What is eutrophication?

Harmful algal blooms, dead zones, and fish kills are the results of a process called eutrophication—which begins with the increased load of nutrients to estuaries and coastal waters.

satellite view of Mississippi River meeting the Gulf of Mexico

This NASA Earth Observatory image shows the region where the Mississippi River meets the Gulf of Mexico. It illustrates how sediment is moved from the land to the sea. The Mississippi River carries millions of tons of nutrient-rich sediment into the Gulf each year.

Eutrophication is a big word that describes a big problem in the nation's estuaries. Harmful algal blooms, dead zones, and fish kills are the results of a process called eutrophication—which begins with the increased load of nutrients to estuaries and coastal waters.

Sixty-five percent of U.S. estuaries and coastal water bodies are moderately to severely degraded by excessive nutrient inputs, which lead to algal blooms and low-oxygen (hypoxic) waters that can kill fish and seagrass and reduce essential fish habitats. Many of these estuaries also support bivalve mollusk populations (e.g., oysters, clams, scallops), which naturally reduce nutrients through their filter-feeding activities.

The primary culprits in eutrophication appear to be excess nitrogen and phosphorus—from sources including fertilizer runoff and septic system effluent to atmospheric fallout from burning fossil fuels—which enter waterbodies and fuel the overgrowth of algae, which, in turn, reduces water quality and degrades estuarine and coastal ecosystems.

Eutrophication can also produce carbon dioxide, which lowers the PH of seawater (ocean acidification). This slows the growth of fish and shellfish, may prevent shell formation in bivalve mollusks, and reduces the catch of commercial and recreational fisheries, leading to smaller harvests and more expensive seafood.

In recent years, NOAA's National Centers for Coastal Ocean Science (NCCOS), in collaboration with NOAA's Northeast Fisheries Science Center, has enlisted estuaries' indigenous residents, namely, bivalve mollusks, to help slow and, in some cases, reverse the process of eutrophication, since they efficiently remove nutrients from the water as they feed on phytoplankton and detritus.

A groundbreaking modeling project in Long Island Sound showed that the oyster aquaculture industry in Connecticut provides $8.5 – $23 million annually in nutrient reduction benefits. The project also showed that reasonable expansion of oyster aquaculture could provide as much nutrient reduction as the comparable investment of $470 million in traditional nutrient-reduction measures, such as wastewater treatment improvements and agricultural best management practices.

The NOAA scientists used aquaculture modeling tools to demonstrate that shellfish aquaculture compares favorably to existing nutrient management strategies in terms of efficiency of nutrient removal and implementation cost. Documenting the water quality benefits provided by shellfish aquaculture has increased both communities' and regulators' acceptance of shellfish farming, not only in Connecticut but across the nation. In Chesapeake Bay, for example, nutrient removal policies include the harvesting of oyster tissue as an approved method, and in Mashpee Bay, Massachusetts, cultivation and harvest of oysters and clams are part of the official nutrient management plan.

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In September 2017, New York Governor Andrew M. Cuomo announced a $10.4 million effort to improve Long Island's water quality and bolster the economies and resiliency of coastal communities by restoring native shellfish populations to coastal waters. The state plans to establish five new sanctuary sites in Suffolk and Nassau Counties to transplant seeded clams and oysters, and to expand public shellfish hatcheries in the two counties through a dedicated grant program. Eutrophication has had significant economic impacts on Long Island Sound, where commercial shellfisheries have lost millions of dollars annually since 1985. Recent projections indicate that without intervention, the Sound could lose all of its seagrass beds by 2030, and that two-thirds of the Sound could lack enough oxygen for fish to survive.

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Last updated: 10/10/17
Author: NOAA
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