Effect of Sargassum pacificum on the calcification of juvenile Porites rus in elevated CO2 conditions

Ocean acidification (OA), the decrease in the pH and carbonate ion concentration due to the uptake of anthropogenic CO₂, poses a severe threat to coral reefs worldwide as studies predict the decline of calcifying organisms and increasing growth of non-calcified algae. Future recovery and resilience of coral reefs will depend on the ability of juvenile corals to survive in association with macroalgae. Since, dissolved inorganic carbon can limit macroalgal photosynthesis, elevated CO₂ levels may increase photosynthesis, carbon storage and overall growth of fleshy macroalgae and increase the pH, thereby potentially offsetting the impacts of OA on nearby corals. Preliminary data using an integrated water sampler revealed a slightly higher pH inside the canopy of Sargassum pacificum patches (20-30 holdfasts/.25 m²) compared to outside the canopy. I tested the hypothesis that S. pacificum, by changing the carbonate chemistry, creates a more favorable microhabitat for juvenile Porites rus calcification by conducting a field experiment with three caged treatments (S. pacificum + P. rus, P. rus, P. rus + algal mimic) placed in a blocked design on top of bommies. To further elucidate and isolate the effect of macroalgae on coral calcification in elevated CO₂ conditions, I placed juvenile P. rus in mesocosms in a fully factorial experimental design crossing two pCO₂ levels (380, 800 µatm) with and without S. pacificum. This research will provide a unique opportunity to explore the possibility of macroalgal species providing a valuable refuge for juvenile corals to survive and calcify under future CO₂ conditions.