Issues : Figure Sets

Figure Set 1: Evidence for Decrease in Coral Diversity in the Florida Keys

Purpose: To demonstrate the changes in corals in the Florida Keys and motivate students to find out why.
Teaching Approach: "Turn to Your Neighbor"
Cognitive Skills: (see Bloom's Taxonomy) — knowledge, comprehension, interpretation
Student Assessment: newspaper article


      The only coral reef community in the continental US is in the waters off the southern tip of Florida. Stretching for more than 200 miles the Florida Keys and Florida Bay contain a mosaic of seagrass beds and corals with mangrove forests at the coastal edge. During the late 1980's and into the 1990's noticeable changes occurred in the Florida Keys area. In 1987 the seagrass Thalassia (turtle grass) began to disappear in Florida Bay. Blooms of phytoplankton became more common and severe. In the reef tract live coal coverage decreased and the corals were plagued with several diseases, some not seen before. Most researchers have looked to the Florida Bay watershed as the source of these problems. For decades freshwater flow from the Everglades into Florida Bay has been greatly changed; water has been diverted, retained, and polluted with herbicides and other agricultural chemicals. As a result of water diversions and also droughts, salinity in parts of the Bay has been reported to be as high as 70ppt (seawater is about 30 ppt). Another likely reason is increased nutrient loading (mainly from agriculture, sewage, and septic systems) to the Bay. Nearshore nutrient concentrations have risen and phytoplankton biomass plus water column turbidity has increased in some locations. Eutrophication, the increase of organic matter in water, can lead to low oxygen events; in the Florida Bay hypoxia (oxygen concentration > 2 ppm) and anoxia (no oxygen) have become more frequent.

      The Coral Reef Monitoring Project

      The figures student work with here are from the Coral Reef Monitoring Project (EPA 2000; www.reefrelief.org/coralreef/monitoring/project5.html) which was supported by the EPA. Because the Florida Keys National Marine Sanctuary and Protection Act designated over 2800 square miles of coastal waters as the Florida Keys National Marine Sanctuary, EPA was required to conduct a monitoring program. This extensive study included 40 reef sites where permanent station markers were established in 1995. Sampling began in 1996 and 160 stations in 40 sites were sampled through 2000.

      For the hard coral diversity surveys presented here two divers conducted simultaneous timed (15 min) inventories in 22 X 2 m stations and; number of species were counted as species diversity measurements. Diseases were also measured. Fig. 1A shows percent change in coral diversity from 1996-2000 in 3 locations: Upper Keys (north Key Largo to Conch Reef), Middle Keys (Alligator Reef to Molasses Reef), Lower Keys (Looe Key to Smith Shoal), and Tortugas (Dry Tortugas to Tortugas Bank). Fig. 1B is change in percent cover of corals in the same stations. Percent cover was measured in 60 frames of video images also taken at permanent locations.

      In addition to the data in Fig. 1 the EPA Executive Summary notes that: "When the first and last years data was compared, eight stony coral species Acropora cervicornis, Leptoseris cucullata, Millepora alcicornis, and Millepora complanata, Mycetophyllia danaana, ferox and lamarckiana and Porites porites) had experienced a significant loss. Only Agaricia fragilis showed a significant gain over the time span sampled... Over the 5 years, there was a significant increase in White Complex disease, and other diseases significantly increased. Overall, there were increases in the number of stations containing diseased coral, the number of species with disease, and the different types of diseases that were observed... The greatest mean percent stony coral cover was consistently observed at patch reef stations. Sanctuary-wide, a general trend of decline in stony coral cover was observed. For all years except 1996 to 1997, the decrease in mean percent coral cover was significant..."

      Other Studies by J.W. Porter et al.

      Other research shows similar results. Porter and Meier (1992) studied changes in coral species in 6 sites between Miami and Key West from 1984 to 1991. These sites were marked with stakes underwater and the same frame was rephotographed each time. During this 8 year period the number of coral species decreased at each station. Other data show that % cover, in addition to species number, decreased in these sites and that there was no recruitment of new, young corals over this time. In the abstract to the paper Porter and Meier say that "mortality of this magnitude is often associated with hurricane damage, but in this survey the losses occurred during a period without catastrophic storms." Sources of mortality identifiable in the photographs include 1) black band disease and 2) bleaching. The authors note that "loss rates of this magnitude cannot be sustained for protracted periods if the coral community is to persist in a configuration resembling historical reef community structure in the Florida Keys."

      More recent work by James W. Porter demonstrates the continuing spread of coral disease. The abstract of Porter et al. 2001 states that "In extensive surveys from Key Largo to Key West in 160 stations at 40 randomly chosen sites, there has been a dramatic increase in1) the number of locations exhibiting disease (82% of all stations are now affected, a 404% increase over 1996 values), 2) the number of species affected (85% of all species are now affected, a 218% increase over 1996 values), and 3) the rate of coral mortality."

      Porter et al. 2001 are cautious about identifying causal factors, saying in the abstract that "The central question of why so many corals are becoming simultaneously susceptible to a host marine pathogen remains unanswered". In the discussion section of the paper they note that areas closest to cities had more stations affected by disease, and therefore proximity to human centers may increase the likelihood of disease. Interestingly they do not themselves suggest that N and P exacerbate diseases and instead quote from a paper on infection of sea fans. The paper also includes a brief discussion of elevated temperature as a stress for corals.

Literature Cited