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The fate of Africaís Lake Tanganyika lies in the balance
Andrew Cohen, University of Arizona
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Danger of sediment
The biodiversity of Lake Tanganyika can be imagined like a thin bathtub ring. It hugs the shallow zones around a deep and steep bottomed lake, up to 1470m in its deepest parts. The hundreds of species that inhabit the sunlit shallows give way to a dark expanse of water lacking oxygen and, so, animal life. This narrow strip of extraordinary biodiversity is on the front line. Eroded sediments are being carried into the lake, affecting this strip.

Researchers have begun to document where the impact is being felt. They are also looking back in time by collecting sediment cores with fossils of the many endemic animals to see when the impact was first felt. They have found that some heavily populated regions lost much of their diversity more than 150 years ago. Other regions, particularly in the more southerly past of the lake, are seeing these effects unfold only in recent decades.

Other pressures
Excess sedimentation is just one problem. Fishing pressure and climate change are also affecting the lake. Large scale fisheries for the lakeís small sardines started in the 1950s, quickly mushrooming into a major industry. They export up to 200,000 tons of fish per year and make up a very large portion of the average personís animal protein intake in the surrounding regions.

In recent years this fishing yield has declined dramatically. This has been partially caused by the unsustainable growth in fisheries, and exacerbated by large numbers of refugees flooding into the region because of conflicts in Rwanda, Burundi and the DRC during the 1990s.

Itís now increasingly clear that another factor has also been at play. Starting in the early 2000s, scientists began to document that the surface waters of Lake Tanganyika were warming rapidly. This is most likely because of global climate change related to an increase in greenhouse gas emissions. This warming has had serious consequences for the lakeís fragile ecosystems.

Warming lake
Warm water is relatively light and struggles to mix with the deeper layers of the lake. This in turn keeps the vast pools of nutrients from being churned back to the surface by waves. It cuts down on the growth of floating plankton, which is what the lakeís many fish populations eat. Scientists have been able to show that the decline in fish populations began well before the onset of commercial fishery in the 1950s. This implicates climate change and lake warming as the probable cause for much of the fisheryís long-term decline. Unfortunately, this trend is unlikely to be reversed as long as the climate in the region continues to warm.

A related consequence of the reduction of mixing in the lake, is a continuous shallowing of the transition from the oxygenated to deoxygenated waters on the lake floor. This means thereís less of an oxygenated ring, reducing the habitat area within the bathtub ring of biodiversity from below. As if scientists and lake managers at Lake Tanganyika didnít have enough on their plates, a new problem has emerged: the search for oil and gas deposits.

Rift lake sediments of the type found in Lake Tanganyika are well known among geologists as reservoirs of hydrocarbons, as over millions of years vast quantities of plankton have died and settled on the lake floor. The consequences of actual production are still unknown. But the recent record of catastrophic oil spills, for example along the Niger River Delta, highlight the critical need for very careful study and environmental planning before production proceeds in fragile Lake Tanganyika.

The biological and economic riches produced by 10 million years of evolution could lie in the balance.The Conversation

This article was originally published on The Conversation. Read the original article.

Andrew Cohen, University Distinguished Professor Joint Professor, Geosciences and Ecology and Evolutionary Biology, University of Arizona

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