Tropical lakes in East Africa don’t grab headlines the way polar bears do, but climate change is having an effect on them, too. Although the changes are not as visible as melting polar ice caps, they are no less real.
As in many lakes around the world, water temperature is on the rise in Lake Tanganyika. This and other climate-related factors are causing subtle but significant changes that threaten the ecological stability of the lake and the livelihoods of people who depend on it.
With air temperatures across tropical Africa expected to rise as much as 2–5 degrees Celsius (3.5–9 degrees Fahrenheit) over the next 50-100 years, warming lake trends are also expected to continue. The changes underway serve as early warning signs, not just for the lake region, but perhaps, for the planet as a whole.
Considered one of the African Great Lakes, Tanganyika is the world’s second largest lake (by volume) and second deepest, after Russia’s Lake Baikal. It contains 17 percent of the world’s surface freshwater – almost as much water as all five of the North American Great Lakes combined. At more than 10 million years old, Tanganyika is among an elite group of only 20 or so ancient lakes in the world.
Lake Tanganyika also ranks in the top tier of some 250 lakes found to have globally significant freshwater biodiversity.
Four countries border the lake: Burundi, Democratic Republic of Congo (DR Congo), Tanzania, and Zambia. This poses a major challenge for lake conservation and management efforts.
About 10 million people live in the lake’s drainage basin, and the population is growing rapidly (by a rate of 2.5 percent per year), according to a 2006 Brief about the lake. In the nearshore areas of the lake, the rate of growth is nearly double the national average at 4 percent per year.
For the most part, the lake has relatively high quality water, except in a few areas where urban and industrial runoff has affected the lake. This is in part due to the lake’s enormous volume, which acts as a buffer to problems that plague some of the other African Great Lakes, such as overfishing and invasive weed growth on Lake Victoria.
Over-exploitation of the fishery and siltation caused by erosion from deforested areas are considered the main threats to the health of the lake. With increased population pressure, the ongoing problem of siltation, and now climate change added to the mix, fish stocks, biodiversity, and water quality are expected to decline.
Given the importance of Lake Tanganyika, I wondered why we haven’t heard more about the effects on lakes such as this in the climate change dialogue. To learn more, I spoke with two scientists who conduct research on the lake.
The Effects of Climate Change
O’Reilly began studying the lake as part of a team looking at the impacts of deforestation in the Tanganyika watershed, where an astounding 100 percent of the native vegetation has been cleared in the northern portion of the watershed. It might not seem obvious, but if you’re interested in learning more about historical land use changes in the lake’s watershed or about the biological communities that have lived in the lake, a good place to look is in the mud at the bottom of the lake.
The particles in the lake and those washing into it, through rivers, streams, shoreline erosion, and even pollution discharges, eventually settle out to the bottom along with the decomposed remains of aquatic organisms. In a lake as deep as Tanganyika, these particles are essentially locked away in layers of mud. The bottom sediments are like a secure vault, storing the ecological history of the lake and its surrounding watershed. In the case of Lake Tanganyika, this process has been occurring for millions of years, making it a treasure trove of information for scientists to study trends, such as the effects of climate change.
By examining sediment cores taken from the lake’s depths, O’Reilly noticed a chemical signal (the carbon isotope signal) that didn’t seem to make sense. Upon further examination, she discovered that this signal suggested that the lake responded to climate warming. Interestingly enough, the 2003 results of her team’s research were published in Nature simultaneously with another team’s results (Verburg et al.) in Science. They had independently reached the same conclusions using slightly different methods.
Lake Tanganyika is Warming
O’Reilly found that water temperatures in Lake Tanganyika have warmed 0.1 degrees Celsius (0.18 degrees Fahrenheit) per decade or 1 degree Celsius (1.8 degrees Fahrenheit) over for the past 100 years. Not only is this affecting the ecological stability of the lake, it has resulted in a 20 percent reduction in biological productivity in the lake.
Scientists are concerned about how continued warming will affect fish stocks and the lake’s rich biodiversity. Reduced fish catches would impact millions of people living in the lake region, many whom live on less than a dollar a day and depend on the lake for basic human needs – the protein from fish and clean water to drink.
More recent global assessments show that the rate of warming in Lake Tanganyika is consistent with other lakes around the world. Although it is not warming as rapidly as some lakes in the northern hemisphere, O’Reilly told me that even small changes in lake temperature can cause major disruptions in the lake’s ecological stability.
An Ancient Lake with Globally Significant Biodiversity
Peter McIntyre, an aquatic ecology professor at the University of Wisconsin-Madison’s Center for Limnology, spoke with me about why scientists are so concerned about the changing dynamics in Lake Tanganyika. His research focuses on conserving both biodiversity and ecosystem dynamics, and he specializes in freshwater fish.
The lake was formed slowly over millions of years as the East African plate separated from the main African plate, creating a massive rift valley that continues to expand today (though very slowly), McIntyre explained. The lake now sits at the headwaters of the mighty Congo River, which carries the water from the west side of Lake Tanganyika on a journey nearly 3,000 miles (4,700 km) across the African continent to the Atlantic Ocean.
“The emergence of a massive lake in the Upper Congo basin provided a perfect opportunity for all kinds of river animals to diversify into hundreds and hundreds of species endemic to the lake,” says McIntyre. Large numbers of fish, snails, and tiny crustaceans (known as ostracods) flourished. Smaller evolutionary radiations of crabs, shrimp, and leaches also occurred, resulting in a lake with some of the greatest freshwater biological diversity on the planet. There is even an endemic sub-species of aquatic cobra, McIntyre told me.
Surface Water Warming More Rapidly Than Its Depths
The lake is remarkable in other ways, not the least of which is its extreme depth. At almost a mile deep (1.472 km), it is among a special class of lakes that remains permanently stratified. The warmer surface waters never mix completely with the cooler water at depth. Although partial mixing events occur from time to time, these are relatively short-lived.
“Because of the temperature differences, the bottom water is effectively isolated from the surface water,” said McIntyre. He describes the temperature boundary in the lake, typically at a depth of 60 or 70 meters, as acting like a drain for nutrients and energy from the surface. When these materials reach the oxygen-starved bottom of the lake, the difference in temperature between the upper and lower layers acts as a barrier – like oil on water – that inhibits the mixing that could replenish nutrients in brightly-lit surface waters.
Lake Tanganyika happens to have one of the longest temperature records of any lake in the world. A century ago, scientists began measuring temperature profiles to a depth of 1,000 meters using insolated bottles and specialized thermometers. Temperature studies were also conducted in the 1920s, 1940s, 1970s, and then more frequently in the 2000s. Although a sparse record, O’Reilly says that it paints a clear picture.
Nowadays, the technology has greatly advanced. Researchers use fancy instruments to precisely measure the temperature change from the surface all the way to the bottom of the lake. For the past four years, McIntyre and his colleagues have continued to collect temperature profiles of the lake.
From these records, scientists are finding that Lake Tanganyika’s surface waters are warming more rapidly than its depths. This has the effect of creating an even sharper gradient between the upper and lower layers of the lake, and thus creating an even greater barrier to wind-induced mixing.
Small Changes in Temperature Can Have Big Impacts on Tropical Lakes
From the standpoint of lake physics, it turns out that warm, tropical lakes are more sensitive to changes in temperature than lakes in cooler climates. O’Reilly explained that the warmer the water is, the more energy is required to mix it. In a typical northern lake that cools in the fall and winter, the energy from wind is enough to mix the lake. “The amount of energy required to maintain mixing, which is very important for circulating nutrients, is huge in Lake Tanganyika,” she says.
Finding the ideal nutrient concentrations in a lake is a delicate balancing act. With too little, the lake cannot support aquatic life. With too much, the lake becomes overly productive and could develop harmful algae blooms and excessive weed growth. Another unique feature about Lake Tanganyika is that its surface waters are low in nutrients yet support an incredibly productive fishery. This is possible because of an occasional upwelling of nutrient rich water from the bottom of the lake.
McIntyre describes the upwelling as being like a small burp of deep water into the surface. “The burps provide a critical annual injection of nutrients to the nearshore area where most of the lake’s species are found,” he said, “and these species are very good at storing the nutrients until the next upwelling occurs.” He says that researchers are already seeing signs that the amount of mixing has decreased. And help is unlikely to come from the winds; O’Reilly has gathered evidence of decreasing winds in the region.
Is the Lake Approaching a Tipping Point?
McIntyre worries that a reduction in the magnitude and frequency of upwelling events caused by climate-related changes could undercut the stability of the entire ecosystem. “The lake’s incredible biodiversity depends on the high productivity fueled by the annual upwelling of nutrients,” he says. If that aspect of the lake’s annual cycle is lost, the whole system could cross a ‘tipping point’ where the changes become permanent.
O’Reilly expressed grave concerns, too. When analyzing the data, she noticed that the lake has not recovered as well as it used to from more extreme conditions, such as an El Niño event. Rather than returning to its pre-El Niño state, the lake recovers only slightly. “This has the effect of stepping up stairs toward becoming a different kind of lake,” she explains, “and at some point Lake Tanganyika will be forever changed.”
Both O’Reilly and McIntyre are members of the Global Lake Temperature Collaboration. They say more research is needed to determine how the lake’s resilience is being affected and whether it will recover from these step-like changes. Although the science is rapidly developing, they think it will be several years before researchers have the capability, assuming that needed data are available, of predicting how close the lake is to reaching a critical “tipping point.”
For the people living on inland lakes, this sounds like the equivalent of the apocalyptic scenarios for sea level rise and more intense storms we’ve all heard about and begun to see in coastal regions. And yet many sources of funding for research and conservation programs on Lake Tanganyika have fallen off in recent years.
What the Future Holds
By studying Lake Tanganyika, not only do scientists learn more about how climate change is affecting one of the world’s largest, deepest, and oldest lakes, their interdisciplinary research provides tremendous insight into how warming trends affect the entire planet, from the tiniest microscopic organisms and schools of colorful fish to the top of the food chain: we humans ourselves.
The four riparian nations surrounding Lake Tanganyika are committed to taking action, but they sorely lack financial resources to get much done. Government representatives met in February 2012 under the auspices of the Convention on the Sustainable Management of Lake Tanganyika and renewed their commitment to an updated Strategic Action Programme. They also pledged to embark on an international fundraising campaign to secure funds for the conservation and management of the lake’s rich natural resources.
Recognizing the impacts of a warming climate, The Nature Conservancy (TNC) launched a project in 2012 with other local and international partners, which includes establishing effective climate change adaptation strategies and actions in the Lake Tanganyika region. The Tuungane (Kiswahili for “let’s unite”) project uses a participatory approach and is based “on the premise that the most effective way to combat climate change is to empower the local communities to sustainably manage their own resources.”
According to Colin Apse, a senior freshwater adviser at TNC, most of these actions do not involve radical departures from actions people are already taking to improve their livelihoods and economies and to protect the ecosystems they depend on. He says there is a wide range of adaptation strategies available, such as implementing village land use plans, improving fisheries practices, and limiting further habitat destruction. Apse is hopeful that these and other strategies can help the people living around Lake Tanganyika prepare for a rapidly changing future.
Unlike retreating glaciers, there is actually something that can be done to lessen the impacts of climate change in lake regions. By recognizing climate change as a major factor, not only for how it affects the health of the lake but the welfare of the people living around it, researchers and conservationists working on Lake Tanganyika are already leading the way forward.
Lisa Borre is a lake conservationist, freelance writer, and avid sailor based in Annapolis, Maryland. With her husband, she co-founded LakeNet, a world lakes network, and co-wrote a sailing guide called “The Black Sea” based on their voyage around the sea in 2010. She is a native of the Great Lakes region and served as coordinator of the Lake Champlain Basin Program in the 1990s.