Spreading Dead Zones and Consequences for Marine Ecosystems

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Science  15 Aug 2008:
Vol. 321, Issue 5891, pp. 926-929
DOI: 10.1126/science.1156401

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  1. Fig. 1.

    Global distribution of 400-plus systems that have scientifically reported accounts of being eutrophication-associated dead zones. Their distribution matches the global human footprint [the normalized human influence is expressed as a percent (41)] in the Northern Hemisphere. For the Southern Hemisphere, the occurrence of dead zones is only recently being reported. Details on each system are in tables S1 and S2.

  2. Fig. 2.

    Conceptual view of how hypoxia alters ecosystem energy flow. The green area indicates the range of energy transferred from the benthos to higher-level predators under normoxia, typically 25 to 75% of macrobenthic carbon. As a system experiences mild or periodic hypoxia, there can be a pulse of benthic energy to predators. This “windfall” is typically short-lived and does not always occur. With declining oxygen, higher-level predation is suspended, benthic predation may continue, and the proportion of benthic energy transferred to microbes rapidly increases (orange). Under persistent hypoxia, some energy is still processed by tolerant benthos. Microbes process all benthic energy as hydrogen sulphide, and anoxia develops (red).

  3. Fig. 3.

    Generalized pattern of benthic community response to hypoxia (34). As DO declines to <0.7 ml of O2/liter and extends through time, mass mortality of both equilibrium (stage III) and opportunistic (stage I) species occurs (red). If anoxia is reached, benthos are eliminated. The recovery path from severe hypoxia is different than the decline path because of the hysteresis-like progression of successional dynamics. When exposed to mild hypoxia, mortality is moderate, and the recovery path is closer to the response path (blue) as fauna restart from midsuccessional stage II. When exposed to intermediate oxygen conditions, the response is minor (green) and not hysteresis-like.