High numbers of Trichodesmium and diazotrophic diatoms in the southwest Indian Ocean

We observed high cell numbers of large (>50m) nitrogen-fixing phytoplankton to the south and to the east of Madagascar in February 2005. The distribution of the diazotrophic taxa we found was markedly different: Trichodesmium was most abundant (500-1000 trichomes L-1) in the waters at the southern tip of Madagascar, while diazotrophic diatoms (125-500 cells L-1) were found to be abundant to the southeast of Madagascar. From these cell numbers, using literature values of nitrogen-fixation per cell, we estimate potential rates of nitrogen-fixation for Trichodesmium (south of Madagascar) of 1-5 mmol N m-2 d-1 and for diazotrophic diatoms (east of Madagascar) of 0.4-2.4 mmol N m-2 d-1. These cell-based estimates highlight the potential for significant nitrogen-fixation in the southwest Indian Ocean. High numbers of diazotrophic diatoms in subtropical waters to the east of Madagascar may have important implications for the biogeochemistry of the austral phytoplankton blooms that occur in this area.