The functional response of the Antarctic bivalve Laternula elliptica to ocean warming and acidification

Marine life is currently under threat from large-scale, long-term changes to the marine environment. Anthropogenic emissions of greenhouse gases, particularly carbon dioxide (CO₂), are causing ongoing change to global marine systems, particularly through ocean warming and acidification. Greenhouse gases in the atmosphere are trapping radiation and heating the entire Earth surface, including the ocean. At the same time, oceanic uptake of CO₂ through absorption by surface waters is altering ocean chemistry, increasing acidity, reducing availability of carbonate ions (CO₃²⁻), and causing increasing dissolution of calcium carbonate (CaCO₃) structures.

Because atmospheric CO₂ diffuses more readily into cold water, the Southern Ocean (SO) will experience ocean acidification in a matter of decades. Warming in the SO is also occurring rapidly and represents a comparatively greater increase in temperature than elsewhere. SO marine fauna have evolved in constant, stable, cold conditions, and as a result are stenothermal and particularly at risk from ocean warming and acidification. The large infaunal bivalve Laternula elliptica is a prevalent keystone species found throughout the Antarctic benthos in high numbers, and contributes significantly to biodeposition and bentho-pelagic coupling.

This thesis examines how L. elliptica adults are affected over medium-term (5-mo) timescales by SO warming and acidification. Adult L. elliptica collected from Cape Evans in McMurdo Sound, Antarctica, were subjected to combinations of temperatures and pHs predicted for the SO by 2050 and 2100 (Temperatures: -1.4°C (control); -0.5°C; +0.5°C. pHs: pH 8.00 (control); pH 7.85; pH 7.65). L. elliptica were assessed at 5 wk and 5 mo to determine their cellular, metabolic, and whole-organism responses to temperature increase and/or pH decrease. Survival parameters such as final survival percentage, survival curves, and time to 50% survival (LD₅₀) were compared among treatments. L. elliptica survival was severely reduced by warming of only 1-2°C above summer ambient temperatures. Physical and physiological condition indices were calculated to assess health, and show changes in shell and body tissue mass. Physical condition stayed similar amongst all treatments at both time points, while physiological condition decreased significantly at 5 mo with elevated temperature. Oxygen (O₂) consumption was measured as a proxy for standard metabolic rate to show whether animals had acclimatised to conditions. O₂ consumption was significantly negatively correlated with physiological condition, and increased, becoming more variable, with both elevated temperature and lowered pH. This indicated that L. elliptica experienced increased metabolic demand in response to these conditions, and there was a general lack of acclimation to these conditions over time.

Overall, pH had no significant effect on survival, metabolic rate, or condition. Heat shock protein 70 (HSP70) gene expression levels were measured to provide a preliminary indication of how the heat shock response of L. elliptica responds to both elevated temperature and reduced pH. Lowered pH appeared to stimulate an up-regulation of HSP70 gene expression at both time points, although this was smaller at 5 mo. L. elliptica did not seem to display a heat shock response at environmentally realistic levels of warming. Overall, warming resulted in lowered survival and condition loss with no sign of acclimation after 5 mo. These responses occurred at smaller degrees of warming than are typically considered lethal for L. elliptica, indicating that successful longer-term maintenance is more thermally limited than short-term survival in this species. While physical (shell) condition was maintained in undersaturated conditions under both elevated temperature and reduced pH, this maintenance occurred alongside increased O₂ demand. Maintaining the aragonitic shell in combination with increased metabolic activity may have contributed to the decline in physiological (body mass) condition observed in L. elliptica. In combination, the results of this experiment indicate that warming of the SO may be more important than ocean acidification to the survival and functioning of adult L. elliptica.

Everett Hempel S. M., 2016. The functional response of the Antarctic bivalve Laternula elliptica to ocean warming and acidification. MSc thesis, Victoria University of Wellington, 126 p. Thesis.

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