Effects of nitrate, phosphate and iron on the growth of macroalgae and benthic cyanobacteria from Cocos Lagoon, Guam

The observed high abundance of algae and cyanobacteria on Guam’s coral reefs raises concern regarding a possible shift from coralto algal-dominated communities. Possible increased nutrient supply to macroalgae and cyanobacteria via the watershed due to anthropogenic disturbance could be a partial cause. In this study, 2 outdoor microcosm experiments are used to test the effects of iron, nitrate and phosphate on 3 species of algae (Halimeda incrassata, Padina tenuis and Dictyota bartayresiana) and 3 species of cyanobacteria (Tolypothrix sp., Schizothrix sp. and Lyngbya majuscula) from Cocos Lagoon, Guam. The 6 species were cultured together sewn to an artificial substrate for 9 d with either nitrate(∼6 μM), phosphate(∼1 μM), iron(∼0.5 μM) enriched or control (ambient nutrients) conditions. Overall gram-specific growth was greatest for L. majuscula, which grew at 9 times the rate of the other species. Algae did not show statistically significant nutrient limitation, although results with D. bartayresiana and P. tenuis suggested iron and nitrate limited growth in the first and second experiment, respectively. Two species of cyanobacteria showed phosphate limitation. The growth of L. majuscula was enhanced with phosphate enrichment, whereas the release of hormogonia by Tolypothrix sp., not the growth of the colonies themselves, may also have been enhanced. Patterns of Tolypothrix sp. hormogonia release also suggested possible direct competition between algae and cyanobacteria; the hormogonia aggregated upon some species but not others. The results of this study suggest that L. majuscula may have more efficient growth and/or nutrient uptake mechanisms compared to the other species, and that it is capable of increased growth in response to phosphate in the water column.