Chair: Sean Connell

Karen J. Westwood⁽¹⁾, Paul G. Thomson⁽²⁾, Rick L. van den Enden⁽¹⁾, Lynsey E. Maher⁽³⁾, Simon W. Wright⁽¹⁾, Andrew T. Davidson⁽¹⁾

¹ Australian Antarctic Division, Hobart, Tasmania, 7050, Australia
² The University of Western Australia, Perth, Western Australia, 6009, Australia
³ City of Hobart, Hobart, Tasmania, 7000, Australia

Background
Polar waters are at increased risk of ocean acidification (OA) due to the higher solubility of CO₂ in colder waters.

Methods
Three experiments examining the influence of OA on primary and bacterial production were conducted during summer at Davis Station, Antarctica (68°35′ S, 77°58′ E). For each experiment, six 650 L tanks were simultaneously filled with 200 µm filtered coastal seawater and incubated for 10 to 12 days, with CO₂ concentrations ranging from pre-industrial to post-2100. Primary and bacterial production rates were determined using NaH¹⁴CO₃ and ¹⁴C-Leucine, respectively. Net community production (NCP) was determined using dissolved oxygen.

Findings
For all experiments, maximum photosynthetic rates (mg C mg chl a^-1 h^-1) decreased with enhanced CO₂, reducing rates of gross primary production (mg C L^-1 h^-1). Rates of bacterial production (µg C L^-1 h^-1) and growth were faster under enhanced CO₂ from Days 0-4, but became more similar between treatments thereafter. Conversely, rates of cell-specific productivity (µg C cell^-1 h^-1) decreased with enhanced CO₂ and initial increases in bacterial production and growth were associated with fewer heterotrophic nanoflagellates. Reductions in primary and bacterial productivity with enhanced CO₂ occurred at concentrations greater than 2X present day (> 780 ppm). The effect of OA on NCP varied with nitrate+nitrite (NOx) availability. At NOx concentrations < 1 µM photosynthesis to respiration ratios declined, with primary production reduced and responses to CO₂ consequently suppressed.

Conclusions
OA may reduce primary production in Antarctic coastal waters, thereby reducing food availability to higher trophic levels and reducing draw-down of atmospheric CO₂. NOx limitation may override this OA effect but cause a similar response. Under a global warming scenario where CO₂ is expected to increase and nutrient fluxes to surface waters are expected to decrease due to strengthened stratification, NCP is predicted to decline.