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°N 20°W in the northeast 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 the export of carbon using 15N stable isotope and 14C radioisotope studies. Good quality f-ratio field data is of fundamental importance to multi-compartment nitrogen-based upper ocean ecosystem modelling of carbon fluxes: this study provides an appraisal of current north Atlantic field measurements making comparisons with our findings. A phytoplankton bloom was tracked for 18 d from 1 May 1990. Waters were initially well-mixed with surface nutrients 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 flagellate 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 (using 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-estimated new production to carbon export values, even over time scales of a month. Primary production values are comparable to 1989 North Atlantic Bloom Experiment (NABE) data, despite the fact that phytoplanktonic taxonomic data for the 1990 bloom were very different from the more typical bloom conditions of 1989. Throughout the experiment the greatest biomass occurred in the 1–5 μm size range, which accounted for an average of 50% of the total production and >50% of nitrate uptake and ammonium uptake. In addition, both the net- and nanoplankton communities preferentially assimilated nitrate, which challenges the assumption that new production can be directly equated to export production on the time-scale of weeks.