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Seasonality, sinking and the chlorophyll maximum of an oligotrophic British Columbia lake Jackson, Leland J. 1988

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•AEGNA-LITY, SINKING AND THE CHLOROPHYLL MAXIMUM OF AN OLIGOTROPHIA BRITISH COLUMBIA LAKE by Leland J . Jackson B.Sc.(Hons.)., Queen's University, 1984 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of Oceanography) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1988 © Leland J . Jackson, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written perrfiission. Department of Oceanography The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 13 January, 1988 DE-6(3/81) i i ABSTRACT A f i e l d i nvestigation was ca r r i e d out over two seasonal periods on an oligotrophic coastal B r i t i s h Columbia lake to determine the rol e of sinking i n the formation of the chlorophyll maximum as well as some aspects of phytoplankton seasonality. Sinking rates of two diatoms were measured and found to be highest i n the epilimnion and lowest at the depth of the chlorophyll maximum. Light affected sinking rate as well as the pos i t i o n of the chlorophyll maximum. The chlorophyll maximum formed at 10-12 m following the onset of seasonal thermal s t r a t i f i c a t i o n and descended to ca. 22 m for the summer. A major factor i n the formation of the chlorophyll maximum i s the decrease of phytoplankton sinking rate at depth. Rhizosolenia e r i e n s i s i s one of the f i r s t phytoplankters to bloom i n the spring. Small f l a g e l l a t e s (3-15 um) and occasionally Dinobryon sp. were also important numerically. In the summer C y c l o t e l l a spp. displaced R. e r i e n s i s as the dominant diatom i n the epilimnion. The r e l a t i v e timing of seasonal maxima of blooms of various species remained s i m i l a r during the two years investigated. Lake f e r t i l i z a t i o n affected the phytoplankton standing stock. R. e r i e n s i s did not greatly benefit from f e r t i l i z a t i o n since i t sank out of the epilimnion and became a major constituent of the chlorophyll maximum before f e r t i l i z a t i o n . Because of i t s large s i z e and low C : c e l l volume r a t i o due to a large vacuole, R. e r i e n s i s i s probably not a good food source for zooplankton. i v TABLE OF CONTENTS PAGE ABSTRACT i i LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS x i i INTRODUCTION 1 Lake Enrichment Programme 1 Chlorophyll Maxima 3 Sinking 8 Study Objectives 10 MATERIALS AND METHODS 10 STUDY AREA 10 Sampling and Analysis 13 chlorophyll 13 l i g h t 14 nutrients 14 temperature 15 in vivo fluorescence 16 phytoplankton p r o f i l e s 16 i n s i t u sinkincr 18 sediment traps 19 F e r t i l i z a t i o n Regime 22 RESULTS 25 V e r t i c a l P r o f i l e Replication 25 V P h y s i c a l O b s e r v a t i o n s 29 t empera ture 2 9 l i g h t 31 Water C h e m i s t r y 3 2 n i t r a t e 32 t o t a l f i l t e r e d phosphorus 34 B i o l o g i c a l O b s e r v a t i o n s 3 6 i n v i v o f l u o r e s c e n c e 3 6 c h l o r o p h y l l 4 0 p h y t o p l a n k t o n 4 3 s i n k i n g r a t e d e t e r m i n a t i o n 50 s e d i m e n t a t i o n 52 DISCUSSION 68 S e a s o n a l C y c l e s 68 s p r i n g 68 summer 7 2 R h i z o s o l e n i a e r i e n s i s a u t e c o l o g y 7 5 Two R i v e r s arm as a c o n t r o l s i t e 78 Lake f e r t i l i z a t i o n 80 G r a z i n g 81 F u t u r e work 82 CONCLUDING REMARKS r 84 REFERENCES 86 APPENDICES 91 V I LIST OF TABLES Page Table 1. Details of the f e r t i l i z a t i o n regime i n Sproat Lake during 1985 and 1986 (N:P r a t i o by atoms). 25 V l l LIST OF FIGURES Page Figure 1. Map of B r i t i s h Columbia, Canada i n d i c a t i n g the location of Sproat Lake. 11 Figure 2. Map of Sproat Lake, i n d i c a t i n g the sampling locations i n Two Rivers (control) and Taylor (experimental) arms. 12 Figure 3 Cross section of a sediment trap used to c o l l e c t sedimenting phytoplankton. 20 Figure 4. Schematic representation of the arrangement of sediment traps and components of mooring l i n e s . 21 Figure 5. Map of Sproat Lake, i n d i c a t i n g (black section) the area of Taylor arm receiving nutrient enrichment. 23 v i i i Figure 6. Soluble reactive phosphorus (a,c) and n i t r a t e (b,d) concentrations i n u n f e r t i l i z e d (a,b) and f e r t i l i z e d (c,d) areas of Great Central Lake, B r i t i s h Columbia on the day before f e r t i l i z a t i o n (•), day of f e r t i l i z a t i o n (•), and one (•) and two (•) days following f e r t i l i z a t i o n (from Stockner et a l . , 1980). 26 Figure 7. Map of Sproat Lake, i n d i c a t i n g p r o f i l e stations for i n vivo fluorescence p r o f i l e comparison of 5 May 1987. 27 Figure 8. In vivo fluorescence p r o f i l e s at s i x stations on 5 May 1987. Numbers i n the bottom r i g h t corner of each plot correspond to a st a t i o n number of Figure 7. 28 Figure 9. Temperature p r o f i l e s for Two Rivers (a) and Taylor (b) arms, 1986. 30 Figure 10. Temperature p r o f i l e s for Two Rivers (a) and Taylor (b) arms, 1987. 31 i x F i g u r e 11. 1 % l i g h t depth (a) and e x t i n c t i o n c o e f f i c i e n t (b) i n Two R i v e r s (•) and T a y l o r (o) arms, 1986. 33 F i g u r e 12. 1 % l i g h t depth (a) and e x t i n c t i o n c o e f f i c i e n t (b) i n Two R i v e r s (#) and T a y l o r (o) arms, 1987. 35 F i g u r e 13. N i t r a t e c o n c e n t r a t i o n d u r i n g 1986 (a) and 1987 (b) i n Two R i v e r s (•) and T a y l o r (o) arms. 37 F i g u r e 14. T o t a l f i l t e r e d phosphorus c o n c e n t r a t i o n d u r i n g 1986 (a) and 1987 (b) i n Two R i v e r s (•) and T a y l o r (o) arms. 3 9 F i g u r e 15. In v i v o f l u o r e s c e n c e p r o f i l e s f o r Two R i v e r s (a) and T a y l o r (b) arms, 1986. 40 F i g u r e 16. In v i v o f l u o r e s c e n c e p r o f i l e s f o r Two R i v e r s (a) and T a y l o r (b) arms, 1987. 42 F i g u r e 17. C h l o r o p h y l l a c o n c e n t r a t i o n s d u r i n g 1986 (a) and 1987 (b) i n Two R i v e r s (•) and T a y l o r (o) arms. 4 4 Figure 18. Depth p r o f i l e s of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) i n Two Rivers (•) and Taylor (o) arms, 1986. 48 Figure 19. Depth p r o f i l e s of small f l a g e l l a t e s (a) and Dinobryon sp. (b) i n Two Rivers (•) and Taylor (o) arms, 1986. 50 Figure 20. Depth p r o f i l e s of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) i n Two Rivers (#) and Taylor (o) arms, 1987. 52 Figure 21. Depth p r o f i l e s of small f l a g e l l a t e s (a) and Dinobryon sp. (b) i n Two Rivers (•) and Taylor (o) arms, 1987. 54 Figure 22. Sinking rates of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) on 9 and 20 May 1987 i n Two Rivers (RR) and Taylor (T) arms. Error bars represent +/- 2 SD; n = 2. 57 Figure 23. Sedimentation of Rhizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Two Rivers arm, 1986. 58 Figure 24. Sedimentation of Rhizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Taylor arm, 1986. 59 x i Figure 25. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Two Rivers arm, 1986. Figure 26. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Taylor arm, 1986. Figure 27. Sedimentation of Rhizosolenia e r i e n s i s at 7 (a) and 15 m (b) i n Two Rivers arm, 1987. Figure 28. Sedimentation of Rhizosolenia e r i e n s i s at 7 (a) and 15 m (b) i n Taylor arm, 1987. Figure 29. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Two Rivers arm, 1987. Figure 30. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Taylor arm, 1987. x i i ACKNOWLEDGEMENTS I express gratitude to the members of my thesis committee for t h e i r encourgement and the freedom they allowed me. Dr. P.J. Harrison provided laboratory space and was continuously available for both academic and non-academic advice. Dr. J.G. Stockner generously provided equipment and space from h i s laboratory at the West Vancouver Laboratories while Dr. W.E. N e i l l was always generous with h i s time and advice. The members of my laboratory were of great benefit i n helping me form a method of s c i e n t i f i c inquiry; t h i s has undoubtably been a p o s i t i v e influence i n writing t h i s t h e s i s . The time and help from P.A. Thompson and M. Soon i n t r a v e l l i n g to and from my f i e l d s i t e i s p a r t i c u l a r l y appreciated. I also thank P. J . C l i f f o r d , W.P. Cochlan and G. Doucette for t h e i r input during the l a s t 2 years. The Lake Enrichment Programme of Fisheries and Oceans Canada was extremely generous i n providing equipment, l o g i s t i c a l support and data from t h e i r 1986 Sproat Lake programme. I thank B.H. Nidle and K. Shortreed for many suggestions and comments. F i n a l l y , major f i n a n c i a l support was provided by a F i s h e r i e s and Oceans Canada post-graduate scholarship, for which I am very g r a t e f u l . 1 Introduction Lake Enrichment Programme The federal government of Canada and p r o v i n c i a l government of B r i t i s h Columbia j o i n t l y run the Salmonid Enhancement Programme (SEP) which has as i t s goal, the enhancement of de c l i n i n g sockeye salmon (Oncorhynchus nerka) stocks. Krokhin (1967) suggested that the comercial salmon fishery may cause a nutrient d e f i c i t i n lakes by decreasing the number of salmon carcasses ava i l a b l e for decay and release of nutrients. Therefore, f e r t i l i z a t i o n of such lakes may replace t h i s p o t e n t i a l nutrient d e f i c i t . One of three major sections of SEP i s the Lake Enrichment Programme (LEP). Through addition of inorganic nitrogen and phosphorus (in the form of a g r i c u l t u r a l grade f e r t i l i z e r ) to selected u l t r a - o l i g o t r o p h i c lakes i n which sockeye salmon spawn, LEP has increased primary production, which has i n turn increased secondary production and resulted i n more food being ava i l a b l e to juvenile sockeye (Barraclough and Robinson, 1972; LeBrasseur and Kennedy, 1972; Parsons et. a l . , 1972; Stockner and Shortreed, 1985). Larger juvenile sockeye r e s u l t from an increase i n available food (zooplankton) i n LEP's study lakes. Ricker (1962) noted that larger juvenile sockeye salmon have a higher "ih-lake" s u r v i v a l and those migrating to sea have a higher "at-sea" s u r v i v a l rate 2 than smaller counterparts. Additionally, Hyatt and Stockner (1985) suggest that larger sockeye smolts may decrease t h e i r age-at-return as adults. I t i s desirable to have an enrichment regime which ultimately r e s u l t s i n enhancement of sockeye salmon populations only. The cost-effectiveness of LEP's programme can be maximized i f nutrient added to lakes i s incorporated into the sockeye salmon food chain. The e f f i c i e n c y of t h i s process w i l l be high i f there are only two steps i n the r e s u l t i n g food chain ( i e . , phytoplankton > zooplankton > juvenile sockeye salmon). A decreaease i n cost-effectiveness may occur i f there are more than two steps i n the r e s u l t i n g food chain or i f predators other than juvenile sockeye consume the enhanced zooplankton stock. A common and abundant pelagic diatom, Rhizosolenia e r i e n s i s has increased notably i n some lakes following nutrient enrichment (Parsons et a l . , 1967; J.G. Stockner, pers. comm.). Following bloom conditions i n the spring t h i s diatom sinks and becomes a major constituent (both i n terms of numbers and biovolume) of a hypolimnetic chlorophyll maximum (Stockner and Hyatt, 1984) which t y p i c a l l y p e r s i s t s through the summer u n t i l f a l l overturn. Incorporation of R. e r i e n s i s into the chlorophyll maximum t y p i c a l l y occurs at the bottom of the euphotic zone (defined as 1% surface irradiance or greater) concurrent with the n i t r a c l i n e (J.G. Stockner, pers comm.). Prevailing l i g h t and nutrient 3 conditions at depth may play a ro l e i n chlorophyll maximum formation by a l t e r i n g c e l l u l a r buoyancy of R. e r i e n s i s as i t encounters d i f f e r e n t environmental conditions while sinking through the mixed layer and thermocline. Rhizosolenia e r i e n s i s i s a large c e n t r i c diatom with a length of of 100-150 um. F i l t e r feeding zooplankton may not be capable of e f f i c i e n t l y grazing t h i s alga due to i t s r e l a t i v e l y large s i z e (Parsons et a l . , 1967). Hence, blooms and subsequent incorporation into the chlorophyll maximum may represent an energy loss (as nutrients) or carbon sink through diversion from food chains leading to juvenile sockeye i n f e r t i l i z e d lakes. To understand the poten t i a l impact of R. e r i e n s i s i n r e l a t i o n to the cost-effectiveness of the Lake Enrichment Programme, an understanding of the biology of t h i s diatom i s required. A useful s t a r t i n g point i s an investigation of the phys i o l o g i c a l ecology of R. e r i e n s i s , concentrating on the effect(s) of various environmental parameters on sinking rate and chlorophyll maximum formation and maintenance. Chlorophyll maxima Phytoplankton physiologists studying pelagic communities are often interested i n obtaining a convenient, semi-quantitative measurement of phytoplankton biomass. In vivo chlorophyll fluorescence (Lorenzen, 1966) has become a standard measurement used to estimate the concentration of chlorophyll a, which serves 4 as a rough index of photosynthetically active biomass. One disadvantage of using an i n vivo fluorescence technique i s that i t does not include heterotrophic phytoplankton. A feature c h a r a c t e r i s t i c of many v e r t i c a l i n vivo fluorescence p r o f i l e s i s a subsurface fluorescence maximum (Cullen, 1982). This feature e x i s t s i n oceans (Anderson, 1969; Venrick et a l . , 1973), large lakes (Kiefer et a l . , 1973; Moll et a l . , 1984) and small lakes (Fee, 1978). Despite the fact that chlorophyll maxima are frequent features observed i n v e r t i c a l fluorescence p r o f i l e s , t h e i r contribution to net areal primary production i s poorly understood. The subsurface chlorophyll maximum i s often found associated with low l i g h t l e v e l s . This layer t y p i c a l l y occurs at about the 1% surface irradiance l e v e l (Fee, 1976) but i t may occur at le v e l s up to 5% surface irradiance (Moll et a l . , 1984). In addition to low l i g h t l e v e l s , subsurface chlorophyll maxima often occupy ( v e r t i c a l l y ) r e l a t i v e l y t h i n layers (ca. 5-20 m; Postel, 1975) i n close proximity to strong v e r t i c a l gradients. Such gradients vary and may include the n u t r i c l i n e (Kiefer et a l . , 1975), thermocline (Cullen, 1982) or pycnocline (Postel, 1975). Cullen (1982) has c l a s s i f i e d four types of chlorophyll maxima based on the method of formation i n each case. This c l a s s i f i c a t i o n i s b r i e f l y outlined below: 5 I. Chlorophyll maximum and primary production maximum near the n i t r a c l i n e : t y p i c a l t r o p i c a l structure (TTS). This c l a s s i f i c a t i o n follows from Dugdale's (1967) concept of the euphotic zone as a two-layered system where phytoplankton growth i s n u t r i e n t - l i m i t e d i n the upper layer and l i g h t - l i m i t e d i n the lower layer. The chlorophyll and primary production maxima occur in the t r a n s i t i o n zone where phytoplankton switch from l i g h t -l i m i t e d to nutri e n t - l i m i t e d growth. One can v i s u a l i z e such a chlorophyll maximum i n the tro p i c s or temperate zone during summer s t r a t i f i c a t i o n . I t i s necessary for the euphotic zone depth to exceed the depth of the thermocline so l i g h t i s present below the mixed layer. This assumes the n i t r a c l i n e i s coincident with the thermocline and that nitrogen i s the nutrient which l i m i t s phytoplankton growth. I I . Physiological adaptation of C : Chi a. One phys i o l o g i c a l adaptation to low l i g h t i n t e n s i t i e s by phytoplankton i s an increase i n Chi a per c e l l (Beardall and Morris, 1976; Banse, 1977; Prez e l i n and Matlick, 1980). An observed chlorophyll maximum may simply be an adaptation to the low l i g h t i n t e n s i t i e s present at the stratum where the chlorophyll maximum i s located and may not necessarily be a biomass (= carbon) maximum. I I I . Behavioral aggregation. While any type of aggregation 6 might broadly be considered a behavioral response, the present context equates behavior to m o t i l i t y . Therefore, behavioral aggregation would be almost exclusively exhibited by f l a g e l l a t e d c e l l s . Exceptions may be envisioned such as with the c i l i a t e Mesodinium rubrum, which i s known to contain a l g a l pigments (Holm-Hansen et a l . , 1970). M. rubrum may contribute s i g n i f i c a n t l y to organic production i n some regions (Smith and Barber, 1979) and may therefore have a s i g n i f i c a n t impact on chlorophyll maximum dynamics. Many d i n o f l a g e l l a t e s exhibit phototactic diurnal v e r t i c a l migration. Such c e l l s are t y p i c a l l y found i n surface waters during daylight hours and deeper during the night. Movement of whole populations on a diurnal basis r e s u l t s i n movement of the chlorophyll maximum, as demonstrated for Gymnodinium splendens i n the Southern C a l i f o r n i a bight (Lasker, 1975; Fielde r , 1982). Generally, motile phytoplankton are capable of v e r t i c a l movement and of forming t h i n layers (Harris et a l . , 1979; Falkowski et a l . , 1980). IV. Decrease i n sinking rate. Steele and Yentsch (1960) demonstrated that nu t r i e n t - l i m i t e d diatoms sink and accumulate at a subsurface n i t r a c l i n e . Sinking rate of Th a l a s s i o s i r a  pseudonana i s s i g n i f i c a n t l y lower at l i g h t i n t e n s i t i e s c h a r a c t e r i s t i c of the subtropical chlorophyll maximum than at l i g h t i n t e n s i t i e s c h a r a c t e r i s t i c of the overlying mixed layer (Bienfang et a l . , 1983). Such a response suggests that growth at low irradiances may create physiological changes which are 7 manifested, i n part, by a l t e r a t i o n of c e l l buoyancy. Bienfang et a l . , 1983, also found a step function i n the response of T. pseudonana to changes i n irradiance; c e l l s at high l i g h t — 9 — 1 . . • • • (> 64 uE«m ^»s •*•) exhibited one sinking rate while c e l l s at low l i g h t (< 27.4 uE«m~ 2«s - 1) exhibited a s i g n i f i c a n t l y lower sinking rate. Cullen's (1982) c l a s s i f i c a t i o n scheme i s useful because i t allows an understanding of important dynamic processes, although i t does not attempt to quantify such processes. I f a chlorophyll maximum i s formed as a r e s u l t of physiological adaptation to low l i g h t by phytoplankton then the d i r e c t importance to production i n the euphotic zone may be small. However, i f i n s i t u primary production i s occurring, the chlorophyll maximum may be extremely important as a food source for higher trophic l e v e l s . A good example i s f i r s t - f e e d i n g anchovy larvae feeding on Gymnodinium  splendens o f f the coast of C a l i f o r n i a (Lasker, 1975). I f c e l l s sink and aggregate at a n i t r i c l i n e (Steele and Yentsch, 1960) they may not be l o s t from the euphotic zone and may be re c i r c u l a t e d into the epilimnion during storm a c t i v i t y . In t h i s s i t u a t i o n the chlorophyll maximum may be an important seed population leading to phytoplankton blooms subsequent to intense mixing events. I t i s of in t e r e s t to know whether a chlorophyll maximum also represents a carbon maximum. Carbon i s a useful common 8 denominator to use when considering transfer of energy through food chains. I f a chlorophyll maximum i s formed as a r e s u l t of physi o l o g i c a l adaptation by phytoplankton c e l l s manifested as an increase i n Chi a per c e l l then one would expect a decrease i n the C : Chi a l e v e l s . Normalizing p a r t i c u l a t e carbon measurements to chlorophyll w i l l eliminate problems of increased carbon values due to increases i n chlorophyll. Sinking Phytoplankton sinking has received much attention i n oceanographic, and to a lesser extent, freshwater l i t e r a t u r e because sinking a f f e c t s the v e r t i c a l d i s t r i b u t i o n of phytoplankton and, as a re s u l t , the carbon budget of the euphotic zone. The standing crop of phytoplankton i n the euphotic zone i s a s t a t i c measure of a number of dynamic processes. Such processes include increases due to growth and losses due to c e l l mortality, grazing and sinking. Thus, understanding the importance of standing stock necessarily requires understanding growth, mortality, grazing and sinking functions which combine to r e s u l t i n the observed standing stock. The importance of sinking has been r e a l i z e d and many mathematical models for primary production (Steele, 1956, 1961, 1962; Ryther and Yentsch, 1957; Anderson, 1974; Bannister, 1974; and many more) include a term for phytoplankton sinking. Buoyancy regulation may play an important r o l e i n determining the 9 v e r t i c a l d i s t r i b u t i o n of phytoplankton. A v a r i e t y of mechanisms may be used to a l t e r c e l l u l a r buoyancy. Physiological processes have the most dramatic e f f e c t on sinking rate (Bienfang et a l . , 1982, 1983) while other processes, such as a l t e r i n g c e l l u l a r l i p i d component (Anderson and Sweeney, 1977) or production of spines or ridges (Lannergren, 1979) play a smaller r o l e . Mediation of turgor pressure on gas vacuole membranes i s an important buoyancy regulation mechanism i n some cyanobacteria; however, t h i s mechanism would tend to be more important i n freshwater rather than marine systems because lakes t y p i c a l l y contain more filamentous cyanobacteria. Study Objectives There are two main objectives of t h i s study: 1. The f i r s t i s to determine whether Rhizosolenia e r i e n s i s i s a sink for nutrients added to the f e r t i l i z e d arm of Sproat Lake; and 2. To determine i f low l i g h t and high nutrient concentrations act together to s i g n i f i c a n t l y decrease sinking rate of R. e r i e n s i s u n t i l i t becomes neu t r a l l y buoyant and forms a layer at depth. These findings w i l l be related to the cost effectiveness of the Lake Enrichment Program. 10 Materials and Methods Study Area Sproat Lake (49°14 /N, 125°06'W; Fig. 1) i s located on central Vancouver Island, B r i t i s h Columbia. The lake i s approximately 22.5 km long and varies between 1 and 1.5 km wide. The surface area i s 41 km2 and elevation of the lake i s 26 m above sea l e v e l . The mean depth i s 56 m, with maximum depth of about 2 60 m. River flow into the lake i s primarily v i a the Taylor River but numerous streams and creeks are present during spring melt. Most of the shoreline slopes very abruptly into deep water. Sproat Lake i s a warm monomictic coastal lake. Water residence time i s ca. 8 y. Sproat Lake has a small l i t t o r a l zone, low inorganic nutrient l e v e l s and low phytoplankton and zooplankton biomass (Stockner and Shortreed, 1985). The s p e c i f i c dates of sampling various parameters are outlined i n Appendix I. Two stations were sampled: Stn 1 i n Taylor Arm which received nutrient enrichment i n 1986 but not i n 1987 and Stn 2, the control s t a t i o n i n Two Rivers Arm which received no nutrient enrichment (Fig. 2). Two seasonal periods are considered i n t h i s study: 1 A p r i l to 9 August 1986 and 7 March to 1 June 1987. 11 Figure 1. Map of B r i t i s h Columbia, Canada, i n d i c a t i n g the location of Sproat Lake. Figure 2. Map of Sproat Lake, i n d i c a t i n g the sampling locations i n Two Rivers (control) and Taylor (experimental) arms. 13 Samples for chlorophyll, nutrients and phytoplankton were obtained from 5 or 6 discrete depths while samples for temperature were obtained from up to 17 discrete depths using either a 3 L or 6 L van Dorn PVC water sampler. Chlorophyll During the 1986 season LEP s t a f f measured chlorophyll, l i g h t i n t e n s i t y and nutrient concentrations i n Sproat Lake as part of t h e i r complete programme. In 1987, I measured chlorophyll and l i g h t i n t e n s i t y and took samples for n i t r a t e and t o t a l phosphate. The nutrient analysis for 1987 was performed by LEP s t a f f . An e f f o r t was made to sample and measure chlorophyll, l i g h t i n t e n s i t y and nutrients so the data from both years could be pooled and differences i n values would r e f l e c t true i n s i t u differences and not differences i n experimental design or technique. Samples were f i l t e r e d onto 47 mm c e l l u l o s e acetate/cellulose n i t r a t e mixed ester membrane f i l t e r s ( M i l l i p o r e Corp.). Following f i l t r a t i o n , f i l t e r s were folded i n h a l f , placed into l a b e l l e d glassine envelopes and stored frozen i n a sealed glass container containing dessicant u n t i l extraction and analysis (maximum storage time ca. 14 weeks). At the time of analysis, f i l t e r s were ground i n a t i s s u e homogenizer with 90 % c h i l l e d acetone, extracted for 20 h and read on a fluorometer following the method of Parsons et a l . , 14 1984 (duplicate samples indicated 6.5 % v a r i a b i l i t y ) . LEP s t a f f used a 2 h extraction and analysed the chlorophyll extracts with a Turner Designs fluorometer (model 111), f a c t o r y - f i t t e d with a red s e n s i t i v e phototube (R136), equipped with an F4T5 blue fluorescent lamp, a Corning CS5-60 primary f i l t e r and a Corning CS2-64 secondary f i l t e r . The equations used to cal c u l a t e chlorophyll and phaeophytin concentrations for 1986 are outlined i n Stephens and Brandstaetter (1983). My chlorophyll methodology d i f f e r e d from LEP's since I used a 20 h extraction and followed the equations of Parsons et a l . , 1984, to determine pigment concentrations. Light Intensity Light i n t e n s i t y was measured with a Li-Cor quantum l i g h t meter (model 185A) equipped with an underwater cosine c o l l e c t o r . Measurements were made just below the surface (representing the surface value) and at every metre to approximately 2 0 m. Values of l i g h t i n t e n s i t y p r o f i l e s were used to measure the 1 % l i g h t depth as well as the l i g h t attenuation c o e f f i c i e n t , k e. Nutrients Samples were placed i n acid washed (a 10% acid wash followed by three rinses with deionized d i s t i l l e d water) 1 L polyethylene bottle s (Nalgene Corp.) for transport (storage time up to ca. 2 h) to the laboratory. Water samples were f i l t e r e d through preashed and prewashed 47 mm glass f i b r e f i l t e r s (Whatman GF/F; average pore si z e 0.7 um) and stored i n acid washed glass bottles 15 with f o i l - l i n e d screw-top caps at 2°C for 8 to 20 weeks. Nitrate was analysed by a modified seawater technique (Stephens and Brandstaetter, 1983) of Brewer and Riley (1975). Buffered samples containing n i t r a t e were passed through a cadmium column reducing n i t r a t e to n i t r i t e . The reduced sample reacted with sulphanilamide and N(l-napthyl) ethylene diamine (NNED) forming a coloured azo dye. The azo dye was quantified colourmetrically using a Technicon Autoanalyzer II system equipped with a 54 0 nm f i l t e r . The l i m i t of detection was 1 ug N0 3~N«L . Total f i l t e r e d phosphorus was analysed by a modified method (Stephens and Brandstaetter, 1983) of Traversy (1971). Samples were digested with a persulphate-sulphuric acid solution, converting p a r t i c u l a t e phosphorus, polyphosphates and organically bound phosphorus to orthophosphate. Orthophosphate then reacted with ammonium molybdate and stannous chloride to form a blue phospho-molybdenum complex. This complex was quantified colourmetrically using a Technicon Autoanalyser II system equipped with a 660 nm f i l t e r . The l i m i t of detection was 1 ug P« I T 1 . Temperature Early i n the f i e l d season, before thermal s t r a t i f i c a t i o n occurred, temperature was measured at 0.5, 5, 10 45 and 50 m. Af t e r the surface water had begun to warm, temperature was measured at 0.5, 2.5, 5, 7.5 30, 35, 40, 45 and 50 m. 16 Temperature values were plotted and the r e s u l t i n g curves were used to define the epilimnion, thermocline and hypolimnion depths. In vivo fluorescence In vivo fluorescence was measured on a r e l a t i v e scale and since i t was not standardized and converted to known values, eg., chlorophyll a, i t was reported without units . In vivo fluorescence p r o f i l e s were measured with a Turner Designs fluorometer (Model 10) equipped with f i l t e r s to measure chlorophyll a and modified for flow through operation. A diaphragm pump (Jabsco e l e c t r i c b i l g e pump, model 34600-0000 ) was run for a minimum of 60 s p r i o r to reading a sample to flush the hose of the previous sample. When the pump was shut o f f , 3 0 s was allowed to elapse to ensure the same time had passed for each reading and to allow the fluorometer signal to s t a b i l i z e . Depths sampled were 0.5, 2.5, 5 27.5, 30, 35, 40, 45 and 50 m. The time between successive sample dates varied between 3 and 5 days (Appendix I ) . In vivo fluorescence p r o f i l e s were obtained i n the morning and Two Rivers arm was sampled f i r s t . Phytoplankton Depths chosen depended on the v e r t i c a l d i s t r i b u t i o n of chlorophyll as estimated by the i n vivo fluorescence p r o f i l e on each sample date. Nalgene bottles (1 1) were f i l l e d and stored cool and i n the dark. Upon return to the laboratory, subsamples were taken for immediate enumeration (the volume depended on the 17 amount required for enumeration) and for long term storage (25 ml). These samples were fixed and preserved with Rhodes Lugol's solution plus acetic acid (Sournia, 1978). The following groups were enumerated: Rhizosolenia e r i e n s i s , C y c l o t e l l a spp. (3 species) A s t e r i o n a l l a formosa, F r a q i l a r i a sp., Synedra sp., Melosira sp., Dinobryon sp., c i l i a t e s , cysts, large f l a g e l l a t e s (15 um and l a r g e r ) , small f l a g e l l a t e s (3-15 um) and others (up to four groups i n some samples and primarily diatoms). Phytoplankton c e l l density p r o f i l e s generally corresponded to s p e c i f i c i n vivo fluorescence p r o f i l e s although phytoplankton samples were not taken for every fluorescence p r o f i l e . I d e n t i f i c a t i o n and enumeration of phytoplankton was by Utermohl technique (Utermohl, 1948) using a Wild M40 inverted microscope equipped with phase contrast optics at either 200X or 600X magnification. During the 1987 f i e l d season two comparisons of phytoplankton c e l l density ( v a r i a b i l i t y between duplicate counts was 8.5 %) and in vivo fluorescence p r o f i l e s were performed for both the control and experimental arms. Phytoplankton samples were taken every 3 m from 0.5-3 0 m. This comparison was performed before and af t e r the establishment of the .fluorescence maximum at both Stn 1 and 2. The f i r s t comparison was performed on 9 A p r i l while the second was performed on 2 0 May. 18 Sinking rates A SETCOL apparatus (Bienfang, 1981) was used to determine the sinking rates of the phytoplankton sampled from various depths before and a f t e r the chlorophyll maximum formation during the 1987 f i e l d season. Depths chosen were 5, 10, 17.5 and 22.5 m. Sinking rate measurements were performed i n duplicate. A cooling water bath and water jackets around the SETCOL apparatus ensured that temperature was constant during the experiments. Natural l i g h t was used and varied by using cheesecloth as screening. Temperature and l i g h t conditions used during the sinking rate experiments were chosen to approximate values found at the depths from which water samples for the experiment were obtained. Samples were obtained early i n the morning and kept cool and i n the dark u n t i l they were transported to the laboratory (60 min maximum elapsed time). The experiments were run for 4 h following which subsamples from the columns were taken, preserved i n Rhodes a c i d i c Lugol's solution and enumerated following the procedure outlined for phytoplankton enumeration. C e l l density was used as the measure of biomass required i n c a l c u l a t i n g sinking rate when using a SETCOL apparatus (Bienfang, 1981); hence, i t was possible to calculate a sinking rate for each diatom species t y p i c a l l y enumerated i n the b o t t l e samples. Sediment traps The sediment traps (Fig. 3) were 51 cm high with a mouth diameter of 12.5 cm. These traps were constructed of PVC p l a s t i c . Both 19 the b a f f l e s and the c o l l e c t i o n cup were made of p l a s t i c . Once material sedimented past the column mouth, the column b a f f l e s prevented removal by currents outside the sediment traps. S i m i l a r l y , while the column was draining the c o l l e c t i o n cup b a f f l e s prevent loss of material that had sedimented into the c o l l e c t i o n cup. The rubber O-ring provided a t i g h t seal between the c o l l e c t i o n cup and column so water did not leak out of the traps while the traps were out of the water but s t i l l f u l l . The top of the c o l l e c t i o n cup was beveled so sedimenting material would f a l l into the cup. Pairs of sediment traps were deployed at 7 and 15 m (Fig. 4) at both Stn 1 and Stn 2. The c o l l e c t i o n cup (Fig. 3) contained a brine solution of 1.5 % NaCl to k i l l grazers and to maintain a high density solution i n the c o l l e c t i o n cup. At sampling time (7 d periods during 1986 and 4 d periods during 1987) the traps were brought to the surface and the upper portion drained (Fig. 3). The contents of the c o l l e c t i o n cup were transferred to a 1 L Nalgene b o t t l e (with the aid of a funnel to avoid s p i l l i n g ) and stored i n a cool, dark styrofoara chest u n t i l they were returned to the laboratory. In the laboratory, each sample was mixed and 100 mL was transferred to a glass b o t t l e before f i x i n g and preserving with Lugol's solution. Samples were enumerated at 200X magnification following the procedure outlined for phytoplankton. Rhizosolenia e r i e n s i s was enumerated as whole c e l l s and f r u s t u l e 20 pieces containing a spine; the l a t t e r was counted as ha l f a c e l l . C e l l f lux determinations are based on R. e r i e n s i s c e l l counts including both whole c e l l s and fru s t u l e pieces. Additional groups enumerated were C y c l o t e l l a spp., A s t e r i o n e l l a formosa, Fragilaria/Synedra, and others. F e r t i l i z a t i o n regime During 1985 and 1986 Taylor arm was f e r t i l i z e d while Two Rivers arm was not f e r t i l i z e d . Neither arm was f e r t i l i z e d during 1987. F e r t i l i z e r was purchased as granular NH4NO3 and ( N H 4 ) 2 P 0 4 and dissolved i n water before i t was sprayed on the lake by a DC-6B water bomber. Details of the f e r t i l i z a t i o n regime are outlined i n Table 1. The area of Taylor arm which received nutrient enrichment i s i l l u s t r a t e d i n Figure 5. Nutrient addition was weekly. The a i r c r a f t flew at a low a l t i t u d e and made a number of passes, on d i f f e r e n t paths, to help maximize f e r t i l i z e r areal d i s t r i b u t i o n . Mixing i n the epilimnion and variable f l i g h t patterns helped ensure an even nutrient d i s t r i b u t i o n i n the top portion of the epilimnion i n the treatment area. 21 Column baffle Column Outflow port 0—ring Collection cup baffle Collection cup End closure Figure 3. Cross section of a sediment trap used to c o l l e c t sedimenting phytoplankton. Surface float 22 Subsurface float 7 m traps 15 m traps Mooring line Anchor F i g u r e 4. S c h e m a t i c r e p r e s e n t a t i o n o f t h e arrangement o f sediment t r a p s and components o f t h e mooring l i n e . F i g u r e 5. Map o f Sproat Lake, i n d i c a t i n g (b l ack s e c t i o n ) the a rea o f T a y l o r arm r e c e i v i n g n u t r i e n t enr i chment . 24 Table 1. Details of the Sproat Lake f e r t i l i z a t i o n regime during 1985 and 1986 (N:P r a t i o by atoms). Year f e r t i l i z a t i o n duration P-load N:P 1985 weekly 18 weeks 3.0 mg P.m-2.wk-1 50:1 1986 weekly 8 weeks 5.6 mg P«m~ 2.wk - 1 50:1 25 Results The fate of nitrogen and phosphorus following f e r t i l i z a t i o n i n Great Central Lake i s i l l u s t r a t e d i n Figure 6 (from Stockner et a l . , 1980). Great Central Lake shares many s i m i l a r physical, chemical and b i o l o g i c a l c h a r a c t e r i s t i c s with Sproat Lake and i s therefore useful for comparison. Figure 6 i l l u s t r a t e s that soluble reactive phosphorus (SRP) returned to p r e - f e r t i l i z a t i o n l e v e l s a f t e r one day and n i t r a t e a f t e r two days following f e r t i l i z a t i o n . The exact time for a l l f e r t i l i z e r to be consumed w i l l depend on the phytoplankton biomass, the nutrient demand of the phytoplankton and phytoplankton nutrient uptake c a p a b i l i t i e s . V e r t i c a l P r o f i l e Replication On 5 A p r i l 1987, the v a r i a b i l i t y i n i n vivo fluorescence within and between Two Rivers and Taylor arms was assessed. Figure 7 i l l u s t r a t e s the p r o f i l e s i t e s . In Taylor arm the stations were a l l within the boundaries of that portion of the area receiving nutrient enrichment during 1986. Generally, p r o f i l e s i n each arm were s i m i l a r while those between arms were not (Fig. 8). The Two Rivers arm p r o f i l e s a l l peaked at 15 m and a l l surface fluorescence values were s i m i l a r , i n d i c a t i n g that the within s i t e v a r i a b i l i t y was low. The Taylor arm p r o f i l e s had more v a r i a b i l i t y than the Two Rivers arm p r o f i l e s but they were a l l generally s i m i l a r . 26 F i g u r e 6. S o l u b l e r e a c t i v e phosphorus (a,c) and n i t r a t e (b,d) c o n c e n t r a t i o n s i n u n f e r t i l i z e d (a,b) and f e r t i l i z e d (c,d) areas of Great C e n t r a l Lake, B r i t i s h Columbia on the day b e f o r e f e r t i l i z a t i o n (A), day of f e r t i l i z a t i o n (•), and one (•) and two (•) days f o l l o w i n g f e r t i l i z a t i o n (from Stockner e t a l . , 1980). Figure 7. Map of Sproat Lake, i n d i c a t i n g p r o f i l e stations for i n vivo fluorescence comparison of 5 May 1987. 29 IN VIVO FLUORESCENCE F i g u r e 8. In v i v o f l u o r e s c e n c e p r o f i l e s a t s i x s t a t i o n s on 5 May 1987. Numbers i n the bottom r i g h t c o r n e r correspond t o s t a t i o n numbers of F i g u r e 7. 30 Comparisons between s i t e s revealed that the Two Rivers arm p r o f i l e s had a sharper peak than the p r o f i l e s i n Taylor arm. The maximum value of the fluorescence p r o f i l e s of Taylor arm was only 64 % that of Two Rivers arm. Both sets of p r o f i l e s had s i m i l a r fluorescence p r o f i l e s i n the surface waters as well as s i m i l a r fluorescence values from 20-50 m. Two seasonal periods are considered i n t h i s t h e s i s . Results for the 1986 f i e l d season (23 A p r i l to 6 August) w i l l be presented followed by the resu l t s for the 1987 f i e l d season (7 March to 1 June). Where possible, an attempt w i l l be made to consider the data for both f i e l d seasons together. Since the 1986 f i e l d season started l a t e and the 1987 was terminated early, the period of overlap between the two years was from 2 3 A p r i l to 1 June. Physical observations Temperature 1986 On 27 May, the beginning of the f i r s t seasonal period, the temperature p r o f i l e was not isothermal (Fig. 9). Surface temperature was approximately 8.0°C with a near constant decrease of temperature with depth to 5.4°C at 17.5 m i n both arms. Through the sampling period s t r a t i f i c a t i o n i n t e n s i f i e d . The maximum surface temperature was ca. 23.1°C i n Taylor arm on 11 August. From the temperature p r o f i l e s the epilimnion was Figure 9. Temperature p r o f i l e s for Two Rivers (a) and Taylor (b) arms, 1986. Figure 10. Temperature p r o f i l e s for Two Rivers (a) and Taylor (b) arms, 1987. 33 estimated to be 0-10 m, the thermocline 10-17.5 m, and the hypolimnion 17.5-bottom i n both Two Rivers and Taylor arms. 1987 The f i r s t p r o f i l e (Fig. 10), of 7 March, shows isothermal conditions from 0-50 m of ca. 6.0°C. By 1 A p r i l thermal s t r a t i f i c a t i o n had commenced. There was a temperature inversion in the surface water on 18 A p r i l ; t h i s followed three nights of sub-zero temperatures and no apparent wind during e i t h e r the night or day. S t r a t i f i c a t i o n continued to i n t e n s i f y through 1 June, at which time surface temperatures were ca. 15°C. The epilimnion was s l i g h t l y shallower i n Taylor arm, possibly due to more mixing early i n the spring delaying the onset of permanent seasonal s t r a t i f i c a t i o n . In general, the temperature p r o f i l e s for both arms were s i m i l a r . Depths of the epilimnion, thermocline and hypolimnion were estimated to be 0-7.5 m, 7.5-12.5 m and 12.5-bottom respectively, i n Two Rivers arm and 0-5 m, 5-7.5 m and 7.5-bottom respectively, i n Taylor arm on 1 June. Light 1986 The 1 % l i g h t depth was generally deeper i n Two Rivers than Taylor arm (except for l a t e summer) in 1986 (Fig. 11). Values increased from 2 0.5 m and 18.3 m i n the spring to 22.5 m and 19.2 m i n the f a l l i n Two Rivers and Taylor arms respectively. The Two Rivers arm extinction c o e f f i c i e n t varied between 0.26 m _ 1 on 23 A p r i l to 0.21 m"1 on 17 September (Fig. 11). The extincti o n c o e f f i c i e n t values for Taylor arm were generally lower than those of Two Rivers arm. 1987 In Two Rivers arm the 1 % l i g h t depth decreased from 20.4 m on 15 March to 14.4 m on 23 A p r i l and increased to 22.3 m on 1 June (Fig. 12). The l i g h t compensation depth was not as deep i n Taylor arm as i n Two Rivers arm. The ex t i n c t i o n c o e f f i c i e n t i n Two Rivers arm increased from 0.21 m - 1 on 15 March to 0.3 3 m - 1 on 23 March and then decreased to 0.21 ra-1 by 1 June (Fig. 12). In Taylor arm k e increased from 0.24 m - 1 on 15 March to ca. 0.32 m - 1 on 1 A p r i l , then decreased to 0.25 m - 1 by 1 June. Water chemistry Nitrate 1986 In Two Rivers arm n i t r a t e was below detection from 0 to 15 m by 28 May and remained low to undetectable throughout the f i e l d season (Fig. 13 a). Nitrate was not sampled on 23 A p r i l . The n i t r a c l i n e began at ca. 15 m and continued through 40 m, the lowest depth sampled during 1986. Nitrate concentration ranged from ca. 30-40 ug N « L _ 1 at 40 m. 35 Figure 1 1 . 1 % l i g h t depth (a) and extin c t i o n c o e f f i c i e n t (b) i n Two Rivers (•) and Taylor ( o ) arms, 1 9 8 6 . 36 0 30 1 ' 1 ' 1 1 1  March April May June 0.15 0.35 1 1 1 ' —•—1 1 ' March April May June TIME F i g u r e 12. 1 % l i g h t d e p t h (a) a n d e x t i n c t i o n c o e f f i c i e n t (b) i n Two R i v e r s (•) a n d T a y l o r (o) a r m s , 1987. NITRATE (/vg-L1) figure 13. Nitrate concentration during 1986 (a) and 1987 (b) i n Two Rivers (•) and Taylor (o) arms. 38 In Taylor arm n i t r a t e was present i n the epilimnion u n t i l sometime between 25 June and 23 July. Epilimnetic n i t r a t e concentrations ranged between 10-40 ug N « L _ 1 . The n i t r a t e p r o f i l e i n Taylor arm was s i m i l a r to that of Two Rivers arm once n i t r a t e concentrations had become undetectable i n surface waters by 23 July. On 23 July there appeared to be a n i t r a t e maximum at 22.5-25 m i n both arms. Considering the 25 June and 20 August p r o f i l e s , an error i n the 30-40 m samples on July 23 r e s u l t i n g i n low n i t r a t e values at these depths may have caused such an apparent maximum. 1987 In Two Rivers arm n i t r a t e showed l i t t l e v a r i a b i l i t y with depth on — i 7 March, with the concentration ranging from 9-12 ug N«L (Fig. 13 b). Generally, n i t r a t e decreased over time but never became undetectable as i t did during 1986. The same trend — 1 occurred i n Taylor arm; however, there was s t i l l ca. 13 ug N«L at 25 m on 1 June. Total F i l t e r e d Phosphorus (TFP) 1986 The maximum TFP measured was 6 ug P r L - 1 i n Two Rivers arm on 28 May (Fig. 14 a). The maximum TFP i n Taylor arm was 4 ug P « L - 1 (measured on both 25 June and 23 J u l y ) . Generally, TFP was extremely low throughout the 1986 f i e l d season. 39 Figure 14. Total f i l t e r e d phosphorus concentration during 1986 (a) and 1987 (b) i n Two Rivers (•) and Taylor (o) arms. 40 1987 The maximum TFP measured i n Two Rivers arm was 6 ug P « L - 1 (at 5 m on 23 March), the same as i n 1986 (Fig. 14 b). In contrast, maximum TFP was 3 ug P « L - 1 at 25 m on 7 March i n Taylor arm. At other times during the 1987 sampling period TP < 2 ug P«L - 1 , with the exception of 20 m on 13 A p r i l i n Two Rivers arm. B i o l o g i c a l Observations In vivo fluorescence 1986 The 8 May p r o f i l e (Fig. 15) i l l u s t r a t e s that a fluorescence maximum was already present at the beginning of the 1986 sampling period i n both Two Rivers and Taylor arms. The value of the fluorescence maximum increased i n Two Rivers arm u n t i l 12 June, where i t reached a value of 0.410 at 22.5 m. The fluorescence maximum varied s p a t i a l l y and temporally but was t y p i c a l l y at 20-25 m. The maximum fluorescence value slowly decreased through the sampling period subsequent to 12 June. In Taylor arm the fluorescence p r o f i l e i n i t i a l l y formed slower than i n Two Rivers arm (compare p r o f i l e s of 8 May). A maximum value of 0.490 occurs on 12 June at 22.5 m. On 3 July there was a second peak of fluorescence at 10 m i n addition to the seasonal fluorescence maximum at 22.5 m. The occurrence of t h i s second fluorescence peak coincided with an epilimnetic bloom of a l g a l picoplankton. By 2 6 July the value of both fluorescence maxima Figure 15. In vivo fluorescence p r o f i l e s f o r Two Rivers (a) and Taylor (b) arms, 1986. 42 had dropped to 0.160 for the 10 m maximum and 0.212 for the 22.5 m maximum. By 6 August there was r e l a t i v e l y l i t t l e change with depth i n the Taylor arm p r o f i l e . 1987 The f i r s t Two Rivers arm p r o f i l e (Fig. 16, 11 March) i l l u s t r a t e s that there was very l i t t l e change i n i n vivo fluorescence with depth. From the 15 March p r o f i l e onward there was a fluorescence p r o f i l e c h a r a c t e r i s t i c of a subsurface chlorophyll maximum. The fluorescence p r o f i l e became thicker with the maximum value for each p r o f i l e increasing u n t i l 13 A p r i l when i t reached a value of 0.550. The fluorescence maximum was situated at 12.5-15 m u n t i l 13 A p r i l , a f t e r which i t descended u n t i l i t reached 2 0 m by 1 June. The p r o f i l e changed shape a f t e r 13 A p r i l with proportionally more fluorescence below the peak than above i t . In Taylor arm the same series of events occurred but l a t e r i n time. A fluorescence peak did not r e a l l y s t a r t to form u n t i l 1 A p r i l . This was approximately 2 weeks l a t e r than i n Two Rivers arm. The fluorescence maximum continued to increase i n in t e n s i t y through June. The fluorescence maximum formed at 10-12.5 m (e.g., 22-30 Ap r i l ) and sank deeper to 20 m by 1 June, s i m i l a r to Two Rivers arm. In a l l fluorescence p r o f i l e s there was a baseline of fluorescence ranging from ca. 0.070-0.090. 4 3 Figure 16. In vivo fluorescence p r o f i l e s for Two Rivers (a) and Taylor (b) arms, 1987. 44 Chlorophyll 1986 On 23 A p r i l i n Two Rivers arm there were greater chlorophyll concentrations i n the epilimnion compared to deeper values (Fig. 17). By 28 May, a subsurface chlorophyll maximum was beginning to form at 15-20 m and by 25 June a well-defined chlorophyll maximum (ca. 2.40 ug Chi a « L _ 1 and the highest value during 1986) was situated at ca. 18 m. The chlorophyll maximum remained at ca. 16 m throughout July and August with a value ranging from 1.57-1.97 ug Chi a^L - 1. By 17 September the chlorophyll maximum was located at ca. 22.5 m and had a value of 1.75 ug Chi a.IT 1. Epilimnetic chlorophyll was variable i n Two Rivers arm throughout the 1986 seasonal sampling period. Generally, epilimnetic chlorophyll increased u n t i l 14 July and then decreased through the remainder of the sampling period. Trends i n Taylor arm were s i m i l a r to those of Two Rivers arm. Following formation at ca. 22.5 m the subsurface chlorophyll maximum remained s l i g h t l y deeper than i n Two Rivers arm. The f i r s t date of sampling (28 May) i l l u s t r a t e s a broad subsurface chlorophyll maximum, which appeared to have two small peaks at 18 and 24 m. The maximum subsurface chlorophyll value was 2.63 ug Chi a « L - 1 on 20 August. At other times the maximum value was s i m i l a r i n both Taylor and Two Rivers arm. 45 CHLOROPHYLL (/yg-L"') 0 2 4 2 4 2 4 2 4 2 4 2 4 E 50L L L L L L 231V 28V 25VI 23 VII 20VIII 17IX X Q- 0 0.8 1.6 0.8 1.6 0.8 1.6 0.8 1.6 0.8 1.6 0.8 1.6 7111 23111 13-IV 261V 20V IVI TIME Figure 17. Chlorophyll a concentrations during 1986 (a) and 1987 (b) i n Two Rivers (•) and Taylor (o) arms. 46 Epilimnetic chlorophyll was considerably less i n Taylor arm than Two Rivers arm through 23 July. The maximum value of 0.63 ug Chi a « L - 1 occurred on 28 May, approximately 6 weeks p r i o r to the occurrence of the maximum i n Two Rivers arm. Values were r e l a t i v e l y constant throughout the seasonal sampling period, ranging from 0.44-0.63 ug Chi a « L _ 1 . Through July such values were much less than those of Two Rivers arm. Values i n August and September were s i m i l a r i n both arms. 1987 The 7 March p r o f i l e (Fig. 17), during conditions of isothermal mixing, showed nearly uniform concentrations of chlorophyll with depth. By 2 3 March there was an increase i n surface chlorophyll i n Two Rivers arm (1-10 m) but not i n Taylor arm. The 13 A p r i l p r o f i l e i l l u s t r a t e s a maximum of ca. 1.70 ug Chi a«L at 15 m. By 26 May the Two Rivers chlorophyll p r o f i l e showed progressively more chlorophyll below the chlorophyll maximum as well as a deepening of the chlorophyll maximum to 20 m by 1 June. In Taylor arm chlorophyll concentrations had not reached the same maximum concentrations that occurred i n Two Rivers arm by 1 June. However, the i n vivo fluorescence p r o f i l e s i l l u s t r a t e d a trend of an increasing maximum value through 1 June. The chlorophyll maximum was located at 20 m but there was also high chlorophyll at 10 m. Since my method was not d i r e c t l y compared to LEP's, possible d i f f e r e n t extraction e f f i c i e n c i e s due to d i f f e r e n t extraction times can not be ascertained. 47 Phytoplankton Many species and/or groups were enumerated during the two f i e l d seasons. These data are separated into two groups: i) data presented i n the text of the thesis and i i ) data contained i n Appendix I I . Therefore i n the re s u l t s section only data for R. e r i e n s i s , C y c l o t e l l a spp., small f l a g e l l a t e s and Dinobryon sp. are presented. 1986 Rhizosolenia e r i e n s i s was present on 19 May sampling date i n both Two Rivers and Taylor arms (Fig. 18 a). Through 9 June a large peak formed at 22.5 m with upper and lower l i m i t s of 10 and 27.5 m respectively. The peak was smaller i n magnitude by 23 June and the upper l i m i t was at 15 m. By 26 July R. e r i e n s i s was v i r t u a l l y absent from 0-30 m. In Taylor arm R. e r i e n s i s c e l l density reached a maximum at 12.5 m on 19 May. The peak migrated to 10 m by 29 May, and the c e l l density at other depths decreased. There was an increase i n c e l l density at 10 m on 9 June. C e l l density increased at 15, 20 and 25 m and decreased at 5 m. The peak diminished and was absent by 2 6 July as was the case i n Two Rivers arm. C y c l o t e l l a spp. C y c l o t e l l a spp. c e l l density formed a pattern opposite to R.eriensis (Fig. 18 b). On 19 May C y c l o t e l l a spp. was v i r t u a l l y absent and on 29 May c e l l densities were s t i l l extremely low. By 48 CELLS x IO3 • mL" 0 2 4 2 4 2 4 2 4 2 4 0 I 1 1 — I 1 1 — I 1 1 — I 1 1 — I 1 r TIME Figure 18. Depth p r o f i l e s of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) i n Two Rivers (•) and Taylor (oj arms, 1986. 49 9 June, Two Rivers arm had s l i g h t l y higher c e l l d e nsities than Taylor arm, and the maximum c e l l density of either arm was only ca. 0.7 x 10 3 c e l l s • m l - 1 . There was a s l i g h t peak i n C y c l o t e l l a spp. at 10 m i n Two Rivers arm but otherwise c e l l d e nsities were low. In Taylor arm there was a large peak at 5 m on 23 June with c e l l density f a l l i n g to near zero by 15 m. The peak i n Taylor arm at 5 m had decreased but c e l l densities at 10, 15 and 20 m were a l l larger than on 23 June. F l a g e l l a t e s (3-15 um) In Two Rivers arm, f l a g e l l a t e s were present at a l l the sampled depths (5-35 m) and c e l l densities were higher around 22.5-25 m (Fig. 19 a) than above and below. On 29 May c e l l density had increased from 5-15 m but remained s i m i l a r at other depths. The 9 June p r o f i l e shows that c e l l density at 5 m had declined dramatically but increased at 10, 15 and 20 m. This trend continued and by 23 June there was only a peak at 2 0 m. By 2 6 July the 20 m peak had disappeared but surface concentrations were again elevated. The largest f l a g e l l a t e c e l l density measured i n Taylor arm was at 12.5 m on 19 May (5.9 x 10 3 c e l l s . m l - 1 ; Fig. 19 a). For other sampling dates, p r o f i l e s were s i m i l a r with the exception of the 20 m sample on 23 June; there was no peak i n Taylor arm. 50 Figure 19. Depth p r o f i l e s of small f l a g e l l a t e s (a) and Dinobryon sp. (b) i n Two Rivers (•) and Taylor (o) arms, 1986. 51 Dinobryon sp. Dinobryon sp. was present at very low c e l l d e nsities i n Two Rivers arm at the beginning of the 1986 sampling period (Fig. 19 b; 19 May). C e l l density increased at 10-25 m by 29 May and by 9 June a maximum had formed at 22.5 m. The c e l l density maximum was at 20 m on 23 June and while the c e l l density remained unchanged at 10 m, c e l l density at 15 m had decreased. By 26 July c e l l density was s i m i l a r from 5-30 m. On 19 May there was a maximum of Dinobryon sp. i n Taylor arm at 10 m. Concentrations at other depths were low but s t i l l higher than those observed i n Two Rivers arm. The maximum declined by 29 May, with concentrations at other depths remaining s i m i l a r to 19 May. From 9 June through 26 July c e l l density was variable from 5-30 m, with no large increases. 1987 Rhizosolenia e r i e n s i s On 15 March R. e r i e n s i s c e l l density was low (< 0.02 x 10 3 c e l l s - m l - 1 ) at a l l depths i n Two Rivers arm (Fig. 20 a). On 13 A p r i l there was a pronounced peak i n c e l l density at 10 m (0.26 x 10 3 cells«ml _ 1) and a small increase at 5 m but t h i s peak was absent on 30 A p r i l . On 28 May a large peak was evident at 15 m (0.61 x 10 2 cells»ml - 1) and c e l l densities had also increased at 10 and 2 0 m. 52 Figure 20. Depth p r o f i l e s of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) i n Two Rivers (•) and Taylor (o) arms, 1987. 53 In Taylor arm, R. e r i e n s i s c e l l density was low compared to that of Two Rivers arm throughout the sampling period. There was a small c e l l density maximum on 28 May at 20 m (0.12 xlO 3 c e l l s . m l - 1 ) ; otherwise c e l l density was usually below 0.05 x 10 3 c e l l s . m l " 1 . C y c l o t e l l a spp. C y c l o t e l l a spp. i n Two Rivers arm was found i n low concentrations early i n the sampling period s i m i l a r to R. e r i e n s i s . Densities remained below 0.02 x 10 3 c e l l s . m l - 1 u n t i l some time a f t e r 13 A p r i l (Fig. 20 b), where two small peaks were apparent on 30 A p r i l at 10 and 20 m; however, by 28 May these peaks had disappeared. Taylor arm began the sampling period with low C y c l o t e l l a spp. c e l l density u n t i l 30 A p r i l . Between 30 A p r i l and 28 May there was a large increase i n the surface layer and down to and including the 20 m depth. The maximum c e l l density was at 15 m while concentrations at 5, 10 and 20 m were lower. F l a g e l l a t e s (3-15 um) On 15 March i n Two Rivers arm, f l a g e l l a t e s were evenly d i s t r i b u t e d from 5-25 m (Fig. 21 a) at ca. 0.5 x 10 3 c e l l s - m i - 1 . Through 30 A p r i l a broad peak developed at roughly 10-2 0 m with maximum c e l l density of 2.51 x 10 3 cells.ml"" 1. This broad peak decreased i n siz e and in t e n s i t y and by 28 May i t was located at 54 Figure 21. Depth p r o f i l e s of small f l a g e l l a t e s (a) and Dinobryon sp. (b) i n Two Rivers (•) and Taylor (O) arms, 1987. 55 10-15 m. On 28 May the value at 5 m was s i m i l a r to that of 30 A p r i l but values at 20, 25 and 30 m decreased. In Taylor arm f l a g e l l a t e s never reached the c e l l d e nsities that occurred i n Two Rivers arm (Fig. 21 a). C e l l d e nsities were s i m i l a r to Two Rivers arm through 13 A p r i l . On 13 A p r i l there were small peaks at 5 and 15 m but by 30 A p r i l these had decreased ( p a r t i c u l a r l y at 15 m) so that from 5-3 0 m c e l l density ranged from 0.69-1.22 x 10 3 cells.ml"" 1. These c e l l d e nsities had decreased even further by 28 May. Dinobryon sp. Dinobryon sp. was present i n extremely low concentrations (Fig. 21 b) u n t i l 28 May (there were measurable concentrations of Dinobryon sp. between 3 0 A p r i l and 28 May but none exceeded the concentrations on 28 May). In Two Rivers arm there was a peak of c e l l density at 15 m and lower concentrations at other depths. Similar to Two Rivers arm, Taylor arm had a broader peak at 15 and 20 m but low concentrations at other depths. Sinking rate determination Sinking rate was determined by using a SETCOL apparatus twice (9 and 20 May) during 1987 representing two d i f f e r e n t stages of the spring bloom. Sinking rate was calculated for both R. e r i e n s i s and C y c l o t e l l a spp.. Results for these two groups were considered separately. 56 Rhizosolenia e r i e n s i s Sinking rate of R_^  e r i e n s i s was s i m i l a r for both sampling dates and both s i t e s at 5 and 10 m and ranged from 1.8-2.2 m « d _ 1 (Fig. 22). One exception was the 10 m sample from Taylor arm on 20 May which had a value of 1.06 +/- 0.08 m«d - 1 . The lowest sinking rates occurred at 22.5 m during both 9 and 20 May. On 9 May, R. e r i e n s i s sank s i g n i f i c a n t l y faster (p < 0.10) at 17.5 m than at the same depth on 20 May at both s i t e s . C y c l o t e l l a spp. On 9 May C y c l o t e l l a spp. had a faster sinking rate at 5 m than at 10 m i n Two Rivers arm while on 20 May there was no s i g n i f i c a n t difference between the two depths. The sinking rates at 5 and 10 m were ca. 0.8-1.0 m«d _ 1, approximately h a l f the sinking rate of R. e r i e n s i s . There were no differences i n the sinking rate of C y c l o t e l l a spp. between the 17.5 m samples on either date i n either arm. The lowest sinking rates occurred at 22.5 m, s i m i l a r to R. e r i e n s i s . Sedimentation 1986 C e l l flux data r e s u l t from sediment trap measurements at 7 and 15 m. The highest c e l l flux of Rhizosolenia e r i e n s i s occurred i n Two Rivers arm on 2 6 May at 7 m (Fig. 2 3 a). Generally, c e l l flux decreased through to 3 0 June. A s i m i l a r trend occurred for R. e r i e n s i s at 15 m i n Two Rivers arm (Fig. 23 b). However, following 9 June there was a sharp decrease i n c e l l flux by 450%. 17.5 22.5 RR ^ T 9 May 9 May RR 20 May T 20 May Figure 22. Sinking rates of Rhizosolenia e r i e n s i s (a) and C y c l o t e l l a spp. (b) on 9 and 20 May 1987 i n Two Rivers (RR) and Taylor (T) arms, Error bars represent +/" 2 SD; n = 2. 58 Figure 23. Sedimentation of Rhizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Two Rivers arm, 1986. 59 R. e r i e n s i s c e l l flux was always greater at 7 m than at 15 m i n Two Rivers arm during the sampling period of 1986. The general pattern of c e l l f lux for C y c l o t e l l a spp. was quite d i f f e r e n t than that of R. e r i e n s i s . R. e r i e n s i s c e l l f lux generally decreased through the sampling period while C y c l o t e l l a spp. generally increased (Figs. 23 to 26). A d d i t i o n a l l y , C y c l o t e l l a spp. c e l l flux at 15 m always exceeded that at 7 m i n both arms. At 7 m i n Two Rivers arm (Fig. 25 a) c e l l f lux of C y c l o t e l l a spp. increased gradually from 26 May to 30 June. At 15 m there was a s i m i l a r pattern of increasing c e l l flux over time but i t d i f f e r e d because the rate of increase was s l i g h t l y greater at 15 m. The i n i t i a l c e l l f lux was lower at 15 m than 7 m on 26 May (by 25%) and by 23 June c e l l flux at 15 m was ca. 300% that at 7 m. C e l l flux for C y c l o t e l l a spp. followed a s i m i l a r pattern i n Taylor arm when compared to Two Rivers arm (Fig. 2 6). At 7 m the i n i t i a l c e l l f lux i n Taylor arm was 40% that of Two Rivers arm. At 15 m, C y c l o t e l l a spp. flux was again lower i n i t i a l l y i n Taylor arm but exceeded c e l l flux i n Two Rivers arm by 3 0 June. 1987 There was a lower flux of R. e r i e n s i s during the sampled portion of 1987 than during 1986 (Figs. 23 and 27). There was no overlap 60 of sampling time but the end of the 1987 f i e l d season approached the beginning of the 1986 season (24 and 26 May r e s p e c t i v e l y ) . R. e r i e n s i s c e l l flux doubled at 7 m i n Two Rivers arm from 2 6 A p r i l to 4 May 1987. At 15 m the pattern remained s i m i l a r to that at 7 m but with a larger increase i n c e l l f l u x on 4 May. In Taylor arm R. e r i e n s i s c e l l flux again showed s i m i l a r trends to that of Two Rivers arm (Fig. 28). In Two Rivers arm at 15 m C y c l o t e l l a spp. c e l l flux decreased i n i t i a l l y but increased near the end of the sampling period (Fig. 29 b). There was a sharp increase on 24 May at both 7 and 15 m (Fig. 29). At both 7 and 15 m the pattern of C y c l o t e l l a spp. c e l l flux i n Taylor arm followed no o v e r a l l trend and i t fluctuated through the sampling period (Fig. 30). 61 Figure 24. Sedimentation of Rhizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Taylor arm, 1986. 62 Figure 25. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Two Rivers arm, 1986. 63 Figure 26. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Taylor arm, 1986. 64 CM i <0 K CO "55 X 3 15 23 I March 9 17 26 4 16 24 April May 15 23 I March 9 17 26 4 16 24 April May TIME Figure 27. Sedimentation of Rhizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Two Rivers arm, 1987. 6 5 CVJ CO g K X 3 15 23 I March 9 17 26 4 16 24 April May 15 23 March TIME Figure 28. Sedimentation of Phizosolenia e r i e n s i s at 7 m (a) and 15 m (b) i n Taylor arm, 1987. 66 CM i 1 0 O K CO o X 3 15 23 March 9 17 26 4 16 24 April May 15 23 I March 9 17 26 4 16 24 April May TIME Figure 29. Sedimentation of C y c l o t e l l a spp. at 7 m (a) and 15 m (b) i n Two Rivers arm, 1987. CVJ CO O cn "5) X 3 15 23 I March 9 17 26 4 16 24 April May 15 23 I March 9 17 26 4 16 24 April May TIME Figure 30. Sedimentation of C y c l o t e l l a spp. at 7 m (a) 15 m (b) i n Taylor arm, 1987. 6 8 Discussion The d e f i n i t i o n s of spring and summer are operational at best. V a r i a b i l i t y of physical, chemical and b i o l o g i c a l processes between years prevents a r i g i d time-dependent d e f i n i t i o n . In t h i s thesis and re f e r i n g to water column temperature structure, spring i s defined as the period of time from isothermal temperature conditions to development of seasonal s t r a t i f i c a t i o n . Summer i s the period of time following the establishment of seasonal s t r a t i f i c a t i o n to f a l l overturn. I t i s useful to discuss the data set i n a context of seasonality in order to pool both year's data together. Since the o r i g i n a l questions focused on gaining an understanding of the r o l e of R. er i e n s i s to the Sproat Lake phytoplankton community, t h i s species w i l l be discussed i n more d e t a i l than other species. Throughout the discussion an emphasis w i l l be placed on Taylor arm since i t was the experimental s i t e . Reference i s made to Two Rivers arm where appropriate and comments regarding the usefulness of Two Rivers arm as a control s i t e are also included. Spring The i n vivo fluorescence p r o f i l e s were c l o s e l y spaced v e r t i c a l p r o f i l e s taken more frequently than either chlorophyll or phytoplankton c e l l counts and therefore i l l u s t r a t e the spring bloom i n more d e t a i l . Appendix III contains a comparison of phytoplankton c e l l counts and i n vivo fluorescence p r o f i l e s . 69 This comparison demonstrates that the i n vivo fluorescence measurements were consistent with phytoplankton b o t t l e counts i n ind i c a t i n g major p r o f i l e features. On 11 March 1987 i n vivo fluorescence was nearly uniform from 0-50 m (Fig. 16) as was chlorophyll and temperature on 7 March and phytoplankton p r o f i l e s of 15 March. I t i s cle a r that the beginning of March 1987 was p r i o r to the spring bloom i n Sproat Lake. Phytoplankton growth was slow u n t i l 23 March when there were increases i n in vivo fluorescence and chlorophyll i n the surface water to a depth of 10-12 m. At t h i s time s t a t i f i c a t i o n had begun to occur and the 1 % l i g h t depth was increasing. This was the i n i t i a t i o n of the spring bloom. A subsurface fluorescence maximum formed quickly, as i l l u s t r a t e d by the 28 March i n vivo fluorescence p r o f i l e (Fig. 16). This maximum may have resulted from increases i n chlorophyll and/or changes i n fluorescence per unit chlorophyll. On 13 A p r i l a well defined subsurface chlorophyll maximum formed at 15 m. This early peak may have been the r e s u l t of growth of R. e r i e n s i s and small f l a g e l l a t e s at 15 m or growth and subsequent sinking to 15 m. Nutrients were also present i n the epilimnion during t h i s period, but they were low. In Sproat Lake, the i n vivo fluorescence maximum formed around 10-12 m, and the 1 % l i g h t depth was deeper than the depth of the fluorescence maximum. Presumably, p o s i t i v e net rates of photosynthsis, and growth, were maintained i n the chlorophyll 70 maximum throughout the spring. Light has been demonstrated to be important i n determining the v e r t i c a l p o s i t i o n of the chlorophyll maximum (Moll et a l . , 1985). C e l l s were capable of growing i n the subsurface chlorophyll maximum at low l i g h t l e v e l s . Moll et a l . , 1985 determined that c e l l s within the chlorophyll maximum of Lake Michigan contributed ca. 60 % of areal (m ) primary production. Throughout the 1987 sampling season, n i t r a t e was measurable throughout the water column; however, the trend of decreasing n i t r a t e would probably have led to n i t r a t e depletion i n the epilimnion by the early summer. Previous studies measuring soluble reactive s i l i c o n (SRS) values from Sproat Lake (Nidle et a l . , 1974; Nidle et a l . , i n press) reported mean annual epilimnetic SRS concentrations of ca. 1050 — 1 4 ug Si«L . Considering the low concentrations of both phosphate and n i t r a t e , Sproat Lake phytoplankton growth i s rar e l y , i f ever, s i l i c a t e - l i m i t e d . During May nutrient concentrations and 1 % l i g h t depth were decreasing and i n vivo fluorescence, R. e r i e n s i s c e l l density and s t r a t i f i c a t i o n were increasing. R. e r i e n s i s sank s i g n i f i c a n t l y faster (p < 0.05) under conditions of high l i g h t and low nutrients than i t did under conditions of low l i g h t and high nutrients (Fig. 20). The same was true for C y c l o t e l l a spp.. R. er i e n s i s and C y c l o t e l l a spp. should have decreased t h e i r sinking 71 rates i f a major factor leading to chlorophyll maximum formation was a decrease of sinking rate at depth. In fact, t h i s was observed. On 9 May, when C y c l o t e l l a spp. was s t i l l increasing there were higher sinking rates i n surface waters than at depth. Clearly, sinking rate i s not always controlled by nutrient l i m i t a t i o n . Two major components of the spring and summer phytoplankton community were R. e r i e n s i s and C y c l o t e l l a spp.. C e l l flux at 7 and 15 m was low for both of these groups early i n 1987 before the spring bloom. C e l l s sedimenting out of the top 15 m early in the 1987 season were from the winter population while following the spring bloom, sedimenting c e l l s were from the spring bloom. Since sinking was a major mechanism of chlorophyll maximum formation and the chlorophyll maximum formed at 10-12 m, there was a higher c e l l f l ux at 7 m than at 15 m during chlorophyll maximum formation. This was p a r t i c u l a r l y evident for R. er i e n s i s , which bloomed e a r l i e r than C y c l o t e l l a spp.. Later i n May, when R. e r i e n s i s growth rate slowed down, c e l l f lux at 15 m approached that of 7 m (Fig. 23). C y c l o t e l l a spp. bloomed l a t e r i n A p r i l and May, when the chlorophyll maximum was below 15 m and therefore more c e l l flux occurred at 15 m than at 7 m. This assumes that c e l l s sank u n t i l they reached the depth of the chlorophyll maximum and then became neutrally buoyant. C e l l s caught i n the 7 m traps represent c e l l flux out of the top 7 m while at 15 m, c e l l flux represents the whole epilimnion. 72 Summer The 1986 summer began with s t r a t i f i c a t i o n already developed and nutrient concentrations low i n the epilimnion. During June and the remainder of the summer s t r a t i f i c a t i o n i n t e n s i f i e d . By the end of June the n i t r a c l i n e began at 15 m. The i n vivo fluorescence maximum was well developed at 22.5 m. Chlorophyll, R. e r i e n s i s and small f l a g e l l a t e s were maximal at 20 m. A large proportion of phytoplankton biomass was therefore located i n the chlorophyll maximum, an area of high nutriens and low l i g h t . In l a t e June and July C y c l o t e l l a spp. displaced R. e r i e n s i s i n the epilimnion as the dominant diatom species. C y c l o t e l l a spp. probably benefited more from the nutrient enrichment than other species given the considerably higher increase i n c e l l density during l a t e June and July. Following the cessation of f e r t i l i z a t i o n on 7 July 1986, nutrient l e v e l s would have remained low u n t i l f a l l overturn. When comparing the p r o f i l e s of i n vivo fluorescence, chlorophyll and phytoplankton i t i s c l e a r that events measured by these parameters did not occur at the same time each year. A possible reason i s that the lake was at a d i f f e r e n t point of i t s seasonal cycle each year. I t i s probable that both these factors were important. The 1986 i n vivo fluorescence and chlorophyll p r o f i l e s had deeper maxima than the 1987 p r o f i l e s on the same date. Since these maxima formed at 10-12 m and then sank with time, i t i s suggested that the seasonal cycle had progressed further by 1 June i n 1986 than 1987. The phytoplankton i n the chlorophyll maximum, located at 20-22.5 m, probably consumed n i t r a t e as i t was advecting through the hypolimnion toward the epilimnion assuming growth was not li m i t e d by phosphate. Measurements of orthophosphate i n Sproat Lake are t y p i c a l l y below the l i m i t s of detection (K. Shortreed, pers. comm.) and i t i s d i f f i c u l t therefore to determine the amount of inorganic phosphorus available for phytoplankton growth. Suttle (1987) suggested that i n Sproat Lake d i f f e r e n t s i z e f r a c t i o n s of phytoplankton may be growth lim i t e d by d i f f e r e n t nutrients. I t i s therefore possible that c o - l i m i t a t i o n by nitrogen and phosphorus may occur. Suttle (1987) demonstrated that a v a r i e t y of factors were important to phytoplankton community composition i n Sproat Lake, including N:P supply r a t i o , temporal patchiness of nutrient supply and size f r a c t i o n of phytoplankton considered (Suttle, 1987). Following the establishment of the chlorophyll maximum during A p r i l and May, i t descended to 20-25 m for the duration of the summer. In the absense of measurable phosphate and n i t r a t e , phytoplankton growth may have occurred at low rates i n the epilimnion possibly u t i l i z i n g remineralized phosphate, ammonium and urea. 74 At the chlorophyll maximum there was measurable n i t r a t e ; therefore, n i t r a t e probably did not l i m i t phytoplankton growth. Phytoplankton above the 1 % l i g h t depth (ca. 15 m) would have been capable of net photosynthesis. C e l l s between ca. 15 i and the 1 % l i g h t depth were probably phosphate-limited. Below the 1 % l i g h t depth growth was l i g h t - l i m i t e d . Most of the chlorophyll maximum (not j u s t the depth at which the chlorophyll maximum occurs) existed above the 1 % l i g h t depth and within the n i t r a c l i n e . At such depths, phosphate may have been supplied by zooplankton excretion and sloppy feeding (Williams, 1981). In l a t e June 1986 the abrupt loss of the chlorophyll maximum may have resulted from the picoplankton bloom, which caused a subsequent decrease i n l i g h t at depth. This bloom occurred throughout Sproat Lake and i t s occurrence was not the r e s u l t of lake f e r t i l i z a t i o n but, i t i s l i k e l y that the f e r t i l i z a t i o n increased the magnitude of the bloom i n Taylor arm. (The picoplankton bloom consisted of more than one species but two large components were Synechococcus sp. and a "Synechococcus-l i k e " gelatinous species; K. Shortreed, pers. comm.). The 3 July i n vivo fluorescence p r o f i l e had, i n addition to the fluorescence maximum at 22.5 m, a smaller maximum at 10 m i n Taylor arm. This may have been the beginning of the picoplankton bloom. C y c l o t e l l a spp. increased t h e i r c e l l density i n the epilimnion at t h i s time; l i k e l y benefiting from the f e r t i l i z a t i o n , because a concurrent increase i n c e l l density i n Two Rivers arm did not 75 occur. A decrease i n the 10 m maximum by 26 July was not accompanied by an increase i n the maximum at 22.5 m as c e l l s sank to depth. C e l l s must either have sank out of the region of the 22.5 m maximum or have been grazed. Without a sediment trap moored below the chlorophyll maximum i t was not possible to ascertain the flux of c e l l s out of the chlorophyll maximum. After the disappearance of the 10 m maximum and concurrent decrease i n k e, l i g h t penetrated deeper and a fluorescence maximum began to re-es t a b l i s h i t s e l f at 25 m (Fig. 13) by the end of August. During July and August, nutrients remained s i m i l a r at depths of the chlorophyll maximum; therefore, i t appears that the po s i t i o n and maintenance of the chlorophyll maximum was affected more by the l i g h t environment than the nutrient environment. Rhizosolenia e r i e n s i s autecology During early lake f e r t i l i z a t i o n projects i n Great Central Lake, R. e r i e n s i s was shown to benefit greatly from nutrient enrichment (Parsons et a l . , 1972). I t was important to know: 1) i f R. er i e n s i s was s i m i l a r l y enhanced i n Sproat Lake, and 2) i f R. er i e n s i s was a nutrient sink. A consideration of the seasonal cycle of R. e r i e n s i s i s important to understanding i t s importance to the Sproat Lake phytoplankton community and therefore i n d i r e c t l y to lake f e r t i l i z a t i o n . 76 In the winter, during conditions of isothermal mixing, growth rates of R. e r i e n s i s were probably low. Nutrients were available for growth but the temperature was low (ca. 5°C) and l i g h t was l i m i t i n g . R. e r i e n s i s existed at t h i s time as an overwintering population. This population, or s p e c i f i c a l l y what remained of i t through the winter, formed the seed population for the following year's spring bloom. In Taylor arm, where the maximum depth i s 270 m, c e l l s which sank to the bottom would probably not have been resuspended. In the l i t t o r a l zone, c e l l s may be resuspended and form part of the seed for the spring bloom. During the spring bloom R. e r i e n s i s reached i t s maximum c e l l density. The success of R. er i e n s i s during the early spring and in the region of the chlorophyll maximum through the summer also indicated that i t i s well adapted to lower temperatures, i . e . , 5-15°C. Since R^ . e r i e n s i s was one of the f i r s t species to bloom i n the spring i t may have a lower l i g h t requirement than other species at low temperatures. Following the spring period, R. e r i e n s i s declined i n the epilimnion and i t s importance centred around i t s contribution to the chlorophyll maximum. I f storm events occurred during the summer i t i s u n l i k e l y that R. e r i e n s i s would have been resuspended into the epilimnion because s t r a t i f i c a t i o n was strong and the chlorophyll maximum was located i n the hypolimnion. As previously mentioned, growth of R. e r i e n s i s was not n i t r a t e - l i m i t e d i n the chlorophyll maximum 77 and growth was probably regulated by the supply rate of phosphorus and l i g h t i n t e n s i t y . In 1986, the epilimnetic a l g a l picoplankton bloom i n the upper part of the water column caused a breakdown i n the chlorophyll maximum, including losses of R. e r i e n s i s . The r e s u l t i n g decrease i n the l i g h t reaching R. e r i e n s i s i n the chlorophyll maximum, due to the bloom above i t , argues for the importance of enough l i g h t reaching the chlorophyll maximum to allow R. e r i e n s i s to grow slowly and maintain i t s e l f at 20-22 m. The time period including f a l l overturn was not monitored during 1986. However, i t i s cle a r that R. e r i e n s i s was present i n s u f f i c i e n t numbers through the October 1986 - February 1987 period to form a seed population for the 1987 spring bloom. Ec o l o g i c a l l y , R. e r i e n s i s i s a species which i s well adapted to conditions of low temperature and l i g h t , c h a r a c t e r i s t i c of the early spring. In l a t e spring i t sank out of the epilimnion, probably achieved neutral buoyancy, and became a major constituent of the hypolimnetic chlorophyll maximum. I t i s therefore u n l i k e l y that R. e r i e n s i s i s a large nutrient sink for nutrients added as part of the lake f e r t i l i z a t i o n programme because R. e r i e n s i s i s present i n substantial numbers i n the epilimnion only during the early portion (March-May) of the spring bloom, p r i o r to f e r t i l i z a t i o n (at the end of May). R. er i e n s i s may have been a small nutrient sink i f , through the 7 8 summer, small numbers of c e l l s sank out of the epilimnion to the chlorophyll maximum and were not grazed. Rhizosolenia e r i e n s i s contains a large vacuole (ca. 90 % of the c e l l volume) and therefore has a low C : c e l l volume - 1. The large s i z e of R. e r i e n s i s probably makes t h i s c e l l d i f f i c u l t to graze by f i l t e r - f e e d i n g zooplankton. Rhizosolenia e r i e n s i s therefore probably does not represent a good food item for zooplankton. Two Rivers arm as a control s i t e In using a control s i t e i t i s desirable that one can compare differences between these observations and observations of the experimental s i t e and a t t r i b u t e the differences to the experimental protocol. In fact, Two Rivers arm i s not a good control s i t e . A shallower 1 % l i g h t depth i n Taylor arm i n the spring and higher e x t i n c t i o n c o e f f i c i e n t s suggest Taylor arm had more non-b i o l o g i c a l suspended material than Two Rivers arm. I f phytoplankton growth was the only factor a f f e c t i n g l i g h t penetration, both arms should have had s i m i l a r properties early i n the spring, before the spring bloom occurred. Logging occurs within the catchment basin of Taylor arm. Recently logged areas during one year may produce high l e v e l s of suspended material i n the snow melt of the following spring, which i n turn decrease l i g h t penetration. 79 Taylor arm i s less sheltered than Two Rivers arm and was therefore more susceptible to breakdown of thermal s t r a t i f i c a t i o n early i n the spring. In 1986, both arms were beyond the onset of thermal s t r a t i f i c a t i o n by the f i r s t sampling date however, i n 1987, development of seasonal s t r a t i f i c a t i o n was observed. Taylor arm s t r a t i f i e d l a t e r than Two Rivers arm and u n t i l strong s t r a t i f i c a t i o n occurred had a shallower epilimnion. The i n i t i a t i o n of the 1987 spring bloom was not as well defined i n Taylor arm as i t was i n Two Rivers arm. Events occurred i n Two Rivers arm ca. 2-4 weeks p r i o r to t h e i r occurrence i n Taylor arm. More mixing i n Taylor arm early i n the spring caused a delay i n the onset of seasonal s t r a t i f i c a t i o n . Comparing physiological processes must be done with caution i f Taylor and Two Rivers arms are compared. On a seasonal basis Two Rivers arm may be an adequate control s i t e but on shorter time scales, i e . , less than a few months, the v a l i d i t y of Two Rivers arm as a control s i t e must be questioned. A better control s i t e might be outside the f e r t i l i z e d portion of Taylor arm and towards the west end of the arm. Lake F e r t i l i z a t i o n In addition to a f f e c t i n g the physical environment, thermal s t r a t i f i c a t i o n i n d i r e c t l y controls the temperature to which 80 phytoplankton c e l l s are exposed. F e r t i l i z a t i o n of Sproat Lake t y p i c a l l y commences around mid/late-May (e.g., 20 May i n 1986). Takahashi and Nash (1973) demonstrated temperature i n h i b i t i o n of photosynthesis i n nearby Great Central Lake between ca. mid-October and la t e May, while through the summer nutrients limited photosynthesis. The exact time of switch-over between temperature and n u t r i e n t - l i m i t a t i o n on photosynthesis w i l l vary. F e r t i l i z a t i o n p r i o r to t h i s point i n time may r e s u l t i n some f e r t i l i z e r loss because phytoplankton demand for nutrients ( s p e c i f i c a l l y nitrogen and phosphorus) may not be s u f f i c i e n t to use the additions. One of the premises behind lake f e r t i l i z a t i o n i s that the f e r t i l i z e r i s added when the spring bloom i s dec l i n i n g due to nutrient l i m i t a t i o n so that the e f f e c t of the nutrient addition i s to maintain bloom l e v e l s of phytoplankton growth through the summer but not to a l t e r the trophic status of the lake. S t r a t i f i c a t i o n should be necessary for n u t r i e n t - l i m i t a t i o n to occur because s t r a t i f i c a t i o n creates a b a r r i e r to nutrient flux into the surface layers from depth. The N and P-load was doubled i n 1986 and the r e s u l t i n g increase i n primary production was obvious by the occurrence of the al g a l picoplankton bloom i n July. During 8 weeks i n 198 6 almost the same amount of phosphorus was added as during 18 weeks i n 1985. While the 1986 loading l e v e l s resulted i n an al g a l picoplankton bloom, which may be undesirable, C y c l o t e l l a spp., a very grazable species, was also enhanced. 81 F e r t i l i z a t i o n was e f f e c t i v e i n increasing phytoplankton standing stock i n 1985 but did not cause unwanted increases i n standing stock (Nidle and Shortreed, i n press). A shallower 1 % l i g h t depth, higher extinction c o e f f i c i e n t and greater maximum fluorescence values i l l u s t r a t e d that more plant material was present i n Taylor arm throughout the summer of 1985 and can be d i r e c t l y attributed to lake f e r t i l i z a t i o n . I t i s therefore suggested that lake f e r t i l i z a t i o n can be an e f f e c t i v e method of increasing phytoplankton standing stock, consistent with e a r l i e r reports (Parsons et a l . , 1972; Schindler and Fee, 1974; Stockner and Hyatt, 1984; Stockner and Shortreed, 1985). Grazing The observed standing stock of phytoplankton i s a s t a t i c measure of dynamic processes including growth, water transport, sinking and grazing. With respect to food chains grazing i s p a r t i c u l a r l y important because i t represents a l i n k between phytoplankton and higher trophic l e v e l s . Growth of phytoplankton, with the exception of the picoplankton bloom i n 1986, was low throughout the summer. Net growth of phytoplankton would primarily be the difference between growth and sinking plus grazing. In an ide a l system a l l phytoplankton growth would be consumed by zooplankton grazing. Since grazing was not assessed i n t h i s study i t s importance as a phytoplankton 82 loss term cannot be determined although grazing was undoubtably occurring. The chlorophyll maximum may represent a large food source for zooplankton. Changes i n phytoplankton species composition with time and the shape and pos i t i o n of the chlorophyll maximum would be influenced by both grazing and sinking. The importance of the chlorophyll maximum as a food source cannot be determined without d i r e c t measurements of grazing. Future work The importance of picoplankton to marine and limnetic systems has become apparent (Stockner and Antia, 1987) but was not addressed i n the present study. The r e l a t i v e importance of picoplankton to ecosystem and chlorophyll maximum dynamics should be considered i n future studies. Physiological rate processes of c e l l s within the chlorophyll maximum have r a r e l y been measured. To f u l l y understand the contribution of the chlorophyll maximum requires q u a n t i f i c a t i o n of such rates, which can then be used i n dynamic models and energy budgets. It i s probable that zooplankton grazing plays a r o l e , i n various degrees, i n c o n t r o l l i n g formation and influencing species composition with time by s e l e c t i v e l y grazing some species. What i s g r a z e d and t h e r a t e s o f g r a z i n g i n b o t h t h e e p i l i m n i o n and c h l o r o p h y l l maximum need t o be d e t e r m i n e d . 84 CONCLUDING REMARKS F i e l d measurements made during t h i s study enabled observation of the spring and summer phytoplankton community i n an oligotrophic coastal B r i t i s h Columbia lake. Over two seasonal periods i t was cl e a r that some events repeated themselves although the exact nature and timing of such events varied. Sproat Lake phytoplankton form a seasonal hypolimnetic chlorophyll maximum following the onset of seasonal thermal s t r a t i f i c a t i o n i n la t e A p r i l and early May. Growth of phytoplankton within the chlorophyll maximum i s probably phosphate-limited. Formation of the chlorophyll maximum began at 10-12 m and i n late-May or early-June i t descended to ca. 22.5 m for the duration of the summer. Some sinking phytoplankton (mainly R. eriensis) become neutrally buoyant at depth and play a major r o l e i n chlorophyll maximum formation. The p o s i t i o n and maintenance of the chlorophyll maximum i s possibly mediated by the l i g h t regime. Nitrate was present throughout the water column during mixing but became depleted during the spring bloom. A n i t r a c l i n e was present throughout the summer, beginning at 15 m. Phosphorus was always present i n very low or undetectable concentrations. The sinking rate of two diatoms was measured and found to be highest i n the epilimnion and lowest at the depth of the chlorophyll maximum. 85 A seasonal cycle of phytoplankton species succession and community structure was observed. R. e r i e n s i s and C y c l o t e l l a spp. were the two major diatom constituents during 1985 and 1986. R. e r i e n s i s bloomed p r i o r to C y c l o t e l l a spp. i n the spring. Throughout the spring and summer, medium-sized f l a g e l l a t e s (3-15 um) were important numerically as was, on occasion, Dinobryon sp.. Later i n the summer other diatoms, including A s t e r i o n e l l a formosa, Synedra sp. and F r a q i l a r i a sp. became r e l a t i v e l y more important numerically, but were s t i l l not as important as R. e r i e n s i s and C y c l o t e l l a spp. F e r t i l i z a t i o n of the lake for enhancement of sockeye salmon can a l t e r phytoplankton standing stock. I f changes to the phytoplankton community are large, i n d i r e c t changes, e.g., to the l i g h t regime, may occur. R. e r i e n s i s was not considered to be a large nutrient l i n k or sink i n Sproat Lake due to temporal and s p a t i a l separation from the portion of the lake d i r e c t l y affected by f e r t i l i z a t i o n . 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Plant production of the Fladen ground. J . mar. b i o l . Assoc. U.K. 35: 1-33. Steele, J.H. 1961. Primary production, i n : Oceanography. AAAS publ. 67: 519-538. Steele, J.H. 1962. Environmental control of photosynthesis in the sea. Limnol. Oceanogr. 7: 137-150. Steele, J.H. and C S . Yentsch. 1960. The v e r t i c a l d i s t r i b u t i o n of chlorophyll. J . mar. b i o l . Assoc. U.K. 39: 217-266. Stephens, K. and R. Brandstaetter. 1983. A laboratory manual, c o l l e c t e d methods for the analysis of water. Can. Tech. Rep. Fish. Aquat. S c i . 1159: i v + 68 pp. Stockner, J.G., Shortreed, K.S.and K. Stephens. 1980. The B r i t i s h Columbia lake f e r t i l i z a t i o n program: Limnological r e s u l t s from the f i r s t two years of nutrient enrichment. Can. Tech. Rep. Fish. Aquat. S c i . 924: 91 pp. Stockner, J.G. and K.D. Hyatt. 1984. Lake f e r t i l i z a t i o n : state of the a r t a f t e r 7 years of application. Can. Tech. Rep. Fish. Aquat. S c i . 1324: 33 pp. Stockner, J.G. and K.S. Shortreed. 1985. Whole-lake f e r t i l i z a t i o n experiments i n coastal B r i t i s h Columbia lakes: Empirical relationships between nutrient inputs and phytoplankton biomass and production. Can. J . Fish. Aquat. S c i . 42: 649-658. Stockner, J.G. and N.J. Antia. 1986. Algal picoplankton from marine and freshwater ecosystems: a m u l t i d i s c i p l i n a r y perspective. Can. J . Fish. Aquat. S c i . 43: 2472-2503. Strickland, J.D.H. and T.R. Parsons. 1972. A p r a c t i c a l handbook of seawater analysis. B u l l . Fish. Res. Board Can. 167: 311 pp. Suttle, C. 1987. Ef f e c t s of Nutrient Patchiness and N:P Supply Ratio on the Ecology and Physiology of Freshwater Phytoplankton. Ph.D. thes i s . University of B r i t i s h Columbia. 176 pp. Takahashi, M. and F. Nash. 1973. The e f f e c t of nutrient enrichment on al g a l photosynthesis i n Great Central Lake, B r i t i s h Columbia, Canada. Arch. Hydrobiol. 71: 166-182. 90 Traversy, W.J. 1971. Methods for chemical analysis of waters a n d waste-waters. Water Quality Di v i s i o n : Inland Waters Branch, Dept. of Fish, and For., Ottawa, Ontario. 169 pp. Venrick, E.L., J.A. McGowen and A.W. Mantyla. 1973. Deep maxima of photosynthetic chlorophyll i n the P a c i f i c Ocean. Fish. B u l l . U.S. 71: 41-52. Walsh, J . J . , Kelly, J.C., Dugdale, R.C. and B.W. Frost. 1971. Gross features of the Peruvian upwelling system with special reference to possible d i e l v a r i a t i o n . Investig. Pesq. 35: 25-42. Walsh, J . J . , Whitledge, T.E., Kelly, J . C , Huntsman, S.A. and R.D. P i l l s b u r y . 1977. Further t r a n s i t i o n states of the Baja C a l i f o r n i a upwelling ecosystem. Limnol. Oceanogr. 22: 264-280. Williams, P.J. LeB. 1981. Incorporation of microheterotrophic processes into the c l a s s i c a l paradigm of the planktonic f o o d web. K i e l e r Meeresforsch. Soderh. 5: 1-28. 91 A P P E N D I C E S 92 Appendix I. Sampling dates for various parameters during 1986 and 1987. Table 1. Sampling dates phytoplankton, for i n 1986. vivo fluorescence and May 8 12 15 19 22 26 29 June 2 5 9 12 19 23 30 July 3 17 21 26 29 August 6 Table 2. Sampling dates for i n vivo fluorescence and phytoplankton, 1987. March 11 15 19 23 28 A p r i l 1 5 9 13 18 22 26 30 May 4 8 12 16 20 June 1 93 Table 3. Sampling dates for l i g h t and temperature, 1987. March 7 15 23 A p r i l 1 18 26 May 6 12 20 June 1 Table 4. Sampling dates for t o t a l f i l t e r e d phosphate, n i t r a t e and chlorophyll , 1987. March 7 23 A p r i l 13 26 May 12 June 1 94 Appendix I I . C e l l density ( c e l l s x 10 2«mL - 1) for groups enumerated and discussed i n the thesis text (lg = large, sm = small, and depths i n metres). Table 1. C e l l densities for Two Rivers arm, 1986. 19 May 5 10 17.5 22.5 Other diatoms 11 18 13 13 l g f l a g e l l a t e s 17 5 12 17 c i l i a t e s 4 5 3 3 26 May 5 10 17.5 20 25 Other diatoms 38 2 40 19 21 l g f l a g e l l a t e s 21 43 64 24 11 30 May 5 10 15 20 25 35 Other diatoms 15 14 15 15 12 22 l g f l a g e l l a t e s 6 26 24 28 9 20 c i l i a t e s 4 1 2 1 3 1 95 2 June 5 10 15 20 25 30 Other diatoms 8 17 22 21 26 24 l g f l a g e l l a t e s 12 56 60 48 6 5 c i l i a t e s 1 0 6 4 4 2 5 June 10 15 20 25 Other diatoms 9 2 0 31 18 l g f l a g e l l a t e s 36 48 50 17 c i l i a t e s 5 1 4 2 9 June 5 10 15 22.5 27.5 32.5 Other diatoms 5 10 15 11 14 9 l g f l a g e l l a t e s 8 35 56 92 4 8 c i l i a t e s 2 3 2 17 1 1 16 June 5 10 13 16 19 Other diatoms 13 18 12 23 3 6 l g f l a g e l l a t e s 5 10 76 57 32 c i l i a t e s 0 1 5 8 4 96 19 June 5 10 15 20 25 30 Other diatoms 23 16 7 18 13 15 l g f l a g e l l a t e s 13 29 16 53 32 28 c i l i a t e s 0 1 1 6 2 2 23 June 5 10 15 20 25 30 Other diatoms 13 19 15 17 29 19 l g f l a g e l l a t e s 10 36 19 89 64 34 c i l i a t e s 0 3 2 5 4 1 30 June 5 10 15 20 25 30 Other diatoms 43 46 24 37 45 60 l g f l a g e l l a t e s 11 i 30 17 29 29 16 c i l i a t e s 1 2 1 4 3 2 17 J u l y 5 10 15 20 25 30 Other diatoms 18 20 24 31 24 23 l g f l a g e l l a t e s 13 18 34 38 22 10 c i l i a t e s 1 2 4 2 1 2 26 July 5 10 15 20 25 30 Other diatoms 51 57 50 51 43 33 l g f l a g e l l a t e s 17 24 12 27 11 11 c i l i a t e s 1 4 4 1 3 2 Table 2. C e l l densities for Taylor arm, 1986. 23 A p r i l 5 10 17.5 22.5 35 Other diatoms 17 18 21 3 3 l g f l a g e l l a t e s 23 36 24 12 1 c i l i a t e s 9 2 4 3 3 98 19 May 5 12.5 17.5 22.5 35 Other diatoms 0 63 8 19 5 l g f l a g e l l a t e s 44 196 19 32 21 c i l i a t e s 1 6 1 3 3 26 May 5 10 17.5 22.5 27.5 Other diatoms 19 21 18 14 11 l g f l a g e l l a t e s 20 33 12 15 16 c i l i a t e s 4 9 5 1 2 30 May 5 10 15 20 25 30 Other diatoms 28 13 21 6 7 27 l g f l a g e l l a t e s 38 54 40 26 11 26 c i l i a t e s 7 13 4 1 1 2 99 2 June 5 10 15 20 25 35 Other diatoms 31 19 26 3 7 13 l g f l a g e l l a t e s 7 84 91 15 12 16 c i l i a t e s 0 6 6 3 1 1 5 June 5 10 15 20 25 30 Other diatoms 169 32 18 37 34 32 l g f l a g e l l a t e s 97 69 38 35 17 23 c i l i a t e s 0 10 6 7 0 3 9 June 5 10 15 20 25 30 Other diatoms 20 43 11 31 25 6 l g f l a g e l l a t e s 6 53 27 20 13 5 c i l i a t e s 4 4 14 10 2 1 100 16 June 5 8 11 14 17 20 Other diatoms 29 21 27 49 38 14 l g f l a g e l l a t e s 30 19 56 57 26 19 c i l i a t e s 3 1 5 7 7 6 19 June 1 5 10 15 20 Other diatoms 50 18 41 10 44 l g f l a g e l l a t e s 21 11 50 37 67 c i l i a t e s 0 2 3 3 10 23 June 5 10 15 20 25 Other diatoms 51 16 21 31 35 l g f l a g e l l a t e s 47 40 21 15 20 c i l i a t e s 1 1 3 3 1 101 1 July 5 10 15 20 25 Other diatoms 37 114 13 11 5 l g f l a g e l l a t e s 11 47 9 5 1 c i l i a t e s 2 11 1 1 1 17 July 7.5 15 20 25 30 35 Other diatoms 77 51 21 30 22 25 l g f l a g e l l a t e s 9 19 11 7 7 15 c i l i a t e s 6 0 1 1 2 1 26 July 5 10 15 20 25 30 Other diatoms 155 105 77 39 25 22] l g f l a g e l l a t e s 18 24 42 13 9 11 c i l i a t e s 0 4 6 4 1 2 102 6 August 5 10 15 20 25 30 Other diatoms 33 36 49 41 26 13 l g f l a g e l l a t e s 18 16 25 28 7 3 c i l i a t e s 1 1 0 4 0 1 Table 3. C e l l i densities for Two Rivers arm, 1987 • 11 March 5 10 15 20 25 Other diatoms 64 80 86 126 120 l g f l a g e l l a t e s 65 98 54 37 54 d i n o f l a g e l l a t e s 0 4 0 0 4 c i l i a t e s 0.8 0 0 4 4 15 March 5 10 15 20 25 Other diatoms 87 25 70 53 35 l g f l a g e l l a t e s 91 51 24 47 16 d i n o f l a g e l l a t e s 8 7 3 3 4 c i l i a t e s 12 7 10 8 5 19 March 5 10 15 20 25 Other diatoms 40 64 76 49 51 l g f l a g e l l a t e s 166 126 54 43 45 d i n o f l a g e l l a t e s 8 0 3 6 0 c i l i a t e s 8 19 10 12 6 23 March 5 10 15 20 25 Other diatoms 22 98 90 45 97 l g f l a g e l l a t e s 192 107 126 163 119 d i n o f l a g e l l a t e s 13 0 0 5 0 c i l i a t e s 22 13 0 10 13 28 March 5 10 15 20 25 Other diatoms 21 101 89 66 44 l g f l a g e l l a t e s 98 218 124 73 62 d i n o f l a g e l l a t e s 4 23 29 4 3 c i l i a t e s 9 0 10 0 0 104 1 A p r i l 5 10 15 20 25 Other diatoms 71 127 65 71 118 l g f l a g e l l a t e s 226 142 94 72 111 d i n o f l a g e l l a t e s 18 8 8 0 7 c i l i a t e s 6 0 0 13 11 5 A p r i l 5 10 15 20 25 Other diatoms 47 280 120 60 40 l g f l a g e l l a t e s 226 433 113 46 43 d i n o f l a g e l l a t e s 6 0 0 4 0 c i l i a t e s 18 38 12 6 12 9 A p r i l 0.5 3 6 9 12 15 Other diatoms 244 83 67 110 268 198 l g f l a g e l l a t e s 202 235 279 316 224 595 d i n o f l a g e l l a t e s 0 0 0 7 30 0 c i l i a t e s 8 12 0 13 0 0 9 A p r i l con't 18 21 24 27 30 Other diatoms 69 112 41 49 66 l g f l a g e l l a t e s 157 96 89 95 110 d i n o f l a g e l l a t e s 5 0 4 0 0 c i l i a t e s 21 38 0 4 0 13 A p r i l 5 10 15 20 25 Other diatoms 205 108 158 526 321 l g f l a g e l l a t e s 312 562 318 95 68 d i n o f l a g e l l a t e s 0 14 0 11 7 c i l i a t e s 18 28 0 0 4 18 A p r i l 5 10 15 20 25 Other diatoms 95 130 112 130 45 l g f l a g e l l a t e s 220 142 257 119 134 d i n o f l a g e l l a t e s 6 0 0 12 4 c i l i a t e s 0 0 12 12 18 106 22 A p r i l 5 10 15 20 25 30 Other diatoms 37 180 152 89 67 39 l g f l a g e l l a t e s 96 304 728 162 82 96 d i n o f l a g e l l a t e s 0 0 0 0 4 4 c i l i a t e s 0 9 14 16 0 4 26 A p r i l 5 10 15 20 25 30 Other diatoms 107 34 123 123 63 75 l g f l a g e l l a t e s 128 23 234 275 115 82 d i n o f l a g e l l a t e s 12 0 0 7 21 4 c i l i a t e s 12 0 23 34 0 12 30 A p r i l 5 10 15 20 25 30 Other diatoms 74 59 155 45 73 71 l g f l a g e l l a t e s 147 428 273 144 195 78 d i n o f l a g e l l a t e s 0 36 0 0 0 0 c i l i a t e s 5 6 24 5 12 6 107 4 May 5 10 15 20 25 30 Other diatoms 48 125 123 70 69 53 l g f l a g e l l a t e s 226 257 187 155 131 147 d i n o f l a g e l l a t e s 0 0 0 0 5 0 c i l i a t e s 0 5 0 4 0 0 8 May 5 10 15 20 25 30 Other diatoms 85 64 115 105 100 89 l g f l a g e l l a t e s 127 236 146 105 82 121 d i n o f l a g e l l a t e s 0 0 0 0 4 0 c i l i a t e s 0 0 0 5 0 11 12 May 5 10 15 20 25 30 Other diatoms 12 154 200 127 95 195 l g f l a g e l l a t e s 127 321 278 225 145 315 d i n o f l a g e l l a t e s 0 0 0 0 0 16 c i l i a t e s 0 0 30 0 0 5 108 16 May 5 10 20 25 30 Other diatoms 84 62 55 79 55 l g f l a g e l l a t e s 145 318 184 84 123 d i n o f l a g e l l a t e s 0 0 5 0 0 c i l i a t e s 0 0 5 0 4 20 May 0.5 3 6 9 12 15 Other diatoms 48 39 43 54 32 157 l g f l a g e l l a t e s 35 56 63 120 125 228 d i n o f l a g e l l a t e s 4 5 6 0 0 8 c i l i a t e s 2 5 2 8 8 14 2 0 May con't 18 21 24 27 30 Other diatoms 147 89 63 93 37 l g f l a g e l l a t e s 103 150 62 95 70 d i n o f l a g e l l a t e s 0 8 3 0 2 c i l i a t e s 27 21 0 10 7 109 24 May 5 10 15 20 25 Other diatoms 65 127 416 157 108 l g f l a g e l l a t e s 149 200 394 206 64 d i n o f l a g e l l a t e s 0 22 64 34 9 c i l i a t e s 0 21 25 17 3 28 May 5 10 15 20 25 30 Other diatoms 32 54 145 112 141 21 l g f l a g e l l a t e s 43 134 212 72 89 51 d i n o f l a g e l l a t e s 0 0 0 0 4 0 c i l i a t e s 0 0 9 17 0 0 Ta b l e 4. C e l l d e n s i t i e s f o r T a y l o r arm, 1987. 11 March 5 10 15 20 Other diatoms 32 36 82 102 l g f l a g e l l a t e s 73 78 43 61 d i n o f l a g e l l a t e s 5 0 0 0 c i l i a t e s 5 0 4 4 15 March 5 10 15 20 25 Other diatoms 136 122 38 12 15 l g f l a g e l l a t e s 93 82 61 53 21 d i n o f l a g e l l a t e s 7 12 0 0 0 c i l i a t e s 5 0 7 3 0 19 March 5 10 15 20 25 Other diatoms 52 70 94 52 24 l g f l a g e l l a t e s 30 94 61 29 7 d i n o f l a g e l l a t e s 3 0 4 4 0 c i l i a t e s 3 12 12 0 4 23 March 5 10 15 20 25 Other diatoms 143 62 63 49 6 l g f l a g e l l a t e s 172 101 100 77 50 d i n o f l a g e l l a t e s 0 0 0 0 0 c i l i a t e s 12 5 5 8 4 28 March 5 10 15 20 25 Other diatoms 44 121 79 175 48 l g f l a g e l l a t e s 357 111 33 75 45 d i n o f l a g e l l a t e s 0 0 0 3 0 c i l i a t e s 6 0 6 4 4 1 A p r i l 5 10 15 20 25 Other diatoms 129 31 101 90 86 l g f l a g e l l a t e s 192 34 71 58 56 d i n o f l a g e l l a t e s 22 9 4 4 9 c i l i a t e s 4 4 7 0 6 5 A p r i l 5 10 15 20 25 Other diatoms 78 154 71 78 78 l g f l a g e l l a t e s 162 389 138 54 7 d i n o f l a g e l l a t e s 16 32 0 4 7 c i l i a t e s 5 24 9 0 4 112 9 A p r i l 0.5 3 6 9 12 15 Other diatoms 145 46 49 85 35 59 l g f l a g e l l a t e s 173 128 104 178 112 188 d i n o f l a g e l l a t e s 5 0 0 0 0 5 c i l i a t e s 5 0 5 4 0 5 9 A p r i l con't 18 21 24 27 30 Other diatoms 48 38 79 93 64 l g f l a g e l l a t e s 89 58 61 164 75 d i n o f l a g e l l a t e s 0 4 4 4 7 c i l i a t e s 4 0 4 0 0 13 A p r i l 5 10 15 20 25 Other diatoms 131 54 104 43 50 l g f l a g e l l a t e s 165 99 208 50 70 d i n o f l a g e l l a t e s 0 0 0 0 4 c i l i a t e s 21 10 15 4 0 113 18 A p r i l 5 10 15 20 25 Other diatoms 46 49 123 38 71 l g f l a g e l l a t e s 43 80 134 105 56 d i n o f l a g e l l a t e s 4 4 5 0 4 c i l i a t e s 4 4 0 11 0 22 A p r i l 5 10 15 20 25 30 Other diatoms 145 589 111 70 89 43 l g f l a g e l l a t e s 288 95 154 73 48 116 d i n o f l a g e l l a t e s 28 8 0 4 4 0 c i l i a t e s 13 37 18 4 15 4 26 A p r i l 5 10 15 20 25 30 Other diatoms 23 129 85 127 47 36 l g f l a g e l l a t e s 81 257 139 108 72 39 d i n o f l a g e l l a t e s 12 0 0 0 4 0 c i l i a t e s 4 0 0 4 4 0 114 30 A p r i l 5 10 15 20 25 30 Other diatoms 163 104 63 56 62 54 l g f l a g e l l a t e s 71 243 120 128 93 120 d i n o f l a g e l l a t e s 0 0 9 0 4 4 c i l i a t e s 6 15 0 4 4 11 4 May 5 10 15 20 30 Other diatoms 78 56 77 75 54 l g f l a g e l l a t e s 220 150 178 75 75 d i n o f l a g e l l a t e s 0 0 0 0 0 c i l i a t e s 0 4 26 9 14 8 May 5 10 15 20 25 30 Other diatoms 85 106 78 134 67 59 l g f l a g e l l a t e s 38 190 74 168 108 113 d i n o f l a g e l l a t e s 0 0 0 0 0 0 c i l i a t e s 0 5 12 0 12 4 115 12 May 5 10 15 20 25 30 Other diatoms 89 83 48 54 123 52 l g f l a g e l l a t e s 143 172 148 128 157 48 d i n o f l a g e l l a t e s 0 0 0 0 0 0 c i l i a t e s 6 6 0 7 4 4 16 May 5 10 15 20 25 30 Other diatoms 23 59 80 65 51 74 l g f l a g e l l a t e s 178 345 248 113 78 55 d i n o f l a g e l l a t e s 4 0 0 6 0 0 c i l i a t e s 4 12 0 0 0 0 20 May 0.5 3 6 9 12 15 Other diatoms 38 55 66 116 86 71 l g f l a g e l l a t e s 35 37 66 75 143 232 d i n o f l a g e l l a t e s 3 3 3 0 0 10 c i l i a t e s 3 5 5 6 8 9 116 20 May con't 18 21 24 27 30 Other diatoms 57 43 71 97 22 l g f l a g e l l a t e s 75 64 74 121 64 d i n o f l a g e l l a t e s 0 0 0 0 0 c i l i a t e s 14 4 3 5 4 24 May 5 10 15 20 25 30 Other diatoms 10 59 178 112 95 48 l g f l a g e l l a t e s 92 132 278 89 89 78 d i n o f l a g e l l a t e s 0 10 0 0 11 9 c i l i a t e s 7 10 24 9 7 6 28 May 5 10 15 20 25 30 Other diatoms 34 63 88 146 115 83 l g f l a g e l l a t e s 34 43 147 139 86 36 d i n o f l a g e l l a t e s 0 18 20 4 4 0 c i l i a t e s 4 0 15 22 17 3 117 Appendix I I I . Comparison of i n vivo fluorescence (o) to c e l l density of Rhizosolenia e r i e n s i s (•), C y c l o t e l l a  SPP- (•) and small f l a g e l l a t e s ( A ) . IN VIVO FLUORESCENCE 9 APRIL RR 2 0 MAY RR 8 1 0 0 .1 .2 .3 .4 .5 I o I 1 I 1 9 APRIL T e I O 20 MAY T - i i 1 8 10 CELL DENSITY x Icf-mL* 

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