A COMPARISON OF ISOTOPIC AND CHEMILUMINESCENT METHODS OF ESTIMATING MICROALGAL NITRATE UPTAKE IN THE NE ATLANTIC

Microalgal nitrate uptake rates derived from measurements using incorp oration of isotopic N-15 tracer were compared with those estimated fro m the disappearance of nitrate measured by the chemiluminescent techni que during 2 cruises investigating phytoplanktonic processes in the NE Atlantic. Nitrate uptake was assessed during the development of the s pring bloom in May 1990 and also shortly after the decline of a mesosc ale coccolithophore bloom in July 1991. In May 1990, surface ambient n itrate concentrations ranged from 6 mu mol l(-1) to 20 nmol l(-1), whi le concentrations of <1 pmol l(-1) were recorded during July 1991. Reg ression analysis of all data from both years reveals a significant cor relation between the 2 techniques (p<0.005; r(2)>0.67, n = 28). This s ignificant correlation suggested the absence of potential artifacts no ted for these techniques such as nitrification. Thus, in the majority of our comparisons, the observed near 1:1 relationship between these 2 independent methods of estimating nitrate uptake gives us confidence in our current ability to estimate 'new production' over a wide range of ambient nitrate and chlorophyll concentrations in the NE Atlantic.