Responses of oysters to a changing climate in southeast Australia

Abstract

Securing economically and ecologically significant oyster species, as our oceans warm and acidify from climate change, is a priority. Native oysters along the southeast coast of Australia will be particularly vulnerable to ocean change due the strengthening of the East Australian Current. Oysters form complex aggregations and reef structures which provide ecosystem services and habitat for several species in coastal and estuarine areas. The Sydney rock oyster Saccostrea glomerata and flat oyster Ostrea angasi are two native species with overlapping distribution in the state of New South Wales (NSW). Currently, both species are commercially cultivated and restoration projects are in progress in southern states. The overall aim of this thesis was to determine the impact of climate change stressors (warming and acidification) on these species to support their persistence and oyster reef restoration as anthropogenic actions modify our oceans. To detect effects of stressors in the capacity of energy gain through feeding, a laboratory experiment was done to evaluate responses of S. glomerata. Oysters responded to stressors by increasing standard metabolic rates (SMR), clearance, ingestion and absorption rates. Such responses suggest that climate change will alter feeding and metabolism of S. glomerata. To predict species responses to changing climate, experiments need to approximate conditions relevant to the region and species natural habitat. It was also investigated the impact of elevated temperature and pCO2 on O. angasi using outdoor flow through mesocosms in Sydney Harbour, NSW. Elevated temperature caused high mortality and decreased the condition of oysters. Elevated pCO2 increased SMR almost four-fold and lowered the extracellular pH. Based on these responses, Ostrea angasi will be living near the limits of its thermal tolerance as climate change worsen by 2050. The potential pathways oysters will use to cope and acclimate to climate change may “climate proof” aquaculture species and ensure reef restoration efforts. An experimental study was done where both species, O. angasi and S. glomerata, were given a mild dose of thermal stress in the laboratory (“stress inoculation”) and then transferred and exposed to warm seawater at Lake Macquarie, NSW for seven months. Shell growth, condition index, lipid content and survival of O. angasi and condition of S. glomerata were all significantly reduced by warming. Overall, in this thesis, S. glomerata were more resilient in their response to elevated pCO2 and temperature. Ostrea angasi had the greatest vulnerability to warming, which may be ameliorated by elevated pCO2. If we are to secure the great benefits oysters bring to coastal ecosystems, we need realistic experiments to predict their responses to climate change stressors. This thesis findings reinforce that project managers need to consider the current and future climate change in sustaining oyster reef restoration.