Octocoral colony density by pooled taxa, life stage, and year from surveys conducted in St. John, US Virgin Islands from 2014 to 2017 (NCEI Accession 0291433)

Acquisition Description:
The following methodology applies to this dataset in addition to other datasets published in Edmunds and Lasker (2019).

Sampling and analytical procedures:

Surveys were completed at six sites on shallow (7–9-m depth) fringing reefs on the south shore of St. John, between Cabritte Horn and White Point. In 1992, these sites were randomly selected on hard substrata along 4.5 km of shore between these headlands, and they have been censused annually to present. Each site consists of a permanently marked transect that has been 40-m long since 2000. The present project began in 2014 with the objective of augmenting a long-standing analysis of benthic community structure (which emphasized scleractinians with new analyses focused on octocorals. As part of this effort, arborescent octocorals were surveyed in situ with genus resolution, using 40 quadrats (0.5 × 0.5 m) placed at random, non-overlapping positions along the same transect (and re-randomized annually) located at each of the six sites. Surveys were completed over four weeks beginning on ~ 20th July of each of 2014–2017, and were conducted by counting and measuring the height of octocorals attached by holdfasts within each quadrat.

Height was determined (± 1 cm) using a flexible tape measure stretched from the holdfast to the colony apex. Abundances were analyzed separately for adults (> 5-cm tall), and recruits (≤ 5-cm tall), with this size cut-off based on the maximal height to which the recruits of most octocoral species are likely to grow in one year. While the benthos was inspected for all small octocorals, sampling efficiency probably was low for recruits consisting of only a few polyps (i.e., < 1-cm tall). Analyses testing for the effects of density dependence (DD) and self-thinning (ST) were first, completed for octocorals pooled among taxa, and second, for the three most common genera of octocorals. Evidence of DD recruitment also was sought from analyses of per capita recruitment by site, with these values obtained by dividing the density of recruits by mean density of adults.

For more information about statistical analyses performed using these data see Edmunds and Lasker (2019).