Biogeochemical responses to late-winter storms in the Sargasso Sea, I—Pulses of primary and new production

We have hypothesized that the weekly/biweekly passage of winter storms in the subtropical open ocean destabilizes the water column leading to pulsed NO3− inputs, resulting in new production that is not accounted for in most annual estimates. This paper presents data on nitrogen and carbon cycling in the Sargasso Sea at approximately daily resolution, during the period prior to seasonal stratification in 2004 and 2005; these data permit us to assess the importance of winter storms for introducing NO3− and the contribution of these inputs to annual new and export production. The two sampling years were in stark contrast to each other with 2004 characterized by periods of relative calm between winter storms, and 2005 characterized by nearly continuous storm activity. As a result, temporal variability in mixed layer depth (MLD) and euphotic zone [NO3−] were very different between years. MLDs in 2004 increased to >150 m in response to the passage of storms and then rapidly shoaled to <100 m leading to the pulsed injection of NO3− (∼100 nmol l−1) into the lower half of the euphotic zone, while in 2005 MLDs were consistently >300 m and euphotic zone [NO3−]>100 nmol l−1. Despite the very different [NO3−], rates of daily NO3− uptake were similar from year to year because of significant nocturnal uptake in 2004. Similar rates of new production did not translate into similar rates of particulate nitrogen and carbon export however, as observed export from the upper 200 m was 2–5-fold greater in 2004 than in 2005. Furthermore, the decrease of particulate nitrogen and carbon flux with depth between 200 and 400 m in 2004 was substantially lower than in 2005; this is consistent with the observed biological response in which diatoms and coccolithophores exhibited rapid growth following pulsed NO3− inputs in 2004. A combination of data from the Bermuda Testbed Mooring, which provides a longer temporal record than the cruise, and the observations presented in this study show that in the winter of 2004, there were 8–10 storm events that likely resulted in pulsed NO3− inputs. Summed over all the events, new production prior to seasonal stratification was estimated to be ∼0.12–0.18 mol N m−2 or ∼14–21% of current annual estimates.