title: In situ changes of tropical crustose coralline algae along carbon dioxide gradients.
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.
authors.affiliation: Australian Institute of Marine Science, PMB 3, Townsville Qld 4810, Australia.
authors.forename: K E
authors.initials: KE
authors.lastname: Fabricius
authors.affiliation: 1] Australian Institute of Marine Science, PMB 3, Townsville Qld 4810, Australia [2] Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, D-27568 Bremerhaven, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Kluibenschedl
authors.affiliation: James Cook University, School of Marine and Tropical Biology, Townsville, Qld 4811, Australia.
authors.forename: L
authors.initials: L
authors.lastname: Harrington
authors.affiliation: Australian Institute of Marine Science, PMB 3, Townsville Qld 4810, Australia.
authors.forename: S
authors.initials: S
authors.lastname: Noonan
authors.affiliation: Australian Institute of Marine Science, PMB 3, Townsville Qld 4810, Australia.
authors.forename: G
authors.initials: G
authors.lastname: De'ath
mesh_terms: D002245:Carbon Dioxide; D017753:Ecosystem; D006863:Hydrogen-Ion Concentration; D000461:Rhodophyta; D012623:Seawater / Q000737:chemistry / Q000382:microbiology
chemical_list: D002245:Carbon Dioxide
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