Seasonal Oxygen Depletion in the Bottom Waters of a Danish Fjord and Its Effect on the Benthic Community

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Abstract
The bottom water in local areas of Limfjorden (Denmark) frequently becomes anoxic during warm summer periods. The water may be stagnant for 1 to several weeks due to stratification in temperature and salinity. The anoxia stimulates the anaerobic metabolism in the sediment and H2S accumulates. Normally the H2S does not penetrate up into the bottom water because the ferric iron pool functions as a buffer binding the transient excess of sulfide as FeS and perhaps FeS2. Benthic animals react to the lack of O2 by creeping out of the mud and may survive lying on the mud surface. Mussel beds increase the benthic respiration per m2 10-fold and thereby enhance O2 depletion of the bottom water. Their high metabolic rate may regulate the size of the mussel beds to the limit at which animals in the center become choked. A survey is presented of the frequency and distribution of anoxia which leads to mass mortality of the bottom fauna in Limfjorden. Due to the seasonal O2 depletion, the composition of the benthic community is in some areas regulated by alternating sequences of extinction and recolonization.

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