Spatial and temporal variability of food web structure during the spring bloom in the Skagerrak

Abstract

During the intense spring diatom bloom, the spatial (km) and temporal (h) variability of the pelagic food web structure was investigated in the Skagerrak. Three surface water masses could be identified in the study area based on their temperature, salinity and biogeochemical characteristics. When the measured biological parameters were separated into water masses instead of stations, food web variability decreased by up to 41%. However, specific egg production rate by Acartia clausi and Oithona similis did not correlate with water masses and was probably influenced by other factors, e.g. small-scale turbulence. Bacterial production and Calanus-grazing followed a diurnal cycle with increased activity during the night. The grazing impact by the copepod community was assessed by the egg production and the gut fluorescence method. Despite the Calanus population, the copepod community only grazed <3% of daily primary production. The protozooplankton, however, ingested 2 to 4 times more of primary production than the copepods. Overall, the zooplankton community only grazed 17 % of daily primary production, and consequently the majority of the spring bloom will leave the euphotic zone ungrazed as a potential food source for the benthos.