Lake Victoria is the source of the Nile and is the largest tropical lake in the world.
With a surface area of 68,800 km² it is considered to be one of the largest water and fishery resources in East Africa, supporting more than 47 million people in the three neighbouring countries (Uganda, Tanzania and Kenya).
In 2004 a documentary, Darwin’s Nightmare, highlighted the environmental and social effects of the Nile perch fishing industry. The Nile perch is the largest freshwater fish and was introduced into Lake Victoria in the 1950s. By a decade it had wiped out the native fish species living in the lake, causing a major ecological disaster.
Today, the Nile perch population has declined slightly due to overfishing, allowing some species to partially recover.
Unfortunately there are other problems- the water quality of Lake Victoria caused by eutrophication.
Alberto Borges, FNRS Research Director at the Laboratory of Chemical Oceanography from the University of Liege explained that eutrophication:
” is caused by increased inputs of nutrients (nitrogen and phosphorus) into the water bodies (rivers and lakes) as a result of increased human activities in the catchment area (intensive agriculture with fertilisers or domestic wastewater) resulting from population growth and economic development.”
This eutrophication leads to a significant development of micro-algae (phytoplankton).
Alberto Borges and his research team was able to study the biomass and composition of phytoplankton as well as the nutrient status of the lake during three scientific missions (2018 -2019). This study shows that the phytoplankton biomass has decreased by about seven times compared to the 1990s, and that the species composition has also changed in a subtle way. What seems to be good news for the environment of Lake Victoria may only be so on the surface.
In addition to nutrients, phytoplankton (like all plants) also need light to grow. In lakes, the amount of light for phytoplankton obviously depends on the solar radiation at the surface of the lake, but also on the depth of the water on which the phytoplankton cells reside.
This depth, known as the mixing layer, depends mainly on the intensity of the wind. If the wind is intense, the depth of the mixing layer is greater, and the phytoplankton cells spend less time near the surface where the light is more intense, and do not develop as well.
Alberto Borges continued: “Our work shows that current weather conditions are windier than in the 1990s, so the depth of the mixed layer is greater and phytoplankton growth less intense than in the 1990s.
” The weaker winds of the 1990s were related to the prevailing conditions of El Niño, a natural oscillation in global climate that originates from the large-scale atmospheric circulation over the Pacific Ocean and affects climate worldwide.”
This rather complex story shows that the established climate regime in the Pacific Ocean (El Niño) affects the ecology of a lake in Africa, on the other side of the planet.
More specifically, it shows that the growth of phytoplankton – and therefore the rest of the food chain – in large tropical lakes responds to eutrophication in a complex way and is strongly modulated by climate.
“This means that the current improvement in water quality in Lake Victoria may only be temporary, and that conditions could deteriorate again in the future if vertical mixing in the lake decreases due to reduced wind intensity (a new period of prevailing El Niño conditions) or due to continued climate warming, added Alberto Borges .