PLANKTON DYNAMICS AND CARBON FLUX IN AN AREA OF UPWELLING OFF THE COAST OF MOROCCO

A carbon flux study was carried out off the coast of Morocco, at 31 de grees N, in a region characterized by the presence of a persistent cyc lonic eddy. Two short-term (4 and 3 day) deployments of free-floating sediment traps were combined with water column sampling and rate proce ss measurements as the ship followed the traps. For a period of 36 h b etween trap deployments, a hydrographic section was run along 31 degre es 30'N as part of a larger scale survey being carried out simultaneou sly on the R.V. A. von Humboldt. The first trap deployment was near th e eastern margin of the eddy and the traps moved to the north and west in a frontal jet associated with its northern boundary. After the sec ond deployment, which was at the recovery point of the first, the trap s moved to the west and then to the southwest. Throughout the study, c hlorophyll concentrations varied between 27 and 125 mg m(-2) (0-100 m) , with highest concentrations in the upwelled water nearest the coast and in upwelled water generated within the cyclonic eddy. Particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrat ions were relatively uniform (13.6+/-1.8 and 1.63+/-0.28 g m(-2)), wit h phytoplankton carbon accounting for 16-85% of total POC. Bacterial c arbon was similar to 5% of total POC and mesozooplankton carbon concen trations were equivalent to similar to 9% of total POC. Microzooplankt on biomass was not assessed but POC:PON ratios in the water column wer e often high, suggesting there was sometimes a large detrital componen t in the POC. Primary production rates varied between 1.0 and 2.5 g C m(-2) day(-1). Bacterial consumption accounted for similar to 50% of p rimary production. Metabolic rates suggested that copepods were ingest ing more than 0.4 g C m(-2) day(-1) while filtration rates suggested t hat ingestion of phytoplankton carbon was only similar to 0.2 g C m(-2 ) day(-1), even when phytoplankton constituted similar to 85% of the P OC.f-ratios (based on uptake rates for N-15-nitrate and ammonia) were between 0.1 and 0.4, and excretion by mesozooplankton could account fo r similar to 40% of the dairy ammonium uptake by phytoplankton. HPLC p igment analysis showed that when chlorophyll biomass was high, diatoms were dominant, whereas when it was low, small prymnesiophytes, chloro phytes and diatoms were all important. The composition of the fluorese cent pigments in material in the sediment traps indicated that intact phytoplankton and copepod faecal pellets were the main sources but the relative rates of sedimentation of pigment, POC and PON for the two t rapping periods did not reflect differences that were observed in the overlying water column. This was likely to be the result of spatial he terogeneity and strong horizontal currents within the euphotic zone. T hus, material collected at 100 m probably did not originate in the wat er column immediately overlying the traps and trapping efficiencies mi ght also have been variable. Copyright (C) 1996 Elsevier Science Ltd