By Louise Kim and Jessica Savage
Medill Reports, Dec. 23, 2023
Understanding how climate change is impacting some of the world’s largest lakes sheds light on how critical needs for livelihood and water could be altered. Tumaini Kamulali, a PhD candidate in the Department of Geosciences at the University of Arizona, led a reteam of researchers in a project that models circulation patterns of Lake Tanganyika, the world’s longest lake that has a volume greater than Lake Superior. Kamulali’s models can help explain how climate is changing circulation and, in turn, how now endless summers are harming the surrounding villages’ chances of catching fish.
Lake Tanganyika is an essential source of life for the region. It’s the second largest lake in the world by volume and the second deepest. The lake reaches depths of 1,470 meters, with an average depth of 570 meters. It’s also the second oldest lake – it formed between nine and 12 million years ago, according to the African Center for Aquatic Research and Education.
More than 10 million people depend on the lake for food and for their livelihoods, and the fish offer 40% of the community’s protein intake.
Because the lake and the fish are so vulnerable to overheating, the lives of millions of people are hanging in the balance.
“The people who depend on them [the lakes] for fisheries are way more than the people who are living in California,” he said. The other two lakes in the region are Lake Victoria and Lake Malawi. “So what does that mean if climate change affects such lakes?”
According to Kamulali, catch in this lake has been decreasing, despite increasing efforts to reel in fish. Fishing effort is up 300% while catch has decreased by 80%, according to the team’s research. This indicates there’s less fish in the lake.
While much of the globe is experiencing devastating impacts of climate change, the especially large Lake Tanganika is heating up due to what researchers call an endless summer effect.
Some people have pointed to overfishing as the problem, but Kamulali and his team think there’s something else.
“This decline actually started before even people started fishing, right before industrial fishing,” he said. “The problem seems to be climate change.”
The lake is experiencing an endless summer effect, according to Kamulali, which means it’s warm all year round.
Lake Tanganyika is reliant on wind-driven air circulation, but rising air temperature since 1960 has been continually heating up the lake.
Typically, during the Southern Hemisphere winter, winds are strong, blowing from south to north. As a result, warm surface waters are pushed from the south to the north. Upwelling happens in the south, and downwelling happens in the north. When winds slacken, secondary upwelling occurs in the north.
“So it used to be warm for some part of the year,” Kamulali said. “And then the other part of the year, during the Southern Hemisphere winter, it would mix. Then, nutrients come out most efficiently. But now, it’s more like it’s an endless summer. It’s just too hot.”
Whenever upwelling occurs, nutrients come up and feed the phytoplankton. When temperatures increase, the warm layer of the lake becomes thicker, and it becomes more difficult for winds to move and mix the layers of the lake to generate productivity. Lake Tanganyika is unique in its deepness. Very strong winds are essential for the nutrients, which are far down in the lake’s depths, to mix properly.
Lake Tanganyika is enormous and extremely difficult to capture in its entirety with observational data, but with computer modeling and analysis, Kamulali was able to show the circulation of this lake.
The model showed that productivity in Lake Tanganyika heavily relies on weather, including wind and air temperature. With temperatures rising globally, this could have serious implications for residents who depend on the lake for food.
Kamulali worked for 8 years in the lake, conducting research with the Tanzania Research Institute and interviewing the lake’s fishermen. He traveled the lake’s entire 420-mile length a dozen times, translating for other English-speaking researchers and collecting data.
When the fishermen saw Kamulali and his team coming in the government boat, they asked him what was happening to the fish.
“‘What can you tell us? We’re not getting any fish.’” Kamulali said of his interactions with the fishermen. “And then we are like, ‘We are trying to find a reason, but we also don’t know. We just have an idea.’”
The communities must adapt to the changing conditions, and The Nature Conservancy is one organization that is funding part of the solution for this biodiversity hotspot.
The Nature Conservancy, the Tanzania Research Institute, and fishermen are working together as collaborators to support the communities that depend on the lake’s fish.
Researchers show fishermen particular spots where the species breed, and have installed buoys around these critical areas. They also rely on the fishermen’s own expertise of the lake, from years of working out on its waters.
“Then the fishermen themselves have what they call beach management units,” Kamulali said. “So the fishermen within a community are in charge of protecting that place.”
Fishermen are paid for their work installing buoys and for participating in the management units.
Kamulali hopes that the combined knowledge of both the fishermen and the researchers will direct rehabilitating the lake. Each group is deeply connected to the ecosystem and they both have much to offer for Lake Tanganyika’s future.
Caption for photo at top: Researcher Tumaini Kamulali and fisherman install buoys to demarcate fish breeding sites as “no fishing zones” and protect Lake Tanganika’s fishery, depleted by climate change.