NUTRIENT CONTROL OF PHYTOPLANKTON PHOTOSYNTHESIS IN THE WESTERN NORTH-ATLANTIC

LIMITED understanding of the influence of the environment on the relation between photosynthesis and light in the ocean impairs our capacity to estimate primary production from remotely sensed data on ocean colour 1 and to model the role of the marine biota in the ocean carbon cycle 2. Here we report results from several years of oceanographic cruises, showing that the parameters of the photosynthesis-light curve for the flora of the North Sargasso Sea are remarkably constant in magnitude, except during the spring phytoplankton bloom when their magnitudes are noticeably higher. We interpret these results as providing direct evidence for nutrient control of photosynthesis in the open ocean. Our findings also reinforce the plausibility of using biogeochemical provinces to partition the ocean into manageable units for basin- or global-scale ana lysis 1,3,4, show that seasonal changes in critical parameters should not be overlooked if robust carbon budgets are to be constructed, and illustrate the value of attacking the parameters that control the key fluxes, rather than the fluxes themselves, when investigating the ocean carbon cycle.