Spatial variation in the dynamics and synchrony of coral reef communities in the US Virgin Islands

Most tropical coral reefs are experiencing declining coral cover, yet interpretation of this generality is tempered by spatial variation in coral cover among reefs separated over 20–200 km. This study addresses such landscape scale variation in coral reefs at 12 sites (7–10 m depth) around St. John (18° 18´ 37.04 N, 63° 43´ 23.17 W) and St. Thomas (18° 20´ 43.57 N, 65° 55´ 13.88 W), US Virgin Islands. Surveys completed from 2011 to 2021 were used to test for spatial variation in community dynamics among islands, shores, and sites. Community synchrony (φ) was used to evaluate portfolio effects in mediating changes in coral communities. From 2011 to 2021, changes in benthic communities differed among sites and times (i.e., there were site × year interactions), and, while coral cover declined at 11 sites, the decline was 2.6-fold faster around St. Thomas than St. John. The loss of coral cover was driven by multiple taxa that differed among sites, thus revealing asynchronous responses to prevailing conditions. Asynchrony suggests that coral communities at some of the sites have the capacity to exploit portfolio effects to modulate stability, yet these effects did not prevent declines in coral cover that reflect island-scale phenomena. In the US Virgin Islands, coral death is overwhelming the capacity for resilience of coral communities that historically may have benefitted from portfolio effects. Until coral assemblages are depleted through taxonomic extirpation, maintenance of the assemblage composition retains the possibility that increased resilience might emerge if environmental challenges can be alleviated.