Experimental coral reef communities transform yet persist under mitigated future ocean warming and acidification

Abstract

Coral reefs are among the most sensitive ecosystems affected by ocean warming and acidification, and are predicted to collapse over the next few decades. Reefs are predicted to shift from net accreting calcifier-dominated systems with exceptionally high biodiversity to net eroding algal-dominated systems with dramatically reduced biodiversity. Here, we present a two-year experimental study examining the responses of entire mesocosm coral reef communities to warming (+2 °C), acidification (-0.2 pH units), and combined future ocean (+2 °C, -0.2 pH) treatments. Contrary to modeled projections, we show that under future ocean conditions, these communities shift structure and composition yet persist as novel calcifying ecosystems with high biodiversity. Our results suggest that if climate change is limited to Paris Climate Agreement targets, coral reefs could persist in an altered state rather than collapse.

Keywords: climate change; coral reef; mitigation; ocean acidification.

Fig. 1.

Coral community survivorship and 3-D coral cover. Plots show time series of coral survivorship (A) and live 3-D coral cover (B) over the course of the experiment according to treatment. Coral cover (the area of skeleton covered by live tissue) is shown as a percentage of the interior surface area of the uncolonized mesocosms (bottom, walls, and standpipe, 0.88 m²). Where individual lines and points are not visible, it is because they are overlapping. Data shown as mean ± SD (n = 10 mesocosm communities per treatment). Both coral survivorship and the rate of increase in coral 3-D cover were reduced under warming and neither responded to acidification. SI Appendix, Table S2 for test results.

Fig. 2.

Effects of experimental warming, acidification, and combined future ocean conditions on net community calcification. (A and B) Entire mesocosm communities (sometimes referred to as net community calcification, NCC, or net ecosystem calcification, NEC, in other studies), (C and D) coral communities, and (E and F) rubble-associated communities. Net community calcification was determined by the total alkalinity anomaly technique in (A and B) and by the buoyant weight technique in (C, D, E, and F). Time-integrated overall mean values are shown on the left (A, C, and E) whereas time series are on the right (B, D, and F). Data bars and points show the mean ± SD (n = 10 mesocosm communities per treatment). Mesocosm calcification declined with acidification and warming, whereas coral calcification declined only with warming and rubble calcification declined only with acidification. SI Appendix, Table S2 for test results.

Fig. 3.

Benthic community structure in the mesocosms at the end of the study. Nonmetric multidimensional scaling ordination illustrating the effects of ocean warming and acidification on benthic community structure from settlement tiles (ARMS) colonized in the mesocosms for the control (blue), ocean acidification (light blue), ocean warming (red), and combined future ocean treatments (purple) (n = 6 tile arrays per treatment). Scatterplots are overlaid on ellipses which show the SD around the centroid for each group. Ellipses are semitransparent to allow visualization of overlapping distributions. Community structure differed according to temperature but not pH, which was driven largely by separation of the ocean warming treatment from the control. Fig. 4 and SI Appendix, Table S3 for additional results.

Fig. 4.

Benthic cover of functional groups on settlement tiles colonized in the mesocosms at the end of the study. (A), vermetid gastropods (B), crustose coralline algae (CCA), (C), biofilm/turf algae, (D), encrusting green algae, (E), motile fauna (n = 6 ARMS tile arrays per treatment). Vermetid gastropods, CCA, biofilm/turf algae, and motile fauna showed significant responses to warming, whereas only vermetid gastropods responded negatively to acidification. Encrusting green algae attained significantly higher abundance under the combined future ocean scenario as compared to the other treatments. None of the other functional groups (anemones, bivalves, macroalgae, sediment, serpulid worms, sponges, tunicates, and uncolonized space) responded significantly to treatment conditions. Box-plots show the median as center line, box limits are upper and lower quartiles, whiskers are 1.5× interquartile range, and open circles as outliers. Boxes are not visible where values were below detection limits of 0.09%. SI Appendix, Tables S2 and S4 for test results.

Fig. 5.

Overall species richness in the mesocosms at the end of the study. Treatment effects on overall species richness in the mesocosms (data shown as proportional variation in species richness relative to the maximum observed richness among treatments) derived from samples of sponges, crustose coralline algae (CCA), and metabarcoding of metazoans from settlement tiles (ARMS), coral-associated microbes, water column-associated microbes, coral-associated algal endosymbionts, fleshy algae, and corals (n = eight datasets representing dozens of species of algae, hundreds of types of animals, and thousands of types of microbes). Box-plots show the median as center line, box limits are upper and lower quartiles, whiskers are 1.5x interquartile range, and there were no outliers. Treatment effects were not significant.