Response of tropical marine macroalgae to thermal stress

Short-term increases in temperatures above 31°C have been shown to influence the structure and diversity of coral reefs through coral bleaching episodes, but quantitative information on how these thermal events affect macroalgae on coral reef communities has rarely been described. Using laboratory mesocosm experiments, I examined six species of commonly occurring macroalgae from two functional form groups on the Florida Keys reef tract to determine their temperature thresholds, quantified by the point at which photosynthesis, quantum yield (Fv/Fm), and mortality rates were significantly affected. Red crustose coralline sp., classified in the calcareous group, showed no significant changes in Fv/Fm, and experienced no mortality at elevated temperatures of 34°C. However, Halimeda incrassata, another calcareous species showed significant changes in photosynthesis at thermal treatments of 32°C, and experienced 75% mortality at 34°C. Sargassum fluitans, a branching fleshy macroalgae maintained photosynthetic rates up to 32°C, but at 34°C succumbed to 100% mortality. Laurencia chondroides, also a branching fleshy macroalgal form experienced 50% mortality at 34°C. My results, among the first high temperature experiments on dominant tropical macroalgae, indicate that macroalgae are more robust than corals (∼32°C) in their thermal tolerance, but have varying thresholds among species.