WHAT DOES IT TELL US?

The amount of algal growth in a lake depends on many factors including: water temperature, water transparency (this determines how far down in the water sunlight can reach for photosynthesis), amount of zooplankton and fish that are eating algae, and the amount of nutrients, such as nitrogen and phosphorus. Changes in the lake, such as higher temperatures or increased nutrients, can cause a shift in the algae type.

Photo courtesy Kennebec Soil and Water Conservation District

Too much algal growth, or too much of a certain type of algae (such as blue-green algae), can cause problems in the lake ecosystem. When there is a rapid growth of a certain type of algae it is called an algal bloom.

Algal blooms reduce the transparency of the lake water, they can produce a foul smell and can add a bad taste to the lake water. These effects of an algal bloom can negatively impact recreational tourism and reduce lake-front property values.

Beyond these effects, as the algae die off they sink to the bottom of the lake and decompose. Decomposition uses up oxygen in the deeper waters (hypolimnion) that are valuable habitat for cold water fish communities. This may force these fish into warmer waters near the surface, where conditions are not optimal for growth and reproduction. Cold water species are often highly valued by anglers.

Illustration courtesy Water on the Web

The figure above shows what happens to cold water fish species when there is a loss of oxygen in the lower levels of a lake. Here the red layer shows the warmer surface waters while the blue layer is the hypolimnion which is colder water. When oxygen is used up by decomposition on the lake bottom, fish are forced into warmer waters (yellow) to get the oxygen they need. This can adversely affect cold-water species.

What do algal blooms tell us about a lake?

Conditions need to be just right for an algal bloom to occur (certain levels of water temperature, water transparency, wave action, nutrients and food web conditions).

Sometimes, algal blooms may indicate that increased amounts of nutrients are entering the lake system. These increases in nutrients may be due to land use activities in the area surrounding the lake. In Maine, examples of these land use activities include construction of camps and new development that reduce the amount of vegetation on the shoreline (this vegetation helps to hold the shoreline where it is, filter pollutants, and take up nutrients before they enter the lake), poorly maintained camp or town roads that empty sediments into the lake, or agricultural practices that introduce nutrients from farm fields or animal wastes from feed-lots. For more information see the Maine Department of Environmental Protection page on non-point source pollution.

What are the average Chlorophyll levels for lakes in Maine?

Chlorophyll levels in Maine lakes vary from a minimum of 1 ppb (parts per billion) to a maximum of 51 ppb, with an average value of 4 ppb. Source: 2002 Maine Volunteer Lake Monitoring Program annual report.