| Summary: | Ocean acidification (OA), the decrease in the seawater pH and carbonate ion concentration due to the uptake of anthropogenic carbon dioxide (CO2) with no change in total alkalinity, threatens coral reefs worldwide as studies predict the decline of calcifiers and increasing growth of non-calcified macroalgae. Future recovery of coral reefs may depend on the ability of corals to survive in association with macroalgae. However, few studies have incorporated multiple species into OA experiments. This research sought to explore the understudied combined impact of macroalgae and elevated pCO2 on corals. Macroalgae can indirectly (e.g., shading, allelochemicals) and directly (e.g., abrasion, overgrowth) harm corals. Alternatively, algae could facilitate coral calcification by reducing pH through photosynthesis. The reef crest surrounding Moorea, French Polynesia, is covered by the fleshy, macroalgal species Sargassum pacificum. Results from photoquadrat surveys showed that scleractinian corals and other biogenic calcifiers covered 15% of the sub-canopy habitat. The same microenvironment experiences on average, increases in pH by 0.031 ± 0.012 pHT and decreases in irradiance by ~98% compared to surrounding, above-canopy conditions. On a larger scale, diel fluctuations in pH (up to 0.197 pHT) were recorded behind the S. pacificum-dominated reef crest. To help predict the future of coral reef dynamics accurately, the present research examined how non-calcifying macroalgae could mitigate, by metabolically reducing CO2 levels, or exacerbate, by shading, the effects of OA on associated scleractinian corals. In Chapter 2, the role of macroalgae as a chemical refuge from OA was addressed through a combined laboratory and field experiment testing the effect of the presence/absence of Sargassum pacificum on juvenile Porites rus exposed to ambient and elevated pCO2. In the field, corals with S. pacificum exhibited less bleaching but did not significantly alter their net calcification rates. P. rus in the laboratory experiment ...
|
|---|