Hydrographic conditions during the 2004 SBI process experiments

Abstract

Abstract Western Arctic Shelf-Basin Interactions (SBI) process experiment cruises were conducted during spring and summer in 2002 and 2004. A comparison of the 2004 data with the results from 2002 reveals several similarities but also some distinct differences. Similarities included the following: (1) Dissolved inorganic nitrogen (DIN) (ammonium+nitrate+nitrite) limited phytoplankton growth in both years, suggesting that the fixed-N transport through Bering Strait is a major control on biological productivity. (2) The head of Barrow Canyon was a region of enhanced biological production. (3) Plume-like nutrient maxima and N** minima (a signal of sedimentary denitrification) extending from the shelf into the interior were common except at our easternmost section where the nearshore end of these features intersected the slope. (4) Particularly during summer, oxygen supersaturations were common in or just above the shallow nitracline. (5) Surface waters at our deepest stations were already depleted in nitrate, ammonium and urea during our springtime observations. A major difference between the 2 years was the greater influence of warm, relatively low-nutrient Alaska Coastal Water (ACW) during 2004 entering the region via Bering Strait. This increased inflow of ACW may have reduced photic zone nutrient concentrations. The differences in water temperature and nutrients were most pronounced in the upper ∼100 db, and the increased influence of warm water in 2004 relative to 2002 was most evident in our East Barrow (EB) section. Although the EB data were collected on essentially the same year-days (29 July–4 August 2002 vs. 29 July–6 August 2004), the surface layers were up to 5° warmer in 2004. While the stronger inflow of ACW in 2004 may have reduced the autochthonous nutrient supply, rates of primary production, bacterial production, and particulate organic carbon export were higher in 2004. This conundrum might be explained by differences in the availability of light. Although, springtime ice thicknesses were greater in 2004 than in 2002, snow cover was significantly less and may have more than compensated for the modest differences in ice thickness vis a vis light penetration. In addition, there was a rapid and extensive retreat of the ice cover in summer 2004. Increased light penetration in 2004 may have allowed phytoplankton to increase utilization of nutrients in the shallow nitracline. In addition, more light combined with warmer temperatures could enhance that fraction of primary production supported by nutrient recycling. Enhanced subsurface primary production during summer 2004 is suggested not only by the results of incubation experiments but by more extreme dissolved oxygen supersaturations in the vicinity of the nitracline. We cannot, however, ignore aliasing that might arise from somewhat different station distributions and timing. It is also possible that the rapid ice retreat and warmer temperatures lead to an acceleration in the seasonal progression of biological processes such that the summer 2004 SBI Process Cruise (HLY 04-03) experiment was observing a state that might have existed a few weeks after completion of the 2002 summer cruise (HLY 02-03). Despite these complications, there is little doubt that biological conditions at the ensemble of hydrographic stations occupied in 2004 during the SBI Process Cruises differed significantly from those at the stations occupied in 2002.