Location: St. John, US Virgin Islands 18.316°N, -64.725°W.

Excerpt from Stockton and Edmunds (2021) in the publication Journal of Experimental Marine Biology and Ecology

Overview

Research was completed at < 7-m depth on shallow reefs in Great Lameshur Bay on the south shore of St. John, US Virgin Islands (18.316°N, -64.725°W). Surveys were completed in July and August of 2019 and continued in January 2020. The study sites are in the Virgin Islands National Park, where they have been protected since 1956. The density of small corals inside and outside of Diadema halos was measured along the shores of two headlands, Yawzi Point and Cabritte Horn, along which spatially aggressive peyssonnelid algal crusts (PAC) differed in abundance. At the tips of the headlands, a zone of high PAC cover was found, within which in situ surveys using subdivided quadrats (described below) revealed that PAC cover was ≥ ~ 10%, with some areas of benthos fully covered by PAC. Northward of this zone at each headland, the cover of PAC was lower < ~10% (also surveyed using subdivided quadrats), and the distinction between the two zones was visible as a transitional area several meters in width. Surveys were completed along ~ 100 m of shore within each PAC zone, and the transitional area was avoided for sampling. The two PAC hereafter are referred to as “Low-PAC” (< ~10% cover) and “High-PAC” (≥ ~10% cover). The causes of this gradient in PAC distribution remain unclear, but extensive surveys in this location have demonstrated higher abundance at exposed sites, which suggests that growth and recruitment of PAC is promoted by water motion.

Within each PAC zone, Diadema clusters were identified based on the presence of ≥ 4 sea urchins located ≤ 0.5 m from one another in shallow water (1.5-7.0 m depth), and they were haphazardly selected for the quantification of small corals in Diadema halos and on adjacent substrata. In this experimental design, each Diadema halo and the adjacent substrata were treated as plots nested within PAC zones, and they were sampled as encountered while working along each headland. Sampling initially was conducted along each headland to maintain a balanced design between zones, although fieldwork in January 2020 resulted in Cabritte Horn being sampled more extensively. Overall, 60 clusters of Diadema were sampled at Cabritte Horn (30 in each PAC zone), and 40 clusters at Yawzi Point (20 in each PAC zone), with each cluster sampled in a paired design with quadrats surveyed within the Diadema halo and on adjacent substrata.

Densities of small corals in Diadema halos

To test the hypothesis that Diadema halos provide a refuge for small corals in habitats dominated by PAC, clusters of D. antillarum were haphazardly selected in the Low-PAC and High-PAC zones of each headland. For each cluster, the abundance of small corals was quantified in the Diadema halo, and also on adjacent substrata outside the Diadema halo. PAC cover was quantified adjacent to the Diadema halo using the same quadrats used to determine the abundance of small corals. Although the sizes of the D. antillarum were not quantified, the clusters were created by adult sea urchins estimated to have test diameters > ~ 2 cm. Each cluster was surrounded by a halo (~ 50-cm width) of cleared substratum that was conspicuous in locations with high cover of PAC and other macroalgae.

Clusters of Diadema were common at Yawzi Point and Cabritte Horn, and were composed of 4–28 sea urchins. Twenty clusters of Diadema (and their halos) were surveyed in the Low-PAC and High-PAC zones at Yawzi Point and Cabritte Horn in July and August 2019. At Cabritte Horn, 10 additional Diadema clusters in each PAC zone were surveyed in January 2020, with these surveys were completed in areas differing from those surveyed in the previous summer. Summer and winter samplings at Cabritte Horn were considered a single sampling, based on the assumption that the 5 months between them was not ecologically meaningful in terms of changes in density of small corals.

The density of small corals was measured using quadrats (0.25 × 0.25 m) that were haphazardly placed on the substratum, either within the Diadema halo, or on adjacent substrata outside of the halo (n = 5 within the Diadema halo, n = 5 on adjacent substrata). Haphazard placement was ensured by deploying the quadrat without looking at the substratum, and in the halos, haphazard placement was modified with the restriction that one side of the quadrat was in contact with the tips of the spines of the sea urchins; this ensured the quadrats were within the halos. Outside the halo, quadrats were randomly located < 5 m from the halo margin. Small corals were counted when ≥ 50% of their area was within each quadrat, and their densities were expressed per square meter, which assumes that the density in the 0.25 × 0.25 m quadrats is 1/16th of that recorded in 1 x 1 m areas. In each quadrat, the density of small corals (≤ 4-cm diameter) was recorded by genus or species depending on the capacity to identify each taxon based on morphology. The percentage cover of PAC outside the halo was quantified using the same quadrats, which were subdivided into 25 sub-squares (each 5 × 5 cm). Sub-squares were scored for dominance of PAC, thereby allowing this substratum category to be quantified with 4% resolution.

Temporal stability of D. antillarum clusters

Testing of the hypothesis that Diadema halos provided refuges for small corals in PAC-dominated seascapes relies on Diadema clusters (and their halos) remaining in the same location long enough for corals to recruit to this microhabitat. To quantify the persistence of Diadema halos (hereafter “cluster stability”), the positions of haphazardly selected clusters were recorded over 9 days in January 2020 at Yawzi Point, and over 6 months at Cabritte Horn. At Yawzi Point, the positions of Diadema clusters (n = 6) were temporarily recorded with markers to allow subsequent relocation; their positions were evaluated every few days using photographs, and at the end of the 9 day period. Longer-term cluster stability was evaluated at Cabritte Horn where two Diadema clusters were photographed in July 2019 so that physical features of the benthos could be used to identify the same locations in January 2020. Over both time scales, it was not possible to determine whether the clusters of D. antillarum were composed of the same individuals, but this possibility was not relevant to evaluating whether the product of clustering (i.e., Diadema halos) remained in the same location.

Pooled data

See Supplementary Files for access to the data tables published in Stockton and Edmunds (2021) which aggregated this dataset. These data were aggregated by site, zone, and taxon and published as table S1 "Taxa Specific Densities" and aggregated by Site, Zone, Cluster, Position, and Quadrat and published as Figure 2 "Pooled taxa for both sites."

See Stockton & Edmunds (2021) for results and a description of the statistical analyses performed on this dataset.

total_coral_densities.csv