WATER QUALITY MONITORING 2006
by Jerry Trent, Water Quality Chair
The water quality team of Judy and Dean Yorston and Jerry Trent has used the new Hach electronic monitoring system three times at six sites on Cross Lake and several sites on the Snake River. We were assisted
by Carla Hugill, the 2006 CLA scholarship winner, who plans to study environmental science next year in college.
We were able to test for a number of important factors at various
depths, thereby obtaining profiles of the water column.
With the monitoring device, digital data was stored and then transferred
to tables and graphs. A number of trends we revealed in the water
column from top to bottom at each site:
– Decreasing temperature.
– Decreasing dissolved oxygen.
– Decreasing pH (acid/base condition).
– Some increase in specific conductance (relates to dissolved minerals
in ionic form from sediments).
– There were significant differences between sites on a given day.
– There were significant differences for a given site from month to
The lake and river are complex, dynamic environments. The physical
and chemical factors listed above all interact. Chemical biochemical reactions that occur are driven or affected by temperature and pH.
For example, increasing temperature not only speeds up biochemical changes such as photosynthesis (which releases oxygen to the water)
but also decreases the amount of oxygen that the water can hold. Photosynthesis changes the pH of the water.
The point is, there are reasons that the water column changes from
top to bottom, is different for various sites, and changes as the
summer season progresses.
Dissolved Oxygen (DO)
Dissolved oxygen in the lake and river is a critical factor. All organisms, except some bacteria, require it. Most of the lake near the surface was super-saturated with oxygen in May, June, and July due mostly to very active photosynthesis.
The amount of DO, however, decreased with depth, and this could
become a problem for game fish such as walleye, northern and
muskie in 18 feet of water as early as May at the north sites in the
lake and at 12 feet in the south bay (south of the river channel).
In June, a low level of DO was noted at about 11 feet in the bay north
of Norway Point, and below 20 feet west of sunken island, and at other sites further south. On the other hand, the south bay had high levels
of DO at all depths in June.
In July, the area 1/2 mile north of sunken island had inadequate DO
for game fish below about 12 feet. The water depth in this area is approximately 22 feet. West of sunken island in 25 feet of water, the
DO was inadequate at about 13 feet. In south bay, DO dropped off dramatically below 11 feet.
In the river, there was adequate DO for game fish at all depths
measured near the highway 61 bridge in about 8-9 feet of water
Previous professional study
We will be comparing our data for 2006 with a previous study done for
the Minnesota Pollution Control Agency about 10 years ago. At that
time, it was stated that the north and south ends of the lake and the
river act like three different bodies of water. Our data ased on
additional sites so far suggests that there are at least three rather
distinct areas in the lake. The professional study found that the river
acts like a distinct body flowing through the lake.
There is much more to be learned!
Why Do Lakes Turn Over?
The answer to the question lies in how water density varies with water temperatures. Water is most dense at 39 F (4 C) and as temperatures increases or decreases from 39 F it become increasingly less dense. In summer and winter, lakes are maintained by climate in what is called a stratified condition. Less dense water is at the surface, and more dense water is near the bottom.
In late summer and fall, air temperatures cool the surface water causing its density to increase. The heavier water sinks, forcing its density to increase. The heavier water sinks, forcing the lighter, less dense water to the surface. This continues until the water temperature at all depths reaches approximately 39 F. Because there is very little difference in density at this stage, the water is easily mixed by the wind. The sinking action and mixing of the water by the wind results in the exchange of surface and bottom water, this is called “turnover.”
This pattern of spring turnover, summer stratification, fall turnover, and winter stratification is typical for lakes in temperate climates, such as Minnesota. Lakes with this pattern of two mixing periods – in spring and fall – are referred to as dimictic lakes. Many shallow lakes, however, do not stratify in the summer, or stratify for short periods only, throughtout the sujmer. Lakes that stratify and destratify numerous times within a summer are knows as polymictic. Both polymictic and dimictric lakes are common in Minnesota. A more detailed description of the physical characteristics of lakes, including temporal and density interactions can
be found at the Water on the Web page by clicking here.
Reprinted from the REPORTER, published by the Minnesota Lakes Association.