Ecosystem Observations in Barrow Canyon:
A Focus for the International Distributed Biological Observatory (DBO)
J. Grebmeier1, R. Pickart2, C. Ashjian2, L. Cooper1, K. Frey3, J. He4,
M. Itoh5, M. Kedra1, T. Kikuchi5, S. Moore6, J. Nelson7, S. Vagle7
1University of Maryland Center for Environmental Science, Solomons, MD, USA
2Woods Hole Oceanographic Institution, Woods Hole, MA, USA
3Graduate School of Geography. Clark University, Worchester, MA, USA
4Polar Research Institute of China, Shanghai, People's Republic of China
5Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
6NOAA/Fisheries, Office of Science & Technology, Seattle, WA, USA
7Institute of Ocean Sciences, Dept. Fisheries and Oceans, Sidney, BC, Canada
November 11, 2012
- Since 1980, sea ice persistence in the Barrow Canyon (BC) region of the Distributed Biological Observatory (DBO) has declined by ~3 days per year.
- Heat flux during the 2010 DBO BC section was 3 times higher compared to that in 1993; heat flux was particularly high in the Alaska Coastal Water. The ACW was warmer in July 2011 than July 2010, suggesting a continued warming trend.
- Zooplankton and benthic species composition vary by water mass type in BC; total zooplankton abundance was greater in 2011 than in 2010.
The Chukchi Sea continental shelf in the Pacific Arctic region (Fig 3.5) is influenced by the northward transport of nutrient-rich Pacific water via the Bering Strait (see the Ocean essay for more information about Pacific Water flow through the Bering Strait), which supports areas of high water column and benthic production on the southeast and northeast portions of the shelf (citations in Grebmeier, 2012). Dramatic, broad temporal and spatial variation in chlorophyll biomass in the Chukchi Sea has coincided with seasonal sea ice retreat and increases in seawater temperatures. One of the key uncertainties in this region is how the marine ecosystem will respond to seasonal shifts in the timing of sea ice retreat and/or delays in fall sea ice formation.
The Distributed Biological Observatory (DBO; Fig. 3.5) is being developed by an international consortium of scientists in the Pacific Arctic as a change detection array to systematically track the broad biological response to sea ice retreat and associated environmental change that is occurring (Grebmeier et al., 2010). The DBO relies on coordinated, international sampling by a network of ships from Canada, China, Korea, Japan, Russia and the United States. Specific high productivity locations in the Bering and Chukchi seas are sampled on a repeated basis as research vessels transit the Pacific sector of the Arctic. Additional measurements by satellite and moorings at the designated sites are providing important time series data to develop an early detection system for biological and ecosystem response to climate warming. The following report highlights specific findings at the DBO Barrow Canyon site (Fig. 3.5).
Sea Ice and Hydrography
Barrow Canyon (BC) in the northeast Chukchi Sea is one of the major conduits for Pacific water into the interior Arctic Basin. Shelf-basin exchange is strongly wind-forced, which leads to upwelling in the canyon (Aagaard and Roach, 1990; Pickart et al., 2012) and along the continental slopes of the Chukchi and Beaufort seas (e.g., Nikolopoulos et al., 2009; Llinas et al., 2009). Upwelling occurs in all seasons and under ice conditions that vary between open water and 100% cover (Schulze and Pickart, 2012). Annual sea ice cover at the DBO BC site has declined dramatically during recent decades, being nearly year round in the early 1980s and declining to only ~9 months by 2009. This amounts to a reduction rate of ~2.95 days per year (Cavalieri et al., 2008), which translates into annual sea ice cover reduction of nearly three months (~88.5 days) over the past three decades. This reduction in annual sea ice persistence is due, in part, to earlier sea ice retreat in spring, but even more so because of later sea ice formation during late autumn. These dramatic shifts in sea ice cover undoubtedly have significant impacts on primary production and ecosystem function throughout the region. See the Sea Ice essay for a pan-Arctic perspective on the changing ice cover, and the Arctic Ocean Primary Productivity and Nutrient Variability essay.
As part of the DBO pilot program in 2010, six repeat high-resolution ship surveys were conducted across the canyon (Fig. 3.6). Total northeastward transports were between 1.0 and 1.7 Sv (Sverdrup), which consist of 0.4-1.0 Sv of warm, fresh Alaskan Coastal Water (ACW) and 0.3-0.6 Sv of cold, salty Pacific Winter Water (PWW). Measured BC transport and the local along-canyon winds are related such that, under southwesterly winds, the northeastward flow of water through the canyon increases. The nearshore ACW supplied 9-27 TW (Tera Watts) of heat via BC into the Arctic Basin during summer 2010 (Fig. 3.6). This was three times larger than the heat flux measured in 1993 (Münchow and Carmack, 1997), mainly due to the higher temperature of the ACW. CTD data (not shown) reveal that the ACW was warmer in July 2011 than July 2010, suggesting a continued warming trend. Current meter sampling along the DBO BC line in July 2011, combined with salinity measurements, indicated a strong eastward flowing current. Stratification was greatest on the western side of the canyon, with higher nitrate and silicate in the deeper PWW on that side (Fig. 3.7). Chlorophyll a was relatively high in the center of the canyon, supporting the findings of the fluorescent sensor on the CTD, whereas ammonium regeneration was greatest at depth in the center of the canyon.
The DBO stations occupied in BC during July 2010 and 2011 show regional structuring. A clear front between the arctic zooplankton community (western side) and Pacific expatriates (eastern side) was observed. During August-September, zooplankton species composition varied both within and across-stream location and water mass type as well as inter-annually, with total zooplankton abundance greater in 2011 than in 2010. The large copepod Calanus glacialis/marshallae was more abundant in 2010, associated with cold PWW in the west, than in 2011. Comparatively, higher abundances for the very small copepod Oithona similis and the small copepod Pseudocalanus spp. characteristic of ACW were observed in 2011. High benthic infaunal biomass, species richness and biodiversity occur in the central region of BC in the northeast Chukchi Sea (see the Arctic Benthos essay for additional information about the Chukchi Sea benthos and the pan-Arctic benthos). Species richness was correlated with water salinity and food quality (chlorophyll a and total organic carbon in the sediments). Benthic community structure and biomass indicated that the western side was dominated by surface deposit feeders (Ennucula tenuis) and the subsurface deposit feeding polychaete Maldane sarsi, with the deepest portion dominated by the suspension feeding bivalve Mytilus spp. Suspension feeding sea cucumbers and ascidians were closer to the shore, indicating stronger currents above these animals in the ACW. High abundance of suspension feeding amphipods (Byblis spp.) close to the shore provided food for gray whales.
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