Coral communities worldwide are undergoing intense degradation in response to natural and human disturbances, and many reef systems have already experienced significant declines in live coral cover associated with an increase in macroalgal abundance. Here, we document the seasonal dynamics of the macroalgal communities of the Northern Florida Reef Tract, providing a baseline for long-term studies of coral-algal competition in the area. Both macroalgal biomass and percent cover on reefs showed an increasing trend from January to July, when both light and temperature conditions were favorable for growth. Maximum percent cover (56.7 %) was found in July and minimum levels in December (25.8 %). During these peaks in algal cover, many corals were completely covered by dense mats of algae. Two genera, Halimeda and Dictyota, represented the largest proportion (77299 %) of the total algal biomass. In the summer, Dictyota spp. dominated the algal community, occupying up to 40 % of the reef bottom with a dry biomass of up to 20 g.m 22 . In addition, two species, Stypopodium zonale and Trichogloea requienii, showed a significant bloom in April 1998, covering a significant percentage of the bottom (up to 25 %) at an inshore reef. Species that exhibited rapid space monopolization on Florida reefs, such as Dictyota spp. and Stypopodium zonale, also showed rapid growth in microcosm and field growth studies. No correlations were found between fish grazer abundance and algal biomass or percent cover, indicating that present grazer population abundance and composition are not adequate to prevent space monopolization and coral overgrowth by algae such as Dictyota spp. and Halimeda spp. Only a continued monitoring effort will determine whether the seasonal dynamics of the algal community may result in the decline of coral populations in the Northern Florida Reef Tract.