Effects of future climate on coral-coral competition

The combined effects of OA and warming on competition were evaluated by comparing dif- ferences in endosymbiont photosynthetic efficiency between points of direct coral-coral contact and points on the same piece of coral that were 3 cm distant from contact with the other species and evaluating these patterns across the two environment scenarios (present vs future). The effects of competition and environment on coral health beyond the areas of direct contact were evaluated by comparing photosynthetic efficiency values taken from areas of the competing cor- als that were 3 cm away from the direct contact area and comparing these values to values from corals in the same tank but not in contact with a competitor (the “no-contact” controls). Lesion presence and size were recorded through daily photos (including a scale) using a Nikon Coolpix W300 camera to evaluate the effects of competition and OA/warming on coral tissue health. Lesions formed first via visible discoloration of coral tissue followed by tissue mortality and tissue loss. Distinctions were made between areas of the lesion that were discol- ored (i.e. areas where the color differed from other parts considered healthy) and areas with exposed coral skeleton and tissue mortality. Total lesion size was quantified as the combined size of the discolored area and the area with tissue mortality. The total area of tissue mortality was also quantified. After the first 2–3 days of lesion formation, lesion size stabilized, so the size of the areas that were discolored and/or contained dead tissue were statistically evaluated

only at day 7 using ImageJ software. Experiment two: Porites astreoides vs Orbicella faveolata Because coral competitive outcomes can change over time as sweeper tentacles form in response to competition [31, 37] and because we wanted to evaluate among-species differences in competitive outcomes, we conducted a follow-up experiment using P. astreoides and O. faveolata. This experiment mimicked the previous experiment’s design with minor variations (Table 1). Three colonies of each species acclimated in the original raceways for an additional period of three weeks while we conducted the first experiment. As with the first experiment, they were then cut into 8 fragments and acclimated for 48 hours before being transferred into the 12 tank experimental system. After acclimating to the experimental system for 7 days, interspecific coral interactions were initiated and monitored over a 14-day period (7 days lon- ger than the previous experiment) to determine whether the effects of future conditions altered the outcomes of coral-coral competition. Once again, daily photos recorded lesion presence and changes in size. Photos were analyzed from day 7 and from day 14, at the end of the exper- iment. Fv/Fm was recorded every other day as in the first experiment. Statistical analyses All analyses were conducted in R version 3.4.3 using the car 2.1–5 [52], lmPerm 2.1.0 [53], and multcomp 1.0 packages [54]. Data were evaluated for normality and equality of variance prior to analyses using the Bartlett’s test and Q-Q plots to analyze plots of residuals. Two way-permutation

ANOVAs were used when data did not meet the assumptions of equality of variance and trans- forming the data failed to meet these assumptions using climate (present vs. future) and contact (contact vs no-contact) as fixed effects. The effect of individual tanks on the data was analyzed using a one-way ANOVA and, lacking significance, was removed from the analysis. All other analyses were conducted using two-way ANOVAs with climate and contact area as fixed effects. Post-hoc tests were completed using one-way ANOVAs with the Bonferroni correction to evalu- ate differences among treatments. Contrasts of physical parameters (temperature, OA, salinity, etc.) between treatments were conducted by averaging the many measures for each tank across time into one mean for each tank (i.e., using tanks as independent replicates) and comparing the six present treatment tanks to the six future treatment tanks via t-tests. PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 5 / 16