16. Community Interactions


16.5. Major Aquatic Ecosystems


Terrestrial biomes are determined by the amount and kind of precipitation and by temperatures. Other factors, such as soil type and wind, also play a part. Aquatic ecosystems also are shaped by key environmental factors. Several important factors are: how far the sun’s rays penetrate the water, the depth of the water, currents, the nature of the bottom substrate, the water temperature, and the amount of dissolved salts.

An important determiner of the nature of aquatic ecosystems is the amount of salt dissolved in the water. Those that have a high salt content (35 parts per thousand or greater) are called marine ecosystems, and those that have little dissolved salt (less than 0.5 parts per thousand) are called freshwater ecosystems.


Marine Ecosystems

Just like terrestrial ecosystems, marine ecosystems are quite diverse. Ecologists recognize several categories of marine ecosystems.


Pelagic Marine Ecosystems

In the open ocean, many kinds of organisms float or swim actively. Shrimp, squid, fish, and whales swim actively as they pursue food. Organisms that are not attached to the bottom are called pelagic organisms, and the ecosystem they are a part of is called a pelagic ecosystem.

The term plankton is used to describe aquatic organisms that are so small and weakly swimming that they are simply carried from place to place by currents. As with all ecosystems, organisms that carry on photosynthesis are the base of the energy pyramid. Phytoplankton are planktonic organisms that carry on photosynthesis. Most phytoplankton are microscopic, single-celled algae and bacteria. The upper layer of the ocean, where the sun’s rays penetrate, is known as the euphotic zone. It is in this euphotic zone where phytoplankton are most common. The thickness of the euphotic zone varies with the degree of clarity of the water but in clear water can be up to 150 meters (500 feet) in depth.

Zooplankton are small, weakly swimming animals of many kinds (crustaceans, jellyfish, and juvenile fish), and several kinds of protozoa, that feed on the phytoplankton by filtering the phytoplankton from the water. Zooplankton are often located at a greater depth in the ocean than the phytoplankton but migrate upward at night and feed on the large population of phytoplankton. The zooplankton are in turn eaten by larger animals such as fish and larger shrimp, which are eaten by larger animals such as salmon, tuna, sharks, squid, whales and seals. (See figure 16.27.)

A major factor that influences the nature of a marine community is the kind and amount of material dissolved in the water.

Coral reef ecosystems are produced by coral animals that build cup-shaped external skeletons around themselves. Corals protrude from their skeletons to capture food and expose themselves to the sun. Exposure to sunlight is important because corals contain single-celled algae within their bodies. These algae carry on photosynthesis and provide both themselves and the coral animals with the nutrients necessary for growth. This mutualistic relationship between algae and coral is the basis for a very productive community of organisms.

The skeletons of the corals provide a surface upon which many other kinds of animals live. Some of these animals feed on corals directly, while others feed on small plankton and bits of algae that establish themselves among the coral organisms. Many kinds of fish, crustaceans, sponges, clams, and snails are members of coral reef ecosystems. Because they require warm water, coral ecosystems are found only near the equator. Coral ecosystems also require shallow, clear water since the algae must have ample sunlight to carry on photosynthesis. Coral reefs are considered one of the most productive ecosystems on Earth (see figure 16.28).



FIGURE 16.28. Coral Reef

Corals are small sea animals that secrete external skeletons. They have a mutualistic relationship with certain algae, which allows both kinds of organisms to be very successful. The skeletal material serves as a substrate upon which many other kinds of organisms live.


An abyssal ecosystem is a benthic ecosystem that occurs at great depths in the ocean. In such deep regions of the ocean there is no light to support photosynthesis. Therefore, the animals must rely on a continuous rain of organic matter from the euphotic zone above them. Essentially, all of the organisms in this environment are scavengers that feed on whatever drifts their way. Many of the animals are small and generate light that they use for finding or attracting food.



An estuary is a special category of aquatic ecosystem that consists of shallow, partially enclosed areas where freshwater enters the ocean. The saltiness (0.5-30 parts per thousand) of the water in the estuary changes with tides and the flow of water from rivers. The organisms that live here are specially adapted to this set of physical conditions, and the number of species is less than in the ocean or in freshwater.

Estuaries are particularly productive ecosystems because of the large amounts of nutrients introduced into the basin from the rivers that run into them. This is further enhanced by the fact that the shallow water allows light to penetrate to most of the water in the basin. Phytoplankton and attached algae and plants are able to use the sunlight and the nutrients for rapid growth. This photosynthetic activity supports many kinds of organisms in the estuary.

Estuaries are especially important as nursery sites for fish and crustaceans such as flounder and shrimp. The adults enter these productive, sheltered areas to reproduce and then return to the ocean. The young spend their early life in the estuary and eventually leave as they get larger and are more able to survive in the ocean. Estuaries also trap sediment. This activity tends to prevent many kinds of pollutants from reaching the ocean and also results in the gradual filling in of the estuary, which may eventually become a salt marsh and then part of a terrestrial ecosystem.


Human Impact on Marine Ecosystems

Since the oceans cover about 70% of the Earth’s surface, it is hard to imagine that humans can have a major impact on them. However, we use the oceans in a wide variety of ways. The oceans provide a major source of protein in the form of fish, shrimp, and other animals. However, overfishing has destroyed many of the traditional fishing industries of the world such as cod fishing off the east coast of North America. Fish farming in the ocean involves the use of pens to enclose fish. The dense populations in the pens result in pollution of the ocean from the food that is provided to the fish and the waste products the fish produce. These captive populations have also caused diseases to spread from farmed species to wild fish. Estuaries are important fishing areas but are impacted by the flow of fertilizer, animal waste, and pesticides down the rivers that drain farmland and enter estuaries. The use of the oceans as transportation results in oil pollution, and trash regularly floats onto the shore. Coral reefs are altered by fishing and siltation from rivers. Mangrove swamps are destroyed as they are converted to areas for the raising of fish. It is clear that humans have a great impact on marine ecosystems.


Freshwater Ecosystems

Freshwater ecosystems differ from marine ecosystems in several ways. The amount of salt present is much less, the temperature of the water can change greatly, the water is in the process of moving to the ocean, oxygen can often be in short supply, and the organisms that inhabit freshwater systems are different.

Freshwater ecosystems can be divided into two categories: those in which the water is relatively stationary, such as lakes, ponds, and reservoirs, and those in which the water is running downhill, such as streams and rivers.


Lakes and Ponds

Large lakes have many of the same characteristics as the ocean. If the lake is deep, there is a euphotic zone at the top, with many kinds of phytoplankton, and zooplankton that feed on the phytoplankton. Small fish feed on the zooplankton and are in turn eaten by larger fish. The species of organisms found in freshwater lakes are different from those found in the ocean, but the roles played are similar, so the same terminology is used.

Along the shore and in the shallower parts of lakes, many kinds of flowering plants are rooted in the bottom. Some have leaves that float on the surface or protrude above the water and are called emergent plants. Cattails, bulrushes, arrowhead plants, and water lilies are examples. Rooted plants that stay submerged below the surface of the water are called submerged plants. Elodea and Chara are examples. This region, with rooted vegetation, is known as the littoral zone, and the portion of the lake that does not have rooted vegetation is called the limnetic zone. (See figure 16.29.)



FIGURE 16.29. Lake Ecosystem

Lakes are similar in structure to oceans except that the species are different because most marine organisms cannot live in freshwater. Insects are common organisms in freshwater lakes, as are many kinds of fish, zooplankton, and phytoplankton.


Many kinds of freshwater algae also grow in the shallow water, where they may appear as mats on the bottom or attached to vegetation and other objects. Associated with the plants and algae are a large number of different kinds of animals. Adult and larval insects are particularly common in freshwater ecosystems along with fish, crayfish, clams, and many birds and mammals.

Although the water molecule (H2O) has oxygen as part of its structure, this oxygen is not available to organisms. The oxygen that they need is dissolved molecular oxygen (O2), which enters water from the air or when it is released as a result of photosynthesis by aquatic plants and other photosynthetic organisms. When water tumbles over rocks in a stream or crashes on the shore as a result of wave action, air and water mix, which allows more oxygen to dissolve in the water. The amount of dissolved oxygen affects the kind of organisms that live in the water.


Streams and Rivers

Streams and rivers are a second category of freshwater ecosystem. Since the water is moving, planktonic organisms are less important than are attached organisms because plankton are swept downstream. Most algae grow attached to rocks and other objects on the bottom. Since the water is shallow, light can penetrate easily to the bottom (except for large or extremely muddy rivers). Even so, it is difficult for photosynthetic organisms to accumulate the nutrients necessary for growth, and most streams are not very productive. As a matter of fact, the major input of nutrients is from organic matter that falls into the stream from terrestrial sources. These are primarily the leaves from trees and other vegetation, as well as the bodies of living and dead insects. Within streams there is a community of organisms that is specifically adapted to use the debris from terrestrial sources as a source of food. Bacteria and fungi colonize the organic matter, and many kinds of insects shred and eat this organic matter along with the fungi and bacteria living on it. The feces (intestinal wastes) of these insects and the tiny particles produced during the eating process become food for other insects that build nets to capture the tiny bits of organic matter that drift their way. These insects are in turn eaten by carnivorous insects and fish.

Organisms in larger rivers and muddy streams, which have less light penetration, rely in large part on the food that drifts their way from the many streams that empty into the river. These larger rivers tend to be warmer and to have slower moving water. Consequently, the amount of oxygen is usually less, and the species of plants and animals change. Any organic matter added to the river system reduces the oxygen in the water as it decays. Plants may becomes established along the river bank and contribute to the ecosystem by carrying on photosynthesis and providing hiding places for animals.


Human Impact on Freshwater Ecosystems

Freshwater resources in lakes and rivers account for about 0.02% of the world’s water. Most freshwater ecosystems have been heavily impacted by human activity. Any activity that takes place on land ultimately affects freshwater because of runoff from the land. Agricultural runoff, sewage, sediment, and trash all find their way to streams and lakes.



11. How do phytoplankton and zooplankton differ?

12. Describe how the producers of benthic and pelagic ecosystems differ.

13. List two ways in which the kinds of organisms present in lakes differ from those in shallow parts of the ocean.

14. Describe two abiotic differences between an estuary and the ocean.