NUTRIENT-LIMITED PRODUCTIVITY OF CALCAREOUS VERSUS FLESHY MACROALGAE IN A EUTROPHIC, CARBONATE-RICH TROPICAL MARINE-ENVIRONMENT

The results of a study of nutrient enrichment with nitrogen (N) and ph osphorus (P) on productivity and calcification of fleshy and calcareou s algae are reported in this study. Plants were collected from a nears hore eutrophic site in the Florida Keys (USA) and experimentally pulse d during the night with combinations of N and P. After several days of pulsing (7-10 days), net productivity, calcification, and alkaline ph osphatase activity (APA), were measured. Productivity of fleshy algae were frequently enhanced by N, P, and N + P, during both summer and wi nter. Phosphorus limited the productivity of Hydroclathrus clathratus during winter and Ulva spp. during summer, whereas nitrogen limited th e productivity of Laurencia intricata during both seasons. During summ er, Dictyota cervicornis productivity was not enhanced by any nutrient enrichment. Nitrogen limited the productivity of the three calcareous species Penicillus capitatus, Penicillus dumetosus and Halimeda opunt ia during winter and that of H. opuntia during summer. Neither N nor P enrichment increased calcification of calcareous species, and P enric hment greatly inhibited calcification of P. dumetosus during winter. N utrient enrichment enhanced the productivity of the fleshy species to a greater extent than that of calcareous algae. The seawater DIN:SRP m olar ratio was low at our eutrophic study site (molar ratio average of 3:1 during winter and 9:1 during summer) compared to more oligotrophi c sites in the Florida Keys, suggesting that in carbonate-rich environ ments, eutrophication shifts nutrient regulation of productivity from P to N. APA activities of fleshy macroalage were higher than calcareou s algae, and rates of all macro algae were 2- to 7-fold higher in summ er compared to winter. Productivity was also about 3-fold higher in fl eshy compared to calcareous species and about 2-fold higher in summer compared to winter. These results suggest that nutrient enrichment enh ances productivity of fleshy algae to a greater extent than that of ca lcareous algae. Thus, overgrowth of calcareous algae by more opportuni stic fleshy forms could reduce carbonate accretion in tropical coastli nes experiencing increased eutrophication.