An annual study of inorganic and organic nitrogen and phosphorus and silicic acid in the southeastern Beaufort Sea

[1] Water column samples from the Mackenzie Shelf, the Beaufort Sea, and the Amundsen Gulf were obtained during 2003–2004 to investigate nutrient dynamics in an Arctic ecosystem influenced by a large river and flaw lead polynya. Nutrient inventories in the upper water column showed a significant seasonal drawdown of nitrate (NO3 ) and silicic acid (Si(OH)4) (1:1.75 mol/mol) and subsequent accumulation of nitrite (NO2 ) and ammonium (NH4 ). Dissolved organic nitrogen and phosphorus concentrations (DON and DOP) were elevated in surface waters during the time of peak phytoplankton growth (bloom and postbloom phases) and covaried in a 17:1 molar ratio. Vertical profiles showed the typical middepth maxima in NO3 , PO4 3 , and Si(OH)4 at salinity 32–33.1. The concentration of DOP (0.76 ± 0.27 mmol P L ) was also 2 times higher in this layer compared to the surface average and inversely correlated with the water mass tracer N* [N* = (NO3 16 PO4 + 2.9) 0.87], suggesting that Pacific-derived waters were a source of the enrichment. Below the mixed layer, DON was generally constant with depth, although averaged profiles (like those of urea) suggested the presence of subsurface maxima at 50 m and between 250 and 300 m. Regeneration ratios varied with depth and were approximately 9.0 mol O2/mol NO3 and 122.5 mol O2/mol PO4 in shallow Pacific-derived waters (halocline layer) and 17.4 and 193.5 in deep Atlantic waters (Atlantic layer), respectively. Deep waters of the Amundsen Gulf contained an excess of 1.7 mmol NO3 L , 0.12 mmol PO4 3 L , and 6.2 mmol Si(OH)4 L 1 and a deficit in O2, relative to waters of similar density in the Beaufort Sea, in proportions consistent with the remineralization ratios derived from oxygen-nutrient regressions. Enhanced export of particulate matter from the overlying polynya and subsequent remineralization at depth are hypothesized to create this nutrient enrichment.