Abundance, biomass and growth rates of pelagic microorganisms in a tropical coastal ecosystem

In situ growth rates of ciliates and heterotrophic nanoflagellates were det ermined after size fractionation during rainy and dry seasons at 2 coastal stations off Zanzibar Island, Tanzania. In addition, bacterial and primary production and the biomass components in the pelagic microbial community we re measured. The bacterial and primary production, and growth rates of hete rotrophic nanoflagellates were significantly higher during the rainy season . For the ciliate community the growth rate between the seasons did not dif fer, but the number of positive growth estimates increased by a factor of 5 in the rainy seasons compared with the dry seasons. Further, the diversity of ciliates increased with the amount of rain, and the size distribution i ndicated a shift towards a community with larger species. Data obtained fro m these experiments are summarised in a carbon budget for each season. Carb on budgets indicate that the carbon flow through the heterotrophic microbia l food web was approximately 3 times higher during the rainy season compare d with the dry season. However, due to the relatively lower production of l arger diatoms, heterotrophic microplankton may be relatively more important as a carbon source for higher trophic levels, such as copepods, during the dry season.