Invertebrate re-colonisation in Mariager Fjord (Denmark) after severe hypoxia. I. Zooplankton and settlement

A severe hypoxic event was observed in the innermost 20 kin of the Danish estuary Mariager Fjord during August 1997 where the entire water column was totally anoxic for two weeks causing extinction of almost all metazoa. A sampling programme was initiated the following spring and summer at three stations in the anoxic part of the estuary. We describe the re-colonisation by zooplankton and in particular meroplankton, and the subsequent potential settlement by bivalves, gastropods, polychaetes and cirripeds. Due to extinction of the benthic community the zooplankton organisms were considered recruited entirely from outside the study area. Two water intrusions from the open water Kattegat were identified based on calculations of mass balance of salt and on phytoplankton composition. Rotifers were the first zooplankters to appear followed by cladocerans and copepods. The meroplanktonic bivalve larvae were followed by polychaete larvae, gastropod larvae and last cirriped nauplii, all with a maximum density of <100 larvae l(-1). Mortality of meroplankton comprised 59-99.5% during the study period when the population is considered to develop in a closed system. However, a simple model including estimated in- and outflow of water and hence larvae forecast 13-69% mortality depending on simulation time and instantaneous mortality rate. In the absence of predators, the disappearance of larvae is suggested to be due to functional food limitation and settlement. Potential settlement was observed at all three stations after 3 weeks of sampling. The succession observed among settlers was: bivalves and polychaetes gastropods - crustaceans (cirripeds). This is comparable to the succession of taxa in the plankton. The potential settlement rate of polychaetes increased from the outermost station towards the innermost station and a tendency to initiate settlement earlier at the innermost station was observed. The polychaete settlement rate comprised up to 30-40 x 10(3) ind. m(-2) d(-1) and was evident mainly in relation to the first water intrusion. A similar pattern, although with a lower settlement rate, was found for bivalves. The settlement rates of the other taxonomic groups revealed no systematic pattern and were in the range of >5 x 10(3) - 10 x 10(3) individuals m(-2) d(-1). Due to hypoxia the inner section of Mariager Fjord is susceptible to defaunation regularly, but the benthic diversity is remedied after a surprisingly short time by recolonisation from adjacent sections of the estuary.