Before he retired as Director of the Duke University Marine Lab, Joe Ramus was fond of telling the undergraduate students that an estuary was where “the surf meets the turf.” It is the juxtaposition of the land and sea that makes estuaries so productive along the passive continental margins of the east coast of the Americas and Asia, the northwestern coast of Europe, and other coastlines worldwide.
Freshwater rivers dump their nutrients into estuaries, where they mix with nutrients delivered by tidal exchange with the sea. The result is a proliferation of plant life—both floating algae (phytoplankton) and submerged vegetation, such as eelgrass. This plant production supports higher trophic levels, including crabs, oysters, fishes, and people. Chesapeake Bay, the largest estuary of the east coast of the United States, is famous for its blue crabs and oysters. A century ago, millions of waterfowl gathered in the Chesapeake Bay estuary during fall migration.
But, worldwide, estuaries are in trouble. While some nutrient loading is good, too much leads to an overproduction of floating algae, which periodically sink to the bottom waters where they decompose consuming most or all the oxygen. The resulting condition, known as hypoxic, is lethal to shellfish and fishes in these waters.
Estuaries have also been subject to pollution by toxic metals, chemicals, and excessive sediment. And, their popularity as recreational waters has led many estuaries to lose their natural shoreline in favor of coastal development behind hardened seawalls, which are inappropriate habitat for shellfish. In the face of rising sea level, hardened seawalls do not allow estuaries and their coastal wetlands to expand upslope—what is normally known as marine transgression. In the past Chesapeake Bay was able to expand its wetland area in response to rising sea level; this is now less likely.
Chesapeake Bay is a prime example of a degraded estuary, having lost a large amount of its crab and oyster harvest to a combination of overfishing, loss of shoreline and hard-bottom habitat, and hypoxia. The loss of oysters is particularly important inasmuch as oysters each normally filter about 50 gallons of water each day, cleaning the estuarine waters. Fortunately, we may have recognized the problem just in time, as recent efforts to reduce the excessive nutrient loading from fertilizer and animal runoff have reduced algal productivity and hypoxia, allowing submerged vegetation, such as eelgrass, to return and expand their area, creating new underwater habitat for other creatures. While seagrass habitat has been expanding, managers have been creating new oyster reefs and tidal marshes, and refining limits on harvesting. The result is a stabilization of crab, oyster, and some fish populations and a continued livelihood for coastal fishermen. Scientists, policy makers, and interest groups across several states have worked together to achieve these early successes.
Chesapeake Bay represents an environmental success in progress, but the efforts to understand and restore this and other estuaries must not stop. The continual arrival of exotic fish, crabs, and plants disrupts the delicate ecological balance of estuaries and marshes. And a recent paper has documented the importance of atmospheric deposition of nitrogen on the surface of Chesapeake Bay, where it now almost matches the delivery of this nutrient in surface runoff and rivers. Like forests, estuaries suffer from the effects of air pollution, which is a source of nitric oxide in the air and nitrate in rainwater (See: http://blogs.nicholas.duke.edu/citizenscientist/diversity/).
The surf, turf and atmosphere form a tripartite association that can maintain or kill the productivity of an estuarine ecosystem and the services it provides for humans.
References
Da, F., M.A.M. Friedrichs, and P. St.-Laurent. 2018. Impacts of atmospheric nitrogen deposition and coastal nitrogen fluxes on oxygen concentrations in Chesapeake Bay. Journal of Geophysical Research Oceans doi: 10.1029/2018JC014009.
Iverson, R.L. 1990. Control of marine fish production. Limnology and Oceanography 35: 1593-1604.
Lefcheck, J.S. and 13 others. 2018. Long-term nutrient reductions lead to the unprecedented recovery of a temperate coastal region. Proceedings of the National Academy of Sciences 115: 3658-3662.
Nixon, S.W. and 15 others. 1996. The fate of nitrogen and phosphorus at the land sea margin of the North Atlantic Ocean. Biogeochemistry 35: 141-180.
Schieder, N.W., D.C. Walters, and M.L. Kirwan. 2018. Massive upland to wetland conversion compensated for historical marsh loss in Chesapeake Bay, USA. Estuaries and Coasts 41: 940-951.
Waycott, M., and 13 others. 2009. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the National Academy of Sciences, U.S. doi: 10.1073/pnas.0905620106
