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Some Theoretical Considerations of Thermal Discharge in Shallow Lakes

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Some Theoretical Considerations of Thermal Discharge in Shallow Lakes 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<in%An A(i n %An higher than normal and ice will disappear. No estimates of possible winter Figure 1 - Relationship between photosynthesis and depth temperatures seem to have been made. A. Relationship between light intensity and depth The lake is alkaline and rich in humic B. Relationship between light intensity and photoassimilation substances like most in this district and is C. Summation of curves A and B (lin scale) comparable with Lake Tjeukemeer, which D. Summation of curves A and B (log scale) is the IBP site of the Limnological Institute in the Netherlands. The lake is rich in synthesis can be described by a normal 2 A = Az dz (4) phytoplankton (Chlorophylla concen­ saturation curve (see fig. 1). 3 (Where 2A = integral photosynthesis trations reach levels of about 100 mg.m~ al g.m-2 • t"1) in summer). 7 A — A„ (2) As the energy for the whole aquatic 2 Az = A at depth Z. / 1+ (aI z) ecosystem enters foodchains primarily via where A = photoassimilation or the surface between the y-axis and the the phytoplankton and then dispersed via A = Light saturated photo- A-depth curve. the zooplankton or bacteria, the theoretical max assimilation The mathematical solution of (4) is effects of heat disposal on phytoplankton 1 and zooplankton populations will be 2 A = ZiAmax (5) a = — discussed in this paper. It is hoped to 1 where Z = depth of Y2 Ik (Vollenweider demonstrate the effects in future research Ik Ik = light intensity where 1965).
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