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Chapter 1 General Introduction
CHAPTER 1
GENERAL INTRODUCTION Limnology can be defined in two ways, either as aquatic ecology or as the science of freshwater systems or “inland oceanology”. Limnology - from the greek prefix “limn-” meaning “marsh” - is an interdisciplinary science combining aspects of hydrobiology, hydrochemistry, hydrophysics and geology. Odum (1971),envisaging the structure of biology as a cylindrical block, considered disciplines such as entomology, microbiology, botany to be vertically cut wedges and taxonomy, physiology and ecology to be horizontal layers, of which ecology is the basic one. Aquatic ecology is thus the science of mutual relationships between organisms and of interactions between organisms and their environment. In this respect limnology is very much like ecology in general. There are differences in the nature of the ecosystem, however. The aquatic ecosystem is usually a closed one. Organisms cannot normally leave it and, with the exception of some insects, are confined within it throughout their entire life cycle. This closed structure results from the fact that watercombines the functions of soil, air and water of the terrestrial ecosystem. In lakes and rivers all organisms use water for their transport, either actively as in the case of swimming fish or passively in the case of some algae. Algae take their nutrients, both minerals and C 0 2 , from the water, and release the products of their metabolism back into the water, either immediately or eventually during post-mortem mineralisation. Other organisms hunt for their food through the water and excrete their waste products into it. Heat and light are transmitted through the water which, due to its physicochemical properties, absorbs them more strongly than does the air. Great diversity - the occurrence of a large number of different organisms - is a feature common t o both the terrestrial and the aquatic ecosystem. In the aquatic habitat high turnover rates of individual organisms and also of chemicals result in a rapid succession of types of organisms, this effect being called the periodicity of phyto- and zooplank€on. These high turnover rates and periodicity effects are due to the fact that the great majority of the organisms in the system are of only microscopically visible size and also because of the variations in the physical and chemical environment which result from seasonal changes. In a study of the aquatic ecosystem it is useful to subdivide it into several component features. Because this subdivision will be used throughout this book, the general outline is given here. Abiotic features. These include physical phenomena such as radiant energy, temperature and other aspects of water physics such as density, viscosity, sur-
2
face tension etc., together with properties of the water molecule itself which result in the peculiar characteristics of water such as its freezing point and certain of its chemical properties. Also included here are the chemical elements and compounds which are dissolved or suspended in the water and which enable most natural waters t o support life. The suspended matter is termed allochthonous if it originates from sources outside the lake or autochthonous if it derives from sources within the lake system. Abiotic factors are rarely uniform with depth. Irradiance decreases with depth, and temperature profiles often show layers of relatively rapid change. It is often useful t o consider effects as occurring in a water column of e.g. 1m 2 . Loadings can be expressed in quantity per volume or per unit surface per year. The loading per surface is the loading per volume times depth. If biological processes must be expressed as units per surface, but have been measured per unit volume, then the unit per surface can be found by summing results in all layers, or mathematically by integrating the function describing the process over depth. Photosynthetic yield can then be expressed as e.g. mg mP2dp1 of C and can thus be related to incoming light, which is expressed in e.g. cal. c m p 2 C 1 , or t o fish yield, which is expressed in kg ha-' yr-' . Primary producers (see Plates 1-111). Primary producers are those organisms which use solar radiation as their only energy source and in this context include the macrophytes, the algae, and the photosynthetic bacteria. Except within the littoral zone of lakes, the algae are quantitatively the most important of these groups. Many algae of the open water are unicellular but multicellular colonies and filaments d o occur. Morphological and functional differentiation into colonies is fairly common amongst the multicellular forms, but no highly differentiated organs such as leaves, stems, or roots are found. In some colonies, e.g. Pediastrum, the outer cells have a markedly different shape from the inner ones, and this feature imparts to the colonies an enlarged surface area which allows them t o float better and increases the efficiency of gas exchange and nutrient uptake. In all colonies or filaments each single cell retains the capacity for reproduction (cell multiplication). The algal groups mentioned below will often be referred to throughout the different chapters of this book, amongst these the diatoms, the green algae and the blue-green algae are quantitatively the most impohant groups. A more detailed list of most genera mentioned is given as Appendix I of this book. The diatoms (Bacillariophyceae) contain both chlorophyll and brownishgreen pigments which often mask the green chlorophyll colour. When wetted with ethanol or methanol they turn green. The silicified cell wall is characteristic, and it may easily constitute more than 50% of the dry weight of the organism. (The structure of the cell wall is described in more detail in Chapter 7.) The skeletons are often beautifully ornamented with transverse lines, projections, or depressions, the detailed patterns of which provide a basis for spe-
3
cies recognition. They remain identifiable in stratigraphic deposits of sediments of great age and enable details of species successions and inferred environmental changes to be discerned, often for a long time span in the history of a lake. The diatcms commonly occur during spring time in temperate waters, either as.free-floating cells or chains (filaments) or attached to plants or stones. They are divided into two broad groups: Centrales, with valves usually circular, and Pennales, with elongated valves. The freshwater species of the genus Melosira are probably amongst the commonest diatoms occurring in Lake Baikal (Siberia) and in shallow ponds or ditches. The green algae (Chlorophyceae) in which the chlorophyll is not masked by other pigments are bright green and have rigid cell walls made of celluloselike carbohydrate. A common genus is the flagellate Chlamydomonas, a freeswimming single-celled organism with two flagella and an eyespot; i t can develop rapidly in ditch water which has been left in the laboratory or classroom. Volvox is one of the genera which forms the largest colonies, sometimes with 20 000 cells. Beating of the flagella, which are possessed only by the outermost cells of the Voluox colony, enables this organism to move by slow rotations through the water. One of the smallest algae is Chlorella, a single cell. It is often used in physiological studies because it can be easily maintained in culture and because of its rapid growth rate. Cladophora, Oedogonium and Spirogira are familiar green filamentous algae growing on stones, poles, etc. Asexual cell division is the commonest method of reproduction by green algae. Sexual reproduction, which occurs usually in adverse conditions, is often rather complicated. The blue-green algae (Cyanophyceae or Myxophyceae) are the “simplest” of all algae and are perhaps more closely related to the bacteria with their procaryotic cells. They also resemble bacteria in some of their chemical properties. Their characteristic colour is due to the pigment phycocyanin but otherpigments can also occur and impart a violet, brown, or yellow colour to any species possessing them. The pigments are evenly distributed throughout the protoplasm, although some infrastructure has been discerned by use of the electron microscope. The cells have no nucleus and reproduce by simple cell division. Certain schools classify them as bacteria and not as algae. One of the more easily recognisable genera is Microcystis, which forms masses of free-floating cells readily visible to the naked eye. The cells multiply abundantly and may form a scum on the surface of smaller lakes or bays. Oscillatoria is a filamentous type and occurs in large quantities during the summer in many eutrophic lakes, thus causing many problems in water management. Other genera which commonly cause blooms of nuisance proportions are Nostoc and Anabaena. Unlike most autotrophic organisms, many of the bluegreen algae are able to use N2 in addition to, or instead of, the more usual NH; or NO, ions, a property referred to as nitrogen fixation. (They are discussed in detail in Chapter 6.) Other important groups are the Cryptophyceae, the Dinoflagellatae and the Chrysophyceae.
4 Consumers. These organisms derive their energy from other organisms. Included in this group are the zooplankton (Plate IV), the insects, fish, etc. The main groups are outlined in Chapter 15. Primary consumers are those consumers that feed on the producers, whereas the secondary consumers feed on the primary ones. Terminal consumers are those organisms that are not used by any other organism, except after their death. They consist mainly of the larger fishes, which are also called predators, although this word is often used for any organism that actively hunts its prey. (Feeding relations between aquatic organisms are outlined and discussed in Chapter 15.) Decomposers. These are the microorganisms that mineralise all dead organic material. Included in this category are bacteria, fungi, and certain yeasts. Their main function in the ecosystem is to mineralise the organic matter back into simple inorganic compound which are then recycled. Strictly speaking, decomposers are consumers, but since they mostly digest dead organisms without ingesting them, it is useful to distinguish them. Each organism has a habitat, i.e. the type of place where it occurs. For example, a young trout has the cool mountain stream for its habitat. Each type of organism is also said to live in a niche, which could be described as its physiological habitat, i.e. the “place” where it can carry out its essential physiological processes. In this context the word “place” should not be taken only in its physical sense but also as place in a food chain, a place restricted by tolerance limits and historical and geographical factors. An association of plants and animals is called a community or biocoenose. A biotope is the geographic entity in which a biocoenose can be found. In the present state of limnological knowledge, quantitative investigations are important since many aquatic processes must first be qua-tified in order to understand how they work. Although qualitative descriptions of organisms and processes are necessary as a first stage, the aim of many current studies is to achieve an understanding of the nature and rate of changes, the quantitative interrelationships in competition and the rate of predation. As a result of the study of factors affecting the regulation of quantities in biological yields and rates, two important principles have been formulated: the law of Liebig (1840) and the law of tolerance (Shelford, 1913). Liebig’s law of the minimum factor states that the yield of a plant crop is determined by that element of which relatively the smallest quantity is available for that plant. If e.g. a wheat crop could be as large as 1000 tonnes based on the amounts of C, N and other elements present, but only 500 tonnes, because no more iron is available, then 500 tonnes it will be. A minimum factor is nowadays often called a limiting factor, i.e. the factor which controls the rate of a process such as the growth of an algal culture. (The difference is extensively discussed in Chapter 10.) Much confusion is caused because the two concepts are muddled. Results of an experiment in which growth rate was
5 measured are often used in terms of, or to predict, the total yield of the ex-. periment. Thus in the study of eutrophication (Chapter 17), the results of a 4-h bottle test for measuring the photosynthetic rate, are often used to predict the density of the final algal crop. (Growth rate and yield are discussed in Chapter 10.) The mere presence of a substance may not necessarily allow growth of an organism. The compound must be present in such a form that the organism can obtain and utilise it, that form being said t o be “available”. The second principle in quantitative ecology was called by Shelford.(l913) the law of tolerance. Factors are not necessarily present only in a minimum quantity; they may in fact be present “in excess”. Many organisms can only tolerate certain quantities of some substances, and larger quantities than the tolerance level may be toxic and the growth rate may decrease. There is often an optimal quantity of any substance which permits the greatest growth of an organism and the presence of lesser or greater quantities than this both result in decreased growth rate or yield. As an example, in Fig. 1.1growth rates of Chlorella and Nostoc are shown as a function of temperature. It can be seen that growth takes place above a minimum “threshold” temperature, T,, ,and below a maximum temperature, T,,, . At one temperature within this range growth is optimal. Above the op-
30
-
25
-
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9
L a
c Y
F!
15-
Fig. 1.1. Temperature relations of photosynthesis in Nostoc muscorum and Chlorella pyrenoidom. (From Clendenning et al., 1956.)
6 timum “the excess is tolerated”. It should be noted that growth optimum curves are seldom bilaterally symmetrical. The decrease in growth is frequently much steeper above the optimum than below it. Temperature optima are often quite different for different species of alga; they determine relative yields of different species in different circumstances and operate to produce certain features of competition and succession. For example the diatom Fragilaria crotonensis has an optimal temperature of 13-16” C. WesenbergLund (1904) found that in the cold summer of 1902 this species was not replaced by blue-green algae as was normally the case, the blue-green algae having a higher optimum temperature than the diatom. Competition as one of the factors in algal succession is probably much more complicated than this example suggests (see Chapter 14). A similar effect may be seen in the response of organisms to different pH conditions (Fig. 1.2). E.g., the green alga Scenedesmus cannot grow at pH = 4 but can grow excellently even above pH = 11, while Chlamydomonas may thrive at pH = 4. Therefore, in a mixed community or culture the pH can have a selective effect, being optimal for one species at a given value, whereas it is minimal for another one. The pH has been shown to rise to high values during bloom conditions (e.g Tjeukemeer, Loch Leven; subsections 14.2.2 and 15.5). It may therefore be a factor through which one algal species may inhibit other ones. Complications arise here also because the pH is not a property in itself, but is related to the CO,--HCO,-CO~system, so that, if the pH changes, the whole balance of the carbondioxide/bicarbonate system is automatically changed. As a final example a harmless compound such as phosphate may be men-
PH
Fig. 1.2. The pH stability relations of photosynthesis in Nostoc muscorum. (From Clendenning et al., 1956.)
tioned. Rodhe (1948) noted that Asterionella formosa is never found in lakes with a PO4-P concentration above 10 mg m-’ . Even in his cultures no growth took place at or above this phosphate concentration, but it is still difficult to prove that phosphate actually inhibits growth. In order to prove this, one must have a good growing culture and measure inhibition following the addition of further quantities of phosphate. If, however, the PO4-P optimum lies near 2 or 5 mg m-’ , it is rather difficult to measure growth accurately because of the small quantity of algae which is present at that concentration. The situation may actually be more complicated in lake conditions and when Asterionella was found to disappear from Lake Windermere when phosphate increased above 10 mg m-’ ,it seemed too good to be true. Possible explanations in this case could be that phosphate concentrations above 10 mg m-’ inactivated the iron by forming insoluble FePO, . Recently Moed (1973) achieved a good growing culture of Asterionella formosa with PO4-P = 1 g m-’ ,but it could be argued that this may be an adaptation to unnatural conditions. Growth rate measurements of this culture as a function of the phosphate concentrab tion may provide convincing proof of the regulating role of phosphate for this species of alga. Limnology may be regarded therefore as a constant attempt t o explain field observations by measuring the isolated process both in natural conditions and in culture. A culture is an oversimplified model, but nevertheless its use can indicate whether a process is possible or not, while sometimes - if one is lucky information about the rate can also be obtained. One should be extremely cautious, however, in attempting to use rates obtained from experiments in culture (or in vitro as this is often referred to) as a basis for predicting or describing rates occurring in natural conditions. The aims of ecological research are twofold. It is done firstly as an attempt to satisfy some of our curiosity about our environment, and secondly to attain understanding and perhaps control over part of that exciting but often unfrienc ly environment which relates directly to man’s life and well-being. This may en able us to utilise our natural resources more effectively and may prevent us from harming them. The splendour of limnological research is that both of these aims can be pursued simultaneously, and perhaps there are few other scientific disciplines in which pure and applied research are so strongly interwoven.
REFERENCES Clendenning, K.A., Brown, T.E. and Eyster, H.C., 1956. Comparative studies of photosynthesis in Nostoc muscorum and Chlorella pyrenoidosa. Can. J. Bot., 34: 943-966. Mued, J.R., 1973. Effect of combined action of light and silicon depletion on Asterionella formosa Hass. Verh. Int. Ver. Theor. Angew. Limnol., 1 8 (prt. 3 ) : 1367-1374. Odum, E.P., 1971. Fundamentals o f Ecology. Saunders, Philadelphia, Pa., 3rd ed., 5: 4 pp.
Rodhe, W., 1948.Environmental requirements of fresh-water plankton algae. Experimental studies in the ecology of phytoplankton. Symb. Bot. Ups., lO(1):149 pp. Shelford, V.E., 1913.Animal Communities in Temperate America. University of Chicago Press, Chicago, Ill. Wesenberg-Lund, C., 1904.Studies over de Danske s@ersplankton. Specielle del. Copenhagen.
PLATE I (p. 9) Blue-green algae (photographs by J.W.G. Lund) 1.Anabaena flos aquae. 2. Anabaena flos aquae, detail; h , heterocyst; s, spore (akinete). 3. a. Microcystis aeruginosa. b. Gomphosphaeria naegeliana. c. Aphanizomenon flos aquae. 4.a. Anabaena flos aquae. b. Oscillatoria agardhii var. isothrix. 5. a. Aphanizomenon flos aquae. b. Gomphosphaeria naegeliana. 6. a. Oscillatoria agardhii var. isothrix. b. Oscillatoria bourrellyi. PLATE I1 (p. 10) Diatoms (photographs by E. Mols) 1.Diatoma elongatum, chain, girdle view. 2. Periphytonic diatom community: Gomphonema olivaceum, G. paruulum, G. constrictum, Synedra acus, Diatoma elongatum, Navicula gracilis, Achnanthes microcephala, Cymbella ventricosa. Light microscope. 3. Same as 2, electron scanning microscope. 4.Stephanodiscus astraea. 5. Gomphonema olivaceum; G. longiceps var. subclavata. 6 . Gomphonema constrictum. PLATE I11 (p. 11) Green algae (photographs by E. Mols) 1. a. Scenedesmus quadricauda. b. A s a, abnormal 8-celled coenobia and large swollen mature cells, cultivated in Rodhe 8, continuous light, 30 000 lux. 2. Phytoplankton of the Tjeukemeer, i.a. Pediastrum boryanum, Scenedesmus acuminatus, S. opoliensis, Oocystis crassa, Micractinium pusillum, Ankistrodesrnus arcuatus. 3. Spirogyra species, conjugating. 4.Pediastrum boryanum, old and just-formed young coenobium still laying in its vesicle. PLATE IV (p. 12) Primary consumers (photographs by E. Mols) 1. Diaptomus gracilis d, a common calanoid copepod in the plankton. 2. Bosmina coregoni, a common cladocer in large lakes. 3. Keratella quadrata, free-living rotifer with egg. 4.Collotheca monoceros, attached rotifer.
9 PLATE I
10 PLATE I1
PLATE I11
12 PLATE IV
GENERAL INTRODUCTION Limnology can be defined in two ways, either as aquatic ecology or as the science of freshwater systems or “inland oceanology”. Limnology - from the greek prefix “limn-” meaning “marsh” - is an interdisciplinary science combining aspects of hydrobiology, hydrochemistry, hydrophysics and geology. Odum (1971),envisaging the structure of biology as a cylindrical block, considered disciplines such as entomology, microbiology, botany to be vertically cut wedges and taxonomy, physiology and ecology to be horizontal layers, of which ecology is the basic one. Aquatic ecology is thus the science of mutual relationships between organisms and of interactions between organisms and their environment. In this respect limnology is very much like ecology in general. There are differences in the nature of the ecosystem, however. The aquatic ecosystem is usually a closed one. Organisms cannot normally leave it and, with the exception of some insects, are confined within it throughout their entire life cycle. This closed structure results from the fact that watercombines the functions of soil, air and water of the terrestrial ecosystem. In lakes and rivers all organisms use water for their transport, either actively as in the case of swimming fish or passively in the case of some algae. Algae take their nutrients, both minerals and C 0 2 , from the water, and release the products of their metabolism back into the water, either immediately or eventually during post-mortem mineralisation. Other organisms hunt for their food through the water and excrete their waste products into it. Heat and light are transmitted through the water which, due to its physicochemical properties, absorbs them more strongly than does the air. Great diversity - the occurrence of a large number of different organisms - is a feature common t o both the terrestrial and the aquatic ecosystem. In the aquatic habitat high turnover rates of individual organisms and also of chemicals result in a rapid succession of types of organisms, this effect being called the periodicity of phyto- and zooplank€on. These high turnover rates and periodicity effects are due to the fact that the great majority of the organisms in the system are of only microscopically visible size and also because of the variations in the physical and chemical environment which result from seasonal changes. In a study of the aquatic ecosystem it is useful to subdivide it into several component features. Because this subdivision will be used throughout this book, the general outline is given here. Abiotic features. These include physical phenomena such as radiant energy, temperature and other aspects of water physics such as density, viscosity, sur-
2
face tension etc., together with properties of the water molecule itself which result in the peculiar characteristics of water such as its freezing point and certain of its chemical properties. Also included here are the chemical elements and compounds which are dissolved or suspended in the water and which enable most natural waters t o support life. The suspended matter is termed allochthonous if it originates from sources outside the lake or autochthonous if it derives from sources within the lake system. Abiotic factors are rarely uniform with depth. Irradiance decreases with depth, and temperature profiles often show layers of relatively rapid change. It is often useful t o consider effects as occurring in a water column of e.g. 1m 2 . Loadings can be expressed in quantity per volume or per unit surface per year. The loading per surface is the loading per volume times depth. If biological processes must be expressed as units per surface, but have been measured per unit volume, then the unit per surface can be found by summing results in all layers, or mathematically by integrating the function describing the process over depth. Photosynthetic yield can then be expressed as e.g. mg mP2dp1 of C and can thus be related to incoming light, which is expressed in e.g. cal. c m p 2 C 1 , or t o fish yield, which is expressed in kg ha-' yr-' . Primary producers (see Plates 1-111). Primary producers are those organisms which use solar radiation as their only energy source and in this context include the macrophytes, the algae, and the photosynthetic bacteria. Except within the littoral zone of lakes, the algae are quantitatively the most important of these groups. Many algae of the open water are unicellular but multicellular colonies and filaments d o occur. Morphological and functional differentiation into colonies is fairly common amongst the multicellular forms, but no highly differentiated organs such as leaves, stems, or roots are found. In some colonies, e.g. Pediastrum, the outer cells have a markedly different shape from the inner ones, and this feature imparts to the colonies an enlarged surface area which allows them t o float better and increases the efficiency of gas exchange and nutrient uptake. In all colonies or filaments each single cell retains the capacity for reproduction (cell multiplication). The algal groups mentioned below will often be referred to throughout the different chapters of this book, amongst these the diatoms, the green algae and the blue-green algae are quantitatively the most impohant groups. A more detailed list of most genera mentioned is given as Appendix I of this book. The diatoms (Bacillariophyceae) contain both chlorophyll and brownishgreen pigments which often mask the green chlorophyll colour. When wetted with ethanol or methanol they turn green. The silicified cell wall is characteristic, and it may easily constitute more than 50% of the dry weight of the organism. (The structure of the cell wall is described in more detail in Chapter 7.) The skeletons are often beautifully ornamented with transverse lines, projections, or depressions, the detailed patterns of which provide a basis for spe-
3
cies recognition. They remain identifiable in stratigraphic deposits of sediments of great age and enable details of species successions and inferred environmental changes to be discerned, often for a long time span in the history of a lake. The diatcms commonly occur during spring time in temperate waters, either as.free-floating cells or chains (filaments) or attached to plants or stones. They are divided into two broad groups: Centrales, with valves usually circular, and Pennales, with elongated valves. The freshwater species of the genus Melosira are probably amongst the commonest diatoms occurring in Lake Baikal (Siberia) and in shallow ponds or ditches. The green algae (Chlorophyceae) in which the chlorophyll is not masked by other pigments are bright green and have rigid cell walls made of celluloselike carbohydrate. A common genus is the flagellate Chlamydomonas, a freeswimming single-celled organism with two flagella and an eyespot; i t can develop rapidly in ditch water which has been left in the laboratory or classroom. Volvox is one of the genera which forms the largest colonies, sometimes with 20 000 cells. Beating of the flagella, which are possessed only by the outermost cells of the Voluox colony, enables this organism to move by slow rotations through the water. One of the smallest algae is Chlorella, a single cell. It is often used in physiological studies because it can be easily maintained in culture and because of its rapid growth rate. Cladophora, Oedogonium and Spirogira are familiar green filamentous algae growing on stones, poles, etc. Asexual cell division is the commonest method of reproduction by green algae. Sexual reproduction, which occurs usually in adverse conditions, is often rather complicated. The blue-green algae (Cyanophyceae or Myxophyceae) are the “simplest” of all algae and are perhaps more closely related to the bacteria with their procaryotic cells. They also resemble bacteria in some of their chemical properties. Their characteristic colour is due to the pigment phycocyanin but otherpigments can also occur and impart a violet, brown, or yellow colour to any species possessing them. The pigments are evenly distributed throughout the protoplasm, although some infrastructure has been discerned by use of the electron microscope. The cells have no nucleus and reproduce by simple cell division. Certain schools classify them as bacteria and not as algae. One of the more easily recognisable genera is Microcystis, which forms masses of free-floating cells readily visible to the naked eye. The cells multiply abundantly and may form a scum on the surface of smaller lakes or bays. Oscillatoria is a filamentous type and occurs in large quantities during the summer in many eutrophic lakes, thus causing many problems in water management. Other genera which commonly cause blooms of nuisance proportions are Nostoc and Anabaena. Unlike most autotrophic organisms, many of the bluegreen algae are able to use N2 in addition to, or instead of, the more usual NH; or NO, ions, a property referred to as nitrogen fixation. (They are discussed in detail in Chapter 6.) Other important groups are the Cryptophyceae, the Dinoflagellatae and the Chrysophyceae.
4 Consumers. These organisms derive their energy from other organisms. Included in this group are the zooplankton (Plate IV), the insects, fish, etc. The main groups are outlined in Chapter 15. Primary consumers are those consumers that feed on the producers, whereas the secondary consumers feed on the primary ones. Terminal consumers are those organisms that are not used by any other organism, except after their death. They consist mainly of the larger fishes, which are also called predators, although this word is often used for any organism that actively hunts its prey. (Feeding relations between aquatic organisms are outlined and discussed in Chapter 15.) Decomposers. These are the microorganisms that mineralise all dead organic material. Included in this category are bacteria, fungi, and certain yeasts. Their main function in the ecosystem is to mineralise the organic matter back into simple inorganic compound which are then recycled. Strictly speaking, decomposers are consumers, but since they mostly digest dead organisms without ingesting them, it is useful to distinguish them. Each organism has a habitat, i.e. the type of place where it occurs. For example, a young trout has the cool mountain stream for its habitat. Each type of organism is also said to live in a niche, which could be described as its physiological habitat, i.e. the “place” where it can carry out its essential physiological processes. In this context the word “place” should not be taken only in its physical sense but also as place in a food chain, a place restricted by tolerance limits and historical and geographical factors. An association of plants and animals is called a community or biocoenose. A biotope is the geographic entity in which a biocoenose can be found. In the present state of limnological knowledge, quantitative investigations are important since many aquatic processes must first be qua-tified in order to understand how they work. Although qualitative descriptions of organisms and processes are necessary as a first stage, the aim of many current studies is to achieve an understanding of the nature and rate of changes, the quantitative interrelationships in competition and the rate of predation. As a result of the study of factors affecting the regulation of quantities in biological yields and rates, two important principles have been formulated: the law of Liebig (1840) and the law of tolerance (Shelford, 1913). Liebig’s law of the minimum factor states that the yield of a plant crop is determined by that element of which relatively the smallest quantity is available for that plant. If e.g. a wheat crop could be as large as 1000 tonnes based on the amounts of C, N and other elements present, but only 500 tonnes, because no more iron is available, then 500 tonnes it will be. A minimum factor is nowadays often called a limiting factor, i.e. the factor which controls the rate of a process such as the growth of an algal culture. (The difference is extensively discussed in Chapter 10.) Much confusion is caused because the two concepts are muddled. Results of an experiment in which growth rate was
5 measured are often used in terms of, or to predict, the total yield of the ex-. periment. Thus in the study of eutrophication (Chapter 17), the results of a 4-h bottle test for measuring the photosynthetic rate, are often used to predict the density of the final algal crop. (Growth rate and yield are discussed in Chapter 10.) The mere presence of a substance may not necessarily allow growth of an organism. The compound must be present in such a form that the organism can obtain and utilise it, that form being said t o be “available”. The second principle in quantitative ecology was called by Shelford.(l913) the law of tolerance. Factors are not necessarily present only in a minimum quantity; they may in fact be present “in excess”. Many organisms can only tolerate certain quantities of some substances, and larger quantities than the tolerance level may be toxic and the growth rate may decrease. There is often an optimal quantity of any substance which permits the greatest growth of an organism and the presence of lesser or greater quantities than this both result in decreased growth rate or yield. As an example, in Fig. 1.1growth rates of Chlorella and Nostoc are shown as a function of temperature. It can be seen that growth takes place above a minimum “threshold” temperature, T,, ,and below a maximum temperature, T,,, . At one temperature within this range growth is optimal. Above the op-
30
-
25
-
ul
5 $20-
9
L a
c Y
F!
15-
Fig. 1.1. Temperature relations of photosynthesis in Nostoc muscorum and Chlorella pyrenoidom. (From Clendenning et al., 1956.)
6 timum “the excess is tolerated”. It should be noted that growth optimum curves are seldom bilaterally symmetrical. The decrease in growth is frequently much steeper above the optimum than below it. Temperature optima are often quite different for different species of alga; they determine relative yields of different species in different circumstances and operate to produce certain features of competition and succession. For example the diatom Fragilaria crotonensis has an optimal temperature of 13-16” C. WesenbergLund (1904) found that in the cold summer of 1902 this species was not replaced by blue-green algae as was normally the case, the blue-green algae having a higher optimum temperature than the diatom. Competition as one of the factors in algal succession is probably much more complicated than this example suggests (see Chapter 14). A similar effect may be seen in the response of organisms to different pH conditions (Fig. 1.2). E.g., the green alga Scenedesmus cannot grow at pH = 4 but can grow excellently even above pH = 11, while Chlamydomonas may thrive at pH = 4. Therefore, in a mixed community or culture the pH can have a selective effect, being optimal for one species at a given value, whereas it is minimal for another one. The pH has been shown to rise to high values during bloom conditions (e.g Tjeukemeer, Loch Leven; subsections 14.2.2 and 15.5). It may therefore be a factor through which one algal species may inhibit other ones. Complications arise here also because the pH is not a property in itself, but is related to the CO,--HCO,-CO~system, so that, if the pH changes, the whole balance of the carbondioxide/bicarbonate system is automatically changed. As a final example a harmless compound such as phosphate may be men-
PH
Fig. 1.2. The pH stability relations of photosynthesis in Nostoc muscorum. (From Clendenning et al., 1956.)
tioned. Rodhe (1948) noted that Asterionella formosa is never found in lakes with a PO4-P concentration above 10 mg m-’ . Even in his cultures no growth took place at or above this phosphate concentration, but it is still difficult to prove that phosphate actually inhibits growth. In order to prove this, one must have a good growing culture and measure inhibition following the addition of further quantities of phosphate. If, however, the PO4-P optimum lies near 2 or 5 mg m-’ , it is rather difficult to measure growth accurately because of the small quantity of algae which is present at that concentration. The situation may actually be more complicated in lake conditions and when Asterionella was found to disappear from Lake Windermere when phosphate increased above 10 mg m-’ ,it seemed too good to be true. Possible explanations in this case could be that phosphate concentrations above 10 mg m-’ inactivated the iron by forming insoluble FePO, . Recently Moed (1973) achieved a good growing culture of Asterionella formosa with PO4-P = 1 g m-’ ,but it could be argued that this may be an adaptation to unnatural conditions. Growth rate measurements of this culture as a function of the phosphate concentrab tion may provide convincing proof of the regulating role of phosphate for this species of alga. Limnology may be regarded therefore as a constant attempt t o explain field observations by measuring the isolated process both in natural conditions and in culture. A culture is an oversimplified model, but nevertheless its use can indicate whether a process is possible or not, while sometimes - if one is lucky information about the rate can also be obtained. One should be extremely cautious, however, in attempting to use rates obtained from experiments in culture (or in vitro as this is often referred to) as a basis for predicting or describing rates occurring in natural conditions. The aims of ecological research are twofold. It is done firstly as an attempt to satisfy some of our curiosity about our environment, and secondly to attain understanding and perhaps control over part of that exciting but often unfrienc ly environment which relates directly to man’s life and well-being. This may en able us to utilise our natural resources more effectively and may prevent us from harming them. The splendour of limnological research is that both of these aims can be pursued simultaneously, and perhaps there are few other scientific disciplines in which pure and applied research are so strongly interwoven.
REFERENCES Clendenning, K.A., Brown, T.E. and Eyster, H.C., 1956. Comparative studies of photosynthesis in Nostoc muscorum and Chlorella pyrenoidosa. Can. J. Bot., 34: 943-966. Mued, J.R., 1973. Effect of combined action of light and silicon depletion on Asterionella formosa Hass. Verh. Int. Ver. Theor. Angew. Limnol., 1 8 (prt. 3 ) : 1367-1374. Odum, E.P., 1971. Fundamentals o f Ecology. Saunders, Philadelphia, Pa., 3rd ed., 5: 4 pp.
Rodhe, W., 1948.Environmental requirements of fresh-water plankton algae. Experimental studies in the ecology of phytoplankton. Symb. Bot. Ups., lO(1):149 pp. Shelford, V.E., 1913.Animal Communities in Temperate America. University of Chicago Press, Chicago, Ill. Wesenberg-Lund, C., 1904.Studies over de Danske s@ersplankton. Specielle del. Copenhagen.
PLATE I (p. 9) Blue-green algae (photographs by J.W.G. Lund) 1.Anabaena flos aquae. 2. Anabaena flos aquae, detail; h , heterocyst; s, spore (akinete). 3. a. Microcystis aeruginosa. b. Gomphosphaeria naegeliana. c. Aphanizomenon flos aquae. 4.a. Anabaena flos aquae. b. Oscillatoria agardhii var. isothrix. 5. a. Aphanizomenon flos aquae. b. Gomphosphaeria naegeliana. 6. a. Oscillatoria agardhii var. isothrix. b. Oscillatoria bourrellyi. PLATE I1 (p. 10) Diatoms (photographs by E. Mols) 1.Diatoma elongatum, chain, girdle view. 2. Periphytonic diatom community: Gomphonema olivaceum, G. paruulum, G. constrictum, Synedra acus, Diatoma elongatum, Navicula gracilis, Achnanthes microcephala, Cymbella ventricosa. Light microscope. 3. Same as 2, electron scanning microscope. 4.Stephanodiscus astraea. 5. Gomphonema olivaceum; G. longiceps var. subclavata. 6 . Gomphonema constrictum. PLATE I11 (p. 11) Green algae (photographs by E. Mols) 1. a. Scenedesmus quadricauda. b. A s a, abnormal 8-celled coenobia and large swollen mature cells, cultivated in Rodhe 8, continuous light, 30 000 lux. 2. Phytoplankton of the Tjeukemeer, i.a. Pediastrum boryanum, Scenedesmus acuminatus, S. opoliensis, Oocystis crassa, Micractinium pusillum, Ankistrodesrnus arcuatus. 3. Spirogyra species, conjugating. 4.Pediastrum boryanum, old and just-formed young coenobium still laying in its vesicle. PLATE IV (p. 12) Primary consumers (photographs by E. Mols) 1. Diaptomus gracilis d, a common calanoid copepod in the plankton. 2. Bosmina coregoni, a common cladocer in large lakes. 3. Keratella quadrata, free-living rotifer with egg. 4.Collotheca monoceros, attached rotifer.
9 PLATE I
10 PLATE I1
PLATE I11
12 PLATE IV