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