Quantitative relationships between phytoplankton, bacteria and heterotrophic microflagellates in shelf waters

Abstract

Estimates of the numbers and biomass of bacteria as a function of depth in coastal and upwelling waters off the western approaches to the English Channel and in the southern Benguela upwelling region off the Cape Peninsula, South Africa, show that the numbers of bacteria are correlated with the standing stocks of phytoplankton as assessed by chlorophyll a concentration. Standing stocks of heterotrophic microflagellates in the size range 3-10 .mu.m, amount to some 16.9% on average, of bacterial standing stocks (mg C m-3) estimated by direct microscopy. Calculations of C flow through the microheterotrophic consumer community suggest that .apprx. 20-60% of primary production, possibly representing the dissolved components leaching out of, and lost from phytoplankton cells during zooplankton grazing, enters the microbial food chain. Much of this appears to be dissipated by bacteria, with some 5.2-8.1% of the photoassimilated C being incorporated into bacterial C-production. At least 66% of this is exploited by the heterotrophic microflagellates leaving a maximum of 34% of bacterial production for the larger bactivorous suspension feeders.