Coastal Ecosystems

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An ecosystem is an interacting community of organisms and their non-living physical environment occupying a certain place and time. Coastal ecosystems occupy the margins of the land and the sea. There are many different types: salt marshes, mangrove swamps, sand dunes, seagrass meadows, coral reefs, kelp forests, tidal flats, rocky intertidal, maritime forests, and coastal heathlands. All are heavily influenced by some combination of saltwater, ocean waves, currents, and ocean breezes, though not necessarily all of these.

The major interactions of organisms and their environment in coastal ecosystems include energy transfer and cycling of materials. These involve several functional groups of organisms. Plants and algae are the major primary producers, that is, organisms that produce their own food through the process of photosynthesis. They use the energy from the sun and the nutrients washed down to the coast from the surrounding land or brought to the coast by the ocean.

The plants living in constant or periodic contact with ocean water are called halophytes ("salt plants"). They must have special adaptations to be able to thrive because saltwater is toxic to most plants.

Plants and algae are the bases of the coastal food chain. They may be consumed by herbivores, such as insects or geese that feed on salt marsh grasses, snails that consume seaweeds on rocky shores, or fish that graze on tropical seagrass beds. Except for the intertidal marshes and mangrove swamps, the place of insects in coastal ecosystems is minor, their ecological role being replaced by crustaceans (such as crabs, shrimp, lobsters, and beach fleas) and mollusks (snails, clams, mussels, etc.) All these, in turn, may become food for carnivores, such as birds (shorebirds, waterfowl, hawks, etc.) or fish. Many animals living in coastal ecosystems do not feed directly on plants or other animals but feed on detritus, nonliving plant material that may contain a large amount of bacteria and fungi. The bacteria and fungi that colonize particles of detritus act to break down this material to simple chemical compounds that can be recycled.

Coastal ecosystems differ from terrestrial ones in several significant ways. The ocean contributes to the exchange of materials, bringing nutrients and removing waste products. In terrestrial ecosystems, the exchange of materials between organisms and their environment does not involve this major mediating agent.

The dominant types of producer organisms in terrestrial ecosystems are plants. In coastal ecosystems they include plants, macroalgae (seaweeds), and phytoplankton (unicellular algae). Seaweeds reach their greatest level of diversity and productivity in coastal ecosystems.

An estuary is a semienclosed body of water where freshwater meets the sea. Typically located at the mouth of rivers, estuaries have characteristics of both fresh and marine habitats and serve as a vital ecological link between the two realms. One of the major factors that determines the place where different organisms can live within an estuary is the gradient of salt concentration, that is, the salinity. The upper reaches of the estuary are most influenced by the river and therefore may be almost completely freshwater. As one moves downstream the influence of the sea becomes increasingly dominant. The salinity of the water gradually increases until at the mouth of the estuary, it is similar to that of the surrounding coastal ocean.

There are daily changes in the movement of water and the salinity profile within an estuary. At high tide the estuary is flooded with higher salinity seawater, and at low tide the river water may dominate and the flow is in a downstream direction. Seasonal changes in response to times when greater rainfall and snowmelt wash down the rivers also strongly influence the estuary.

Another characteristic of estuaries is the salt wedge. Since saltwater is more dense than freshwater, the saltwater tends to underlie the river water where the two meet. Thus the surface water of the estuary is usually much fresher than that at the bottom.

These changes in salt concentration within the estuary present a real challenge to plants and animals. They not only have to be salt tolerant, but they also have to be able to tolerate changes in salinity, thus estuaries have their own unique species that differ from those of wholly freshwater or marine habitats. Those few plants that have been able to adapt to life in the estuaries, such as seagrasses, salt marsh plants, and different types of algae, are often extremely productive because having adapted to tolerate the stresses of changing salinities, growing conditions are ideal. Intertidal plants, such as salt marsh grasses and mangrove trees, submerged sea grasses, and algae, are constantly moist with a steady supply of nutrients coming from the sea or the river. As a result, estuaries are among the most productive ecosystems on Earth in terms of the amount of organic matter produced by plants and algae. Estuaries are home to abundant fish, bird, and invertebrate populations, which take advantage of this tremendous plant and algal productivity. Many species of ocean fishes, including a number that are commercially important, spend their juvenile stages in the relative safety of estuaries where the abundance of life sustains their growth to adulthood.

Located at the end points of watersheds, estuaries are often sites where pollutants accumulate and thus the estuaries are very sensitive to human activities. Pollutants generated in the watershed and transported downstream by rivers tend to settle out once they reach estuaries. Thus estuaries serve as barometers of the health of entire watersheds.

Coastal dunes are an unstable, shifting habitat whose very structure is a product of ocean currents, winds, and storms. Currents and waves along the shore deposit sand on the beach, then winds shape the sand into series of small hills that often gradually migrate inland to be constantly replacedat the beachfront by new dunes. Winter storms may completely reshape the landscape, blowing holes in the dunes closest to the ocean and starting the process over.

Sand is unstable, which is why dunes can achieve a maximum stature of only several hundred feet. Dune plants have to be able to tolerate life in shifting sands where water rapidly percolates through the soil and out of the reach of plant roots. Plants that grow on sand dunes must be able to tolerate harsh, desert-like conditions where, as any beachgoer who has walked barefoot on hot sand will attest, there is no shade and daytime temperatures can be extremely hot. Dune plants have a lot in common with desert plants, in which fresh water loss and overheating are real problems. Thus many are succulent or have thick cuticles on their leaves and deeply sunken stomata to prevent water loss. These same kind of adaptations are found in cacti and other desert plants.

The roots of some dune plants play a role in stabilizing sand dunes, helping to shape the nature of this ecosystem. Beach grass is particularly notable in this regard and is often planted deliberately by people to keep dunes in place. The rapidly growing network of roots produced by beach grass penetrates deep into the dune, lending structural support that can keep the dune in place except under the most severe coastal storms. Beach naupaka, a shrub, is valued the same way on Hawaii and other Pacific islands. A dune initially covered by these stabilizing plants is ripe for colonization by other plants, thus the original plant colonizers set the stage for a successional cycle.

Aquatic Ecosystems; Halophytes.

Bertness, Mark. The Ecology of Atlantic Shorelines. Sunderland, MA: Sinauer Associates, 1999.

Carson, Rachel. The Edge of the Sea. Boston: Houghton-Mifflin, 1979.

Teal, John, and Mildred Teal. Life and Death of the Salt Marsh. Boston: Little Brown, 1969.

Valiela, Ivan. Marine Ecological Processes, 2nd ed. New York: Springer-Verlag, 1995.

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