Biodiversity of coral reef cryptobiota shuffles but does not decline under the combined stressors of ocean warming and acidification

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.

Growth and survival among Hawaiian corals outplanted from tanks to an ocean nursery are driven by individual genotype and species differences rather than preconditioning to thermal stress

“…1A ) and housed in flow-through seawater tanks at HIMB. Over the 2-year mesocosm study, many of these corals grew into larger colonies from which fragments for the present study were derived (described in Bahr et al, 2020 ; McLachlan et al, 2022 ; Timmers et al, 2021 ).…”

“…Briefly, each mesocosm from the previous experiment experienced natural daily and seasonal fluctuations in light, seawater temperature, and carbonate chemistry with the temperature treatments set to either the present-day 2-week average of O‘ahu (hereafter referred to as ambient) or +2 °C elevated seawater temperatures (high temperature) that simulated future ocean conditions with approximately 24 degree heating weeks per year of bleaching stress ( McLachlan et al, 2022 ; Timmers et al, 2021 ). Buoyant weight converted to dry weight was used to determine growth ( Jokiel, Maragos & Franzisket, 1978 ).…”

“…1B , 2A ). For both species, one colony of each genet (genotype) remaining in both ambient and higher temperature treatments in the original mesocosm study ( McLachlan et al, 2022 ; Timmers et al, 2021 ) was selected to fragment into ten smaller ramets (replicate fragments) to be deployed to the coral trees. Care was taken to make ramets from each colony approximately the same size, with each initially no larger than ~7 × 10 cm.…”

“…Found in a diverse array of Hawaiian reef habitats, Montipora capitata is a dominant species that has both branching and plating morphologies, whereas M. flabellata displays an encrusting growth form that is found primarily in shallow areas of high wave action and irradiance ( Fenner, 2005 ; Hunter & Evans, 1995 ; Rodgers et al, 2015 ). Using small fragments of M. capitata and M. flabellata from a prior 2-year mesocosm study ( Bahr et al, 2020 ; McLachlan et al, 2022 ; Timmers et al, 2021 ), we tracked the growth of corals on the trees through an additional year and compared that to their previous performance in the mesocosm system. Additionally, because subsets of corals were also grown under present-day average and high temperature conditions as part of the original study, we investigated what effect, if any, the exposure to heat stress had on growth rate and survival of corals from the high temperature tanks compared to those from the ambient system when exposed to a natural warming event while growing on the coral trees.…”

Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

“…. If the combined OA and thermal stress facilitate the dominant gammarids (e.g.,Timmers et al, 2021) and microbial decomposition (e.g.,Kelaher et al, 2018), …”