Responses of native Hawaiian calcifying macroalgae to naturally occurring ocean acidification conditions

Rising atmospheric carbon dioxide (CO2) levels from anthropogenic emissions have elevated oceanic CO2 concentrations through air-sea exchange, which lowers pH and decreases calcium carbonate (CaCO3) saturation, including aragonite saturation (one of the mineral forms of CaCO3). These conditions, named ocean acidification (OA), impair the ability of calcifying marine algae to form CaCO3 and maintain physiological structures essential for growth, reproduction, and survival. In the Hawaiian Islands, submarine groundwater discharge (SGD) has high partial pressure of CO2 (pCO2), average concentration 3000 ppm. This study used natural variations in pCO2 caused by SGD to examine effects of lowered aragonite saturation on Native Hawaiian calcifying macroalgae: Halimeda macroloba (Chlorophyta), Padina australis (Phaeophyta), Dichotomaria marginata (Rhodophyta), and Galaxaura rugosa (Rhodophyta). Temperature, salinity, pH, total alkalinity, total CO2, pCO2, and aragonite saturation states were quantified at four sites on Hawaii Island. Four calcifying algal species were transplanted within and between sites with differing pCO2 and aragonite saturation states. Percent CaCO3 content and photosynthetic activity were assessed before, during, and after transplantation. After ten days, all algal species showed a greater change in percent CaCO3 content at the experimental site, the site representative of OA conditions including high pCO2, low pH, and low aragonite saturation, than the corresponding control sites. Padina australis experienced the greatest percent change in CaCO3, -60% (± 63% propagation of error); whereas, G. rugosa experienced the least change, -4% (± 5% propagation of error). Photosynthetic activity in H. macroloba and G. rugosa had no significant change during transplant experiments; however, P. australis did have a significant change in photosynthetic activity at the experimental site, and thalli were dead by day ten. Halimeda and Galaxaura may be more resistant to OA than Padina. Different responses among the algal species may be related to differences in their morphology and anatomy. Results suggest that OA has the potential to shift nearshore macroalgal community structure and reduce biodiversity in the Hawaiian Islands.

Hart K. M., 2016. Responses of native Hawaiian calcifying macroalgae to naturally occurring ocean acidification conditions. MSc thesis, University of Hawaii at Hilo, 59 p. Thesis.

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