Uptake of carbon, nitrogen and phosphorus by phytoplankton along the 20 degrees W meridian in the NE Atlantic between 57.5 degrees N and 37 degrees N

Phytoplankton biomass and rate of production were measured along a transect from 57.54 degreesN to 37.01 degreesN in the northeast Atlantic during Jul y 1996 and at a series of stations over a 7-day period at 37 degreesN 20 de greesW. Surface nutrient concentrations ranged from 4 mu mol l(-1) NO3-, an d 0.35 mu mol l(-1) PO43- at 57.54 degreesN to < 10 nmol l(-1) NO3- and sim ilar to 10 nmol l(-1) PO43- at 37.01 degreesN. The greatest phytoplankton b iomass and production were measured in the vicinity of a frontal system at 50 degreesN, and there was a general decline in total phytoplankton biomass and production to the south of the transect. Production was measured in th ree size fractions. At the station with the highest chlorophyll concentrati ons (50.34 degreesN), phytoplankton cells larger than 5 mum dominated the a ssemblage, accounting for 72% of the chlorophyll concentration (22.9 mg m(- 2)) and 51% of primary production (54.1 mmol Cm-2 d(-1)), but picophytoplan kton production was also high (43%). At 57 degreesN, carbon fixation by the > 5 mum fraction accounted for 75% of the daily production of 60.75 mmol C m-2 d(-1). At 37 degreesN, picophytoplankton was the dominant group, accoun ting for similar to 58% (10 mg m(-2)) of chlorophyll and similar to 64% (46 mmol Cm-2 d(-1)), of primary production. Nitrate, ammonium and phosphate u ptake rates also were determined. Although high nitrate uptake rates were m easured in the surface water at similar to 50 degreesN, the greatest uptake rates of both depth-integrated nitrate and ammonium were at the south of t he transect. At 37 degreesN, a deep euphotic zone was present and light pen etrated through the nitracline; total nitrate uptake was enhanced because o f assimilation at the base of the euphotic zone. As a consequence, high val ues of depth-integrated f-ratio were measured in the oligotrophic waters at the south of the transect. Phosphate was predominantly incorporated into t he picoplankton fraction, which included heterotrophic and autotrophic comp onents, at all stations and a significant proportion of phosphate uptake oc curred in the dark. The C:N:P assimilation ratios were variable throughout the region; phosphate uptake was generally greater than would be expected i f nutrient assimilation were in proportion to the Redfield ratio. (C) 2001 Elsevier Science Ltd. All rights reserved.