Overview
This study took place on the south shore of St. John, and surveys were completed in August 2019 and January 2020 at Cabritte Horn and Tektite on the eastern side of Great Lameshur Bay. These sites were selected because the abundance of Peyssonnelid Algal Crusts (PAC) has been measured in these locations since 2015, and the high abundance of PAC created a tractable system to test for the outcomes of PAC-coral interactions. The coral community structure in St. John from 1987-present is described elsewhere, but in brief, coral cover has been < 4.5% at six sites since 1992, but at two other sites, it has declined from 45% to 4% (Yawzi Point) and 32% to 27% (Tektite) from 1987–2019. Over the same period, the cover of macroalgae has increased, and the rest of the hard substratum has remained covered by crustose coralline algae, turf algae, and bare rock (combined as “CTB”). The high abundance of igneous rock on these reefs provides substratum suitable for growth of PAC. As PAC in St. John is more abundant in shallow (3–5 m) versus deep (5–9 m) water, surveys were designed to contrast PAC between depths. Sampling along a 15 m transect at each site and depth was used to evaluate PAC abundance, growth, and competitive encounters.

PAC abundance
Peyssonnelid Algal Crust (PAC) was surveyed in August 2019 in quadrats placed at random positions along transects positioned haphazardly along the 3 meter and 9 meter isobaths at Tektite and Cabritte Horn (n = 20 quadrats per transect, with one transect at each site and depth). PAC abundance was determined by planar cover, which was evaluated using a quadrat (0.5 × 0.5 m) subdivided into 25 equal squares, each of which was categorically scored for planar dominance by “PAC” or benthic taxa considered together as “other”; octocorals, CTB, macroalgae (mostly Halimeda, Dictyota, and Lobophora), and scleractinians were scored as “other”. With this approach, PAC abundance was resolved with 4% resolution.

Statistical analyses
To test for variation in PAC abundance among sites and depths, a two-way fixed effects ANOVA was used in which site and depth were fixed effects, and percent cover of PAC (arcsine transformed) was the dependent variable. Statistical analyses were completed using the open-source software R ver. 3.5.1, with lme4 and Matrix packages for log-linear analysis, and DescTools for the G-test. Statistical assumptions of ANOVA were tested using graphical analysis of the residuals.

Figure_2a.csv