Lack of strong responses to the Pacific marine heatwave by benthivorous marine birds indicates importance of trophic drivers

Abstract

The Pacific marine heatwave (PMH) of 2014-2016 was an intense, long-lasting environmental disturbance expressed throughout the north Pacific. While dramatic consequences of the PMH on pelagic food webs have been well documented, effects on nearshore food webs, i.e. those based on macroalgal primary productivity, benthic invertebrate intermediate consumers, and specialized benthivorous top predators including some marine birds, are not well understood. We conducted summer and winter coastline marine bird surveys in 2 National Parks in the northern Gulf of Alaska from 2006 to 2022. We evaluated changes in abundance of benthivorous marine birds in relation to the PMH, after accounting for effects of season and region. We also evaluated changes in abundance of nearshore benthic invertebrate prey to allow specific consideration of a prey-based mechanism for effects of the PMH across food webs. We found that benthivorous marine birds, consisting of sea ducks and shorebirds, did not show a strong response to the PMH, unlike significant effects demonstrated by piscivorous birds in pelagic biomes. In contrast to extreme reductions in quantity and quality of forage fish documented elsewhere, we found that common benthic invertebrate prey abundance remained relatively stable. Our results support the hypothesis that, across food webs, top predator responses to the PMH were driven primarily by how and whether the PMH affected their prey availability. These findings show how a large-scale environmental perturbation affects biological communities differently through various trophic pathways, which provides insight into ecosystem resiliency and can inform management strategies in the face of persistent climate change.