Phytoplankton dynamics in the Congo River

Abstract

Summary 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 13C 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). Chl a concentrations differed between the two periods studied: in the mainstem, they varied between 0.07 and 1.77 μg L−1 in HW conditions and between 1.13 and 7.68 μg L−1 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. Daily integrated production measured in the mainstem (n = 15) varied between 64.3 and 434.1 mg C m−2 day−1 in FW conditions and between 51.5 and 247.6 mg C m−2 day−1 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. 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.

Evaluation of Primary Production in the Lower Amazon River Based on a Dissolved Oxygen Stable Isotopic Mass Balance

“…Our mass balance results are in agreement with observations of a significant abundance of phytoplankton biomarkers in the mainstem of the Amazon (Mortillaro et al, 2011). Likewise, primary production has recently been found to be important in the main channel of the Congo River (Descy et al, 2017).…”

“…However, several studies have more recently found evidence for a higher relative contribution of primary production (and subsequent breakdown of algal OM) to ecosystem metabolism in the Amazon River based on the stable isotopic signature of dissolved oxygen (δ 18 O-O 2 ) and the presence of algal biomarkers in the mainstem (Quay et al, 1995;Mortillaro et al, 2011;Ellis et al, 2012). Similar observations of high rates of primary production and algal abundance have also been made in the mainstem of the Congo River, a similar large tropical system (Descy et al, 2017).…”

Carbon dynamics and CO<sub>2</sub> and CH<sub>4</sub> outgassing in the Mekong delta

“…Similarly, the correlation between pCO 2 and δ 13 C-DIC (Fig. 4) (Reynolds and Descy, 1996), as also observed in other tropical rivers (for example Descy et al, 2017). Phytoplankton development during low water was also reported in the Upper Mekong River (confluence with the Tonle Sap River) by Ellis et al (2012), based on elemental and lignin analyses.…”

Carbon dynamics in the Mekong Delta

“…phytoplankton development during low water, probably related to an increase of water residence time related to low freshwater discharge (Reynolds and Descy, 1996), as also observed in other tropical rivers (for example Descy et al, 2017).…”