Search

In marine benthic habitats, structural complexity is positively associated with diversity, and the area of hard surfaces modulates community structure. These principles are well developed on tropical coral reefs, where stony corals create complex surfaces that are favored for settlement. This study explores the relationship between substratum orientation and the distribution of arborescent octocorals on shallow reefs (< 10-m depth) in St. John, U.S. Virgin Islands. To test the hypothesis that octocorals are distributed unequally among microhabitats, their abundance on vertical, horizontal, and inclined substrata (i.e., different microhabitats) was measured. To evaluate whether octocorals were crowded in favored microhabitats, the shortest distance between pairs of octocoral was compared among microhabitats, and the consequences of microhabitat location were evaluated by measuring colony sizes for Gorgonia ventalina . The most common octocorals were G . ventalina , Eunicea spp., and Antillogorgia spp., and these taxa were distributed unequally among microhabitats, even though the microhabitats were equally abundant. Relative to a random distribution among microhabitats, more G . ventalina than expected was found on vertical substrata, more Antillogorgia spp. was found on horizontal substrata, and more Eunicea spp. was found on vertical substrata. On horizontal and inclined substrata, the mean distance between G . ventalina and the nearest octocoral was 25 ± 4 cm, but on vertical substrata, this distance was 8 ± 2 cm; most of the nearest neighbors to G . ventalina were Eunicea spp. on vertical substrata, and the mean distance between these colonies was 5 ± 1 cm (all ± SE). There was no difference in size of G . ventalina among microhabitats, suggesting that adult success was unrelated to microhabitat for this species. Together, these results show that most arborescent octocorals favor vertical substrata on the shallow reefs of St. John, and because of this preference, colonies are more crowded on vertical versus horizontal or sloping substrata. [ABSTRACT FROM AUTHOR]

Abstract: In this study, coral recruitment was measured on a kilometer-wide scale over two years on shallow (5–6m depth) fringing reefs in St. John, US Virgin Islands, with the objective of determining the extent to which variation in recruitment was affected by biophysical coupling involving temperature and flow. Coral recruitment was measured using settlement tiles deployed at 10 sites along 10km of shore. The tiles were first deployed in August 2006, and thereafter replaced every ≈6months to sample from either August to January, or January to August over 2years. Seawater temperature was recorded at the 10 sites using logging thermistors, and flow was quantified using drogues. Overall, corals recruited at a rate equivalent to 76 corals m−2 6months−1, and were represented mostly by poritids (43% of recruits), agaricids (29%), faviids (17%) and siderastreids (7%). Although the density of recruits differed among sites in a pattern that varied among periods and years, there was a consistent trend for mean density to decline from ≈4 corals tile−1 at eastern sites, to ≤1 coral tile−1 at western sites. One aspect of seawater temperature – the daily range – differed among sites and was greater at western compared to eastern sites, and while it was related inversely to recruitment over one of the sampling periods, it was equivocal as a physical process affecting recruitment. Instead, our results are consistent with biophysical coupling involving patch depletion and downstream filtering, whereby patches of coral larvae are delivered to the south shore of St. John and depleted of larvae through settlement as the water progresses westward. [ABSTRACT FROM AUTHOR]

The article presents a study on long-term changes in the concentration of zooplankton and particulate material over a fringing reef in Saint John, U.S. Virgin Islands. Seawater quality was evaluated from concentrations of particulate matter (PM), particulate organic matter (POM) and zooplankton. The hypothesis that seawater quality over Caribbean reefs has changed over forty years is said to be consistent with the results of the study.

In this study, long-term (1987-1998) dynamics are described on a local scale (<20 km) for coral reefs in St. John, US Virgin Islands, which are located in a marine protected area (MPA). The study consists of two sites (Yawzi and Tektite) which were selected in 1987 based on relatively high coral cover (=32%), and six sites that were randomly selected in 1992. Over 12 years, mean coral cover at Yawzi (9 m depth) changed significantly, declining from 45 to 20% cover between 1987 and 1998 (a 56% reduction). Less than 1 km away at Tektite (14 m depth), coral cover also changed significantly, but here it increased 34% (from 32 to 43% cover). Over the same period, macroalgal cover showed a significant upward trend at both sites, increasing from 2 to 26% at Yawzi, and from 6 to 13% at Tektite. The random sites (7-9 m depth) differed from the initial sites in both community structure and dynamics. Mean coral cover at the random sites (~8%) was less than one third of that at Yawzi and Tektite, and varied significantly among sites and years in an idiosyncratic pattern. The percentage cover of macroalgae and the pooled coverage of crustose coralline algae, algal turf, and bare space showed a strong site 2 time interaction, illustrating that the sites differed in dynamics, but that the differences varied among times. Thus, as has been reported elsewhere in the Caribbean, serious reef degradation has occurred on at least one reef in St. John, but the patterns of change vary markedly on a kilometer-wide scale. In comparison with other long-term studies of Caribbean coral reefs, the degradation of a coral reef in an MPA around St. John is noteworthy since there are few local anthropogenic disturbances that can be held responsible for the decline. The strong possibility that large-scale events such as hurricanes and global warming have played a pivotal role in the decline of at least one reef in St. John emphasizes the need to embrace landscape- and regional-scale phenomena... [ABSTRACT FROM AUTHOR]

The article discusses a study on the growth of large numbers of Acropora palmata in shallow water in Saint John, U.S. Virgin Islands. The study conducted snorkel surveys to estimate the population density of the coral. The surveys revealed the abundance of the coral in the marine area between Cabritte Horn and White Point.