Phytoplankton dynamics in the Congo River JEAN-PIERRE DESCY* , † , FRANC ß OIS DARCHAMBEAU † , THIBAULT LAMBERT † , MAYA P. STOYNEVA-GAERTNER ‡ , STEVEN BOUILLON § AND ALBERTO V. BORGES † *Research Unit in Organismal Biology (URBE), University of Namur, Namur, Belgium † Unit  e d’Oc  eanographie Chimique, Universit  e de Li  ege, Li  ege, Belgium ‡ Department of Botany, University of Sofia St. Kl. Ohridski, Sofia, Bulgaria § Department of Earth & Environmental Sciences, KU Leuven, Leuven, Belgium SUMMARY 1. We report a dataset of phytoplankton in the Congo River, acquired along a 1700-km stretch in the mainstem during high water (HW, December 2013) and falling water (FW, June 2014). Samples for phytoplankton analysis were collected in the main river, in tributaries and one lake, and various relevant environmental variables were measured. Phytoplankton biomass and composition were determined by high-performance liquid chromatography analysis of chlorophyll a (Chl a) and marker pigments and by microscopy. Primary production measurements were made using the 13 C incubation technique. In addition, data are also reported from a 19-month regular sampling (bi-monthly) at a fixed station in the mainstem of the upper Congo (at the city of Kisangani). 2. Chl a concentrations differed between the two periods studied: in the mainstem, they varied between 0.07 and 1.77 lgL À1 in HW conditions and between 1.13 and 7.68 lgL À1 in FW conditions. The relative contribution to phytoplankton biomass from tributaries (mostly black waters) and from a few permanent lakes was low, and the main confluences resulted in phytoplankton dilution. Based on marker pigment concentration, green algae (both chlorophytes and streptophytes) dominated in the mainstem in HW, whereas diatoms dominated in FW; cryptophytes and cyanobacteria were more abundant but still relatively low in the FW period, both in the tributaries and in the main channel. 3. Daily integrated production measured in the mainstem (n = 15) varied between 64.3 and 434.1 mg C m À2 day À1 in FW conditions and between 51.5 and 247.6 mg C m À2 day À1 in HW. Phytoplankton biomass in the Congo River mainstem was likely constrained by hydrological factors (accumulation due to increased retention time during FW, dilution by increased discharge during HW), even though increased nutrient availability in the FW period might have also stimulated phytoplankton production. 4. In contrast to other tropical river systems where connectivity with the floodplain and the presence of natural lakes and man-made reservoirs play a prominent role in the recruitment of phytoplankton to the main river, our results show that phytoplankton growth in the Congo River can take place in the main channel, with hydrological processes allowing maintenance of phytoplankton biomass even during HW. Keywords: diatoms, green algae, large tropical river, physical control, potamoplankton Introduction Most studies on potamoplankton, i.e. the assemblage of suspended algae in flowing water, have been conducted in temperate lowland rivers, starting at the end of the 19th century with the first comprehensive reports on the presence and often substantial amounts of suspended algae in river waters (review by Welch, 1952). These early reports highlighted the role of physical factors in controlling growth and losses of algae during down- stream transport, a view which was largely confirmed afterwards (e.g. Reynolds, 1988; Reynolds & Descy, Correspondence: Jean-Pierre Descy, Professor emeritus, University of Namur, Belgium. E-mail: jpdescy@unamur.be © 2016 John Wiley & Sons Ltd 1 Freshwater Biology (2016) doi:10.1111/fwb.12851