The role of zooplankton in the biological carbon pump of the subantarctic Southern Ocean
Creators: name: Halfter, Svenja
Publisher: name: University Of Tasmania
Year: 2023

description: Initiatives to study the impact of climate change on carbon sequestration in the subantarctic Southern Ocean have led to regular research voyages and the establishment of long-term moorings at the Southern Ocean Time Series site (47oS, 140oE) in the region. The subantarctic zone plays an important role in the physical uptake and sequestration of carbon dioxide due to the formation and subduction of water masses. However, while extensively studied in other parts of the world's ocean, the biological production, transformation, and transport of particulate material is not well documented for the subantarctic zone. In particular, the role of zooplankton in the biological gravitational, mesopelagic-migrant, and seasonal lipid pumps has received less attention. Current knowledge gaps extend across zooplankton species composition in subsurface waters and its seasonal development, zooplankton mortality that leads to downward carbon ux by carcasses and quanti _cation of zooplankton respiration in the water column. This thesis synthesises information on zooplankton and biogeochemistry in the subantarctic Southern Ocean to study the zooplankton-mediated carbon pump, including data from samples collected by deep-sea sediment traps, on _eld campaigns and from laboratory measurements. The thesis is composed of a general introduction (Chapter 1), a literature review (Chapter 2), three analysis chapters (Chapter 3-5) and a discussion, containing synthesis and future research priorities (Chapter 6). The second chapter reviews the role of zooplankton in establishing characteristic carbon export regimes in the Southern Ocean. Two case studies, the Kerguelen Plateau (high productivity, relatively low export) and the High-Nutrient Low-Chlorophyll waters south of Australia (low primary productivity, relatively high downward export), illustrate the importance of the zooplankton community composition, biomass and grazing for downward carbon export. The third chapter presents a long-term time-series (&gt;20 years) of deep-sea zooplankton community composition data, collected as swimmers in sediment traps. Results indicate a decrease of abundance and diversity with increasing depth, low seasonality in zooplankton abundances and the absence of a long-term trend in the zooplankton community. The fourth chapter shows the importance of zooplankton carcasses for downward biological carbon export in the subantarctic zone. Estimations of the carcass ux are sensitive to alteration of mortality rates and sinking speed, but less sensitive to a change in microbial decomposition rates. Finally, a newly developed research instrument to measure zooplankton respiration in the _eld is presented in Chapter 5. The light trap attracts zooplankton into the main chamber of the ZOORESPIRE, which closes after a pre-set time period. During the incubation phase their respiration in-situ as a decline in oxygen concentration over time, which will enable better quanti_cation of respiration throughout the water column and especially in the under-studied mesopelagic zone.