Effective marine resource management begins with knowing the distribution of resources within the region. Minimal data, and thus limited knowledge, exists about the reef resources of Martin County. The marine benthic habitats in Martin County need to be mapped to characterize and quantify the distribution of its coral and other benthic communities, therefore, the Florida Department of Environmental Protection – Coral Reef Conservation Program, FL Fish and Wildlife Research Institute and the National Coral Reef Institute at Nova Southeastern University have partnered to expand upon previous mapping efforts to identify and classify the benthic habitats in the southeast Florida region (Miami-Dade, Broward, Palm Beach, and Martin Counties). The maps will provide critical information needed to understand the extent of the coral reef habitat throughout Martin County and the southeast Florida region. They will enable managers to enforce impact avoidance and assist in the development of conservation action strategies.
Updating the existing maps is also essential to the region to monitor changes to the resources and provide current data for management decisions. Southeastern Florida has a very dynamic marine system influenced by high energy weather systems (e.g. hurricanes), ship groundings, various construction projects, and artificial reef deployment which change the morphology of the sea floor and thus affect the benthic habitats. Existing Broward benthic habitat maps were drawn based on 2001 LIDAR data, therefore a new LIDAR survey in Broward County will facilitate updating these maps.
The Broward LIDAR dataset was collected by Tenix LADS Inc. between July and August 2008. The data were obtained and processed into high resolution hill-shaded topographic maps. Detailed information regarding this survey can be obtained by contacting Ken Banks at Broward County’s Environmental Protection and Growth Management Department Natural Resources Planning and Management Division.
The marine benthic habitats in Martin County were mapped using the same combined technique approach as was done in the other southeast Florida counties (Walker, Riegl, and Dodge 2008). The mapping area extended seaward from shore to the 30 m depth contour where possible and covered an area of ~350 sq km. Image-based analyses in deeper water were not ideal in Martin County due to poor water clarity; therefore, a high resolution (4 m) LIDAR bathymetric survey was conducted to image the sea floor. This effort was conducted in two phases. Phase 1, where a LIDAR bathymetric survey of the seafloor was conducted, and Phase 2 where habitat maps were created by outlining and defining the features within the bathymetric survey.
Phase 1 mapping began when the project area in Martin County was flown in December 2008 by Blom Aerofilms, Ltd. LIDAR for the project area was acquired over a period of four days and included both topographic and bathymetric LIDAR as well as vertical aerial imagery. These data were processed by Blom. Deliverables for the project included cleaned point cloud, DTM 5m grid, hillshaded geotifs, seabed reflectance data, and 25cm GSD orthophotos.
Gaps in the initial LIDAR data coverage were evident mainly due to poor water quality, temporal, and meteorological conditions. Of the total 341.5 km² surveyed, 51.5 km² contained data holidays and coverage gaps; 15% of the total survey area. Therefore Blom Aerofilms re-flew the areas with major gaps in December 2009 coincident with other work in the United States. The re-flights included a collection of similar data types. The re-flight scheduling and data processing significantly delayed the project, thus a no-cost extension was granted by FWC to extend the project to December 2012.
Benthic habitat maps were produced by delineating seafloor features evident in multiple datasets including the 2008 and 2009 high resolution LIDAR bathymetry and aerial photography collected from Phase 1. Phase 2 started in April 2010 and continued until August 2012. The habitats were classified according to established NOAA guidelines in coordination with the NOS Coral Mapping Program and use a similar classification scheme when possible.
Of the 374 km² seafloor mapped in Martin County, the polygon totals indicated 95.2% was Sand, 4.1% was Coral Reef and Colonized Pavement, and 0.7% was Other Delineations. The Martin County benthic habitat morphology is very different than the other counties further south. Hardbottom habitats are sparse outside of a shallow, near shore area around St. Lucie Inlet and a few thin deep ridge lines which taper or are buried further north. All of these features are thought to be cemented beach dunes submerged during the last Holocene sea level transgression. Although not confirmed by coring, they do not appear to be composed of a coral-derived framework and they do not exhibit any morphologic signs of historic reef growth like the spur and groove formations of the Outer Reef which terminates in Palm Beach County near Lake Worth inlet (Banks et al. 2007; Walker 2012).
The most extensive, deep hardbottom was the northern end of the Deep Ridge Complex which extends from Palm Beach into southern Martin for about 2 km before it appears to be covered with sediments. Only small, thin portions of the tallest ridges are exposed further north. In southern Martin there are three shore-parallel deep ridge lines. The first deep ridge, nicknamed Three Holes, is located approximately 2 km from shore in 18 m water depth and extends approximately 3.5 km northward in a mostly continuous arrangement. The second deep ridge appears at the same latitude that Three Holes terminates, but it is approximately 6 km from shore in 22 m of water. This feature extends northward in a mostly continuous fashion for about 6 km. The third deep ridge, nicknamed 7-Mile Ledge, is the most conspicuous deep hardbottom feature. Despite its name, in southern Martin this feature is located approximately 6 km (~ 4 miles) from shore in 22 m of water. This is also its widest portion at just about 0.5 km. This ridge extends northward over 23 km with relatively few (4) small breaks or gaps. At its northern terminus, it is located about 12.8 km (8 miles) from shore in 25 m water depth.
Shallow hardbottom habitats extended throughout much of the county, but the majority of the habitat existed near St. Lucie inlet. This was comprised of two habitats, Colonized Pavement-Shallow and Ridge-Shallow. The differences between their delineations were mainly morphological. The Ridge-Shallow has an obvious linear morphology and usually contains higher relief, at least at larger scales. The Colonized Pavement-Shallow is typically lower relief and has no distinct linear morphology. The shallow Martin County ridges extend 2.5 km north of the inlet and 11.5 km south in a shore-parallel orientation. The eastern side resides in about 10m depth, it crests near 3m and the western side remains shallow in some parts and drops back to 10m in others. The shallow colonized pavement is located westward of the shallow ridge in waters 10m to 4m deep, sloping upward toward shore. As with other features along the northern Florida Reef Tract, these ridges terminate at the shoreline. The northern terminus is known as Bath Tub Reef and the southern end slips under the shoreline just off Bridge Road on Jupiter Island. Small portions of shallow ridge appear north of the inlet off Jensen Beach. These appear to be ephermeral communities affected by high wave energy and shifting sediments. Beach construction, storm activity, and natural littoral drift all have an effect on the type and arrangement of near shore sea floor habitats and depending on their magnitudes may cause large-scale changes through time.
Approximately 357 km² were identified as unconsolidated sediments that contained different sediment features that were not part of the mapping. The most evident features were large sand dunes throughout the county extending to the northeast. In the south, these dunes appear to be partially or totally burying portions of deep ridge habitats. Elevation profiles revealed these features were up to 11 m high extending over 2.25 miles wide. Little is known about the movement of these features, but given the dynamic environment and the frequently high currents, it is likely that they are migrating across the seafloor, including over the deep ridges.
In collaboration with FWC, FDEP-CRCP, and NCRI, NOAA funded quantitative ground truthing to provide a rigorous determination of habitat types beyond qualitative efforts and valuable information about the composition of the benthic communities for resource management. This was accomplished in August 2012. Data were collected on 16 sites: 7 Ridge-Deep sites, 5 Ridge-Shallow sites, and 4 Colonized Pavement-Shallow sites. The sites were distributed across the seascape as much as possible to provide data on all the main hardbottom habitats and account for latitudinal variation. A cluster analysis and corresponding non-metric, multi-dimensional scaling (MDS) plot showed that the sites were more similar than not, yet subtle distinctions were evident when the sites were categorized by habitat. The Ridge-Deep sites all plotted on one side of the graph and the two shallow habitats on the other, showing there are likely differences between shallow and deep habitats. Furthermore apart from one site, colonized pavement and ridge did not cluster, indicating a wide range of benthic communities between shallow sites. A summary of the mean percent cover data by habitat showed many differences in cover. Turf algae were more abundant on the shallow colonized pavement (41.4% ± 11.1) and ridge (52.4% ± 19.6) than the deep ridge (19.1% ± 9.5) and vice versa for cyanobacteria. Cover varied greatly within habitat categories and most cover types were low (> 5%) making it difficult to detect differences at the habitat level. Although percent cover between habitats was muddled by within-habitat variability, the number of biotic cover categories (e.g. macroalgae, hydroids, coral) were significantly different. Colonized Pavement-Shallow had significantly fewer biotic cover categories (5.5 ± 0.96 SEM) than the Ridge-Shallow (7 ± 0.45 SEM) and Ridge-Deep (7.4 ± 0.72 SEM). The number of biotic categories ranged from 4 to 8 on the Colonized Pavement-Shallow, from 6 to 8 on the Ridge-Shallow, and from 4 to 9 on the Ridge-Deep. This indicates the shallow colonized pavement may have less taxonomic diversity than the other habitats. Rugosity significantly varied between habitats. The Ridge-Shallow mean rugosity significantly higher than the Colonized Pavement-Shallow which was significantly higher than the Ridge-Deep. This result was not surprising because feature relief (albeit at a larger scale) was one of the main criteria used to distinguish between the two shallow habitats. Although univariate differences between habitats were found (e.g. MDS separation, rugosity, number of biotic categories), multivariate differences of cover types and amounts among sites were not statistically strong between the habitat categories. A one-way analysis of similarity (ANOSIM) was performed to statistically determine the strength of the site categorization by habitat. The strongest result was between the Ridge-Deep and Ridge-Shallow indicating these were most different and supporting the MDS results, however the difference was not very strong. Furthermore the results between Deep-Ridge and Colonized Pavement-Shallow and between Colonized Pavement-Shallow and Ridge-Shallow were very weak. The lack of strong ANOSIM groupings was likely due to not distinguishing between algal species. Although no species data were collected, it was recognized anecdotally that the algal communities between the deep and shallow hard bottoms were distinct. Previous research showing distinct differences in the macroalgal communities in southeast Florida supports these observations (Lapointe 2007). Lapointe’s data show that shallow ridge sites had a large component of Phaeophyta cover (> 50% during certain times) that was not present in the deep habitats, where Chlorophyta was dominant. This was further exemplified by the five sites on the Deep Ridge Complex in north Palm Beach that were dominated by Chlorophyta and Rhodophyta and had very little Phaeophyta if any. Therefore, if macroalgal communities were distinguished in the Martin County quantitative ground truthing, it is likely that the cluster analysis between habitats would have been much more robust.
The MDS plot scatter indicated there may be a cross-shelf pattern to the communities in the Nearshore Ridge Complex ((NRC) combination of Ridge-Shallow and Colonized Pavement-Shallow habitats). A site located on the eastern side of the shallow ridge had a distinct community comprised mostly of macroalgae, turf algae, and palythoa. Sites associated with the shallowest top portion of the ridge (the crest) were most similar to each other. And all of the other shallow sites located on the western side of the shallow ridge grouped in a central axis. It is likely that the distinct ridge profile is providing different conditions across the shelf that are shaping the benthic communities. This could account for larger within-habitat variations because the shallow ridge was not divided into separate habitats to account for the differences across the fore-ridge, crest, and back-ridge.
NSUOC Martin County Mapping Final Report
Brian K. Walker, Amanda Costaregni, Ian Rodericks, Greg Lewis, and Nick Gadbois. 2012. Characterizing and Determining the Extent of Coral Reefs and Associated Resources in Southeast Florida through the Acquisition of High-Resolution Bathymetry and Benthic Habitat Mapping .NSUOC Martin County Mapping Final Report : 1 -103. http://nsuworks.nova.edu/occ_facreports/35.