Implications of density driven currents for interaction between jellyfish (Aurelia aurita) and zooplankton in a Danish fjord

The distribution of Aurelia aurita in a shallow fjord with a local population of numerous small jellyfish was influenced by density driven currents created by frequent salinity changes in the adjacent open sea. When new water of either higher or lower salinity entered the fjord this gave rise to changes in the jellyfish distribution. Likewise, the presence and distribution of zooplankton was influenced. Incoming copepods disappeared simultaneously with the invasion of the new water mass by the jellyfish. The time it took for A. aurita to enter a new water mass of higher or lower salinity was dependent on the salinity difference. Laboratory experiments indicated that the adaptation time for equilibrium buoyancy and normal swimming of the jellyfish was directly proportional to the salinity difference. Osmoconforming times of2 to 4 h were found for salinity gradients of2 to 4 psu typically found in the fjord. The population filtration impact exerted by A. aurita was usually able to keep the zooplankton biomass low (mean residence time of a zooplankton organism varied between 10 and 20 h). Similar dynamics governed by density driven currents may be expected in other shallow waters influenced by frequently changing salinities. Such knowledge about the dynamic interactions between hydrography and biology, explaining otherwise unaccountable large variations in phyto- and zooplankton biomasses may be of importance for a general basic understanding of the dynamics in coastal ecosystems.