Caribbean octocoral communities: finding the forest for the trees?

Abstract

Octocorals have increased in abundance on many Caribbean coral reefs, and at some sites, “octocoral forest” may be a better community descriptor than “coral reef.” Implicit to the concept of a forest is that structural elements, i.e., trees, colonies, etc., alter the environment in ways that affect the structural elements themselves and the organisms that inhabit the forest. At what density do the structural elements create the emergent properties of a “forest?” Communities traditionally characterized as hardgrounds and coral reefs around Puerto Rico and St John, US Virgin Islands, varied in density of octocoral colonies from a few to > 100 colonies m −2 in surveys conducted in 2021 and 2022. Canopy cover was correlated with the density of colonies, but among the quadrats with the highest octocoral densities, there was no significant correlation between numbers of colonies and canopy cover. Frontal area, a measure related to the community’s effect on water flow, as well as the volume occupied by colonies followed patterns similar to canopy cover. Vertical profiles of flow velocity were measured from the substratum to 2.3 m above the bottom on a reef on St John where octocoral population density ranged from 0 to 16 colonies m −2 . Profiles of orbital velocity exhibited perturbations which were more pronounced in locations with > 10 colonies m −2 . Using the effect on flow as a criterion, four of the eight surveyed sites would function as octocoral forests. Understanding the density of organisms at which emergent properties appear is essential to understanding the bio-physical interactions affecting the community. Developing explicit definitions of animal forests such as octocoral forests and coral reefs is critical to following the trajectories of these ecosystems in a world of changing climate.

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