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 Engllsh 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 to 10 pm, amount to some 16.9 % on average, of bacterial standing stocks (mg C m-3) estimated by direct microscopy. Calculations of carbon flow through the microheterotrophic consumer community suggest that approximately 20 to 60 % of primary production, posslbly 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 to 8.1 % of the photoassimilated carbon being incorporated into bacterial carbon production. At least 66% of this is exploited by the heterotrophic n~icroflagellates leaving a maximum of 34 % of bacterial production for the larger bactivorous suspension feeders