* Corresponding author. Fax: # 44-1752-633101. E-mail address: kdon@pml.ac.uk (K.M. Donald). Deep-Sea Research II 48 (2001) 873}897 Uptake of carbon, nitrogen and phosphorus by phytoplankton along the 203W meridian in the NE Atlantic between 57.53N and 373N Kirsten M. Donald*, Ian Joint, Andrew P. Rees, E. Malcolm S. Woodward, Graham Savidge Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK Queen's University Belfast, Marine Laboratory, Portaferry, Co Down, Northern Ireland, UK Received 8 October 1997; received in revised form 11 August 1998; accepted 5 January 1999 Abstract Phytoplankton biomass and rate of production were measured along a transect from 57.543N to 37.013N in the northeast Atlantic during July 1996 and at a series of stations over a 7-day period at 373N 203W. Surface nutrient concentrations ranged from 4 mol l NO  , and 0.35 mol l PO  at 57.543N to (10 nmol l NO  and &10 nmol l PO  at 37.013N. The greatest phytoplankton biomass and production were measured in the vicinity of a frontal system at 503N, and there was a general decline in total phytoplankton biomass and production to the south of the transect. Production was measured in three size fractions. At the station with the highest chlorophyll concentrations (50.343N), phytoplankton cells larger than 5 m dominated the assemblage, accounting for 72% of the chlorophyll concentration (22.9 mg m) and 51% of primary production (54.1 mmol Cm d), but picophytoplankton production was also high (43%). At 573N, carbon "xation by the '5 m fraction accounted for 75% of the daily production of 60.75 mmol Cm d. At 373N, picophytoplankton was the dominant group, accounting for &58% (10 mg m) of chlorophyll and &64% (46 mmol Cm d), of primary production. Nitrate, ammonium and phosphate uptake rates also were determined. Although high nitrate uptake rates were measured in the surface water at &503N, the greatest uptake rates of both depth-integrated nitrate and ammonium were at the south of the transect. At 373N, a deep euphotic zone was present and light penetrated through the nitracline; total nitrate uptake was enhanced because of assimilation at the base of the euphotic zone. As a consequence, high values of depth-integrated f-ratio were measured in the oligotrophic waters at the south of the transect. Phosphate was predominantly incorporated into the picoplankton fraction, which included heterotrophic and autotrophic components, at all stations and a signi"cant proportion of phosphate uptake 0967-0645/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 1 0 2 - 8