PRODUCTIVITY REGIME AND PHYTOPLANKTON SIZE STRUCTURE IN THE TROPICAL AND SUBTROPICAL NORTH-ATLANTIC IN SPRING 1989

Productivity regime and phytoplankton size structure are described for two different epipelagic systems in the tropical/subtropical Northeast Atlantic Ocean investigated during 9-11 day drift studies in spring 1989 in the JGOFS North Atlantic Bloom Experiment, 18-degrees-N, 30-degrees-W and 33-degrees-N, 20-degrees-W. At the 18-degrees-N study site, an oligotrophic system was encountered. The water column above the main pycnocline at about 50-60 m depth was nutrient-depleted, and both chlorophyll and primary production displayed subsurface maxima at the nutricline. Picoplankton was the dominant size fraction, accounting for 78-90% of chlorophyll and 83-98% of primary production. Synechococcus-type coccoid cyanobacteria were the dominant picoplankters. The hydrographic situation was characterized by high small-scale variability: the most interesting feature was the intrusion of nutrient-depleted Subtropical Salinity Maximum Water into the euphotic zone, whose impacts on the productivity regime are discussed. At 33-degrees-N study site, a post-bloom situation was encountered. Although the euphotic zone was nutrient-depleted, higher amounts of larger phytoptankton were present, the contribution of picoplankton being 42-53% of chlorophyll and 42-86% of primary production. Over the course of the drift study, subsurface maxima of chlorophyll and productivity evolved. the contribution of picoplankton having increased. Picocyanobacteria again were the dominant picoplankters. At both study sites the profiles of abundance ratios of picocyanobacteria to picoeucaryotes cell numbers proved to be a useful tool to characterize water masses.