Rates of nitrification and ammonium dynamics in northeastern Chukchi Sea shelf waters

Nutrient concentrations are often depleted in surface waters during the late summer open-water period in the northern Chukchi Sea. Yet the rate of re-supply of nutrients to the pelagic environment controls phytoplankton community abundance and productivity, which in turn influences the benthic components of this relatively shallow ecosystem. We measured nitrogen cycling rates at four experimental stations on the northeastern Chukchi Sea shelf in the western arctic. At each station, rates for net NH 4 + regeneration, actual NH 4 + uptake, and nitrification were measured using 15N isotope enrichment methods with 24-h bottle incubations under both light and dark conditions. Net NH 4 + regeneration rates throughout the water column ranged between −0.25 and 0.23 μmol N L−1 h−1, with highest net positive regeneration occurring under light exposure in surface waters. Actual uptake rates ranged between −0.41 and −0.01 μmol N L−1 h−1. Nitrification rates (measured at two stations) were generally highest in the sediment overlying waters, ranging between 0.14 and 0.67 μmol N L−1 h−1, and appear to account for most NH 4 + uptake. Calculations of turnover time indicate that N is recycled within one day throughout the water column under natural conditions in the northern Chukchi Sea. Our findings confirm the results of previous studies that suggest a significant portion of surface nutrients on the Chukchi Shelf are products of horizontal advection through the Anadyr Current but also highlight the role of sediment regenerated N in supporting water-column primary production.