Aquatic Ecosystems

Introduction

Communities of plants and animals living in water are known as aquatic ecosystems. They are divided into two main groups. Freshwater ecosystems are found in water containing low concentrations of salts, from ponds to estuaries. Marine ecosystems are found in the saltwater of seas and oceans. Most of us are not far away from an aquatic ecosystem of some kind, whether it be in the ocean or a local pond.

The nature of an aquatic ecosystem is shaped, as on land, by the availability of food, oxygen, and the prevailing temperature. Added to this is salinity, which is the salt concentration of the water. Aquatic ecosystems in shallow waters, where there is plenty of sunlight, generally tend to be the most productive. Water pollution, generally coming from human activities, comprises the greatest pressure on aquatic ecosystems. For instance, fish can be killed by acid rain in lakes or lack of oxygen where excess nutrients have been dumped in an estuary.

Historical Background and Scientific Foundations

Aquatic ecosystems are the communities of organisms, their surrounding watery environment, and the relationship between the two. Aquatic, or watery, environments are divided into freshwater and marine. Freshwater has less than one gram per liter of dissolved solids, mainly salts, of which sodium chloride is the most important as far as living organisms are concerned. Salinity is the term given to the concentration of salts in a body of water. Saltwater contains more than one gram per liter of dissolved solids.

Freshwater ecosystems are found in ponds, lakes, reservoirs, rivers, and streams. Marine, or saltwater, ecosystems are found in the seas and oceans. Estuaries, which are places where a river meets the sea, such as San Francisco Bay, are part freshwater and part marine in their makeup. The diversity of an aquatic ecosystem depends upon temperature, availability of light, nutrients, oxygen, and salinity. This makes for very different ecosystems in, for instance, a shallow pond in a temperate climate with plenty of light compared to the deep ocean where conditions are dark and cold.

A wide range of plants, animals, and microbes are found in aquatic ecosystems, from bacteria that tolerate high temperatures in hydrothermal vents at the bottom of the ocean to the blue whale, which is the world’s largest animal. The smallest are the phytoplankton and zooplankton, the microscopic animals and plants forming the bottom layer of the aquatic food chain. There are also many aquatic invertebrates including worms, insects, and crustaceans. Among the aquatic vertebrates, amphibians, such as frogs, live on land and water, while fish are purely water-dwelling animals. Many birds, such as kingfishers, gulls, and ducks, live on or near water. Whales are one of the few mammals that can live in the marine environment.

Rivers and streams are lotic, or flowing, freshwater environments. Their organisms tend to be small, with flattened bodies, so they are not swept away. Fallen leaves, insects, and other detritus are important food sources. Lakes and reservoirs are lentic, or layered, systerms with still water. The littoral layer, near the edge of a lake, generally has a large ecosystem with organisms that can use both land and water such as dragonflies, frogs, ducks, and turtles.

Estuaries are regions where the river meets the sea, or ocean. They comprise a semi-enclosed coastal area, open to the marine environment. An estuary has different ecosystems, such as a salt marsh, which is farthest from the sea. Mudflats are tidal and are therefore, sometimes aquatic and sometimes terrestrial. The range of species in mudflats tends to be narrow, but there can be many individuals within this range. The channels are the areas of an estuary that connect to the ocean and are generally rich in fish and crustaceans like crabs. Overall, an estuary is a rich ecosystem because it tends to be shallow and there is a good mixing of nutrients.

The main differences between freshwater and marine environments are the salinity of the water, the depth of the water, and the availability of sunlight for photosynthesis. Freshwater organisms need mechanisms to prevent water loss, because water flows from regions of high salt concentration to those of low salt concentration. Marine organisms at depth have to deal with the pressure of water on their bodies and lack of sunlight. These factors mean that freshwater and marine ecosystems can be very different in their diversity.

Thousands of different invertebrates are found in the seas and oceans. There are sponges, echinoderms (which include starfish, sea urchins, and sea cucumbers), and cnidarians, such as jellyfish, corals, and sea anemones. Mollusks, which are found on land and sea, include the gastropods (limpets, slugs, and snails), cephalopods (octopus, squid, and cuttlefish), and bivalves (clams, mussels, cockles, oysters, and scallops). Most crustaceans are marine animals and include crabs, lobsters, shrimps, barnacles, and woodlice. There are also many species of marine worms living on the sea floor or in the sand of coastal areas.

Around half of the 25,000 known species of fish live in the marine environment, mainly in shallower, warmer

WORDS TO KNOW

ECOSYSTEM: The community of individuals and the physical components of the environment in a certain area.

EUPHOTIC ZONE: The uppermost layer of a body of water in which the level of sunlight is sufficient for photosynthesis to occur.

EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.

FRESHWATER: Water containing less than one gram per liter of dissolved solids.

LITTORAL: The region of a lake near the shore.

LOTIC: Flowing water, as in rivers and streams

SYMBIOSIS: A pattern in which two or more organisms of different species live in close connection with one another, often to the benefit of both or all organisms.

waters. Meanwhile, whales are marine mammals, breathing with lungs, and the largest animals in the oceans. In the open ocean, the nature of an ecosystem is dependent on depth. In the euphotic zone, there is net primary pro-

IN CONTEXT: DIATOMS IN AQUATIC ECOSYSTEMS

Diatoms belong to the taxonomic phylum Bacillariophyta. There are approximately 10,000 known diatom species. Diatoms are microscopic, single-celled algae that have intricate glasslike outer cell walls partially composed of silicon; they can be identified based on the structure of these walls. Many diatom species are planktonic, suspended in the water column moving at the mercy of water currents. Others remain attached to submerged surfaces. One gallon of water may contain millions of diatoms; their abundance makes them important food sources in aquatic ecosystems. When diatoms die, their cell walls are left behind and sink to the bottom of bodies of water. Massive accumulations of diatom-rich sediments compact and solidify over long periods of time to form rock rich in fossilized diatoms that is mined for use in abrasives and filters.

duction of food by photosynthesis carried out by 4,000 or so species of phytoplankton. These are eaten by slightly larger animals, such as tiny crustaceans called copepods. These, in turn, will be eaten by animals bigger than they are. Farther down, at 4 miles (6.5 km) deep and beyond, there is no light in these zones close to the ocean floor except for that which is emitted from the organisms themselves. Thousands of species, including bacteria, squid, and fish, emit flashes of light by a process known as bioluminescence. They feed on one another and on detritus falling from the upper layers.

The marine environment contains some important ecological niches. For instance, coral reefs consist of animals called stony corals, each of which is a polyp with tentacles that can trap organisms. These polyps live in symbiosis with photosynthetic algae. Coral reefs tend to occur in warm, shallow, clear waters where they provide a home for a diverse community of fish, worms, and crustaceans.

Impacts and Issues

Many human activities threaten the health of aquatic ecosystems. For instance, acid rain created from sulfur and nitrogen oxide emissions have turned many lakes and streams acidic, so they no longer support various fish species. Meanwhile, the building of dams to create hydroelectric power plants can block the routes of migratory fish like salmon.

In the marine environment, the coral reefs are among the world’s most threatened ecosystems. They are affected by a range of factors, including destructive fishing practices, pollution, sewage, and global warming. Estuaries and shore areas are also at risk from pollution, which can cause eutrophication by raising nutrient levels in the water. Eutrophication encourages the growth of plant decomposers, which consume available oxygen in the water, affecting fish, and other marine organisms.

See Also Estuaries; Freshwater and Freshwater Ecosystems; Marine Ecosystems; Oceans and Coastlines; Rivers and Waterways

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann, R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

Web Sites

Environment Canada. “Aquatic Ecosystems.” http://www.ec.gc.ca/water/en/nature/aqua/e_ecosys.htm (accessed March 10, 2008).

U.S. Environmental Protection Agency. “Aquatic Biodiversity.” http://www.epa.gov/bioiweb1/aquatic/index.html (accessed March 10, 2008).

Susan Aldridge