Coral Reef Research at St. John Island
The data available through this project describe the long-term dynamics of shallow coral reefs along the south coast of St. John from as early as 1987. Five groups of data are available: (1) descriptions of coral reef community structure (as % cover) based on the analysis of photoquadrats, (2) density, growth, and survivorship of juvenile corals (~ 5 m depth), (3) coral recruitment to settlement tiles (~ 5 m depth), (4) physical environmental conditions (seawater temperature, rainfall), and (5) octocoral community structure (by genus, retrospectively using photoquadrats, and by species through in situ counts, 2014-).
Coral Reef Community Structure
Most biological data come from photoquadrats recorded annually (usually in the summer) from as early as 1987. The core ecological data describing benthic community structure are grouped by habitat in one data package:
- Tektite – this habitat is at 14 m depth on the eastern side of Great Lameshur Bay and is the original site of the Tektite underwater habitat in 1968 and 1972. The Tektite project marked the birth of the Virgin Islands Ecological Research Station (later the Virgin Islands Environmental Resource Station) that hosts the present project. The reef in this location consists of a single buttress that has remained dominated by Orbicella annularis since at least 1968. Surveys at this site consist of 30 photoquadrats (1 x 1 m) along three, 10 m transects.
- Yawzi Point – this habitat is at 9 m depth and is on the western side of Great Lameshur Bay and has been recorded photographically since 1987. This reef also started the study dominated by Orbicella annularis, but has rapidly lost coral cover over the last 30 y. Surveys at this site consist of 30 photoquadrats (1 x 1 m) along three, 10 m transects.
- Random Sites – were added in 1992 to address the concern that the original sites (Yawzi Point and Tektite) were selected on “good” areas of reef and, therefore, could only remain the same or decline in coral cover. The Random sites were selected using random coordinates in 1992, and consist of 6 sites (at 7-9 m depth) scattered between Cabritte Point and White Point. All lie a little shoreward of Yawzi Point and Tektite, and have been characterized by low cover of scleractinians since at least 1992 (< 5% cover). The surveys consist of 18-40 photoquadrats (0.5 x 0.5 m; with sample sizes determined by the use of 35 mm cameras [1992-1999] versus digital photography [2000-present]) placed at random points along the transect (and are re-randomized every year).
Juvenile Coral Dynamics
The studies described above report the patterns of change in coral community structure. To evaluate the processes driving the community dynamics, in 1994 annual surveys for the density, growth and mortality of juvenile corals were begun. These consist of in situ surveys of 0.5 x 0.5 m quadrats for the presence of small corals on natural surfaces, and have been completed at up to six sites; 5 sites were censused in 1994, 2 sites in 1995, 5 sites in 1996-1998, and 6 sites thereafter. Additionally, ~500 juvenile corals (< 40 mm diameter) are tagged annually and assayed for growth and mortality every ~12 months.
Starting in 2007, the recruitment of sceractinian corals has been evaluated at 12-month intervals using settlement tiles. Settlement tiles are unglazed terracotta (15 x 15 cm) and are installed at 5 sites, each of which has 15 tiles installed independently in a horizontal orientation. Coral recruits (typically < 2 mm diameter) are scored to family following bleaching and drying of the tiles.
Octocoral Community Structure
Octocoral community structure is quantified retrospectively using photoquadrats (1987-present), and through in-water assays (2014-). Retrospective analyses are conducted by counting colonies based on the presence of holdfasts in each quadrat and are completed with genus resolution. In water assays are completed to mostly species resolution (often through the use of voucher specimens and analysis of sclerites) and include both density (colonies per area) and measurements of colony height.
Physical Environmental Conditions
Select environmental conditions are measured as part of this project. Seawater temperature has been measured since 1989 at multiple depths, rainfall over > 30 y, and surface light intensity since 2011; in situ measurements (14 m depth) of light intensity began in 2014. Most seawater temperature has been measured using Hobo temperature loggers (± 0.2°C), the long-term record has been assembled using data from multiple generations of instruments, some of which have been collected by the Vi National Park. Rainfall is collated from multiple stations reported at the Southeastern Regional Climate Center and courtesy of R. Boulon.