Compared to external loads from tributaries and sediment recycling, the role of waterbirds as phosphorus (P) sources in estuaries is overlooked. We performed monthly ship-based surveys of waterbird abundance in the Lithuanian part of the Curonian Lagoon, calculated their potential P excretion, and compared its relevance to the riverine inputs. Phosphorus excretion rates for the censused species were assessed accounting for variations of body weights, daily feces production and their P content, and assigning species to different feeding and nutrient cycling guilds. During the study period (March-November 2018), 32 waterbird species were censused, varying in abundance from ~18,000-32,000 (October-November) to ~30,000-48,000 individuals (June-September). The estimated avian P loads during the whole study period varied between 3.6 and 25 tons, corresponding to an area load between 8.7 and 60.7 mg P m-2. Waterbird release of reactive P to the system represented a variable but not negligible fraction (1%-12%) of total external P loads, peaking in June-September and coinciding with cyanobacterial blooms. This study is the first in the Baltic Sea region suggesting that waterbirds are potentially important P sources to phytoplankton in large estuaries, in particular, during low discharge periods.
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