Fig 1.
Standardised geomorphic classification of coral reefs showing the subcategories of linear breakwater reefs.
There are 4 levels: The first is based on depth, with two categories, intertidal breakwater reefs, and subtidal submerged reefs. The second is based on form, with two categories, linear BW reefs and non-linear or dispersed BW reefs. The third, for linear BW reefs, is based on physiographic setting with four categories, Interior (protected Lagoons and Bays), Coastal (continental or insular shelves ≤ 5 km wide), Bank (waters with a shelving seafloor shallower than 200 m, including shelves >5 km wide) and Oceanic (waters lacking a shelving seafloor shallower than 200 m). The fourth is based on shallow geomorphology with two categories, Crest-type (sloping to subtidal depths on either side of the crest) and Flat-type (with an intertidal platform behind the crest). A final modification term for Fringing reefs is Attached to the coast (with no lagoon substrates, only back-reef) or Detached from it (with a lagoon). In total, this produces 32 possible categories of linear BW reef.
Fig 2.
Breakwater reef terminology and geomorphic zonation of shallow benthic seascapes in the Greater Caribbean.
The upper profile shows that BW reefs are three-dimensional bio-accretionary benthic structures (geoforms) composed of two geomorphic units—the back-reef and the reef-front—either side of the crestline, where fair-weather waves break. These two units constitute the BW reef deposit. As well as covering the reef deposit, corals are also found veneering non-reefal benthic zones in the wider ecological seascape, such as in the adjacent lagoon and inner fore-reef shelf. The outer fore-reef shelf can also be a depocentre for sand and submerged reef development. The lower image illustrates how a 1-km long section of each reef is sampled to determine 10 morphometric parameters: polygons delineate back-reef and reef front areas, which combine to give total deposit area and size ratio; actual total crestline length is compared to end-to-end length to derive sinuosity, and its persistence is derived from summing the gaps as a percentage of the total length; the reef-front slope is derived from the average distance from the crestline to the mid-shelf break (or 15 m isobath); These 10 morphometric parameters are used to statistically analyse the geomorphology of reef types. Drone image courtesy of Dr. L. Alvarez-Filip.
Table 1.
Number and length (km) of linear breakwater reefs (>500 m) and their subtypes in the Greater Caribbean.
Only 16 out of the possible 32 reef subtypes are present and, of those, 9 are common (bold) and 7 are uncommon (kmz data for all reefs provided in the S1 File).
Fig 3.
Distribution of breakwater reef subtypes in 8 ecoregions of the Greater Caribbean.
a. Ecoregions, including the Gulf of Mexico South, Florida-Bahamas, and the West, East, Central (divided into Central East and Central West), Southeast, and Southwest Caribbean (modified after Spalding et al. 2007). b. Reef subtypes in each ecoregion, showing the total length of breakwater reefs (km), and their relative abundance considering the total coastal length (%). Note a bipartite distribution, with flat-type coastal reefs (11 and 13) dominating the Central East, East, Southeast and Southwest Caribbean ecoregions, and crest-type reefs (14, 20, 22) dominating the Greater North, West, and Central-West Caribbean. Basemap data: NASA Visible Earth.
Table 2.
Mean morphometric parameters of common reef subtypes.
Internal morphological variation within each subtype is represented by the average deviation from the group centroid. Many morphometric variables have non-normal sampling distributions causing the standard deviation to be larger than the mean of the variable.
Fig 4.
Kernel density estimates of morphometric data described in Table 3 with quartiles.
Panels a and b show density curves corresponding to crestline sinuosity and persistence (% gaps) by reef subtype (in percentages). Panels c, d and e show density curves in reef-front and back-reef areas (av. length) and reef-front slope (Crestline-MSB distance). Of those with consistent morphologies (11, 13, 15, 22) all three flat types (11, 13, 15) show back-reef areas with the same normal distribution, central tendency and narrow dispersion, and steep reef-front slopes.
Fig 5.
Multivariate analysis of dispersion and central tendencies of morphometrics in the seven common reef subtypes.
a: Boxplot of multivariate dispersion (variance), showing the median and interquartile ranges of distances to the group centroid. Note: dashed line divides flat and crest subtypes, and crest types have outliers in 3 of the 4 groups. b. Multivariate Central Tendency of morphometrics. The mMDS plot shows bootstraps averages together with 95% region estimates of uncertainty for ‘mean settings’ (black symbols) of each reef subtype. Bootstrapping approximation is obtained from the distances in an m = 5-dimensional mMDS space, for which the Pearson correlation to the dissimilarities is = 0.993. The overlapping diffuse subtypes (14, 16, 20, 22), with a high average squared Euclidean distance, indicate higher uncertainty in the central tendency (implying indistinct morphologies) compared to the 3 other subtypes (11, 13, 15) where the uncertainty in central tendency is considerably lower (implying distinct morphologies).
Table 3.
Relative contribution of morphometric parameters to common reef subtypes.