An ecosystem is a community of organisms that live and interact within a particular environment. In an aquatic ecosystem, that environment is water, and all the system’s plants and animals live either in or on that water. The specific setting and type of water, such as a freshwater lake or saltwater marsh, determines which animals and plants live there.
Marine, or ocean, systems cover about 70 percent of the Earth’s surface and are identified by the presence of dissolved salts in the water. The level of salinity averages about 35 parts per thousand g of water, but it can vary in response to climate or a nearby source of freshwater. Marine organisms must adapt to either a constantly changing or stable level of salt content and cannot move successfully from one to the other.
Saltwater ecosystems range from the abundant life of coastal areas to the nearly barren ocean bottom. In marine habitats the food chain begins with plankton, microorganisms that require sunlight for energy and growth, so systems closest to the surface or in relatively shallow water support more life. These include estuaries, salt marshes, coral reefs and other tropical habitats, and intertidal areas such as lagoons and kelp beds. Animal life in marine ecosystems ranges from microscopic zooplankton through fish of all sizes to marine mammals, including seals, whales and manatees.
Freshwater--water that is either drinkable or has little or no salt content--supports its own aquatic ecosystems. These include rivers and streams, lakes and ponds, wetlands and even groundwater. Each of these systems is unique, and even within categories, any specific habitat is affected by altitude, temperature and humidity. For instance, a plant native to a warm shallow lake in the tropics could not survive on the steep banks of a cold, fast-moving mountain stream.
Freshwater ecosystems provide homes for a wide variety of animal life including insects, amphibians and fish. One estimate of fish species puts the number that live in freshwater at 40 percent of the Earth’s total. According to Brian Richter of The Nature Conservancy, at least 45,000 freshwater fish species have been cataloged. Worms, mollusks, algae and bacteria all live in freshwater systems, as do innumerable varieties of plants. In addition, animals such as birds, otters and bears use freshwater ecosystems as a food source.
Human use of aquatic ecosystems also plays a role in their health and survival. Freshwater systems provide water for drinking, agricultural and industrial use, and sanitation, while marine systems provide fertilizers, food additives and cosmetics ingredients. Both types of systems provide food, transportation and recreation. However all of this is threatened by pollution caused by agricultural and urban runoff, the introduction (inadvertent or not) of exotic species to specific habitats, overfishing, coastal development and even global warming.