Microplankton community and their contribution to carbon and nitrogen cycles in the Easter Indian Ocean

Student thesis: Master's Thesis


The composition of microplankton (eukaryotic phytoplankton and micro-heterotrophs) in different water masses off the coast of Western Australia were examined on two research voyages in Austral autumn and winter; one with a latitudinal gradient (33˚S to 13˚S), the other with an onshore/off shore gradient (125˚E to 100˚E). Dinoflagellates were the most abundant specie, and ciliated were relatively more abundant in cold core eddies (offshore water) than warm core water masses. The contribution of the microplankton community to carbon was also calculated. Ciliates contributed over three times more carbon per cell than dinoflagellates. Overall, this study has revealed the distribution of microplankton being highly influenced by Leeuwin Current, and as the primary producers control biogeochemical cycles, this study provides knowledge to predicting future climate scenarios that have implications for the understanding of the earth’s biogeochemical cycle. As the bioavailable nutrients are scares in the Eastern Indian Ocean, the conditions of biological fixation from atmospheric resources are good. However, only a handful of research regarding the biological fixation of dinitrogen has been conducted in this region. My project also investigated diel nitrogen fixation rates by diazotrophs. Nitrogen fixation rates varied across latitudinal and longitudinal gradient. Highest rates were found in warm core water mass, and the constituted mostly of fixation in light condition. However, fixation was also found in dark conditions, which suggests that the community of nitrogen fixers is diverse. Though precise community was not measured, the strategies of how diazotrophs fix nitrogen give indication on who could be fixing in this different water masses. Nitrogen fixation rates had a positive relationship with Chlorophyll a measurements, suggesting a potential enhancement in primary production by the diazotroph community.
Date of Award2013
Original languageEnglish
Awarding Institution
  • University of Western Australia
SupervisorAnya M. Waite (Supervisor)

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