Short‐term Effects of Great Cormorant Droppings on Water Quality and Microbial Community of an Artificial Agricultural Reservoir

Agricultural reservoirs are established to improve the management of water resources. Waterbirds in protected waters have become a nuisance, however, as nutrients from fecal deposits transported by the waterbirds have served to severely deteriorate water quality. Despite the importance of clean water resources, the microecology of small agricultural reservoirs regularly colonized by transitory waterbirds are seldom reviewed. To improve our understanding of the influence of waterbirds on small bodies of water, a microcosm study was conducted using water and sediment from an agricultural reservoir inhabited by 300 to 500 great cormorants. Temporal changes in total nitrogen, total phosphorous, chemical oxygen demand, NH-N, PO-P, and chlorophyll-a concentrations, in addition to the microbial community, were evaluated for microcosms containing 0, 0.5, 1.0, and 5.0 g of feces collected from a great cormorant colony. Chemical analysis of the water microcosm revealed that all microcosms showed both immediate and prolonged increases in nutrients due to the addition of feces. Additionally, a mere 0.5 g of feces doubled the concentration of chlorophyll-a from 2.1 ± 0.99 to 5.2 ± 1.1 μg L within 1 mo. Nonmetric multidimensional scaling of the microbial community structure revealed disturbances in both water and sediment microcosms. Disturbances to the microbial community in the water microcosm were significant only when 5.0 g of feces was added; however, disturbances to sediment microbial communities were induced by a smaller mass of feces. These results confirm the short-term water quality impairment and shift in microbial community structure caused by waterbird droppings and bird colony surface runoff in an agricultural reservoir.