Is Lake Tanganyika in trouble?

Andreas Wilson-Späth

Lake Tanganyika is magnificent. Stretching for 676km from north to south, it’s the longest freshwater lake in the world. It’s also the second deepest (with a maximum depth of 1470m), and the second most voluminous (it holds about 18 900 cubic kilometres of water), surpassed in those categories only by Russia’s Lake Baikal.

Bordered by the DRC, Burundi, Tanzania and Zambia, Lake Tanganyika contains some 16 to 18% of the planet’s available fresh water. What’s more, it also hosts some of the most varied aquatic ecosystems anywhere.

Recognised as an important global biodiversity hotspot, it provides a home for at least 1 500 different species, some 600 of which occur nowhere else. Among the many different kinds of creatures are 250-odd species of cichlid fish, 75 other types of fish, as well as large numbers of snails, bivalves, crustaceans (including shrimps and crabs), sponges, worms, leeches and more.

This ancient body of water is of vital significance for the growing human population along its shores. An estimated 100 000 people in the region are involved in fishing and for about a million locals 25 to 60% of the protein in their daily diet is derived from fish caught in the lake. The lake helps to support the livelihood of over 10 million people and plays an important role in the economies of the surrounding countries.

For some time now the health of Lake Tanganyika has been under increasing pressure from a variety of human activities in and around it, including deforestation, invasive species, pollution and agriculture. The overexploitation of its fish stocks by local fisher folks has been considered as a major threat to its biodiversity.

It’s becoming increasingly clear, however, that global climate change – caused mainly by the burning of oil, coal and natural gas thousands of kilometres away – are to blame for the lake’s dwindling biological productivity and biodiversity.

In June, a team of US scientists published a paper that confirms this theory. By looking at paleoclimate data and historic temperature measurements they found that the water temperature of the lake has increased steadily over the last century and a half, and that the water column has become more and more stratified (i.e. separated into distinct layers). This lack of mixing of the water has very significant effects on the plants and animals that live in it.

Normally, the oxygen-rich surface water of the lake decreases in temperature when air temperatures drop during the changing of the seasons. This cooler surface water thus becomes denser and starts to sink, pushing up water from the bottom of the lake to replace it. The deeper water contains lots of nutrients and mixing it with shallower water provides much needed food for algae which in turn feed the animals inhabiting the lake.

Not only does the lack of water mixing lower the productivity of the shallower reaches by depriving them of nutrients, but it also stops oxygenated surface water from reaching the lakebed, making less of it liveable for bottom-dwelling species.

By disrupting this regular turnover of the water column, preventing nutrients to reach the top and oxygen from getting to the bottom, global warming is essentially threatening to shut down the lifecycle of the lake and consequently endangering its biodiversity and the sustainability of the human settlements that depend on it.

It’s not the first time that researchers have warned of this problem. Earlier studies have shown that the mixing of the water was being impeded by warmer air temperatures as well as decreasing speeds of the winds that aid in the process, concluding “that climate warming is diminishing productivity in Lake Tanganyika”. More recent work simply adds weight to these findings.

This is just one more illustration of the fact that the effects of climate change have no boundaries. Rising temperatures caused by greenhouse gas emissions from the developed world – as well as by us here in South Africa – have direct and detrimental impacts on natural environments and the lives of people in distant countries and on different continents.

- Andreas is a freelance writer with a PhD in geochemistry. Follow him on Twitter: @Andreas_Spath

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