Africa, a continent beset by poverty, civil wars and a catastrophic AIDS epidemic, now has one more worry - global warming. A team of researchers, led by University of Arizona geosciences Professor Andrew Cohen, has determined that regional climate shifts paralleling global warming patterns have caused fish yields from Africa's Lake Tanganyika to drop by about 30 percent over the past 80 years, and could produce further declines in the future. The findings appear in the Aug. 14, 2003 issue of the journal Nature.
Lake Tanganyika is one of the largest lakes in the world, holding about 18 percent of the Earth's liquid fresh water. Its rich fish population provides between 25 and 40 percent of the animal protein supply, as well as significant revenue for residents of its bordering nations of Burundi, Tanzania, Zambia and the Democratic Republic of Congo.
The lake is so large that it has its own "water climate" governed by natural forces similar to those that control atmospheric weather patterns, Cohen explains. Because cool water is heavier than warm water, it sinks, creating a thick, cool-water layer in the deeper parts of the lake. Other large lakes also develop thermal layers, but in lakes located in temperate regions of the world seasonal variations cause dramatic mixing between the warm and cool layers, stirring up the nutrients that accumulate in the cool deep layer. Lying near the equator, Lake Tanganyika experiences only minor seasonal fluctuations, so its water layers remain remarkably stable, effectively trapping most of its critical nutrients in the lake's depths.
During the mild winter season the cool and warm water layers in Lake Tanganyika do experience some mixing, which keeps algae and phytoplankton in the surface waters supplied with the crucial nutrients they need to flourish and support the food chain. Lower air temperatures cool the lake's surface waters, causing them to sink and stir the nutrients upward from the lake bottom. Exerting even more influence on water circulation in Lake Tanganyika are the strong winter winds. The southerly winds generate waves that help mix the water layers as surface water moving north is replaced by deep, nutrient rich water from below. And the winds' persistent bearing down on the water surface piles water up on the north end of the gigantic lake. When the winds die down in the spring, Cohen explains, gravity forces the water back, producing sloshing waves that further aid in mixing.
Cohen and his research team - UA geosciences graduate students Catherine M. O'Reilly (the lead author on the Nature article and now an assistant professor at Vassar College) and Simone R. Alin (now a post-doctoral fellow at the University of Washington), and colleagues from Louisiana and Belgium - report that the upper-water temperatures in Lake Tanganyika have risen steadily by one-tenth of a degree Celsius per decade since 1913. While this small temperature change may seem barely noticeable, Cohen says it can have drastic effects on fluid dynamics in the lake because as the water warms, the more difficult it becomes to mix with the cooler water below. "Think about how hard it is when you make a bath too hot to cool it down by turning up the cold water. The colder water just sinks rather than mixing. When you are dealing with a big body of water, it is really hard to mix below the zone where the direct influence of wind operates."
Compounding the problem, the winter winds over Lake Tanganyika have mysteriously diminished by 30 percent since the late 1970s. The combined effect of warmer waters and calmer winds has been to reduce the mixing between water layers. And that, say the researchers, has hampered nutrients from entering the surface waters from the deeper cool layer, leading in turn to decreased populations of algae and phytoplankton, and ultimately, dwindling fish populations.
To reach these ominous conclusions, Cohen's team analyzed historical records of water mixing in addition to their own recent measurements. Water mixing is apparent by measuring the amount of dissolved oxygen at different water depths. Since most oxygen enters the water from the atmosphere, the more mixing, the deeper the oxygen penetrates into the lake. The researchers found that this oxygenated zone has been getting shallower at the alarming rate of more than one-and-a-half meters per year since 1946.
To assess the history of algal growth in Lake Tanganyika, the researchers analyzed the chemical composition of core samples of sediments buried at the bottom of the lake. The sediments contain organic material from decayed algae and phytoplankton. Its chemical makeup at different core depths indicates how productive the algal growth in the lake was during different time periods. The analysis indicated that algal and phytoplankton growth diminished by about 20 percent over the past century, explaining declining fish yields.
Have the people in the region felt the decline of their natural resource? It's difficult to tell, says Cohen. "The situation is horribly complicated by the civil unrest in the region, especially in Burundi and Congo, which have completely altered shipping, fishing and trade practices. I suspect we won't have a clear answer to this until the fighting in both countries subsides.
The research was supported in part by the UA Nyanza Project, a research and education program funded through a $1 million grant from the National Science Foundation. Major support also came from the United Nations Lake Tanganyika Biodiversity Project.