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Modern Shelf Anoxia |
Institute of Zoology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
The Northern Adriatic Sea is characterized by the full range of features associated with sensitive marine ecosystems. In the Gulf of Trieste, mortalities due to anoxia have been reported in 1974, 1983 and 1988. The anoxic event in September 1983 led to mass mortality in a highly developed and wide-ranging community in the Gulf. This destroyed over 50% of the epifaunal biomass in only two days, and over 90% within four days.
The increased frequency, severity, and range of recent oxygen deficiencies suggest that, in addition to meteorological and hydrographic factors, anthropogenic influences are a contributing or enhancing factor in the onset of anoxic conditions and associated mass mortalities. These anthropogenic influences may be subsumed under the heading eutrophication. If eutrophication only slightly increases the rate of oxygen decrease below the thermocline, it may advance the point at which critical oxygen levels are attained by several days. Since only a few days of anoxic conditions are required to destroy this benthic community (with an equally short time required for reoxygenation) the degree of eutrophication may determine whether an ecological catastrophe occurs or not.
The recolonization process between 1984 and 1987 has been slow: the total biomass and the relative contribution of the designating species have remained distinctly below previously recorded values. Additional anthropogenic influences (intensive dredging) may further impede recolonization. A further, small-scale mortality was registered in 1988.
