Production of pelagic Sargassum and a blue‐green epiphyte in the western Sargasso Sea1

North of 30°N Sargassum production averaged about twice that to the south, possibly related to the degree of winter mixing. Production rates were relatively high in continental shelf areas or near land masses but showed no major seasonal variations nor was there an obvious relationship with concentrations of NO2, NO3, NH3, and PO4. Under normal daylight conditions in October Sargassum photosynthesizes at a maximum rate. Average carbon turnover time is no more than 40 days and Sargassum and its epiphytes contribute no more than 0.5% of the total primary production in the western Sargasso Sea. Carbon turnover rate of the epiphyte Dichothrix exceeded that of its host Sargassum by an average of fourfold. Dichothrix production on the shelf averaged as much as 15% of the pelagic macroalgal production in October. Dichothrix is most abundant in the continental shelf waters and lowest in the Sargasso Sea; this distribution may be related to the available iron in the seawater.