Functional structure of microbial food web in the Senegal River Estuary (West Africa): impact of metazooplankton

Abstract

We studied the impact of various metazooplanktonic predators (two calanoid copepods and barnacle larvae) on microbial plankton isolated from the Senegal River Estuary. Experiments performed in microcosms were based on size-class fractionation (3, 12, 20 and 60 µm size classes) of the microbial community in order to obtain different assemblages, including bacteria, heterotrophic nanoflagellates (HNF), ciliates, pico- and nanophytoplankton. Removal of bacterial predators >3 µm increased the net growth rate of heterotrophic bacteria from 0.437 day−1 in the presence of all predators (fraction <60 µm) to 0.682 day−1 (fraction <3 µm). Removal of protozoa >12 µm caused an increased net growth rate of flagellates from 0.102 day−1 (fraction <60 µm) to 0.332 day−1 (fraction <12 µm). The same results were also observed on protozoa dynamics in the presence of Temora stylifera (0.498 day−1), suggesting a trophic control of the protozoa >12 µm by this copepod, allowing an increase in HNF growth rates through indirect effect. The highest ingestion rate over 60 h of experiment was recorded for the copepod Temora (61.1 µm3 ngC−1 h−1) and the lowest value for barnacle larvae (25.6 µm3 ngC−1 h−1). An intermediate value (47.5 µm3 ngC−1 h−1) was recorded for the copepod Acartia clausi. All these ingestion rates were mainly channeled in the size classes between <3 and 15–18 µm equivalent spherical diameter (ESD) (6 and 2435 µm3, respectively), with the highest values noted between <3 and 6–9 µm classes. In the presence of Temora, the cascading effects can explain both the decreases in mean volume and in growth rate for bacteria, due to the filtration of ciliates by the zooplanktonic predator, allowing the development of the HNF. The presence of a large size class of bacteria (among the five classes identified by flow cytometry) and a high abundance of HNF suggest an efficient heterotrophic pathway within the microbial loop of this sub-saharan estuary.