For Great Lakes' Sakes, continued
The world’s deepest lakes have formed in geologic faults, or “rifts.” These include Lake Baikal in Siberia, more than a mile deep with a fifth of the world’s freshwater (twice Superior’s volume and four times its depth). Lake Tanganyika, largest of the so-called rift valley lakes in East Africa, is nearly as voluminous. Moreover, these lakes are old—30 million years for Baikal, 20 million for Tanganyika. With a long history of evolution, they’re home to species found nowhere else. Baikal has 2,500 endemic species. Of its 29 species of sculpins, a deepwater fish, 27 are endemic. Baikal even has a unique freshwater seal.
Large North American lakes, scoured out by glaciers, are comparatively shallow (though Great Slave, the deepest of the lot, reaches to more than 2,000 feet). They are also young, dating to the retreat of the big ice scarcely 10,000 years ago. Most are connected by rivers to nearby lakes. As a result, their species, from diatoms to predatory fish such as lake trout, are widespread.
Big differences also exist between northern and tropical lakes. In temperate climes, cooling water sinks toward the bottom as it reaches its greatest density just above freezing temperature, bringing oxygen to the depths and forcing nutrients to the shallows. Tropical lakes stratify—warm water on top and cool dense water below. Wind mixes the layers but never to great depths. As a result, deep water in these lakes is devoid of oxygen and most life.
Valued by Millions
The world’s great lakes provide valuable services to the millions who live nearby. Companies locate on the shores of the North American Great Lakes for a supply of clean water. The lakes have important fisheries (though recreational fishing has far surpassed commercial fishing). They provide low-cost transportation of taconite and grain. They influence the region’s climate, boosting snowfall to the ski hills of Michigan’s Upper Peninsula and moderating the climate of the cherry- and grape-growing Michigan Mitten.
“One of the things about the Great Lakes is that they are used for so many different industries,” says Lynn Vaccaro, coastal research specialist for Michigan Sea Grant in Ann Arbor. In fact, Vaccaro has calculated that more than 1.5 million jobs are directly connected to the North American Great Lakes, generating $62 billion annually in wages. “The Great Lakes are used for so much more than just tourism and fisheries,” she says.
But if the North American Great Lakes are valuable, Africa’s great lakes are imperative to survival. The number of people who live near the African lakes and depend on them for food and employment is staggering. According to the United Nations, more than 30 million live in the Lake Victoria watershed, 10 million near Lake Tanganyika, and more than 10 million in the Lake Malawi catchment. These numbers are growing 2 to 3 percent per year.
And these lakes mean everything to those who live near them. As one example, lakes Malawi (a deep rift lake with more than 1,000 fish species), Chilwa, Chiuta and Malombe provide food, irrigation, recreation and transportation to more than 13 million in Malawi, Tanzania and Mozambique, according to Daniel Jamu, senior scientist at the WorldFish Center in Zomba, Malawi. Fisheries employ 60,000, and an additional 450,000 work in fish processing, distribution and associated trades. Fish from these lakes supply Malawians 40 percent of their total protein.
“Fish is a critical ingredient for nutritional security in Malawi, where human diets are dominated by maize and cassava,” Jamu says.
Feeling the Strain
Big as they are, the world’s large lakes are not invulnerable. Dobiesz and colleagues identified 25 metrics—predator-prey balance, food-chain length, number of exotic species, phytoplankton abundance, pollutants and others—to establish trends and gauge the health of ecosystems for Baikal, the African rift lakes and the North American Great Lakes.
The healthiest of the big lakes Dobiesz measured were Tanganyika, Baikal and Superior—not coincidentally the lakes with smaller human populations. Lakes Malawi, Huron and Erie were in a middle, “transitional” group. At the “disturbed” end of the spectrum were Ontario, Victoria and Michigan.
The force driving disturbance? “People,” sighs Dobiesz, “everything we do. What we really found is it all starts with people. The metrics that changed the most and led to problems with other things were related to how big was the population around the lake.”
Dobiesz found the lakes had many problems in common. Overfishing drove down the average size of exploited species. Increasing nitrogen from fertilizer and other runoff fed harmful algae blooms. Shoreline development consumed fish nurseries and wildlife habitat.
In some lakes, industrial pollutants such as PCBs discharged in the water or deposited atmospherically accumulated up the food chain, reaching worrisome concentrations in top predators such as lake trout in North America or seals in Lake Baikal.
The Lake Baikal seal, Pusa sibirica, is the only exclusively freshwater seal. Some 80,000 to 100,000 seals are thought to live on or near the big lake. They are found nowhere else in the world.
Nearly three football fields long, the Lee A. Tregurtha is one of more than 100 lake freighters, or “lakers,” that carry iron ore, grain and other goods among the ports lining the North American Great Lakes. A laker can carry a ton of cargo 600 miles on a single gallon of fuel.
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Last modified on January 23, 2012