Scientists studying lakes in northern and tropical latitudes are finding that rising lake water temperatures are affecting the ecosystems of the lakes. "Changing Planet" is produced in partnership with the National Science Foundation.
ANNE THOMPSON, reporting:
Lake Superior is the greatest of the Great Lakes, the deepest, the coldest, and the largest. On the border between the United States and Canada, it covers more square miles than any other freshwater lake in the world.
Dr. JAY AUSTIN (University of Minnesota Duluth): Lake Superior is a very large, very cold, and in some sense, understudied lake.
THOMPSON: Physicist Jay Austin is trying to uncover what makes large lakes like Superior get hot and cold. Scientists know that climate change affects the temperature of such lakes, but Austin's research reveals that the warming is much greater than anyone expected.
AUSTIN: Those surface waters are very clearly warming.
THOMPSON: Leading a team from the Large Lakes Observatory in Duluth, Minnesota, Austin oversees the retrieval of this giant 300 kilogram, or about 660 pound, yellow buoy. Fifteen kilometers, or about 9 miles, from Duluth, it has recorded important climate information.
Unidentified Man: Ok, that's good, Johnny. Set it right down there.
JAY AUSTIN: We are looking at things that you would expect, like the wind speed and direction, we are looking at air temperature, humidity, cloud cover, all the things that control how energy is transferred from the atmosphere to the lake.
THOMPSON: Austin says the summer temperatures of Lake Superior jumped nearly 2.5 degrees Celsius, or about four to four-and-a-half degrees Fahrenheit, over the last 30-years.
This trend Austin says is due to a combination of factors, starting with less winter ice and warmer air during the summer, and resulting in shorter winters and longer summers.
AUSTIN: It turns out that the lake sort of has a memory and if you have a very high ice year, the following summer ends up being very, very cool and if you have very little ice, like we did last year, you end up with a very warm summer, like we did this year.
THOMPSON: Less ice cover means more water evaporates and lake levels decrease. Warmer waters can lead to an increase in populations of invasive species that feed on native fish, like sea lampreys, and an increase in harmful algal blooms that can make the lake toxic to fish.
AUSTIN: The systems are really so complex, that it's very difficult to say "well, if the temperature goes up then x is going to happen," but these are the sorts of things we're starting to study.
THOMPSON: What's happening to Lake Superior isn't an isolated case.
Scientists are finding rising water temperatures in 100 other large lakes around the world.
At Bard College in New York, biologist Catherine O'Reilly studies East Africa's tropical Lake Tanganyika, another of the world's largest freshwater lakes.
Dr. CATHERINE O'REILLY (Bard College): And we know that, that water temperature changes over the last two decades are much, much greater than any temperature changes in the lake over the past 1,000 years.
THOMPSON: Can you attribute the changes that you're seeing in Lake Tanganyika to climate change?
O'REILLY: Yes, we definitely can. We know that the water temperature's increasing at the same rate that the air temperature's increasing. So the very close connection between how the air temperatures are changing and how the way the lake works is changing.
THOMPSON: Changes in the lake's ecosystem are evident in tests O'Reilly conducts in the lab. Lake sediment and water samples reveal that Tanganyika is less productive than it was 150 years ago. One reason is a process called "water stratification."
When lake water heats up, it separates into layers, with warm water on top and cool water below. As the temperature difference between the two layers increases, the waters don't mix together, and nutrients from the deep water don't cycle up to the surface.
THOMPSON: So when phosphorus and nitrogen decline, what happens?
O'REILLY: When you don't have a lot of nutrients in the water, there's not a lot of food for algae to grow on, and the whole food chain in the lake is based on the algae at the base of the food web. So if you start to have fewer algae, then you're gonna have fewer things that eat algae, and that includes fish.
THOMPSON: More than 10 million people depend on the fish in Lake Tanganyika for food and income, and with the lake population growing, human activity like deforestation also becomes a problem.
O'REILLY: This is sort of a potential crisis, right? Because we have so many people living in that region, and yet the food that they depend on is probably declining substantially.
THOMPSON: But the changes threaten more than just the food web. They also threaten the cultural identity of all people who live beside these great lakes.
AUSTIN: Lakes gives us a sense of place. It's what really defines where we are, and we should understand if those systems are changing, and understand what the implications of that change are.
THOMPSON: Changes that could impact how people eat, make money, and live, making it crucial that we understand the secrets held by these magnificent bodies of water.
A lake is a large body of water that is surrounded by land. Lakes contain less than 1 percent of the world’s fresh water, but they are a very important freshwater source. Almost all of the world’s fresh water is either frozen in huge masses of ice or buried underground. Lakes contain more than 98 percent of the fresh water that is available for use.
Lake Superior, Lake Tanganyika, Africa, Great Lakes, Water, Climate Change, Global Warming, Temperature, Stratification, Jay Austin, Catherine O'Reilly, Minnesota, Large Lakes Observatory, Population, Ecosystem, Food Web, Phosphorus, Nitrogen, Nutrients, Algae, Biology, Physics