UNIVERSITY PARK, Pa. — Climate change could put a major fishery — and the millions of people who depend on it — at risk in one of the world’s most vulnerable regions, according to researchers.
“Fisheries are important to food security throughout the world,” said Sarah Ivory, assistant professor of geosciences at Penn State. “Developing nations, like those surrounding Lake Tanganyika in eastern Africa, depend on fish as an important protein source. It’s also an important part of their cultural identity.”
Lake Tanganyika is home to small pelagic fish, known locally as dagaa, that feed millions of people in Tanzania, the Democratic Republic of the Congo, Burundi and Zambia. The fish’s conservation is crucial to the food security and economy of rapidly growing and largely impoverished segments of these nations, but certain climatic changes may place the fishery at risk.
Dagaa feed on algae and plankton, which means greater algae production in the lake results in more fish. Understanding how this aquatic food web responds to external forces, like climate, is critical for identifying vulnerabilities and maintaining healthy fish stocks. But until now, limited information existed on how Lake Tanganyika may respond to such forces.
To understand how the lake reacts to climatic changes, the scientists analyzed well-preserved sediment cores taken from the lake for information on the lake’s upwelling — the process by which deep waters rise and fertilize surface waters, thereby increasing algae and photosynthesis. They report their findings today (Oct. 9) in Science Advances.
The chemistry and fossil content of each layer of the sediment cores tell a specific story about how the lake functions, said Michael McGlue, Pioneer Natural Resources Professor of Stratigraphy at the University of Kentucky and lead author of the study.
“Our study revealed that high frequency variability in climate can lead to major disruptions in how the lake’s food web functions,” he said.
Until now, sedimentary records from Lake Tanganyika lacked the resolution needed to accurately measure the influence of frequent climatic events, such as the El Niño Southern Oscillation. Most sedimentary datasets are low resolution, meaning that changes can only be detected over wide intervals of time, such as thousands of years. Conditions within certain areas of Lake Tanganyika converged to provide high temporal resolution of its sediment, which the team was the first to sample.
“We were able to detect changes that were happening in Lake Tanganyika over very short intervals of time (e.g., months or years) using these sediments,” McGlue said. “This is quite rare — and crucial — for using the data to guide fisheries management and conservation practices. Designing effective strategies for fisheries management using low resolution data is a challenge because environmental changes that affect the food web can occur rapidly.”
Statistical analyses of the sediment records revealed cycles of climate processes that resulted in changes to the aquatic communities, said Ivory. The team tied those cycles to climate mechanisms influencing the lake system.
The researchers observed increases in algae production due to high solar irradiance — the amount of energy from the sun that reaches Earth’s atmosphere. The convergence of high solar irradiance and La Niña results in a strong monsoon and upwelling, which increases algae in southern Lake Tanganyika, according to the researchers. In contrast, a monsoon weakened by low solar irradiance and El Niño, as well as warmer surface waters, results in weak or absent upwelling and low algae production.
The sediment samples provided the detail necessary to capture abrupt change associated with climate processes, said Jeffery Stone, an associate professor from Indiana State University and co-author on the paper.
The team said the socioeconomic threat these conditions create for sub-Saharan Africa is the most severe of any region on Earth, but they believe their findings can help guide long-term management practices.
Ismael Kimirei, director-general of the Tanzania Fisheries Research Institute (TAFIRI), said the study’s findings are a critical building block toward research-informed policymaking in the Lake Tanganyika region.
“The importance of fisheries to the food security of the east and central African nations cannot be overemphasized,” Kimirei said. “There is a growing body of research on declining fish production from Lake Tanganyika and other great lakes, which, coupled with the findings of this study and the ever-increasing fishing pressure, paint a gloomy future for the region. Therefore, sustainable fisheries of the lake can be achieved/maintained only if conventional fisheries management marries with ecosystem management and conservation approaches.”
The results make clear that changes in climate can have a cascading effect on the food webs in large tropical lakes, said Ivory.
“The impacts of this are akin to multi-year or multi-decade droughts in agricultural systems, from a food security perspective,” she said.
Andrew Cohen, with the University of Arizona, said the findings have implications beyond tropical lakes, as climate impacts on freshwater resources in the tropics are a bellwether for global change.
Future work needs to take into account how landscape changes as well since changes in vegetation can also alter nutrient and sediments into lakes, added Ivory.
“This work is important because climate changes that affect food security disproportionately hurt the poor,” McGlue said. “This is one way science and social justice can become interwoven.”
Additional researchers included Jennifer Latimer, Indiana State University; Michael Soreghan, University of Oklahoma; McKenzie Brannon, a University of Kentucky alumna who contributed to the study during her time as an undergraduate; and Tumaini Kamulali, a University of Arizona graduate student and researcher from TAFIRI in Tanzania.
The University of Kentucky-Pioneer Endowment, the Society of Exploration Geophysicists/Geoscientists Without Borders and the National Science Foundation supported this research.