Rising Temperature in Large LakesThere is a scientific consensus to the fact that the global climate is warming because of the addition of heat-trapping greenhouse gases which are increasing dramatically in the atmosphere as a result of human activities. A lot of focus has been placed on the effects of global warming on the ocean -- sea level is rising, sea water is becoming more acidic, and ocean circulation is changing due to melting sea ice. But did you know that the Earth's freshwater lakes are being affected by global warming as well?
A NASA study that was released in November 2010, surveyed the surface temperature trends in 167 large lakes worldwide. They reported an average warming rate of 0.45 degrees Celsius (0.81 degrees Fahrenheit) per decade, with some lakes warming as much as 1 degree Celsius (1.8 degrees Fahrenheit) per decade. The warming trend was global. While the greatest increases were found in lakes found in mid- to high-latitudes of the Northern Hemisphere, increases were seen in North America, South America, Europe, Asia, Africa and Australia.
Scientists are just starting to study and understand the implications of rising temperatures on lake ecosystems. One area of concern is the fact that rising lake temperatures result in increased algal blooms. Algae is naturally found in lake ecosystems and is in fact the base of the aquatic food web. But when the numbers of algae in a lake rise dramatically, a bloom results. Some algal blooms are harmless to life, but are simply unappealing. Water in that area might look terrible, smell foul or even taste bad (when water is drawn for drinking and purification from that source). Other times, algal blooms can be toxic to fish, other aquatic organisms, wild and domestic animals that use that source of water, and humans. Humans can experience gastroenteritis (if the toxin is ingested), lung irritations (if the toxin becomes aerosolized) or skin irritation (if the algae/toxin is touched for instance while swimming) .
Rising lake temperatures have also been shown to favor invasive species found in lakes. In the Great Lakes region, two examples of invasive species under scrutiny are zebra mussels and lampreys. Zebra mussels have been seen to thrive in warmer and warmer waters, which means they can extend their living range to higher and higher latitudes. Lampreys seem to thrive in warmer waters growing bigger and bigger and are staying active for more of the year. Both of these invasive species are extreme pests that are killing off native species, eating the food of native species, or in the case of zebra mussels, causing billions of dollars of damage to structures and aquatic vehicles.
NASA was able to survey a large number of lakes all in one study with the help of satellite data. These findings are in line with what is being reported 'on the ground'. For instance, Russian and American scientists have been studying the Siberian Lake Baikal for over 60 years. Lake Baikal contains 20 percent of the world's freshwater, and it is large enough to hold all of the water in the U.S. Great Lakes. It is the world's deepest lake as well as its oldest (25 million years old). The data from the lake shows that the surface waters have warmed significantly and that the food web in this lake has already experienced dramatic changes. Many are concerned that global warming has reached this most remote of locations, especially since Lake Baikal is home to over 2,500 plant and animal species, with most, including the freshwater seal, found nowhere else in the world.
In about as different a location as you could find from Siberia, scientists have found more evidence of global warming -- in East Africa's Lake Tanganyika. Geologists have determined that the lake has experienced unprecedented warming during the last century; in fact according to core samples, it is the warmest the lake has been for 1,500 years. Scientists expect that as the lake gets warmer, fish productivity will decline. This has been attributed in a large part to water stratification that many large lakes are experiencing. The lake, one of the richest freshwater ecosystems in the world, is divided into two levels. Most of the animal species live in the upper 100 meters, including valuable sardines which are a source of food for many living in the area. The lake depends on wind to churn its waters and send nutrients like nitrogen and phosphorous from the depths toward the surface. These nutrients are food for algae, which supports the lake's entire food web. But as Lake Tanganyika warms, the difference in temperature (and density) increases and the mixing of waters is lessened; fewer nutrients are funneled from the depths to the surface. An estimated 10 million people live near the lake, and depend on it for drinking water and for food. Temperature rises in Lake Tanganyika will change how people eat, live and make money.
Many of the same issues are being studied by scientists studying Lake Superior, the deepest, the coldest, and the largest of the Great Lakes. By surface area, it is the biggest freshwater lake in the world. Dr. Jay Austin leads a team of researchers from the Large Lakes Observatory (University of Minnesota) who are studying Lake Superior. One of the ways they study the lake is with buoys like the one shown in the image on this page. The buoys house instruments that measure things like air temperature, water temperature, cloud cover, humidity, wind speed and direction, all the things that control how energy is transferred from the atmosphere to the lake. Scientists then access the buoy data remotely. Austin says the summer temperatures of Lake Superior jumped 4-4.5 degrees Fahrenheit over the last 30 years. In this complex system, there are obviously many interconnected factors to gauge. One basic assumption is that warmer temperatures mean less winter ice in Lake Superior. With less ice, the lake surface is actually darker, the albedo is lower, more solar energy is absorbed, and less is reflected (than it would have been by the white ice). Holding onto the energy causes more ice to melt, which, in turn, lowers the albedo, causes more energy to be absorbed and more warming. With this warming comes evaporation of the lake's waters. This in turn, results in lower water levels for the lake overall. Low lake levels are a concern to property owners on lakes, those in the shipping industry, and low lake levels affect water outlets and inlets into the lake (even groundwater seepage), and wildlife too.
Lake Baikal, Lake Tanganyika and Lake Superior aren't isolated cases - global warming affects the temperature of lakes around the world. Many of these lakes are experiencing the undesirable effects of global warming such as an increase in algal blooms, the rise of invasive species, decreased fish productivity and lowered lake levels. Obviously, more study and attention is due these important areas where so many people live, work and make their homes.