THE ROLE OF MESOSCALE HYDROGRAPHY ON MICROBIAL DYNAMICS IN THE NORTHEAST ATLANTIC - RESULTS OF A SPRING BLOOM EXPERIMENT

During RV Meteor cruise No. 10 from May to June 1989 (JGOFS pilot stud y) bacterial and picocyanobacterial abundance, biomass, and bacterial production were estimated at two drift stations close to 47N, 20W and 58N, 20W in the northeast Atlantic. At 47N two different mesoscale hyd rographic structures were sampled which divided the drift experiment i nto a cyclonic and an anticyclonic circulation phase. Transition from one phase to the next was clearly reflected by changes of the biologic al structure in the upper water column. Phytoplankton stocks maintaine d during the cyclonic phase were about 1.8 times higher than those of the anticyclonic phase (1552 mg C m(-2) and 880 mg C m(-2), resp., int egrated over the mixed layer, Deckers, 1991). Integrated stocks of bac teria showed an opposite pattern of distribution. Picocyanobacterial b iomass (PCB) was 3.4 times higher during the anticyclonic phase than d uring the cyclonic phase (96 mg C m(-2) and 28 mg C m(-2), resp.), and the respective factor for total bacterial biomass (TBB) was 3.7 (830 mg C m(-2) and 225 mg C m(-2), resp.). Our analysis indicates that the combined bacterial biomass dominated within the mixed layer during th e anticyclonic phase, while the cyclonic phase was clearly dominated b y eucaryotic phytoplankton. Additional evidence for a shift of biology toward the microbial food web was indicated by a strong increase of b acteria during the anticyclonic phase. Thus, simultaneously and side b y side, an autotrophic and a heterotrophic system were supported by th e prevailing hydrographic conditions. At 58N within an anticyclonic me soscale hydrographic structure the phytoplankton bloom was at a develo ping stage, characterized by low biomass (730 mg C m(-2) in the mixed layer, Deckers, 1991) but relatively high primary production. In contr ast, bacterial stocks were quite high, but bacterial production was lo w in comparison to the anticyclonic phase at 47N (90 mg C m(-2) d(-1) and 153 mg C m(-2) d(-1), resp., integrated from 0-300 m). It was calc ulated that bacterial gross production averaged 42% (47N, anticyclonic phase) and 25% (58N) of primary production. These results suggest tha t within a specific type of hydrographic structure either a heterotrop hic or an autotrophic system can be established, depending on the stag e of bloom development. In conclusion: Depending on their origin and a ge, mesoscale hydrographic structures can be correlated with different stages of biological development. This leads to the mesoscale patchin ess of biological measurements, which is a characteristic feature of t he northeast Atlantic.