The role of Sargassum pacificum in ocean acidification effects on corals.
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 displayed ~23% higher net calcification rates in elevated pCO2 and ~16% reduced calcification in the macroalgal treatments. P. rus was resistant to OA but sensitive to S. pacificum. These results suggest the potential ability of S. pacificum to outcompete corals for a shared carbon resource (bicarbonate) or to induce other harmful chemical changes on the coral. The hypothesis that Sargassum pacificum could positively (chemical refuge) or negatively (shading) alter the effects of OA on the growth and photosynthesis of the coral species Acropora pulchra was tested in Chapter 3 with a combined field and laboratory experiment. A fluctuating pCO2 treatment was used in both experiments to replicate the natural pH fluctuations created by S. pacificum on the reef crest. Similar to Porites rus, A. pulchra did not respond significantly to the manipulated CO2 treatments. In the mesocosm experiment, light played a substantial role in controlling coral growth; shaded corals exhibited significantly reduced net calcification, linear extension and photosynthesis rates, and increased chlorophyll a. In addition, shading slightly exacerbated the impact of fluctuating and elevated pCO2 on coral linear extension and photosynthesis rates. In the field, corals displayed somewhat higher net calcification rates in fluctuating pCO2 over stable pCO2. Thus, the presence of a macroalgal-induced chemical refuge from OA is only apparent qualitatively for A. pulchra while light appeared to be the main driving factor affecting coral growth and metabolism. Across both field experiments, corals with Sargassum pacificum consistently exhibited reduced calcification rates compared to algal mimic treatments. This suggests that S. pacificum may have other harmful chemical effects on corals. The present study provides novel approaches to creating more ecologically relevant OA experiments and motivates OA research to examine the complex relationships between marine organisms and the abiotic environment of a coral reef.
Thesis or Dissertation