This old lake
Lake Tanganyika is the world’s longest lake and at 10 million years, one of the oldest. Straddling four countries in East and Central Africa, Tanganyika spans 418 miles and plunges 1,870 feet deep, making it an important source of freshwater and fish for millions of people.
It so happens that its features also make it a geologic gold mine. Deep in these waters, researchers are finding out more about modern day climate change.
Taking sediment cores from the bottom of the lake, Brown University researcher Jessica Tierney and her colleagues are finding the geological evidence that can give perspective on modern day observations of climate change.
The lake has been getting warmer over the last century and in tandem there has been less mixing of surface water levels with the deeper ones. A drop in phytoplankton biomass – the lake’s primary producers — has likely reverberated up the food chain to impact fish numbers (although overfishing is another culprit). But are these changes part of longer term variability, or can they be linked to modern climate change?
In a paper published in this month’s Nature Geoscience, the team attempted to reconstruct the lake’s ecological and climatic conditions over the past 1,500 years. The old lake has extensive sedimentary deposits, up to five kilometers thick in certain places, making it possible to reconstruct the paleo-climate record for tropical Africa, and much of recent history as well.
Looking at charcoal deposits and biogenic silica (to infer the abundance of phytoplankton), the researchers found periods of warm and cold around lake. Beginning around 1900, lake surface temperatures have increased 3.6 degrees F, an unprecedented rate in the past 1500 years and “uncharacteristic of the preceding natural variability.”
In other words, the lake’s changes are the result of climate change. Warm surface waters have decreased the mixing of the water columns, thereby locking down nutrients and impacting the productivity of phytoplankton. The researchers write:
“Apart from fishing intensity, the present decline in primary productivity is likely to further impact the clupeid (sardine) fishery, with potentially dire implications for the communities and the regional economy that depend on it.”
The lake supports some 200,000 tons of sardine catch a year. With productivity dropping, climate change could take away a primary source of protein and employment to the lake’s people.
– Alison Hawkes