Seabird responses to ecosystem changes driven by marine heatwaves in a warming Arctic

Abstract

ABSTRACT: In the Pacific Arctic, the Chukchi Sea has been warming for decades, and exhibited an exceptionally warm period from 2015 to 2021. We examined changes in seabird distribution and abundance in the Chukchi Sea, and their relationships to environmental and prey conditions between 2 contrasting periods. We sampled systematically placed stations in late summer during 2 years before (2012, 2013) and 2 years during the warm period (2017, 2019; characterized by multiple marine heatwaves). Ship-based bird counts were used to model at-sea density of 5 seabird foraging guilds relative to oceanographic (water temperature, salinity, chlorophyll) and prey (large copepods, euphausiids, 3 forage fish taxa) variables. Relative to cool years, heatwave years were characterized by warmer, saltier waters, low abundance of large copepods and euphausiids, and elevated fish abundance, including an unprecedented abundance of age-0 walleye pollock Gadus chalcogrammus. Seabird species richness was higher during heatwave years but diversity was lower, driven by an influx of shearwaters. The best models for surface feeding and diving piscivores and diving planktivores included oceanographic and prey variables, plus a heatwave interaction term, indicating that responses to variables differed between cool and heatwave periods, with greatest disparity exhibited by diving planktivores. Models for surface planktivores were inconclusive, whereas shearwater distribution was associated with geographic variables (latitude, distance offshore), with relationships differing during cool and heatwave periods. We propose a conceptual model of how a prolonged period of marine heatwaves may affect the offshore seabird community via changes in prey species composition and distribution.