New production in the Irminger Basin during 2002

Abstract

New production in the Irminger basin over the annual cycle 2001–2002 is estimated by considering changes in inorganic nutrient standing stocks between April and August 2002, with additional terms for nutrient fluxes across the thermocline and atmospheric deposition. New production before April and after August is argued to be negligible, and we estimate an annual new production rate of 36 gC m −2 year −1 within the basin. Approximately 30% of the nitrogen removed in new production accumulates as total organic nitrogen and is not exported over seasonal time scales. No systematic spatial variability in export was observed, probably due to smoothing by mesoscale activity and the widely spaced observations. Examination of satellite-derived sea surface chlorophyll levels and vertical nutrient data suggests that production begins at the margins of the basin, in particular on the Greenland shelf where it is dominated by non-siliceous phytoplankton, and then spreads to its centre. Our value for new production is lower than estimates predicted from satellite images of chlorophyll, but consistent with estimates from temperature/nitrate (T/N) regressions and Argo float data. Interannual variability in mixed layer depth is unlikely to be the source of this discrepancy, suggesting that factors not considered in the algorithms to derive export production from satellite chlorophyll concentrations, such as species succession, may play an important role in high latitudes. Satellite ocean colour data show persistently high chlorophyll concentrations from June–August suggesting that new production may follow a similar seasonality. However, T/N relationships suggest that most nitrate is removed before surface seawater temperature reaches 6 °C, a situation which satellites and Argo floats suggest occurs by the end of May; thus, most new production is likely to occur before this time. Nitrate/silicate drawdown ratios, cross-thermocline fluxes and initial pool sizes suggest that new production is likely to have been dominated by diatoms which become silicate limited with new production failing to exhaust nitrate stocks for, as yet, unclear reasons.