CHAPTER 7 Freshwater Ecology Ours is a water planet. Water covers three quarters of its surface, makes up two-thirds of our bodies. It is so vital to life we can't live more than four days without it. If all the earth's water-an estimated 325 trillion gallons-were squeezed into a gallon jug and you poured off what was not drinkable (too salty, frozen or polluted) you'd be left with one drop. And even that might not pass U.S. water quality standards.95 7.1 NORMAL STREAM LIFE N ORMAL stream life can be compared to that of a "balanced aquarium."96 That is, nature continuously strives to provide clean, healthy, normal streams. This is accomplished by maintaining the stream's flora and fauna in a balanced state. Nature balances stream life by maintaining both the number and the type of species present in any one part of the stream. Such balance ensures that there is never an overabundance of one species. Nature structures the stream environment so that plant and animal life is dependent upon the existence of oth- ers within the stream. Thus, nature has structured an environment that provides for interdependence, which leads to a balanced aquarium in a normal stream. 7.2 FRESHWATER ECOLOGY To this point, the fundamental concepts of ecology, which are generally re- lated to both terrestrial and freshwater habitats, have been discussed. Before narrowing the focus to the topic of freshwater ecology and, more particularly, stream ecology and self-purification, the two different ecosystems, land and freshwater habitats, will be contrasted. 95~arr, J., Design for a Livable Planet. New York: Harper & Row, p. 61, 1990. 96~~~~. Manual on Watel: Philadelphia: American Society for Testing and Materials, p. 86, 1969. Copyright © 2001 by Technomic Publishing Company, Inc. 74 FRESHWATER ECOLOGY The major difference between land and freshwater habitats is in the medium in which both exist. The land or terrestrial habitat is enveloped in a medium of air, the atmosphere. The freshwater habitat, on the other hand, exists in a water medium. Although the two ecosystems are different, they both use oxygen. Contrast exists in how the oxygen is formulated in each system and how organ- isms utilize it. The following data clearly illustrate this contrast. For example, atmospheric air contains at least twenty times more oxygen than does water. Air has approxi- mately 210 m1 of oxygen per liter; water contains 3-9 m1 per liter, depending on temperature, presence of other solutes, and degree of saturation. Moreover, freshwater organisms must work harder for their oxygen. That is, they must ex- pend far more effort extracting oxygen from water than land animals expend re- moving oxygen from air.97 Other contrasts between land and water ecosystems can be seen in other comparisons. For example, water is approximately 1000 times denser than air and approximately 50 times more viscous. Additionally, natural bodies of wa- ter have tremendous thermal capacity, with little temperature fluctuation, as compared to atmospheric air. Freshwater ecology is the branch of ecology that deals with the biological aspect of limnology. Limnology, as defined by Welch, "deals with biological productivity of inland waters and with all the causal influences which deter- mine it."9"imnology divides freshwater ecosystems into two groups or classes, lentic and lotic habitats. The lentic (Lenis = calm) or standing water habitats are represented by lakes, ponds, and swamps. The lotic (Lotus = washed) or running water habitats are represented by rivers, streams, and springs. On occasion, these two different habitats are not well differentiated. This can be seen in the case of an old, wide, and deep river where water velocity is quite low, and the habitat, therefore, becomes similar to that of a pond. 7.3 LENTIC HABITAT Lakes and ponds range in size of just a few square feet to thousands of square miles. Scattered throughout the earth, many of the first lakes evolved during the Pleistocene Ice Age. Many ponds are seasonal, just lasting a couple of months, such as sessile pools, while lakes last many years. There is not that much diver- sity in species, because lakes and ponds are often isolated from one another and from other water sources such as streams and oceans. Lakes and ponds are divided into four different "zones" that are usually de- termined by depth and distance from the shoreline. The four distinct 97~ickman, C. P., Roberts, L. S., and Hickman, F. M,, Integrated Principles of Zoology St. Louis: Times Mir- rorA4osby College Publishing, p. 161, 1988. 98~elch, P. S., Limonology New York: McGraw-Hill, p. 10, 1963. Copyright © 2001 by Technomic Publishing Company, Inc. Lentic Habitat 75 zones-littoral, limnetic, profundal, and benthic-are shown in Figure 7.1. Each zone "provides a variety of ecological niches for different species of plant and animal life."99 The littoral zone is the topmost zone near the shores of the lake or pond with light penetration to the bottom. It provides an interface zone between the land and the open water of lakes. This zone contains rooted vegetation such as grasses, sedges, rushes, water lilies and waterweeds, and a large variety of or- ganisms. The littoral zone is further divided into concentric zones, with one group replacing another as the depth of water changes. Figure 7.1 also shows these concentric zones-emergent vegetation, floating leaf vegetation, and submerged vegetation zones-proceeding from shallow to deeper water. The littoral zone is the warmest zone because it is the area that light hits, it contains flora such as rooted and floating aquatic plants, and it contains a very diverse community, which can include several species of algae (like diatoms), grazing snails, clams, insects, crustaceans, fishes, and amphibians. The aquatic plants aid in providing support by establishing "excellent habitats for photosynthetic and heterotrophic (requires organic food from the environment) microflora as well as many zooplankton and larger invertebrate^."^^^ In the case of insects, such as dragonflies and midges, only the egg and larvae stages are found in this zone. The fauna includes such species as turtles, snakes, and ducks that feed on the vegetation and other animals in the littoral zone. Figure 7.2 shows a top view of the other zones making up the littoral zone. Figure 7.1 Vertical section of a pond showing major zones of life. (Source: Modified from E. Enger, J. R. Kormelink, B. F. Smith, and R. J. Smith, Environmental Science: The Study of Interrelation- ships. Dubuque, IA: Williarn C. Brown Publishers, p. 77, 1989.) 99~iller, G. T., Environmental Science: An Introduction. Belrnont, CA: Wadsworth, p. 77, 1988. 'Wetzel, R. G., Limonology Orlando, FL: Harcourt Brace, p. 519, 1983. Copyright © 2001 by Technomic Publishing Company, Inc. FRESHWATER ECOLOGY Figure 7.2 View looking down on concentric zones that make up the littoral zone. From Figure 7.2, it can be seen that the limnetic zone is the open-water zone up to the depth of effective light penetration; that is, the open water away from the shore. The community in this zone is dominated by minute suspended or- ganisms, the plankton, such as phytoplankton (plants) and zooplankton (ani- mals), and some consumers such as insects and fish. Plankton are small organ- isms that can feed and reproduce on their own and serve as food for small chains. J Note: Without plankton in the water, there would not be any living organ- isms in the world, including humans. In the limnetic zone, the population density of each species is quite low. The rate of photosynthesis is equal to the rate of respiration; thus, the limnetic zone is at compensation level. Small shallow ponds do not have this zone; they have only a littoral zone. When all lighted regions of the littoral and limnetic zones are discussed as one, the term euphotic is used, designating these zones as hav- ing sufficient light for photosynthesis and the growth of green plants to occur. The small plankton do not live for a long time. When they die, they fall into the deep-water part of the lakelpond, the profundal zone. The profundal zone, because it is the bottom or deep-water region, is not penetrated by light. This zone is primarily inhabited by heterotrophs adapted to its cooler, darker water and lower oxygen levels. The final zone, the benthic zone, is the bottom region of the lake. It supports scavengers and decomposers that live on sludge. The decomposers are mostly large numbers of bacteria, fungi, and worms, which live on dead animal and plant debris and wastes that find their way to the bottom. Copyright © 2001 by Technomic Publishing Company, Inc. Lotic Habitat 7.4 LOTlC HABITAT Lotic (washed) habitats are characterized by continuously running water or current flow. These running water bodies, rivers and streams, typically have three zones: riffle, run, and pool. The rifle zone contains faster-flowing, well-oxygenated water, with coarse sediments. In the riffle zone, the velocity of current is great enough to keep the bottom clear of silt and sludge, thus provid- ing a firm bottom for organisms. This zone contains specialized organisms that are adapted to live in running water. For example, organisms adapted to live in fast streams or rapids (trout) have streamlined bodies, which aid in their respi- ration and in obtaining food.lol Stream organisms that live under rocks to avoid the strong current have flat or streamlined bodies. Others have hooks or suckers with which to cling or attach to a firm substrate to avoid the washing-away ef- fect of the strong current.Io2 The run zone (or intermediate zone) is the slow-moving, relatively shallow part of the stream with moderately low velocities and little or no surface turbu- lence. The pool zone of the stream is usually a deeper water region where velocity of water is reduced and silt and other settling solids provide a soft bottom (more homogeneous sediments), which is unfavorable for sensitive bottom-dwellers. Decomposition of some of these solids causes a lower amount of dissolved oxy- gen (DO). It is interesting to note that some stream organisms spend some of their time in the rapids part of the stream and other times in the pool zone (trout, for example). Trout typically spend about the same amount of time in the rapid zone pursuing food as they do in the pool zone pursuing shelter. Organisms are sometimes classified based on their modes of life. The fol- lowing section provides a listing of the various classifications based on mode of life. 7.4.1 CLASSIFICATION OF AQUATIC ORGANISMS BASED ON MODE OF LIFE (1) Benthos (Mud Dwellers): this term originates from the Greek word for bot- tom and broadly includes aquatic organisms living on the bottom or on sub- merged vegetation. They live under and on rocks and in the sediments. A shallow sandy bottom has sponges, snails, earthworms, and some insects. A deep, muddy bottom will support clams, crayfish, nymphs of damselflies, 'Ol~rnith, R. L., Ecology and Field Biology. New York: Harper & Row, p. 134, 1974. Io2~llen, J. D., Stream Ecology: Structure and Function of Running Waters. London: Chapman & Hall, p. 48, 1996. Copyright © 2001 by Technomic Publishing Company, Inc. 78 FRESHWATER ECOLOGY dragonflies, and mayflies. A firm, shallow, rocky bottom has nymphs of mayflies, stone flies, and larvae of water beetles. (2) Periphytons or Aufiuchs: the first term usually refers to microfloral growth upon substrata (i.e., benthic-attached algae). The second term, aufwuchs (pronounce: OWF-vooks; German: "growth upon"), refers to the fuzzy, sort of furry-looking, slimy green coating that attaches or clings to stems and leaves of rooted plants or other objects projecting above the bottom without penetrating the surface. It consists not only of algae like Chlorophyta, but also diatoms, protozoans, bacteria, and fungi. (3) Planktons (Drifters): they are small, mostly microscopic plants and animals that are suspended in the water column; movement depends on water cur- rents. They mostly float in the direction of the current. There are two types of planktons: phytoplanktons are assemblages of small plants (algae) with lim- ited locomotion abilities (they are subject to movement and distribution by water movements) and zooplankton are animals that are suspended in water with limited means of locomotion (examples include crustaceans, protozo- ans, and rotifers). (4) Nektons or Pelagic Organisms (capable of living in open waters): they are distinct from other planktons in that they are capable of swimming inde- pendent of turbulence. They are swimmers that can navigate against the cur- rent. Examples of nektons include fish, snakes, diving beetles, newts, turtles, birds, and large crayfish. (5) Neustons: they are organisms that float or rest on the surface of the water. Some varieties can spread their legs so that the surface tension of the water is not broken; for example, water striders (see Figure 7.3). (6) Madricoles: organisms that live on rock faces in waterfalls or seepages. Figure 7.3 Water strider. (Source: Standard Methods, 15th Edition. Copyright 0 198 1 by the Amer- ican Public Health Association, the American Water Works Association, and the Water Pollution Control Federation; reprinted with permission.) Copyright © 2001 by Technomic Publishing Company, Inc. Limiting Factors 7.5 LIMITING FACTORS An aquatic community has several unique characteristics. The aquatic com- munity operates under the same ecologic principles as terrestrial ecosystems, but the physical structure of the community is more isolated and exhibits limit- ing factors that are very different from the limiting factors of a terrestrial eco- system. Certain materials and conditions are necessary for the growth and reproduc- tion of organisms. If, for instance, a farmer plants wheat in a field containing too little nitrogen, it will stop growing when it has used up the available nitro- gen, even if the wheat's requirements for oxygen, water, potassium, and other nutrients are met. In this particular case, nitrogen is said to be the limiting fac- tor. A limiting factor is a condition or a substance (the resource in shortest sup- ply) that limits the presence and success of an organism or a group of organisms in an area. There are two well-known laws about limiting factors: (1) Liebig's Law of the Minimum: Odum has modernized Liebig's Law in the following: "Under steady state conditions the essential material available in amounts most closely approaching the critical minimum needed, will tend to be the limiting one."Io3 Liebig's Law is normally restricted to chemicals that limit plant growth in the soil, for instance, nitrogen, phosphorus, and potassium. It does not deal with the excess of a factor as limiting. (2) Shelford's Law of Tolerance: although Liebig's Law does not deal with the excess of a factor as limiting, excess is or can be a limiting factor. The pres- ence and success of an organism depends on the completeness of a complex of conditions. Odum describes Shelford's Law of Tolerance as follows: "Absences or failure of an organism can be controlled by the qualitative and quantitative deficiency or excess with respect to any one of the several fac- tors which may approach the limits of tolerance for that For instance, too much and too little heat, light, and moisture can be limiting fac- tors for some plants. Price points out that "these two laws actually relate to individual organisms, and the survival of an individual in a given set of conditions, independent of others in the same niche."lo5 Expressed differently, both of these laws state that the presence and success of an organism or a group of organisms depend upon a complex of conditions, and any condition that approaches or exceeds the limits of tolerance is said to be a limiting condition or factor. 1030dum, E .P,, Fundamentals of Ecology. Philadelphia: Saunders College Publishing, p. 106, 1971. '040dum, E .P., Fundamentals of Ecology. Philadelphia: Saunders College Publishing, p. 107 1971. '''price, P. W., Insect Ecology New York: John Wiley & Sons, Inc., p. 415, 1984. Copyright © 2001 by Technomic Publishing Company, Inc. 80 FRESHWATER ECOLOGY Several factors affect biological communities in streams. These include the following: water quality temperature turbidity dissolved oxygen acidity alkalinity organic and inorganic chemicals heavy metals toxic substances habitat structure substrate type water depth and current velocity spatial and temporal complexity of physical habitat flow regime water volume temporal distribution of flows energy sources type, amount, and particle size of organic material entering stream seasonal pattern of energy availability biotic interactions competition predation disease parasitism mutualism The common physical limiting factors in freshwater ecology important to this discussion include the following: (1) Temperature (2) Light (3) Turbidity (4) Dissolved atmospheric gases, especially oxygen (5) Biogenic salts in macro- and micronutrient forms macronutrients, such as nitrogen, phosphorus, potassium, calcium, and sulfilr micronutrients such as iron, copper, zinc, chlorine, and sodium (6) Water movement-stream currents, especially rapids 7.5.1 TEMPERATURE Aquatic organisms are very sensitive to temperature change, as water tem- Copyright © 2001 by Technomic Publishing Company, Inc. Limiting Factors 81 perature generally does not change rapidly. It should be noted, however, that surface waters can be subject to great temperature variations. Tchobanoglous and Schroeder point out that across the United States, for ex- ample, surface water temperatures can vary from OS°C to 27"C.lo6 Water has some unique properties, such as very high molar heat of fusion (1 -44 kcal) and molar heat of vaporization (9.70 kcal), which allow a very slow change in water temperature. Aquatic organisms often have narrow temperature tolerance and are known as stenothermal (narrow temperature range). The limits for abrupt changes in water temperature are -5°F. Water has its greatest density (1 g/cm3) at 4°C. Above and below this temperature, it is lighter. Temperature changes, there- fore, produce a characteristic pattern of stratification of lakes and ponds in trop- ical and temperate regions, which helps the aquatic life to survive under severe winter and summer conditions (see Figure 7.4). Figure 7.4 shows the effect temperature has on thermal stratification, which causes turning over of lakes and ponds. During the summer, turning over occurs because the top layer of water be- comes warmer than the bottom; and as a result, there are two layers of water, the top one lighter and the bottom one heavier. With the further rise in temperature, the top layer becomes even lighter than the bottom layer, and a middle layer with medium density is created. These layers, fiom top to bottom, are known as epilimnion, thermocline, and hypolimnion. They are lightest and warmest, me- dium weight and warmer, and heaviest and cool, respectively. There is a strong drop in temperature at the thermocline. There is no circulation of water in these three layers. If the thermocline is below the range of effective light penetration, which is quite common, the oxygen supply becomes depleted in the hypolimnion, because both photosynthesis and the surface source of ox- ygen are cut off. This state is known as summer stagnation (see Figure 7.5). During the fall, as the air temperature drops, so does the temperature of the epilimnion until it is the same as that of the thermocline. At this point, the two layers mix. The temperature of the whole lake is now the same, and there is a complete mixing. As the temperature of the surface water reaches 4"C, it be- comes more dense than water below, which is not in direct contact with the air and does not cool as rapidly at the lower levels. The denser oxygen-rich surface layer stirs up organic matter as the water sinks to the bottom; this is known as fall turnover.lo7 During the winter, the epilimnion, which is ice-bound, is at the lowest tem- perature and is thus lightest; the thermocline is at medium temperature and me- dium weight; and the hypolimnion is at about 4°C and heaviest. This is winter 'O~chobanoglous, G. and Schroeder, E. D., Water Qualiq. Reading, MA: Addison-Wesley, p. 132, 1985. '07~orthington, D. K. and Goodin, J. R., The Botanical World. St. Louis: Times MirrorNosby College Press, p. 69, 1984. Copyright © 2001 by Technomic Publishing Company, Inc. Figure 7.4 Thermal stratification of a lake. (Source: J. M. Morgan, M. D. Morgan, and J. H. Wiersma, 1986, Introduction to Environmental Science, Copyright 01986 by W. H. Freeman and Company; used with permission.) 1-3C 7-18C Thennocline Figure 7.5 Thermal stratification of a pond. (Source: Adapted from E. P. Odum, Fundamentals of Ecology. Philadelphia: Saunders College Publishing, p. 3 10, 197 1 .) Copyright © 2001 by Technomic Publishing Company, Inc. [...]... classes: and habitats 7. 5 A sessile pond is a pond 7. 6 The topmost zone near the shores of a lake is known as the zone Copyright © 2001 by Technomic Publishing Company, Inc 92 7. 7 FRESHWATER ECOLOGY are small organisms that can feed and reproduce on their own and serve as food for small chains 7. 8 The zone of a lake not penetrated by light is called the zone zone supports scavengers and decomposers 7. 9... bottom Run08 dissolution of rocks, and sewage discharge are the three primary sources of basic nutrients in streams and rivers 7. 10 CHAPTER REVIEW QUESTIONS 7. 1 Explain the "balanced aquarium" concept 7. 2 The major difference between land and freshwater habitats is in the in which they both exist 7. 3 Atmospheric air contains at least does water times more oxygen than 7. 4 Limnology divides freshwater... mayflies, darnselflies, caddis flies, and some beetles Making up the nektons are organisms such as fish, amphibians, reptiles, large crustaceans, water beetles, and water scorpions Neustons are represented by water striders and whirligig beetles Planktons, actually, zooplanktons, such as water fleas, copepods, rotifers and protozoans, make up the final class Decomposers consist of fungi and bacteria and are... halo of white fungi filaments Fungi and bacteria work together to reduce and transform dead animals and plants into humic substances Although all five classes of organisms are found in lentic communities, their distribution by zone varies For example, most of the organisms are found in the littoral zone, whereas the limnetic zone has phytoplanktons, zooplanktons, and fish The profundal zone has some... silts and settleable solids that cause lowered DO due to decomposition Aquatic organisms-are classified according to their mode of life There are five classes: benthos, periphytons, plankton, pelagic (nektons), and neustons -Benthos (mud dwellers) live within or on bottom sediments -Periphytons (Aufiucks) live attached to plants or rocks -Planktons (drifters) are small microscopic plants (phytoplankton)... The 7. 10 Name the three zones of a lotic habitat 7. 1 1 float or rest on the surface of water 7. 12 Define limiting factor 7. 13 List at least twelve factors that affect biological communities in streams 7. 14 What causes winter kill of fish? 7. 15 Oxygen and are often limiting factors in ecosystems 7. 16 The amount of DO is affected inversely by the amount of the water in 7. 17 Explain eutrophication 7. 18... complete depletion of dissolved oxygen (DO) in spite of organic pollution, whereas lakes and ponds can go anaerobic 1080dum,E P., Fundamentals of Ecology Philadelphia: Saunders College Publishing, p 31 1, 1 971 Copyright © 2001 by Technomic Publishing Company, Inc FRESHWATER ECOLOGY 7. 5.4 TURBIDITY The waters of a lake or stream are often transparent The rate of penetration of light is affected inversely... microscopic plants (phytoplankton) and animals (zooplankton) that move about freely with the current Copyright © 2001 by Technomic Publishing Company, Inc Chapter Review Questions -Pelagic (nektons) are organisms swimming freely -Neustons are organisms that live on the surface of the water Limiting factors-such as temperature, light, turbidity, dissolved gases, biogenic salts, and water movements, limit the... eutrophication Sources of human wastes and pollution are sewage, agricultural runoff, mining, industrial wastes, urban runoff, leaching from cleared land, and landfills 7. 7.1 CLASSIFICATION OF LAKES BASED ON EUTROPHICATION Lakes can be classified into three types based on their eutrophication stage ( 1 ) Oligotrophic lakes Cfew foods): they are young, deep, crystal-clear water, nutrient-poor lakes with little... small quantity of organic matter grows in an oligotrophic lake; phytoplankton, zooplankton, attached algae, macrophytes (aquatic weeds), bacteria, and fish are present as small populations "It's like planting corn in sandy soil, not much growth."110Lake Superior is an example from the Great Lakes (2) Mesotrophic lakes: it is hard to draw distinct lines between oligotrophic and eutrophic lakes, and often . suspended or- ganisms, the plankton, such as phytoplankton (plants) and zooplankton (ani- mals), and some consumers such as insects and fish. Plankton are small organ- isms that can feed and reproduce. Habitat 75 zones-littoral, limnetic, profundal, and benthic-are shown in Figure 7. 1. Each zone "provides a variety of ecological niches for different species of plant and animal life."99. rocks, and sewage discharge are the three primary sources of basic nutrients in streams and rivers. 7. 10 CHAPTER REVIEW QUESTIONS 7. 1 Explain the "balanced aquarium" concept. 7. 2