Impact of mixing on the seasonal variations of productivity and phytoplankton communities of Lake Edward (East Africa)

In this study, we conducted monthly sampling from January 2017 to October 2018 in the pelagic zone of Lake Edward, Uganda to determine seasonal variability in environmental characteristics, and changes in phytoplankton composition and abundance expected from seasonal changes. The phytoplankton was examined using marker pigment analysis by high performance liquid chromatography (HPLC) and subsequent CHEMTAX processing for estimating abundance of phytoplankton groups. In addition, microscopy was used to identify the dominant taxa. Two mixing events were observed in the dry season (July to September) of 2017 and 2018, with temperature of about 26 – 26.5°C along a total depth of 20 m. This had a strong influence on the primary production signal of the lake with chlorophyll a (Chla) as high as 20 µg/l in 2017, with cyanobacteria ranging from 10 to 14 µg Chla /l, diatoms 4-6 µg Chla/l and green algae <3 µg Chla/l. This was also observed in the overall higher δ13C-POC and lower δ13C-DIC, showing uptake of dissolved inorganic carbon (DIC) for primary production. The relatively high C:N ratio (~10±1.9) indicate a moderate N limitation in the pelagic zone. However, these observations were only true for 2017. In the dry season of 2018 this influence was not clear with chlorophyll a barely higher than 10 µg/l despite the strong complete mixing. A multivariate analysis (RDA) was applied to identify the main environmental variables involved in the success of cyanobacteria and diatoms, broadly confirming the conclusions of a previous study in Lake Edward.