Seasonal dynamics of macroalgal communities of the northern Florida reef tract

Coral communities worldwide are undergoing intense degradation in response to natural and human disturbances, and many reef systems have already exper ienced significant declines in live coral cover associated with an increase in macroalgal. abundance. Here, we document the seasonal dynamics of the m acroalgal communities of the Northern Florida Reef Tract, providing a basel ine for long-term studies of coral-algal competition in the area. Both macroalgal biomass and percent cover on reefs showed an increasing tre nd from January to July, when both light and temperature conditions were fa vorable for growth. Maximum percent cover (56.7%) was found in July and min imum levels in December (25.8%). During these peaks in algal cover, many co rals were completely covered by dense mats of algae. Two genera, Halimeda a nd Dictyota, represented the largest proportion (77-99%) of the total algal biomass. In the summer, Dictyota spp. dominated the algal community, occup ying up to 40% of the reef bottom with a dry biomass of up to 20 g.m(-2). I n addition, two species, Stypopodium zonale and Trichogloea requienii, show ed a significant bloom in April 1998, covering a significant percentage of the bottom (up to 25%) at an inshore reef. Species that exhibited rapid spa ce monopolization on Florida reefs, such as Dictyota spp. and Stypopodium z onale, 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 c omposition are not adequate to prevent space monopolization and coral overg rowth by algae such as Dictyota spp. and Halimeda spp. Only a continued mon itoring effort will determine whether the seasonal dynamics of the algal co mmunity may result in the decline of coral populations in the Northern Flor ida Reef Tract.