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We investigated the microzooplankton assemblage and the grazing impact of microzooplankton on phytoplankton in two contrasting eddies in an oligotrophic region of the eastern Indian Ocean during October 2003. Each eddy had a unique microzooplankton assemblage that did not conform to the expectations of downwelling and upwelling eddies. The warm-core (WC) eddy was ~130km in diameter and was well mixed to ~250km in the centre, while the cold-core (CC) eddy was smaller (~100km) and had a warm cap and a well-developed deep chlorophyll a maximum (DCM). Both eddies had formed within the Leeuwin Current at the continental shelf break off Western Australia. Total microzooplankton biomass ranged from 0.5 to 3.8μg CL^-1, and was significantly (F_1,10 = 5.6, p<0.05) greater in the WC eddy. The WC eddy contained a significant population of large diatoms, which were grazed by dinoflagellates; the CC eddy resembled a nutrient-limited, oceanic environment with small phytoplankton cells (90% ,5 μm) and was dominated by ciliates. The growth rate of phytoplankton derived from dilution experiments using chlorophyll a analysis ranged between -0.09 and 0.12d^-1, and phytoplankton mortality due to microzooplankton grazing was between 0 and 0.23d^-1. The growth rate of specific picoplankton genera, using significant flow-cytometry data, ranged between 0 and 1.3d^-1, and mortality due to microzooplankton grazing ranged between 0 and 0.42d^-1 source. In most cases there was a close balance between growth and mortality in both eddies. Although the eddies differed in their physico-chemical properties and community structures, microzooplankton grazing could account for >100% of primary production in experiments from both eddies. However, the structure of the phytoplankton and microzooplankton assemblages were not typical for either up- or downwelling eddies, and as such the fate of organic carbon will not reflect that of classic eddies.

Item Type:	Refereed Article
Keywords:	Mesoscale eddies; Heterotrophic organisms; Grazing; Picoplankton; Ciliate; Leeuwin Current
Research Division:	Biological Sciences
Research Group:	Ecology
Research Field:	Marine and estuarine ecology (incl. marine ichthyology)
Objective Division:	Environmental Management
Objective Group:	Marine systems and management
Objective Field:	Marine biodiversity
UTAS Author:	Paterson, H (Dr Harriet Paterson)
ID Code:	59781
Year Published:	2007
Web of Science® Times Cited:	33
Deposited By:	Research Division
Deposited On:	2009-12-17
Last Modified:	2013-10-29
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