Effects of future climate on coral-coral competition

considered to be relatively resistant to several common stresses (e.g. bleaching, predation [42]), and may be the most common remaining competitors on modern, and likely future, Caribbean reefs. Materials and methods Ethics statement Field research was conducted under FKNMS-2015-078-A1 and FKNMS-2017-128 research permits granted through the Florida Keys National Marine Sanctuary. Experiments assessing the combined effects of OA and warming on coral-coral competition were conducted at the Smithsonian Marine Station in Fort Pierce, Florida. Two experiments were run sequentially over a period of seven weeks, with the first experiment running for 16 days between 2 and 18 October 2017 and the second experiment running for 23 days between 18 October and 9 November 2017. Three colonies of M. cavernosa and O. faveolata, and six colonies of P. astreoides (each 30–45 cm in diameter) were collected from the Florida Keys National Marine Sanctuary coral nursery in Key West, FL, maintained in indoor raceways with running seawater for at least 2 months, and then used in the experiments. Experiment one: Porites astreoides vs Montastraea cavernosa After the initial two-month acclimation period, three colonies of P. astreoides and three of M. cavernosa were each divided into eight fragments (each fragment 7–10 cm). Coral fragments recovered in the raceways for 48 hours prior to being moved to 12 tanks of 37-L designed to manipulate carbon dioxide levels and temperature individually in each tank. This short recov-

ery period (48 hours) may have led to some additional coral stress, but it was consistent across all treatments and no apparent signs of stress such as tissue loss or bleaching were observed. Seawater was collected from 0.4 km offshore from Fort Pierce, Florida, filtered (<10 μm), and water recirculated within each tank using a 473 LPH powerhead. Tanks were randomly assigned to either present (targeted to be ~400 μatm pCO2 and ~28.5˚ C) or future (targeted to be ~1000 μatm pCO2 and ~30.0˚ C) oceanic conditions (Table 1, n = 6 per treatment). Present treatments are based on current conditions, while the future treatments were designed to mimic the predicted most extreme scenario by the IPCC (RCP 8.5) ([43]) (Table 1). This design was chosen over a fully factorial experiment because temperature and OA are predicted to both change over time, thus representing future conditions, and to allow for greater sample size and power to detect effects on coral-coral interactions. pH was continuously controlled Table 1. Average (±SE) calculated carbonate chemistry parameters from the measured parameters of pH, total alkalinity (TA), temperature, and salinity (n = 6). Experiment Treatment Salinity Temp (˚C) TA (μmol kg- 1) pHNBS pCO2 (μatm) CO2 (μmol kg- 1) HCO3- (μmol kg- 1) Oca Oar P. astreoides vs. M. cavernosa Present 36.18 28.60 2340.49 8.18 373.75 9.55 1639.80 5.68 3.80 ±0.15 ±0.07 ±33.12 ±0.02 ±21.82 ±0.56 ±90.40 ±0.40 ±0.26 Future 35.62 30.24 2474.24 7.89 1083.4 26.68 2125.99 3.49 2.35 ±0.22 ±0.09 ±50.51 ±0.01 ±52.61 ±1.21 ±47.05 ±0.07 ±0.05

p-values p = 0.032 p<0.001 p = 0.094 p<0.001 p<0.001 p<0.001 p = 0.001 p = 0.001 p = 0.001 P. astreoides vs. O. faveolata Present 36.30 28.47 2137.73 8.17 460.70 11.93 1655.87 4.60 3.07 ±0.17 ±0.05 ±23.87 ±0.01 ±14.93 ±0.40 ±21.69 ±0.11 ±0.07 Future 36.59 30.28 2352.72 7.86 1107.99 27.31 2025.81 3.21 2.16 ±0.39 ±0.11 ±25.56 ±0.01 ±47.53 ±1.12 ±20.02 ±0.15 ±0.10 p-values p = 0.660 p<0.001 p = 0.002 p<0.001 p<0.001 p<0.001 p<0.001 p<0.001 p<0.001 https://doi.org/10.1371/journal.pone.0235465.t001 PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 3 / 16