Abstract
Abstract. Ocean acidification (OA) poses a severe threat to tropical coral reefs, yet much of what is know about these effects comes from individual corals and algae incubated in isolation under high pCO2. Studies of similar effects on coral reef communities are scarce. To investigate the response of coral reef communities to OA, we used large outdoor flumes in which communities composed of calcified algae, corals, and sediment were combined to match the percentage cover of benthic communities in the shallow back reef of Moorea, French Polynesia. Reef communities in the flumes were exposed to ambient (~ 400 μatm) and high pCO2 (~ 1300 μatm) for 8 weeks, and calcification rates measured for the constructed communities including the sediments. Community calcification was reduced by 59% under high pCO2, with sediment dissolution explaining ~ 50% of this decrease; net calcification of corals and calcified algae remained positive but was reduced by 29% under elevated pCO2. These results show that, despite the capacity of coral reef calcifiers to maintain positive net accretion of calcium carbonate under OA conditions, reef communities might transition to net dissolution as pCO2 increases, particularly at night, due to enhanced sediment dissolution.
Highlights
The calcium carbonate framework produced by coral reefs hosts the highest known marine biodiversity and protects tropical shores from wave erosion (Ferrario et al, 2014)
Net calcification was higher at ambient versus high pCO2 (Fig. 2a), both during the day and night; there were no differences between flumes within each treatment, so the nested factor was removed from the final analysis
Treatment effects were more striking than during the day, as calcium carbonate dissolution exceeded precipitation at high pCO2 (−1.6 ± 0.9 gCaCO3 m−2 d−1), whereas net calcification remained positive at ambient pCO2 (2.6 ± 0.6 gCaCO3 m−2 d−1)
Summary
The calcium carbonate framework produced by coral reefs hosts the highest known marine biodiversity and protects tropical shores from wave erosion (Ferrario et al, 2014). Most of the studies on coral reef organisms have been performed on individuals maintained in isolation in laboratory conditions, and studies performed at the scale of whole communities are scarce (Leclercq et al, 2002; Jokiel et al, 2008; Andersson et al, 2009; Dove et al, 2013). There are three complementary approaches for studying the responses of coral reef communities to OA: firstly, in situ observations of communities living in naturally acidified water (Fabricius et al, 2011) due to volcanic activities or local conditions (Shamberger et al, 2014); secondly, carbonate chemistry can be manipulated directly in situ (Kline et al, 2012), this approach is challenging technically and has not yet been used to study intact communities; and thirdly, reef communities can be constructed ex situ (Andersson et al, 2009; Dove et al, 2013) to allow precise control of the physical parameters predicted under future OA conditions. We chose to construct ex situ communities and used, for the first time, large outdoor flumes (after Atkinson and Bilger, 1992) to investigate the effects of OA on coral reef communities
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