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

Abstract

[1] 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 NO 3 À in the upper mixed layer were otherwise low and increased slowly from January to April. The coincident decline of NO 2 À suggested nitrification near the surface. The vernal drawdown of NO 3 À in 2004 began at the ice-water interface during May, leaving as little as 0.9 mM of NO 3 À 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 NO 3 À consumption while generating the primary NO 2 À maximum. Dissolved inorganic carbon and soluble reactive phosphorus above the SCM continued to decline after NO 3 À was depleted, indicating that net community production (NCP) exceeded NO 3 À -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 NO 3 À . The projected increase in the supply of heat and freshwater to the Arctic should bolster vertical stability, further reduce NO 3 À -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.

Role for urea in nitrification by polar marine Archaea

“…Therefore, we believe that our finding that polar Thaumarchaeota do not contribute significantly to the uptake of labile monomers such as leucine is more representative for the low to moderate productivity conditions found in most polar marine waters. Reports of archaeal amoA genes in polar environments (28), and recent evidence of active nitrification in Arctic winter waters (31,37), suggest that polar Thaumarchaeota may be nitrifiers. To test whether polar Thaumarchaeota were growing autotrophically, we measured their activity in bicarbonate uptake.…”

“…The water column in the Southeast Beaufort Sea is perennially stratified, with low-salinity Pacific waters forming a halocline between the surface Polar Mixed Layer and deeper warmer Atlantic waters. This oceanic area is oligotrophic (31), and during the sampling period, there were seasonal fluctuations in surface light availability (from 0.01 to 3.29 μmol quanta m −2 s −1 ), ice cover, and temperature (from −1.7 to −0.4°C; SI Appendix, Table S1). Chl a values were generally low (<0.05 μg/L during the winter; SI Appendix, Fig.…”

Seasonal variability of the inorganic carbon system in the Amundsen Gulf region of the southeastern Beaufort Sea

“…Increased consumption of silicate relative to nitrate has been linked to iron or nutrient limitation in phytoplankton (Hutchins and Bruland 1998;Takeda 1998). The southern Beaufort Sea is nitrate-limited after sea-ice break-up and the onset of the spring and summer phytoplankton blooms (Tremblay et al 2008). However, NCP can be sustained upon nitrate depletion by regenerated nitrogen, nitrogen fixation, or by an allocthonous nitrogen source.…”

“…Whereas runoff from the Mackenzie River contributes significantly to the input of terrestrial freshwater in the North American sector of the Arctic, much of this runoff is directed to the west, making its way into the surface waters of the Beaufort Gyre Yamamoto-Kawai and Tanaka 2005). In contrast, the freshwater input to Amundsen Gulf is dominated by the formation and melting of sea-ice, with runoff playing a more minor role (Tremblay et al 2008;Lanos 2009). This is also reflected in the lack of terrigenous material accumulation in the Amundsen Gulf sediments (Magen et al 2010).…”

“…This method is reliable when nitrate and phosphate are not depleted, and DIC, nitrate, and phosphate are taken up in Redfield proportions (Arrigo 2005;Bozec et al 2006). However, DIC may still be consumed when nutrients are depleted, and carbon-based estimates of NCP often exceed nitrogenbased estimates (Thomas et al 1999;Bozec et al 2006;Tremblay et al 2008). Monthly values of DDIC bio , DNO Table 4.…”