Increases in Benthic Particulate Export and Sedimentary Denitrification in the Northern Chukchi Sea Tied to Under‐Ice Primary Production

Abstract

AbstractChanging sea ice conditions have led to increases in net primary production (NPP) in the northern Chukchi Sea, driven in part by massive under‐ice phytoplankton blooms. These blooms increase particle export to the sediments and could affect the rate of sedimentary denitrification. We use a 1‐D coupled ecosystem model forced with satellite‐derived sea ice conditions to quantify changes in particle export, nitrification, and denitrification on the northern Chukchi shelf. Between 1988 and 2018, increases in annual NPP drove secular increases in particle export to the benthos (1.8 ± 0.8 mmol m−2 yr−1), water‐column and sedimentary nitrification (1.2 ± 0.4 and 1.1 ± 0.4 mmol m−2 yr−1, respectively), and sedimentary denitrification rates (1.3 ± 0.5 mmol m−2 yr−1). Increased annual export to the benthos and denitrification were driven by higher rates early in the year (from January to June) and are highest in years where under‐ice blooms (UIBs) dominate. Greater denitrification rates in the northern Chukchi Sea would likely reduce NPP in downstream regions such as the Greenland Sea and promote greater N2 fixation in the North Atlantic. Furthermore, sea ice loss and a change in advection of nitrogen (N)‐replete waters through the Bering Strait will likely increase winter N concentrations in the northern Chukchi Sea. Through N sensitivity experiments, we found that 30% of all added N was lost through denitrification, diminishing the N supply available downstream of the Chukchi Sea. Thus, increased particle export associated with UIBs has the potential to markedly alter the N cycle both in the northern Chukchi Sea and in adjacent waters.