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                The Curious Case of Deep Lakes and The Disappearing D.O.

By David Van Wie


It is late June and the lake looks beautiful. Gentle puffs of wind dance down the shore, leaving tell-tale ripples in dark patches set off from the smooth water shimmering in the sunlight. Meanwhile, deep beneath the calm surface, something mysterious is happening that may have a disturbing effect on the trout and salmon living in lake.

The process of thermal stratification is underway. The lake is undergoing its seasonal transformation that will create an invisible barrier between the cold deep water, and the warmer water near the surface. It could turn into a frightening and desperate situation.

Thermal stratification is especially important to cold water fish species, like trout and landlocked salmon. These species require colder water during the warmer months, usually below 55 degrees Fahrenheit. They also need high amounts of dissolved oxygen (DO) in the water.  How much DO is available in the cold deep water will determine whether these sensitive species can thrive through all stages of reproduction and growth.

Maine has some of the best water quality in the world in its lakes and ponds. But even absent obvious sources of pollution, our prized trout and salmon fisheries can be affected in ways that are less visible, especially in our lakes that stratify in the summer.

The problem occurs when algae growth increases due to nutrient pollution, especially phosphorus, in stormwater runoff. When the algae dies, sinks to the bottom, and decays, the concentration of DO in the deepest layers is depleted, which puts trout and salmon at risk.

The Trouble with Phosphorus

Phosphorus is known as a “limiting nutrient.” In freshwater, the limited availability of naturally-occurring phosphorus determines how much algae can grow in the water. In our most pristine lakes, the tiny amount of phosphorus available means hardly any algae in the water. This is why the water in many lakes is crystal clear.

By measuring the clarity or transparency of water using something called a Secchi disk lowered into the water, scientists can get a good idea how much algae or sediment is in the water. In half of Maine lakes, you can see to the bottom in over 15 feet of water. This is good! According to the Maine Volunteer Lake Monitoring Program, over 50 lakes in Maine have transparency of over 9 meters, or more than 30 feet!

In lakes that are surrounded by or undergoing development, excess phosphorus enters the water from septic systems, from fertilizer washed off of lawns in the rain, and from eroding camp roads, driveways and shorelines. Tributaries and roadside ditches transport soil particles and sediment containing phosphorus into the water. Even adding sand to a beach will contribute large quantities of phosphorus to the lake.

As additional phosphorus is introduced, the algae happily blossom and multiply. We may not notice it at first, but eventually the water looks cloudier and greener. Using a Secchi disk to measure transparency, volunteers might record that they can only see the disk 10 feet down, instead of 12 or 15 feet earlier in the year. Heavy rainstorms and heat waves sometimes cause severe algae blooms that turn the water green and scummy, like pea soup.

Algae doesn’t live forever. When it dies, the algae carcasses sink to the bottom, where bacteria start to consume the dead algae. In doing so, the bacteria also consume large amounts of oxygen. As a result, the DO level in the water declines, with less oxygen available for the trout and salmon seeking refuge in the cold water. In some lakes, the DO levels go rapidly to zero near the bottom, especially after the lake stratifies.

The Science of Stratification

Most of us have noticed thermal stratification in lakes when swimming in summer. Diving beneath the surface, we feel the water temperature plummet at the “thermocline” - a line (really a thin layer of water) where the temperature drops 20 or more degrees (Fahrenheit) in just a few feet.  The thermocline can be anywhere from five to ten or more feet beneath the surface. The top layer above the thermocline (called the epilimnion) might top 70 degrees, while the temperature in the deeper layer (hypolimnion) hovers in the 40s.

Not every lake stratifies. Shallow lakes and ponds are less likely to stratify. Wind and currents (in impounded lakes) also affect stratification.  In some lakes, the deeper areas of lakes will stratify while shallow bays or arms do not.

Separation Anxiety

Once a lake stratifies, the top layer and the bottom layer become physically separated. The warmer water on top circulates with the wind, and continues to be oxygenated by waves and photosynthesis from algae and plants. But the epilimnion usually becomes too warm for trout and salmon to thrive. At the same time, the deeper colder hypolimnion layer becomes isolated, receiving no new oxygen from the surface.

The dissolved oxygen (DO) levels in the hypolimnion decline over the summer as biological processes consume oxygen. Bacteria in the sediment relentlessly break down decaying organic matter consuming oxygen.

Fisheries biologists have determined that cold water species are stressed if the DO level in the water goes below 5 parts per million (ppm). For lakes with high water quality like Kennebago Lake, the amount of oxygen available is well above this level (7 to 10 ppm) all summer even in the deepest parts of the lake.

In other lakes, such as Sabbathday Lake in New Gloucester, the DO level in the lower hypolimnion layer often falls below 5 ppm for days or weeks at a time. Fish seeking higher levels of oxygen must remain near the thermocline to find the right balance between temperature and oxygen levels, if they can.

Mousam Lake in southern Maine has a more complex geography where large parts of the lake remain sufficiently oxygenated while other deeper parts suffer depletions in DO.

China Lake east of Augusta once sustained healthy populations of trout and salmon, but years of massive algae blooms due to phosphorus runoff resulted in the complete loss of these cold water species due to low DO levels. In other lakes, the effect may not be so dramatic and complete, but it sure can hurt the fishing!

Fear The Zombie Trout

We can’t do anything about lake stratification, nor would be want to. But we can control soil erosion, stormwater pollution, and phosphorus that ultimately result in low oxygen in the deep waters. The DEP Lakes Program and the Volunteer Lake Monitoring Program, which keeps close tabs on these impacts, and our local watershed associations need our support.

Whatever we can do to prevent another Algae Apocalypse will keep the zombie trout and salmon from swimming in a living hell after the lake stratifies. I am sure they will appreciate the effort.


Reprinted with permission from The Maine Sportsman, June 2016

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