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.

Growth of PAC
Linear growth of PAC on natural substrata
Along the same transects used to quantify PAC cover, tags (n = 20 tags per site) were placed next to corals that were interacting with PAC, and they were used to measure both the growth rate of PAC and the outcome of the interactions with corals (described in dataset 836304). Corals for tagging were selected haphazardly as encountered along the transect line, and in each case, the margin of PAC engaged in the coral-PAC encounter was marked with a numbered aluminum tag (32-mm diameter) epoxied (Z-Spar Splash Zone A-788) to non-living substratum adjacent to the interaction. When the tags were deployed in August 2019, the shortest distance between the tag and the margin of the PAC was measured (± 0.1 mm) using calipers. In January 2020, the tags were located using a metal detector (Vibra-Probe 580, Treasure Products, Inc.), and for each coral, the tag was used as a fixed reference towards which the linear growth of PAC was recorded. The distance between the tag and PAC was measured again, and the growth of PAC recorded as the change in distance between the PAC and tag was expressed as µm d-1. This method resolved the capacity of PAC to spread over rock, but it did not explicitly evaluate growth towards the coral.

Statistical analyses
Differences in linear growth rate of PAC on natural substrata were analyzed using a two-way fixed effects ANOVA, with site and depth as fixed factors. As testing for an effect of depth was a primary objective of this study, growth rates were compared between depths using planned comparisons.

Assuming that the coral-PAC interactions encountered along the transect lines effectively were randomly selected, their frequency of occurrence in August 2019 was tested for independence among depths (3 m vs 9 m), sites (Tektite vs Cabritte Horn), and interaction type (described above) using log-linear analysis. This was used to determine whether the frequency of each interaction type could be compared between times (August 2019 vs January 2020) with a model simplified by pooling among depths and sites, with the rationale that these effects were not significant.

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.