Seasonal and Interannual Variability of Nitrate in the Eastern Chukchi Sea: Transport and Winter Replenishment

Calvin Mordy1,2, Shaun W Bell2,3, Edward D Cokelet2, Carol A Ladd2, Geoffery T Lebon2,3, Peter Proctor2,3, Phyllis J Stabeno2, David Strausz2,3, Eric Wisegarver2 and Kevin R Wood2,3, (1)Joint Institute for the Study of the Atmosphere and Ocean, Seattle, WA, United States, (2)NOAA Pacific Marine Environmental Laboratory, Seattle, WA, United States, (3)JISAO/University of Washington, Seattle, WA, United States
Abstract:
Rapid changes in sea ice and ocean properties are occurring in the Chukchi Sea, and there is considerable uncertainty how these changes might influence nutrient distributions and ultimately primary productivity. Although inorganic nitrogen is a limiting nutrient, there are few reports on seasonal or interannual variability of nitrate, especially those focused on wintertime replenishment of nitrate. This study examined six years of hourly measurements of nitrate at multiple mooring locations off Icy Cape between 2010 and 2018 with a focus on winter replenishment in relation to transport. As winter water (WW) warms in spring and summer to above -1.6°C, it has been categorized as remnant WW. This definition does not distinguish between water advected through Bering Strait during winter and remnant summer water that has locally cooled, transitioned into winter water, and, in some years, remains over the northern shelf. This distinction is important because nitrate concentrations are lowest in newly formed WW, and rates of local nitrate replenishment appear low relative to the nutrient flux through Bering Strait.

There is considerable interannual variability in transport over the northeastern shelf of the Chukchi Sea that is driven by northerly (weakens transport) and southerly (strengthens transport) wind events. In recent years, there has been an increase in southerly wind events that may be reinforced by warming of arctic shelves. As these conditions enhance total transport and nutrient flux through Bering Strait, contemporary Bering Sea water is advected onto the northern Chukchi Sea shelf. During winters with highest transport (2010 – 2011, 2017 – 2018), pre-bloom (May 15) nitrate concentrations were high and closely resembled nitrate concentrations in the Bering Sea. Anomalously low nitrate concentrations were observed in the winter of 2011-2012 when transport was negligible. In the presence of southerly wind events, nutrient measurements in the northern Bering Sea can be used to forecast pre-bloom nitrate concentrations available for sustaining primary production in the eastern Chukchi Sea. Since 2005, inorganic nitrogen concentrations in the northern Bering Sea have varied between 11 – 22 µM, an indication that net community production over the eastern Chukchi Sea may have varied by 50% during this time.