| dc.description.abstract | Approximately 25 % to 40 % of anthropogenic carbon dioxide (CO2) emissions have been absorbed by the oceans, influencing the physical and chemical properties of seawater and contributing to ocean warming (OW) and acidification (OA). This thesis investigated the impact of long-term exposure to OW and OA, at levels predicted for up to the years 2050 and 2100 on the physiological responses of the sea urchin, Heliocidaris erythrogramma. This species is an ecologically important benthic ecosystem engineer in south-east Australia with the ability to reshape community assemblages through its herbivory. Experimental studies on adult H. erythrogramma were conducted using experimental designs which involved a gradual introduction to future OW and OA conditions, long-term incubation periods to accommodate acclimatory responses and temperature adjustments over time to incorporate seasonal change. The influence of parental acclimation effects on offspring responses to OW and OA were also investigated to determine the potential for transgenerational effects. Temperature is the most important factor in determining the performance and fitness of marine ectotherms. In the southeast Australian OW hotspot, up to +6 °C of warming is projected for the region by the year 2100. The consequences of exposure to warming were investigated in H. erythrogramma in experiments that employed gradual adjustment over 6 weeks to four temperature levels (ambient 20 °C, 22 °C, 24 °C, 26 °C). This was followed by an acclimation period of 3 months at these temperatures in constant conditions. Metabolic rate, Q10, heat-shock protein (HSP70) expression, gonad index, gonad histology and survival were measured. There were no significant effects of elevated temperature on metabolic rate, HSP70 protein expression and survival at 22 °C, indicating that H. erythrogramma was able to tolerate a +2 °C increase in temperature. However, metabolic depression was evident at the highest warming level of +6 °C (26 °C) which caused decreased survival and elevated HSP70 protein expression. Heliocidaris erythrogramma responded to the stress of increased temperature by altering its physiology, but long-term exposure to +4 °C and +6 °C levels of warming incurred unsustainable metabolic costs and led to trade-offs between metabolism, HSP70 expression and survival. Decreased pH through uptake of CO2 affects a wide range of physiological functions in sea urchins due to alteration of the carbonate chemistry of seawater which affects pH balance, mineral saturation for skeletogenesis and physiological hypercapnia (CO2). The thermal and metabolic responses of H. erythrogramma are likely to be influenced by OA, with the combined impact of OW and OA being difficult to predict. To investigate the effects of both stressors on the physiology of H. erythrogramma, OA was incorporated with OW in a multistressor experimental design. Heliocidaris erythrogramma collected in winter were gradually adjusted over 7 weeks to three temperature (ambient, +2 °C and +3 °C) and two pH (ambient 8.0, low 7.6) treatments. Temperature was adjusted weekly to reflect the seasonal temperature change that occurs over time from winter to summer (17 °C to 22 °C). The elevated temperature treatments were adjusted to offset the temperature profile. The sea urchins were incubated in those conditions for 22 weeks. Metabolic rate was quantified at 4 and 12 weeks of acclimation and feeding and ammonia excretion rates were measured at 12 weeks. Survivorship was recorded daily for up to 22 weeks. In single stressor treatments metabolic rate increased with either OW or OA. In contrast, the two stressors interacted when combined causing a decrease in metabolism as temperature increases at low pH. There was no evidence of interaction between the two stressors for feeding rate, ammonia excretion rate, or survival. Thus, while pH influenced metabolic rate causing metabolic depression at +3 °C, there was no evidence of pH affecting the other parameters measured. Survival was significantly reduced at +3 °C, regardless of pH, but not at +2 °C. It appeared that the earlier arrival of warmer temperature in the +3 °C treatment was stressful, especially during a time of increasing gonad development. H. erythrogramma may be living close to its lethal limit in Sydney Harbour since it already experiences the temperatures used in the study. This study highlighted the importance of investigating multiple physiological traits in characterising organism responses to climate change. Current populations are likely to be vulnerable to the combined stress of OW and OA in the near future. While investigations of climate change effects at the adult stage provide insights into the resilience of established adult populations, it is also important to understand how their offspring may be affected. These transgenerational carry-over effects are influenced by a combination of parental environmental history and developmental plasticity and can convey resilience to offspring between generations. To investigate the influence of parental environment on offspring performance, adult H. erythrogramma were acclimated to OW and OA conditions over the gonad developmental period (winter to summer) and then spawned after 3 months in the adult treatments. The resulting embryos were incubated over 14 days to the juvenile stage at four temperatures (18 °C, 20 °C, 22 °C, 24 °C) and two pHT levels (8.0, 7.6). Juvenile metabolic rate and test diameter (as a proxy for growth) were measured. Parental exposure to warming had a net increased effect on the metabolic rate responses of juveniles to temperature, whereby they respired more (up to 30 %) when incubated in warmer treatments. There were no effects of parental environment or juvenile thermal environment on growth. Parental exposure to low pH had no effect on metabolic rate or growth. However, exposure to low pH during offspring development significantly affected the growth of the juveniles (up to 4 % decrease). This thesis uses metabolic rate as a key indicator of physiological response to projected climate change conditions. Metabolic rate measurements are well-established as a key parameter to assess the biological and ecological consequences of environmental stress. However, current methods of estimating metabolic rates from respirometry data are difficult and are performed manually, which introduces subjectivity to the estimates. To address problems associated with the lack of standard methods to analyse increasingly complex respirometry data, a new package for the R statistical computing language, called respR, was developed. This software package provides a range of functions for analysing respirometry data. Importantly, this package simplifies handling and analysis of respirometry data and can be used to reliably and rapidly generate reproducible analyses of metabolic rates. The respR package provides new methods to accurately detect maximum and minimum rates, linear sections of previously intractable data, and critical oxygen tension, based novel techniques of rolling regression and machine learning. The package has been published as an open-source project to address data reproducibility and transparency when processing respirometry data. For H. erythrogramma, an ecologically important species on Australia’s temperate reefs, there were clear long-term physiological consequences of acclimation to OW and OA on physiological condition and fitness. This species is vulnerable to a +3 °C increase in temperature but appears resilient to decreased pH. The response of H. erythrogramma to the combined effect of both stressors appeared to be dominated by changes in temperature. The outcomes of this research have significant implications as to the resilience of H. erythrogramma to climate warming, especially where it resides in the global warming hotspot of temperate Australia. However, there was evidence of carry-over effects on juvenile physiology shaped by parental environment, which suggests a capacity for transgenerational resilience. Further work is required to determine the multi-generational response of this species to climate change. | en_AU |
