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

Observations of wintertime nutrient concentrations in surface waters are sc arce 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 nutri ent concentrations and oxygen saturation. A south-north increase in surface water winter nutrient concentration is observed. Winter nitrate concentrat ions range from very low levels of about 0.5 mu mol dm(-3) at 33 degreesN t o about 13.5 mu mol dm(-3) at 60 degreesN. Previous estimates of winter nit rate concentrations have been overestimates by up to 50%. At the Biotrans S ite (47 degreesN, 20 degreesW), a typical station in the temperate Northeas t Atlantic, a mean winter nitrate concentration of 8 mu mol dm(-3) is estim ated, 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 rem ineralised nitrate that had not been recognized in previous winter nutrient estimates. Mesoscale variation of wintertime nitrate concentrations at Bio trans 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 du e to restricted deep mixing during an atypically warm winter. The significa nce of winter nitrate estimates for the assessment of spring-bloom new prod uction and the interpretation of bloom dynamics is evaluated. Applying esti mates 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 spri ng-bloom new production (0.3 mol N m(-2)). Using previous estimates of wint ertime nitrate yields unrealistically high estimates of pre-bloom new produ ction (1.21-1.79 mol N m(-2)) which are inconsistent with observed levels o f primary production and the seasonal development of biomass. (C) 2001 Else vier Science B.V. All rights reserved.