Abstract
Crustose coralline algae (CCA) fulfill important ecosystem functions in coral reefs, including reef framework stabilization and induction of larval settlement. To investigate in situ the effects of high carbon dioxide on CCA communities, we deployed settlement tiles at three tropical volcanic CO2 seeps in Papua New Guinea along gradients spanning from 8.1 to 7.4 pH. After 5 and 13 months deployment, there was a steep transition from CCA presence to absence around pH 7.8 (660 μatm pCO2): 98% of tiles had CCA at pH > 7.8, whereas only 20% of tiles had CCA at pH ≤ 7.8. As pH declined from 8.0 to 7.8, the least and most sensitive CCA species lost 43% and 85% of cover, respectively. Communities on upward facing surfaces exposed to high light and high grazing pressure showed less steep losses than those on shaded surfaces with low grazing. Direct CO2 effects on early life stages were the main mechanisms determining CCA cover, rather than competitive interactions with other benthic groups. Importantly, declines were steepest at near-ambient pH, suggesting that CCA may have already declined in abundance due to the recent seawater pH decline of 0.1 units, and that future severe losses are likely with increasing ocean acidification.
Highlights
Crustose coralline algae (CCA) fulfill important ecosystem functions in coral reefs, including reef framework stabilization and induction of larval settlement
Declines were steepest at near-ambient pH, suggesting that CCA may have already declined in abundance due to the recent seawater pH decline of 0.1 units, and that future severe losses are likely with increasing ocean acidification
The individual tiles were exposed to seawater chemistry conditions ranging from median values of pH 8.08 to 7.37, 1838 to 2236 mmol kg[21] dissolved inorganic carbon (DIC), and saturation state of calcite (VCalc) of 7.6 to 1.73 (Fig. 1, Supplementary Table S1)
Summary
Crustose coralline algae (CCA) fulfill important ecosystem functions in coral reefs, including reef framework stabilization and induction of larval settlement. Crustose coralline algae (CCA, family Corallinaceae, Rhodophyta1) play an important role in many marine ecosystems, providing reef framework, shore protection and carbonate sediments in shallow water[2], and facilitating the settlement and survival of larvae of numerous other benthic taxa[3,4,5,6]. One study showed that in tropical Indo-Pacific CCA, thin encrusting and early successional species, which are important as settlement substrata, appear to be more OA-sensitive than more robust taxa with thicker crusts[6]. Others have documented an unusually high CO2 tolerance in thick crusts of the tropical Porolithon onkodes, and suggested that this common species will likely continue to provide essential reef and shore protection in future oceans under OA8,21.
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