Agaricia spp. recruitment failure and subsequent recovery in the wake of mass bleaching and disease outbreak on reefs of St. Thomas, US Virgin Islands

Caribbean coral reefs have declined severely under cumulative stressors including hurricanes, thermal anomalies, disease outbreaks, and local anthropogenic impacts. In early 2019, stony coral tissue loss disease (SCTLD) emerged in the northern U.S. Virgin Islands (USVI), followed later that year by anomalously high temperatures and mass bleaching across the territory, providing an opportunity to assess the resilience of recruitment to acute disturbance. This study quantified recruitment at three shallow reef sites (7–14 m) in St. Thomas from 2019–2022 and compared results with pre-disturbance data (2015–2017). Recruitment tiles (15 × 15 cm) were deployed across eight time periods and photographed to determine recruit density and taxonomic identity. Bleaching and disease prevalence were calculated from coral health assessments along six 10-m transects as part of the Territorial Coral Reef Monitoring Program. All recruits observed were Agaricia spp. Recruitment rates in 2019 and 2020, following these disturbances, declined by an order of magnitude relative to pre-disturbance surveys. Because the six environmental predictors vary to differing degrees among versus within sites, their correlations with recruitment were decomposed into between-site and within-site components. Coral cover and Agaricia spp. abundance varied predominantly among sites (86.9% and 51.9% of variance between sites, respectively); their apparent positive associations with recruitment reflected differences among three site means rather than temporal variation within sites, and neither persisted once site means were removed. In contrast, Degree Heating Weeks (DHW) and combined bleaching and paling prevalence varied almost entirely within sites (8.7% and 1.3% between-site variance), and both showed marginal negative within-site relationships with recruitment (DHW: r = -0.43, p = 0.054; bleaching/paling: r = -0.41, p = 0.064). Thermal stress was therefore the only environmental variable with a detectable within-site signal, consistent with the temporal concordance between the 2019 disturbance and recruitment collapse. By 2021–2022, recruitment rebounded toward pre-disturbance levels at most sites, indicating recovery of recruitment potential within approximately two years. These findings demonstrate the sensitivity of Agaricia spp. recruitment to disturbance and underscore monitoring recruitment as a key indicator of reef resilience, as continued thermal stress and persistent disease threaten the natural recovery capacity of Caribbean reefs.