Abstract
Abstract. The susceptibility of crustose coralline algae (CCA) skeletons to dissolution is predicted to increase as oceans warm and acidify. Skeletal dissolution is caused by bioerosion from endolithic microorganisms and by chemical processes associated with undersaturation of carbonate minerals in seawater. Yet, the relative contribution of algal microborers and seawater carbonate chemistry to the dissolution of organisms that cement reefs under projected pCO2 and temperature (pCO2-T) scenarios have not been quantified. We exposed CCA skeletons (Porolithon onkodes) to four pCO2-T treatments (pre-industrial, present-day, SRES-B1 "reduced" pCO2, and SRES-A1FI "business-as-usual" pCO2 emission scenarios) under natural light cycles vs. constant dark conditions for 8 weeks. Dissolution rates of skeletons without photo-endoliths were dramatically higher (200%) than those colonized by endolithic algae across all pCO2-T scenarios. This suggests that daytime photosynthesis by microborers counteract dissolution by reduced saturation states resulting in lower net erosion rates over day–night cycles. Regardless of the presence or absence of phototrophic microborers, skeletal dissolution increased significantly under the spring A1FI "business-as-usual" scenario, confirming the CCA sensitivity to future oceans. Projected ocean acidity and temperature may significantly disturb the stability of reef frameworks cemented by CCA, but surficial substrates harbouring photosynthetic microborers will be less impacted than those without algal endoliths.
Highlights
Crustose coralline algae (CCA) contribute significantly to the construction of coral reef frameworks by depositing CaCO3 and binding reef components (Littler and Littler, 1984; Perry and Hepburn, 2008)
Our results reveal a remarkable ecological role of phototrophic microborers in counteracting the susceptibility of CCA skeletons to chemically driven dissolution, but they reveal a significant increase in skeletal dissolution under future pCO2 and temperature (pCO2-T) climate scenarios
Offsets for “reduced” and business-as-usual” conditions were based on Intergovernmental Panel of Climate Change (IPCC) scenarios (SRES B1 and A1FI respectively) projected for the end of the century (Meehl et al, 2007). pCO2-T regimes followed daily and seasonal variability based on a reference site (Harry’s Bommie, http://www.pmel.noaa.gov/co2/story/Heron+Island) and were achieved by a controlled system described previously (Dove et al, 2013; Reyes-Nivia et al, 2013)
Summary
Crustose coralline algae (CCA) contribute significantly to the construction of coral reef frameworks by depositing CaCO3 and binding reef components (Littler and Littler, 1984; Perry and Hepburn, 2008). Along with the environmental effect of altered HMC, recent evidence suggests that bioerosional processes driven by endolithic algae further increase the dissolution of CCA as pCO2 and temperature rise (Diaz-Pulido et al, 2012). Both biological (bioerosional) and environmental (chemical) processes play a role in weakening the structural integrity of reef structures, their relative contribution to the destruction of major reef cements has not been investigated. Empirical and field studies have further shown how a drop in seawater intensifies the dissolution of reef sediments, those dominated by HMCs (Yamamoto et al, 2012; Morse et al, 2006) These studies offer valuable insights into the vulnerability of magnesium calcite-rich
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