The 2013 nearshore mapping project conducted by Walker and Klug expanded the previous knowledge on the amount, location, and species type of ecologically important large (>2 m) coral colonies in southeast Florida. They discovered over 110 previously undocumented large corals of which 60 were dead and 50 were still alive; 40 of the living corals were larger than 2 m wide and up to 5 m in diameter. Because these corals are the largest and oldest organisms on our reefs, they deserve special attention.
Currently there is unprecedented disease and bleaching in the northern portion of the Florida Reef Tract. It is imperative that the large coral baseline condition is documented to understand the present condition of the large corals in southeast Florida. Understanding how the coral populations are affected by this outbreak and identifying which individuals were resilient enough to recover is critical to the management of the SE FL coral reef ecosystem therefore the objective of this project was to achieve recommendation four from Walker and Klug (2014): conduct a full inventory study to understand the extent, size, condition of the large (> 2 m diameter) corals.
Live corals greater than 2 m diameter identified during reconnaissance were assessed by SCUBA divers. High resolution photographs and video were collected of the coral as a permanent record of its condition. Photographs were taken systematically at each of the four main compass headings (north, east, south, and west) and from overhead. In cases where the coral was too large or the visibility was poor, multiple pictures of the coral were taken at a closer distance. Divers then estimated the percent live tissue cover and percent recent and old dead skeleton remaining, percentage of bleached tissue, percentage of diseased tissue, and the number of tissue isolates. Each coral was then measured using a rigid meter stick was used to measure height, the linear distance along the longest axis, and the widest axis perpendicular to the first axis and a measuring tape to measure the distance over the surface of the coral. In areas with multiple large corals, a Garmin 76csx GPS in an underwater housing with a floating antenna was used to collect the coordinate of each coral.
Surveys were conducted over eleven days between September and November 2015. Additional reconnaissance surveys were conducted to assess sixty-two new targets that were not previously visited due to poor visibility during Government Cut channel dredging. A total of 115 corals were inventoried and measured. See Appendix 1 for images and data collected on each coral. The majority of corals were Orbicella faveolata (78.2%), followed by Montastrea cavernosa, Siderastrea siderea, Colpophyllia natans, Orbicella annularis, and Pseudodiploria strigosa. Corals were found between 4.6 m and 8.8 m depth predominantly in the nearshore colonized pavement and shallow ridge habitats at an average depth of 6.4 m. Colonies were evenly distributed between Miami-Broward and Biscayne Coral Reef Ecosystem Regions. A few corals were spread out but most were clustered into smaller areas.
There was no apparent pattern of size with latitude. Eight corals, all O. faveolata, were measured larger than 4 m and spanned from Key Biscayne to Hollywood. The two largest corals, which measured 5.6 m long, were located off Key Biscayne and contained 50% and 70% live tissue. One other coral measured 5.1 m long located near Bal Harbor and had 30% live tissue.
Almost half of all large corals did not show signs of stress from bleaching or disease, however all of the M. cavernosa, O. annularis, and C. natans had either or both conditions. Thirty-seven percent of all corals had some recent mortality, including all four O. annularis and C. natans colonies and about half of the M. cavernosa, S. siderea, and P. strigosa colonies. Twenty-three percent of all corals had some bleaching, but M. cavernosa appeared to be affected more than other species. There were many smaller M. cavernosa colonies not captured in this study with extensive bleaching, especially in the Biscayne region. Eight percent of all colonies had both bleaching and disease.
The diseases visually observed in this study were white plague, black band, dark spot and possibly Caribbean yellow band. Coral diseases are very difficult to identify precisely in the field and require histological and genetic analyses to be conducted.
Changes in condition were noted between the reconnaissance and the surveys. In 2015, bleaching recovery was noted within 41 days on recently surveyed corals near Key Biscayne. This coincided with a period of noticeable cooler water temperatures and is likely indicative that the 2015 bleaching event was subsiding accordingly. Conversely, the halting of disease progression was not noted in our surveys. For example white plague disease on a C. natans had killed significant tissue over 27 days. The condition and fate of that colony is presently unknown.
Changes in coral condition and live tissue cover were noted between 2014 and 2015. In 2015 corals were found completely bleached that were not bleached in 2014. Colonies were also found fully and partially bleached in 2014 and 2015 where portions of the partially bleached areas were bleached in both years and portions were not. The timing of these changes is worth noting because in south Florida corals usually bleach from heat stress later in the summer around August and September. Corals originally surveyed in June 2014 may have still bleached in 2014, recovered, and bleached again in 2015. Without regular monitoring this cannot be determined.
Disease was not noted to occur in corals between years through our initial photo and video documentation evaluations but it was observed in 2015 when not present in 2014.
Percent mortality was high in all corals combined. When including all of the dead corals found in the reconnaissance, 100% mortality was the highest (34%). However the partial mortality percentages were also high with 43% of corals between 25% and 99% partial mortality and 31% at least half dead. Twenty-three percent were less than one quarter dead including 5% that were more than 90% living.
This study documented baseline conditions of the largest and oldest corals of the southeast Florida reefs which are analogous to the “redwoods” of our nearshore community. In southeast FL, corals grow about 1 cm per year. Corals greater than 2 meters in diameter can be hundreds of years old. The largest corals in a population are the oldest and have exponentially more reproductive capacity than smaller ones, making them the most important demographic of their respective species. Their age indicates that they have persisted through the multitude of anthropogenic impacts and stressors that have occurred in the region since the western colonization of Florida. Their size also provides habitat for a diverse and abundant assemblage of fish. A large proportion of the large corals are in the relatively flat, nearshore habitats, and thus provide an oasis for many fish species.
High partial mortality is an indicator of more stressed systems. We found 65% of large corals were either dead or had less than half of their live tissue remaining. The dead ones are difficult to assess as one must collect samples to identify the species and we do not know when they died. This would be valuable information because it would allow us to determine if the frequency of mortality in these corals is increasing through time. In other words conditions are more stressful today causing more frequent mortality. This can be determined by drilling the corals and determining their ages by comparing them to a reference coral.
Assessing these corals through time is important. We can identify which events reduce their tissues, whether they recover from bleaching, the frequency of bleaching and disease for each coral and the total population, and how resilient they are to stress events. The overwhelming majority of these corals were O. faveolata, a reef-building species listed as threatened under the Endangered Species Act. These resilient corals might give clues to the ability of certain corals to recover from adversity and help in the restoration of the species across the reef tract. Further, large coral colonies are more fecund, giving an exponentially increased amount of reproductive output also making these colonies particularly important in the species’ recovery.
A list of recommendations of work critical to the understanding and management of the Southeast Florida coral populations, especially for O. faveolata, which is threatened under the Endangered Species Act includes: (1) Spatial analysis of large coral distribution, (2) Regular assessments of the large live corals, (3) Identifying the dead coral species and timing of death, (4) Histology and reproductive study, (5) Genetic studies, and (6) Restoration.
Brian K. Walker and Katelyn Klug. 2015. Southeast Florida Large Coral Assessment 2015 : 1 -151. https://nsuworks.nova.edu/occ_facreports/125.