On the Relations of Vertical Distribution,Diurnal Migration and Nutritional State of Herbivorous Zooplankton in the Northern North Sea during FLEX 1976

At a fixed station in the northern North Sea the occurrence of herbivorous and omnivorous zooplankton and the changes in its vertical distribution were studied over 2 1/2 months during the spring phytoplankton bloom in 1976. From a sporadical distribution of relative few plankton organisms in the mixed water column at the end of winter, in the middle of April a significant accumulation of organisms began to occur in the upper layers parallel to the development of stratification and the phytoplankton spring bloom. As exceptions, the populations of Microcalanus pusillus and Metridia lucens withdrew to below the thermocline. About ten days after the decline of the phytoplankton bloom, a few zooplankton species—especially wax ester producing copepods like Calanus finmarchicus, Pseudocalanus elongatus, Microcalanus pusillus, aad Metridia lucens—started diurnal vertical migrations. The movements of the species and stages occurred synchronously, leading in the same direction. Nevertheless, the amplitude of the migrations increased with each maturity stage reached. The vertical migrations, characterized by actual locations and velocities of the centers of gravity of the populations, are interpreted in terms of metabolism leading to fat storage, decomposition and reconstruction of lipids and proteins, in terms of a periodical change in the density of the organisms. Further data sets about the vertical distribution of organisms are cited to support the hypotheses stated.