Dead Zone Consequences on Marine System
Dead Zone Consequences on Marine Ecosystem
Dead zones have developed in continental seas, such as Baltic, Kattegat, Black Sea, Gulf of Mexico, and East China Sea, all fishery areas (Diaz). Eutrophication, greening of the water column, causes dissolved oxygen (DO) in bottom waters and restricts water exchange that, in turn, causes dead zones. DO as low as 0.1 ml of O2/liter has led to mass mortality and major community structure changes with dead zones.
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Progression
Causes
Declines of DO were first observed in the 1950s with the 1960s becoming widespread with hypoxia. This was discovered after the increased use of industrially produced nitrogen fertilizer. The number of dead zones has approximately doubled each decade since the 1960s (Diaz). Oxygen depletion of marine systems became a major environmental problem by the end of the 20th century. Improvements that have been made have been related to organic and nutrient loadings, stratification strength, freshwater runoff, and nutrient and carbon input management. As oxygen improves the ecosystem function improves.
Types
Seasonal oxygen depletion, the most common form of eutrophication-induced hypoxia, occurs in summer. Periodic oxygen depletion can occur more often and has been reported in a fourth of the hypoxia systems. It can be influenced by winds and tides. Infrequent episodic oxygen depletion occurs less than once per year. It is the first signal a system has reached a critical point of eutrophication, which combined with physical processes causes hypoxia. Persistent hypoxia occurs in systems prone to persistent stratification. It accounts for 8% of the dead zones (Diaz).
Progression
Phase one of coastal hypoxia enhances the deposition of organic matter that promotes microbial growth and respiration and produces greater demand for oxygen. DO levels deplete with stratification. Phase two hypoxia will become transiently causing mass mortality of benthic animals. Phase three, after time and continued buildup of nutrients and organic matter, hypoxia becomes seasonal or periodic. Phase four, if conditions persist, causes the hypoxia zone to expand and, as DO levels fall, anoxia establishes and releases microbial generated H2S. The critical point is the appearance of severe seasonal hypoxia.
Causes
Hypoxia and anoxia becoming widespread influences on estuarine and marine environments, overfishing, habitat loss, and harmful algal blooms are all causes of dead zones.
References
Diaz, R.J. & Rosenberg, R. "Spreading Dead Zones and Consequences for Marine Ecosystems." Science, vol 321 (2008): 926-929. article.
