Nitrogen uptake dynamics in landfast sea ice of the Chukchi Sea

The coastal Chukchi Sea has a particularly productive landfast ice ecosystem but is currently lacking direct measurements of nitrogen (N) cycling within the ice. Using stable isotopic tracers, we measured uptake and regeneration of ammonium and nitrate, along with primary and secondary production at three depths within landfast sea ice in the Chukchi Sea near Barrow, Alaska, during April 2011 and January 2012. These are the first data of N uptake and regeneration rates in landfast Arctic sea ice during spring and winter. Both inorganic and organic nutrient concentrations in the ice were generally higher than the water column, with the exception of phosphate, which may have limited production in certain sections of ice. Primary production at all ice depths was higher than the water column during April, but below the detection limit in January. Bacterial production in the bottom ice (0-10 cm from the ice-water interface) was consistently higher than the water column across seasons. Absolute uptake of ammonium was highest in the bottom ice and higher than absolute uptake of nitrate at all depth horizons except in the upper ice (30-40 cm from ice-water interface) during January. While N uptake rates in the ice were higher than the water column, nitrification rates were lower. Regeneration of ammonium and nitrate far exceeded uptake within the ice. The magnitude of N uptake and regeneration rates in landfast ice highlights the importance of the biological sea ice community to the region.