Contemporary agriculture uses huge amounts of fertilizer (largely responsible for the doubling of global Nitrogen flows and the tripling of global Phosphorus flows).  The over application and lack of finesse with which fertilizer is used produces a tradeoff between increased agricultural production and coastal ecosystem production.

Agricultural runoff is the main driver of low oxygen coastal ‘dead zones,’ which greatly reduce fish, shellfish, and most other living things, consequently reducing coastal fisheries, recreation opportunities, and sometimes, when there are toxic algae blooms, endangering human health.

A Science magazine news article (Oct 26, 2006) reports on an update to a UNEP assessment of the world’s dead zones.

The number of oxygen-starved “dead zones” in global marine waters has jumped by more than a third in the last 2 years, according to a United Nations Environment Programme (UNEP) report released last week. The latest figures reveal some 200 dead zones worldwide, up from 149 since 2004. The affected waters are robbed of fish, oysters, sea grasses, and other marine life, damaging food supplies for millions of people worldwide, the report warns.
Dead zones form when microscopic marine plants called phytoplankton explode in number. When the phytoplankton die, bacteria feast on them and consume vast amounts of dissolved oxygen. The resulting oxygen depletion–or hypoxia–kills fish, oysters, sea grasses, and other marine life. Although phytoplankton are the backbone of marine food chains and their populations naturally wax and wane, abnormally large “blooms” have been on the rise since the 1970s. According to the UNEP report, this has been due to skyrocketing marine levels of nutrients such as phosphorus and nitrogen from fertilizers, sewage, animal wastes, and other sources.

Marine biologist Robert Diaz of the Virginia Institute of Marine Sciences in Williamsburg compiled much of the findings on dead zones from exhaustive reviews of scientific journals around the world. Better scientific reporting in recent years likely accounts for some of the apparent increase in the phenomenon, he says; “however, there’s no mistaking the consistent upward trend over the last 50 years.” It is difficult to estimate the total area affected worldwide, but he believes the total is “on the order of” 300,000 square kilometers. About 80% of the zones occur every summer and autumn, he says. Some, such as the Baltic Sea’s 80,000-square-kilometer zone, even persist year-round.

The situation may well worsen. The UNEP report projects that the volume of nitrogen alone dumped by rivers into the oceans will climb 14% by 2030, compared to mid-1990s levels. However, not all dead zones are linked to human activities, says paleoceanographer Kjell Nordberg of Göteborg University in Sweden. His historical and geological studies indicate that natural changes in climate and ocean conditions have caused oxygen depletion in some North Sea estuaries and fjords. Not all hope is lost, however. In some areas where sewage discharge and agricultural practices are implicated, regulations to curb the impacts have helped improve oxygen levels over the last few years, Nordberg says.