Seasonal changes of benthic algal assemblages have been studied mainly in temperate and sub-tropical areas. It is not clear how natural processes contribute to the seasonal dynamics of algal assemblages on coral reefs, particularly in areas influenced by relatively cold upwelling waters. To investigate the seasonality in algal assemblages we monitored the percent cover of species and algal groups over one year (1994–1995) at two rocky-coral reefs at depths of 9–12 m on the Colombian Caribbean coast (Bahía Chengue, Tayrona National Natural Park, TNNP). The presence of relatively cold waters with temperatures of 25 °C and salinities of 36 ppt is indicative of upwelling events, while warm waters of 28–29 °C and salinities of 33 ppt indicate the rainy seasons in the area. The algal assemblage changed in composition and abundance throughout the year with a bimodal cover pattern observed for macroalgae and turf algae. During the rainy seasons (May to June and October to December) the assemblage was dominated by algal turfs (up to 43% cover) and showed low macroalgal cover (< 20%). In contrast, during the two upwelling periods (February and July to August) it was dominated by macroalgae (up to 44% cover). Cover of brown macroalgae Dictyota spp. (mainly D. bartayresiana) declined from ≈34% during the upwelling to only 5% in the non-upwelling rainy seasons. Environmental data revealed a significant inverse correlation between water temperature and macroalgal cover, suggesting that increases in macroalgae are favoured by the presence of cold, upwelling events. It is likely that upwelling events bring nutrient rich waters that stimulate macroalgal growth, however, the role of nutrients regulating the abundance of macroalgae in the TNNP is yet to be investigated. The results of this study support the argument that benthic algal assemblages of coral reefs are a highly dynamic component of these ecosystems.
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