SIZE-FRACTIONATED PRIMARY PRODUCTIVITY IN THE NORTHEAST ATLANTIC IN MAY JULY 1989

Size-fractionated primary productivity measurements were done in the northeast Atlantic from May to July 1989 as part of the Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment. Samples were taken on the 20-degrees-W meridian at 47, 52, 56 and 60-degrees-N, with most intensive sampling at 47 and 60-degrees-N. At the time of the first sampling at 47-degrees-N, the bloom was over but, at the other stations, substantial phytoplankton biomass and productivity were measured. The northward progression in maximum phytoplankton biomass was associated with increasing water stability and resulted in a decline in the concentration of nitrate and silicate in the surface mixed layer. The maxima in phytoplankton biomass and productivity were associated with phytoplankton cells that were larger than 5 mum diameter, but the immediate pre-bloom conditions at 60-degrees-N were dominated by small nanoplankton (<5 mum), which accounted for >50% of the daily productivity of 500-800 mgC m-2 day-1. In the post-bloom conditions at 47-degrees-N, nanophytoplankton again became increasingly important. Picophytoplankton (<1 mum) accounted for a constant proportion (10-15%) of the total productivity. The implications of the measured productivity of the different phytoplankton size fractions are discussed in relation to vertical flux of organic carbon and the potential transport into deep water of carbon dioxide, which has been incorporated into particulate organic matter.