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Recent declines in sea ice extent and warming seawater temperatures in the Arctic have the potential to impact regional and pan-Arctic marine ecosystems. To investigate marine biological response to these key drivers and other environmental factors, we undertook a robust trend analysis of benthic macrofaunal populations and environmental drivers in the Bering Strait region. Our focus was on the waters of the northern Bering and southern Chukchi Seas, which are shallow (<100 m) and seasonally productive, with strong pelagic-benthic coupling between water-columnderived organic matter and the seafloor. Studies indicate that both in situ production and advection of upstream phytodetritus support persistent biologically productive regions, termed hotspots, in the greater Bering Strait region. The benthic marine ecosystem is dominated by macroinvertebrates (e.g., clams, polychaetes, and amphipods) that in turn serve as food resources for diving mammals and seabirds, thus allowing for changes to cascade strongly through the food web from prey to predator. During our study, the persistence of seasonal sea ice significantly declined; trend analyses indicate both earlier sea ice breakup and later fall freeze-up in recent years. When combined with warming seawater temperatures in the region, these changes have ramifications for water column processes that influence benthic faunal biomass and composition, which can transfer to upper trophic level predators. We studied these changes by evaluating time series sites in three benthic biomass hotspots starting in 1998 (Southeast Chukchi Sea region), 1999 (Chirikov Basin region), and 2000 (St. Lawrence Island Polynya region). We present these data within a broader evaluation of benthic biomass results from prior cruises dating as early as the 1970s. The current study focuses on the period 1998–2015 at sites occupied annually each July using CCGS Sir Wilfrid Laurier. Since 2010, these time series sites have become part of the international Distributed Biological Observatory (DBO), a network of standard time series stations and transect lines in the Pacific Arctic that is used for evaluating changes within the biological system. We found that these regions have experienced northward shifts in high benthic biomass and changes in dominant macrofaunal composition that are coincident with recent reduced sea ice cover and variable warming of seasonal water column temperatures. Hydrographic changes can influence chlorophyll a inventories in surface sediments and total organic carbon content, both of which are indicators of food supply to the benthos. In addition, sediment grain size reflects variable current flow that in turn influences faunal composition. Time series studies are essential for evaluating whether this region is transitioning or even reaching a “tipping point” that could shift the benthic-dominated system to a pelagic one, with large-scale ramifications for ecosystem structure in this highly productive Pacific Arctic ecosystem. INTRODUCTION A key organizing principle for the ecosystem of the northern Bering Sea and adjoining southern Chukchi Sea is a strong link in the shallow, seasonally productive waters between spring-summer pelagic production and transfer of that organic matter to the underlying sediments as sea ice retreats (Grebmeier et al., 2015a). Over the last decade, the seasonally variable sea ice cover in this Pacific Arctic region has been retreating earlier and forming later (Frey et al., 2015). Three major water masses influence the northern Bering Sea: Anadyr Water (AW) in the west, characterized by cold, high salinity water, with seasonally high nutrients; Alaska Coastal Water (ACW) to the east, characterized by warmer, less saline water; and Bering Shelf Water (BSW) that occurs between these two water masses, with intermediate seawater temperature and salinity values (Figure 1; Danielson et al., 2017). North of Bering Strait, AW SPECIAL ISSUE ON OCEAN WARMING By Jacqueline M. Grebmeier, Karen E. Frey, Lee W. Cooper, and Monika Kędra Photo of clams, polychaetes, tunicates, and brittle stars, all collected from one 0.1 m2 van Veen grab in the benthic hotspot north of Bering Strait. Photo credit: Lee Cooper

Trends in Benthic Macrofaunal Populations, Seasonal Sea Ice Persistence, and Bottom Water Temperatures in the Bering Strait Region