Copepods and mixotrophic Rhizaria dominate zooplankton abundances in the oligotrophic Indian Ocean

The large, tropical oligotrophic gyres of the world's oceans are understood to be relatively unproductive, with low phytoplankton and zooplankton biomass, and inefficient food webs, although data are scarce. Here we investigate changes in the zooplankton assemblage along a 15 °C temperature gradient from 20 stations on the 110°E transect as part of the second International Indian Ocean Expedition. To ensure that we reflect most of the zooplankton biodiversity, we used a 100 μm mesh net to capture the small copepod and microzooplankton components that are important and often overlooked in tropical systems. Further, to obtain a synoptic view, we towed the Continuous Plankton Recorder between stations across 30° of latitude. We found that copepod assemblages clustered into four groups, with the most distinct being at the highest latitudes south of the sub-tropical Front. As the ocean temperature increased from south to north along the transect, zooplankton abundance and diversity also increased. The dominant copepod species changed accordingly and were predominantly those with the ability and/or preference to selectively feed on microzooplankton. Although copepods were the most abundant taxon, the proportion of microzooplankton, particularly, mixotrophic Rhizaria, was consistently high. Thus, our study found a highly mixotrophic system supporting secondary production in the oligotrophic Indian Ocean.