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

Abstract

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.

Changing Occurrences of Fall Blooms Associated With Variations in Phytoplankton Size Structure in the Pacific Arctic

“…In the Pacific Arctic food webs, even small changes in the lower trophic levels would have an influence on the distribution and abundance of higher trophic levels as a consequence of the short and efficient energy-transfer pathways (Grebmeier et al, 2010), and thus the bottom-up control on higher trophic levels is crucial (Schonberg et al, 2014). This study revealed an increased occurrence of fall blooms, which has the potential to explain recent variations in benthic macrofaunal biomass at the DBO sites (Grebmeier et al, 2018;Waga et al, 2019). Consequently, our findings contribute to facilitate an holistic understanding of ocean dynamics and complex trophic linkages from primary production to higher trophic levels, and improve our understanding of processes causing variations within marine ecosystems, as might occur in the Pacific Arctic.…”

“…As phytoplankton size structure during the post-bloom period strongly determines the amount of the flux of organic carbon into the sediment (Waga et al, 2019), interannual variations in phytoplankton size structure during the post-bloom period could influence not only the occurrence of a fall-bloom but also increased activity of higher trophic levels in the food web. Indeed, several studies have reported decreasing and increasing trends in benthic macrofaunal biomass at DBO1 and DBO3, respectively, while that of DBO2 showed an insignificant trend at DBO2 (Grebmeier et al, 2018;Waga et al, 2019). Fall blooms are observed to occur more frequently in the Arctic Ocean during the last decade.…”

Responses of the Northern Bering Sea and Southeastern Bering Sea Pelagic Ecosystems Following Record‐Breaking Low Winter Sea Ice

“…This suggest that with future warming, the Chukchi Sea may bloom prior to an ice‐free Bering Sea. In addition, the absence of sea ice and the associated flux of ice algae to the bottom likely had a profound impact on the relatively high, but varying, benthic community biomass found over the northern shelf (Grebmeier et al, ; Grebmeier et al, ).…”

Seasonal and latitudinal variations in sea ice algae deposition in the Northern Bering and Chukchi Seas determined by algal biomarkers

“…Trends in sea ice extent and duration are variable from year-to-year and throughout the Arctic [31]. Across the Pacific Arctic region (Bering, Chukchi and Beaufort Seas), sea ice break-up is occurring earlier and forming later, leading to younger and thinner sea ice annually with persistence declining by 9 to 30 days per decade over the satellite record [15,[31][32][33]. Two record low maximum winter extent periods for the Bering Sea occurred in 2018 and 2019, along with a record low summer minimum extent for the Chukchi Sea in 2019 [33][34][35].…”