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Pond Life

A Guide to Common Plants and Animals of North American Ponds and Lakes

St. Martin's Press

PONDS AND LAKES

In spring and summer, activity is ceaseless in a pond or small lake. Water striders skate over the surface. Dragonfly nymphs, which will soon change into swift, darting adults, climb the stems of cattails. Close to shore a crayfish grabs and eats a worm — and moments later a bass devours the crayfish. A turtle plops off a log and begins to browse on plants. Countless small shrimplike copepods strain their microscopic plant food from the water and then become food for small fishes that, in turn, are eaten by large fishes or by wading birds. As the water cools in autumn, animals become less active. In winter, the pond or lake is rather quiet, and in the North, only a few animals still stir under the ice.

To understand the teeming life in a pond, one must know what conditions are necessary to support it. This study of inland waters — ponds, lakes, and streams — is called limnology. It is a division of the broader science of ecology, which deals with the ways plants and animals live together in particular environments. Limnology is concerned with all the interrelated factors that influence the inland-water environment. Thus it treats not only biology but also chemistry, geography, weather, climate, and other similar factors or conditions. Only a few of the many thousands of plants and animals that live in or near ponds, lakes, and streams are described and illustrated in this book. As your interest in "pond probing" grows you can consult the books listed on page 155 for more detailed information.

A POND is commonly described by limnologists as a quiet body of water so shallow that rooted plants grow completely across it. Its water temperature is fairly uniform from top to bottom and tends to change with the air temperature. There is little wave action, and the bottom is usually covered with mud. Typically, plants grow all along the shore. The amount of dissolved oxygen may vary greatly in a 24-hour period.

A lake is usually larger than a pond. The water is too deep for plants to grow except around the shore. The temperature of the water is relatively stable from day to day, but in northern lakes, temperature "layering" occurs in summer. The amount of dissolved oxygen during a 24-hour period remains about the same. Because of the broad expanse of water exposed to the wind, shores on the down-wind side are commonly wave-washed, barren strands of sand or rocks.

What people know as a pond or a lake locally, however, differs from one region to another. In some places, for example, the pond illustrated here would be called a marsh. Many of the broad lakes in Florida are shallow, and though they lack plants in the middle, their water temperature and amount of dissolved oxygen follow the pattern of ponds.

SEVERAL DIFFERENT KINDS of bodies of water are called ponds. The basins of some ponds are potholes in glacial debris and are filled by seepage and runoff water from the surrounding land. Others are cutoffs from old stream channels. Some are temporary, others permanent. Despite their differences in origin and age, all are much alike in size, depth, and similar features. A few of the more common and distinctive kinds of ponds are shown here.

CYPRESS PONDS are common in the central and lower Mississippi Basin and along the coastal plain of the southeastern U.S. Their water is often brownish, and many are dry during parts of the year. Along the shore, willows and bay trees are mixed with cypresses, which often grow out into the water.

BOG PONDS are found in moist temperate regions over most of N.A. Their water is usually highly acidic and is often muddy. Cedars dominate the high ground, and alders grow profusely on the shore. Thick beds of sphagnum extend outward from shore. Floating-leaf plants may cover the surface.

MEADOW-STREAM PONDS develop where a stream widens and the speed of its current drops sharply. Pondweeds, stoneworts, cattails, and others with emergent leaves grow in the shallows. Water lilies, water shield, and others send floating leaves to the surface. Above.

MOUNTAIN PONDS are often formed by glaciers. In some, the bottom is pure rock; in others, deep, soft mud. Many are ice-free only briefly and dry up in summer. Sedges grow along margins. Despite the short summer season, many kinds of animals live in these icy waters.

FARM PONDS are built as a part of good farming practices. They also provide fishing and boating. A farm pond should be at least 3 feet deep at the shoreline to prevent plant growth and have a spillway to control water level. It should fill from seepage, not from a stream that will soon fill the basin with silt.

CHARACTERISTICS OF WATER

Water will dissolve more substances than will any other liquid, and for this reason it is called "the universal solvent." Oxygen, carbon dioxide, and nitrogen are absorbed from the atmosphere. Oxygen is also contributed as a by-product of photosynthesis (here), and carbon dioxide is released by both plants and animals in respiration (here). Phosphates, chlorides, and similar mineral salts are dissolved in run-off and seepage water.

WATER MOLECULES are strongly attracted to one another through their two hydrogen atoms. At the surface this attraction produces a tight film over the water. A number of organisms live both on the upper and on the lower sides of this surface film.

DENSITY OF WATER is greatest at 4°C (39.2°F). It becomes less as water warms and, more important, as it cools to freezing at 0°C. Then it changes to ice which floats as its density is only 0.917. Ice is also a poor conductor and thus reduces heat loss from below. Only very shallow ponds ever freeze solid.

HEAT-HOLDING CAPACITY of water is great. It absorbs and releases heat much more slowly than does air. For this reason plants and animals of most ponds and lakes are not usually subjected to suddenly varied temperatures. Though the air temperature may change rapidly and greatly, the water temperature of a deep lake changes slowly. In a shallow pond or lake, the water temperature varies with the air temperature, as above.

TRANSPARENCY OF WATER permits enough light to penetrate for plants to carry on photosynthesis. The depth to which light can penetrate decreases as water becomes more turbid or contains more suspended materials. Few plants grow in muddy ponds because the silt absorbs light.

OXYGEN AND CARBON DIOXIDE are passed back and forth between plants and animals and their environment. The proportion of these gases in the atmosphere is generally constant: oxygen, 21 percent; carbon dioxide, .03 percent. In pond, lake, and stream waters they usually vary greatly, even from night to day.

Oxygen, which is necessary for the survival of nearly all plants and animals, is quite soluble in water, but the amount dissolved in fresh water is much lower than in the atmosphere. Oxygen from the air is absorbed slowly, but the process is speeded when wind and waves disturb the water surface. Also, the cooler the water, the more dissolved oxygen it will hold.

During the day, when sunlight penetrates the water, plants give off oxygen as a by-product of photosynthesis more rapidly than it is used in respiration by plants and animals. A reserve of oxygen builds up. In darkness, when photosynthesis stops, both plants and animals use this oxygen. For this reason the oxygen content in ponds and shallow lakes varies greatly in a 24-hour period.

Carbon dioxide, more soluble in water than oxygen, comes both from the decay of organic material and from respiration of plants and animals. Some is also contributed by ground water and from the atmosphere, either directly or with rain. Near the bottom of deep lakes the amount of dissolved carbon dioxide may be quite high. Few plants and animals can survive in this region. Carbon dioxide is used by plants in photosynthesis. It is the source of the carbon found in fats, proteins, and carbohydrates, the basic food substances of animals.

Carbon dioxide is important also in determining the water's pH — its degree of acidity or alkalinity. It combines with water to form weak carbonic acid, which in turn reacts with limestone or dissolved lime, if present, to form carbonates and bicarbonates. These compounds are indirect sources of carbon and serve also as "buffers" that regulate pH. The pH of water often determines what animals and plants live there (below). For example, mollusks with limy shells cannot live in acid waters.

DISSOLVED MINERALS in pond, lake, and stream waters include phosphates, nitrates, chlorides, sulfates, carbonates, and others. These "minerals" originated in chemical combination with such elements as potassium, magnesium, calcium, sodium, and iron. In solution these compounds are separated into their component ions. (For example, calcium carbonate, CaCO3 becomes Ca++ and CO3--.) The minerals are absorbed by plants as ions rather than as salts. All plants and animals require small amounts of these minerals in building cell protoplasm and body tissues. Floating plants take their minerals directly from the water; rooted aquatic plants, from the pond bottom. Animals get minerals from their plant and animal foods. Minerals are released also from plants and animals that die and decay in the pond or lake. Thus the minerals are kept in cycle.

TEMPERATURE LAYERING occurs in deep lakes of temperate zones. In summer, the surface water absorbs the sun's heat and warms faster than the water below. The warmed water is less dense than the cold water, so it floats on the cool lower layers. By midsummer there are three distinct layers.

Not enough light penetrates the middle and lower layers to permit photosynthesis, and a mixing or circulation of water between the upper and lower levels is prevented by the thermocline, a layer of sudden temperature drop that acts as a barrier to vertical movements. Also, decomposition of organic debris in the lower layer increases the amount of carbon dioxide and reduces oxygen there. In these deep bodies of water nearly all of the fishes and other animals live above the thermocline, where food and oxygen are plentiful.

SPRING AND FALL

During spring and fall overturns, the temperature of the water is equalized throughout the lake or pond. Fishes and other animals are active and distributed widely.

WINTER

Activity is greatly reduced in winter, under the ice. Many animals hibernate in the mud or debris at the bottom. Some fishes continue to feed but less actively.

The upper layer of water cools in autumn until it approaches the temperature of the water in the middle and lower layers. Aided by winds, the surface water sinks, causing circulation from top to bottom. This is called "fall overturn." In winter, the cold surface water continues to sink until, near the freezing point, its density begins to decrease. The near-freezing water (below 39.2°F.) eventually turns to ice at the surface.

The cover of ice prevents the wind from circulating the water, and "winter stagnation" occurs. Thick ice or snow also screens out the light and may stop photosynthesis. Plants and animals may die due to a lack of oxygen; this is called "winterkill." When the ice melts in spring and the surface water warms above 39.2°F., it becomes less dense. Aided by winds, another circulation and mixing, called "spring overturn," occurs until the water temperature is again relatively uniform.

HABITATS IN PONDS AND LAKES

Habitats are places in which rather distinctive groups of plants and animals are found. In lakes and large ponds, the four easily recognized habitats are the surface film, open water, bottom, and shore.

THE SURFACE FILM is the habitat of air-breathing, floating animals and of those animals with special devices that permit them to walk on the surface without breaking through. Some kinds of beetles, water bugs, and free-floating plants are adapted to life only on the upper side of the film. The larvae of some beetles and flies spend much time hanging on the underside of the film. Surface-dwelling animals feed on the floating plants, prey on one another, or eat insects or other animals that drown and then float on the surface.

OPEN-WATER life consists of large, free-swimming animals, such as the fishes, and of small microscopic plants and animals that drift suspended in the water. The drifters, called plankton, vastly outnumber all the larger pond inhabitants. Small suspended plants, or phytoplankton, are mostly algae, the basic food in ponds and lakes. At times, some phytoplankton species multiply in great numbers, forming a "bloom" that clouds the water. The zoo-plankton consists of small suspended animals — numerous tiny crustaceans, rotifers, some insect larvae, and other invertebrates. The kinds and numbers of plankton organisms vary seasonally but are usually most abundant in spring.

Turtles, birds, and large fishes frequent the open-water area. Small fishes usually remain among plants near shore. At night, some kinds of insect larvae and crustaceans migrate from the bottom toward the surface, then return to deeper water as daylight comes.

The open-water area ends where plants become rooted. Ponds and shallow lakes with emergent plants extending from shore to shore lack an open-water area. Deep open water of large lakes supports little life.

THE BOTTOM of ponds and lakes offers a variety of living conditions from the shore outward to the deepest regions. A sandy bottom in the shallows (here) may be inhabited by sponges, snails, earthworms, and insects. If the water is quiet, the bottom is usually covered with mud or silt and contains much organic debris. Crayfish and the nymphs of mayflies, dragonflies, and damselflies are some of the many kinds of animals that burrow into the bottom muds. Others live among the plants, where food is usually plentiful and where they find protection from predators.

Living conditions on the bottom in deep open water are quite different. If the lake is very deep or the water turbid, light does not penetrate to the bottom and plants cannot grow. Animals can find little shelter, the amount of dissolved oxygen is low, and the carbon-dioxide concentration is high. Among the few larger animals that can live in the deep bottom zone are some of the earthworms, small clams, and fly larvae ("bloodworms" and "phantoms"). Bacteria of decay are commonly abundant in the deep region. They are important in returning chemicals to the cycle of life.

THE LITTORAL HABITAT extends from the water's edge outward as far as rooted plants grow. In most ponds and in many shallow lakes, this area may stretch from shore to shore. In many lakes there are typically three distinct concentric borders of flowering plants, except where the shore is so rocky or swept by waves that plants cannot grow.

Closest to shore is the emergent plant zone. It is dominated by plants that are rooted to the bottom and have stems and leaves above the surface. Grasses, sedges, and rushes are plants typical of the emergent zone in ponds and lakes the world over. Many kinds of frogs, birds, and mammals find food and shelter here. A variety of algae, protozoans, worms, insects, snails, and small fishes live among underwater plant stems.

Broad, flat-leaved water lilies and such floating plants as water ferns and duckweeds characterize the floating-leaf plant zone. Because the masses of floating leaves shade out the light, bottom plants may be scarce. Some snails, bugs, and mayflies lay their eggs on the underside of leaves. Many kinds of algae live in this zone, either attached to larger plants or floating free. Sun-fishes and others breed and nest here.

The innermost band of vegetation forms the submersed plant zone. Pondweeds, hornworts, and waterweeds are typical of this zone. All have leaves that are either long and sinuous or bushy and very branched, characteristic adaptations of those few flowering plants that grow completely submerged. The flowers are pollinated above the surface; the seeds germinate and the young plants develop only under the water.

The littoral zone, dominated by higher plants is the richest in the pond community. Here are the greatest number of species, both plant and animal. The littoral zone is easiest for the observer to visit and to study.

FOOD WEBS

Except for sunlight, the source of energy needed by green plants for photosynthesis, a pond or a lake either contains or produces everything necessary for the survival of the plants and animals that live in or near it. Ponds, because they are usually small, are especially good places in which to learn the relationships of plants and animals to their environment and to one another. The most common linkage is through production or consumption of food.

All green plants, from floating microscopic plants to such flowering plants as pond lilies, manufacture food. Plants become food for plant-eating (herbivorous) animals, such as mayfly nymphs, small crustaceans, and some kinds of beetles. These animals are in turn preyed upon by small flesh-eating (carnivorous) animals including fishes, dragonfly nymphs, and beetle larvae. Larger fishes eat the smaller fishes, crustaceans, and insects. If not eaten, every plant and animal eventually dies and decomposes. Its protoplasm is reduced to the basic materials that green plants need for growth. In this way the cycle of foods is continuous.

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Excerpted from Pond Life by George K. Reid, Herbert S. Zim, George S. Fichter, Sally D. Kaicher, Tom Dolan. Copyright © 2001 St. Martin's Press. Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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