1994 0 
Zieman, J. C. (1994) A conceptual model of seagrass dieoff in Florida Bay. Abs., ASLO/PSA 
Joint Mtg., Miami, FL. a-85. 
[ABSTRACT ONLY. DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] Since the 
summer of 1987, Florida Bay has experienced a major dieoff of seagrasses totaling 
tens of thousands of hectares, nearly all Thalassia testudinum. Dieoff has been 
accompanied by eutrophic epiphyte and algal growth, apparently due to the release of 
nutrients from the nutrient rich barren sediments and the thousands of metric tons of 
decreasing seagrass. Distal causes of dieoff include long-term curtailment of water 
flow into the Everglades and resulting change in communities, coupled with an abnormal 
(>3 times normal) time between perturbations by tropical storms. Potential proximal 
triggers include: (1) abnormally high temperatures in summer and fall of 1987, (2) 
extreme salinities, and (3) possibly disease. Seagrass photosynthetic capacity is 
compromised by eutrophic algal growth mud suspended sediments from the unstabilized 
sediments. Plant death would then result from an inability of the plants to meet 
respiratory requirements, or hypoxic stress. As the dieoff process continues, the 
additional dead seagrass leaves and barren sediments maintain a positive growth loop 
which has not yet reached equilibrium, but the conditions sustaining the dieoff are not 
the conditions that initiated it. A model of this process is presented. 
1994 0 
Zieman, J. C., R. Davis, J. W. Fourqurean, and M. B. Robblee (1994) The role of climate in 
the Florida Bay seagrass dieoff. Bull. Mar. Sci. . 54(3): 1088. 
[ABSTRACT ONLY. DATE OF SAMPLING UNKNOWN OR NOT APPLICABLE.] Since the fall 
of 1987, Florida Bay has experienced a major die-off of seagrasses and benthic 
macrophytes totaling tens of thousands of hectares. After several years, dieoff of 
Thalassia continued at a reduced rate, while colonization and growth of the colonizer 
Halodule became widespread. Anomalies in the recent climate record may have played a 
significant part in the dieoff initiation. Retrospective analysis of earlier data coupled 
with current studies show 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 (maximum salinities >70, max. yearly station 
averages >50 %o), which are partly anthropogenically derived, were prevalent during 
much of the dieoff. Climatic stresses are: (1) excessively warm waters in the late 
summer and fall of 1986 - 1988, and 1990; and (2) a reduction of historic tropical 
storm frequency and intensity. Increased temperatures and decreased day length in the 
fall negatively impact seagrass P/R. Historical and anecdotal evidence suggests a 
continuing shift over the past to decades from a mixed habitat to an increasingly 
monospecific Thalassia community. While recolonization processes are establishing a 
more diverse mixture of habitats with the potential of enhanced secondary 
productivity, in some areas, in 1992, major dieoff expansion has occurred in western 
Florida Bay. 
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