Program     Optical sensing of plankton communities and dynamics       1010 Seasonal cycles of optical properties at the Australian Integrated Marine Observing System Southern Ocean Time Series Tuesday 8th @ 1010-1030, Conference Room 7 Christina Schallenberg, Antarctic Climate and Ecosystems CRC, University of Tasmania James Harley, Antarctic Climate and Ecosystems CRC Peter Jansen, Antarctic Climate and Ecosystems CRC Diana M. Davies, Antarctic Climate and Ecosystems CRC Thomas W. Trull* , CSIRO Oceans and Atmosphere Presenter Email: Tom.Trull@utas.edu.au Seasonal biomass cycles provide insight into the importance of physical versus ecological controls of ocean productivity and export, and potential responses to climate change. The Southern Ocean Time Series is a set of moorings serviced annually that observes these dynamics at for four (NPZD) trophic levels commonly used in biogeochemical models: Nutrient depletions and Phytoplankton community structure from automated sample collections; Zooplankton from four-frequency acoustics; POC and ballast mineral Detrital fluxes from sediment traps, as well as net community production (NCP) from O2/N2. Chlorophyll fluorescence and optical backscatter show persistent seasonality, with the chlorophyll fluorescence to particulate optical (700 nm) backscatter ratio increasing in summer. Using calibrations of fluorescence to chlorophyll and backscatter to POC, and phytoplankton counts and cell volumes from microscopy, we evaluate multiple working hypotheses for the origins of this seasonality, including trophodynamic changes in the ratios of detritus to phytoplankton; per cell chlorophyll content responses to light levels, and phytoplankton seasonal successions.

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