Assessing the effects of migratory waterbird droppings on potential lake eutrophication using water quality models: A case study of Yangming Lake on Kinmen Island, Taiwan.

ABSTRACT Eutrophication in freshwater lakes and reservoirs is a key threat to aquatic ecosystems and water supply systems. In some lakes and reservoirs, waterbirds might be an important nutrient source. The effects of waterbirds on water quality have been studied in temperate regions, but few studies have been conducted in tropical or subtropical regions. This study is the first to demonstrate the effects of migratory birds and cormorants on an Asian subtropical lake, Yangming Lake, Kinmen Island, Taiwan. Water quality model tools were applied with field investigations. During the study period, rainfall was relatively low and water storage was very low, resulting in cormorants contributing a high percentage of the phosphorus (P) loads, ∼70% of the annual external nutrients. However, when simulating normal rainfall records, the simulated total P (TP) concentration decreased, and the waterbirds contributed 55% of the external nutrients. Without the birds, the TP concentration is expected to be <50 μg/L under normal water volume. This study showed that migratory waterbird roosting could bring excess nutrients to the lake and significantly affect the water quality during the migratory period. However, the status of the lake water volume might dilute or strengthen the impacts of waterbirds. The random and nonlinear effects of waterbird droppings can be regarded as nonpoint source pollution, and management strategies are suggested to reduce the transport of waterbird droppings to receiving waterbodies.