Ocean acidification Impacts Primary and Bacterial Production in Antarctic Coastal Waters during Austral Summer

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.