Microzooplankton community structure and grazing on phytoplankton, in an eddy pair in the Indian Ocean off Western Australia

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 ∼ 130 km in diameter and was well mixed to ∼ 250 m in the centre, while the cold-core (CC) eddy was smaller ( ∼ 100 km ) 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 3.8 μ g C L - 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 μ m ) and was dominated by ciliates. The growth rate of phytoplankton derived from dilution experiments using chlorophyll a analysis ranged between - 0.09 and 0.12 d - 1 , and phytoplankton mortality due to microzooplankton grazing was between 0 and 0.23 d - 1 . The growth rate of specific picoplankton genera, using significant flow-cytometry data, ranged between 0 and 1.3 d - 1 , and mortality due to microzooplankton grazing ranged between 0 and 0.42 d - 1 . 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.