The Not-So-Great Lakes: Warming Waters Make Great Lakes Vulnerable
September-October 2008
This is the fourth in a series of five articles about major problems in the Great Lakes.
Scientists have been tossing thermometers into the frigid waters of Lake Superior for a long time. But about 10 years ago they started seeing some shocking numbers. In 1998, the summer surface temperatures were in the 70s—20 degrees above normal. And a hundred feet down, the water was 15 degrees warmer than usual.
Such measurements have caught the attention of the National Oceanic and Atmospheric Administration because even a few degrees difference in water temperatures can affect the ecology of what many think of as the “Greatest of the Great Lakes.” Cold water has kept zebra mussels, round gobies and many other exotic species in check in Lake Superior, whereas these exotics have wreaked havoc on the warmer Lakes Michigan, Huron and Erie. Cool water is also important to certain microscopic plankton. When temperatures rise, they are forced deeper where there is less light. That limits production of food for fish and other organisms. Lake Superior’s food chain is fragile—a drop in plankton production could have severe repercussions. And warmer water leads to less ice, more evaporation, and lower water levels. (See the May-June 2008 issue of The Wildlife Volunteer for an article on water levels in the Great Lakes.)
University of Minnesota researchers have found that Lake Superior’s surface waters have warmed an average of about 4.5 degrees Fahrenheit since 1979. This is almost double the change in average air temperatures for the Great Lakes Region. This warming has set off a chain reaction of phenomena. For example, with less ice cover reflecting less sunlight, the lake absorbs more heat from the sun. That causes more water loss and lower lake levels.
There have been similar increases in water temperature in the other Great Lakes, and the biological impacts are becoming obvious. In Lake Erie, blooms of some of the more noxious algae forms like Lyngbya wollei are occurring as early as April. Lyngbya has a competitive advantage over other algae forms in warmer waters. It often reaches nuisance levels in the warm water discharges of power plants.
Cynobacteria—a harmful class of algae with some toxic forms—are also increasing in the Great Lakes. Some types prevalent in Florida and other Southeastern states have become established in the Great Lakes in the last decade. There is no doubt that rising temperature is the root of the problem.
Algae blooms brought about by warming water could negate the potential benefits of billions of dollars spent on municipal sewage treatment over the past 40 years. As temperatures warm, more zones of oxygen-depleting algae will likely occur regardless if we continue to limit loading of phosphorus and nitrogen—the nutrients associated with nuisance plant growths in the 1960s and 70s. The dead zones will return and efforts to curb pollution will be thwarted by a few degrees of change in temperature. This is the great irony of the Great Lakes. Despite their vastness, the water bodies are sensitive ecosystems and nothing makes them more sensitive than heat.
In addition to affecting water levels and algae growth, temperature increases even change the way water circulates in the Great Lakes. In summer, the warm surface water is less dense and doesn’t mix easily with the cooler water below. This “layering” of water is well known to scientists and fishermen. Beneath a layer of warm surface water there is a “thermocline”—a zone in which there is a fairly rapid decrease in temperature as depth increases. A layer of cold water is found below the thermocline. When wave action or cooling weather breaks down the thermocline, “upwellings” occur with cold water moving to the surface. The algae, zooplankton, and fish of the Great Lakes are finely tuned to this layering, the structure of the thermocline, and the frequency and duration of the upwellings. A few degrees of warming can throw everything out of synch, and some scientists fear, threaten the entire food chain.
While no one knows exactly how much temperature change the Great Lakes can handle without severe algae blooms and collapsing fisheries, some researchers think we are already at or close to the “tipping point.” They plan to put a lot more thermometers in the Great Lakes in the next few years while drawing public attention to temperature increases as a somewhat invisible but critical threat to the largest body of fresh water on earth.
Dr. Patrick J. Rusz
Director of Wildlife Programs
This is the fourth in a series of five articles about major problems in the Great Lakes.
Scientists have been tossing thermometers into the frigid waters of Lake Superior for a long time. But about 10 years ago they started seeing some shocking numbers. In 1998, the summer surface temperatures were in the 70s—20 degrees above normal. And a hundred feet down, the water was 15 degrees warmer than usual.
Such measurements have caught the attention of the National Oceanic and Atmospheric Administration because even a few degrees difference in water temperatures can affect the ecology of what many think of as the “Greatest of the Great Lakes.” Cold water has kept zebra mussels, round gobies and many other exotic species in check in Lake Superior, whereas these exotics have wreaked havoc on the warmer Lakes Michigan, Huron and Erie. Cool water is also important to certain microscopic plankton. When temperatures rise, they are forced deeper where there is less light. That limits production of food for fish and other organisms. Lake Superior’s food chain is fragile—a drop in plankton production could have severe repercussions. And warmer water leads to less ice, more evaporation, and lower water levels. (See the May-June 2008 issue of The Wildlife Volunteer for an article on water levels in the Great Lakes.)
University of Minnesota researchers have found that Lake Superior’s surface waters have warmed an average of about 4.5 degrees Fahrenheit since 1979. This is almost double the change in average air temperatures for the Great Lakes Region. This warming has set off a chain reaction of phenomena. For example, with less ice cover reflecting less sunlight, the lake absorbs more heat from the sun. That causes more water loss and lower lake levels.
There have been similar increases in water temperature in the other Great Lakes, and the biological impacts are becoming obvious. In Lake Erie, blooms of some of the more noxious algae forms like Lyngbya wollei are occurring as early as April. Lyngbya has a competitive advantage over other algae forms in warmer waters. It often reaches nuisance levels in the warm water discharges of power plants.
Cynobacteria—a harmful class of algae with some toxic forms—are also increasing in the Great Lakes. Some types prevalent in Florida and other Southeastern states have become established in the Great Lakes in the last decade. There is no doubt that rising temperature is the root of the problem.
Algae blooms brought about by warming water could negate the potential benefits of billions of dollars spent on municipal sewage treatment over the past 40 years. As temperatures warm, more zones of oxygen-depleting algae will likely occur regardless if we continue to limit loading of phosphorus and nitrogen—the nutrients associated with nuisance plant growths in the 1960s and 70s. The dead zones will return and efforts to curb pollution will be thwarted by a few degrees of change in temperature. This is the great irony of the Great Lakes. Despite their vastness, the water bodies are sensitive ecosystems and nothing makes them more sensitive than heat.
In addition to affecting water levels and algae growth, temperature increases even change the way water circulates in the Great Lakes. In summer, the warm surface water is less dense and doesn’t mix easily with the cooler water below. This “layering” of water is well known to scientists and fishermen. Beneath a layer of warm surface water there is a “thermocline”—a zone in which there is a fairly rapid decrease in temperature as depth increases. A layer of cold water is found below the thermocline. When wave action or cooling weather breaks down the thermocline, “upwellings” occur with cold water moving to the surface. The algae, zooplankton, and fish of the Great Lakes are finely tuned to this layering, the structure of the thermocline, and the frequency and duration of the upwellings. A few degrees of warming can throw everything out of synch, and some scientists fear, threaten the entire food chain.
While no one knows exactly how much temperature change the Great Lakes can handle without severe algae blooms and collapsing fisheries, some researchers think we are already at or close to the “tipping point.” They plan to put a lot more thermometers in the Great Lakes in the next few years while drawing public attention to temperature increases as a somewhat invisible but critical threat to the largest body of fresh water on earth.
Dr. Patrick J. Rusz
Director of Wildlife Programs