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Coastal Dead Zones

  • By: Robert W. Christopherson
  • In: Encyclopedia of Geography
  • Edited by: Barney Warf
  • Subject:General Geography, Earth & Environmental Science

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 ...

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