Some theoretical considerations of thermal discharge in shallow lakes
Heating of freshwater lakes or streams (so on its influence on fish spawning and to study those natural algae that are called thermal pollution) is an incidental behaviour. present normally in smaller amounts but result of many industrial processes, but Much of this work involves the effect on are extinguished due to their lack of mainly of the production of electricity. single species, but thermal discharges adaptation to changing environment. We do In this paper we try to identify the areas of may be important in changing the species not know if the early spring blooms of greatest concern in this problem. We like composition of population, especially diatoms are unimportant for the food to start with a few introductory remarks where this is regulated by competition, chains of lakes, because they appear too about the similarities and contrasts to grazing, and prédation. early for the zooplankton. It is possible thermal pollution of seawater. One may know the lethal temperatures and that they are important because in early The natural cycle of water is on a scale minimum temperatures, and the minimum spring they suppress the blue green algae which is more than sufficient to supply duration of a given temperature for egg and through their indirect effect on the man's needs; about 100,000 kma flows production. And in spite of this, no algae that follow the diatom bloom they downrivers each year and is adequate for prediction can be made on the ecological may ultimately affect the zooplankton. changes induced by thermal pollution in To overcome the need for so many studies freshwater ecosystems. The reason why one might feel inclined to use data from one such predictions cannot be made is because ecosystem for predicting changes in another DR. H. L. GOLTERMAN we lack a coherent theory. one. But in doing so we must be very Limnologisch Instituut We do not even know whether or not laws careful. Rivers with their ever flowing water controlling ecosystems exist. and rapidly changing populations are Rigler (1974) compared in this context clearly distinct from lakes. Tropical lakes predictions made by biologists with those should not be used to predict phenomena made by physicists. Highly accurate in heated temperate lakes, because their predictions e.g. of electrical current can be temperature depends on or is coupled the needs of even the most densely based on Ohm's law, while gas pressure with the irradiance in a tropical lake so that populated areas. Problems do occur in those can be predicted from the laws of Boyle evolution has led to species depending on parts of the world where man tries to live and Gay Lussac. The problems are simple concomitant temperature and irradiance in semi-deserts, and in temporarily arid as the two processes are not related. In an values. areas such as India where there are ecosystem, even a simple one, predictions In an artificially heated lake temperature recurrent seasonal shortages. must be made on photosynthesis of algae, and irradiance are not related; temperature egg laying of zooplankton, and survivorship Water supply problems in other areas are no longer depends only on irradiance as a of these eggs. But these processes are usually a result of using water for waste source of heat. related. Therefore we need theories for disposal too. The need for electricity is Organisms have a long history of evolution. the separate problems and a general theory especially high in populous regions where In this long period of evolution organisms linking the several chains together. It seems the generation of electricity produces have found their way of living in a to me not likely that such a theory will thermal pollution, which is only one aspect certain light and temperature regime, easily if ever be discovered. of the wider pollution and eutrophication sometimes by producing species with plank- One of the essential constituents of such problems caused by industrial, tonic larvae, sometimes by carrying egg a theory must be competition. agricultural and other activities of these sacs, sometimes by adjusting the number Competition is a main feature controlling dense populations. These pollution problems of reproductive cycles in a year. The growth in a natural ecosystem. affect freshwaters to a greater extent than variation in life histories show the answers Phytoplankton species compete for their they do the seas. Thermal pollution in to a large number of combinations of mineral constituents, zooplankton compete particular is more serious in W.Europe environmental factors. for their food etc. Very few, if any studies (except perhaps in Britain) where fresh- In conclusion is seems clear that: deal with the influence of temperature waters are used for cooling than in the on groups of organisms competing with USA where most heat is discharged into 1. There is a great need for basic research each other, and the number of possible the sea — where the heat will be in applied ecology. Ecosystems must be effects and situations that may occur may diluted by currents or tidal activities — or understood, before they can be used often be well beyond the practical limits into rivers in sparsely populated regions. rationally. for study. If for example two organisms are Even in the USA there are problems together in a lake it will make a large 2. There is a great need for education. Man however. Eutrophication of the Great Lakes difference if one or both are in their must realise that any waste of energy is is marked and thermal pollution may make (exponential) growing or in their decline deplorable. Methods of using waste heat the situation worse: these forms of phase. Such a study should be made at least should be sought; electricity made from pollution are probably synergistic. throughout a total growing season and the natural gas or oil should not be used for A lot is known about the influence of numbers of organisms that may be heating as the loss of energy is considerable temperature on growth processes of a large ecologically important may be quite large. and can easily be avoided. number of organisms. Temperature has It has been suggested that only the most for a long time been recognised as a major important organisms should be studied. 3. The aquatic ecosystem should not be biological parameter. There is an explosion How does one define importance, however? considered as a dustbin or a garbage of knowledge on the influence of Those organisms having the greatest biomass collector. Nuclear, or even the larger temperature on one species, on its life are not always the most important. Often conventional plants, should not be built on history, its growth rate, migration, spawning the nuisance algae such as blue green algae the shores of lakes. and on other behavioural aspects. have the largest biomass. Their importance lies of course in the fact that they dominate In the following paragraph theoretical Several reviews exist already in the literature and thus the ecosystem, but one also needs considerations are given for a study on the on the lethal effects of heat disposal and influence of temperature on phytoplankton 20
and zooplankton populations, which 25 50 75 100, together form the basic part of the food DEPTH pyramid in the aquatic ecosystems. At present in The Netherlands a plant is being built that will dispose 300 Cal.cm 2 day1 in Lake Bergumermeer, which has a surface area of only 5 km2 and a depth of about 2 m. The natural irradiance is given in table 1. From this table it can be seen that the 12.5 25 50 100' 'Alk Ik TABLE 1 - Total solar energy at latitudes 50° i OR* North (in Cal.cm-%d-l) DEPTH 50 100 DEPTH 10 .100 Jan. March May July Sept. Nov.
110 310 550 570 350 120 Znd
human heat input is equal to the natural input in March. Owing to the greater losses of heat in December and January the winter temperatures will not actually reach the 25 50 75 100 125 25 50 100 March values, but they will be considerably A
Iz=l0e-ez (1) the photosynthesis is no longer dependent growth and photosynthesis is physiologically on the light intensity, but on nutrient well established. Photosynthetic rate will where I0 = light intensity at surface Iz = light intensity at depth = d supply, for instance the amount of available increase with temperature up to an optimal nitrogen or phosphorus. value. Above that temperature the photo- E = extinction coefficient 2 Z = depth The total photosynthesis per m lake surface synthetic rate will decrease and often much is the summation of the photosynthesis in more steeply than below the optimum The relationship between light and photo- the different water layers, (fig. 3). Tailing (1955) demonstrated that H.20 (9) 1976, nr. 1 21
the influence on photosynthesis is different The temperature may have an effect on the from that on growth rate probably due to depth of the so called compensation point, the influence on respiration (fig. 4). i.e. the point where the light intensity is Other studies are those of j0rgensen and so low that the photosynthesis is equal to Steemann-Nielsen (1965) and Thomas the respiration. This effect which is well (1966), but very little information is known in physiology, (see figure 8) means available on the influence of temperature that in a lake the photosynthetic layer on the growth of algae under nutrient becomes shallower as the compensation limited conditions. point shifts to higher light values while the layer of (dark) 02-uptake becomes deeper. Aruya (1965) worked with natural popu It is not known how nutrient depletion will lations from a pond and showed that high affect this phenomenon. optimal temperatures occur at high water temperatures (fig. 5). Figure 2 - Limiting factors for algal growth as A close approximation in the warmer function of time and place (Takahashi and Nash, 3. Effect of temperature on algal periodicity months and discrepancy in the colder 1973). months was demonstrated indicating that Most of the primary production in lakes is in the warmer months temperature increase performed by unicellular algae (sometimes may have a strong inhibitive effect. From arranged in colonies), which even in p. -- I i i nature may reach high growth rates; division the high values for the photosynthetic rates 35 - it can be deduced that this phytoplankton (doubling) times of once a week or even population was not growing under nutrient higher will often be found. 30 - limiting conditions. - The population reach their maxima in a few weeks; after that the population Eppley (1972) reviewed the influence of the normally disappears equally rapidly. temperature on phytoplankton growth in 25 - As an example, the periodicity of the sea and found that temperature set an Nostoc-»/ k Asterionella jormosa will be discussed upper limit on the growth rate and on the / & \\ ÜO briefly (Lund 1949, 1950) (See figure 9). rate of photosynthesis per weight of P M Starting in February, the cell number chlorophyll. The upper limit of the increases exponentially following the temperature effect on the growth rate is 15 • - formula: given in figure 6, which shows much dN variation in the specific growth rate. 10 The drawn line indicates the upper limit, J U - = iiN (6) CMorello-»/, an approximate equation is dt ô\ logio n = 0.0275 T — 0.070 5 - - .s'y where N = number of cells where /i = doublings per day (see below). I«, = division per day Other factors such as light and nutrient t P"t i i i i i i , , I It seems likely that the start of growth is supply overrule the temperature reduction 10 20 30 40 Temperoture, "C, mainly due to the increasing light quantity of the rates of growth (see figure 7). (cal.day-1). Although temperatures from Temperature may have a further effect on February till May become higher as well, growth rate as it also influences the rate Figure 3 Temperature relations of photosynthesis in Nostoc muscorum and Chlorella pyrenoidosa the growth rate remains linear if plotted of the nutrient supply by mineralization (from Clendenning et al 1956). on a log scale. It seems therefore likely (see subsection 4). that the growth rate is nutrient dependent In natural lake water the influence of as stated above for photosynthesis. As soon F!-c*esynthetic rate Relative qrowth rate temperature is obviously complex although (j-;r,0,/tO\cll,hr) ,1 (d^.mom/da,) as silica is depleted, growth ceases; the there does seem to be an upper limit set to maxima is controlled by the silica concen algal growth by temperature. This tration in winter, or effect (figure 6) could be described as dN K —N a temperature limited upper growth rate, = ßS. (7) which in nature is seldom reached due to dt N the depletion of nutrients. It could be that this effect plays a role in the transistory where K = the maxima number that N can layer between light and nutrient limitation reach. but very little evidence for this hypothesis K can be calculated from the winter SiOa concentration (10e cells = 0.14 mg SiOa). is available. Takahashi and Nash (1973) -J studied the influence of temperature on Relative liqhr intensity It can be seen from figure 9 that the cells natural populations by plotting photo Figure 4 - Comparison of the variation, recorded disappear in roughly the same time that synthesis at different dates against the under laboratory conditions, of the photosynthetic they took to develop. The disappearance temperature; their results are already shown rate and the relative growth rate of cultured rate again follows an exponential relation Asterionella population in relation to light and is therefore different from the cease of in figure 2. It may be argued however, that intensity and temperatures. Each relative growth temperature was not the only other para rate indicated is derived as a mean from two bacteria growth, which is often described meter in the different samples, and that duplicate cultures. (Tailing 1955). by the formula: photosynthesis and temperature might both dN . = /3N — N2 (8) be dependent on another factor, e.g. regulated by mineralization as well as by r dt nutrient supply which well may be photosynthesis. This aspects will be discussed dependent on the season. Algal growth is in subsection 4. where y = inhibition rate y « ß 22
TEMPERATURE 'C
Figure 7 - Growth rate vs. temperature relationship predicted by the Baly equation as used by Riley, Stommel and Bumpus (1949). Three different levels of total radiant energy are included for the Baly equation: 1.0 (circles), 0.53 (triangles), and 0.05 lyjmin (squares). The line of maximum Figure 5 - Seasonal changes in photosynthesis temperature curves (solid lines) and respiration tempera expectation predicted by Equation (1) is drawn for ture curves (dashed lines) of phytoplankton from Shinjiike Pond. Photosynthesis was measured at comparison (no symbols). 15200 lux by the Winkler method from April to November and by the C14 method from December to March.
(I) P [high temp .ample COj)
/ (3) P (high temp, little CO*)
/ ^^"^ (2) P (low temp, ample CO2)
/y< flfhigh temp)
P Mow temp.) A
Figure 8 - Influence of temperature on compen sation point (Rabinowitch, 1951).
cells killed per unit time would thus be proportionnai to the number itself and not to its second power. After the Asterionella growth either Fragillaria or Tabellaria appear. These have been growing at the same time TEMPERATURE °C but could not compete with Asterionella. Figure 6 - Variation in the specific growth rate (n) of photoautotrophic unicellular algae with Lund (1964) has explained how Tabellaria temperature. Data are all for laboratory cultures. Growth rate is expressed in doublings Iday. From cannot compete with Asterionella due to Epply 1972). the lower growth rate of Tabellaria, while Fragillaria, having the same growth rate Death rate of the Asterionella population high temperature during the period when cannot compete due to the low number must therefore be described by the algae are severely nutrient limited of overwintering cells. (Lund 1965). The fact that the disappearance dN Competition can be described mathemati must be described with equation (9) after = S N (9) cally as follows: dt equation (8) might be explained by assuming that cells are killed in a period that they dNi KI — Ni — «Na The death of the cells is caused by the are present in the upper water layers where 0iNi (10a) combined action of high light intensity and light might be harmful. The number of dt Ki H20(9) 1976, nr. 1 23
1 ' ' | ' ' 44*- ' ' I ' ' ) ' ' I ' ' 1 'I I ' ' 'I ' ' I ' ' 'I ' I' ' ) ' ' 14^- 1 " I ' ' 1 ' 'I
Figure 9 - Periodicity of Asterionella formosa and .silicain Windermere and Esthwaite Water (Lund 1949, 19750). Solid line: number of live cells per ml Upper series: Esthwaite water Interrupted line: nitrate nitrogen (mg.l—1 x 10) Middle series: Windermere South Basin Solid black: SiO^ cone. (mg.l~y) 0.5 mg.l—1 cross hatched Lower series: Windermere North Basin
dNo K2 — N2 — yNi Temperature will have a more dominant occur, which gradually will be replaced by B3N2- effect on the outcome of this type of blue greens (See figure 10). dt K2 competition than can be explained by its (10b) influence on the growth constants themselves. Competition is believed to be one of the where « and y are competition coefficients It is hoped to demonstrate these effects most important factors regulating this (Slobodkin 1961) with simplified computer models. In shallow succession and it seems likely that changes It can be shown that survivorship is eutrophic lakes the normal algal successions in spring temperature of 2-3°C will Ki are often as follows: after the diatoms drastically change this succession pattern: controlled by a, ß, y and . spring bloom peaks of Chlorococcales disposal of heat will have a synergistic Ko (often Scenedesmus and or Pediastrum) effect to that of eutrophication. 24
4. Influence of temperature on 20000 mineralization After the death of the algal cells they will be rapidly mineralized by bacteria (Golter man, 1973). The bacterial mineralization is 15000 • strongly dependent on the temperature. In figure 11 thetemperatur e effect onth e ammonia production from dead algal cells by natural populations is demonstrated. 10000 The algal cells were killed by autolysis and suspended in lake water at different temperatures. Cells were also suspended in lake water that had been heated at 65° C to 5000 kill protozoa. It canb e seen in figure 11 that protozoa didno t contribute toth e mineralization and that the temperature has its effect viath e bacteria. In figure 12 it can be seen that a single strain of JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Pseudomonas sp,whic h digest the algal cells andrelease s ammonia, shows the same Figure 10- Periodicity of dominant algae in Tjeukemeer in1970 temperature effect asth enatura l population. Pennate diatoms number of cells/ml It has been stated above that the growth Centric diatoms excluding Melosira number of cells /ml rate of the phytoplankton is limited by Melosira, number of cells /ml nutrient supply. More rapid mineralization Scenedesmus number of col/ml therefore automatically means a higher growth rate, butno ta higher standing crop. Nevertheless the diurnal oxygen variation in eutrophic shallow lakes, which often % shows large supersaturations in day time _____— — and a decreased oxygen concentration at light will show more pronounced differences. This will be due,no t only to increased
/ S \\ ••* bacterial mineralization, but also by \ S increased algal respiration. f / / / ,• Both respiration and photosynthesis will increase with rising temperature, but often /l / ''/ -•-" photosynthesis will reach its optimum at a / // I '' -y lower temperature than will respiration / / .--" .--• (see figure 13). This effect is probably 'I / --•-•' ..---" more important inth e competition between // ,••-.. different algal species than the temperature [/'••''''""' • \ effect on photosynthesis alone. It seems likely for instance, that the optimum Figure 11 - Ammonia produced from 'leached' algal cells after suspension in lake water at different temperatures o o: no treatment; • •: lake water heated to 65°C for 1 hour; : temperature in theP- R curve for diatoms 30 °C; : 20 °C; : 10°C (Golterman '73). will be exceeded byth eincreasin g spring temperatures andtha t these will then approach the optimum for blue-green algae. From this and other factors the replacement of diatoms by blue greens seems to be a predictable effect of artificial heating. Sorokin (1971) warned that with increasing temperatures theaquati c habitat will notb e automatically colonized by high temperature phytoplankton butprobabl y by blue green algae andsuggeste d rather optimistically that artificial restocking of such habitats with high-temperature algae of less nuisance species would be feasible and suggested the establishment of a collection of high temperature algal strains. Disregarding problems during thewinte r period it should be realized that the preparation of the inoculum for a relative small lake would Figure 12- Release of ammonia from 'leached' Scenedesmus cells during mineralization in lake involve several thousands of liters of algal water (o o) or in cultures of P. boreopolis (* •) at three temperatures; : 30 °C; cultures. : 20°C; :10°C. H20 (9) 1976, nr. 1 25
o S/C T • HAUPltEN •C0PCP0DI7C ^ 2.0 10
«•
6- *
10 20 —I —I— —1 1 TEMPERATURE, *C. 70ol 900 1 1000 1100 1200 1300 1*00 STUNDEN 10 50 TA8E Figure 13- Influence of temperature on photo Figure 14- Development time of different stages of Eudiaptomus gracilis (Eckstrin, cited in synthesis and respiration of Ramalina farinacea Nauwerck, 1963). (Rabinowitch 1956).
Very few — if any — data exist on the influence of increasing temperatures on competition and growth (photosynthesis— respiration) of natural phytoplankton populations. This will be more difficult to study than measuring theinfluenc e of temperature on isolated samples oflake - phytoplankton. Practically, the only way to study rather subtle effects is inlarg e experimental ponds built inth e lake itself, which can be kept ata nelevate d tem perature fora long time, orb y comparing an artificially heated lake with similar lakes in the same area.
5. Influence onzooplankto n Figure 15- Development time of different stages of Eudiaptomus graciloides inlake Erken. (Nauwerck, 1963). The same effects oncompetitio n between zooplankton species and onne t growth (assimilation —respirator y losses) of these t organisms may be expected as have been 26 described forth e phytoplankton. Again, 24 a small increase of the temperature may 22- completely change theoutcom e of compe EUDIAPTOMUS ORACILOIDES (ERKE N) 20' CRACiLis (SCHLUCHSEE.nach ECKSTEIN) tition and through this change, secondary ( BODENSEE,nac h ELSTER ) effects may affect the phytoplankton 18 again, asman y examples are now available 16 of the specificity of grazing ofth e zooplank 14 ton. 12 A special feature with the zooplanktonha s 10 to be taken into account as well; that of the e influence of temperature onth e life cycle e of the zooplankton. During the summer, 4' eggs develop via different nauplii and 2' copepodite stages into fertile females, which —1— 1 —1 1 1 1 200 I 300 400 PM 70o| 600 S00 I 1000 STUNDEN will produce eggs again. 10 40 TAGE Temperature has a major effect on the Figure 16 -Development time of different stages of Eudiaptomus gracilis and Eudiaptomus graciloides duration of each stage. Examples are given in different lakes (Nauwerck, 1963). in figures 14, 15, 16 which show how consistent this effect is. as theshortenin g of the different stages periodicity of both phyto- and zooplankton Eudiaptomus graciloides follows the same will be additive, and number of generations will bedisturbed , it seems possible that the type of curve inLak e Erkena s per year will increase. Consideringth e production of specific food forsmalle r Eudiaptomus gracilis in Schluchseean d influence of temperature on primary stages (e.g. nauplii) may take place at the Bodensee. Other examples can befoun d in production it does not seem likely that the wrong time. It also seems likely that in the Nauwerck's paper (1963). Furthermore it primary production will be increased,s o evolution of dominant populations in a can besee n how anincreas e of 5°C will that more food for the extra generations given lake, these timing effects of the food considerably shorten the whole life cycle will be present. Furthermore, as the natural supply will be important. The fact that one 26
physical factor (temperature) will be selection of dominant species. The influence western North Atlantic, Bull. Bingham Oceanogr. changed without the naturally controlling will have a synergistic effect with Collect. Yale Univ., (1949), 12 (3), 1-169. one, (irradiance) is a pollution different eutrophication, both stimulating the pro 13. Slobodkin, L. B., Growth and regulation of animal populations. New York, Chicago, etc., from the nutritional effects of sewage for duction of blue green algal blooms. Holt, Rinehart and Winston, 1961, (1961), 184 pag. example, in which the nutrients are often 3. Increasing temperatures will increase 14. Sorokin, C, Calefaction and phytoplankton. in balance for algal growth, so that the Bioscience, (1972), 21 (23), 1153-1159. the number of generations of zooplankton whole community will be influenced to the 15. Takahashi, M. and Nash, F., The effect of per year, without increased food production. same extent. The effect of temperature on nutrient enrichment on algal photosynthesis in The effect on overwintering stages of the life cycles can be more easily demonstrated Great Central Lake, British Columbia, Canada, fauna may be detrimental, ultimately Arch. Hydrobiol., (1973), 71 (2), 166-182. than the effect of temperature on leaving open niches in the ecosystem. 16. Tailing, J. F., Relative growth rates of three phytoplankton by comparing warm with Other predator-prey relationships and plankton diatoms, Ann. Bot., (1955), 19, 331-341. temperate lakes because the phytoplankton competitive processes will be disturbed. 17. Tailing, J. F., The phytoplankton population will also be influenced by the different as a compound photosynthetic system, New erosion patterns in tropical environments. 4. The above mentioned points together Phytol., (1957), 56, 133-149. It is known however that warm water lakes with better known effects such as influence 18. Thomas, W. H., Effects of temperature and produce smaller zooplankton organisms with on spawning and migration and other illuminance on cell division rates of three species shorter life cycles and that higher respiration of tropical oceanic phytoplankton, J. Phycol., physiological or even lethal effects, make (1966), 2, 17-22. losses do indeed occur, but total produc heat disposal not permissible if this causes 19. Vollenweider, R. A., Calculation models of tivity tends to be higher. temperature elevations larger than those photosynthesis-depth curves and some implications If the shortening of the life cycle occurs occuring naturally. Attention should be regarding day rate estimates in primary produc in the winter too, a rather special problem given especially to winter temperatures as tion measurements. In: Proceedings IBP Symposium on 'Primary productivity in aquatic could develop. Normally the populations the tolerance of the ecosystem is smaller environments', Pallanza (Italy), April 1965. survive the winter as 'winter eggs' than during summer. Is: Mem. 1st. Ital. Idrobiol., (1965), 18 suppl., ('Dauereier). If these should start 425-457. developing e.g. in December or lanuary only a small quantity of food will be present References • • • so that the population might starve. 1. Aruga, Y., Ecological studies of photosynthesis The recruitment for a new population would and matter production of phytoplankton, Bot. then depend on migration from other Mag. Tokyo, (1965), 78, 280-288. lakes in the same area, where the over 2. Clendenning K. E., Brown T. E. and Fyster H.C., wintering populations were not affected. Comparative studies of photosynthesis in Nostoo muscorum and Chlorella pyrenoidosa, Can. J. Bot., It is possible that in the Frysian district (1956), 34, 943-966. this migration would not be a problem as 3. Eppley, R. W., Temperature and phytoplankton all lakes are interconnected through open growth in the sea, Fish. Bull., (1972), 70 (4), canals, and there are no differences in 1063-1085. elevation. 4. Golterman, H.L., The role of phytoplankton in detritus formation. In: Proceedings IBP-UNESCO If this migration did not occur for some Symposium on 'Detritus and its role in aquatic reason this would leave the lake with an ecosystems', Pallanza (Italy), May 1972. Is: Mem. open niche and the introduction of a new 1st. Ital. Idrobiol., (1973), 29 Suppl., 89-103. species would be possible. This introduc 5. Jorgensen, E. G. and Steemann Nielsen, E., tion may have a favourable or a harmful Adaptation in plankton algae. In: Proceedings IBP Symposium on ,Primary productivity in effect (introduction of a vector for a new aquatic environments', Pallanza (Italy), April 1965. disease). It should be realized that several Is: Mem. 1st. Ital. Idrobiol., (1965), 18 Suppl., organisms that occur only in warm lakes do 37-46. not occur in cooler lakes as they have no 6. Lund, J. W. G., Studies on Asterionella. I. The possibility of overwintering, because the origin and nature of the cells producing seasonal maxima, I. Ecol., (1949), 37 (2), 389-419. water temperature falls below 4°C. 7. Lund, J. W. G., Studies on Asterionella formosa Similar effects on life cycles (especially Hass. II. Nutrient depletion and the spring during winter) may occur for benthic maxima, J. Ecol., (1950), 38 (1), 1-35. organisms such as Chironomids and even 8. Lund, J. W. G., Primary production and on fish spawning. If this happened in periodicity of phytoplankton. Verh. int. Verein, conjunction with the timing effect on food theor. angew. Limnol., (1964), 15, 37-56. production serious disturbances in several 9. Lund, J. W. G., The ecology of the freshwater food chains would occur. phytoplankton. Biol. Rev., (1965), 40, 231-293. 10. Nauwerck, A., Die Beziehungen zwischen Zooplankton und Phytoplankton im See Erken, Summarizing remarks Symb. bot. upsal., (1963), 17 (5), 1-163. 11. Rabinowitch, E. I., Photosynthesis and related 1. Increasing temperatures may effect net processes. 2 vols. New York, London, 1945-1956. growth of phytoplankton positively or Vol. 2, part 1, Spectroscopy and fluorescence of negatively depending on the species. photosynthetic pigments, kinetics of photosyn thesis, 1951. Vol. 2, part 2, Kinetics of photo Mineralization of dead phytoplankton will synthesis, 1956. occur more rapidly, and may lead to faster Rigler, F. H., 1974, Contribution to XIX SIL growth rates, but does not necessarily lead Congress, Winnipeg. Verh. Int. Verein, their angen. to higher biomass. Limnol. 19, in press. 12. Riley, K. A., Stmmel, H. and Bumpus, D. F., 2. Larger effects may be expected on the Quantitative ecology of the plankton of the