(Fig. 1.1) (Dayton et al., 1995; Auster, 1998; NRC, 2002). The magnitude of effects increases with the frequency and geographic scale of trawling. The most striking data are from New England and the Gulf of Mexico (NRC, 2002), although the situation is almost certainly comparable on continental shelves around the world (Dayton et al., 1995). In New England, the total area fished (TAF) by trawling is 138,000 km2, and 56% of the sample areas are fished more than once a year, so that the equivalent of 115% of the TAF is fished every year. In the Gulf of Mexico, the TAF is 270,000 km2, 57% of the sample areas are fished more than once a year, and trawls sweep 255% of the TAF each year. Thus, trawling has drastically degraded most of the sea floor in these huge regions, and with multiple trawling episodes per year at favored sites, there is obviously no opportunity for ecosystem recovery.
Nutrient runoff is naturally greatest, and eutrophication, hypoxia, and toxic blooms are most intense, in estuaries and coastal seas like the Adriatic and Baltic seas and Chesapeake and San Francisco bays (Diaz, 2001; Jackson, 2001; Jackson et al., 2001; Lötze et al., 2006). However, major river systems like the Amazon, Yangtze, and Mississippi–Missouri also discharge vast amounts of nutrients, sediments, and organic matter into relatively small areas of open coast and surrounding continental shelves. The enormous increase in the use of chemical fertilizers in the drainage basins of these great rivers over the past 50 years (Tilman et al., 2002), coupled with the virtual elimination of suspension feeding oysters and wetlands along their delta margins, has resulted in the formation of vast eutrophic and hypoxic regions comparable with the worst conditions in estuaries (Diaz, 2001).
The iconic American example is the hypoxic “dead zone” that extends some 500 km west of the Mississippi delta. The area of the hypoxic zone has doubled in the past 20 years to ≈20,000 km2, and the rate of increase in area is a linear function of nitrogen loading from the Mississippi drainage (Fig. 1.2) (Rabalais et al., 2007; Turner et al., 2008). Analyses of the geochemistry and mineralogy of cores shows that hypoxic conditions were uncommon before the 1950s, strongly supporting the hypothesis that their formation is due to comparatively recent human impacts and is not a natural phenomenon. The dead zone expands during the summer, when hypoxia extends from shallow depths to the sea floor, and there is mass mortality of most animals that cannot swim away, including major fisheries species like shrimp. The dead zone is hardly dead, however, but supports an extraordinary biomass of diverse microbes and jellyfish that may constitute the only surviving commercial fishery. In addition,