From: Andy Soos, ENN
Published May 17, 2010 03:09 PM

Lake Tanganyika is Getting Hot

Lake Tanganyika is an African Great Lake. It is estimated to be the second or third largest freshwater lake in the world by volume, and the second deepest, after Lake Baikal in Siberia. It is situated in the Great Rift Valley. Geologists led by Brown University have determined the east African rift lake has experienced unprecedented warming during the last century, and its surface waters are the warmest on record. That finding is important, the scientists write in the journal Nature Geoscience, because the warm surface waters likely will affect fish stocks upon which millions of people in the region depend.

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The Lake lies in the East African Rift which is an active continental rift zone in eastern Africa that appears to be a section of Africa splitting off from the rest of Africa. The Rift runs from the Afar Triple Junction in the Afar Depression southward through eastern Africa and the Lake.

It is estimated that 25—40% of the protein in the diet of the approximately one million people living around the lake comes from lake fish. Currently, there are around 100,000 people directly involved in the fisheries operating from almost 800 sites. Lake Tanganyika is bordered by Burundi, the Democratic Republic of Congo, Tanzania, and Zambia which are among the poorest countries in the world.

The research team took core samples from the lake bed that laid out a 1,500-year history of the lake's surface temperature. The data showed the lake's surface temperature, 26 degrees Celsius (78.8°F), last measured in 2003, is the warmest the lake has been for 1500 years. The team also documented that Lake Tanganyika experienced its biggest temperature change in the 20th century, which has affected its ecosystem that relies upon the natural conveyance of nutrients from the depths to jump start the food chain upon which the fish survive.

"Our data show a consistent relationship between lake surface temperature and productivity (such as fish stocks)," said Jessica Tierney, a Brown graduate student who this spring earned her Ph.D. and is the paper's lead author. "As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the [fishing] industry."

Lake Tanganyika, one of the richest freshwater ecosystems in the world, is divided into two
general levels. Most of the animal species live in the upper 100 meters, including the valuable sardines. Below that, the lake holds less and less oxygen, and at certain depths, it is anoxic, meaning it has no oxygen at all. What this all means is the lake is highly stratified and depends on the wind to churn the waters and send nutrients from the depths toward the surface as food for algae, which supports the entire food web of the lake. But as Lake Tanganyika warms, the mixing of waters is lessened, the scientists find, meaning less nutrients are funneled from the depths toward the surface. Worse, more warming at the surface magnifies the difference in density between the two levels; even more wind is needed to churn the waters enough to ferry the nutrients toward the fish dwelling upper layer.

"The people throughout south central Africa depend on the fish from Lake Tanganyika as a crucial source of protein," noted Cohen, an author on the paper. "This resource is likely threatened by the lake's unprecedented warming since the late 19th century and the associated loss of lake productivity."

Climate change models show a general warming in the region, which, if accurate, would cause even greater warming of the Lake Tanganyika's surface waters and more stratification in the lake as a whole. "So, as you move forward, you can imagine that density gradient increasing," said Russell, an author on the paper.

Some researchers have posited that the declining fish stocks in Lake Tanganyika can be attributed mainly to overfishing, and Tierney and Russell say that may be a reason. But they note that the warming in the lake, and the lessened mixing of critical nutrients is exacerbating the fish stock decline, if not causing it in the first place. 

Climate change is not just the air temperature going higher. Water temperatures can increase too and in the case of Lake Tanganyika the effects can be quite dramatic,

For further information: http://www.sciencedaily.com/releases/2010/05/100516195655.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29&utm_content=Google+Reader or   http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo865.html   

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