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 (N), and phosphorus (P).
3. 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.
4. 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.
5. 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.

中文翻译：

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20 世纪中叶以来维多利亚湖湖沼学变化

1. 维多利亚湖经历1960和1990之间和水质的强降解，富营养化的结果，从改变硅藻到N的主导浮游植物组₂ -定影蓝藻和有在叶绿素2至10倍的增加一个。本研究的目的是确定维多利亚湖2018-2019 年的物理（光、分层）和生态（营养、叶绿素a、浮游植物组成）条件是否自 1990 年代以来发生了变化。
2. 样本于 2018-2019 年在近岸和近海水域（乌干达）收集，在三个截然不同的季节：大雨（3 月）、低雨（10 月）和干旱（6 月），分别对应于不同的水柱混合状况，晚分层、早分层和混合政体。在每个站点（近岸 48 个和近海 25 个），我们测量了温度、氧气、浮游植物生物量和组成、无机营养物、颗粒有机碳、颗粒氮 (N) 和磷 (P) 的垂直剖面。
3. 2018-2019 年近岸和近海地表水的叶绿素a浓度分别为 10.3 ± 7.1 和 2.8 ± 1.1 µg/L，接近富营养化前 1960 年代的测量值，但明显低于 1990 年代的测量值（71 ± 100 和 14 ± 6 µg/L）。2018-2019年维多利亚湖浮游植物仍以硅藻和蓝藻为主。然而，我们观察到更多的非异囊菌丝状和球菌/菌落蓝藻类群比在 1990 年代占主导地位的气体空泡异囊菌类群更适应混合条件。2018-2019 年的颗粒氮显着低于 1990 年代，表明固氮效率较低。2018-2019 年溶解的二氧化硅浓度显着更高Aulacoseira spp.，在 1990 年代未观察到，可能是由于溶解的二氧化硅浓度低。
4. 由于缺乏长期监测数据，与 1990 年代相比，2018-2019 年叶绿素a浓度较低的原因尚不清楚。然而，气候控制（厄尔尼诺/拉尼娜条件）可能是影响叶绿素a历史趋势的重要因素。2018-2019 年更高的风促进了垂直混合，导致更深的温跃层和地表混合层，最终与 1990 年代相比降低了浮游植物的产量。相比之下，1990 年代的温跃层和地表混合层较浅，使浮游植物能够悬浮在光照良好的上层水中，从而提高生产力。在2018年至2019年的湖方面依然是P饱和，这表明高的叶绿素另一个情节一个 如果未来风力条件变小，或者地表水的持续变暖最终克服了当前多风条件的混合，则浓度可能会增加。
5. 这项研究提供了关于维多利亚湖当前生态功能的见解，并强调了气候变化对改变浮游植物光环境的湖泊物理的影响。由于新的厄尔尼诺现象或气候变化，未来可能出现少风期，这可能会导致维多利亚湖再次发生富营养化。由于在 2018-2019 年，湖泊仍被营养物质饱和，因此需要降低营养物质浓度（尤其是 P），以防止未来因混合减少而造成的破坏性富营养化时期。