The effects of acidification and warming on marine calcifying biota

The Earth’s climate is changing at an unprecedented rate due to increasing use of fossil fuels and widespread deforestation. This means that the concentration of atmospheric carbon dioxide is increasing, elevating mean global temperatures. In addition, the oceans act as a huge carbon sink and are absorbing more carbon dioxide than they have in the last 650, 000 years causing the oceans to become more acidic. At the beginning of this study the guidelines stated that the control pCO2 concentration for laboratory studies should be 380 µatm (Meehl et al., 2007), concordant with the then current atmospheric pCO2. Four years later, the most recent literature reports that current concentrations of atmospheric pCO2 have now risen to 400 µatm (IPCC, 2013). This demonstrates the unprecedented speed at which our climate is changing and highlights the urgency for research into the potential implications that this change may have on marine systems. In many coastal marine systems calcifying organisms construct biogenic formations that can underpin the ecosystem and form biodiversity hotspots. Calcifying algae and cold water corals are two such organisms. These habitats are economically, politically and socially important. However our knowledge of how these keystone marine organisms may respond in the future is still ambiguous. In general marine calcifiers are likely to be negatively affected, although within some taxa there is considerable variability in their response during climate change studies (e.g. coralline algae and corals). Furthermore, natural variability will overlay the environmental changes associated with anthropogenic global climate change, and as such is likely to significantly influence the response of marine biota to the projected environmental changes. Currently, there are few studies that consider global climate change in the context of natural variability and/ or run long enough to assess acclimatisation potential. Thus, this research aims to provide a better understanding of the impact global climate change may have on key marine calcifiers in the context of natural variability and acclimatisation. This was achieved through a number of laboratory- and field-based studies, utilising well established and adapted techniques. This research focused primarily on red coralline algae. Projected changes in seawater carbonate chemistry mean that marine organisms that utilise dissolved inorganic carbon species as a substrate for multiple physiological processes (i.e. photosynthesis and calcification) are likely to be amongst those most greatly impacted by environmental change. Chapter 3 is the longest laboratory study (24 months), to date, investigating the effects of global climate change on a marine calcifier. Results suggest that seasonal variability in environmental conditions will greatly impact the response of coralline algae to elevated temperature and pCO2. In addition, while calcification may be maintained or increase in response to elevated temperature and/ or pCO2 (chapter 3 and 5) and despite evidence of acclimatisation potential, overall growth was significantly hampered by elevated temperature in the long term. This supports the hypothesis that dissolution may in fact be the primary threat to marine calcifiers, as opposed to impaired calcification. Irradiance is key in coralline algal photosynthesis yet the role of light availability on mediating coralline algal responses to multiple stressors remains scant. The present study examined net photosynthesis and photosynthetic characteristics in the free-living coralline algae, Lithothamnion glaciale in response to sub-diel changes in irradiance in algae exposed to elevated temperature and pCO2 (chapter 4). Observations suggest that light availability will mediate the response of coralline algae to global climate change in the future, as optimal light for photosynthesis increases with increasing temperature and pCO2 (chapter 3, 4 and 5). Cold-water corals make up some of the most heterogeneous, biologically diverse, three-dimensional ecosystems known in the deep sea. However, due to the difficulty in accessing these habitats, to date there is little information about how these organisms may respond to global climate change. The present study provides evidence of intraspecific variability in the response of cold water corals to global climate change that may be dependent upon their prior environmental experience (chapter 7). In situ acclimatisation to variable and low pH may provide cold water corals with the physiological flexibility to acclimatise and adapt to global climate change in the future. Evidence of intraspecific differences in physiology and morphology were also observed in situ in the brown partially calcifying alga, Padina gymnospora between algae located on the reef crest and more environmentally variable reef flat (chapter 6).