A quarter-century of variation in sponge abundance and community structure on shallow reefs in St. John, US Virgin Islands

Abstract

This study tested the hypothesis that sponge assemblages on the reefs of St. John, US Virgin Islands (18.315°N, 64.716°W), changed from 1992 to 2017. Sponges were identified to species or genus in photoquadrats and were quantified at 2–3 y intervals by density, with linear dimensions used to estimate volume as a proxy for biomass. From 1992 to July 2017, overall sponge density (pooled among taxa) increased, although trends varied among the most common species for which density increased (four species), decreased (one species), or showed no linear relationship with time (2 species). Hurricanes Irma and Maria caused a 51% decline in overall sponge density from July to November 2017, with density in November 2017 being 31% lower than in 1992. For the common sponges, densities of 6 declined by 82–100% between July and November 2017, but densities in November 2017 were 13–736% higher than in 1992 for four species, and 47–100% lower than in 1992 for 3 species. Sponge volume did not change linearly between 1992 and November 2017, but it was reduced by 54% following the hurricanes. Multivariate variation over time in benthic community structure was not strongly associated with sponge density alone, although a significant association with rainfall highlighted the potential roles of this environmental factor in directly, or indirectly, mediating changes in community structure. Overall, the mean density of sponges, as well as most of the common sponge species, trended upwards prior to the hurricanes, but these trends reversed following the storms.