MEPS 233:73-88 (2002)  -  doi:10.3354/meps233073

Contribution of algal sinking and zooplankton grazing to downward flux in the Lazarev Sea (Southern Ocean) during the onset of phytoplankton bloom: a lagrangian study

E. A. Pakhomov1,2,*, P. W. Froneman1, P. Wassmann3, T. Ratkova4, E. Arashkevich4

1Southern Ocean Group, Department of Zoology and Entomology, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
2Department of Zoology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
3Norwegian College of Fishery Science, University of Tromsø, 9037 Tromsø, Norway
4P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovsky Avenue 36, 117541 Moscow, Russia
*Address for correspondence: University of Fort Hare. E-mail:

ABSTRACT: Hydrography, chlorophyll a, phytoplankton and zooplankton dynamics and the vertical flux of particulate organic carbon (POC) and pigments in the upper 200 m were investigated for 12 consecutive days during a drogue study conducted in the open waters of the ice-edge zone of the Lazarev Sea during the austral summer (December/January) 1994/95. Results of the study indicate that during the experiment, primary production, although variable, increased from ~300 to ~800 mgC m-2d-1. This increase could likely be related to development of a shallow pycnocline. Analysis of sediment trap data showed that the vertical carbon flux resulting from sedimentation and grazing activity was greatest in the upper water column (<80 m). The importance of grazers to total POC flux was highest at the beginning and the end of the investigation and accounted for up to 15% of total carbon flux. The contribution of grazers to vertical flux was negligible (<2%) during the intermediate part of the Southern Ocean Drogue study. Lower contribution of grazers to sedimentation of POC at depth can likely be related to community composition of zooplankton. Sedimentation of phytoplankton cells from the upper water column increased during the study. Here, downward POC flux resulting from sedimentation of microphytoplankton was equivalent to 15-75% of the total. Increase in sedimentation of phytoplankton during the study can be related to an increase in the average size of phytoplankton cells. Transport of POC from surface waters to deeper depths resulting from sedimentation or grazing activity was equivalent to <48% of total daily primary production, measured at 50 m, while the same value for phytoplankton flux did not exceed 27% of the total. Zooplankton density was insufficient to exert either a positive (via faecal pellets) or negative (via reducing suspended phytoplankton concentration) effect on particulate carbon sedimentation. This resulted in algal sink being the most important mechanism in downward POC flux during the onset of the phytoplankton bloom period in the Marginal Ice Zone, even in the presence of pelagic tunicates.

KEY WORDS: Seasonal ice zone · Antarctica · Lazarev Sea · Vertical flux · POC · Phytoplankton · Zooplankton · Grazing

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