Resuscitating Coastal Dead Zones Starts Far From Shore
Grace S. Brush, the foremost ecological historian of Chesapeake Bay, America’s largest estuary, examines the historical connections between land use, nitrogen and coastal dead zones.
Today, chemically synthesized nitrogen is recognized as a major cause of aquatic eutrophication ( a state where excess nutrients spur excessive plant growth) and anoxia (the absence of oxygen, which results in “dead zones”) in many coastal regions of the world.
A large effort is directed toward preventing nitrogen from getting into aquatic systems to restore coastal shellfish populations and the coastal fishery — important food sources for much of humanity. Current monitoring indicates that most of the nitrogen responsible for coastal deterioration originates as runoff from fertilized agricultural fields, with secondary sources being animal, municipal and industrial waste as well as atmospheric contributions from cars and industry.
Hence, numerous attempts to decrease nitrogen are directed toward fertilizer reduction, but fertilizer use is particularly intractable because the sources are diffuse — or “non-point.” (That’s in contrast to “point” sources such as human, animal and industrial effluents, which can be removed by waste treatment technologies.) Fertilizers enter aquatic systems primarily through agricultural runoff, leaching into the soil and then into surface and ground waters, which eventually discharge into coastal regions.
An important question in developing methods to break this cycle involves knowing the history of coastal deterioration and its relation to the use of nitrogen.
Not Doomed to Repeat It
Biological and geochemical profiles from sediment cores throughout the Chesapeake Bay illustrate a history of land use and the deterioration of estuaries. These studies show that in pre-colonial times (prior to late 17th century) nitrogen influxes were very low, suggesting that biological nitrogen fixation (the only nitrogen available at that time) was balanced by the natural loss of nitrogen, or denitrification. During this time the landscape consisted of a diversity of forests, coastal marshes, floodplains and inland wetlands, many of them created by beavers. As a result, the pre-colonial landscape presented many opportunities for denitrification, which occurs in wet, anaerobic environments.
Coastal conditions did not change much in early post-colonial times, when agriculture consisted of small farms separated by patches of forest. By the middle 19th century and continuing into the 20th century, more than three-fourths of eastern North America was deforested, primarily for agriculture. Deforestation was accompanied by draining many of the wetlands for arable land.
Thus, landscape vegetation, hydrology and geochemistry were changed, and areas where denitrification could occur greatly decreased. At the same time, other sources of nitrogen fertilizer became available, including guano, nitrate deposits and, after World War I, synthetic nitrogen.
These changes are recorded in the sediment cores by an increase in indicators of deteriorating water quality: increasing sedimentation rates, high nitrogen influxes and a decrease in bottom-dwelling, or benthic, organisms – all of which reached a maximum in the late 20th century. Paleoecological records compiled for other hypoxic regions throughout the world show a similar pattern, suggesting that coastal dead zones are a 20th-century global phenomenon, largely related to the increasing use of chemically produced nitrogen.
You Can’t Go Home Again
How can the coastal regions be restored?
Returning the landscape to pre-colonial conditions is not an option. The denitrifying capacity of the pre-colonial landscape has been altered by fundamental hydrologic changes such as the burial of headwater streams, drained wetlands and grasslands replacing forests.
However, there may be ways to reverse aquatic deterioration through various engineering technologies and landscape management designs that prevent nitrogen used on the land from finding its way into aquatic systems, and also by enhancing conditions for denitrification on the landscape and not just near shores. After all, the nitrogen entering aquatic systems originates throughout the watershed and prevention should not be restricted to buffering riparian areas bordering streams.
One possibility for reducing the leaching of nitrogen into soils and surface or ground waters might be by planting forest stands on appropriate soils throughout the watershed. It has been shown that nitrogen leaching from soils into streams is greater in sugar maple forests than in red oak forests in the Catskills of New York, for example.
In the Chesapeake watershed, many forest species are restricted to specific soil types. The paleoecological record of the Chesapeake system indicates that nitrogen influxes did not increase in the estuary until approximately 40 percent of the land was deforested. Forest stands on appropriate soils throughout the drainage area might be one way to limit leaching of nitrogen (and other substances) into surface and groundwater — and ultimately into aquatic systems.
In addition, denitrification can be increased throughout the watershed by restoring wetlands wherever it is hydrologically feasible, as well as by using technologies such as retrofitting sewage waste systems for denitrification.
Efforts to remove nitrogen from the environment are expensive and are designed primarily for restoring fisheries. However, society’s needs have changed, and coastal systems are valued for aesthetics, recreation and other qualities as well as for providing food. That little progress has been made in restoring the health of coastal regions suggests that objectives serving multiple goals are what the public might want and will pay for.
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