Chair: Sean Connell

Andrew Davidson ^(1,2,3), Karen Westwood ^(1,2,3), Paul Thomson ⁽⁴⁾, Stacy Deppeler ⁽³⁾, Penelope Pascoe ⁽¹⁾, Simon Wright ^(1,2,3), Imojen Pearce ^(1,2,3), Rick van den Enden ^(1,2,3), Rob Johnson ⁽⁵⁾, Kai Schulz⁽⁶⁾, Bronte Tilbrook ⁽⁷⁾, Miguel DeSalas ^(8)
1Australian Antarctic Division, Kingston, Tasmania, 7050, Australia
²Antarctic Climate and Ecosystems Cooperative Research Centre, University of Tasmania, Hobart, Tasmania, 7001, Australia
³Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, 7001, Australia
⁴The University of Western Australia, Crawley, Western Australia, 6009, Australia
⁵Bureau of Meteorology, Melbourne, Victoria, 3001, Australia
⁶Southern Cross University, East Lismore, NSW, 2480, Australia
⁷Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research, Hobart, Tasmania, 7001, Australia
⁸Tasmanian Herbarium, Tasmanian Museum and Art Gallery, Hobart, Tasmanian 7005, Australia

Background
The response of organisms to enhanced pCO ₂ can differ with changes in environment and among strains/species. Consequently, spatial and temporal changes in community structure, and physical or chemical environment, can alter the response of organisms to CO₂. Establishing the nature and extent of these changes is vital if we are to predict the effects of ocean acidification on ecosystems and the services they provide.

Methods
The effect of elevated pCO₂ on natural communities of marine microbes was determined within and between seasons at Davis Station, Antarctica. Four experiments were performed, in which coastal sea water was incubated at different CO₂ concentrations in 6 x 650 L minicosm tanks.

Findings
The effects of CO ₂ on coastal Antarctic microbial communities were similar in nature (sign) for all 4 experiments. This was despite changes in the microbial community and the physical and chemical environment among experiments. The threshold CO₂ concentration required to elicited changes in the microbial community was ~2 x current pCO₂ (800-1200 µatm). Above this threshold, rates of biomass accumulation and primary productivity fell and picoplankton abundance (autotrophs and prokaryotes) increased in all experiments. The magnitude of the CO₂-induced change was apparently mediated by nutrient availability, acclimation, community composition and grazing.

Conclusions
The consistent nature of microbial responses to elevated pCO₂ simplifies attempts to model and predict the effects of OA on the microbial loop in Antarctic coastal waters over time.