MACROALGAL BLOOMS IN FLORIDA'S COASTAL WATERS: CODIUM ISTHMOCLADUM

Hanisak, M. D. Harbor Branch Oceanographic Institution, Fort Pierce, FL 34946 USA During the past decade, reports of macroalgal blooms along the reefs of southeast Florida have increased. These blooms may become harmful due to negative impacts associated with excessive biomass, smothering of corals, sponges, and other reef organisms, and benthic hypoxia and anoxia. Since 1989‐1990, recurrent blooms of Codium isthmocladum have occurred on reefs off Palm Beach and Broward Counties in depths between 20 and 60 m. The objectives of this study were to determine the spatial and temporal variability in C. isthmocladum biomass and tissue nutrient (C:N:P) ratios. Studies conducted primarily on populations in northern Palm Beach County showed distinct seasonal and spatial patterns in biomass. The growth of individual thalli followed the classic sigmoidal growth curve, with linear increases in biomass occurring during the summer, and growth terminating in late September/October. During peak growth, thalli doubled ca. every week (i.e., specific growth rate = ca. 0.10 doublings d‐1). Tissue nutrient analyses supported the hypothesis that nitrogen was limiting the growth of this alga. Growth of C. isthmocladum is largely a result of its relationship with light, temperature, and nutrients; when conditions are favorable, and in the absence of physical disturbance, the alga is capable of explosive growth. In the last two years, blooms of Caulerpa verticillata have been occurring on the same reefs where C. isthmocladum is still abundant. Anthropogenic nutrients may be an important part of the reason why macroalgal blooms have been occurring for over a decade in south Florida's coastal waters.