Seasonal dynamics of macroalgal dominance and environmental drivers in a western phase-shifted Caribbean reef lagoon

Tropical reefs are among the most threatened ecosystems on Earth. In the Caribbean, dominance has shifted from coral to macroalgae, due to several proximal and distal causes, such as the decline in the population of herbivorous fish, eutrophication, pollution, and climate change. Macroalgae are essential components of reefs, although their overgrowth results in phase shifts, disrupting the reef community balance. Over the course of a year, we examined the seasonal relationship between environmental factors and changes in macroalgal dominance in a western Caribbean reef lagoon. Sampling yielded 5,160 macroalgae thalli, spanning 40 species. From these, 3,663 thalli (71%) and eleven species corresponded to Ochrophyta (Brown algae), 816 thalli (15.8%) and 12 species to Rhodophyta (Red algae), and 681 thalli (13.2%) and 17 species to Chlorophyta (Green algae). Results showed that macroalgae cover was significantly related to water temperature, nutrient concentration, and photosynthetically active radiation (PAR). Brown algae cover increased during the wet season when water temperature was high, and PAR was low. The cover of the three macroalgae groups was lower during the dry season, concurrently with high PAR values. Nutrient concentrations were found to be within normal ranges, with no signs of eutrophication. The seasonal variations in the environmental conditions, together with the rare occurrence of herbivore fish and sea urchins, support the assumption that these factors are the underlying drivers of the dominance recorded. These findings provide evidence of the relative importance of driving factors and mechanisms ruling the coral–algal phase shift phenomenon in tropical reefs. • Seasonal environmental variation significantly influences the macroalgae assemblage • Macroalgae were influenced by water temperature, PAR, phosphate, and nitrate • Macroalgae cover decreases with high PAR values present in the dry season • High Macroalgae cover is not always related to water eutrophication