Nitrogen Dynamics in a Marginal Sea-Ice Zone

Intense blooms of phytoplankton develop at the edge of the seasonal sea ice in the southeastern Bering Sea in spring. These blooms are closely associated with extremely strong water column density stratification due to ice melt. Here we examine the nitrogen requirements of the phytoplankton and evaluate the importance of nitrogen sources to the surface mixed layer. Ammonium and nitrate uptake rates and inorganic nitrogen concentrations within the mixed layer over a 12-day period in May 1982 showed that the amount of nutrient initially present was insufficient to satisfy the requirements of the phytoplankton. During the late winter, inorganic nitrogen levels over the Bering Sea shelf are approximately 18-20 mg-at. NH4 m-2 and 332-427 mg-at. NO3-N m-2 in a 25 m surface mixed layer. Phytoplankton specific nitrogen uptake rates measured with 15N averaged 0.0071 h-1 for ammonium and 0.0208 h-1 for nitrate at locations representative of early bloom conditions. The total nitrogen productivity over the 12-day period during which a phytoplankton bloom developed was estimated at 370.8 mg-at. NH4 m-2 and 588.5 mg-at. NO3 m-2. Nitrogen was not found to limit the bloom associated with the ice edge and N-uptake rates appeared to be close to saturation values. These uptake rates are used in a budget which provides some general boundary conditions for nitrogen input rates to the mixed layer by different biological and physical mechanisms. The budget was run as an electronic spreadsheet on a personal computer. After examining the various possible sources, we conclude that upwelling associated with the ice edge in conjunction with stationary fronts and in situ ammonification by nanoplankton and bacteria are the significant processes involved in inorganic nitrogen supply. The model presented is an efficient tool for examining physical processes at the ice edge if detailed measurements of biological parameters are available