Coastal Dead Zones

As river drainage basins collect the runoff from the continents, they carry the flush of agricultural runoff, fertilizers, manure, sewage, and other waste. This discharge load acts as nutrient enrichment that forces eutrophication (from the Greek eutrophos, meaning “well nourished”). Huge spring blooms result as phytoplankton in the upper photic zone of coastal waters flourish. Anthropogenic (human-forced) pollution is damaging some 400 estuarian and coastal marine systems worldwide. These damaged ecosystems are Earth's coastal dead zones.

As algae die, they drift to the seabed and accumulate in the bottom sediments, feeding bacterial action in the benthic zone. The bacteria consume oxygen as they process the organic debris. As a function of this bacterial respiration, dissolved oxygen (DO) levels decline. The biological oxygen demand (BOD) of bacteria feeding on the decay exceeds the dissolved oxygen, and hypoxia develops, killing any fish that venture into the area. Benthic-dwelling animals (flounder, crab, lobsters) and immobile animals (worms, clams, oysters) are killed. These low-oxygen conditions act as a limiting factor on marine life.

Conditions are worsened by physical factors and can vary seasonally. Stratification of waters by temperature produces a warmer surface layer and cold deep layers following spring blooms and the approach of summer warmth. Some recovery is possible from seasonal depletions, but hypoxic conditions can spread spatially with the increasing influx of nutrients over time. Thus, the dead zone, a region of oxygen-depleted (hypoxic) water off the coast of Louisiana in the Gulf of Mexico has expanded to more than 22,000 km2 (square kilometers), or 8,500 mi.2 (square miles), each summer (first photo). The Mississippi drainage system handles the runoff for 41% of the continental United States, delivering the nutrient load to this region of the Gulf. In fact, all along the Texas coast, an extensive dead zone prevails, stretching in a discontinuous concentration some 32 km (20 mi.) from the coastline, fortified by nutrient-laden runoff and coastal septic systems.

Various events can worsen conditions, such as heavy precipitation from storms or hurricanes tracking inland, and produce periodic episodes of eutrophication. Dead zones occur in episodes such as the one in North Carolina in 1999. In short succession during September and October, Hurricanes Dennis, Floyd, and Irene delivered several feet of precipitation to the state, each storm falling on already saturated ground. In North Carolina, more than 10 million hogs, each producing 2 tons of waste per year, were located in about 3,000 agricultural factories. These operations collected almost 20 million tons of manure into hundreds of open lagoons, many set on river floodplains. The three-hurricane downpour flushed out waste lagoons into wetlands and streams and on to Pamlico Sound and the ocean, producing a spreading dead zone and an environmental catastrophe.

When the 1993 Midwest floods hit, the Mississippi river's nutrient discharge was such that the size of the dead zone in the Gulf of Mexico doubled. Following Hurricane Katrina in 2005, the dead-zone region was overwhelmed by the [Page 487]discharge carrying all the sewage, chemical toxins, oil spills, household pesticides, animal wastes and carcasses, and nutrients. The agricultural, feedlot, and fertilizer industries dispute the connection between their nutrient input and the dead zone.

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