Interactive effects of ocean acidification and warming on coral reef associated epilithic algal communities under past, present-day and future ocean conditions

• Published in: Coral reefs 2016-06, Vol.35 (2), p.715-728
• Main Authors: Vogel, N., Cantin, N. E., Strahl, J., Kaniewska, P., Bay, L., Wild, C., Uthicke, S.
• Format: Article
• Language: English
• Subjects: Acidification; Algae; Biomedical and Life Sciences; Carbon dioxide; Coral reefs; Ecology; Freshwater & Marine Ecology; High temperature; Life Sciences; Ocean acidification; Ocean warming; Oceanography; Oceans; Peyssonnelia; Photosynthesis
• ISSN: 0722-4028; 1432-0975
• DOI: 10.1007/s00338-015-1392-x

Epilithic algal communities play critical ecological roles on coral reefs, but their response to individual and interactive effects of ocean warming (OW) and ocean acidification (OA) is still largely unknown. We investigated growth, photosynthesis and calcification of early epilithic algal community assemblages exposed for 6 months to four temperature profiles (−1.1, ±0.0, +0.9, +1.6 °C) that were crossed with four carbon dioxide partial pressure (pCO 2 ) levels (360, 440, 650, 940 µatm), under flow-through conditions and natural light regimes. Additionally, we compared the cover of heavily calcified crustose coralline algae (CCA) and lightly calcified red algae of the genus Peyssonnelia among treatments. Increase in cover of epilithic communities showed optima under moderately elevated temperatures and present pCO 2 , while cover strongly decreased under high temperatures and high-pCO 2 conditions, particularly due to decreasing cover of CCA. Similarly, community calcification rates were strongly decreased at high pCO 2 under both measured temperatures. While final cover of CCA decreased under high temperature and pCO 2 (additive negative effects), cover of Peyssonnelia spp. increased at high compared to annual average and moderately elevated temperatures. Thus, cover of Peyssonnelia spp. increased in treatment combinations with less CCA, which was supported by a significant negative correlation between organism groups. The different susceptibility to stressors most likely derived from a different calcification intensity and/or mineral. Notably, growth of the epilithic communities and final cover of CCA were strongly decreased under reduced-pCO 2 conditions compared to the present. Thus, CCA may have acclimatized from past to present-day pCO 2 conditions, and changes in carbonate chemistry, regardless in which direction, negatively affect them. However, if epilithic organisms cannot further acclimatize to OW and OA, the interacting effects of both factors may change epilithic communities in the future, thereby likely leading to reduced reef stability and recovery.