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Human activities in riverine water bodies can cause loading of nutrients and other pollutants in receiving systems e.g. reservoirs, affecting species interactions, population dynamics and ecosystem functioning. The present study aimed to investigate the influence of anthropogenic alteration on an artificial water body, i.e. the Shahid Rajaee reservoir (Mazandaran province, Northern Iran). The reservoir is subject to high anthropogenic nutrient loads from the Shirinroud and Sefidroud rivers, the effect of which needs to be evaluated considering the strategic importance of the reservoir as a source of water for the human population. A field sampling campaign was carried out at seven stations (one in each of the two river mouths and five in the reservoir itself) each month from April to July 2016. The study entailed sampling of benthic macroinvertebrate fauna at the river mouths and plankton communities at the reservoir stations. In addition to macroinvertebrate and plankton collections, physicochemical and nutrient variables were measured at all stations. Substantial loading of organic matter, mainly from the Shirinroud river, was detected throughout the sampling period, with a detrimental effect on both benthic and plankton communities. The decline in plankton diversity was particularly noticeable at stations close to the river mouths during June and July, with a bloom of Cyanobacteria and Dinoflagellates and strong dominance of larger zooplankton taxa, i.e. Cladocera. Our results suggest that river water quality, affected by human activities, alters the planktonic communities in Rajaee reservoir. Our assessment indicates that measures on both the Shirinroud river and the reservoir are required in order to mitigate and prevent even stronger eutrophication of the reservoir ecosystem.