Chair: Sean Connell

James Black ^(1)*, Andrew McMinn ⁽²⁾, John Runcie ⁽³⁾, Jonathan S. Stark ⁽⁴⁾

¹ Institute for Marine and Antarctic Studies, Hobart, Tasmania, 7000, Australia
² Institute for Marine and Antarctic Studies, Hobart, Tasmania, 7000, Australia
³ Aquation Ptd Ltd, Umina Beach, NSW 2257, Australia
⁴ Australian Antarctic Division, Hobart, Tasmania, 7050, Australia

Background
Community respiration and primary production are both essential ecosystem processes, which influence many other aspects of ecosystems. Little is known however about in-situ ecosystem community respiration and benthic diatom photosynthetic responses to ocean acidification (OA), especially from soft sediment communities in the Antarctic. Recent developments in Free Ocean Carbon Enrichment (FOCE) technology has enabled in-situ investigations of ecosystem responses achievable. The current study investigated in-situ acute (<144hrs) and chronic (after 8 weeks) microphytobenthos (MPB) responses to OA using respiration chambers and antFOCE infrastructure, deployed parallel to antFOCE experiments.

Methods
In-situ experimental respiration and photosynthesis chambers were deployed on soft sediment MPB communities in Antarctica. Carbonate chemistry conditions were monitored via antFOCE equipment. Experiments were repeated five times under 400 ppm and 950 ppm CO₂ conditions. Community respiration and the steady state fluorescence yield (ɸPSII) of benthic diatoms was measured with a novel shutter Pulse Amplitude Modulation (PAM) fluorescence device. Two automatic PAM measurements were made per hour measuring the vertical migration of the biomass and Deil yield (ɸPSII). Two PAR sensors recorded light continuously, enabling in-situ determination of relETR. Sediment cores sliced at 2mm intervals recorded biomass, species abundance and diatom cell size changes at the end of each experiment.

Findings
The MPB displayed an initial difference in behavioural photo tactile response and vertical migration, which increased with increasing light levels. Deil yield (ɸPSII) patterns were also higher in the 950 ppm treatment, relETR was unaffected.

Conclusions
Initial yield (ɸPSII) increases are believed to be an alleviation of the carbon limitation that occurs within dense MPB communities. Relaxing this constraint could result in higher growth rates in MPB communities. Furthermore, changes in photo-tactile responses and vertical migration is likely to change the distribution of extra cellular polysaccharides in the sediment, which plays an important role in preventing sediment erosion.