Effects of wastewater treatment plant pollution on in-stream ecosystems functions in an agricultural watershed

摘要

We studied the effect of point-source and non-point-source pollution on the retention capacity ofthe stream and its link with the metabolic state (primary production and respiration) and invertebrates assem-blages in a third order Mediterranean stream. Two experimental sites were chosen: one in the upper part ofthe catchment (Montegut site) characterized by low concentrations in nitrate and phosphate and one in thelower part of the catchment (Lezat site) characterized by high nitrate and phosphorus concentrations. Bothexperimental sites were located on reaches that included a Waste Water Treatment Plant (WWTP) pointnutrient source allowing discussion of the relative effects of point-source and non-point-source nutrientsloads on ecosystem function (respiration and uptake rates) and aquatic organism assemblages. NI-t-N, andPOi--P uptake rates were determined using solute additions conducted at constant rates (short-term nutrientaddition procedure) and NO_3~- -N uptake rates were determined using instantaneous solute addition (slugaddition procedure). Rates of gross primary production (GPP) and ecosystem respiration were determinedusing the open system, two-stations diurnal oxygen change method. Benthic invertebrate communities wereinvestigated for species and functional feeding groups diversities measurements. Results show that autotrophyin the river results from nutrients of two distinct origins: point sources for phosphorus (urban area andWWTP) and non-point sources for nitrogen (agricultural zones) with local additions from WWTP inputs.Comparison between the two sites shows that the WWTP did not affect uptake rates, respiration or primaryproduction of the ecosystem in the low-nutrient Montegut reach despite increase of invertebrates communitiesbiomass density. Inputs from the WWTP, in the high nitrate and phosphate Lezat reach, increased respir-ation, lower benthic biomass and led to changes in the species composition and did not affect uptake rates.