Demands of the herbivore community on phytoplankton production in the Celtic Sea in August

Abstract

Zooplankton species diversity in the Celtic Sea in August 1982 was low; two species of copepod and two species of euphausiid accounted for 90 to 95% of the biomass sampled by a 280 μm-mesh net. Some 75% of the primary production was by phytoplankton smaller than 5 μm. The demands of both the macrozooplankton and the microzooplankton have been examined. If it is assumed that macrozooplankton cannot efficiently graze particles smaller than 5 μm, there was insufficient primary production to meet the demands of the copepods and euphausiids; however, there would have been sufficient if these animals could graze phytoplankton 5 to 1 μm. Ciliates were in competition with the macrozooplankton for phytoplankton and could not have been significant grazers of bacterial biomass. The majority of microflagellates were autotrophic; less than 10% of the population did not possess a chloroplast and were presumably heterotrophs. Bacterial production was low and was insufficient to meet the demands of the heterotrophic microflagellates, but there was sufficient production by the picophytoplankton to meet microflagellate requirements. The data do not appear to support the ideas of a significant flow of energy through the “microbial loop” in the Celtic Sea in August.