Effects of pCO2 on the interaction between an excavating sponge, Cliona varians, and a hermatypic coral, Porites furcata

Currently, atmospheric CO2 levels fluctuate around 390 ppm and continue to rise at an accelerating rate (Caldeira and Wickett 2003, 2005); projected scenarios indicate that levels will approach 450–600 ppm by 2050 and 750– 1,000 ppm by the end of the twenty-first century (IPCC 2007; gattuso and lavigne 2009). The resultant increase in dissolved seawater CO2 is expected to reduce surface ocean pH by 0.3–0.4 units, substantially altering the carbonate chemistry of coastal marine ecosystems (Kleypas 1999; guinotte and Fabry 2008; Doney et al. 2009). These changes in seawater pCO2 will affect the physiological ability of organisms to precipitate calcium carbonate (CaCO3) (Feely et al. 2004; Orr et al. 2005) and will therefore disproportionately impact ecosystems relying on the formation of biogenic carbonate structure, such as coral reefs (Hoegh-guldberg et al. 2007; andersson and gledhill 2013). Tropical studies investigating the effects of increasing pCO2 have primarily focused on the biological responses of calcifying species, particularly the calcification processes of hermatypic corals and coralline algae (gattuso et al. 1997; Kuffner et al. 2007; anthony et al. 2008; Jokiel et al. 2008; Kurihara 2008; albright and langdon 2011; Diaz-Pulido et al. 2012). Based on the results of these studies, it is now widely accepted that increases in pCO2 will reduce calcification rates across a range of taxa and, Abstract rising dissolved pCO2 is a mounting threat to coral reef ecosystems. While the biological and physiological impacts of increased pCO2 are well documented for many hermatypic corals, the potential effects on bioerosion processes remain largely unknown. Increases in pCO2 are likely to modify the direct interactions between corals and bioeroders, such as excavating sponges, with broad implications for the balance between biologically mediated deposition and erosion of carbonate in reef communities. This study investigated the effects of three levels of CO2 (present-day, mid-century and end-of-century projections) on the direct interaction between a bioeroding sponge, Cliona varians, and a Caribbean coral, Porites furcata. Increased pCO2 concentrations had no effect on the attachment rates of C. varians to the corals, and we observed no significant impact of pCO2 on the survival of either the coral or sponges. However, exposure to end-of-century levels of CO2-dosing (~750 μatm) reduced calcification in P. furcata and led to a significant increase in sponge-mediated erosion of P. furcata. These findings demonstrate that pCO2 can enhance erosional efficiency without impacting survival or competitive vigor in these two species. While few studies have considered the influence of pCO2 on the competitive outcomes of interactions between corals and other reef organisms, our study suggests that assessing the impacts