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

Abstract

In situ growth rates of ciliates and heterotrophic nanoflagellates were determined 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 were measured. The bacterial and primary production, and growth rates of heterotrophic nanoflagellates were significantly higher during the rainy season. For the ciliate community the growth rate between the seasons did not differ, 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 indicated a shift towards a community with larger species. Data obtained from these experiments are summarised in a carbon budget for each season. Carbon budgets indicate that the carbon flow through the heterotrophic microbial food web was approximately 3 times higher during the rainy season compared with the dry season. However, due to the relatively lower production of larger diatoms, heterotrophic microplankton may be relatively more important as a carbon source for higher trophic levels, such as copepods, during the dry season.