Water Quality Terms - Abbreviated

(from Fusilier's Atlas and Gazetteer of Michigan Inland Lakes)

1.     Temperature and Dissolved Oxygen

Because of the formation of a thermocline (the layer of water in a lake where the temperature changes fastest with depth in summer if the lake is deep enough), the temperature many times determines the amount of dissolved oxygen.  In very high quality lakes, we find dissolved oxygen below the thermocline in late summer.   The reason the lake runs out of oxygen below the thermocline in poorer quality lakes is that bacteria continue to decompose organic material which has precipitated into this deeper water, but the thermocline prevents the surface layer of water from mixing with the bottom layer.  Since the main source of oxygen in a lake is from contact with air, and the thermocline prevents the bottom water from coming in contact with the air at the surface, lake bottom water can become oxygen depleted by bacterial decomposition during the summer months as well as in the winter months when ice prevents air contact with water.

2.     Chlorophyll A

Chlorophyll A, like phosphorus and nitrates, is measured in the parts per billion range.  It is generally used to estimate the amount of algae growing in the water.  Many limnologists felt, and we agree, the lower the better.  We like to see chlorophyll at values below 3 ppb.  In 1993, one lake had a chlorophyll A concentration of 171 micrograms per liter.  That's about as high as we've seen.

3.     Secchi Disk Reading

A Secchi disk is a black and white disk which when lowered into a lake disappears from view at one depth (deeper) and re-appears when raised at another depth (shallower)…the average of the two measurements is recorded.  The larger this recorded measure, the clearer the lake.  Algae, suspended sediments, fragments of acquatic plants, etc. which can cloud water impact this reading.

4.     Total Nitrate

Total nitrate, also measured in parts per billion, is also an important indicator of lake-water quality.  Many limnologists felt that any concentration below 200 ppb indicated excellent lake water quality.  Nitrate concentrations are almost always higher in spring than in summer; the reason for this being that in spring, the water is too cold to permit bacteria, which break down nitrates, to grow; while in the warm summer water, the bacteria flourish.  Although many suggest that phosphorus is the main culprit in lake-water quality degradation, we often find phosphorus is the limiting nutrient in spring, while nitrogen is the limiting nutrient in summer.

Since nitrogen is water-soluble, any fertilizers used which contain nitrogen will dissolve in a rain and wash into the lake, adding immensely to our weed problem.

5.     Alkalinity

The alkalinity test is a common limnological test, which indicates if you have a hard or soft water lake.  Hard water lakes are more resistant to water quality problems, because the chemicals that make water hard have the ability to precipitate phosphorus to the bottom sediments.  They are also more resistant to acid rain problems, because the carbonates buffer the acid and prevent large pH changes.  Alkalinity is generally higher in spring than in summer.  A panel of limnologists felt that any alkalinity between 75 and 200 milligrams per liter indicated excellent quality water for a lake.

6.     Conductivity

Conductivity is a measure of the amount of dissolved materials in a lake.  Many limnologists felt, and we agree, that the lower the amount of dissolved materials in a lake, the better.  Normal conductivity readings for Michigan lakes range from about 200 to 700 micromhos per centimeter at 25 degrees Centigrade.

7.     Total Phosphorus

Total phosphorus, which is measured in parts per billion (or micrograms per liter) is an important indicator of water quality of a lake.  The lower the better.  We like to see phosphorus concentrations below 10 parts per billion.  Like nitrogen, phosphorus is an essential aquatic plant nutrient.  Phosphorus levels in the lake are already more than the aquatic plants, especially in the summer, can absorb (limited by the amount of nitrogen available) and more than adequate to nourish your lawns without the addition of  fertilizers.

Contributed by Charles Pendleton

Last modified: 12/05/03 16:58:26 -0800