Controls on Water Column Nitrification Over the Chukchi Shelf, Arctic Ocean During the SUBICE Campaign (May - June, 2014)

In the highly productive Chukchi Sea, nitrogen (N) is the limiting nutrient during the phytoplankton growth season. Replenishment of nitrate (NO₃^-) by the microbial process of nitrification is partially responsible for resetting the dissolved N pool after the growth season. However, little is known about nitrification in the Chukchi Sea or its controls during this time. Both light and substrate (NH₄⁺) have been shown to limit nitrification rates in the ocean. As part of the SUBICE field campaign (May – June, 2014) we made direct measurements of water column nitrification using the stable isotope ¹⁵N with the goal of better understanding nitrification on the Chukchi shelf, and the role that light and ammonium play in controlling rates. At multiple stations we incubated water from three depths (surface, mid-column, bottom) under both simulated in situ irradiance and in the dark with ¹⁵N-NH₄⁺. Bottles were subsampled every eight hours for 24 hours and the accumulation of ¹⁵N in the NO₃^- pool was measured. Results showed that both light and the ambient concentration of NH₄⁺ were important controls on nitrification rates. Overall, rates of nitrification were lower in the light, relative to the dark, bottles. In the light bottles, rates were lowest at sites with little to no ice cover, were insignificant in surface waters independent of ice cover, and increased with depth. In the dark bottles, rates varied by ~10-fold, increased with depth, and were highly dependent on NH₄⁺ concentration (R² = 0.42, p < 0.001). We will interpret these results with respect to how future changes in ice cover will impact nitrification on the Chukchi Shelf and the Arctic N-cycle.