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.

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Despite their ecological importance, sponges are often avoided in biodiversity studies and monitoring programs because they are notoriously difficult to identify using morphological or molecular methods. Here, we investigate the metabarcoding performance of universal degenerate cytochrome c oxidase subunit I (COI) primers in detecting species from this challenging phylum in a cryptobenthic community. Twenty‐two modified Autonomous Reef Monitoring Structures (ARMS) were deployed for 2 years in mesocosms receiving unfiltered seawater from an adjacent reef slope. Upon recovery, each unit was inspected by a marine sponge taxonomist who used a combination of taxonomy, imagery, and DNA barcoding (28S rRNA and COI) to identify sponges and generate a validated taxonomic richness value for each ARMS unit. A total of 69 unique sponge barcoded morphologies (BMs) were identified from the classes Calcarea, Demospongiae, and Homoscleromorpha. Metabarcoding identified 41 unique sponge molecular operational taxonomic units (MOTUs) from Demospongiae and Homoscleromorpha but the primers failed to amplify any species from the class Calcarea which comprised 22% of the BMs. Sponge richness did not differ between BMs and MOTUs assigned to the classes Demospongiae and Homoscleromorpha. However, assignments at the order and family level in Demospongiae underscore known limitations in sponge taxonomic resolution using the COI gene. The prevalence of false positives within the order Suberitida and the pervasiveness of false negatives within the order Haplosclerida highlighted both technical and biological constraints in the metabarcoding method. Overall, these results confirm the need for discretion in sponge MOTU assignments using universal degenerate barcoding primers that target a short fragment of the COI gene. However, they also demonstrate that COI metabarcoding is capable of capturing sponge richness from a complex natural community.

Coral reefs are among the most sensitive ecosystems affected by ocean acidification and warming, and are predicted to shift from net accreting calcifier-dominated systems to net eroding algal-dominated systems over the coming decades. Here we present a long-term experimental study examining the responses of entire mesocosm coral reef communities to acidification (-0.2 pH units), warming (+ 2°C), and combined future ocean (-0.2 pH, + 2°C) treatments. We show that under future ocean conditions, net calcification rates declined yet remained positive, corals showed reduced abundance yet were not extirpated, and community composition shifted while species richness was maintained. Our results suggest that under Paris Climate Agreement targets, coral reefs could persist in an altered functional state rather than collapse.