Limnological changes in Lake Victoria since the mid-20th century

1. Lake Victoria experienced a strong degradation of water quality between the 1960s and the 1990s and, as a consequence of eutrophication, the dominant phytoplankton group changed from diatoms to N2-fixing cyanobacteria and there was a 2- to 10-fold increase in Chlorophyll-a. The goal of this study is to determine whether the 2018-2019 physical- (light, stratification) and ecological- (nutrient, Chlorophyll-a, phytoplankton composition) conditions in Lake Victoria changed from the 1990s.
2. Samples were collected in 2018-2019 in nearshore and offshore waters (Uganda), during three contrasting seasons: heavy rains (March), low rains (October) and dry (June), which corresponded to distinct water column mixing regimes, respectively, late-stratified, early-stratified and mixed regimes. At each station (48 nearshore and 25 offshore), we measured vertical profiles of temperature, oxygen, phytoplankton biomass and composition, inorganic nutrients, and particulate organic carbon, particulate nitrogen (PN) and phosphorus.
3. Chlorophyll-a concentrations in 2018-2019 were 10.3±7.1 and 2.8±1.1 µg L-1 in the nearshore and offshore surface waters, respectively, close to those measured in the 1960s before eutrophication, but distinctly lower than those measured in the 1990s (71±100 and 14±6 µg L-1). The phytoplankton of Lake Victoria in 2018-2019 still appears dominated by diatoms and cyanobacteria, but diatoms develop better in stratified conditions whereas cyanobacteria develop better in mixing conditions in comparison with the historical observations. Accordingly, we observed more non-heterocystous filamentous and coccal/colonial cyanobacteria taxa that are better adapted to mixing conditions than gas-vacuolated heterocystous taxa, which were dominant in the 1990s. PN in 2018-2019 decreased significantly compared to the 1990s, indicative of less efficient nitrogen (N) fixation. The dissolved silica (DSi) concentrations in 2018-2019 increased significantly with the concomitant reappearance of Aulacoseira spp., which was not observed in the 1990s, presumably due to low DSi concentrations.
4. As data from long-term monitoring are missing, the reasons for the lower Chlorophyll-a concentrations in 2018-2019 compared to the 1990s are unclear. Bottom-up (nutrients) and top-down (grazing) controls on Chlorophyll-a concentrations were both unlikely. Therefore, climatic controls appear (El Niño/ La Niña conditions) as an important factor to understand the historical trend in Chlorophyll-a. Higher wind in 2018-2019 promoted vertical mixing resulting in a deeper thermocline and surface mixed layers, which eventually lowered phytoplankton production in comparison to the 1990s. In contrast, the thermocline and surface mixed layers in the 1990s were shallower, enabling phytoplankton to stay suspended in the upper well illuminated water, allowing greater productivity. The lake in 2018-2019 is still in conditions of phosphorus (P) saturation, which suggests that another episode of high Chlorophyll-a concentrations could develop if less windy conditions occur in future, as during the 1980s and the 1990s, or if continued warming of surface waters eventually overcomes the mixing from present windy conditions.
5. This study gives insights about the present ecological functioning of the lake and stresses the impacts of variations in climate on lake physics that changes the light environment for phytoplankton. We also provide a biogeochemical data set, which could contribute to a baseline survey for following the ecological changes in Lake Victoria.