Invertebrate re-colonisation in Mariager Fjord (Denmark) after a severe hypoxia. II. Blue mussels (Mytilus edulis L.).

A severe hypoxic event was observed in the Danish estuary Mariager Fjord during August 1997. For two weeks the entire water column was anoxic in the innermost 20 km of the estuary and even after 5 weeks the surface water had not attained 100% oxygen saturation. All metazoa were considered extinct. A sampling programme the following spring and summer 1998 was initiated at three stations representing an east- west gradient within the anoxic part of the estuary. With a frequency of 2-4 weeks we sampled larvae and newly settled common blue mussel (Mytilus edulis L.) in order to describe re-colonisation. In July and October 1998 and March 1999 recruitment was estimated from bottom sampling at the experimental stations. Mapping of the blue mussel population was performed in spring and fall of 1999 measuring coverage, abundance and biomass along 14 transects. Due to extinction of the benthic community, mussels were recruited entirely from the outer section of the estuary and/or from the Kattegat. Two distinct cohorts of mussel larvae were identified and shell growth rates of up to 5.8 mum d-1 were observed. Settlement peaked approximately 1 month after the highest densities of the first cohort of mussel larvae were observed. Lack of growth and settlement of the second cohort larvae could be attributed to functional food limitation. Difference in initial recruitment success between stations was levelled by high mortality during the first year. Due to hypoxia the inner section of Mariager Fjord is susceptible to defaunation regularly, but the population of blue mussels, which dominates the benthic fauna of the estuary is remedied after apprx2 years by re-colonisation from adjacent sections of the estuary. The population was, however, not in stable equilibrium.