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.

Coral-PAC interactions
As described above, coral-PAC interactions were haphazardly selected for tagging as encountered along the transect. These interactions were < 1 m from the transect, and represented cases where scleractinian tissue was contacting PAC, and included cases when PAC already was overtopping coral tissue. Interactions were tagged regardless of the species of coral or length of the contact zone with PAC. Therefore, coral species and their interactions with PAC were sampled according to the relative abundances of these interactions in the community. The tagging of coral-PAC interactions was restricted to coral colonies > 4 cm diameter in order to minimize the effects of high coral mortality attributed to small size alone. Corals were identified to species, and interactions were categorized as: (a) “PAC overgrowing coral” when the PAC was on top of coral tissue, (b) “coral overgrowing PAC'' when coral tissue and skeleton were over-topping PAC, or (c) “neutral” when the coral and PAC met, but neither was overgrowing the other.

The scheme of categorical ranking of coral-PAC interactions was used to determine whether different interaction types had equal chances for complete overgrowth of the coral by PAC. In January 2020, the tagged coral-PAC interactions were again located (described above) to evaluate their status. When the study began, we assumed from the 3-fold increase in PAC coverage recorded on these reefs over 2 years that at least some of the tagged corals would be fully overgrown by PAC within 6 months. This assumption was incorrect with respect to our results and, therefore, it was not possible to score the tagged corals for the number that had become overgrown by PAC by January 2020. Instead, the corals were categorized on the same scale as used in August 2019 to test for changes in their relative rankings of interactions with PAC.

Statistical analyses
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.

Tables1and2.csv