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 reef s raises concern regarding a possible shift from coral- to algal-dominated communities. Possible increased nutrient supply to macroalgae and cyanobact eria 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 incr assata, Padina tenuis and Dictyota bartayresiana) and 3 species of cyanobac teria (Tolypothrix sp., Schizothrix sp. and Lyngbya majuscula) from Cocos L agoon, Guam. The 6 species were cultured together sewn to an artificial sub strate for 9 d with either nitrate- (similar to6 muM), phosphate- (similar to1 muM), iron- (similar to0.5 muM) enriched or control (ambient nutrients) conditions. Overall gram-specific growth was greatest for L. majuscula, wh ich grew at 9 times the rate of the other species. Algae did not show stati stically significant nutrient limitation, although results with D. bartayre siana and P. tenuis suggested iron and nitrate limited growth in the first and second experiment, respectively, Two species of cyanobacteria showed ph osphate limitation. The growth of L. majuscula was enhanced with phosphate enrichment, whereas the release of hormogonia by Tolypothrix sp., not the g rowth of the colonies themselves, may also have been enhanced. Patterns of Tolypothrix Sp. hormogonia release also suggested possible direct competiti on between algae and cyanobacteria; the hormogonia aggregated upon some spe cies but not others, The results of this study suggest that L. majuscula ma y have more efficient growth and/or nutrient uptake mechanisms compared to the other species, and that it is capable of increased growth in response t o phosphate in the water column.