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  1. Abstract

    Declines in abundance of scleractinian corals on shallow Caribbean reefs have left many reefs dominated by forests of arborescent octocorals. The ecological mechanisms favoring their persistence require exploration. We quantified octocoral communities from 2014 to 2019 at two sites in St. John, US Virgin Islands, and evaluated their dynamics to assess whether portfolio effects might contribute to their resilience. Octocorals were identified to species, or species complexes, and their abundances and heights were measured, with height2serving as a biomass proxy. Annual variation in abundance was asynchronous among species, except when they responded in similar ways to hurricanes in September 2017. Multivariate changes in octocoral communities, viewed in 2-dimensional ordinations, were similar between sites, but analyses based on density differed from those based on the biomass proxy. On the density scale, variation in the community composed of all octocoral species was indistinguishable from that quantified with subsets of 6–10 of the octocoral species at one of the two sites, identifying structural redundancy in the response of the community. Conservation of the relative colony size-frequency structure, combined with temporal changes in the species represented by the tallest colonies, suggests that portfolio effects and functional redundancy stabilize the vertical structure and canopy in these tropical octocoral forests.

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  2. null (Ed.)
    Recent shifts in the presence and abundance of species on shallow Caribbean coral reefs have left octocorals as the dominant functional group on some reefs, creating an “animal forest” with an associated canopy. This transition changes the reef profile potentially affecting flow and sedimentation. We examined the effects of an octocoral forest on the depositional environment on a shallow-water fringing reef system on the south shore of St John, USVI. The depositional environment was characterized as canopy or non-canopy based on octocoral density. The effect of the octocoral canopy on flow and sedimentation was assessed using clod cards and sediment traps at 15 paired locations. The octocoral canopy altered flow resulting in greater levels of turbulence within the canopy. Sediment traps in areas of dense octocoral canopy accumulated greater amounts of sediment, with coarser, more rounded grains. Organic content of sediments collected in the traps was greater within the canopy than outside of it. The increase in turbulence within the canopy was likely due to wave driven oscillatory flow interacting with the octocoral colonies. Sediment traps in the canopy likely had greater sediment accumulation due to both resuspension of sediments from within the canopy and deposition of imported sediments as flow decreased within the canopy. The presence of octocoral canopies and the reworking of sediment within them may affect the success of settling larvae and the evolution of reef structure. 
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