Effects of ocean acidification and warming on shallow subtidal temperate seaweed assemblages in eastern Tasmania, Australia : implications for the blacklip abalone (Haliotis rubra)

This study investigated how the combined stressors of ocean warming and acidification (reduced pH and elevated CO\\(_2\\)) impact seaweed species that the commercially exploited blacklip abalone (Haliotis rubra) relies on for all components of its life cycle. The impacts of warming and acidification on seaweeds that provide habitat, a food source or induce settlement of larvae, were assessed through a series of manipulative laboratory experiments. To provide context for experimental work, a field survey was undertaken that examined seaweed biomass, species composition and nutritional quality (fatty acid composition and nitrogen content) in three sites that spanned a latitudinal gradient in eastern Tasmania (Chapter 2). Results showed that the nutritional quality of the understory seaweeds consumed by H. rubra increased from the northern to southern site. This increase was consistent with higher productivity of H. rubra in the southern region and was driven by a higher biomass of red species at the southern sites, which were rich in polyunsaturated fatty acids (PUFA) and nitrogen. Most studies examining the response of seaweeds to ocean acidification in laboratory studies have used experimental treatments based on future projections for the open ocean. This is problematic as pH within seaweed beds is highly variable (compared to stable open ocean), and pH fluctuations can influence the response of seaweeds to acidification. Chapter 3 examined the effect of fluctuating pH on two sympatric red seaweeds (Callophyllis lambertii and Plocamium dilatatum) under both current and future ocean pH. Only C. lambertii was affected by fluctuating pH, with reduced growth and photosynthetic rates relative to the static conditions. The differential responses of two sympatric red seaweeds led to the incorporation of pH fluctuations in the treatments of all subsequent experiments, to provide environmental realism. Chapter 4 investigated the influence of marine heatwaves along with future warming and acidification on the brown seaweed Phyllospora comosa, which forms primary habitat for H. rubra and is an important food source. P. comosa was physiologically tolerant to marine heatwaves under both current and future ocean conditions. This tolerance was likely due to an adjustment in fatty acid composition with a reduction in the proportion of PUFA to saturated fatty acids (SFA) maintaining optimum membrane fluidity at elevated temperatures. Furthermore, energetic savings arising from increased CO\\(_2\\) supply (i.e. acidification) may have facilitated this adjustment when marine heatwaves were superimposed on future ocean warming and acidification. Chapter 5 examined the effects of warming and acidification on crustose coralline algae (CCA) that are known to induce settlement of H. rubra larvae. A scenario-based approach was used in which the responses of CCA (genus Sporolithon) cultured under current ocean temperature and pH were compared to responses under temperature and pH levels expected by 2030, 2050 and 2100 (RCP 8.5 emissions scenario). Results suggest that CCA are likely to be significantly negatively affected by combined warming and acidification as soon as 2030. The findings of this thesis suggest that the key habitat forming seaweed in abalone habitat, P. comosa, is likely to acclimate to future ocean conditions. However, this acclimation mechanism (reduction in PUFA), along with a reduction in nitrogen content observed under global ocean change in both P. comosa and C. lambertii, may lead to a significant reduction the nutritional quality of these seaweeds for H. rubra. Whether other seaweeds utilised as food sources such as the red seaweeds that were abundant in our southern field sites will respond in similar ways requires investigation. Recruitment of H. rubra larvae may be negatively impacted within the next two decades via adverse impacts of climate change on CCA assemblages. These findings highlight the need for managers of commercially exploited species to consider the effects of climate change on the seaweeds they rely on.