Wintertime nutrients in the North Atlantic - New approaches and implications for new production estimates

Observations of wintertime nutrient concentrations in surface waters are scarce in the temperate and subarctic North Atlantic Ocean. Three new methods of their estimation from spring or early summer observations are described and evaluated. The methods make use of a priori knowledge of the vertical distribution of oxygen saturation and empirical relationships between nutrient concentrations and oxygen saturation. A south-north increase in surface water winter nutrient concentration is observed. Winter nitrate concentrations range from very low levels of about 0.5 mu mol dm(-3) at 33 degreesN to about 13.5 mu mol dm(-3) at 60 degreesN. Previous estimates of winter nitrate concentrations have been overestimates by up to 50%. At the Biotrans Site (47 degreesN, 20 degreesW), a typical station in the temperate Northeast Atlantic, a mean winter nitrate concentration of 8 mu mol dm(-3) is estimated, compared to recently published values between 11 and 12.5 mu mol dm(-3). It is shown that most of the difference is due to a contribution of remineralised nitrate that had not been recognized in previous winter nutrient estimates. Mesoscale variation of wintertime nitrate concentrations at Biotrans are moderate (less than +/- 15% of the regional mean value of about 8 mu mol dm(-3)). Interannual variation of the regional mean is small, too. In the available dataset, there was only 1 year with a significantly lower regional mean winter nitrate concentration (7 mu mol dm(-3)), presumably due to restricted deep mixing during an atypically warm winter. The significance of winter nitrate estimates for the assessment of spring-bloom new production and the interpretation of bloom dynamics is evaluated. Applying estimates of wintertime nitrate concentrations of this study, it is found that pre-bloom new production (0.275 mol N m(-2)) at Biotrans almost equals spring-bloom new production (0.3 mol N m(-2)). Using previous estimates of wintertime nitrate yields unrealistically high estimates of pre-bloom new production (1.21-1.79 mol N m(-2)) which are inconsistent with observed levels of primary production and the seasonal development of biomass. (C) 2001 Elsevier Science B.V. All rights reserved.