Plankton abundance and resource exploitation by the endemic sprat <i>Nannothrissa stewarti</i> (Poll &amp; Roberts, 1976) in a large blackwater lake of the middle Congo River basin, Lake Maï-Ndombe

ABSTRACT Lake Maï-Ndombe is a large natural shallow lake of the Congo River basin. We investigated the composition and abundance of plankton and the diet of Nannothrissa stewarti that is at the basis of an important fishery. The lake is not stratified, phytoplankton being well developed during low water (LW) conditions with higher biomass (average value ~ 2.5 ± 1.1 μg chlorophyll a L−1) than in high water (HW) conditions (0.14 ± 0.1 μg chlorophyll a L−1). It is oligotrophic but with an autochthonous algal primary production, in contrast to earlier reports. The metazooplankton comprises 15 species (excluding bdelloid rotifers), among which the dipteran Chaoborus sp. Its biomass is low (~0.20 ± 0.1 g C m−2 in LW vs 0.130 ± 0.02 g C m−2 in HW). Based on gut contents analysis, Nannothrissa is a highly selective feeder in Lake Maï-Ndombe, feeding mostly on Cladocera despite their relatively low abundance. This suggests that only part of the metazooplankton production is being exploited by the sprat. However, stable carbon and nitrogen isotope data are inconsistent with the dominance of zooplankton in stomach contents, and further work should elucidate which 13C-depleted sources further support the diet of this key consumer in Lake Maï-Ndombe.