Nutrient content of macroalgae with differing morphologies may indicate sources of nutrients for tropical marine systems

To investigate whether tissue N and P content of morphologically distinct m acroalgae reflect different processes controlling nutrient availability, we measured water column nutrients and collected 5 species of algae for tissu e 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 sta tions. 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 S tations (HNS), Water column and sediment nutrient concentrations were eleva ted inshore and in HNS. Species were of 3 morphological forms: upright thal li with open branches, densely packed mats, and rhizophytic thalli. In the first category, Acanthophora spicifera had higher N content inshore compare d to offshore while both A. spicifera and Hypnea musciformis had higher tis sue N and P contents in HNS. In contrast, mat-forming algae (Dictyota dicho toma and D. cervicornis) had higher tissue N and P contents offshore compar ed 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 nutr ients 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 a nd open-branching patterns may have a more direct relationship between tiss ue nutrient content and water column nutrient concentration than other form s. In contrast, mat-forming species may deplete nutrients within the mat, r elying on strong currents found offshore to penetrate dense mats and replen ish nutrients. Rhizophytic algae have access to both water column and sedim ent nutrients, and higher inshore tissue contents and at some HNS may refle ct 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 indicato r of different nutrient sources in tropical systems.