Lake Tanganyika Heating Up, Warmest In 1,500 Years
Lake Tanganyika Heating Up, Warmest In 1,500 Years
Lake Tanganyika in east Africa is getting warmer, say geologists from Brown University. Reporting this week in the journalNature Geoscience, the researchers say the lake has experienced unprecedented warming, rising 2 degrees over the past 90 years, making it the warmest it has been in 1,500 years. Warming surface water will affect levels of phytoplankton upon which fish depend, as it has in the past, the researchers say. Jessica Tierney, lead scientist on the project and currently a post-doctoral researcher with National Oceanographic and Atmospheric Administration’s Climate and Global Change program, analyzed core samples from the lake floor to deduce lake surface temperatures over the past 1,500 years.
Lake temperatures have fluctuated in the past but this warming is unprecedented. “The warmest it’s ever gotten was 24.3 degrees Celsius. It’s never happened to the degree it is happening now,” says Tierney. Lake Tanganyika last reached 24.3 degrees Celsius during a warm period between 600 and 900 years ago. It is currently 26 degrees Celsius, or 78.8 degrees Fahrenheit.
To track the temperature change, Tierney and her colleagues looked in the sediment for molecules called glycerol dialkyl glycerol tetraether (GDGTs). These molecules come from Archaea, ancient single-celled organisms that change their membranes in response to the water temperature. This proxy – called a paleothermometer – has been used to reconstruct ocean surface temperatures and was within tenths of a degree when compared to direct temperature readings of Lake Tanganyika.
In addition to reconstructing lake temperatures, the researchers used the core samples to infer past levels of phytoplankton. Diatoms, a common phytoplankton, encase themselves in silica shells and when they die the shells fall to the lakebed. By measuring levels of biogenic silica – silica from biological sources – the geologists can estimate the levels of phytoplankton across time. Whenever lake surface temperatures rose, the researchers saw phytoplankton levels drop.
Warming surface temperatures make it harder for water below 100 meters to circulate and bring up the nutrients phytoplankton rely on, like nitrogen and phosphorous, says Tierney. Lower phytoplankton levels will hurt Lake Tanganyika’s supply of sardines, sleek, perch and cichlids, already declining due to overfishing.
According to a 2001 report from the Lake Tanganyika Biodiversity Project – an international project run by the nations bordering the lake and advised by international agencies like the UN – over 4 million people depend on the lake for food and water and fish make up a substantial portion of the local diet. The four countries bordering the lake are Burundi, the Democratic Republic of the Congo, Tanzania, and Zambia. Lake Tanganyika is the second largest lake in the world, by volume.
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