Bacterial and phytoplankton production in the maximum turbidity zone of three European estuaries: the Elbe, Westerschelde and Gironde

Biomass and production of phytoplankton and heterotrophic bacteria in spring are presented for three turbid European estuaries, the Elbe (Germany), the Westerschelde (The Netherlands) and the Gironde (France), with emphasis on the effect of turbidity on microbial community densities and activities. Total suspended matter (TSM) concentrations were highest in the Gironde estuary and lowest in the Elbe estuary. Maximum concentrations were found in the maximum turbidity zone (MTZ). Both primary production (PP) and bacterial production (BP) showed a longitudinal gradient with lowest PP and highest BP in the MTZ. Production rates of both phytoplankton and bacterioplankton were lowest in the Gironde and highest in the Westerschelde. PP was positively correlated with the depth of the euphotic zone while BP was positively correlated with TSM and particulate organic carbon. The POC/TSM-ratio, which is related to the degradability of organic carbon, was differed between the three estuaries and was highest in the Westerschelde. The ratio BP:PP was generally very high (>1), and maximal in the MTZ (>4), illustrating the heterotrophic nature of the estuarine ecosystems. Due to the extremely high turbidity in the Gironde, the contribution of bacterial carbon to total microbial biomass (bacteria+algae) was >50%. We conclude that the MTZ has a pronounced impact on the structure and functioning of the microbial community leading to an increased importance of heterotrophic processes and increased degradation of organic material.