Plankton response following a spring upwelling event in the Ria de Arosa, Spain

Abstract

plankton and bacterioplankton growth dynamics, and biomass and ingestion rates of zooplankton and mussels hung from rafts in the Ria de Arosa, Spain, were studied after a spring upwelling event. Upwelling of North Atlantic Central Water was confirmed by salinity, temperature and nutrient properties of the bottom waters in the Ria and the positive upwelling index calculated for the study period. On 11 March. 1983, primary production and chlorophyll a concentrations exceeded 30 mg C m-3 h-1 and 10 mg m-3, respectively, in the surface waters. After the upwelling period primary production decreased to <2 mg C m-3 h-1. Our results show that bacterial growth dynamics were spatially (vertically) separated from phytoplankton growth dynamics following the upwelling event. The hghest specific rate of bacterial production occurred below the depth of maximum photosynthesis and above the thermocline. The highest rates of bacterial production were < 10 % of phytoplankton production. Zooplankton (>333 km) grazing experiments indicated an active selection of phytoplankton over bacteria and a sufficient grazing rate (90 to 124% d-1) to crop the daily phytoplankton production. Thus, herbivorous zooplankton may at times be in direct competition for food with raft-mussel aquaculture in the ria de Arosa