Hydrographic conditions during the 2002 SBI process experiments

Abstract

A review of the hydrographic data from the 2002 Western Arctic Shelf–Basin Interactions (SBI) Process Cruises permits the following conclusions. (1) Temperature–salinity relationships were similar to canonical descriptions, but at five stations in the outer shelf/slope region, warm/high-salinity Atlantic Layer Water appeared to have risen, displaced the lower halocline, and mixed with shelf/upper halocline water. (2) Primary production in the SBI study region was strongly influenced by the advection of dissolved inorganic nitrogen (DIN) entering via Bering Strait. This import of DIN (ammonium+nitrate+nitrite) is modified by local processes, but without the Bering Strait inflow, biological productivity in the SBI region would be much lower. (3) In comparison to the inflowing Atlantic waters, DIN+urea/phosphate and DIN+urea/silicate ratios in the Pacific waters that dominated the upper ∼150 m of the water column were low. They were also low relative to Redfield uptake ratios for phytoplankton. (4) Microbial processes continue to destroy DIN in significant quantities as the Pacific waters transit the SBI region. (5) Nitrate and ammonium were the principal contributors to DIN. Nitrite concentrations were always <0.4 μM. With a few exceptions urea concentrations were <0.5 μM. (6) Moderate concentrations of DIN occurred in surface layers over the shelf in spring, but surface concentrations in the adjacent basin were low, suggesting that basin productivity is low. (7) In summer, DIN depletion in the surface layers was widespread, but a nutricline below ∼15 m contained chlorophyll and dissolved oxygen maxima suggesting net primary production in this layer. (8) A comparison of nutrient and dissolved oxygen concentrations in abyssal waters of the Canada Basin with conditions in Fram Strait suggests that the deep metabolism in the SBI region is exceedingly low compared to typical deep-ocean values. (9) The low abyssal metabolism and phosphate–silicate relationships suggest that the maxima in biogenic solutes (ammonium, silicate, etc.) that appear to originate on the shelf and penetrate the interior at halocline depths are not accompanied by comparable concentrations of labile organic matter. Thus, the moderate to high primary production over the shelf and slope supported by the import of DIN from Bering Strait is largely regenerated over the shelf. (10) Our easternmost section (east of Pt. Barrow) displayed nutrient maxima at depths of ∼100 m as did our three sections to the west, but in this section these signals were not connected to the shelf, and were most likely advected by an eastward shelf-break jet.