Abstract
Ocean-warming and acidification are predicted to reduce coral reef biodiversity, but the combined effects of these stressors on overall biodiversity are largely unmeasured. Here, we examined the individual and combined effects of elevated temperature (+2 °C) and reduced pH (-0.2 units) on the biodiversity of coral reef communities that developed on standardized sampling units over a 2-y mesocosm experiment. Biodiversity and species composition were measured using amplicon sequencing libraries targeting the cytochrome oxidase I (COI) barcoding gene. Ocean-warming significantly increased species richness relative to present-day control conditions, whereas acidification significantly reduced richness. Contrary to expectations, species richness in the combined future ocean treatment with both warming and acidification was not significantly different from the present-day control treatment. Rather than the predicted collapse of biodiversity under the dual stressors, we find significant changes in the relative abundance but not in the occurrence of species, resulting in a shuffling of coral reef community structure among the highly species-rich cryptobenthic community. The ultimate outcome of altered community structure for coral reef ecosystems will depend on species-specific ecological functions and community interactions. Given that most species on coral reefs are members of the understudied cryptobenthos, holistic research on reef communities is needed to accurately predict diversity-function relationships and ecosystem responses to future climate conditions.
Highlights
These ecosystems later this century [2, 4, 18, 19]
In experimental flow-through mesocosms that received unfiltered seawater drawn from an adjacent reef slope, we examined the cryptobiota communities that developed on standardized habitats [38] in each of four treatments: present-day pH and temperature (Control treatment), ocean acidification (−0.2 pH units—Acidified treatment), ocean-warming (+2 °C—Heated treatment), and future ocean combined stressors (−0.2 pH units and +2 °C—Acidified-Heated treatment) (SI Appendix, Fig. S1)
A total of 275 species proxies, representative molecular operational taxonomic units (MOTUs), from 354,921 rarefied sequences, representing 13 phyla were obtained from 22 ARMS units after the 24-mo soaking period within the mesocosms (SI Appendix, Tables S1–S3)
Summary
These ecosystems later this century [2, 4, 18, 19]. future projections of the combined effects of increased temperature and acidity on biodiversity have typically been derived from reviews and meta-analyses based on short-term, single-species experimental manipulations [20,21,22,23] or from in situ observations of a handful of taxa along natural gradients of seawater chemistry or temperature [5, 7, 12, 13, 24]. In experimental flow-through mesocosms that received unfiltered seawater drawn from an adjacent reef slope, we examined the cryptobiota communities that developed on standardized habitats (two-tiered Autonomous Reef Monitoring Structures, or ARMS) [38] in each of four treatments: present-day pH and temperature (Control treatment), ocean acidification (−0.2 pH units—Acidified treatment), ocean-warming (+2 °C—Heated treatment), and future ocean combined stressors (−0.2 pH units and +2 °C—Acidified-Heated treatment) (SI Appendix, Fig. S1) These experimental oceanwarming and acidification conditions reflect those predicted for the late 21st century given current commitments under the Paris Climate Accord (roughly intermediate between Representative Concentration Pathways RCP 6.0 and RCP 8.5) [39]. As studies are increasingly pointing toward the critical functional importance of this community in food webs and the maintenance of biodiversity on coral reefs [43, 45, 46], there is a need to diminish the existing knowledge gap on both taxonomic composition and ecosystem function of this community in response to climate change
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