Study On The Seasonal And Interannual Variations In Polarward Nutrient Flux From Bering Sea

The nutrient flux through Bering Strait has significant impacts on the biomass and community composition of phytoplankton in the Chukchi Sea.Nitrate is the limiting nutrient for phytoplankton growth in the study area.Based on a three-dimensional ocean-sea ice-biogeochemical model,the spatial distribution and interannual variations in simulated temperature,salinity,northward current speed in the Bering Strait,and sea ice concentration,temperature,salinity,circulation,chlorophyll concentration,nitrate concentration over the Bering Sea shelf were evaluated by using observed data.The results show that the model reasonably captures the spatiotemporal variations of physical and biogeochemical fields in the Bering Sea.Then the seasonal and interannual variations in nitrate flux through Bering Strait were analyzed,according the hindcast simulations from 1998 to 2015.In addition,the mechanisms for regulating the extreme nitrate fluxes in winter were discussed.The main findings are summarized as follows.（I）The Bering Strait Throughflow is higher from May to August（1.21～1.35 Sv）,and lower in other months（0.85～1.09 Sv）.Nitrate concentration is higher from February to April(12.13～12.28 mmol N m^-3),and remained low from July to August(7.65～7.68 mmol N m^-3).The nitrate flux through Bering Strait reaches the peak(12.14～13.17 kmol N s^-1)during February-May and then decreases to a relatively lower level(8.10 kmol N s^-1)in July and maintains this level until January.Shelf-slope exchange near Cape Navarin plays a key role in poleward nutrient supply since the main nutrient transport belt over the Bering shelf corresponds to the pathway of Anadyr Current from Cape Navarin to Bering Strait.（II）The nitrate fluxes in winter（February-March）and summer（July-August）both show significant interannual variations.In the winter season,the interannual nitrate flux through Bering Strait,mainly controlled by nitrate concentration,has a significant correlation（r=0.80,p<0.01）with that through the section adjacent to Cape Navarin（CN section）in the fall season（October 11-December 10）.In the summer season,biological activity plays a limited role in the interannual variation in poleward nitrate flux.Instead,this interannual variability is determined jointly by the volume flux and nitrate concentration through Bering Strait.The correlation coefficient between the interannual nitrate fluxes through Bering Strait in summer and CN section two months ago is 0.76（p<0.01）.These suggest that the remote interannual signals in winter and summer from upstream appear to regulate the Bering Strait nitrate flux via horizontal advection so that the exchange near Cape Navarin can be considered as a predictor factor.Furthermore,in summer,the interannual nitrate fluxes through Bering Strait and CN section have the highest correlation（r=0.86,p<0.01）at the same period due to the highest correlated volume fluxes.（III）In winter,the nitrate flux through Bering Strait was lowest(4.56 kmol N s^-1)in 2000-2001,and highest(19.77 kmol N s^-1)in 2010-2011.In the fall and winter of2000-2001,the northward flow over the western Bering Sea shelf turned anticlockwise in the Gulf of Anadyr and outflowed the shelf near Cape Navarin due to the center of Aleutian Low moving to the middle of Bering Sea.During this period,the Aleutian Basin water with high nitrate concentration did not reach Bering Strait so that the nitrate flux through the Bering Strait was lowest.However,in the winter of 2010-2011,the increased volume flux and nitrate concentration through Bering Strait under the weaker north wind leaded to the highest nitrate flux among the study period.