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

Abstract

Phytoplankton biomass and rate of production were measured along a transect from 57.54°N to 37.01°N in the northeast Atlantic during July 1996 and at a series of stations over a 7-day period at 37°N 20°W. Surface nutrient concentrations ranged from 4 μmol l −1 NO 3 −, and 0.35 μmol l −1 PO 4 3− at 57.54°N to <10 nmol l −1 NO 3 − and ∼10 nmol l −1 PO 4 3− at 37.01°N. The greatest phytoplankton biomass and production were measured in the vicinity of a frontal system at 50°N, 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 (50.34°N), phytoplankton cells larger than 5 μm dominated the assemblage, 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 picophytoplankton production was also high (43%). At 57°N, carbon fixation by the >5 μm fraction accounted for 75% of the daily production of 60.75 mmol Cm −2 d −1. At 37°N, picophytoplankton was the dominant group, accounting for ∼58% (10 mg m −2) of chlorophyll and ∼64% (46 mmol Cm −2 d −1), of primary production. Nitrate, ammonium and phosphate uptake rates also were determined. Although high nitrate uptake rates were measured in the surface water at ∼50°N, the greatest uptake rates of both depth-integrated nitrate and ammonium were at the south of the transect. At 37°N, 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 significant proportion of phosphate uptake occurred in the dark. The C : N : P assimilation ratios were variable throughout the region; phosphate uptake was generally greater than would be expected if nutrient assimilation were in proportion to the Redfield ratio.