Daily bacterioplankton dynamics in a sub-Saharan estuary (Senegal River, West Africa): a mesocosm study

Temporal variability in bacterial communities was studied on a daily scale in the estuar- ine part of the largest river on the West African coast, the Senegal River. Duplicate mesocosms (3 m 3 volume) were placed in the upper part of the estuary at the end of the dry season (May 2002) and treated with low and high inorganic nutrient (N and P) enrichment. High nutrient additions were followed by a 7-fold increase in phytoplankton biomass and a 6-fold increase in bacterial abundance after 4 and 9 d, respectively. Heterotrophic nanoflagellates (HNF) showed their maximal abundance (1 × 10 6 ml -1 ) 2 d after the bacterial peak. The low bacteria to flagellate ratios recorded on Day 10 may suggest enhanced bacterivory from HNF. Simultaneous measurements of growth and grazing rates on bacteria during the bacterial growth phase were performed with the dilution method and seemed to indicate that the HNF community was capable of quickly controlling bacterial development. How- ever, estimates of the carbon demand of HNF during their growth phase (915 µg C l -1 d -1 ) appeared to be more elevated than the bacterial carbon production (63% of the HNF carbon demand). To cover HNF carbon requirements, an alternative/complementary prey might be the picophytoplanktonic cells, which were very numerous during the study (i.e. 85% of the total phytoplankton count). During the period of high grazing pressure, bacterial populations were characterized by higher specific activity (from tritiated thymidine incorporation) and culturability (from plate counts). The presence of very large bacteria, as detected by epifluorescence microscopy and flow cytometry measure- ments, may be an escape response from flagellate grazing. 16S rDNA sequences from bacterial isolates showed the presence of 2 types of taxonomic units (Vibrio natriegens- and Flexibacter maritimus-like bacteria), which can be considered by their forms and growth rates to be strains that have developed strategies to protect against grazing. Thus, as demonstrated in various temperate systems, predation by HNF in estuarine tropical ecosystems may also be of importance in shaping the structure and functions of the bacterial community.