e r e n t y e a r s . P a r a m e l e t u s P a r a m e l e t u s J o n e s J o n e s N i g h t N i g h t May 25/71 May 31/72 - 0 . 2 3 6 0 . - 0 . 4 3 6 0 24 21 10 14 N o t S i g n i f i c a n t 4 . 2 4 2 . 2 9 g o o d g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t t h e h i g h e r v e l o c i t y d u r i n g d i f f e r e n t y e a r s . P a r a l e p t o p h l e b i a P a r a l e p t o p h l e b i a P a r a l e p t o p h 1 e b i a P a r a l e p t o p h l e b i a J o n e s J o n e s J o n e s G a t e s N i g h t N i g h t Day N i g h t May 25/71 May 31/72 J u n e 2/71 J u n e 1 6 / 7 3 - 0 . 2 1 1 0 - 0.2316 - 0.0S7Q - 0 . 1 6 1 2 6 19 7 6 12 55 8 7 N o t S i g n i f i c a n t 4 . 7 4 3 . 5 5 1 7 . 5 4 6 . 2 0 g o o d g o o d g o o d g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t t h e h i g h e r v e l o c i t y 1 9 7 1 - 7 2 c o m p a r e d - b e t w e e n l o c a t i o n s t h e d i s t a n c e t r a v e l l e d was s h o r t e r a t a h i g h e r v e l o c i t y -b e t w e e n n i g h t and day t h e d i s t a n c e t r a v e l l e d was l o n g e r d u r i n g t h e d a y . E p e o r u s E p e o r u s J o n e s G a t e s N i g h t N i g h t May -31/72 J u n e 1 6 / 7 3 - 0 . 5 7 2 6 . - 0 . 0 4 1 7 . 25 71 113 135 S i g n i f i c a n t 1 . 7 5 2 3 . 9 8 g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t G a t e s C r e e k a t a l o w e r v e l o c i t y t h a n a t J o n e s C r e e k a t a h i g h e r v e l o c i t y . C a e t i s B a e t i s J o n e s G a t e s N i g h t N i g h t May 3 1 / 7 2 J u n e 1 6 / 7 3 - 0 . 9 7 8 1 - 0 . 2 5 6 8 8 24 11 2 5 N o t S i g n i f i c a n t 1 . 0 2 3 . 8 9 g o o d V e l o c i t y a f f e c t s - same a s f o r E p e o r u s . C h l o r o p e r l a J o n e s N i g h t May 25/71 - 0 . 2 0 0 9 5 6 4 . 9 8 g o o d L e p i d o s t o m a L e p i d o s t o m a J o n e s J o n e s N i g h t Day May 25/71 J u n e 2/71 - 0 . 1 1 8 6 - 0 . 1 0 2 2 7 31 10 4 2 N o t S i g n i f i c a n t 8 . 4 3 9 . 7 8 g o o d g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t a h i g h e r v e l o c i t y d u r i n g t h e d a y t h a n t h e l o w e r v e l o c i t y a t n i g h t . . C h i r o n o m i d a e p u p a e G a t e s N i g h t J u n e 1 6 / 7 3 - 0 . 4 8 4 8 13 7 2 . 0 6 g o o d O t h e r s O t h e r s J o n e s G a t e s N i g h t N i g h t May 25/71 J u n e 1 6 / 7 3 - 1 . 2 1 8 9 - 0 . 1 9 8 0 4 5 5 4 N o t S i g n i f i c a n t 0 . 8 2 5 . 0 5 g o o d g o o d V e l o c i t y a f f e c t s - t r a v e l l e d a g r e a t e r d i s t a n c e a t a h i g h e r v e l o c i t y d u r i n g t h e day t h a n t h e l o w e r v e l o c i t y a t n i g h t . CO co 84 TABLE X. Average velocity and l ight intensity in the various blockage experiments. Jones Creek Gates Creek Conditions Night Night Day Night May 25, 1971 May 31, 1972 June 2, 1971 June 16, 1973 Velocity 0 > 6 1 6 Q 5 3 8 Q 7 6 5 Q 3 6 Z m/sec Light Intensity - • : - 1960 f t - c Ephemerel1 a and Chloroperla frequented the d r i f t in substantial numbers during one of the years of observation, but in that year there were differences between the species in the rate of leaving. "Other" species were only evident at Jones Creek and differed in abundance in the two years. The observed natural d r i f t at night of Cinygmula was about eight times greater and the rate of return to the substrate was about half in 1972 of that in 1971. One might conclude that at high densities the d r i f t rate was lowered, which seems incongruous. The 1971 sample, however, was small, and the 1972 sample was probably the better estimator of rate of return. For Parameletus, the observed natural d r i f t was similar during the night t r i a l s in 1971 and 1972, but the rate of return to the substrate in 1971 was twice that in 1972. Some species were relat ively abundant at night at both Jones and Gates Creek. Epeorus and Baetis were three times more abundant in the observed natural d r i f t in 1972. For Epeorus, the estimated natural d r i f t was much larger than the observed natural d r i f t , but no difference was evident for Baetis. For both species, the rate of return to the substrate was much faster at Jones Creek than at Gates Creek for Epeorus and Baetis, respectively. Differences in the estimated rates of return occurred between years for species at each loca l i ty and between l oca l i t i e s . Twice as many Chironomidae were observed in the natural d r i f t as estimated during a night t r i a l at Gates Creek. The estimated rate of return to the substrate was relat ively high with respect to "Other Species" at Gates Creek. The daytime natural d r i f t was four times higher than the night d r i f t for Lepidostoma at Jones Creek, and the rate of return was lower during the day than at night. Presumably, greater act iv i ty during the day would result in fewer sites for occupancy, and the dr i f t ing animals would d r i f t farther to f ind s i tes. Paraleptophlebia was one of the few species which was found both at Jones and Gates Creek, although i t was not abundant in a l l areas of both channel The observed natural d r i f t was f a i r l y low and constant except during the 1972 t r i a l , and the estimated natural d r i f t followed the same pattern. The rate of return was extremely low during the day in 1971 as compared to a l l of the night t r i a l s . During the night, the rate of return at Jones Creek differed between the two years but was almost twice as fast as at Gates Creek in both years. The observed natural and estimated natural d r i f t for the other species summed were l i t t l e different between Jones and Gates Creeks, and a rate of re-turn to the substrate was faster at Jones Creek, where "Others" were mainly case carrying caddisfl ies (those at Gates Creek were mayflies). The faster rate of return at Jones Creek may ref lect the effect of the larval cases on the rate of return. Discussion ) The rate of return of each of the species dif fered, indicating that each species has i ts specif ic return rate, and for some species, the rate of return differed dramatically between years at the same local i ty and between loca l i t ies . Although the velocity was not highly variable, i t was expected that rate of return would be lower when velocity was high. This did not occur - might be attributed to variations in the density of animals in the substrate. Differences in the return rate between night and day t r i a l s were not as predicted for Paraleptophlebia. As Paraleptophlebia was a night-active species, i t was expected that the return rate would be higher during the day, but the reverse occurred. E l l i o t t (1971b) observed no s ignif icant difference between return rates for day and night t r i a l s . For Lepidostoma, a day-active species, i t was expected and observed that the rate of return would be slower during the day than at night. The distance travelled by the individual species in the d r i f t may have been influenced by several environmental and behavioral factors. The response of the organism to dr i f t ing , the number of sites available on the substrate and velocity acted, either singly or in consort, to affect the distance travelled. \ 87 5.3 THE RESPONSE OF THE DRIFT TO SUBSTRATE DENSITY Introduction When dr i f t ing stream organisms are f i l t e red out of the water continuously, animals entering the d r i f t immediately downstream of the blockage can respond in a variety of ways. For example, the dr i f t ing species may enter the d r i f t at a rate dependent on their density on the substrate or the organisms may enter the d r i f t at a constant rate. Additionally, each species may have a minimum thresh-old density below which i t wi l l not enter the d r i f t . When the stream dr i f t is blocked, i t would be expected that the d r i f t be-low the blockage would at f i r s t follow the form of equation (5). With the blockage maintained, the stream bed below the block would become progressively more impov-erished, because dr i f t ing animals are no longer being replaced from upstream. Thus the number leaving N, per unit area per unit time is a constant proportion -R (y) (where y = 1-e" ) of the numbers in the substrate (Ng), i . e . : N L = yN B (20) These animals sett le to the bottom at various distances downstream, the number remaining in the dr i f t being: N = N e " R D ( 2 1 ) "D L The number sett l ing per unit of area Ns is a constant proportion (x) (where x = -R e ) of the number in the d r i f t Np, i . e . : Ns = x N p (22) On the average, at each distance and within each time period, the numbers set-t l ing equals the numbers leaving, hence: yN B = x ND (23) 88 The distance that organisms d r i f t may be expressed as a product of velocity and time D = vt (24) Considering the process as continuous over the whole region below a blockage the number remaining in the substrate (N^ .) below the blockage wi l l de-cl ine according to N T = N e " b t <2S> T s where b is the proportion that leave each day and t is the number of days ( i .e. assuming the blockage prevents movement downstream from both the d r i f t and through the gravel). At each distance below the blockage there wi l l be recruitment (NR) from points closer to the blockage so that at a particular distance (D) below the blockage N R = N t e " R 1 + N t e " R 2 + . . . N t e " R D (26) Combining (25) and (26) poses some problems in analysis, *but for the continuous case, a Bessel function can be used to represent the number of orga-nisms expected at a particular time and distance. The processes involved can be expressed as two di f ferent ia l equations: BN w = ^ B = x N ; - y N B ( 2 ? ) Tt D S N B = xN - yN R (28) T t W B * Mr. N. Gilbert provided the mathematical analysis here presented. 89 where N is the population in the stream = No at t = 0 w • r w (the number in the natural stream dr i f t ) Ng is the population density in the suostrate = NOg at t = 0 (number of animals in the substrate) x is the rate of sett l ing out of the water y is the rate of entering the d r i f t v is the velocity of the stream t is the time The total rate of sett l ing (per unit time per unit area) is xN and the w total rate of entering into the d r i f t (per unit time per unit area) is yNg. The general solution of these equations may be computed for suf f ic ient ly small values of D and t from the series: for the d r i f t 0 0 CO \ _ = ] . e " ( X _ T ) nio T (1 + X + X__ + ... X__) (29) No, n! 2! n! w and for the substrate 00 \ = 1 - e" ( X _ T ) \ n = O T (1 + X + X__ + . . . X n _ 1 ) (30) NoB n! 21 TTTT): where X = xD (31) v and T = y (t-D) (32) v Figure 20 shows the family of curves implied by this model, that can be stretched appropriately to find a best f i t . 0-5 0-6 0-1 0-8 0-9 1-0 DISTANCE (METERS) Figure 20. Theoretical ly, the proportion of animals expected to "put up" into the d r i f t from natural causes on successive days below a site of continuous complete f i l t r a t i on of the d r i f t . Note: Y axis should be proportion of the natural drift. 91 If the rate of entry into the d r i f t is constant and not density dependent or modified by movement in the gravel, one would expect that the series could be f i t ted sat i s factor i ly to empirical data. Given a set of observations of d r i f t density at different times after a blockage, i t is then possible to estimate y and x by a non-linear least squares f i t t i n g procedure. Methods A series of interlocking nets (30 cm by 30 cm opening) was placed on one of the baffles in Jones Creek Spawning Channel completely f i l t e r ing a l l organisms from dr i f t ing downstream. The blockage was maintained continuously between May 30 and June 9, 1972. Each of the blocking nets was cleared of animals and other material every four hours. Below the blockage a series of nets (10 cm by 30 cm opening) was placed one in each of three sections at increasing distances (1, 2, 5, 5, 7.5, 12, 15, 20 meters) below the blockage. Three control nets were placed two meters upstream of the blocking nets. The procedure was to lower the experimental nets between 2250 and 2350 hrs on successive days between May 30 and June 9. Upon raising the nets, the samples were fixed in formalin for later evaluation. Differences in velocity at each net were corrected to an average velocity, and numbers were expressed as a proportion of the number caught in the control nets. Results A blockage was carried out continuously between May 31 and June 8, 1972. After four days the d r i f t had dropped to a low level which was the same from day to day and showed only a s l ight increase in the number of organisms dr i f t ing at various distances below the blockage. The model was f i t ted to only the f i r s t four days of observations. The proportion of the summed species in the d r i f t for each day was obser-ved to be higher than predicted at sites closest to the blockage, and was lower 92 than predicted by the third and fourth day at the sites farthest from the block-age (Figure 21). The estimated rate of leaving the d r i f t f e l l within the range for the summed species fcund in disturb experiments (Section 5.2, Table XI). The rate of "putting up" into the d r i f t was much lower than the rate of leaving. Thus, the proportion of those putting up of the total number available was very small. For each of the individual species, the proportion putting up into the d r i f t was higher each day than was expected for the sampling sites closest to the blockage (Figure 21). By the fourth day, the proportion of several species (Ephemerella, Parameletus, Paraleptophlebia and Epeorus) was lower than predict-ed at the farthest sites from the blockage. For Baetis and "Other Species", the proportion dr i f t ing was more or less constant at each distance for each day. The proportion dr i f t ing of Ephemerella was observed to be higher on the second day than on the f i r s t , then lower on the third day, and on the fourth day, clearly less. For several of the species, Cinygmula, Parameletus, Paraleptophlebia and Epeorus, there was no clear trend in the observed data, except for a lowering of the proportion in the d r i f t . The rate of leaving the d r i f t varied among the various species. The rate of putting up, on the other hand, was low for a l l species, part icular ly so for the Baetis and the "Other Species", for which the l ine of best f i t was estimated to be the same for each day. Discussion /-For each of the taxa observed and thus the summed species, a higher number of animals than predicted was observed in the dr i f t at the sampling stations close to the blockage. One of several factors may have been responsible. F i r s t , the f i l t e r i n g nets providing the blockage may have allowed animals to pass. This was not l i ke ly , as the blocking nets were placed tightly on the top of the baffle (Figure 4 top) which extended from bank to bank. Second, organisms may have moved downstream through the gravel, but this seems unlikely because the baffle 93 Figure 21. The proportion of summed species and various individual species which entered the d r i f t on four successive days downstream of a continuous blockage of the d r i f t . The lines of best f i t are marked 1 to 4 one l ine for each day. The observed data points are marked by: • = f i r s t day, 0 = second day, X = third day, A= fourth day. Note: Y axis should be proportion of the natural drift. 94 95 I • TABLE XI. The values of the rate of leaving and putting up into the dr i f t per unit time and the instantaneous rate of leaving the d r i f t as estimated from the recovery of the d r i f t on successive days below the continuous blockage of the d r i f t . Instantaneous Taxon Rate of Leaving Rate of Putting up Rate of Leaving Remarks Summed Species 1.558 0.247 -0.211 Ephemerella 0.233 0.023 -0.792 Cinygmula 0.402 0.177 -0.669 Convergence was obtained Parameletus . 0.357 0.160 -0.700 between the theoretical Paraleptophlebia 0.621 0.140 -0.537 and observed for the Epeorus 0.482 0.193 -0.618 summed and each individual Baetis 0.124 1.12 x 10" 1 1 -0.883 species. Others 0.075 7.30 x 10" 5 -0.928 extended into the substrate to a hard packed bed and downstream movement thus should have been prevented. Third, an upstream movement in the gravel may have occurred maintaining a higher density than expected at the upper end of the experimental area. This explanation seems l ike ly . Bishop and Hynes (1969) and E l l i o t t (1971b) found that various species of invertebrates may move upstream in the gravel. " If each of the dr i f t ing species were to each 24 hours move upstream the same mean distance travelled (1/R) downstream when dr i f t ing , the relation between numbers in the d r i f t and distance would follow the same pattern on each successive day of blockage. If the animals were to only partly move back to their point ov origin each day, a net displacement downstream would result. On days succeeding the f i r s t , the number in the d r i f t would be higher than predicted, but not as high as for the f i r s t day. Parameletus, Paraleptophlebia and Epeorus were a l l in great-er abundance on the second and succeeding days, than predicted. If the animals moved upstream farther than the mean distance travelled downstream, the numbers in the d r i f t would be higher on the second day than the f i r s t . For example, 96 Emphemerella dr i f ted at a higher level on the second day than on the f i r s t . The model gave a moderately good description of the differing response of benthic in -vertebrates, but evidentally an adequate model would also include upstream move-ment in the gravel. An alternative, and perhaps additional explanation is that the rate of leaving declines with decrease in density in the substrate. The reduction in the numbers entering the d r i f t from day to day was dramatic. By four days the d r i f t had dropped to a relat ively constant low level . The f i r s t four days can be con-sidered to be attributable to behavioral responses. The drop in d r i f t for most species was reflecting a reduction in numbers, with l i t t l e i f any physical change in the area, the density was changing but the carrying capacity was not. Baetis and other less abundant species dr i f ted in constant numbers through-out the experiment, suggesting that these organisms were below their environmental carrying capacity. Other species were also apparently below the behavioral d r i f t threshold by four days. Thus, by the fourth or f i f t h day a l l species were at what Waters (1965) cal ls "constant d r i f t " level . The rate of return to the substrate from the d r i f t occurred at a constant rate, independent of density during the behavioral phase of dr i f t ing. The rate of return to the substrate was constant and the density of the d r i f t was constant during the "constant d r i f t " phase. 97 5.4 SIMULTANEOUS DISTURBANCE OF THE SUBSTRATE AND BLOCKAGE OF THE DRIFT Introduction If the stream d r i f t is blocked, then the number of organisms at various distances downstream is given by the McLay model as equation (5), and i f the sub-strate is disturbed over a width of stream bed (w), then the number of organisms at various distances downstream is given by: N D = w N o e " R D (31) If the width (w) is chosen such that N n = N (32) D max x ' then the resultant d r i f t , the sum of that given by equations (5) and (31) is N = N (1 - e " R D ) + N e " R D (33) T max ' max v ' Thus by disturbing the gravel immediately downstream of a blockage, the model would predict a constant d r i f t of N at a l l distances downstream. max Methods The aim of this series of experiments was to combine the block and disturb types. A set of interlocking blocking nets was put in a l ine across the stream. The experimental nets were placed one in each of the three sections of the stream at increasing distances (1, 2, 3, 4, 10, 20 meters) downstream of the area which was blocked and disturbed. Three control nets were placed two meters upstream of the blocking nets. The nets were lowered at 2145 hrs and raised in the same sequence at 2245 hours and the samples fixed in formalin for later evaluation. Two t r i a l s were carried out, one in which 22 cm width was disturbed across the stream and / 98 in the other, 11 cm. The data was treated by l inear regression analysis to determine the l ine of best f i t through the observations, although the relat ion-ship is known not to be l inear, except i f the width is so chosen that N n = N u max Results For both t r i a l s , a s ignif icant portion of the variance associated with the average of the summed species was not explained by regression. The departure from zero slope was less at 11 cm than at 22 cm (Figure 22). If a greater width was disturbed, i t would be expected that the slope would be increasingly greater, provided the relative abundance of the differing species did not change (Table XII). Thus, greater width would introduce suff ic ient animals to outnumber those naturally dr i f t ing. Conversely, with a narrow width, so few animals would be disturbed that the slope would change from negative to positive. The l inear regression was not a good model for the individual species, and the variance about the l ine of best f i t was large for each of the species. The slopes in most cases, changed in the direction predicted from consideration of the width disturbed. For some of the species, namely Ephemerella, Parameletus, Paralepto- phlebia and Baetis, the slope changed from negative to positive when the distur-bance width was changed from 22 cm to 11 cm. By contrast, for Cinygmula, Epeorus and Chloroperla on the negative slope was lower when the width disturbed was reduced. For Lepidostoma, the slope was changed from positive to negative with a decrease in the width disturbed. Thus, when 22 cm was disturbed the number intro-duced was smaller than the natural d r i f t , but when 11 cm was disturbed the number introduced was greater than the natural d r i f t . 99 Figure 22. Linear least squares l ine of best f i t through the average of the sum of a l l the species taken together and various individual species a-"", increasing distances below a dis-turbance and blockage. Where a 22 cm width was disturbed from one side of the stream to the other and in other cases a 11 cm width was disturbed. r; 100 I I ) I I TABLE XII. The rate of return to the substrate with simultaneous disturbance of the substrate and blockage of stream invertebrates when the total area disturbed was varied. Width Disturbed Rate of Return Species (cm) to Substrate A l l species 22 -1.308 taken together 11 -0.2131 Ephemerella 22 -0.1281 11 +0.0811 Cinygmula 22 -0.4192 11 -0.0683 Parameletus 22 +0.0831 11 -0.1146 Paraleptophlebia 22 -0.2827 11 +0.0367 Epeorus 22 -0.1037 11 -0.1854 Baetis 22 -0.0219 11 +0.0914 Chloroperla 22 -0.5570 11 -0.1751 Lepidostoma 22 +0.2286 11 -0.0444 Discussion If the number disturbed from the substrate below a blockage equals the maximum number in the natural d r i f t the predicted l ine of best f i t would be horizontal. When the area disturbed is too wide, the relationship of numbers to distance should decline and the reverse should occur when the area disturbed is too narrow. The unexplained var iab i l i ty associated with the number caught may ind i -cate a patchy distribution over the experimental area for each of the species. Thus, the value of the maximum natural d r i f t was changing. If each of the species was distributed evenly over the substrate, the width of area disturbed that would result in a horizontal l inear relationship could be easily predicted. 101 The change in declination of the l ine of best f i t was in the direction predicted for the summed species. This change, however, is dependent on the species mix. Additionally, i t is assumed that the disturbed d r i f t does not alter the natural d r i f t pattern. The change in the l ine of best f i t from a declination to an incl ination for some of the species indicates that the width of disturbance was reduced too much and the natural d r i f t outnumbered the disturbed. For those species in which the slopes decreased but remained negative, the width disturbed was apparently too great. For Lepidostoma, the change in l ine of best f i t from incl ination to de-cl ination with a decrease in area disturbed was not expected. Presumably, there was a local concentration in the 11 cm str ip or there was a substantial sampling error. 102 5.5 ESTIMATES OF THE RELATIVE DENSITY OF INVERTEBRATES AT GATES CREEK The abundance of organisms living on and in stream beds has been estimat-ed by many authors. Most have used bottom samples, but more recently attempts have been made to use the drift. The kick technique is known to disturb propor-tionally fewer of the small animals and some adherent animals are greatly under-represented (Macan, 1958; Hynes, 1961; Coleman and Hynes, 1970). The estimated bottom density from disturb and blockage experiments carried out at Gates Creek can be used to calculate the density of each species at the locality of each trial. The method of disturbance was tested for effectiveness (Section 3.1). The width of substrate disturbed was 22 cm. The width of net receiving the intro-duced pulse was 30.48 cm. Thus, if those caught are assumed to have come from an area 0.22 meters by 0.3048 meters, the average number per meter squared can be estimated. (The depth of gravel distrubed was about 8-11 cm.) Estimates of the effectiveness of the method of disturbance were calculated for summed species, Epeorus, Baetis and "Others". From the four trials, an average for the proportion leaving the substrate was used to calculate the number occupying the substrate at the various sites of disturbance. Nymphs were not divided into size categories and were averaged from three samples at each distance. The estimates for the summed species, which include Epeorus, Baetis and "Others", are slightly lower than the total for the three categories (Table XIII), reflecting the geometric averaging procedure in estimating average distance trav-elled. The estimates for the individual species do not suffer from the same bias. Differences in the estimated numbers occupying the substrate in large part reflect the distributions of Epeorus and Baetis along the channel. The experiment on June 11 was carried out near station two on the channel, the trials at earlier dates below the station and trials at a later date above the station. The high abundance estimate coincided with the greatest drift recorded for Epeorus, but not for Baetis. 103 TABLE XIII. Estimates of the relative number per meter squared as determined from the estimated disturbed animals and the effectiveness of the disturbance at Gates Creek Channel. NUMBERS OCCUPYING ONE SQUARE METER OF SUBSTRATE Sum of Estimates Date Summed Species Epeorus Baeti s Others For Last 3 Columns June 2, 1973 5,538 4,079 1,894 2,651 8,624 June 8, 1973 8,227 5,818 2,355 282 8,455 June 9, 1973 5,081 4,016 383 395 4,794 June 11, 1973 3,993 3,826 563 - . 4,389 June 12, 1973 27,041 27,635 793 - 28,428 June 13, 1973 3,902 3,446 282 479 4,207 June 14, 1973 7,683 .7,051 742 - 7,793 June 15, 1973 3,085 1,897 1,612 508 4,017 Drift cannot be used to estimate the absolute density of a l l the stream dwelling organisms l iv ing in the substrate. Only the relat ive abundance can be estimated because there may be certain species (or age groups, etc.) which never enter the d r i f t . Estimates of each dr i f t ing species can be made i f the effect ive-ness of the sampling method can be estimated. Thus, the model can be used to estimate the bottom density of each of those species. 104 6. GENERAL DISCUSSION The dispersal of stream insects is unique by comparison with dispersal of animals in other environments. Where most species can travel in any direction horizontally, or in some cases, both horizontally and ver t ica l ly , stream l iv ing insects are limited to a choice between upstream or downstream, unless they have an adult f ly ing phase. There are some vague paral lels amongst terrestr ia l ani-mals. For example, some species of l inophiid spiders disperse in the spring, while others in the f a l l (Duffey, 1956), but for both, dr i f t ing in the wind is a normal behavior at a particular stage of the l i f ecyc le , just as i t is for stream insects. Stream d r i f t is essential ly a process which serves to disperse individuals throughout a stream system. For many stream insects, the adult f ly ing phase may actively disperse upstream replenishing the populations that were depleted by downstream dr i f t (Roose, 1957), although for some species, dispersal is at the whim of a ir currents and is not necessarily upstream (E l l i o t t , 1971b). Drifting also serves on a more local scale to increase the efficiency of ut i l i zat ion of stream bottoms. Provided d r i f t occurs when vulnerabil ity to pred-ation is low, stream invertebrates can colonize areas which are relat ively sparsely populated. Streams are dynamic and constantly changing in the short term, so that areas become unsuitable for some species and suitable to others. Species that d r i f t at an early l i f e history stage presumably colonize suitable areas quickly. The way in which the d r i f t process occurs can be readily visualized. Either by accident, or as a sort of searching behavior for food, or as a conse-quence of behavioral interactions, organisms detach from the substrate and d r i f t downstream. Then either by an active swimming behavior or by passively waiting for an appropriate change in current or a chance turbulence, the organisms re-attach to the substrate, perhaps repeating the dr i f t ing sequence until a favor-able s ite is obtained. The speed of downstream dispersal is obviously a function of stream velocity, but is also evidently related to the behavior of the individual organism. 105 The McLay model is a f i r s t attempt to give quantitative description to the d r i f t process and simply states that organisms wi l l move downstream various distances in conformity with the rule that the relative rate of return to the substrate is a constant. It might thus be imagined that for species x, there is a f i f t y - f i f t y chance that an individual in the d r i f t wi l l f ind attachment within one meter, so that every meter, half of the remaining dr i f t ing animals wi l l be back on the substrate. For species y, the chances may be only 20 per-cent that attachment wi l l occur in a meter, in which case the mean distance travelled wi l l be greater than for species x. In natural circumstances, this s implif ied model of d r i f t may be inade-quate. Organisms may not be uniformly distributed through the length of a stream, in consequence of which the natural level of d r i f t is not constant as the model would imply. There may be density dependent effects which increase d r i f t once threshold densities are achieved. Upstream migration may also occur, the animals travel l ing in both directions with d r i f t only measuring the downstream movement. Velocity and turbulence vary in parts of streams and in consequence, organisms are carried different distances. For a l l of these reasons, the McLay model does not represent natural circumstances, but rather, is a styl ized version of what would happen in sty l ized conditions. The McLay model nevertheless gives an excellent description of how pulses of introduced animals leave the d r i f t . Different species do leave the d r i f t in conformity with the model, and do so at different rates and are thus swept down-stream different distances. The average distance travelled is not necessarily accomplished in one move. For example, an introduced pulse of Baetis took approximately 15 minutes to move f ive meters (Section 4.1). Assuming that the disturbance did not cause animals to keep leaving the disturbed area for up to half an hour, the animals must have spent part of the time in the substrate be-tween 0-5 meters (the velocity was only 0.45 m/sec). In this study there were many examples of the influence of flow charac-ter i s t ics on distance travelled. For example, Baetis was found to return to the substrate by one meter in laminar flow of 0.45 m/sec (Section 3.0), but the same species in the turbulence of a spawning channel travelled between 20 and 30 meters (Section 5.0). Some species actively move toward the bottom and others are carried there by turbulence. For example, Baetis and Cinygmula actively return to the substrate, but Ephemerella does not (Section 3.0). Live animals return to the substrate faster than dead ones. Dead Epeorus were found to d r i f t much farther (Section 2.2) than was estimated for l ive animals (Section 5.0). During disturbance experiments, there was some indication that an intro-duced pulse was modified by animals that were recruited from areas downstream, - the sett l ing of the introduced pulse of animals presumably disturbing others. Thus, estimates of the relative abundance of animals occupying the disturbed substrate were probably biased in the direction of being larger than actual. S imilar ly, the estimate of the average distance travelled may be influenced by the disturbing effect of the introduced pulse. When the stream was blocked to d r i f t and the stream bed disturbed, there was not a good f i t to the theoretical expectation. Apparently, the pulse of animals was intermittently on the substrate and in the d r i f t , and affected the rate of "putting up" of those animals leaving the substrate due to natural causes. Reassuringly, no consistent differences occurred between distances d r i f t -ed by animals which were disturbed and distances drifted by those leaving the d r i f t naturally below a blockage (Table XIV). Some species, namely Cinygmula, Paraleptophlebia and "Others" travelled distances downstream during the blockage, that were intermediate in value to the distances travelled from disturbances. Parameletus and Chloroperla, however, moved a greater distance (at a slower velocity) during a blockage than as a result of two disturbances. The addit ivity of pulses of introduced animals was also well described by the model. However, the contributions from each source of disturbance were not the same. The density of organisms in the substrate varied along the phys-i ca l l y homogeneous spawning channel at Gates Creek (Section 2.1), eventhough temperature and velocity, substrate type and cover along the channel were similar. Apparently, the various species were distributed according to food ava i lab i l i ty . Carrying capacity also affects the rate at which animals enter the d r i f t naturally. If the natural d r i f t results purely from what is defined as constant d r i f t , the rate of leaving the d r i f t is less than i f the d r i f t is a result of 107 TABLE XIV. The estimated average distance travelled by various species downstream of a disturbance (velocity 0.766 and 0.758) and downstream of a blockage (velocity 0.616) at Jones Creek during periods of darkness. Species Blockage Velocity m/sec 0.616 Disturbance Velocity m/sec 0.766 0.758 Cinygmula 2.25 1.59 14.18 Parameletus 4.25 1.62 0.62 Paraleptophlebia 4.74 8.57 17.70 Chloroperla 4.98 3.36 2.28 Others 0.82 4.18 0.64 behavioral response. For night-active species, the carrying capacity of an area is higher during the day than at night, and the opposite occurs for day-active species. As the individuals grow, the space required to forage increases and the number which occupy an area decreases. E l l i o t t (1967b) found that the density of individual species in the d r i f t was highest during periods of rapid growth when competition for food and space may have been the most severe. The d r i f t rate changed from day to day although no measurable change in conditions was observed. This could not be accounted for by the effect of d r i f t on sub-strate densities. The pattern of substrate distr ibution could be constantly changing within an area but relat ively few of the organisms might move from the preferred area. Dr i f t samples taken each day for a period of 18 days, indicated no d is t inct upstream or downstream movement although large numbers drifted every day for most species. For example, the average number of Epeorus dr i f t ing past station two (where Epeorus was most abundant in the dr i f t ) was 20,000 between 2300 and 2400 hrs. On either side of station two, the d r i f t was less (Figure 14a).. Thus, although potentially Epeorus could have dispersed throughout the channel, i t nevertheless remained most abundant at station two. The changing pattern of the animals occupying chosen sites in an area of stream bed could, in addition to the velocity, contribute to the form of the relationship of the number leaving the d r i f t below a blockage. 1 Many authors have suggested that there is a strong positive correlation between current velocity and the quantity of aquatic d r i f t (Chapman, 1966; Mundie, 1969; Waters, 1969a; Everest and Chapman, 1972). The faster the d r i f t the greater the quantity of d r i f t organisms. Most authors neglect, however, to consider whether the increase in the quantity is because more animals enter the d r i f t , or because those in the d r i f t travel farther or because of a combination of these factors. The power function f i r s t suggested by E l l i o t t (1971b) with modifications, was found to hold in this work, suggesting that animals travel farther at higher velocit ies. The power function adequately describes distance travelled with increasing velocity for the summed species, Baetis and Epeorus. The hypothesized relationship between number of dr i f t ing organisms down-stream on successive days after a blockage did not sat i s factor i ly describe observed data. Apparently, there is inter-gravel movement upstream or changes in the rate of leaving the d r i f t with changes in density (or a combination of both). The latter seems most l i ke ly . Higher numbers entered the d r i f t than was expected below the blockage. The d r i f t did, however, drop rather dramatically between successive days implying that the d r i f t was density-dependent. Presum-ably at higher densities there is competition for space or food, or both. The McLay model thus emerges as a simple conception concerned with the styl ized relationship between the number in the d r i f t and the distance they travel. The host of complicated inter-relationships between such variables, as the swimming behavior, density, dependence and. the carrying capacity under chang ing biotic and abiotic environmental conditions, are considered to be aggregated It i s , however, the interactions between the many components that ultimately ensure the adequacy of the model in describing the d r i f t phenomenon as a whole. Thus, as so often is the case, simple models do not describe the underlying biology of the animals concerned, but they nevertheless explain the gross phenomenon adequately. The McLay model has proved to be no exception. The particular relevance of this discussion is that the McLay model can be used as a "stepping stone" to a better understanding of the components govern ing stream d r i f t . The paucity of information about the quantitative dynamics of dr i f t necessitates more complex approaches to the problem. It is hoped that the quantities and relationships developed here wi l l provide a stimulus to the development of these concepts further and their application to practice. The model might then be used as McLay put i t , "to test a theory concerning the de-l ivery of food items to a feeding f i sh " and could lead to better stocking policies for streams. 110 LITERATURE CITED Albrecht, M.L. 1968. Die Wirkung des Lichtes auf die quantitative Vrteilung der Funa im F l ie Bgewasser. Limnologica (Berlin) 6: 71-82. Al len, K.R. 1941. Studies on the biology of the early stages of the salmon (Salmo salar). 2. Feeding habits. J . Anim. Ecol. 10: 47-76. . 1942. Comparison of bottom faunas as sources of available f ish food. Trans. Am. Fish. Soc. 71: 275-283. 1951. The Horokiwi Stream. A study of a trout population. New Zealand Mar. Dept. Fish. Bul l . No. 10 Amb'uhl, H. 1959. Die Bedentung der Stromung als okologischer Faktor. Schweiz Z. Hydrol. 21: 133-264. 1961. Die Stromung als physiologischer und okologischer Faktor. Verh. internat. Verein."theor^ angew/ Limnol. 14: 390-395 Anderson, N.H. 1966. Depressant effect of moonlight on act iv i ty of aquatic insects. Nature, 209: 319-320. 1967. Biology and downstream dr i f t of some Oregon Trichoptera. Can. Ent. 99: 507-521 and D.M. Lehmkuhl. 1968. Catastrophic d r i f t of insects in a wood-A r- 4- \s\ s\ r-.«i A n . i n n / i n r land stream. Ecol. 49: 198-206. Baily, R.G. 1966. Observations on the nature and importance of organic d r i f t in a Devon river. Hydrobiologia, 27: 353-367. Ba l l , R.C, T.A. Wojtalik and F.F. Hooper. 1963. Upstream dispersion of radio-phosphorus in a Michigan trout stream. Pap. Mich. Acad. Ac i . 48: 57-64. Beauchamp, R.S.A. 1932. Some ecological factors and their influence on competi-tion between stream and lake- l iv ing tr ic lads. J . Anim. Ecol. 1: 175-190. Berner, L. 1959. A tabular summary of the biology of North American mayfly nymphs (Ephemeroptera). Bul l . Fla. State Mus. Bio. Ser. Vol. 4(1). Bishop, J.E. 1969. Light control of aquatic insect act iv i ty and d r i f t . Ecol. 50: 371-380. 1973. Observations on the vertical distribution of the benthos in the Malaysian stream.- Freshwater.Biol. 3: 47-156 and H.B.N. Hynes. 1969a. Upstream movement of the benthic inverte-brates in the Speed River. J . Fish. Res. Bd. Canada 26: 279-298. . 1969b. Downstream d r i f t of the invertebrate fauna in a stream ecosystem. Arch. Hydrobiol. 66: 56-90. I l l Bournauld, M. 1963. Le courant, facte'ur ecologique et ephlogique de la Vie aquatique. Hydrobiologia, 21: 125-165. Brett, J.R. 1957. Salmon research and hydroelectric power development. Bul l . Fish. Res. Bd. Canada 114, 26 pp. Brinck, P. 1949. Studies on Swedish stoneflies (Plecoptera). Opusc. ent. suppl. 11: 1-250. Brinkhurst, R.O. 1967. Sampling the benthos. Cyclostyled MS. Great Lakes Inst. Rep. PR32, University of Toronto, Toronto, 6pp. Bryan, J.E. and P.A. Larkin. 1972. Food special ization by individual trout. J . Fish. Res. Bd. Canada 29: 1615-1624. Cadwallader, P.L. 1973. The ecology of Galaxias vulgaris (Pisces: Salmoniformes: Galaxiidae) in the r iver Glentui, New Zealand. Ph.D. Thesis, University of Canterbury, New Zealand. Unpubl. Casper, H. 1951. Rhythmische Erscheinungen in der Fortpflanzung von Clunio  marinus (Dept. Chiron.) und das Problem der lunaren Periodizetat Organismen. Arch. Hydrobiol. Suppl. XVIII: 415-594. Chapman, D.W. and R. Demory. 1963. Seasonal changes in the food ingested by aquatic insects larvae and nymphs in two Oregon streams. Ecol. 44: 140-146. Chapman, D.W. 1966. The relative contribution of aquatic and terrestr ia l primary producers to the trophic relations of stream organisms. In: Organism-Substrate Relationships in Streams. Cummins, K. et a l . ,~Ted). Spec. Publ. 4, Pymatuning Laboratory of Ecology, Univ. of Pittsburgh. Chaston, I. . 1969. Seasonal act iv i ty and feeding patterns of brown trout (Salmo  trutta) in a Dartmoor stream in relation to ava i lab i l i ty of food. J . Fish. Res. Bd. Canada 26: 2165-2171. C l i f f o rd , H.F. 1966. The ecology of invertebrates in an intermittent stream. Invest. Indiana Lakes, Streams 7: 57-98. . 1972. Dri ft of invertebrates in an intermittent stream draining marshy terrain of west-central Alberta. Can. J . Zool. 50: 985-991. Coffman, W.P. 1967. Community structure and trophic relations in a small wood-land stream. L inesv i l le Creek, Crawford County, Pennsylvania. Ph.D. Thesis, University of Pittsburgh, Pittsburgh. Coleman, M.T. and H.B.N. Hynes. 1970. The vertical distr ibution of the inver-tebrate fauna in the bed of a stream. Limnol. Oceanogr. 15: 31-40. Coutant, C.C. 1964. Insecticides Sevin: effect of aerial spraying on dr i f t of stream insects. Science 146: 420-421. Cummins, K.W. 1964. Factors l imiting the microdistribution of larvae of the caddisfl ies Pycnopsyche 1 i pi da (Hagen) and Pychnopsyche gutt i fer (Walker) in a Michigan stream. Ecol. Monogr. 34: 271-295. 112 Cummins, K.W. 1973. Trophic relations of aquatic insects. Ann. Rev. Entom. 18: 183-206. Dendy, J.S. 1944. The fate of animals in stream dr i f t when carried into lakes. Ecol. Monogr. 14: 333-357. Dimond, J.B. 1967. Evidence that d r i f t of stream benthos is density related. Ecol. 48: 855-857. Dodd, G.S. and F.L. Hisaw. 1923. Ecological studies of aquatic insects. I. Adaptations of mayfly nymphs to swift streams. Ecol. 5: 137-149. ' . 1924. Ecological studies of aquatic insects. III. Adaptations of caddisfly larvae to swift streams. Ecol. 6: 123-137. Duffy, E. 1956. Aerial dispersal in a known spider population. J . Anim. Ecol. 25: 85-111. Edington, J.M. 1965. The effect of water flow on populations of net spinning Trichoptera. Mitt, internat. Verein. theor. angew. Limnol. 13: 40-48. Efford, I.E. 1960. A method of studying the vertical distribution of the bottom fauna in shallow waters. Hydrobiologia, 16: 288-292. Egglishaw, H.J. 1964. The distributional relationship between the bottom fauna and plant detritus in streams. J . Anim. Ecol. 33: 463-476. E l l i o t t , J.M. 1965a. Daily fluctuation of d r i f t invertebrates in a Dartmoor stream. Nature, 205: 1127-1129. . 1965b. Invertebrate d r i f t in a mountain stream in Norway. Norsk. ent. T. 13: 97-99. •_j . 1967a. Invertebrate d r i f t in a Dartmoor stream. Arch. Hydrobiol. 63: 202-237. . 1967b. The l i f e histories and dr i f t ing of the Plecoptera and Ephemeroptera in a Dartmoor stream. J . Anim. Ecol. 36: 343-362. . 1968. The daily act iv i ty patterns of mayfly nymphs. Ephemeroptera. J . Zool. 155: 201-221. . 1969. The diel act iv i ty patterns of caddis larvae. Trichoptera. J . Zool. Lond. 160: 279-290. ' 1970. Diel changes in invertebrate dr i f t and the food of trout Salmo trutta L. J . Fish. Biol. 2: 161-165. . 1971a. Upstream movement of benthic invertebrates in a lake d i s t r i c t stream. J . Anim. Ecol. 40: 235-252. . 1971b. The distance travelled by dr i f t ing invertebrates in a Lake Distr ict stream. Oecologia (Berl.) 6: 350-379. 113 E l l i o t t , J.M. 1972. Effect of temperature on time of hatching in Baetis rhodani. Ephemeroptera: Baetidae. Oecologia. Berlin 9: 47-51. Everest, F.H. and Chapman, D.W. 1972. Differences in l i t t o r a l fauna due to fluctuating water levels below a hydroelectric dam. J . Fish. Res. Bd. Canada 29: 1472-1476. Fager, E.W. 1969. Production of stream benthos: a cr it ique of the method of assessment proposed by Hynes and Coleman 1968. Limnol. & Oceanogr. 14: 766-770. Hamilton, A.L. 1969. On estimating annual production. Limnol. & Oceanogr. 14: 771-782. Harker, J.E. 1953a. The diurnal rhythm of act iv i ty of mayfly nymphs. J . Expl B io l . 30: 525-533. 1953b. An investigation of the distribution of mayfly fauna of a Lancashire stream. J . Anim. Ecol. 22: 1-13. Hartland-Rowe, R. 1955. Lunar rhythm in the emergence of Ephemeroptera. Nature, 176: 756. Harrison, A.D. 1966. Recolonization of a Rhodesian stream after drought. Arch. Hydrobiol. 62: 405-421. Hoffman, C H . and E.W. Surber. 1945. Effects of an aerial application of wettable DDT on f i sh and f ish food organisms in Back Creek, West Virginia. Trans. Am. Fish. Soc. 75: 48-58. Holt, C S . and T.F. Waters. 1967. Effect of l ight intensity on the d r i f t of stream invertebrates. Ecol. 48: 225-234. Hopkins, T.L. 1964. A survey of some marine bottom samplers. Ijn: Progress in Oceanography, Vol. 2 , Sears, M. (ed). Pergamon Press, New York. 213-256. Hora, S.L. 1930. Ecology, bionomics and evolution of the torrential fauna with special reference to the organs of attachment. Phi l . Trans. R. Soc. (B) 218: 171-182. V. Horton, P.A. 1961. The bionomics of brown trout in a Dartmoor stream. J . Anim. Ecol. 30: 311-338. Hubault, E. 1927. Contribution a l'e'tude des invertebres torrenticoles. Bul l . b io l . Fr. Belg. Suppl. 9: 1-390. Hughes, D.A. 1966. The role of responses of l ight in the selection and maintenance of microhabitat by the nymphs of two species of mayfly. Anim. Behav. 14: 17-33. Hultin, L., B. Svensson and S. Ulfstrand. 1969. Upstream movement of insects in a south Swedish small stream. Oikos, 20: 553-557. 114 Hunt, R.L. 1965. Surface-drift insects as trout food in the Brule River. Trans. Wis. Acad. Sc i . Arts Lett. 54: 51-61. Hynes, H.B.N. 1941. The Taxonomy and ecology of the nymphs of Br i t i sh Plecoptera with notes on the adults and eggs. Trans. R. Soc. London 91: 459-557. : • . 1958. The effect of drought on the fauna of a small mountain stream in Wales. Verh. internat. Verein. theor. angrew. Limnol. 13: 826-833. . 1961. The invertebrate fauna of a Welsh mountain stream. Arch. Hydrobiol. 57: 344-388. . 1970a. The ecology of stream insects. Ann. Rev. Entomol. 15: 25-43. . 1970b. The ecology of running waters. Univ. Toronto Press. • and M.T. Coleman. 1968. A simple method of assessing the annual production of stream,benthos. Limnol. & Oceanogr. 13: 568-573. Ide, F.P. T935. The effect of temperature on the distr ibution of the mayfly fauna of a stream. Univ. Toronto Studies Biol . Series 39: 1-76. . 1942. Ava i lab i l i ty of aquatic insects as food of the speckled trout Salvelinus font inal i s . Trans. N. Amer. Wildl. Conf. 7: 442-450. Jaag, 0. and H. Amblihl. 1964. The effect of current on the composition of biocoenosis in flowing water streams. Adv. Int. Conf. Wat. Polln. Res., London. Pergamon Press, Oxford. 31-49. Jenkins, T.M. 1969. Night feeding of brown trout and rainbow trout in an experimental stream. J . Fish. Res. Bd. Canada 26: 3275-3278. Jenkins, T.M. J r . , CR. Feldmeth and G.V. E l l i o t t . 1970. Feeding of rainbow trout (Salmo gairdneri) in relation to abundance of dr i f t ing inverte-brates in a mountain stream. J . Fish. Res. Bd. Canada 27: 2356-2361. Kawai, T. 1959. Foods of Sal velinus pluvius of the stream connecting with Lake Otori- ike. Jap. J . Limnol. 20: 167-173. Lehmkiihl, D.M. and N.H. Anderson. 1972. Microdistribution and density as factors affecting the downstream dr i f t of mayflies. Ecol. 53(4): 661-667. Lennon, R.E. 1941. Drift-borne organisms in Pond Brook, Passaconaway, N. Hamp. Univ. N.H. Ext. Serv. Contrib. No. 2, B io l . Inst. Leonard, J.W. 1942. Some observations on the winter feeding habits of brook trout fingerlings in relation to natural food organisms present. Trans. Am. Fish. Soc. 71: 219-227. and F.A. Leonard. 1962. Mayflies of Michigan trout streams. Bul l . Cranbrook Inst. Sci . 43: 1-139. 115 Levanidova, I.M. and V.Y. Levanidov. 1965. Dirunal migrations of benthal insect larvae in the r iver. I. Migrations of Ephemeroptera larvae in the Khor River. Zool. Zh. 44: 373-385. Macan, T.T. 1957. The l i f e history and migration of the Ephemeroptera in a stony stream. Trans. Soc. Br. Ent. 12: 129-156. . 1958. Methods of sampling the bottom fauna in stony streams. Mitt. Int. Ver. Theoret. Agnew. Limnol. 8. . 1961. Factors that l imi t the range of freshwater animals. B io l . Rev. 36:• 151-198. and J.C. Mackereth. 1957. Notes on Gammarus pulex in the English Lake D i s t r ic t . Hydrobiologia, 9: 1-12. Maciolek, J.A. and P.R. Needham. 1951. Ecological effects of winter conditions on trout foods in Convict Creek, Cal i fornia. Trans. Am. Fish. Soc. 81: 202-217. Madsen, B.L. 1966. 0m rytmisk aktwitet los defgnfluenymfer. Flora Fauna 72: 148-154. . 1968. Comparative ecological investigations of two related mayfly nymphs. Hydrobiologia, 31: 337-349. . 1968. The distributions of nymphs of Branchyptera, r i s i , Mort. and Nemoura flexuosa Aub. (Plecoptera) in relation to oxygen. Oikos 19: 304-310. MacKinnon, D., L. Edgeworth and R.E. McLaren. 1961. An assessment of Jones Creek spawning channel. Cn. Fish. Cult. 30: 3-14. Maitland, P.S. 1965. The distr ibut ion, l i f e cycle and predators of Ephemerella  ignita (Poda) in the r iver Endrick, Scotland. Oikos, 16: 48-57. . 1969. A simple corer for sampling sand and f iner sediments in shallow water. Limnol. Oceanogr. 14: 151-156. McCormack, J.C. 1962. The food of young trout (Salmo trutta) in two different becks. J . Anim. Ecol. 31: 305-316. McLay, C L . 1970. A theory concerning the distance travelled by animals enter-ing the d r i f t of a stream. J . Fish. Res. Bd. Canada 27: 359-370. Metz, J . 1972. Die invertebratendrift an der oberflache eines voral-penflusses und ihre selektive ausnutzung durch die regenbogen-forellen (Salmo gairdneri). Zoologischen Insitut der Universitat, Munchen. Minckley, W.L. 1963. The ecology of a spring stream. Doc. Run, Meade County, Kentucky. Wildl i fe Monogr. 11: 1-124. . 1964. Upstream movement of Gammarus (Amphipoda) in Doe Run, Meade County, Kentucky. Ecology 45: 196-197. 116 Minshall, G.W. 1967. The role of allochthonous detritus in the trophic structure of a woodland springbrook community. Ecol. 48: 139-T49. and R.A. Keuhne. 1969. An ecological study of invertebrates of the Duddon, an English mountain stream. Arch. Hydrobiol. 66: 169-191. . and R.W. Winger. 1968. The effect of reduction in stream flow on invertebrate d r i f t . Ecol. 49: 580-582. Moffett, J.W. 1936. A quantitative survey of the bottom fauna in some Utah streams variously affected by erosion. Bul l . Univ. Utah Bio. Ser. 3: 1-33. Moon, P.O. 1935. Flood movements in the l i t t o r a l fauna Windermere. J . Anim. Ecol. 4: 216-228. 1940. An investigation of the movements of freshwater invertebrate faunas. J . Anim. Ecol. 9: 76-83. Morgan, N.C. and Egglishaw, H.J. 1965. A survey of the bottom fauna of streams in the Scottish Highlands. Part 1, Composition of fauna. Hydrobiologia, 25: 181-211. Muller, R. 1954a. Die Dr i f t in flieBender Gewasser. Arch. Hydrobiol. 49: 539-545. 1954b. Investigations on the organic d r i f t in North Swedish streams. T J- I" L... Pi_ r» . . • . i . - -Rep. Inst. Freshw. Res., Drottningholm 35: 133-148 . 1963a. Tag-Nachtrbythmus von Baetidenlarven in der "Prganischen Dr i f t " . Naturwissenschaften 50: 161. _. 1963b. Temperatur und Tagesperiodik der "Organischen Dr i f t " . Naturwissenschaften 50: 410-411. . 1963c. Diurnal rhythm in "Organic Dr i f t " of Gammarus pulex. Nature 198: 806-807. . 1965. Field experiments on periodicity of freshwater invertebrates. J_n: Circadian Clocks, North-Holland Publ. Co. Amsterdam. 314-317. . 1966a. Die Tagesperiodik von FlieBavasserorganismen. 2. Morph. Okol. Tiere 56: 93-142. . 1966b. Zur Periodik von Gammarus pulex. Oikos 17: 207-211 . Mundie, J.H. 1969. Ecological implications of the diet of juvenile coho in streams. Jjn: Symposium on Salmon and Trout in Streams. H.R. MacMillan Lecture in Fisheries, Univ. of Br i t i sh Columbia, Vancouver, B.C. Neave, F. 1930. Migratory habits of the mayfly Blasturus cupidus Say. Ecol. 11: 568-576. Needham, P.R. 1928. A net for the caputre of stream dr i f t , organisms. Ecol. 9: 339-342. 117 Nielsen, A. 1950. Torrential invertebrate fauna. Oikos 2: 176-196. • . 1951. Is dorso-ventral f lattening of the body an adaptation to torrential l i fe? Verh. internat. Verein. theor. angew. Limnol. 11: 264-267. O'Donnell, D.J. and W.S. Churchi l l . 1954. Certain physical, chemical and biological aspects of the Brule River, Douglas County, Wisconsin. Trans. Wis. Acad. Sci. Arts Lett. 43: 201-255. Ogilvie, R.M. 1971. A study of d r i f t in the Wainuiomata r iver, Wellington. B.Sc. (Hons.) Project. Univ. Wellington. Unpubl. Patrick, R. 1959. Aquatic l i f e in a new stream. Wat. Sewage Wks. December 1959, p. 5. Pearson, W.D. and R.H. Kramer. 1972. Dri ft and production of two aquatic insects in a mountain stream. Ecol. Monogr. 24: 365-385. Percival, E. and Whitehead, H. 1929. A quantitative study of the fauna of some types of stream-bed. J . Ecol. 17: 282-314. Reimers, N. 1957. Some aspects of the relation between fish foods and trout survival. Ca l i f . Fish. & Game 43: 43-69. Roose, T. 1957. Studies on upstream migration in adult stream-dwelling insects. 1. Rept. Inst. Freshw. Res., Drottningholm 38: 167-193. Scott, D. 1958. Ecological studies on the Trichoptera in the River Dean, Cheshire. Arch. Hydrobiol. 54: 340-392. Scott, D.O. 1961. Effects of DDT spraying on aquatic insects in the Bitterot River. Completion Report, U.S. Forest Serv. Mimeo. Pp: 1-15. Slack, H.D. 1936. The food of caddisfly (Trichoptera) larvae. J . Anim. Ecol. 5: 105-115. • Stehr, W.C. and J.W. Branson. 1938. An ecological study of an intermittent stream. Ecol. 19: 294-310. 'Steinmann, P. 1913. Uber Rheotaxis bei Tieren des flieBenden Wassers. Verh. naturforsch. Geo. Basel, 24: 136-158. Stocker, Z.S.J, and Williams, D.D. 1972. A freezing core method for describing the vertical distribution of sediments in a stream-bed. Limnol. Oceanogr. 17: 136-138. Surber, E.W. 1937. Rainbow trout and bottom fauna production in one mile of stream. Trans. Am. Fish. Soc. 66: 193-202. Tanaka, H. 1960. On the daily change of the dr i f t ing of benthic animals in streams, especially on the types of daily change observed in taxonomic groups of insects. Bul l . Freshw. Fish. Res. Lab., Tokyo, 9: 13-24. 118 Thorup, J . 1966. Substrate type and i t s value as a basis for the delimitation of bottom fauna communities in running waters in Organism-Substrate Relationships in Streams, K. Cummins et aK (ed). Spec. Pub. 4. • Pymatuming Laboratory of Ecology, Univ. of Pittsburgh. Tobias, W. and E. Thomas. 1967. Die Oberflachendrift als Indikator periodischer Aktivitatsverlaufe bei Insekten. Ent. Z. 77: 153-163. Ulfstrand, S. 1967. Microdistribution of benthic species (Ephemeroptera, Plecoptera, Trichoptera, Diptera, Simuliidae) in Lapland streams. Oikos 18: 617-660. Waters, T.F. 1961. Standing crop and d r i f t of stream bottom organisms. Ecol. - 42: 532-537. • 1962a. A method to estimate the production rate of a stream bottom invertebrate. Trans. Am. Fish. Soc. 91: 243-250. _. 1962b. Diurnal periodicity in the dr i f t of stream invertebrates. Ecol. 43: 316-320. _. 1964. Recolonization of denuded stream bottom areas by d r i f t . Trans. Am. Fish. Soc. 93: 311-315. . 1965. Interpretation of-invertebrate d r i f t in streams. Ecol. 46: 327-334. . 1966. Production rate, population density and d r i f t of the stream invertebrate. Ecol. 47: 597-604. _ . 1968. Diurnal periodicity in the d r i f t of a day-active stream invertebrate. Ecol. 49: 152-153. ' . 1969a. Invertebrate dri ft—ecology and significance to stream fishes. J_TK Symposium on Salmon and Trout in Streams. T.G. Northcote, ed. Univ. B.C. . 1969b. Diel patterns of aquatic invertebrate d r i f t in streams of northern Utah. Utah Acad. Proc. 46: 109-130. Williams, D.D. and H.B.N. Hynes. 1974. The occurrence of benthos deep in the substratum of a stream. Freshw. Biol . 4: 233-256. Wodsedalek, R. 1912. Natural history and general behavior of the Ephemeridae nymphs Heptagenia interpunctata (Say). Ann. ent. Soc. Amer. 5: 31-40. Wojtalik, T.H. and T.F. Waters. 1970. Some effects of heated water on the dr i f t of two species of stream invertebrates. Trans. Amer. Fish. Soc. 99: 782-788. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            data-media="{[{embed.selectedMedia}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0100064/manifest

Comment

Related Items