Mass mortality of marine birds in the Northeast Pacific caused by Akashiwo sanguinea

Harmful algal blooms (HABs) are dense concentrations of phytoplankton that can have deleterious effects on marine life. We documented two of the largest marine bird mortality events ever definitively ascribed to a single HAB, the cause of which was death resulting from plumage fouling by surfactant-like proteins produced by the dinoflagellate Akashiwo sanguinea. The two mortality events were observed along the coast of Washington State in September and October 2009, collectively representing an estimated deposition of 10 500 carcasses, of which the majority were surf scoters, white-winged scoters and common murres. Each mortality event was coincident in space and time with observed bloom landfall, with each event preceded by a similar chain of environmental conditions. Prior to each event, the presence of A. sanguinea and upwelling-favourable conditions likely led to bloom proliferation. In both cases, this period was followed by conditions that transported the senescent bloom into the nearshore environment, whereupon subsequent wave action lysed A. sanguinea cells, creating foam that contained surfactant- like compounds. This sequence of conditions, exacerbated by the presence of aggregations of marine birds in wing moult, appear to be the necessary requirements for marine bird mortality of this scale due to foam-induced plumage fouling. This mechanism of HAB-induced mortality may become more prevalent in the California Current System given the apparent increasing occurrence of HABs and the broad environmental tolerances exhibited by A. sanguinea.