Limnetica 36

In middle and lower river basin reaches, bacterioplankton communities are usually found and their development and dynamics depend on the physico-chemical characteristics of the water, the inputs from tributaries, as well as the interactions these communities establish with the riverbed. The aim of this study was to assess whether direct inputs of effluents from wastewater treatment plants (WWTP) to the river generated significant changes in the density and viability of bacterioplankton communities. To achieve this objective, prokaryote density and physico-chemical characteristics in river water were analysed just before and after two WWTP located in the middle and lower parts of the Tordera river basin, as well as in each WWTP effluent. Five samplings were carried out during a base flow period.  Moreover, in two occasions, prokaryote density at the riverbed sediment was also measured in order to assess the possible link between benthic and planktonic compartments. Results showed that WWTP effluents carried more prokaryotes and nutrients and had higher temperature than the river reaches receiving such effluents, being their impact more significant in the mid-river than in the low-river reach. In the mid-river reach, WWTP effluent input increased prokaryotic density which may be determined by the higher availability of phosphate and dissolved organic matter which might favour its development, as it is also suggested by the observed increase of prokaryote viability together with a prokaryote cell density which was larger than that predicted from the river and the WWTP effluent prokaryote loads.  In contrast, at the down-river reach, prokaryote density was almost not affected by the WWTP effluent input. In this reach, prokaryote viability decreased at the river waters downstream the WWTP. It is suggested that the low oxygen concentration in the WWTP effluent was not favouring bacterioplankton growth, while the lower flow velocity may have favoured the settlement of prokaryote cells at the riverbed, as suggested by the increase in cell density in river sediments. Results showed that river bacteriplankton state is highly dynamic and responsive (viability ranged from 13 to 57%) when receiving effluents with distinct characteristics. Our results suggest that the expected response of increasing river bacterioplankton density and viability when receiving effluents with high loads of nutrients and organic matter will be alleviated by specific physical and chemical characteristics of the main effluent which may inhibit prokaryote growth and/or by specific riverbed characteristics which hold the development of an active benthic microbial community.