Influence of an estuarine plume and marine sewage outfall on the dynamics of coastal bacterioplankton communities

Marine bacterioplankton in the coastal region off Aveiro (NW Portugal) develop under the influence of the plume of a mesotrophic estuarine system (Ria de Aveiro) and, more recently, under the influence of the discharges of a marine sewage outfall (S. Jacinto). In an attempt to compare the degree of disturbance introduced by these 2 features to the abundance of sewage bacteria and heterotrophic activity of natural bacterioplankton, water samples were collected at 10 offshore and nearshore locations. Rates of ectoenzymatic activity and monomer incorporation were determined as proxies for potential heterotrophic activity of natural bacterial communities. ATP and chl a concentrations were used as estimates of the total abundance of phytoplankton. Faecal coliforms and total colony counts were used as indicators of sewage bacteria contamination. However, ATP, chl a, faecal coliforms, total colony counts, aminopeptidase activity and the maximum rate of leucine incorporation (leucine Vm) correlated negatively with linear distance to the mouth of the estuary, establishing the estuarine plume as an importance source of disturbance. Chl a, aminopeptidase activity and total colony counts also correlated negatively with linear distance to the sewage outfall, but no significant impact on the concentration of faecal bacteria could be detected. Compliance with the European Union Bathing Water Directive 76/160/EEC was achieved in 97% of samples. The ratio between β-glucosidase activity (polymer degradation) and glucose incorporation (monomer uptake) increased as the distance from the mouth of the estuary and from the sewage outfall increased. This indicates that, in addition to inputs of bacteria and nutrients, changes in the quality of the available organic substrates and/or sewage-related toxic effects may impose a shift in the functioning of heterotrophic bacterioplankton communities in this costal area.