OCEANOGRAPHIC TIME SERIES DATA IN THE BERING STRAIT REGION

 

Introduction

Arctic systems can be rich and diverse habitats for marine life in spite of the extreme cold environment. Benthic faunal populations and biogeochemical cycling processes are directly influenced by changing sea ice extent, seawater hydrography (nutrients, salinity, temperature, currents), and water column production. Benthic organisms on the Arctic shelves and margins are long-term integrators of overlying water column processes. Because they have adapted to living under extreme conditions, they are intimately susceptible to climate warming. Recent studies indicate the Arctic sea ice in the western Arctic is melting and retreating northward earlier in the season and the timing of these events can have dramatic impacts on the biological system. Changes in overlying primary production, pelagic-benthic coupling, and benthic production and community structure can have cascading effects to higher trophic levels, such as sediment feeders like walruses, gray whales, and diving seaducks. Recent indicators of contemporary Arctic change in the northern Bering Sea include seawater warming and reduction in ice extent. Our time series studies indicate associated declines in bottom-dwelling clam populations in the shallow northern Bering shelf in the 1990’s. In addition, declines in benthic amphipod populations in the region likely influenced the movement of migrating gray whales to feeding areas north of Bering Strait during this time period. Finally a potential consequence of a seawater warming and reduced ice extent in the northern Bering Sea could be the northward movement of bottom feeding fish currently in the southern Bering Sea that prey on benthic fauna, thus increasing the feeding pressure on the benthic prey base and enhance competition for this food source for benthic-feeding marine mammals and seabirds. The following presentation outlines biological changes observed in the northern Bering Sea ecosystem through a >20 yr environmental time series data set in the Bering Strait region.

Oceanographic time series data in the northern Bering have been collected through a variety of scientific programs over the last twenty years (NSF, NOAA/CIFAR, US Fish & Wildlife Service (BERPAC), and each July since 2000 as part of the NSF-funded Bering Strait Environmental Observatory (BSEO; Fig. 1). Ship-based oceanographic sampling is used to examine the status of benthic communities in the Bering Strait region that form the basis for foodwebs that support benthic feeding apex predators such as gray whale, bearded seal, walrus and diving sea ducks. Our annual sampling since 1998 has been supported in collaboration with Dr. Eddy Carmack (Institute of Ocean Sciences, IOS) and the Canadian Coast Guard ship Sir Wilfrid Laurier enroute to the Canadian Arctic. The long-term record of benthic community status extends as far back as the 1930’s when Soviet studies were conducted in the same area. We have taken advantage in our past work of the longer-term integration of oceanographic change that is reflected in many benthic indices, which vary as a result of changes in particle sedimentation, the strength of pelagic-benthic coupling, changes in apex predator populations, or other physical and biological events. The tools we have been using to evaluate possible changes in the Bering and Chukchi benthic ecosystem have included sediment oxygen demand, deposition patterns of the particle-reactive natural radionuclide ⁷Be, the stable carbon isotope composition of sediment organic matter, elemental carbon/nitrogen ratios, sediment grain size, as well as more direct benthic metrics such as macrofaunal biomass and community ecological structure.

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Figure 1.(Left) Location of time-series oceanographic data for the Bering Strait region, most recently supported by the Bering Strait Environmental Observatory (BSEO, boxed sites) and western Arctic Shelf-Basin Interactions (SBI) projects. (Right) Time series measurements of total sediment oxygen uptake (an indicator of carbon supply to the benthos) in the region southwest of St. Lawrence Island in the northern Bering Sea (BSEO-S), showing a significant decline in carbon deposition at sites influenced more by hydrographic factors (top graphs) than predator-prey interactions (bottom graphs).

Atmospheric forcing and its influence on marine systems varies both with time and space, but throughout this region studies indicate that wind forcing is a major driving force for the transport of Pacific water from the Bering into the Chukchi Sea and northward into the Arctic halocline (Overland and Stabeno 2004, Woodgate et al. 2004). A pressure gradient maintained between the Siberian high and the Aleutian Low is influenced by atmospheric conditions in these two regions. An increase in southerly winds over the Bering Sea has recently occurred, and is related to changes in atmospheric forcing (Overland and Stabeno 2004). Time series studies at the M2 mooring in the SE Bering Sea indicate both an earlier spring retreat of ice and an increase in bottom water temperatures over the last decade (Fig. 2a, b).

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Figure 2. (Left) Environmental times series at the M2 mooring in SE Bering Sea showing earlier sea ice retreat (Stabeno and Overland 2001) and (Right) associated increased surface and bottom water temperatures (http://www.beringclimate.noaa.gov/data/Bcresult.php).

Warmer sea surface temperatures in the St. Lawrence Island region in the 1990’s has led to an earlier transition from winter to spring and an earlier retreat of sea ice (Stabeno and Overland 2001). Our recent hydrographic and sea ice studies (Clement et al. 2004) in the northern Bering Sea documented reduced sea ice thickness and extent in an unusually ice-free year (2001) relative to 1999 and we also evaluated the impacts on water column and benthic productivity (Fig. 3). Unlike in the SE Bering Sea (Hunt et al. 2002), there was no significant difference in the onset of the chlorophyll bloom with the earlier retreat of sea ice in late winter. However, it is possible that the normal May bloom period could be enhanced with warmer spring surface waters, but warmer waters can also lead to an increase in zooplankton populations and overall grazing, thus potentially limiting carbon deposition to the sediments.

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Figure 3. Sea ice concentration during spring and early summer 1999 and 2001. The red box outlines the BSEO-S times series stations (modified from Clement et al. 2004).

In the Bering Sea south and north of St. Lawrence Island (SLI), the transport of Pacific-derived water is predominantly from south to north through Bering Strait, resulting in high primary productivity in the spring (Fig. 4). However, in the ice-covered winter/early spring period, the area south of SLI is influenced by the seasonal SLI polynya (SLIP), an area of open water that develops south of SLI as prevailing northerly winds force sea ice away from wind-sheltering land-masses. The resultant brine injection sets up periodic baroclinic currents that transport water and is hypothesized to entrained organic matter to the south and then west within a “cold pool” that is maintained throughout the summer (Grebmeier and Cooper 1995 and references therein), and is often marked by spatially distinct nutrient distributions (Whitledge et al. 1988). Phytoplankton produced over the shallow Bering Sea shelves sediment only 30-100 m to reach the bottom. Zooplankton population biomass is low under the ice during winter with minimal primary production and very low temperatures (Lovvorn et al. in-press). Thus, a large but brief pulse of carbon from an ice-edge bloom, plus subsequent open water bloom, sinks to the bottom without being grazed by zooplankton (Coyle and Cooney 1988). In the Bering Sea, in general, earlier melting results in lower phytoplankton biomass during the bloom (Saitoh et al. 2002). If an earlier retreat of ice occurs so that there is reduced or no ice-edge bloom, then the open-water bloom subsequently occurs later as observed on the southern Bering Sea, resulting in increasing zooplankton populations that can more fully graze production (Coyle and Pinchuk 2002, Hunt et al. 2002). As a result, much less of the bloom reaches the bottom to support benthic communities in the southern Bering Sea.

Figure 4 (Left) Chlorophyll-a estimate for 27 May 1997. (Right) Correction of the image by S. Saitoh, which lowers the chlorophyll-a concentration to levels consistent with field observations.

High oxygen uptake (an indicator of carbon supply to the benthos) as well as benthic biomass (an inter-annual integrator of overlying water column processes) are observed in persistent “hot spot” zones (Fig. 5; Grebmeier and Dunton 2000, Cooper et al. 2002, Grebmeier and Cooper 2004). Recent benthic studies also show that benthic productivity has been decreasing over the past two decades in these “hot spots” of biological productivity southwest and northeast of Saint Lawrence Island in the northern Bering Sea, and into the southern Chukchi Seas. These changes include decreases in benthic biomass of the dominant bivalve fauna south of St. Lawrence Island (Grebmeier and Dunton 2000, Grebmeier and Cooper 2004) and amphipods north of the island in the Chirikov Basin (Moore et al. 2003). These changes coincide with a reduction in current transport through Bering Strait in the 1990’s (Roach et al. 1995). Measurements of sediment grain size at these sites indicate a change towards finer-grained sediments southwest of St. Lawrence Island, which provides indirect evidence of a reduction in current flow. By contrast, sediments in the Chirikov Basin north of St. Lawrence Island are becoming sandier, which may be occurring due to enhanced deposition southwest of St. Lawrence Island, creating a “sediment starved” scenario. The end result is resuspension of finer-sediment grains that are ultimately deposited as currents reduce in velocity again north of Bering Strait. Another possibility is that hydrographic changes could result in a reduction of nutrient upwelling onto the northern Bering Sea shelf and a decline in pelagic productivity, thus impacting the carbon flux to the sediments and benthic standing stock in the region. Declines in amphipod populations and shifts in gray whale feeding sites from the northern Bering Sea to the southern Chukchi Sea (Moore et al. 2003) also suggest that a larger spatial scale ecosystem change is underway.

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Figure 5. Spatial sampling in the northern Bering and Chukchi seas (Left) for sediment oxygen (mmol O₂ m^-2 d^-1) and (Right) for benthic macrofaunal biomass (gC m^-2) in the Bering, Chukchi and East Siberian Seas, 1984-1995. These parameters provide short-term (oxygen uptake) and long-term (biomass) indications of carbon supply to the benthos (Grebmeier and Dunton 2000).

Our longest time-series data set is southwest of St. Lawrence Island (SLI). Changes in regional oceanography that would influence the Gulf of Anadyr gyre position or size to the southwest of SLI ultimately are related to northward transport of water through Bering Strait, and geostrophic balance within the Arctic Ocean basin. Water-column primary production and the final location of carbon deposition to the benthos are related to ice production and brine formation in the SLI polynya during late winter-early spring. Reduced ice production south of SLI under global change might decrease renewal of nutrients for early-season production by ice algae and phytoplankton, and baroclinic currents that move this material to the southwest (Grebmeier and Cooper 1995). The regional sea surface warming observed south of St. Lawrence Island in the 1990’s vs 1980’s (Stabeno and Overland 2001) may be a factor in the declining productivity in this region (Grebmeier and Dunton 2000, Grebmeier and Cooper 2004). A warming trend in the northern Bering Sea would influence the persistent cold pool, which would diminish in size and strength. Since this localized area of low temperatures is thought to inhibit demersal fish migration northward, any warming trend may allow these fishes to migrate northward and increase the competition between bottom-feeding fish and benthic-feeding marine mammals and birds for food prey. These and other studies indicate that it is timely to continue focused biologically-based sampling in the Bering Strait region in this era of environmental change.

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Supported by U.S.NSF grants: OPP0125082, OPP0125399, OPP0125301, and OPP0229302; NOAA/CIFAR 603015, NOAA/CIFAR 04-0048; USFWS BERPAC project.

Contact information

Jackie M. Grebmeier
University of Tennessee
569 Dabney Hall
Ecology & Evolutionary Biology
Knoxville, TN 37996

Phone: 1-865-974-2592
Fax: 1-865-974-7896
Email: jgrebmei@utk.edu