We conducted a 3-factor nutrient-enrichment experiment (factors: N, P, initial tissuenutrient 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. Acanthophora 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 contrast, 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 tissuenutrient 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' NH 4 . 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.

To investigate whether tissue N and P content of morphologically distinct macroalgae reflect different processes controlling nutrient availability, we measured water column nutrients and collected 5 species of algae for tissue N and P analysis from 18 stations along the southwestern coast of Puerto Rico. Nutrient content of sediments was also determined for a subset of stations. Southwestern Puerto Rico was chosen because the literature suggests that gradients in sediment type and organic matter content, advection, and terrestrial influence occur in this region. Stations were either inshore or offshore areas with 3 stations per area chosen a priori as High Nutrient Stations (HNS). Water column and sediment nutrient concentrations were elevated inshore and in HNS. Species were of 3 morphological forms: upright thalli with open branches, densely packed mats, and rhizophytic thalli. In the first category, Acanthophora spicifera had higher N content inshore compared to offshore while both A. spicifera and Hypnea musciformis had higher tissue N and P contents in HNS. In contrast, mat-forming algae (Dictyota dichotoma and D. cervicornis) had higher tissue N and P contents offshore compared to inshore. Although these species had high nutrients in many of the HNS, samples from some offshore reefs were equally high. Halimeda incrassata, a rhizophytic form, had greater tissue N content inshore than offshore. H. incrassata tissue nutrients were also elevated in some HNS, but not others. There were significant correlations between water column and sediment nutrients and the tissue N and P content of A. spicifera and H. incrassata, but not for either Dictyota. These results suggest algae with upright thalli and open-branching patterns may have a more direct relationship between tissue nutrient content and water column nutrient concentration than other forms. In contrast, mat-forming species may deplete nutrients within the mat, relying on strong currents found offshore to penetrate dense mats and replenish nutrients. Rhizophytic algae have access to both water column and sediment nutrients, and higher inshore tissue contents and at some HNS may reflect enhanced nutrient supplies from these sources. Our findings suggest that if other environmental factors are carefully taken into consideration, the tissue N and P content of macroalgae may prove to be an effective indicator of different nutrient sources in tropical systems. KEY WORDS: Macroalgae · Tropics · Nutrients · Morphological forms · Puerto RicoResale or republication not permitted without written consent of the publisher

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