THE PHYSICAL AND CHEMICAL ENVIRONMENT AND CHANGES IN COMMUNITY STRUCTURE ASSOCIATED WITH BLOOM EVOLUTION - THE JOINT GLOBAL FLUX STUDY NORTH-ATLANTIC BLOOM EXPERIMENT

The physical and chemical structure of the upper 100 m of the water column and biological parameters were measured during spring bloom development along a 20-degrees-W transect from 60 to 47-degrees-N in June 1989. At 60-degrees-N, the situation was that of a late bloom: weak and intermittent stratification, high nutrient concentrations, and high and variable phytoplankton biomass and production. At 47-degrees-N, a post-bloom situation was observed: seasonal stratification in the upper 50 m, surface nutrient depletion, a subsurface chlorophyll maximum and reduced levels of primary production. At all stations, detrital material accounted for more than two-thirds of the >0.8 mum particulate organic carbon within the upper 100 m. Phytoplankton biomass showed no pronounced latitudinal trends, but there was a shift from >5 mum diatoms and prymnesiophytes at 60-degrees-N to dinoflagellates and 1-5 mum prymnesiophytes at 47-degrees-N. Nanophytoplankton comprised <5% of the biomass and <15% of the total production. Heterotrophic biomass was of the same order as phytoplankton biomass, and dominated by bacteria and microzooplankton. Microzooplankton dominated the consumption of primary production, whereas mesozooplankton accounted for <2%. Microzooplankton biomass and grazing rates doubled between 60 and 47-degrees-N, while mesozooplankton grazing rates more than halved. The data suggest a shift in the balance between production and consumption at different stages of the bloom. At 60-degrees-N, a late bloom situation, primary production exceeded consumption, while at 47-degrees-N, under post-bloom conditions, the situation was reversed. Lipid biomarkers indicate that the concentration of labile detrital carbon was significantly higher at the northernmost station, and that this was associated with mesozooplankton grazing. This suggests the efficiency of bloom utilization was low, which together with deeper mixing and the larger size structure of the plankton community, would result in a potential for substantial export flux during the late bloom phase.