A Comparison Between Late Summer 2012 and 2013 Water Masses, Macronutrients, and Phytoplankton Standing Crops in the Northern Bering and Chukchi Seas

Seth L Danielson, UAF, College of Fisheries and Ocean Sciences, Fairbanks, AK, United States, Lisa B Eisner, NOAA - Alaska Fisheries Science Center, Ecosystem Monitoring and Assessment, Seattle, WA, United States, Carol A Ladd, NOAA Seattle, Seattle, WA, United States, Calvin W. Mordy, University of Washington, Joint Institute for the Study of Atmosphere and Ocean, Seattle, WA, United States, Leandra Sousa, Department of Wildlife Management, North Slope Borough, Barrow, AK, United States and Thomas Weingartner, University of Alaska Fairbanks, College of Fisheries and Ocean Sciences, Fairbanks, AK, United States
Abstract:
Survey data from the northern Bering and Chukchi sea continental shelves in August-September 2012 and 2013 reveal interannual differences in the spatial structure of water masses along with statistically significant differences in thermohaline and chemical properties and phytoplankton communities. We find that the near-bottom Bering-Chukchi Summer Water (BCSW) was more saline in 2012 and Alaskan Coastal Waters (ACW) were warmer in 2013. Both carried higher nutrient concentrations in 2012, supporting a larger chlorophyll a standing crop biomass that was comprised primarily of small (<10 μm) size class phytoplankton. The location of phytoplankton biomass concentrations and their size compositions reveal linkages between the wind fields, seafloor topography, water masses, and the pelagic production. The horizontal structure of the shelf water masses differed in part because of the August regional wind field, which was more energetic in 2012 but was more persistent in direction in 2013. ACW were found all along the coast from Nunivak Island to Point Barrow in 2012, but in response to the persistent wind of 2013 ACW was not found north of Ledyard Bay. Instead, the 2013 NE Chukchi shelf was flooded with cold and fresh waters derived from ice melt waters (MW) that resided above cold and salty Bering-Chukchi Winter Waters (BCWW). Similarly, in the northern Bering Sea, low-salinity coastal waters from western Alaska were driven offshore to a greater extent in 2013, while in 2012 they were found more confined to shore and more prominently extended northward through Bering Strait. The water mass distributions together with the winds and limited surface current data suggest that the NE Chukchi Alaskan Coastal Current (ACC) was shut down for a time in August and September 2013. Our results have implications for the fate of fresh water, heat, and pelagic production on the Bering-Chukchi shelves.
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