Long-term variation of octocoral populations in St. John, US Virgin Islands

Abstract

The decline in abundance of scleractinian corals over the past three decades in the Caribbean has raised the possibility that other important benthic taxa, such as octocorals, are also changing in abundance. We used photoquadrats taken over 20 yr from reefs (7-9 m depth) at six sites on the south coast of St. John, US Virgin Islands, to test the hypothesis that octocorals have changed in abundance since 1992. Octocorals were counted in 0.25 m 2 photoquadrats at 2-to 3-yr intervals and identified to genus or family. Overall, there was variation over time in population density of octocorals (pooled among taxa, and also separately for Antillogorgia spp., Gorgonia spp., and plexaurids) at each site, and densities remained unchanged or increased over 20 yr; where increases in density occurred, the effects were accentuated after 2002. The local-scale analysis was expanded to the Caribbean (including the Florida Keys) by compiling data for octocoral densities from 31 studies for reefs at B25 m depth between 1968 and 2013. At this scale, analyses were limited by the paucity of historical data, and despite a weak trend of higher octocoral densities in recent decades, statistically, there was no change in octocoral abundance over time. Together with data from the whole Caribbean, the present analysis suggests that octocorals have not experienced a decadal-scale decline in population density, which has occurred for many scleractinian corals.

Soft coral and sea fan (Octocorallia) biodiversity and distribution from a multitaxon survey (2009–2014) of the shallow tropical Kimberley, Western Australia

“…However, a more balanced approach needs to also include the examination of other benthic taxa including octocorals, which not only play a dominant role in extant benthic community structure, but may well populate and dominate future reefs (Blakeway and Radford 2004;HoeghGuldberg et al 2007;Hughes et al 2010;Edmunds et al 2014;Baum et al 2016). Numerous studies over the last several decades have documented declines in the abundance of reef building corals after disturbance events and a phase shift to dominance by fleshy, non-calcifying invertebrates (Benayahu and Loya 1981;Bradbury and Mundy 1989;Fabricius 1996Fabricius , 1998Fox et al 2003;Przeslawski 2008;Ruzicka et al 2013;Lenz et al 2015;Smith et al 2016). The faunistic importance of octocorals as key contributors to ecosystem services in the Kimberley illustrates the need for further soft coral surveys that will encompass the full spectrum of reef types and habitats, especially inter-reefal and understudied environ ments, such as sof t sedi ment a nd sandy areas.…”

“…2016). Transitions to octocoral dominance have been reported from the IndoPacific following environmental disturbances of coral reef systems, for example caused by bleaching events, blast fishing, coral predation by crown of thorns seastars, or decline in water quality (Benayahu and Loya 1981;Bradbury and Mundy 1989;Fabricius 1996Fabricius , 1998Fox et al 2003;Ruzicka et al 2013;Lenz et al 2015;Smith et al 2016;Baum et. al.…”

Cryptic regime shift in benthic community structure on shallow reefs in St. John, US Virgin Islands

“…The record describes the abundance of octocoral genera, and the cover of scleractinians at mostly species resolution, macroalgae, and CTB (crustose coralline algae, algal turf, and bare space combined; Aronson & Precht 2000). Changes in cover of scleractinians, macroalgae, and CTB are described elsewhere (Edmunds 2013), and a subset of these data has been used to show that octocorals increased in abundance between 1992 and 2013 (Lenz et al 2015). Using all the data from 1992 to 2014, and quantifying octocoral and scleractinian communities, we evaluate whether this record conforms to the commonly referenced 'coral reef crisis' (Hughes et al 2010, Bellwood et al 2004) that refers to sites facing imminent collapse of the coral reef community.…”

“…These alternative transitions include changeovers from scleractinians to corallimorpharians or soft corals in the Indo-Pacific (Work et al 2008), and to sponges (Loh & Pawlik 2014) or gorgonian soft corals (Ruzicka et al 2013, Lenz et al 2015 in the Caribbean. Transitions in community structure favoring octocorals are particularly interesting, be cause octocorals have long been recognized as im portant constituents of coral reef faunas (Kinzie 1973), even though ecological studies of this ecosystem rarely give them much attention.…”

“…In the Caribbean, octocoral populations historically have been recorded over large areas of reef with their colonies interspersed at low densities among scleractinians (Kinzie 1973). Recent analyses indicate that octocorals may be becoming more common throughout the region (Ruzicka et al 2013, Lenz et al 2015, including in habitats formerly dominated by scleractinians (Ruzicka et al 2013). These changes may reflect a widespread and gradual transition in benthic community structure to favor octocorals, which has im portant implications for the structure and function of coral reefs.…”