Dataset: Size and density of Gorgonia ventalina from 2013-2019, St. John, US Virgin Islands

Taken from publication in PeerJ
The study focused on Gorgonia ventalina at Yawzi Point and Tektite within Great Lameshur Bay, and the research was completed under permits issued by the Virgin Islands National Park. These reefs have been monitored from 1987 to present, and the present analysis was superimposed on the same study areas. At each site, three permanent transects were installed in 1987, with each ~ 10 m long, parallel to one another, and ~ 5 m apart. Contiguous photoquadrats (1 × 1 m) have been recorded along each transect to quantify the cover of benthic taxa.

Starting in August 2013, colonies of G. ventalina ± 1 m of each transect were surveyed, with their sizes recorded as height. Each colony was mapped through Cartesian coordinates along each transect, and their height was recorded using a flexible tape measure (± 0.5 cm) as the linear distance from the holdfast to the highest distal portion of living tissue. Fans that were symmetrical or slightly imperfect in shape with small areas of mortality were easily measured, but fans that were torn and were affected by partial mortality (i.e., categorized as "ragged" fans) posed challenges for measurement. The size of ragged fans was recorded as their greatest height, which overestimated their size relative to the amount of G. ventalina tissue. Field logistics prevented finer resolution of tissue area (e.g., through photography, but separate analyses of photoquadrats were used to evaluate the abundance of ragged colonies in each year.

The surveys of G. ventalina were repeated annually in August from 2013 to 2019, and on each occasion colonies were mapped and their sizes recorded as above. Sizes and Cartesian coordinates were compared between consecutive years to identify colonies that were evaluated in both years, colonies that had died between years (lost from the reef or reduced to a horny axis without tissue), or small colonies that recruited between years.

Analyses
Mean densities of G. ventalina were compared among times using repeated measures ANOVA in which transects were repeatedly surveyed over time; a non-parametric Friedman test was used when statistical assumptions were not met. Mean colony sizes were compared over time using one-way ANOVA in which each colony was a replicate. The density of G. ventalina recruits was evaluated from colonies = 5 cm tall, and their densities and heights were compared over time as described above. Colonies were considered to have left the recruiting size class when they were > 5 cm tall. The percentage distribution of colonies among sizes classes over time was evaluated using size (height) classes of 10 cm, with the largest class including colonies between 60 and 100 cm tall. The distribution of colonies among size classes was tested for variation among years using chi-squared contingency tables. Growth was recorded as the change in height between consecutive years for colonies that were located in both years, and growth was compared over time using non-parametric Kruskal-Wallis (three or more groups) or Mann-Whitney U-Tests (two groups).
Density-associated effects were explored through analyses of the relationships between mean height and mean density for self-thinning, and between mean density of recruits and mean density of colonies > 5 cm tall, using transects as statistical replicates. Evidence for self-thinning would be revealed by an inverse relationship size and density, and a slope of the log size versus log density relationship of ~ 1.5. Evidence of density-associated recruitment would be revealed by a linear relationship between the density of recruits and larger colonies, and a linear relationship (either positive or negative) between per-capita recruitment and density of colonies > 5 cm tall. Per capita recruitment was calculated by dividing the density of recruits by the density of colonies > 5 cm tall.