The role of microzooplankton in carbon cycling in the southern ocean

Froneman, P.W. (1996) The role of microzooplankton in carbon cycling in the southern ocean. PhD thesis, Rhodes University.

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Abstract

A 3-year study was carried out on the role of microzooplankton in carbon cycling in the south Atlantic and the Atlantic sector of the Southern Ocean. Microzooplankton grazing impact on phytoplankton was estimated during austral summer and winter employing the dilution technique. Carnivory by larger zooplankton on microzooplankton during- summer was estim_ated using in vitro incubations. Microzooplankton assemblages were always dominated by protozoans comprising ciliates and dinoflagellates. Densities in winter « 1000 cells rl) were, however, approximately 50% lower than summer densities (> 1500 cells 1-1). During summer, when larger microphytoplankton cells (> 20 !lm) dominated total chlorophyll, micro zooplankton removed"" 15% of the initial standing stock or < 25% of the daily potential phytoplankton production. Size selectivity experiments showed that microzooplankton preferentially feed on the nano- (20 - 2.0 Um) and picophytoplankton « 2.0 Um) chlorophyll fractions. Indeed, during summer the grazing impact of micro zooplankton was significantly correlated with the contribution of the < 20 !lm fraction to total chlorophyll (P < 0.05). In the < 20 !lm chlorophyll fraction, microzooplankton grazing was sufficient to control the growth of the nano- and picophytoplankton suggesting that, where larger microphytoplankton cells dominate, micro zooplankton maintain the background concentrations of the nano- and picophytoplankton. During winter, when small nano- and picophytoplankton cells dominate total chlorophyll concentrations, the microzooplankton grazing impact on phytoplankton is dramatically increased. Microzooplankton removed on average 37% of the initial phytoplankton stock or"" 70% of the daily phytoplankton production. These results suggest that in winter, micro zooplankton are the main sink for phytoplankton production. Carnivory experiments conducted with selected meso- (copepods) and macro zooplankton (euphausiids and tunicates) showed that all species examined consumed micro zooplankton in the presence of substantial chlorophyll concentrations. Microzooplankton can, therefore, be regarded as trophic intermediates between bacterioplankton, small phytoplankton cells and larger zooplankton species in the Southern Ocean. The results of this investigation suggest a spatiotemporal shift in efficiency of the biological pump mediated by changes in the size composition of the phytoplankton assemblages. South of the Antarctic Polar Front (APF) large microphytoplankton cells dominate the summer chlorophyll biomass, suggesting that larger zooplankton grazers represent the main sink for phytoplankton production. Under these conditions, carbon flux to the interior of the ocean will be high due to diel vertical migrations by grazers and the production of large, fast sinking faecal pellets. The sedimentation of large phytoplankton cells also contributes to flux. In the permanently open waters south of the APF and throughout the Southern Ocean during winter, small phytoplankton cells dominate total chlorophyll, resulting in the microbial loop being the main sink for phytoplankton production. The c!ose coupling between the micro zooplankton and the microbial loop dramatically reduces the transfer of organic carbon from the surface layers to depth. Carnivory by metazoans on microzooplankton may reduce the high grazing impact of micro zooplankton and, may also represent an important source of carbon flux originating from the microbial loop.

Item Type:Thesis (PhD)
Uncontrolled Keywords:Marine zooplankton, South Atlantic Ocean
Subjects:Q Science > QL Zoology > Invertebrates
Divisions:Faculty > Faculty of Science > Zoology & Entomology
Supervisors:McQuaid, Christopher and Perissinotto, Renzo
ID Code:3298
Deposited By: Mrs Carol Perold
Deposited On:05 Sep 2012 08:24
Last Modified:05 Sep 2012 08:24
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