PLANKTON SUCCESSION AND CARBON CYCLING AT 47-DEGREES-N-20-DEGREES-W DURING THE JGOFS NORTH-ATLANTIC BLOOM EXPERIMENT

An overview is given of the multinational investigations carried out at 47-degrees-N 20-degrees-W for the period 24 April-31 May, with the main emphasis on the succession of plankton communities and the flux of organic carbon through various plankton components. The depth of the upper mixed layer decreased rapidly after 25 April, triggering the start of the spring bloom that developed within a 2-week period. Chlorophyll a stocks integrated to 80 m water depth reached peak concentrations during the first 10 days in May. The observed decline was partly due to the seasonal development, but also may have been influenced by changes in water masses associated with a cyclonic eddy. Primary production ranged from 50 to 150 mmol C m-2 day-1 with highest values in the first half of May. After the first bloom phase, dominated by diatoms, nanophytoplankton gained more importance as primary producers when silicate was depleted. Stocks of bacteria, microzooplankton and mesozooplankton increased in the second half of May. Bacterial production averaged 30% of primary production and probably metabolized a large amount of dissolved organic carbon (DOC) per day, but due to uncertain carbon conversion efficiencies total bacterial carbon consumption is difficult to estimate. Large daily changes in the DOC standing stock could not be reconciled with the measured primary production and are probably an expression of spatial rather than temporal variations. Microzoooplankton was found to consume around 64% of primary production in the second half of May. Grazing estimates for mesozooplankton varied but seem to be small (<5% of primary production) for most of the investigation period. The small mesozooplankton size classes (<1 mm) dominated biomass and grazing. Vertical particulate organic carbon (POC) flux measured by sediment traps in 150 m depth was around 9.8 mmol C m-2 day-1 representing approximately 11% of primary production. The spring bloom pulse of particle flux reached the deep ocean and benthos 4-6 weeks after the surface water peak.