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AQUATIC LIFE ZONES

The most fundamental division in aquatic ecol­ogy is probably between freshwater and saltwater habitats. Salinity (the concentration of dissolved salts, such as sodium chloride, in a body of water) affects the kinds of organisms present in aquatic ecosystems, as does the amount of dissolved oxygen, Water greatly interferes with the penetration of light, so floating aquatic organisms that photosynthesize must remain near the water's surface, and vegetation attached to the bottom can grow only in shallowest water. In addition, low levels of essential mineral nutrients limit the number and distribution of living things in certain aquatic envi­ronments.

Aquatic habitats contain three main ecological categories of organisms: free-floating plankton, strongly swimming nekton, and bottom dwelling benthos. Plankton are small or microscopic organism that are relatively feeble swimmers and thus, for the most part, are carried about at the mercy of currents and waves. They are unable to swim for horizontally, but some species are capable of large daily vertical migrations and are found at different depths of water at different times of the day or sometimes at different seasons. Plankton is generally subdivided into two major categories. Phytoplankton — photosynthetic cyanobacteria and free-floating algae of several types—are pro­ducers and form the bases of most aquatic food chains. Zooplanktons are a nonphotosynthetic organism that includes protozoa (animal-like protists) and small animals, including the larval stages of many animals that are large as adults. Nekton are larger, more strongly swimming organisms such as fish, whales, and turtles. Benthos are bottom dwelling creatures that fix themselves to one spot (such as oysters and barnacles), burrow into the

Sand (such as many worms and echinoderms), or simply walk about on the bottom (such as lobsters and brittle stars).

Rivers and Streams

The nature of a flowing-water ecosystem changes greatly between its source (where it begins) and its mouth (where it empties into another body of water). For example, headwa­ter streams (small streams that are the sources of a river) are usually shallow, swiftly flowing, highly oxygenated, and cold. In contrast, downstream

Lakes and Ponds

A large lake has three basic life zones: littoral, limnetic, and profundal; smaller lakes and ponds typically lack a profundal zone. The littoral zone is the shallow-water area along the shore of a lake or pond. It includes lake-shore vegetation such as cattails and bur-reeds, plus sev­eral deeper-dwelling aquatic plants and algae. The littoral zone is the most productive zone of the lake (that is, photosynthesis is greatest here), in part because it receives nutrients, which stimulate the growth of plants and algae, from the surrounding land. Animals of the littoral zone in­clude frogs and their tadpoles, turtles, worms, cray­fish and other crustaceans, insect larvae, and many fish, such as perch, carp, and bass. Here, too, at least in the quieter areas, one finds surface dwellers such as water striders and whirligig beetles.

    The limnetic zone is the open-water area away from the shore; it extends down as far as sunlight penetrates. The main organisms of the limnetic zone are microscopic phytoplankton and zooplank-ton. Larger fish also spend most of their time in the limnetic zone, although they may visit the littoral zone to feed and breed. Owing to the depth of this zone, less vegetation grows here.

    The deepest zone of a large lake, the profundal zone, is below the limnetic zone. Because of the lack of light, producers do not live in the profundal zone, Much food drifts into the profundal zone from the littoral and limnetic zones. When dead plants and animals reach the profundal zone, decay bacte­ria decompose them, liberating the minerals contained in their bodies. These minerals are not effec­tively recycled, because there are no producers to absorb them and incorporate them into the food chain. As a result, the profundal habitat tends to be both mineral-rich and anaerobic (without oxygen), and hence occupied by few forms of higher life.

Thermal Stratification and Turnover in Temperate Lakes

The marked layering of lakes caused by how far light penetrates is accentuated by thermal stratification, which is characteristic of large lake in temperate areas. Thermal stratification occurs because the summer sunlight penetrates and warms surface waters, making them less dense. In the summer, cool (and therefore denser) water remains at the lake bottom, separated from the warm (and therefore less dense) water above by an abrupt tem­perature transition called the thermocline.

    In temperate lakes, falling temperatures in au­tumn cause a mixing of the lake waters called the fall turnover. (Because there is little seasonal temperature variation in the tropics, turn­overs are not common there.) Fall turnover occurs because, as the surface water cools, its density in­creases and it displaces the less dense, warmer, min­eral-rich water beneath. The warmer water then rises to the surface, where it cools and sinks. This

process of cooling and sinking continues until the lake reaches a uniform temperature throughout.

The area of shoreline between low and high tides is called the intertidal zone. Although the high levels of light and nutrients, together with an abundance of oxygen, make the intertidal zone a biologically productive habitat, it is also a very stressful one. If an intertidal beach is sandy, the inhabitants must contend with a constantly shift­ing environment that threatens to engulf them and gives them scant protection against wave action (Figure 6-18). Consequently, most sand dwelling

organisms, such as mole crabs, are continuous and active burrowers. Because they are able to follow the tides up and down the beach, they usually do not have any notable adaptations to survive desic­cation (drying out) or exposure.

A rocky shore provides a fine anchorage for seaweeds and animals, but is exposed to wave action when immersed during high tides and to dry­ing and temperature changes when exposed to the air during low tides. A typical rocky shore inhabitant has some way of sealing in mois­ture, perhaps by closing its shell if it has one, plus a powerful means of anchorage to the rocks— mussels, for example, have horny, threadlike an­chors, and barnacles have a special cement gland. Rocky shore intertidal algae (seaweeds) usually have thick, gummy coats, which dry out slowly when exposed to air, and flexible bodies not easily broken by wave action. Some rocky shore commu­nity inhabitants hide in burrows or crevices at low tide, and some small semiterrestrial crabs run about the splash line, following it up and down the beach.

    The marine environment has two main divi­sions, the benthic environment (the ocean floor) and the pelagic environment (the ocean water). The upper reaches of the pelagic environment comprise the euphotic region, which extends from the surface to a depth of approximately 100m (325 ft). Light penetrates the euphotic zone in suf­ficient amounts to support photosynthesis. The pe­lagic environment is divided into two provinces, the neritic province and the oceanic province.

    The neritic province is open ocean from the shoreline to a depth of 200 m (650 ft). Nekton (such as sharks, tunas, and porpoises) and larger benthic organisms (such as corals, spiny lobsters, and starfish) are mostly confined to the shallower neritic waters (less than 60 m, or 195 ft, deep) be­cause that is where their food is. Not only are there seaweeds on the bottoms of shallower areas, but there are also large numbers of phytoplankton in the water itself.

    The oceanic province is the part of the open ocean that is deeper than 200m (650ft). This is most of the ocean; in fact, about 88 percent of the ocean is more than 1.5 km (0.9 mi) deep. Because light cannot penetrate to such depths, the oceanic

province supports few organisms. Most of the life that exists under the tremendous pressure and dark­ness of the abysses depends upon whatever food drifts down into its habitat from the upper, lighted regions. The principal exceptions are found at the deep-sea thermal vents.

Animals of the abysses are strikingly adapted to darkness and scarcity of food. Abys­sal fish, for example, have huge jaws that enable them to swallow large food particles they might encounter. (If an organism does not chance upon food very often, it needs to eat as much as possible when food is present.) Many abyssal animals have illuminated organs, enabling them to see one an­other for mating or food capture. A great many are predators or scavengers (there is no other choice) and live in dispersed populations.

province supports few organisms. Most of the life that exists under the tremendous pressure and dark­ness of the abysses depends upon whatever food drifts down into its habitat from the upper, lighted regions. The principal exceptions are found at the deep-sea thermal vents.

Animals of the abysses are strikingly adapted to darkness and scarcity of food. Abys­sal fish, for example, have huge jaws that enable them to swallow large food particles they might encounter. (If an organism does not chance upon food very often, it needs to eat as much as possible when food is present.) Many abyssal animals have illuminated organs, enabling them to see one an­other for mating or food capture. A great many are predators or scavengers (there is no other choice) and live in dispersed populations.

The Importance of Coral

Coral formations are important ecosystems, as rich in specks as a tropical rain forest. Most reefs are between 5,000 and 10,000 years old; some have existed for several million years. A single reef can contain more than 3,000 species of corals, fish, and shellfish. Nearly one-third of all the world's fish live on coral reefs arid many more depend on them at some stage in their life cycle. Reefs are abun­dant fishing grounds, critical to the fishing industries of countries such as the Philippines and Indonesia. They also reduce the energy of waves, thereby protecting shorelines against storms. The destruction of one reef off the coast of Sri Lanka pushed the shore­line back some 350 yards. Yet the world's coral reefs are being degraded and destroyed. Of 109 countries with large reef formations, 90 are damaging them. Silt washing down­stream from clear-cut inland forests has been smothering the world's reefs. Pollution, land reclamation, tourism, and the mining of corals for building material are also taking a heavy toll. Regeneration cannot keep pace: A new coral colony requires 20 years to grow to the size of a human head.