Influence of initial tissue nutrient status of tropical marine algae on response to nitrogen and phosphorus additions

We conducted a 3-factor nutrient-enrichment experiment (factors: N, P, initial tissue- nutrient status) on common species of macroalgae collected from 2 sites along SW Puerto Rico. Our objective was to determine the relative importance of N- or P-limitation among species and sites and to investigate the role of tissue-nutrient status in the response to increased nutrient supply. Acan- thophora spicifera, Dictyota cervicornis, and Hypnea musciformis initially depleted in tissue nutrients responded strongly to either +N or +P alone and both nutrients together by increasing growth. In con- trast, growth of these macroalgae with enriched internal stores of nutrients was generally not as nutrient-limited. In our 3 d experiments, the calcified alga Halimeda incrassata never showed a growth response, perhaps due to the short duration or adaptation to sediment nutrient supply. Tissue N-stores in all algae with low internal nutrient concentrations increased in response to +N, whereas initially higher N-stores were diluted to support growth during the experiment. In addition, all algae depleted the added N and P from the water over the course of the experiment, regardless of tissue- nutrient status. Depleted algae also took up dissolved organic nitrogen (DON) as a secondary source of N, either directly or after mineralization, whereas enriched algae may have 'leaked' NH4. Our results demonstrated that prior storage of nutrients strongly influenced the response of a number of tropical algal species to increased nutrient supply. This variable response to nutrients within and among algae from 3 major algal divisions over small geographical scales may partially explain the mixed results in previous studies of N- and P-limitation.