Hydrographic conditions during the 2004 SBI process experiments

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.