Why do we study Lake Tanganyika?
Lake Tanganyika is the world’s longest freshwater lake, with a north-south expansion of 673 km, covering the distance between Zurich and Berlin, and it sustains the second largest inland fishery of the African continent. Unfortunately, its fish stocks are declining, and previous investigations have identified overfishing and climate change as the main causes. Hence, Lake Tanganyika provides the opportunity to study direct (overfishing) as well as more subtle, global human alterations (climate change) on a large lake ecosystem.
The nutrient dynamics are essential for fish productivity
By heating up the surface waters, global warming, enforces the stable layering of the water column (stratification). The strong thermal layers reduce the vertical transport of nutrients from the deep waters into the nutrient-poor, sunlit surface waters. The nutrient scarcity slows down algal growth, which in turn has implications on the entire food web. In an aquatic food web, all higher organisms depend on the biomass produced by algae and bacteria. Thus, nutrients in the productive surface zone means less algae and ultimately less fish.
But there might be a hero to the story: nitrogen-fixing cyanobacteria. These organisms have the remarkable capability to fix nitrogen from the atmosphere (N2), while other algae need to rely on dissolved inorganic nitrogen (DIN) for growth (Fig. 1). Because nitrogen is the limiting nutrient in Lake Tanganyika, nitrogen-fixing cyanobacteria may compensate for the reduced DIN transport into the surface zone and help maintain an equally productive algal and bacterial community at the base of the food web.