Vertical stability and the annual dynamics of nutrients and chlorophyll fluorescence in the coastal, southeast Beaufort Sea

Abstract

The first quasi‐annual time series of nutrients and chlorophyll fluorescence in the southeast Beaufort Sea showed that mixing, whether driven by wind, local convection, or brine rejection, and the ensuing replenishment of nutrients at the surface were minimal during autumn and winter. Anomalously high inventories of nutrients were observed briefly in late December, coinciding with the passage of an eddy generated offshore. The concentrations of NO3− in the upper mixed layer were otherwise low and increased slowly from January to April. The coincident decline of NO2− suggested nitrification near the surface. The vernal drawdown of NO3− in 2004 began at the ice‐water interface during May, leaving as little as 0.9 μM of NO3− when the ice broke up. A subsurface chlorophyll maximum (SCM) developed promptly and deepened with the nitracline until early August. The diatom‐dominated SCM possibly mediated half of the seasonal NO3− consumption while generating the primary NO2− maximum. Dissolved inorganic carbon and soluble reactive phosphorus above the SCM continued to decline after NO3− was depleted, indicating that net community production (NCP) exceeded NO3− ‐based new production. These dynamics contrast with those of productive Arctic waters where nutrient replenishment in the upper euphotic zone is extensive and NCP is fueled primarily by allochthonous NO3−. The projected increase in the supply of heat and freshwater to the Arctic should bolster vertical stability, further reduce NO3− ‐based new production, and increase the relative contribution of the SCM. This trend might be reversed locally or regionally by the physical forcing events that episodically deliver nutrients to the upper euphotic zone.