Bottom-up effects on zooplankton biomass persist from spring to summer

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Marine zooplankton populations are generally believed to be bottom-up controlled. However, most studies from the Norwegian Sea-Barents Sea area (NS-BS) have focused on top-down control by planktivorous fish. We quantify bottom-up effects on Calanus finmarchicus biomass by including particle tracking in statistical analyses of survey data from NS-BS between 1959 and 1993. Spatial variation in C. finmarchicus biomass in summer is positively related to back-calculated temperature in spring, reflecting that more biomass originates from warmer areas closer to the core overwintering areas in the Norwegian Sea. However, a warm spring does not generally result in overall increased summer biomass. In warm springs, overall biomass is generally high, and population growth from spring to summer is lower than in cold years. Further, C. finmarchicus biomass in summer relates positively to increased wind in spring when the mixed-layer-depth (MLD) is shallow. The same wind-MLD combination relates positively to chlorophyll biomass in spring, possibly by prolonging the spring bloom with nutrient replenishment. These results suggest that C. finmarchicus biomass in summer is influenced by bottom-up effects of food availability in spring.