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Aquatic Ecosystems

Aquatic Ecosystems

Most of the water on our planet is in the oceans that cover 71 percent of Earth's surface. Less than 1 percent of all the water is considered freshwater, and most of that is frozen in the polar ice caps. The study of freshwater aquatic ecosystems , limnology, has often been separated from the study of marine systems, oceanography. Although freshwater and marine ecosystems are extremely diverse in structure, all of the plants and animals must live in or on water. The physical constraints and opportunities for life in water, and how all of these living organisms interact with each other in a liquid medium, give a unifying theme to aquatic studies.

The Influence of Water

Water is a liquid and has a greater density and viscosity than air. It can absorb a large amount of solar radiation with a small increase in temperature. Once heated, it will get cooler at a slower rate than land. Aquatic organisms are therefore somewhat buffered against massive, rapid changes in temperature. Aquatic organisms, however, may have to adapt to the water temperature in hot freshwater springs, to the hot vents in the ocean floor in volcanic areas, and to the chilling cold of the water of the polar oceans and freezing winters in the temperate zones of the world. Cold water is denser than hot water, and this may lead to massive mixing and turnover in lakes. In rivers and streams, the water depth and the water flow rate will determine the structure of the biological communities. Plants have to be attached and be highly specialized in structure to survive in fast-flowing water. In larger bodies of water, such as lakes and oceans, wind and tides will mix waters and carry sediments and organisms over large distances. Wave action will alter the physical structure and the geography of shores and coastlines and have a tremendous effect on the biological communities that can survive there.

Water is not always clear, and photosynthetic plants are limited to growing on or near the water surface, or in shallower coastal zones where they can receive sufficient sunlight. While marine habitats all have high levels of salts, the nutrient ion content and the alkalinity and acidity (pH) of freshwater habitats are variable. Plants in water need to get all of their mineral nutrients from the water or from the sediment below. They must also find sufficient oxygen and carbon dioxide to respire, photosynthesize, grow, and reproduce.

Diversity of Habitats

In both freshwater and marine ecosystems, the majority of the photosynthetic plants are algae. Most of these algae are microscopic. Huge colonies of algal cells, however, can be seen as algal blooms and colonies of filamentous forms are easily observed in patches on rocks and in slow-moving streams. In oceans, microalgae form a major part of the plankton. This phytoplankton is pelagic, living freely in the seawater. Large plants (macrophytes) are less common in oceans, although huge mats of large, floating algae are found in the Sargasso Sea in the mid-Atlantic. Forests of large algae (seaweeds) and some flowering plants (sea grasses) are confined to shallower coastal habitats. These macrophytes are generally attached to rocks or sand on the ocean bottom: they are benthic organisms. Rivers, lakes, and other freshwater habitats contain benthic or pelagic flowering aquatic plants, ferns, mosses, and liverworts that have reinvaded the water from the land at various times during their evolution. In rivers and streams, the water depth and the water flow rate will be a key determinant of plant success. In oceans, currents and tides will carry sediments and organisms over large distances.

All of the aquatic plants are primary producers. Their ability to fix carbon dioxide into carbohydrates by their light-driven photosynthetic reactions makes them the basis of the aquatic food chain. They are grazed upon and eaten, and, when they die, their structures are degraded by a huge variety of dependent organisms in the food web. These freshwater and marine rine organisms will include fish and other strictly aquatic organisms as well as bacteria, fungi, birds, and mammals that are related to land-living forms.

Freshwater aquatic ecosystems are very diverse. They include lakes, ponds, rivers, and streams with a wide range of depth, flow rates, and water chemistry. Aquatic ecosystems include wetlands, where the water is either just below or just above the soil surface. The depth and distribution of this water may change with season. Wetlands are termed bogs, fens, swamps, and marshes. These have been extensively catalogued depending upon the position of the water table , the water chemistry, and the plant communities that grow in them.

Aquatic ecosystems are not isolated from the adjacent land nor from each other. The inputs and exchanges of sediments, water, and organisms are continuous. The marine shorelinethe littoral zoneis particularly rich in species and has a clearly visible zonation of organisms and habitats from the land into the sea. A similar habitat zonation is seen on the shore of a lake. Riverbanksthe riparian zonewill have a specialized plant community influenced by the amount of shading by trees on shore. These interactions between freshwater and marine ecosystems are seen both in estuaries, where rivers flow into the sea, and in coastal salt marshes.

Apart from this wide variety of natural aquatic ecosystems, humans continue to build and develop a large number of artificial aquatic ecosystems. These range from reservoirs, canals, channels for irrigation and drainage, and paddy fields for the production of rice, through lakes and ponds built for landscaping and fish culture, down to the decorative home aquarium. The understanding and management of these natural and artificial aquatic ecosystems, together with the need to slow the destruction and move to restore many of the natural areas, will continue to challenge us all. Human populations have always lived close to water. The ecosystems on seacoasts, by lakes, and in the river valleys are under tremendous pressure from urbanization , industrialization, and commerce.

see also Algae; Aquatic Plants; Coastal Ecosystems; Ecosystem; Wetlands.

Roger F. Horton

Bibliography

Dawes, Clinton J. Marine Botany, 2nd ed. New York: John Wiley & Sons, 1998.

Dobson, Mike, and Chris Frid. Ecology of Aquatic Systems. Harlow, England: longman, 1998.

Giller, Paul S., and Bjorn Malmqvist. The Biology of Streams and Rivers. Oxford: Oxford University Press, 1998.

Wetzel, Robert G. Limnology. Philadelphia: W.B. Saunders, 1975.

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