STOCKS AND DYNAMICS OF BACTERIOPLANKTON CARBON DURING THE SPRING BLOOM IN THE EASTERN NORTH-ATLANTIC OCEAN

Bacterial biomass increased five-fold in the euphotic zone (from 450 to 2250 mgC m-2) in response to the spring phytoplankton bloom in the eastern North Atlantic Ocean (Lat. 47-degrees-N, Long. 20-degrees-W) in 1989. Bacterial biomass accounted for about 20-30% of the particulate organic carbon (POC) above 50 m and a somewhat larger fraction in the layer below. Bacterial production averaged about 30% of primary production and remained rather constant while the primary production varied from 600 to 1500 mgC m-2 day-1 in response to event-scale changes in irradiance. Thus bacterial production varied from 15 to 80% of the concurrent primary production, with peaks occurring on overcast days when photosynthesis was low. Bacterial production in both the euphotic zone and the layer immediately below appeared to respond to the meteorologically-driven variations in photosynthesis with a time lag of 3-4 days, consistent with estimates of turnover rates of 0.2 day-1. In the upper layer incorporation of dissolved free amino acids supported about 20% of the production. The bacterial carbon demand at peak production required subsidies of carbon from the bulk POC and/or DOC pools. In the lower layer, decomposition of the vertical flux of sinking POC may have supported about half the mean production. Our bacterial production estimates for the 50-150 m layer are consistent with vertical flux estimates from drifting sediment traps and support other observations in suggesting that very large amounts of primary production pass through the DOC pool on short time-scales. Our observations suggest a spring-time North Atlantic condition for the microbial foodweb in which bacterial biomass makes up a relatively small part of the plankton carbon stock, but cycles rapidly (0.1-0.3 day-1) and commands a large share of the carbon cycling in the upper ocean. Through extensive metabolism of DOC and sinking POC in the upper 200 m, the bacterioplankton act as a selective gate through which some fraction of the products of primary production are exported into the oceanic interior.