Pelagic Sargassum community metabolism: Carbon and nitrogen

The production, nitrogen fixation, and release rates and fate of dissolved organic matter of a pelagic Sargassum community have been investigated at eight stations in the Gulf Stream and the Sargasso Sea. Net production and gross nitrogen fixation rates of Sargassum and epiphytes varied significantly between stations, 328 ± 114μg C (g dry wt)⁻¹h⁻¹ and 18 ± 7.4μg N g⁻¹h⁻¹, respectively. The net release rates of dissolved organic carbon (287 ± 150μg DOC g⁻¹h⁻¹) also showed the same variability between stations. On the other hand, the community carbon and nitrogen content, 268 ± 4.8 and 16.9 ± 2.4 mg g dry wt⁻¹, respectively, remained constant at all stations. The results of chemical measurements indicate that ≈ 0–50 % of the gross production was lost as a result of photosynthate release. From 14C-tracer experiments it was found that the planktonic and epiphytic heterotrophs mineralized 50–70 % of the photosynthate released by Sargassum and epiphytic algae. Based on the community gross production and fixation rates, carbon and nitrogen content, the amount of nitrogen required for the observed production rates, the Sargassum community appears to obtain a substantial part (40%) of its nitrogen from nitrogen fixation.