Size-fractionated primary production and nitrogen uptake during a North Atlantic phytoplankton bloom: implications for carbon export estimates

A Lagrangian experiment was set up at 47 degreesN 20 degreesW in the northe ast Atlantic as part of the Joint Global Ocean Flux Study (JGOFS) programme . As an integral part of the programme, this study investigated the uptake of carbon and nitrogen into different algal size classes, and quantified th e export of carbon using N-15 stable isotope and C-14 radioisotope studies. Good quality f-ratio field data is of fundamental importance to multi-comp artment nitrogen-based upper ocean ecosystem modelling of carbon fluxes: th is study provides an appraisal of current north Atlantic field measurements making comparisons with our findings. A phytoplankton bloom was tracked fo r 18 d from 1 May 1990. Waters were initially well-mixed with surface nutri ents of 4-6 mmol NO3 m(-3) and 1.5-2.5 mmol SiO4 m(-3) and surface biomass of 1-1.5 mg chl a m(-3), indicating the likely onset of a bloom. Increased insolation and vertical stability produced a diatom-dominated bloom with a surface biomass peak of 3.5 mg chl a m(-3), decreased nutrient values (0.5 mmol NO3 m(-3), 0.35 mmol SiO4 m(-3)), and maximum integrated carbon uptake values of 1970 mg C m(-2) d(-1) on 17 May. Maximum nitrogen uptake values of 251 mg NO3 m(-2) d(-1) were measured on 14 May during the greatest flage llate growth phase. Integrated production rates over the experiment ranged from 0.5 to 1.0 g C m(-2) d(-1), and carbon export was estimated at 480 (us ing NO3 uptake and Redfield ratios) or 570 (using integrated C assimilation values and the mean weighted f-ratio) mg C m(-2) d(-1). The decoupling of maximum carbon and nitrogen uptake rates over a three-day period indicates that it may not be appropriate to use Redfield ratios to convert nitrate-es timated new production to carbon export values, even over time scales of a month. Primary production values are comparable to 1989 North Atlantic Bloo m Experiment (NABE) data, despite the fact that phytoplanktonic taxonomic d ata for the 1990 bloom were very different from the more typical bloom cond itions of 1989. Throughout the experiment the greatest biomass occurred in the 1-5 mum size range, which accounted for an average of 50% of the total production and > 50% of nitrate uptake and ammonium uptake. In addition, bo th the net- and nanoplankton communities preferentially assimilated nitrate , which challenges the assumption that new production can be directly equat ed to export production on the time-scale of weeks. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.