Agriculture Impacts Oceans

Since the 1950’s, agriculture has relied on synthetic chemical fertilizers to ensure high crop yields, often using more than the crops actually need. For example, Larry Heatherly writes in “Nitrogen fertilizer management for corn” (Delta Farm Press, May 17, 2007) that nitrogen fertilizer rates for corn fields in Missouri range from 63-209 kg per hectare.

However, as Paul Hollis writes in his article “Nitrogen efficiency is target in corn” (Southeast Farm Press, January 29, 2008), the corn plants only use about 50-60% of this nitrogen. So, what happens to the remaining 40-50%?

Well, a lot of it enters streams and rivers and eventually coastal ecosystems. In their article “Beyond Science Into Policy: Gulf of Mexico Hypoxia and the Mississippi River” Dr. Nancy Rabalais and her colleagues report that the Mississippi River carries an astounding 1.6 million metric tons of nitrogen into the Gulf of Mexico each year, of which about 650,000 metric tons is from agricultural sources (BioScience 52, February 2001).

How Agricultural Fertilizer Impacts Coastal Ecosystems

The nutrient of most concern for coastal ecosystems is nitrogen. Normally, nitrogen is in short supply. This limits the creation of new organic matter, or biomass, by phytoplankton, the tiny plants at the bottom of the marine food chain. However, when the ecosystem becomes nitrogen enriched by agricultural fertilizers phytoplankton populations can dramatically increase, resulting in what is called an ‘algal bloom’.

When the plankton die they sink to the sea floor where as part of the natural process of decomposition they are eaten by oxygen-consuming bacteria. When plankton die in larger numbers than usual, such as after an algal bloom, these bottom-dwelling bacteria use oxygen faster than it can be replaced from the water layers above, which creates low oxygen, or ‘hypoxic’, conditions.

Hypoxic 'Dead Zones' Impact Marine Biodiversity and Fisheries

While creatures such as fish, crabs and shrimp can usually avoid hypoxic areas, slower less mobile organisms like clams and worms may die or suffer stunted growth and reproduction. So, not only does sea floor habitat become unavailable to fish, but they lose a source of food as well. Long term exposure to hypoxic conditions can also permanently affect an ecosystem’s biodiversity by changing the types of plants and animals able to live there.

The impact can be substantial. In his article for Scientific American (“Oceanic Dead Zones Continue to Spread”, August 15, 2008) David Biello reports that over 212,000 metric tonnes of food is lost to hypoxia each year in the northern Gulf of Mexico.

Management of Agricultural Fertilizers Must Link Terrestrial and Coastal Ecosystems

Thus, while growing food is crucial for feeding growing human populations, agricultural fertilizers are polluting coastal ecosystems, creating hypoxic areas that reduce habitat, lower biodiversity and harm the commercial fisheries. This is a global problem that emphasises the need for large-scale management regimes that explicitly recognize the linkages between terrestrial and coastal ecosystems.

Salmon in Corn Field

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