Abstract
Temporal variations in the numbers of foraging seabirds usually coincide with concurrent variations in physical processes influencing prey availability. Responses to periodic tidal currents are commonly reported, with certain tidal states being favoured. By contrast, responses to intermittent meteorological events have rarely been reported, even though wind-driven exchanges of water masses or intrusion of estuarine plumes could have similar consequences. As large-scale offshore constructions (e.g., aquaculture, coastal defences, ports and marine renewable energy installations) and climate variations alter periodic tidal currents and intermittent meteorological events, respectively, quantifying responses to these physical processes can identify potential impacts on seabird communities. This study quantifies responses of foraging seabirds to physical processes in the Ria de Vigo, northwestern Spain. The numbers of foraging European Shags Phalacrocorax aristotellis and Yellow-legged Gulls Larus michahellis showed no response to variations in tidal current direction and speed. By contrast, both species increased in number during an estuarine plume intrusion (the Western Iberian Buoyant Plume: WIBP) following an extreme river discharge event and a period of southerly winds. These increases in numbers may be explained by the temporary combination of marine and brackish-water fauna, increasing prey biomass. The frequency of extreme river discharge events is likely to decrease in northwestern Spain. If WIBP intrusions consistently enhance prey availability, observations of large numbers of foraging seabirds using the ria could become rarer.