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Productivity of the macrophytes of Marion Lake, B.C. Davies, Gordon Stanley 1968

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THE PRODUCTIVITY OF THE MACROPHYTES OF MARION LAKE, B.C. by GORDON STANLEY DAVIES B.Sc., University of British Columbia, 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF -M. SC. in the Department of Zoology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1968 In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make i t 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 hjis representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of B r i t i s n Columbia Vancouver 8, Canada Department of ABSTRACT The in situ productivity of both the phytoplankton and the macro-phytes in Marion Lake, B.C., was determined from April 1966 through 14 14 September 1966, by using C techniques. The uptake of NaH CO^ was measured in selected macrophytes by enclosing them in plexiglass 14 chambers. These plants were then combusted in oxygen, and the CO., was absorbed in toluene-POPOP-ethanolamine. Radioassay was 14 accomplished by liq u i d s c i n t i l l a t i o n . In addition to the C method, an organic weight method was used to measure macrophytic productivity. The productivity of the macrophytes was always higher than that of the phytoplankton. There was a considerable difference in the estimates of the macrophyte productivity arrived at by the two different methods, and reasons for this are discussed. The total productivity of the lake is very low when compared with lakes of similar latitudes because of low phytoplankton productivity. It is concluded that in Marion Lake the macrophytes are more important primary producers than the phytoplankton. The d i f f i c u l t y of comparing data between this and other studies is discussed, and the need for standardization of methods is emphasized. i i TABLE OF CONTENTS ABSTRACT i TABLE OF CONTENTS i i LIST OF TABLES i i i LIST OF FIGURES i v ACKNOWLEDGMENTS v INTRODUCTION 1 METHODS 2 The Study Area 2 Phytoplankton 4 Rooted Aquatics 6 RESULTS 13 DISCUSSION 24 LITERATURE CITED 27 LIST OF TABLES Table I. Comparison of productivity of macrophytes and phytoplankton in Marion Lake, I966 . Blank values indicate that net respiration equals or exceeds photosynthesis. 18 II. Distribution and cumulative net productivities of the four species of macrophytes i n Marion Lake, 1966. Numbers in parentheses represent number of plants involved in determination. 19 III. Variability i n numbers of macrophytes between quadrats of one square meter. 20 IV. Variability in fresh weights of individual plants. 21 V. Variability in drying and ashing techniques. Each percentage represents a mean value obtained from ten plants. 22 VI. Comparisons of the annual mean primary productivity of Borax Lake, California (Wetzel, 1964) to the values for Marion Lake, British Columbia; data in parentheses are values for the growing season. 23 i v LIST OF FIGURES Figure P a g e 1. Morphometric map of Marion Lake, British Columbia, showing Stations A, B, D, and E. 3 2. Light and opaque chambers used for determining productivity of macrophytes. 8 3. Oxygen combustion flask showing: A - platinum carriage holding encapsulated sample, B - stopcock, and C - capillary tube. 10 4. Vegetation map of Marion Lake, British Columbia, I966 . The lake was surveyed with the aid of SCUBA. 12 5 . Productivity of the aquatic macrophytes in Marion Lake, April to September, 1966, as determined by organic weight method. 15 6. Comparison of the productivity of the phytoplankton and 14 macrophytes as determined by C method. 16 7. Comparison of the productivities of the four important macro-phytes i n Marion Lake, April to September, 1966, as determined by the organic weight method. 17 V ACKNOWLEDGMENTS I wish to thank Dr. Ian E. Efford for his help and encouragement with a l l aspects of the study. Dr. R. G. Wetzel of Michigan State University offered much good advice and his correspondence was greatly appreciated. Drs. D. Chitty, C. V. Finnegan, P. A. Larkin, and E. B. Tregunna c r i t i c a l l y read the manuscript and made many helpful suggestions. Dr. C. T. Beer of the Cancer Research Institute of the University of British Columbia made available an oxygen com-bustion flask which was used during the study, and he was most generous with his time in discussing the handling and assay of radio-isotopes . Many people assisted in the f i e l d observations and collections but I especially thank Mr. Kanji Tsumura who assisted in almost a l l the practical aspects of the study. Messrs. M. Dickman, J. Mathias, R. Armstrong, and P. Richerson provided much in the way of help and discu s s i o n . My wife , Anita Davies, made the illustrations and assisted in a l l phases of the preparation of the manuscript. 1 INTRODUCTION In most of the limnological studies dealing with primary prod-u c t i v i t y , the emphasis has been placed on the productivity of the phytoplankton. Recently Wetzel ( 1964) and others have noted that in shallow lakes the contribution of the periphyton and higher vascular aquatics may be vastly underrated. The present study attempts to evaluate the role played in primary productivity by the macrophytes of Marion Lake, B.C. and is part of a detailed investigation which w i l l describe the flow of energy through a small freshwater lake. The primary productivity of this lake for 1965 was recently described by Efford ( 1967) , but since phytoplankton productivity is apt to fluctuate a great deal from year to year, i t was again determined in the course of the present study. The productivity of the macrophytes was determined by making 14 periodic biomass determinations and also by using a C method. 2 METHODS The Study Area Marion Lake is located 16 km. north of Lang le y , B.C. in the University Research Forest. The lake l i e s in a long narrow valley 300 m. above sea l e v e l , bordered on the west by a steep ridge 300 m. above the lake l e v e l and on the east by the southern slopes of the Coastal Mountains. The basin of the lake is 800 m. long and about 200 m. wide at its maximum. At standard water l e v e l the lake has an area of 13.3 ha., one half of which is less than 2 m. deep ( F i g . 1) . The major inlet 2 stream enters from the north and drains a watershed of 6.5 km. including nearby Eunice Lake. The outlet at the south end feeds into the North Alouette system which is 1 km. away. Both inlet and out-let streams are gravelled, whereas the bottom of the lake consists of a uniform brown ooze. In winter the lake is isothermal, but from spring through f a l l it is weakly stratified. The winters in the lower mainland of British Columbia are relatively mild and consequently the lake does not regularly have a permanent ice sheet but is covered with a thick layer of slush. A detailed physical description of the lake is given elsewhere ( Efford, 1967) . 3 F i g . 1. Morphometric map of Marion Lake, British Columbia, showing Stations A, B, D, and E. MARION LAKE 4 The phytoplankton contains over 200 species (Dickman, MS, 1967) and of these the dominant forms are microflagellates of the Chlorophyceae and Chrysophycae. Efford ( 1967) attributes 95 percent of the phytoplankton productivity to the nannoplankton. He separated the nannoplankton productivity from that of the macroplankton by regularly filtering half a sample normally and half through a#25 net ( 0.064 mm. diameter) . He further states that spring blooms in the macroplankton appear not to change productivity s i g n i f i c a n t l y . The periphyton are dominated in the summer by the algae Mougeotia spp. which are found in dense mats over the bottom ooze in shallow water and adhering to the stems of Nuphar sp. and Potamogeton natans. There are only four rooted aquatics of numerical significance: Vallisneria sp. and Isoetes occidentalis (submerged) , and Potamogeton  natans and Nuphar sp. (floating) . Dense beds of Chara spp. are found in association with the main spring of the lake and also with several of the smaller ones. Isoetes occidentalis is distributed in water over 2 m. in depth, while the other macrophytes are generally found in more shallow areas. Phytoplankton In order to compare the productivity of the phytoplankton with 14 that of the rooted aquatics, the C technique introduced by Steeman-Nielsen ( 1952) and modified by Goldman ( 1963) was used. Paired 5 samples of water were collected from several depths i n a polyethylene bottle and transferred to clear and opaque glass stoppered bottles. 14 14 Each sample was inoculated with 4fj.c of C i n the form of NaH COg. The samples were then incubated at the depths from which they were obtained for a period of 4 to 6 hours. This period i s considered adequate for significant carbon uptake but brief enough to minimize bottle effects (Vollenweider and Nauwerk, 1961) . Ideally, productivity measurements should have been taken at suitable increments from dawn to dusk each day of the study period, but this was impractical. Instead, a four hour mid-day incubation period was selected i n order to average out the variations in the diurnal curve caused by changing meteorological conditions which could influence the photosynthetic pattern. The assumption was made that productivity i s directly proportional to light with f u l l realization that this correlation is not always exact; minimal rates might be obtained, for example, i f the measured rates were at maximal intensity. D a i l y light curves were obtained at Marion Lake with a recording pyrheliometer and the area under these curves was determined by planimetry. The ratio of the area under that portion of the curve representing the incubation period to the total area of the light curve gives a diurnal correction factor which was used to extrapolate d a i l y production values. After incubation, aliquots of the samples were filtered under vacuum at the lakeside onto HA M i l l i p o r e filters of a porosity of 0.45 + 0.02 [3.. The a c t i v i t y of the samples was determined at the 14 International Agency for C Determination i n Denmark. 6 The method involves di f f i c u l t i e s (Thomas, 1961) , but i t is probably the most sensitive now available for measuring carbon fixation rates under natural conditions and i s thought to give values which approxi-mate net productivity (Strickland, i960) . A l l observations were made at Stations A and E, the assumption being that Station A would be t y p i c a l of that part of the lake deeper than 2 m., and Station E of the shallower portion ( F i g . 1) . The areas represented by A and E were equal, so the productivity of the lake on any given day was taken to be the average of the productivities at A and E. Rooted Aquatics There is a vast literature on the investigations of the productivity of higher aquatic plants, much of which has been reviewed by Penfound ( 1956) , Westlake ( 1963) , and Wetzel ( 1964) . U n t i l recently when Wetzel ( 1963) devised a technique for measuring the in situ prod-14 uctivity of aquatic macrophytes with C, most of the studies have been concerned with biomass determinations and, less frequently, techniques involving the changes i n dissolved oxygen i n the water surrounding the plants. Hartman and Brown ( 1966) have shown that dissolved oxygen con-centrations i n the water surrounding vascular plants are not proportional to the production of internal oxygen because of the storage of oxygen i n the internal lacunae. Errors involved when using the oxygen 7 technique may be 200 percent or higher (Wetzel, personal communica-tion) . The above considerations plus the added technical d i f f i c u l t y of obtaining samples while operating at depths of up to 4 m. necessitated eliminating the oxygen technique. Two methods were used to determine the seasonal productivity 14 of the macrophytes; one was based on the calculation of in situ C fixation rates, the other on the rate of increase i n organic carbon as determined by periodic cropping. In both cases, periodic measure-ments were made throughout the growing season. Although the organic weight method was used to describe each of the four important 14 macrophytes i n the lake, the C investigations were carried out only on the two submerged aquatics. With the aid of SCUBA, light and opaque plexiglass chambers were placed over individual plants ( F i g . 2) . The cylinders were similar to those designed by Wetzel ( 1964) and were calibrated so that the enclosed volume of water was known. Depending on the s i z e of the plants, the volumes used were 1.5 or 2.0 1. A known amount of 1 4 C ( 10-20 u.c/1. of water) was injected into the chamber as N a H 1 4 C O g , by means of a hypodermic syringe. After 4 hours of incubation ( 10:00 A.M. until 2:00 P.M.) , the entire plant was removed and epiphytes were stripped from the leaves since they w i l l also f i x carbon. The plant was then blotted, weighed, and placed on dry i c e . In the laboratory the plants were dried at 60°C and then ground i n a m i l l to pass through a feo mesh (0.25 mm. diameter) sieve. Ten mg. aliquots of each plant were then combusted in an oxygen 8 Fig . 2. Light and opaque chambers used for determining productivity of macrophytes. 9 flask designed by Dr. C. T. Beer of the Cancer Research Institute of the University of British Columbia ( F i g . 3) . When the combustion was complete, the flask was water-cooled in order to reduce the internal pressure. The capillary tube of the flask was then inserted in a li q u i d s c i n t i l l a t i o n v i a l which contained a toluene-POPOP-ethanolamine mixture. When the stopcock of the flask was opened, the scintillation-absorbent mixture was drawn up, because of the vacuum, into the fl a s k , where it was trapped by closure of the stopcock. The apparatus was agitated intermittently and after 20 minutes the fluid was returned to the s c i n t i l l a t i o n v i a l for counting. The efficiency of the above assay (38%) was determined by comparing the counts of replicate samples where one of the plants 14 was placed in an induction furnace, combusted and the C 0 2 evolved flushed into an evacuated 500 ml. ionization chamber. This second assay was made with a Dynacon Electrometer which is accurate within 1% by comparison with the American National Bureau of Standards samples. The raw data which are in counts per minute are converted to mgC in exactly the same way as they are for the phytoplankton. In order to - 2 - 1 express these values as mg C-m 'day , it is necessary to know the biomass distribution. A l l the productivity values given in the -2 paper are expressed as mg C-m of lake surface. j A l l observations on Isoetes occidentalis were made at Station IB while those on Vallisneria sp. were made at Station D ( F i g . 1) . On a given sampling day, three light and one opaque chamber measure-ments were made for each species. i i i I 10 F i g . 3. Oxygen combustion flask showing: A - platinum carriage holding encapsulated sample, B - stopcock, and C - capillary tube. A 11 The collection of the biomass data was complicated by the hetero-geneous distribution of the plants. Accordingly, with the use of SCUBA, a thorough map was made of the submergent distribution 2 ( F i g . 4) . Samples were then collected along transects within 1 m. quadrats. Great care was taken to obtain extensive samples for each time period and, further, to obtain the roots or rhizomes where possible as these may represent a considerable portion of the biomass. SCUBA greatly fa c i l i t a t e d t h i s , although the productivity of Potamogeton natans may be underestimated because the roots of this plant break very e a s i l y . From these samples wet and organic weights were obtained. The organic weight is calculated by subtracting the ash weight from the dry weight. It is assumed to be the best criterion of biomass since i t i s free from errors due to variable water and ash contents (Westlake, 1963) . Carbon analyses were performed on the four species of macro-phytes i n order to determine the percentage of organic carbon when related to organic weight. A l l the values obtained fitted i n the range published by Westlake ( 1963) , and i t was decided that the value of 47% was representative. This figure, therefore, was used to convert the biomass figures to mg C. The weighing, drying, and ashing methods used agree with those outlined by Westlake ( 1965a) . 12 Fig. 4 Vegetation map of Marion Lake, British Columbia, 1966. The lake was surveyed with the aid of SCUBA. N u p h a r s p . E S 3 P o t o m o g e t o n n a t a n s m m 13 RESULTS The average productivity of the phytoplankton in Marion Lake was -2 -1 8.04 mg O m -day . The average productivity of the submerged macrophytes was 33.2 mg O m 2-day 1, determined by using the 1 4 C method. The organic weight method indicated that the combined productivities of the submerged and floating macrophytes was 18.8 -2 -1 mg C-m -day , this value being obtained by finding the area under the productivity curve shown in Fig. 5. A l l values are corrected for the total lake area. 14 The C method of measuring the productivity of the macrophytes yields consistently higher results than does the organic weight method, but no matter which method is considered, the productivity of the macrophytes is always higher than that of the phytoplankton (Table I) . The values shown in the third column of the table represent the ratio of the productivities as measured by the two methods. Table II records, for each of the species of macrophytes, the _2 i n i t i a l and maximum biomass in g O m of lake surface. A cumulative net productivity is derived for each species at the time when the total organic weight was at a maximum (mid-August) . Although the area covered by each species of plant was comparable, it can be seen that the productivity of the floating aquatics is greater than that of the submerged plants by a factor of ten. A measure of r e l i a b i l i t y of the sampling program used in the f i e l d , and in the laboratory analyses, is given in Tables III, IV, and . V. A standard error equal to 10% of the mean is considered tolerable in most ecological studies (Watt, 1968) and i t can be seen that this value is never greatly exceeded. Some of the vari a b i l i t y in the Vallisneria sp. material may be attributed to the fact that this species grew throughout the time of the study by vegetative means, and it was often a subjective decision as to what constituted an individual plant. There was a unimodal temporal pattern in the primary productivity of the phytoplankton in Marion Lake, with the peak of production occurring in early August ( F i g . 6) . It i s als;o evident that differences in productivity between cl o s e l y occurring days was sometimes con-siderable. Although the productivity of the macrophytes was based on fewer determinations, a peak was observed in late June followed by a steady decline, until by mid-September there was relatively l i t t l e productivity ( F i g . 6) . The temporal variation in productivity values derived from the organic weight data is somewhat clearer. There is a bimodal curve of productivity with a minor peak occurring i n early June followed by a rapid increase in productivity to a major, peak in mid-August. The individual curves for three of the species ( Potamogeton natans, Nuphar sp., and Vallisneria sp.) reflect, with minor variations, the overall pattern ( F i g . 7) . Isoetes occidentalis apparently reverses this pattern, exhibiting a major peak in its productivity in late May and a minor peak i n mid-August. 15 Fi g . 5. Productivity of the aquatic macrophytes i n Marion Lake, April to September, 1966, as determined by organic weight method. 16 Fi g . 6. Comparison of the productivity of the phytoplankton 14 and macrophytes as determined by C method. 17 Fi g . 7. Comparison of the productivities of the four important macrophytes i n Marion Lake, April to September, 1966, as determined by the organic weight method. APR ' MAY J U N J U L AUG ' ' S E P Table I. Comparison of productivity of macrophytes and phytoplankton in Marion Lake, 1966. Blank values indicate that net respiration equals or exceeds photosynthesis. Productivity i n mg C • m • day Date Submerged aquatics Submerged aquatics Ratio Phytoplankton Total Macrophytes C-14 method organic wt. method C-14 method organic wt.method May 28 44.2 4.2 11/1 2.2 10.8 June 10 40.7 3.2 12/1 1.4* 22.9 June 30 73.8 2.4 31/1 2.3* 16.4 July 17 31.5 1.7 17/1 2.2* 21.3 Aug. 4 32.9 2.7 12/1 2.9* 42. 1 Aug. 18 18.1 • 2.9 6/1 2.7* 34.3 Sept. 5 2.2 - 2.1* -Sept. 15 2.6 - 0.8 -* Values marked with an asterisk are from day closest to sampling date. Table II. Distribution and cumulative net productivities of the four species of macrophytes in Marion Lake, 1966. Numbers in parentheses represent number of plants involved in determination. 2 -2 -1 Species Area % Depth Range Biomass (gC/m ) Productivity (mgC• m -day ) of total in meters Initial Maximum Potamageton natans 7.0 0-2.0 0.5 ( 10) 12.2 ( 10) 7.1 Nuphar sp. 5.0 0-2.5 4.2 ( 10) 34.2 ( 10) 13.2 Isoetes occidentalis 4.4 1.0-4.0 0.8(10) 4.2(10) 1.3 Vallisneria sp 5.8 0-3.0 0.4 ( 10) 2.2 ( 10) 0.9 Total 22.2 5.9 52.8 22.5 20 Table III. Variability i n numbers of macrophytes between quadrats of one square meter. Species Number of quadrats counted Average number of plants per m May Sept. Standard error as % of mean May Sept. Isoetes occidentalis 10 11 10.8 8.5 Vallisneria sp. 10 29 45 8 .4 13.6 Potamogeton natans 10 16 12.2 5.1 Table IV. Variability in fresh weights of individual plants . Species Potamogeton natans Number of plants weighed 10 Average Weight (mg) April 2.4 Aug. 10.9 Standard Error as % of mean April 2.1 Aug. 4.7 Nuphar sp. deep 10 667.4 3644.6 2.3 2.8 shallow Isoetes  occidentalis Vallisneria sp, 10 10 10 265.0 2.6 0.4 1812.0 13.6 0.6 3.4 3.5 17.7 0.6 4.5 3.8 Table V. Variability in drying and ashing techniques. Each percentage represents a mean value obtained from ten plants. Species Dry weight as % of fresh Standard Error as % of mean April Aug. April Aug, Ash weight as % of dry Standard Error as % of mean April Aug. April Aug. Potamogeton natans 15.0 14.1 2.8 1.6 8.2 9.9 2.6 1.7 Nuphar sp. 10.5 7.7 9.7 7.3 6.3 6.9 10.9 5.9 Isoetes occidentalis 11.2 10.7 8.1 8.1 17.9 22.0 0.8 2.9 Vallisneria sp. 13.9 13.0 3.6 4.5 24.0 24.9 11.0 2.9 Table VI. Comparisons of the annual mean primary productivity of Borax Lake, California (Wetzel, 1964) to the values for Marion Lake, British Columbia; data in parentheses are -2 -1 values for the growing season and are in mg C-m -day Annual Mean Productivity mgC-m ^ -day Phytoplankton Periphyton Macrophytes Borax Lake 249.3 731.5 76.5 (372.3) Marion Lake <2.0 7.1 (18.8) 24 DISCUSSION Marion Lake may be characterized by a very low le v e l of phyto-plankton productivity and a relatively high macrophytic productivity. The rates obtained in this study are only estimates, but i t is clear 14 that there is agreement between the C method and the organic weight method, as to the relative importance of the macrophytes in the lake. There i s , however, considerable difference between the estimates of the productivity of the macrophytes made by the two different 14 methods . The C method gives an instantaneous measure of net productivity and there is no correction for respiration in the dark. The organic weight method, since i t involves longer periods of time, corrects for rapidly changing environmental conditions which might influence the rate of carbon fixation and also allows for respiration in the dark. This method is considered to give an estimate of net productivity i f the community experiences a marked annual fluctuation in its biomass and if i t suffers few losses from grazing or death (Westlake, 1963) . There is no quantitative evidence from Marion Lake to justify the acceptance of the latter two assumptions, but repeated observations indicated that there was l i t t l e damage done by grazing. According to Westlake ( 1965a) , some aquatic macrophyte communities are scarcely subject to grazing and their production is mostly consumed by bacteria and detritus feeders. This would appear to be the case in Marion Lake, since most of the herbivores found on the plants appeared to be browsing on the attached algae. 25 It is now clear from a number of phytoplankton studies, where the 14 C method has been used, that there is considerable variation in prod-uctivity both within days and between days (Rhodheet a l , 1958) . It is possible, therefore, to explain the lack of a clear temporal pattern in the data, by the infrequent nature of the sampling periods. It is better to sample every day, or where this is not possible, for several consecutive days each month, in order to make a reasonable estimate of productivity. Another source of variability i n estimating the prod-uctivity of the phytoplankton, arises from the fact that the varia b i l i t y between stations at one time is nearly the same as the va r i a b i l i t y at one station and one depth over a period of ten days (Efford, 1967) . The two index stations used for the phytoplankton tend to give somewhat higher values than the stations farther south, therefore giving a some-what exaggerated estimate of the productivity. The same kind of va r i a b i l i t y is inherent in the rates determined 14 by measuring the productivity of the macrophytes with C. These values are further influenced by the probability that, with few replicates, the experimental plants could be i n different physiological states. The productivity of the phytoplankton in Marion Lake, however, is surprisingly low when compared with that of other temperate zone lakes. Recent unpublished data supports the hypothesis that the rapid flushing of the lake (sometimes within two to three days) prevents any substantial increase in the phytoplankton population, thereby reducing the rate of carbon fixation (Dickman, personal communication) . It is very d i f f i c u l t to compare the data from Marion Lake with that obtained from studies on other lakes because of the wide variety of 26 techniques and sampling procedures involved. Westlake ( 1965b) has recently made an appeal to standardize observational procedures and suggests that one of the main functions of the International Biological Programme should be to recommend methods based on common principles and to obtain internationally comparable observations of the basic biol o g i c a l parameters. Of a l l the freshwater productivity studies, there is only one that lends i t s e l f to comparison with the Marion Lake study, that of Wetzel ( 1964) . From his study, Borax Lake is more productive than Marion Lake, but the relative importance of the macrophytic productivity i s greater in Marion Lake (Table VI) . A considerable growth of the periphytic algae, Mougeotia spp. , was observed in Marion Lake throughout the summer, but their productivities were not monitored owing to the dif f i c u l t y of placing the experimental chambers over the algae without displacing them or covering them with bottom sediments . It is possible that the periphyton is very important in fixing energy; its importance in supporting a large crop of herbivores has already been observed. Emergent communities are generally considered to be more productive than submerged ones (Westlake, 1963) . Much of the l i t t o r a l zone of Marion Lake is covered with emergent plants and it is l i k e l y that a consideration of their productivity, as w e l l as that of the periphyton, would increase considerably the total estimated productivity. Organic weight criteria would have to be used as no convenient way has yet been devised to measure accurately the in situ carbon fixation for these plants. 27 LITERATURE CITED Efford, I. E . 1967. Temporal and spatial differences in phytoplankton productivity in a small lake. J. Fish. Res. Bd. Canada, 24 ( 11) : 2283-2307. Goldman, C . R. 1963. The measurement of primary productivity and limiting factors in fresh water with carbon-14. pp. 103-113 in M . S. Doty (Ed.) Proc. Conf. on Primary Productivity Measurement, Marine and Freshwater U.S. AEC Publ. T1D-7633. Hartman, R. T. and D. L . Brown. 1967. Changes in internal atmosphere of submersed vascular hydrophytes in relation to photosynthesis. Ecology 48 ( 2) :252-258. Penfound, W. T. 1956. Primary production of vascular aquatic plants. Limnol. Oceanogr. 1_: 92-101. Rhodhe,W., Vollenweider, R. A. and Nauwerk, A. 1958. The primary production and standing crop of phytoplankton. pp. 299-322 in A. A. Buzzati-Traverso (Ed.) Perspectives in Marine Biology, Univ. of California Press, Berkeley. Steeman-Nielsen, E . 1952. The use of radioactive carbon (C-14) for measuring organic production in the sea. J . Cons. Internat. Explor. MerJJ.: 117-140. Strickland, J. D. H . i 9 6 0 . Measuring the production of marine phyto-plankton. Bull. Fish Res. Bd. Canada, 122. 172 p. Thomas, William H . 1961. Physiological factors affecting the interpre-tation of phytoplankton production measurements, pp. 147-162 in M . S. Doty (Ed.) Proc. Conf. on Primary Productivity Measurement, Marine andFreshwater U.S. AEC Publ. T1D-7633. 28 Vollenweider, R. A. and A. Nauwerck. 1961. Some observations on the C-14 method for measuring primary production. Verh. int. Ver. Limnol. 14: 134-139. Watt, Kenneth E. F. 1968. Ecology and Resource Management. McGraw-Hill Book Co. , New York. 450 pp. Westlake, D. F. 1963. Comparisons of plant productivity. Biol. Rev. 38: 385-425. Westlake, D. F. 1965a. Some basic data for investigations of the productivity of aquatic macrophytes. Mem. Inst. Ital. Idrobiol., _8 Suppl. , 229-248. Westlake, D. F. 1965b. Theoretical aspects of the comparability of productivity data. Mem. Inst. Ital. Idrobiol., 18 Suppl.:,. 313-322. Wetzel, R. G . 1963. Primary productivity of aquatic macrophytes . Verh. int. Ver. Limnol. 15: 426-436. Wetzel, R. G . 1964. A comparative study of the primary productivity of higher aquatic plants, periphyton, and phytoplankton in a large, shallow lake. Int. Revue ges. Hydrobiol. 4_: 1-64. 

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