[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-05] Coral-algae interactions under ocean acidification

*Joshua Masataro Heitzman¹, Guinther Mitushasi¹, Akihiro Hirata¹, Sylvain Agostini¹ (1.Shimoda Marine Research Center, University of Tsukuba)

Keywords:ocean acidification, Porites heronensis, Acropora solitaryensis, turf algae, CO2 seep analogue, microsensor

Increasing carbon emissions drive climate change at a rapid rate, leading to a global decrease in oceanic pH; a phenomenon termed as ‘ocean acidification’ (OA). Calcifying organisms, including hermatypic corals are directly affected by OA as it causes a decrease in their calcification rate and ecological fitness. While OA acts as a stressor for corals, it can be a resource for primary producers with some marine algae benefitting from the increase in pCO₂. Competition of corals with turf algae may play an increasingly important role under future acidified conditions, with their outcomes dictating potential phase-shifts of current coral ecosystems. However, we do not understand the competitive outcome of coral-algae interactions under OA. To answer this question, coral-algae interaction experiments were conducted under both a controlled environment in aquaria and in situ at a natural analogue for ocean acidification analogue (CO₂ seep; Shikine island, Japan). Both experiments used the coral species A. solitaryensis and P. heronensis which have been described as sensitive and resistant species to OA, respectively, based on ecological surveys conducted at the Shikine island CO₂ seeps and studies on their physiological characteristics. Algal interaction was induced by removing 50% of tissue from each coral fragment, allowing algae to settle onto the exposed coral skeleton. Subsequent change in the coverage of coral tissue and turf algae were monitored regularly through top-down photographs for surface area measurements and compared against each other. Net calcification and skeletal growth were derived from total alkalinity and the buoyant weight difference of samples from the start to end of the experimental period, respectively. Net photosynthesis and photosystem efficiency of both the corals and algae were determined using an O₂ microsensor and PAM (pulse-amplitude modulation), respectively. Finally, the pH and oxygen microenvironment surrounding the coral tissues and the algal mat was determined. Following 60 days of incubation, P. heronensis showed little to no change in coral-algal coverage, regardless of treatment. Although A. solitaryensis colonies showed an increase in coral tissue coverage during the experiment, this recovery was reduced under acidified conditions compared to ambient conditions. This confirm previous studies that described P. heronensis as a resistant but slow growing coral and, show that it may not be severely affected by competition with turf algae under OA. On the other hand, the reduced recovery rates showed for A. solitaryensis under OA, suggests that competition with turf algae could contribute to the decline of fast-growing but sensitive species. This may suggest future acidified oceans will shift towards an algal-state, leaving behind only the most resistant corals.