|Edmunds, Peter J.||California State University Northridge (CSU-Northridge)||Principal Investigator|
|Biddle, Mathew||Woods Hole Oceanographic Institution (WHOI BCO-DMO)||BCO-DMO Data Manager|
The shallow reefs (≤ 14-m depth) of St. John have been subjects of time-series analyses since December 1987. Study plots first were established at 9-m depth at Yawzi Point, and at 14-m depth at Tektite, both of which provide examples of reefs dominated by Orbicella annularis. The initial sampling evaluated the impacts of coral bleaching in 1987, and each site consisted of three, 10-m transects placed at a constant depth, parallel to one another (and 5-m apart), and permanently marked. Ten contiguous photoquadrats (1 × 1 m) have been recorded along each transect annually, and here results from 1988 (March) and 1989 (April and October) are used to evaluate the effects of Hurricane Hugo, from 1995 (May) and 1996 (May) to evaluate the effects of Hurricanes Luis and Marilyn, and from 2017 (July and November) to evaluate the effects of Hurricanes Irma and Maria. The damage from Hurricane Hugo at Yawzi Point was extensive, and to capture this spatial scale of damage, the analysis was expanded with four, unmarked 20-m transects. In 1989, these transects were recorded using Hi-8 video (Sony Corporation), but starting in 1992, they were recorded as contiguous photoquadrats using underwater cameras. Surveys of these 20-m transects from 1989 (October), 1995 (May), 1996 (May), and 2017 (July and November) are used here to expand the spatial scale of the present analysis.
Photoquadrats at Yawzi Point and Tektite were recorded using cameras attached to a framer that held them perpendicular to the reef. A Nikonos V (35-mm format) was used from 1987-1999, and digital cameras thereafter, with 3.3 MP resolution from 2000–2006, 6.1 MP from 2006–2010, 12.1 MP in 2011, 16.2 from 2012–2015, and 36.3 MP from 2016–present. Cameras were fitted with a strobe (Nikonos SB 105) and the images resolved objects ≥ 1-cm diameter.
The analyses at Yawzi Point and Tektite were augmented in 1992 with six additional sites that were selected using random coordinates constrained to hard substrata. This sampling focused on a habitat where boulders and cliffs of igneous rock are common, mean coral cover has remained < 5%, and Orbicella annularis has not been common since at least 1992. Five sites are at 9-m depth, with one at 7-m depth (RS9), and they have been recorded annually. These sites serve as replicates of reefs between Cabritte Horn and White Point, and are analyzed as the pooled random sites (PRS). Results from 1995 (May) and 1996 (May) are used to evaluate the effects of Hurricanes Luis and Marilyn, and from 2017 (July and November) to evaluate the effects of Hurricanes Irma and Maria. Each site consists of a permanently marked transect at a constant depth that was 20-m long from 1992–1999 (n ~ 18 photoquadrats site-1), but was extended to 40 m in 2000 when digital photography was implemented (n ~ 40 photoquadrats site-1). Photoquadrats (0.5 × 0.5 m) were recorded at random positions along each transect (and re-randomized annually) using cameras (as described above) attached to a framer that held them perpendicular to the reef. Cameras were attached to two strobes (Nikonos SB 105), and resolved objects to at least 5-mm diameter.
Photoquadrats were analyzed by overlaying them with a grid of 200 randomly-located dots and identifying the substratum beneath each dot. Images were analyzed manually prior to 2005, from 2005–2011 using CPCe software, and from 2012 to present, using CoralNet software with manual annotations. With this approach, the abundance of each substratum type is defined by the total number of dots that occur on top of it in each image, and when expressed as a percentage of the dot population on each image, provides a measure of percentage cover (hereafter “cover”) (Menge 1976). Two resolutions were applied to the analyses, first to resolve three functional groups (FG), coral (combined cover of scleractinians), macroalgae (algae ≥ 1-cm high, mostly Halimeda, Lobophora, Padina, and Dictyota), and a combined category of crustose coralline algae, algal turf, and bare space (CTB). Second, scleractinians were resolved to the lowest taxonomic level possible, which was genera at Yawzi Point and Tektite, and a combination of species and genera at the PRS.
In addition to the hurricanes described herein, St. John also was impacted by Hurricane Lenny on 17 November 1999 (Table S1). However, underwater damage attributed to this storm was minor, probably due to the modest local wind speeds (150 km h-1), and propagation of damaging waves east and south that reduced their impacts on the southern shore of St. John. The effects of Hurricane Lenny are not considered in the present analysis. Given the vagaries of fieldwork extending over 31 Years, it was not possible to standardize the timing of sampling that took place before and after each storm episode. Sampling took place 6 weeks after Hurricane Hugo, 8 months after Hurricane Luis, and 9 weeks after Hurricane Maria. Sampling after the two most recent storms was comparable to sampling after Hurricane Hugo with regards to the delay following the storms and, therefore, probably quantified mostly coral mortality directly attributable to physical damage, and blooms of macroalgae commensurate with the growth that is possible in two autumn months. The longer delay in sampling after Hurricanes Marilyn and Luis probably resulted in measurements of coral mortality that was caused both by direct physical damage and delayed-onset disease, as well as blooms of macroalgae that can grow over 8 months extending from autumn to spring.
To test for changes over time in the response of benthic communities to hurricanes, the three FG were tested in separate univariate models (in which the dependent variables were cover of coral, macroalgae, or CTB) for the effects of hurricanes (the fixed effect). The effects of Hurricane Hugo were evaluated relative to FG abundance in April 1989, and Hurricanes Luis and Marilyn, and Irma and Maria were each considered single storm episodes because they occurred in quick succession. Percentage cover was arcsine transformed, and the assumptions of normality and equal variance explored through graphical analysis of residuals. Planned contrasts were used to compare cover before and after each storm episode. Changes in absolute and relative coral cover along the four 20-m transects at Yawzi Point were compared between storm episodes using a Mann Whitney U-test. Multivariate community structure was compared before and after storms using non-metric multidimensional scale (nMDS) and Bray Curtis dissimilarities calculated for scleractinian abundance by lowest taxonomic resolution, and for FG. For PRS (both coral and FG), and FG at Yawzi Point and Tektite, data were square-root transformed. For coral cover at Yawzi Point and Tektite, pre-Hurricane Hugo data came from March 1988, and all were standardized and then square root transformed to address the dominance of the communities by Orbicella annularis. Univariate statistical analyses were completed using Systat 13, and multivariate statistics using PRIMER version 6.
BCO-DMO Processing Notes:
|Site||Study site; Polled random sites (6 sites)||unitless|
|Slide||quadrat number; ~ 18 prior to 2000; and ~ 40 after 2000. Each quadrat is 0.5 x 0.5 m in size||unitless|
|Timing||Timing; indicating before and after two episodes of major hurricanes||unitless|
|Orbicella_annularis||Orbicella_annularis percent cover||percent|
|Agaricia||Agaricia percent cover||percent|
|Dendrogyra||Dendrogyra percent cover||percent|
|Dichocoenia||Dichocoenia percent cover||percent|
|Diploria_labrynthiformes||Diploria_labrynthiformes percent cover||percent|
|D_strigosa||D_strigosa percent cover||percent|
|Eusmilia_fastigiata||Eusmilia_fastigiata percent cover||percent|
|Favia_fragum||Favia_fragum percent cover||percent|
|Madracis_decactis||Madracis_decactis percent cover||percent|
|Montastraea_cavernosa||Montastraea_cavernosa percent cover||percent|
|Orbicella_faveolata||Orbicella_faveolata percent cover||percent|
|Orbicella_franksi||Orbicella_franksi percent cover||percent|
|Meandrina_meandrites||Meandrina_meandrites percent cover||percent|
|Mussa||Mussa percent cover||percent|
|Porites_astreoides||Porites_astreoides percent cover||percent|
|Branching_Porites||Branching_Porites percent cover||percent|
|Stephanocoenia_intercepts||Stephanocoenia_intercepts percent cover||percent|
|Siderastrea_radians||Siderastrea_radians percent cover||percent|
|Siderastrea_siderea||Siderastrea_siderea percent cover||percent|
|All_Mycetophylla||All_Mycetophylla percent cover||percent|
|Acropora||Acropora percent cover||percent|
|CTB||combined category of crustose coralline algae; algal turf; and bare space percent cover||percent|
|Macroalgae||summation of macroalgae mostly in the genera Halimeda; Lobophora; Dictyota percent cover||percent|
|Coral||combined coral cover percent cover||percent|
|Dataset-specific Instrument Name|| |
|Generic Instrument Name|| |
|Dataset-specific Description|| |
Photoquadrats at Yawzi Point and Tektite were recorded using cameras attached to a framer that held them perpendicular to the reef.
|Generic Instrument Description|| |
All types of photographic equipment including stills, video, film and digital systems.
|National Science Foundation (NSF)|
|National Science Foundation (NSF)|
|NSF Division of Environmental Biology (NSF DEB)|
|NSF Division of Ocean Sciences (NSF OCE)|