dinoflagellates) and/or loss of photosyntheric pigments from the algae remaining in the coral 
soft tissues. This can be caused by stress. Bleaching events have been noted worldwide and 
some have coincided with ENSO events. The 1987 event was studied mostly oo the outerFlorida 
Keys, Bahamas, Venezuela and St. Croix. 
Black band disease is present throughout the coral reefs of the Florida Keys (Kuta and 
Richardson, 1994). The disease consists of a population of the cyanobacterium Phormidiurn 
corallyticum and associated microbial community, and is characterized by an active season 
which occurs during the warmer months when water temperature is at or above 25°C. No 
reports of the disease in Florida Bay were found. 
6.4. Seagrass dieoffs 
Seagrasses constitute a complex and highly productive ecosystem that predominates in shallow 
marine areas in South Florida. The seagrass ecosystem provides food and habitat for diverse 
species, including important sports and commercial fisheries. Livingston (1987) reviewed 
historic trends of human impacts on seagrass meadows of Florida. Municipalization, 
industrialization, and agricultural activities in coastal drainage systems have been accompanied 
by various impacts in almost every bay system in Florida. Seagrass meadows have been 
virtually eliminated in most portions of the Pensacola Bay and Tampa Bay systems. Significant 
losses have been noted over the past 20 - 40 yrs in Choctawhatchee Bay, Apalachee Bay, 
Charlotte Harbor, Biscayne Bay, and the Indian River. Lack of reliable data precluded 
appropriate evaluations in other areas. However, the two primary concentrations of 
seagrasses in the northern hemisphere, Florida Bay and the northwest Gulf coast (including 
Apalachee Bay), are currently threatened by wide-ranging forms of human activity, and a 
general lack of long-term, multidisciplinary ecological studies has inhibited a thorough 
understanding of the problem. Studies in Apalachee Bay indicated that relatively minor water- 
quality changes can destroy or severely alter seagrass distribution and productivity, and 
recovery after impact appears to be slow. Light penetration is the most important factor 
affecting seagrass growth and survival, and it is reduced directly or indirectly by algal blooms 
caused by nutrient enrichment, suspended sediments, and/or water color due to dissolved 
organic material (Kenworthy and Haugert, 1991). 
More than 4,000 ha of Thalassia beds were lost in recurring dieoffs since 1987, and an 
additional 23,000 were affected to a lesser degree (Robblee et at., 1991). The areas affected 
were Johnson Key Basin, Rabbit Key Basin, Rankin Lake, Cross Bank, and Sunset Cove. Dead and 
dying Thalassia were first observed near Cross Bank and Rankin Lake during the summer of 
1987. By the summer of 1988, the dieoff was evident in about 30% of the Thalassia beds of 
western Florida Bay. The Sunset Cove bed died between November 1988 and January 1989. 
Dieoffs appear to occur most rapidly during the fall and spring. These recurring episodes of fast 
but patchy dieoffs are different from those observed as the result of eutrophication. 
The dieoffs of Thalassia in Florida Bay have been reported since 1987 and continue as of this 
writing. Large areas were affected during the fall of 1987. There may be a seasonal pattern to 
the dieoffs associated with high temperatures and high salinities during periods of low fresh 
water flow. Drought conditions during which rainfall was lower than average contributed to the 
generation of hypersalinity conditions from 1986 to 1989. This was a major climatic stress 
that may have resulted in seagrass die-off (D. Morrison, National Audubon Society, personal 
communication, 1995). Zieman et ai (1992), in a retrospective analysis of earlier data coupled 
with current studies, showed a large increase in seagrass biomass prior to the dieoff and a 
decline in turnover rate or specific plant productivity during the dieoff. External stress in the 
form of hypersaline conditions, which are partly anthropogenically derived, were found to be 
prevalent during much of the dieoff. Climatic stresses were (1) excessively warm waters in 
the late summer and fall of 1986 - 1988, and 1990, and (2) a reduction of historical tropical 
27 
