A possible connection between hydrography and the distribution of Calanus finmarchicus on the Norwegian midshelf in 1997

Three cruises were conducted in April, May/June, and July 1997 in order to describe the demography of Calanus finmarchicus on the Norwegian midshelf. The Norwegian Atlantic Current and the Norwegian Coastal Current dominate t he study region. The differences in the physical conditions of the water ma sses are most evident along an east-west axis, showing cold, less saline wa ter masses closer to the coast, whereas Atlantic water masses dominate at t he shelf break and the open ocean. Seasonal lateral movement mixes coastal and Atlantic Water, and the clearly defined gradients present in spring are gradually eroded. The overall seawater temperature in 1997 appears to have been higher than the mean of the past decades. The observed vertical shift in the chlorophyll a from surface bloom in spring to a deepwater chlorophy ll a maximum around 50 m in July is a consistent feature, but during the pe riod of investigation there was extensive patchiness with respect to the di stribution of chlorophyll. The start of recruitment from G(0) of C. finmarc hicus differs in time over the Norwegian midshelf, with two regions (off Mo re and in Vestfjorden/Helgeland) clearly ahead of the other areas. The form ation of G(1) C. finmarchicus seems to be dictated by the stabilization of water masses on the shelf and by a longitudinal/ latitudinal offset of the ascent of CV to the surface, which was particularly prominent in the open o cean. The broad mesoscale pattern of the temporal propagation of Calanus sh ows that G(1) starts at the shelf, propagates towards the open ocean coveri ng all three regions, and finally returns as G(2) to the shelf with a pulse of CI-II during July and most likely August. (C) 2000 International Counci l for the Exploration of the Sea.