The physical and chemical environment and changes in community structure associated with bloom evolution: the Joint Global Flux Study North Atlantic Bloom Experiment

Abstract

Abstract 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°W transect rom 60 to 47°N in June 1989. At 60°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°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 μm particulate organic carbon within the upper 100 m. Phytoplankton biomass showed no pronounced latitudinal trends, but there was a shift from > 5 μm diatoms and prymnesiophytes at 60°N to dinoflagellates and 1–5 μm prymnesiophytes at 47°N. Nanophytoplankton comprised The data suggest a shift in the balance between production and consumption at different stages of the bloom. At 60°N, a late bloom situation, primary production exceeded consumption, while at 47°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.