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

Abstract 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 N 2 ‐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. 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 (N), and phosphorus (P). Chlorophyll‐ a concentrations in 2018–2019 were 10.3 ± 7.1 and 2.8 ± 1.1  µg/L 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). The phytoplankton of Lake Victoria in 2018–2019 still appears dominated by diatoms and cyanobacteria. However, 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. Particulate N was significantly lower in 2018–2019 than in the 1990s, indicative of less efficient N fixation. The dissolved silica concentrations in 2018–2019 were significantly higher with the concomitant reappearance of Aulacoseira spp., which was not observed in the 1990s, presumably due to low dissolved silica concentrations. As data from long‐term monitoring are absent, the reasons for the lower chlorophyll‐ a concentrations in 2018–2019 compared to the 1990s are unclear. However, climatic controls (El Niño/La Niña conditions) may be an important factor influencing 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 P saturated, suggesting that another episode of high chlorophyll‐ a concentrations could develop if less windy conditions occur in future, or if continued warming of surface waters eventually overcomes the mixing from present windy conditions. This study gives insights about the present ecological functioning of Lake Victoria and emphasises the impacts of variations in climate on lake physics that changes the light environment for phytoplankton. A possible less windy period in the future resulting from a new El Niño phase or from climate change, will probably lead to another episode of eutrophication in Lake Victoria. As in 2018–2019 the lake was still saturated by nutrients, there is need to reduce the nutrient concentrations (especially P) to prevent future destructive eutrophic periods caused by reduced mixing.