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

A severe hypoxic event was observed in the innermost 20 km 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 5X103-10X103 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.