Abstract: Sewage inputs on fluvial ecosystems affect benthic communities and alter trophic networks resulting in changes on river functioning. Functional indicators (e.g. river metabolism) have been proposed as a valuable tool to evaluate ecosystem impairment. In the present study we monitored river metabolism in spring (few days after a major flood) and in summer (after 35 days of low flow conditions) using both single-station and two-stations methods over a 24 h period up and downstream of wastewater treatment plant (WWTP) effluents on three Atlantic river reaches located in northern Spain (Europe). Concurrently with river metabolism, we characterized environmental characteristics (flow, velocity, depth, pH, water temperature, nutrients, etc.), benthic macroinvertebrate communities and biofilm (algae and epilithic biomass). Ecosystem Respiration (ER24) was similar at the different periods and locations, but Gross Primary Productivity (GPP) tended to decrease in impacted reaches (downstream WWTPs) and in summer (except in the Saja River). However, the balance of the metabolic processes showed a trend towards autotrophy in the largest river, while WWTP effluents increased its autotrophy. Chlorophyll a concentration was > 4 times larger in spring than in summer in all river reaches, while epilithic biomass followed a similar but less obvious pattern. Increase of invertebrate scraper densities (mainly, Potamopyrgus antipodarum) seems to be a plausible explanation for biofilm biomass temporal patterns in all sites (higher in spring than in summer), altering GPP and ER24 patterns. Thus, metabolism rates show different responses to WWTP effluents depending on season and on the relationships among functional and structural components, with special focus on the composition and structure of macroinvertebrate communities. Increasing our understanding of cause-effect relationships on the impairment of aquatic ecosystems needs to account for both structural and functional components and their interactions.

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