Lake Tanganyika is at its warmest temperature for 1500 years, threatening both the biodiversity within its waters, as well as the millions of people whose livelihoods depend on it.
This dire warning was made in a paper to be published in an upcoming issue of the journal Nature Geoscience. The study, conducted by Jessica Tierney and coauthors, examined samples of Lake Tanganyika's sediment cores to reconstruct lake surface temperatures and biological productivity for the last 1500 years.
The results indicated that the current lake surface temperatures of about 26°C (last measured in 2003) is the warmest in the last one-and-a-half millennia, and that the largest temperature changes (upwards) have occurred within the last century (at a rate of about 2°C). This suggested that the recent anomalous trend in the lake surface temperature is a response to the production of anthropogenic greenhouse gases.
The warm temperatures spell bad news for the approximately 10 million people living in the vicinity of the lake that depend on it for water and food (particularly the lake sardine fishery), as the researchers found a negative correlation between biological productivity of the lake and surface temperatures for the last 1500 years.
As with many large standing bodies of water, Lake Tanganyika is highly stratified, with the first 100 metres or so having enough oxygen to support life. The lake depends on winds to churn up nutrients from the anoxic depths of the lake for the phytoplankton living on the surface, which anchors the entire ecosystem of the lake.
With increasing temperatures comes increasing stratification, bringing less nutrients from the bottom of the lake. To compound matters, the density differential between the surface waters and deeper waters also increases with increasing surface temperatures, requiring stronger winds to churn up the nutrients from the depths of the lake.
"The people throughout southcentral Africa depend on the fish from Lake Tanganyika as a crucial source of protein," notes Andrew Cohen, one of the authors of the study. "This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."
Although some researchers have blamed increased fishing intensity for the decline in fish stocks within the lake, the authors note that the temperature-induced decline in primary productivity will exacerbate this drop, even if it is not the causal factor.
For more information, see the paper: Tierney, JE, MT Mayes, N Meyer, C Johnson, PW Swarzenski, AS Cohen and JM Russell (2010) Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500. Nature Geoscience, doi:10.1038/ngeo865