Population Dynamics and Predictions for the Calcareous Green Algae, Halimeda, on the Coral Reefs of South Florida

Halimeda (Chlorophyta, Bryopsidales) (Lamouroux 1812) is a genus of calcareous green algae that has become abundant on coral reefs of the tropics and subtropics. Though they serve biologically and geologically important roles in these ecosystems, their presence may be an indicator of declining environmental conditions. This study focused on a population of Halimeda in the shallow coastal waters of Tavernier, Florida. Three species were encountered: Halimeda discoidea, Halimeda opuntia, and Halimeda incrassata; Halimeda discoidea was the most abundant. The study was conducted in two parts. Part one involved the observation and quantification of the life history characteristics (e.g. fertility rate, growth rate, mortality rate) of the collected specimens in an artificial environment. Population numbers increased throughout the five week study as new individuals were added through vegetative reproduction. Sexual reproduction was not observed. In Part two the life history characteristics of H. discoidea were used in the construction of a size-classified transition matrix model. Sensitivity and elasticity analyses suggested that the population is most sensitive to changes in the survival rate of reproductive juveniles, and least sensitive to the survival of post-reproductive adults. A standard Chi-square test was used to examine the accuracy of the population distribution predicted by the model against the observed distribution during each week of the study. The test results confirmed the validity of the model; however further analysis of the matrix revealed that without the input of sexual recruits the population will go extinct. A random sexual component was then added to the matrix. Environmental stochasticity can have a direct affect on sexual recruitment in Halimeda. Recruitment interval and recruitment success influence the ability of the population to maintain itself. These factors were manipulated in the matrix. With a 25% recruitment interval of 50 recruits the simulated population was able to avoid extinction over a 25 year period. These results implicate that Halimeda populations require both sexual and asexual recruitment. Additionally, the patchy distribution of Halimeda on and near coral reefs of south Florida indicates that the population may actually be a metapopulation. Recommendations for further studies include an investigation of additional sub-populations within the metapopulation. A mortality analysis would also be useful for gaining a better understanding of the population dynamics and sediment contributions of Halimeda in south Florida