Baseline mapping and quantitative assessment data are required prior to future permitted or un-permitted impacts in order to determine the pre-existing state of the benthic resources; therefore, it is imperative that these data be collected on the ecologically sensitive and economically valuable shallow-water coral reef habitats in southeast Florida. In southeast Florida, the nearshore reef habitats are most vulnerable to coastal construction activities and other anthropogenic impacts, therefore these habitats were the focus for this study. The study goals were to provide a spatially appropriate map of increased resolution and a regional quantitative characterization of nearshore benthic resources to evaluate differences in benthic communities between habitats and with latitude for the southeast Florida region of the Florida Reef Tract. This study is a snapshot habitat characterization providing the current status of shallow-water coral reef community composition. Additionally, these data can be used to reduce un-permitted impacts by informing marine zoning efforts and aid in the creation of new no-anchor zones.
Detailed 1ft resolution overlapping aerial photographs were collected for the Nearshore Ridge Complex (NRC) and Inner Reef from Key Biscayne to Hillsboro Inlet, 68.5km of coastline by PhotoScience, Inc. on March 8, 2013. The imagery and recent bathymetry were visually interpreted into benthic habitat maps. Quantitative groundtruthing of 265 targeted and randomized sites was conducted between April and June 2014. Five 1km wide cross-shelf corridors were placed as evenly as possible across the mapped space while maintaining consistent habitat types and amounts between corridors and avoiding any major anthropogenic influences like shipping channels and proximity to inlets and outfalls. Survey site locations were stratified across three main habitats within each corridor: Colonized Pavement-Shallow, Ridge-Shallow, and Linear Reef-Inner. Percent cover data at each site was collected. Additionally, species, colony size (length, width, height), percent mortality, condition (pale or bleached), and presence of disease was recorded for stony corals. Gorgonians were categorized by morphology (rod, plume, fan) and counted in four size classes (4-10, 11-25, 26-50, and >50cm). Xestospongia muta and Cliona spp. were also counted. Then an accuracy assessment was performed where drop camera video with GPS data were collected at 494 locations randomly stratified across all habitat types. The overall accuracy was 97.9% at the Major Habitat level.
Of the 172.73km² seafloor mapped, the polygon totals indicated 41.34% was Sand, 47.07% was Coral Reef and Colonized Pavement, 9.35% was Seagrass, and 2.25% was Other Delineations. These totals are estimates due to some habitats having a large mix of sand within. Three habitat types dominated the mapped hardbottom area. The largest was Colonized Pavement (38.36km²), followed by Ridge-Shallow (25.52km²), and Linear Reef-Inner (14.99km²). These comprised 97% of the hardbottom habitats. Seagrass accounted for 9.35% of the map and was solely contained south of Government Cut. Sand comprised 41.34% of the map and Other Delineations accounted for 2.25%.
The clear, high-resolution images enabled the delineation of thirty-five dense Acropora cervicornis patches. Some of these corresponded to known locations of dense patches. These are the largest dense patches in the continental United States. Using aerial photography delineations area estimates, the seven patches near the known existing locations totaled approximately 46,000m² whereas the 28 newly confirmed areas exceed 110,000m². Dense Acropora cervicornis comprised 1% of the mapped hardbottom habitats.
Significant differences in percent benthic cover between habitats occurred in all corridors, however some comparisons were stronger than others. Corridor 1 exhibited clear differences between the colonized pavement and inner reef sites due to the high percentages of seagrass on the colonized pavement that did not occur on the Inner Reef sites (nor any other habitat in the region). Corridor 2 showed much weaker differences between habitat types, however the colonized pavement sites were significantly distinct from the inner reef and ridge sites due to the comparatively high percentage of sand on the colonized pavement versus the inner reef and ridge. Corridor 3 ridge was significantly distinct from the colonized pavement and inner reef sites mostly due to lower percentage of Palythoa spp. on the ridge. Corridor 4 inner reef sites were significantly different from the others driven by much higher percentage of macroalgae and higher Palythoa spp. Corridor 5 exhibited significant differences between all habitat types. Inner reef sites had higher percentages of Palythoa spp., gorgonians, and sponges than any other habitat. Colonized pavement sites had the lowest percentages of gorgonians and Palythoa spp. while having the highest percentages of sand.
Comparisons of benthic cover percentages between all sites in a given habitat type were conducted to evaluate latitudinal community differences. Among colonized pavement sites, Corridor 1 was significantly different from all other corridors due to the presence of seagrass which only occurred in Corridor 1 colonized pavement. Corridor 5 was also significantly distinct from all other corridors due to a low percentage of gorgonians, stony corals, and Palythoa spp. with a high percentage of turf algae. The ridge sites comparisons showed distinct clustering of corridors 2, 3, and 5 in the MDS indicating that there are latitudinal differences in benthic cover in the ridge habitat. The main dissimilarity contributors in corridor 2 were lower percentages of palythoa spp. and macroalgae than corridors 3 and 5 and higher percentages of gorgonians and stony corals than corridor 5. Corridor 3 had higher percentages of macroalgae, stony corals, and gorgonians than corridor 5. The inner reef sites also exhibited latitudinal differences in benthic cover. Corridors 1 and 5 separated out from the other corridors and each other. The main cover classes driving the clustering of corridor 1 sites were high percentages of gorgonians and Palythoa spp, while the main contributor to the corridor 4 cluster was high macroalgae percentages in that corridor.
A total of 4,568 stony coral colonies were identified, counted, and measured. Twenty-two species were found, but Porites astreoides (29.7%), Siderastrea siderea (17.5%), and Acropora cervicornis (10.3%) comprised 57.5% of the total number of stony corals measured in this study. The largest coral measured in the study was a Siderastrea siderea located in corridor 4 which measured 225 cm long, 200 cm wide, 140 cm tall and an estimated 4.1 m² of live tissue. Stony coral density pooled for the entire surveyed area of 4,200m² was 1.09 corals/m². Mean coral density was lowest in the colonized pavement sites and highest in the inner reef sites, however this also varied by corridor. The colonized pavement coral density in Corridors 1 and 5 was lowest and highest in Corridors 3 and 4.
Coral density on ridge habitat had a similar pattern to colonized pavement with corridor 3 having the highest density. Conversely coral density on the inner reef was highest in corridor 1 and corridor 4. Acropora cervicornis was found in higher densities than S. siderea on the colonized pavement but it only occurred in corridors 3 and 4. It was also found in higher density on ridge habitat except for corridor 5. Of the 471 A. cervicornis colonies counted, only 5.3% occurred on the inner reef. Two hundred and thirty-five (49.9%) were found in the colonized pavement and 211 (44.8%) at the ridge sites.
The mean number of coral species (richness) varied by corridor and habitat. Colonized pavement sites had the lowest richness and it was highest on inner reef. Mean richness also varied by corridor within habitats. Among the colonized pavement sites, corridor 3 and corridor 4 had the highest mean richness and corridor 5 the lowest. Similarly, among the ridge site, mean coral richness was highest in corridor 3 and lowest in corridor 5. Mean richness among inner reef sites were not very different however corridor 1 was significantly higher than corridor 3.
A total of 30,076 gorgonians were counted, classified by morpho-type (Fan, Plume, Rod), and binned into size classes. Rods were the most abundant comprising almost 72% of the total number counted and plumes were second-most comprising 24% of the total. This varied by corridor and habitat. With all size classes combined, fans were lowest on the colonized pavement and highest on the ridge. Plumes were higher on the inner reef than the colonized pavement and ridge. Conversely rods were lower on the inner reef than the colonized pavement and ridge. Gorgonians also varied within habitat types by corridor. In colonized pavement, fans were highest in corridors 3 and 4 whereas plumes were more abundant in the southern corridors. Rods were dominantly abundant throughout the colonized pavement except for corridor 5 where they were conspicuously absent. In the ridge habitat, fans varied among corridors without a clear latitudinal pattern. Plumes were more abundant in the southern corridors, while rods were dominantly abundant throughout. The inner reef habitats generally had a higher abundance of plumes and a more even ratio of rod and plume abundance throughout all corridors. Plumes were the most abundant type in corridor 1, but were also high in corridors 3 and 5.
Xestospongia muta colonies were predominantly found at the inner reef sites. Of the 262 total colonies counted, 87.7% were at inner reef sites. Densities were lower than gorgonians and stony corals throughout the study. Mean X. muta abundance varied between corridors. In colonized pavement and ridge habitats, X. muta predominantly occurred on corridor 4 however mean abundance was very low. At the inner reef sites, X. muta was much lower in corridor 1 than all other corridors, which did not significantly vary.
This study elucidated new data on the extent of the Endangered Species Act threatened coral species, Acropora cervicornis. Only approximately 30% of the discovered dense patches were identified as previously known and the total regional area of A. cervicornisdense patches is now estimated at 156,000 m². The condition of the coral in these patches cannot be surmised from the images. Additionally, the polygons depicted in the habitat map are likely under-representative of the shape and sizes of these patches due to their fuzzy boundaries. A detailed study to map their boundaries and characterize their condition is needed to properly inventory these patches and their condition. Furthermore, the only way to fully understand if the net amount is increasing is to investigate it on a regional level. Previous imagery must be identified and used to determine the timing of when these patches came into existence. Unfortunately no consistent data sets have been identified that can be used for this purpose at this time. A compilation of local imagery has been helpful in some cases. It is recommended that a regional set of imagery be repeatedly collected in the future to elucidate the dynamics of dense patches of A. cervicornis and document the current extent of nearshore resources. This is especially important after large storm events.
This study has expanded the present knowledge on the amount, location, and species type of ecologically important large coral colonies. Although smaller than the minimum mapping unit for this study (and thus not in this study’s scope and funded separately), 187 blips in the LIDAR associated with dark specs in the imagery were identified and a portion investigated. Of the 53 that were visited, 47 were stony corals estimated between 2 and 5 m in diameter. Twenty-three (43%) were alive in various conditions. These were predominantly Orbicella faveolata (20), but 2 were Siderastrea siderea and one was a Montastrea cavernosa. Corals of this size are likely to be hundreds of years old, meaning they have persisted through the multitude of anthropogenic impacts that have occurred in the region. Large coral colonies are more fecund, giving an exponentially increased amount of reproductive output making these colonies particularly important in the restoration of the reef system. It is recommended that a host of important studies be conducted to understand the full extent, size, condition of these large, resilient corals and to monitor them through time, investigate their reproduction and genetic diversity, and perhaps use them to help propagate naturally resilient corals in restoration efforts.
Florida DEP Coral Reef Conservation Program report
Brian K. Walker and Katelyn Klug. 2014. Southeast Florida Shallow-Water Habitat Mapping & Coral Reef Community Characterization .Florida DEP Coral Reef Conservation Program report : 1 -71. http://nsuworks.nova.edu/occ_facreports/87.