Coastal environments include three of the four habitats on Earth that are most productive of biomass. Each of these habitats is directly affected by sea level and sea-level rise. Increasing concentrations of dissolved gases in the oceans will affect these ecosystems as well.

The biomass productivity of marshes, estuaries, and coral reefs is comparable to that of tropical rain forests. This high productivity is due in part to the extreme range of environmental conditions at transitions between major habitats: terrestrial to marine, freshwater to seawater, and from the surface to considerable depths. These ecosystems not only are critical for the organisms whose entire lives are restricted to that ecosystem but also serve as nurseries for many of the fish and invertebrate species commercially harvested for human consumption.

Coastal wetlands usually consist of a fringing border of marshlands grading through four general types, from open water toward land: salt marshes, brackish marshes, intermediate marshes, and freshwater marshes. The principal determinants of each grade are soil salinity and elevation. Soil salinity is slightly higher than the salinity of the surrounding water, so salt marsh plants, which are flooded by high tide, must be very salt tolerant. Such plants include salt grass and mangrove. Moving inland, soil elevation increases slightly with a consequent decrease in seawater influence versus freshwater input. The wetlands are integral to the coastal ecosystem. They filter freshwater runoff, trapping nutrients and sediments and thus extending the land seaward. Marshes protect the shore from wave action and storm damage by binding the soil and reducing wave energy. They serve as a nursery for many commercially important species such as shrimp, oysters, and crabs.

Estuaries are semi-enclosed bodies of water where freshwater runoff from land mixes with seawater in a shifting gradient of brackish water with freshwater layered over denser seawater. Depending on the rate of freshwater flow, a wedge of salt water can intrude far upstream. For instance, in dry years with low river flow, salt water can intrude up the Mississippi River as far as New Orleans, threatening the city's water supply. During low water, an underwater sand sill is constructed across the river bottom a few kilometers downstream of the city to prevent this catastrophe. Major estuaries in the United States include Chesapeake Bay, Puget Sound, San Francisco Bay, and much of the Gulf Coast. Estuaries are one of the world's most productive natural environments and serve as the nursery areas for many commercial and sport fisheries. Typically, adult animals spawn at sea, and their eggs and larvae are carried by tide and currents to estuaries, where the young animals grow and develop.

Sea level is not uniform. On most coasts, sea level fluctuates between high tide and low tide twice a day. The extent of these tides also fluctuates on a monthly cycle. Once a month, there is a highest high tide, the spring tide, and two weeks later there follows the lowest low tide of the month, the neap tide. Between the levels of the spring tide and the neap tide is a gradient of conditions that are constantly varying. The intertidal zone is often subdivided into high, middle, and low subzones, each with its characteristic species. Organisms in the high intertidal zone are exposed to air virtually daily. In the low intertidal zone, organisms are exposed only a few days a month. Above the high intertidal zone is a splash zone, where terrestrial organisms must be tolerant of salt water. Below the level of low tide is the subtidal zone, which is further stratified by salinity tolerance and depth of light penetration. The consequences of tidal fluctuation are most dramatic on rocky shorelines, where the stratification of algal and invertebrate animal communities is clearly visible at low tide.

Oceans are the largest reservoir of carbon in the carbon cycle and readily dissolve carbon dioxide (CO2) from the atmosphere. As such, they were thought to provide a buffer against global warming, with algal productivity increasing in proportion to carbon uptake from the atmosphere. However, when CO2 dissolves in water, it forms carbonic acid, which lowers pH to the detriment of marine organisms. In addition, increasing oceanic nitrogen levels from fertilizer runoff and sewage washed by rivers into the sea and from nitrous oxide produced by burning fossil fuels is having a negative impact on marine life. Again, initial thought was that adding nitrogen to the oceans would be a good thing, fertilizing the sea and promoting algal growth. In many places, however, the resulting algal blooms deplete dissolved oxygen in the water, forming extensive dead zones, where fish cannot survive. For instance, every year a dead zone forms off the mouth of the Mississippi River, extending from Louisiana to Texas.

Coral reefs are important coastal habitats in tropical waters, such as off the coasts of Hawaii and southern Florida. They are highly productive and, like estuaries and marshes, serve as a nursery for many marine animals. A reef itself is produced largely by calcium deposition from coral animals and coralline algae. The organisms secrete calcium carbonate, or limestone, around themselves for protection against predators, slowly building new reef upon older, dead layers. Coral animals have a narrow range of optimal temperatures, and scientists think that warming is responsible for some of the bleaching and death occurring on reefs worldwide, but particularly in the Caribbean Sea. Coral animals require symbiotic unicellular algae for calcium secretion, and animals that lose their symbionts quickly beach and die. In addition to temperature change, gradual acidification of the water due to dissolved CO2 also may be a critical factor in coral death. As corals die, they often are replaced by carpets of algae that completely change the community composition of the reef.

Although reef walls may extend to great depths, their most productive zones are just below the surface. Fringing and barrier reefs are often visible from shore because of waves forming breakers as the water suddenly becomes shallower. As a result, the reef crest dissipates wave energy and protects the shoreline from the power of wave and storm action. A shallow lagoon, with its own characteristic species, typically forms between the reef and the shore.

All coastal ecosystems are characterized by gradients of temperature and salinity that are directly related to water depth and distance from freshwater sources. Temperature-induced sea-level rise has an impact on seawater intrusion into freshwater and terrestrial habitats, changing the nature of the ecosystem. Higher temperatures also increase the rate of evaporation, concentrating salts and further raising salinity levels.

References:

1)         Bertness, Mark, Brian Reed Silliman, and Robert Jefferies. "Salt Marshes Under Siege." American Scientist 92, no. 1 (2005): 54-61.

2)         Ricketts, Edward F., and Jack Calvin. Between Pacific Tides. Stanford, Calif.: Stanford University Press, 1962.

3)         Silver, Cheryl Simon, with Ruth S. DeFries. One Earth, One Future: Our Changing Global Environment. Washington, D.C.: National Academy Press, 1990.

4)         Wolanski, Eric, et al. "Mud, Marine Snow, and Coral Reefs." American Scientist 91 (2005): 44-51.

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