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 sprin g are presented for three turbid European estuaries, the Elbe (Germany), th e Westerschelde (The Netherlands) and the Gironde (France), with emphasis o n the effect of turbidity on microbial community densities and activities. Total suspended matter (TSM) concentrations were highest in the Gironde est uary and lowest in the Elbe estuary. Maximum concentrations were found in t he 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 positiv ely correlated with the depth of the euphotic zone while BP was positively correlated with TSM and particulate organic carbon. The POC/TSM-ratio, whic h is related to the degradability of organic carbon, was differed between t he three estuaries and was highest in the Westerschelde. The ratio BP:PP wa s generally very high (> 1), and maximal in the MTZ (> 4), illustrating the heterotrophic nature of the estuarine ecosystems. Due to the extremely hig h turbidity in the Gironde, the contribution of bacterial carbon to total m icrobial biomass (bacteria + algae) was > 50%. We conclude that the MTZ has a pronounced impact on the structure and functioning of the microbial comm unity leading to an increased importance of heterotrophic processes and inc reased degradation of organic material. (C) 1999 Elsevier Science B.V. All rights reserved.