FATE OF A PHYTOPLANKTON SPRING BLOOM - SEDIMENTATION AND CARBON FLOW IN THE PLANKTONIC FOOD WEB IN THE NORTHERN BALTIC

During the spring bloom in 1988, the dynamics of planktonic carbon flow were studied weekly in the euphotic layer in the northern Baltic. The spring bloom developed after the formation of a slight vertical salinity gradient near the surface at the end of April, and a peak in phytoplankton primary productivity and biomass (dominated by the dinoflagellate Peridiniella catenata) was reached about 1 wk later. The biomass of all heterotrophic compartments, especially that of bacteria and copepods, increased strongly during the peak and declining phases of the algal bloom, showing that their success was closely linked with the bloom. During the whole bloom period, the integral primary production (C-14 incorporation) was 45.5 g C m-2, and 'new' (NO3--N-based) production contributed about 80% of this value. The rotifers - copepods grazing chain and the bacteria - heterotrophic nano-flagellates-ciliates 'microbial loop' consumed directly about the same amount (3.5 g C m-2) of phytoplankton carbon. Algae accounted for 64% of the total carbon consumption of zooplankton. Sedimentation corresponded to 72% of the primary production. The sum of algal biomass increase and loss factors (exudation, grazing and sedimentation) was 94% of the integral primary production, which supports our conclusion that there is a strong imbalance between primary and secondary production in the vernal planktonic food web off the SW coast of Finland.