The effect of climate change on Antarctic microphytobenthic biofilm communities

The International Panel on Climate Change (IPCC) predicts that by the year 2100, the oceans will be more acidic (lower by ~ 0.4 pH) and 2 ‚ÄövÑvÆ 3¬¨‚àûC warmer under a business as usual‚ÄövÑvp (RCP 8.5) scenario. These changing environmental drivers may have a synergistic effect on some marine organisms, with changes expected to substantially alter the structure of most marine communities. However, there is a paucity of data on in-situ responses of diatoms in the Microphytobenthos (MPB) to ocean acidification (OA) and climate change. Therefore, the aim of the present thesis was to study the responses of diatoms to climate change to determine; 1) behavioural, photosynthetic and physiological responses of diatoms to climate change, 2) biological feedback mechanisms of diatoms on carbonate chemistry and 3) diatom community responses to climate change. The present thesis used a combination of field and mesocosm laboratory experiments that utilised complete marine communities in Free Ocean Carbon dioxide Enrichment (FOCE) style experiments. In these experiments, measures of diatom behaviour, photo-physiology, and MPB community composition are used to assess impacts of climate change. MPB were found to have a preference for OA with Photo-tactileresponse (PTR) increasing which increased photosynthetic yield (‚Ķ‚àèPSII). However, once temperature increases (2‚ÄövÑvÆ3 ¬¨‚àûC) and OA were combined, MPB yield (‚Ķ‚àèPSII) decreased, especially as temperature increases moved outside the MPB's thermal tolerance zone (TTZ). Important biological buffering feedbacks from MPB on carbonate chemistry were also identified, which under some circumstances may decrease the OA severity on other members of the community. Substantial changes in diatom community composition were also identified, however diatom communities had a differential response to OA, resulting in only some diatom species showing positive responses to OA. The overall conclusions for MPB ecosystems being that if temperature increases are within an autotrophic organism's TTZ, then the photosynthetic performance will increase under OA, resulting in higher primary production. However, if temperature increases move outside of an autotrophic organism's TTZ then we should expect to see a decrease in primary production. Therefore, under the business as usual‚ÄövÑvp scenario of climate change predicted for the year 2100, it is expected that MPB ecosystems will likely undergo changes in community composition and be negatively affected.