Distributional shifts among seabird communities of the Northern Bering and Chukchi seas in response to ocean warming during 2017–2019

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Abstract

In the northern Bering Sea and eastern Chukchi Sea, 2017–2019 were record-breaking years for warm ocean temperatures and lack of sea ice. The region supports millions of seabirds that could be affected by shifts in prey distribution and availability caused by changing environmental drivers. However, seabirds are highly mobile and often flexible in diet, and might alter their foraging distributions accordingly. To determine if there was evidence of long-term changes in abundance of seabirds, or if seabirds used the offshore habitat differently during recent warm years, we compared species richness, community composition, and distribution and abundance of selected species and Total seabirds (all species combined) between two periods, 2007–2016 and 2017–2019. We also evaluated annual changes in abundance during 2007–2019. We used 79,426 km of transects from vessel-based surveys conducted July through September. Total seabird density for the entire study area increased by ~20% during 2017–2019, but changes were not consistent across the study area, nor among species, and species richness declined except for a slight increase in the northern Chukchi Sea. Total seabird density declined most in the northern Bering Sea (−27%), although it increased in the Chirikov Basin by 73%. During 2017–2019, abundance of piscivorous murres (Uria spp.) decreased everywhere, whereas planktivorous Aethia auklet density increased by 70% in Chirikov Basin; auklets apparently abandoned their post-breeding migration to the Chukchi Sea. Short-tailed shearwaters (Ardenna tenuirostris) expanded farther into the northern Chukchi Sea, with nearly twice the density of the previous decade. We identified five seabird community types, three of which (all dominated by an alcid species) contracted spatially in the later period, and shifted south or near colonies. In contrast, a short-tailed shearwater dominated community expanded northward, and a community defined by low seabird density expanded throughout the eastern portion of both the northern Bering and Chukchi seas, suggesting higher-density communities had shifted westward. The variable responses among species correspond to documented changes in the environment as well as their natural history.

Introduction

The Bering and Chukchi seas have been undergoing warming events and subsequent alteration of biological ecosystem components over the last 20 years (Grebmeier et al., 2006; Stabeno and Bell, 2019). However, events during 2017–2019 appear to have been distinctively disruptive of long term physical and biological patterns. Sea ice plays a critical role in primary productivity of these marine ecosystems. The formation of ice algae feeds phytoplankton blooms as the ice retreats (Brown and Arrigo, 2013), supporting zooplankton production (Campbell et al., 2016; Stabeno et al., 2010), and ultimately upper trophic levels. Early ice retreat, or lack of sea-ice formation, impacts these mechanisms with repercussions throughout the food web (Hunt et al., 2011). In the northern Bering Sea, warm conditions lead to early ice retreat, resulting in early and high primary productivity, particularly near the ice edge (Brown et al., 2011; Brown and Arrigo, 2013).

During 2017, sea ice formed over the eastern Bering Sea shelf, but there was an unusual and early retraction of ice over the northwestern Bering Shelf, attributed to persistent southerly winds. As a result, the northern Bering Sea was characterized by ice conditions similar to those of a ‘warm’ year, despite ice coverage farther south (Siddon and Zador, 2018). In 2018 and again in 2019, ocean temperatures were above normal in winter, and ice extent in the Bering Sea was the lowest recorded in four decades. In both years, sea ice retreated north of Bering Strait before spring (Siddon and Zador, 2018; 2019; Cornwall, 2019). The extremely low ice cover during 2017–2019 in the northern Bering Sea and Chukchi Sea resulted in altered oceanographic and biological conditions; these were most evident in 2018, and included impacts to lower and upper trophic levels (Duffy-Anderson et al., 2019).

Seabirds are indicators of ocean conditions (Murphy, 1936; Piatt et al., 2007 and references therein; Velarde et al., 2019). By understanding responses of seabirds to broad-scale ecological shifts we may better predict impacts to upper trophic-level taxa in a rapidly changing environment. In the Bering Sea, recent responses of seabirds to ocean warming have included mass mortality (Jones et al., 2019), failed nesting attempts and low reproductive success (Dragoo et al., 2020; Romano et al., this issue). Since 2015, seabird mass mortality events have occurred almost annually in the Bering Strait region (Duffy-Anderson et al., 2019). Species-specific mortality events and seabird reproductive success at monitored colonies can be indicative of food web changes (Abraham and Sydeman, 2004; Jones et al., 2019; Piatt et al., 2020). However, these metrics do not necessarily provide insight into how the broader seabird community has responded to an altered ecosystem.

Seabirds are long-lived, with adaptations to buffer variability in their environment. Forgoing a breeding season or undergoing a few years of low breeding success may not necessarily lead to substantial population-level repercussions (Cairns, 1992; Velarde and Ezcurra, 2018). Seabirds are also highly mobile, and can search for prey over a large area, particularly when not attending a colony. Further, seabirds spend most of their lives at sea, and their temporal and spatial distribution across the seascape often reflects the productivity and foraging conditions of large marine areas (Ballance et al., 1997; Gall et al., 2013; Suryan et al., 2012; Yen et al., 2006). Here, we examine broad-scale responses of seabirds to a warm period (2017–2019) in the Northern Bering and Chukchi Sea Large Marine Ecosystem (LME) relative to the preceding decade (2007–2016). Specifically, we use vessel-based surveys to assess how seabirds differed in species-specific and community-level abundance and distribution between these two time periods.

Section snippets

Study area

Our study area encompassed offshore waters of two regions, the northern Bering Sea (hereafter, Bering Sea) and eastern Chukchi Sea (hereafter, Chukchi Sea) (Fig. 1), and we considered southern and northern subregions within each region. We refer to the subregions (Fig. 2) as the Northern Bering (59.5°N to St. Lawrence Island; distinct from the general northern Bering Sea), the Chirikov Basin (St. Lawrence Island to Bering Strait at ~65.8°N, including Little Diomede Island), the Southern Chukchi

Species richness

Estimated species richness was higher in the Bering Sea (~40 species) than in the Chukchi Sea (~30 species) during both time periods. Within the two Bering subregions, species richness was slightly lower during 2017–2019, whereas it remained similar overall in the two Chukchi subregions (Fig. 3). However, in both the Bering and Chukchi regions, there was a reversal in richness between subregions; i.e. during the later period the Chirikov Basin had slightly higher species richness than the

Discussion

During the exceptionally warm, low-ice years of 2017–2019, we found evidence of broad-scale shifts in distribution of individual species and of identified seabird communities compared to the previous decade. Sea ice extent in the northern portion of the Bering Sea was the lowest on record during the late period of our study. In 2017, sea ice failed to form over the northwestern Bering Shelf due to atypical southerly wind patterns. Unprecedented open water predominated throughout the Northern

Author Statement

Kathy Kuletz: Funding acquisition, Project administration, Conceptualization, Writing - Original draft preparation, Reviewing & Editing; Daniel Cushing: Conceptualization, Data curation, Methodology, Formal analysis, Visualization, Writing – Reviewing & Editing. Elizabeth Labunski: Conceptualization, Investigation, Data curation, Visualization, Reviewing & Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The seabird survey data was collected by the authors and a number of dedicated observers throughout the years. The surveys were supported by funding from the North Pacific Research Board (NPRB Projects 637,2007–2008 and B64,2008–2010), and by Inter-Agency Agreements from the Bureau of Ocean Energy Management (2010–2019; BOEM IAs M10PG00050, M17PG00017, and M17PG00039). We thank the many research vessel crews, chief scientists, and collaborating researchers that made our surveys possible,

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