Zooplankton dynamics during the onset of winter in a northern Norwegian fjord. Vertical distribution and metabolic activity in relation to phytoplankton and sedimentation

Abstract

Abstract The study was conducted at a permanent station (depth of 80 m) in Ramfjorden (ca. 70°N), a branch of Balsfjorden near Tromso, Norway, in NovemberlDecember 1989. Phytoplankton and zooplankton were collected and sedimentation rates were measured every 3 to 4 days; zooplankton respiration was determined in the laboratory. Phytoplankton concentrations were extremely low and tended to decrease during the investigation period, while zooplankton abundance was still at about 65 % of its annual peak level. Depth distribution was markedly different between copepod species. Pseudocalanus acuspes and Acartia longiremis were more abundant in the surface layer while concentrations of Microsetella norvegica and Microcalanus pusillus were higher in deep water. Weight-specific respiration and faeces concentrations in the surface layer were higher than at depth. It is hypothesized that P. acuspes and A. longiremis remain active in winter and, by feeding, conserve part of the biomass in the pelagial.