This story about the quality of Clark Lake’s water was written by Richard Moyer. Dr. Moyer, and his wife Lois, live on the north shore, west end. He is a professor emeritus of science education and natural sciences at College of Education, Health, and Human Services, University of Michigan-Dearborn.
Last year I summarized the spring and fall “lake checks” completed by PLM on the water quality of Clark Lake. While 2020 hasn’t been the best of years in many ways, Clark Lake has fared pretty well. Once again samples were taken in late April and September. The overall conclusions were the same as last year:
• Conditions are good for fish growth
• Dissolved oxygen levels are adequate for good fish production
• pH is within acceptable limits
• Phosphorus and Nitrogen are within acceptable limits
A year ago, I explained that lakes are classified overall by their trophic level—trophic relating to the amount of plant growth. Left alone, all lakes eventually evolve to eutrophic levels, essentially highly nutrient laden and ultimately filled with plant materials. The opposite is a clear lake with virtually no plant material, known as an oligotrophic lake. The translation from the Greek literally means few feeding (plants). Based on their findings, PLM classifies Clark Lake in three measures as meso-oligotrophic, which means essentially that it is in between eutrophic and oligotrophic leaning toward the latter. One measure found the lake slightly more eutrophic with classification of mesotrophic.
Clark Lake underwater. Photo: Brandi Zak
Healthy lakes have a dissolved oxygen level of greater than 6.5 mg/L (milligrams per liter). Our lowest level, in the fall at a depth of 10 meters was 7.5 mg/L. Spring readings, were found to be in the 10.5 to 11.5 mg/L which is near saturation of dissolved oxygen. As you look at the data (which follows) you may notice that the amount of dissolved oxygen is lower in the fall than the spring. Not anything to worry about—the amount of oxygen that can dissolve varies with temperature and cold water can hold more dissolved oxygen.
Clark Lake’s pH levels are nearly unchanged from a year ago and are well within healthy levels. Recall that pH is a logarithmic measure of how acidic or alkaline a substance is with pure distilled water being neutral at a pH of 7. Clark Lake is (still) slightly alkaline at about a pH of 8.5.
The clarity of the lake is also good as measured by a Secchi Disk, a black and white circle that is lowered into the water until it can no longer be seen. The greater the depth at which it can still be seen, the more clear the water. Here we slipped a bit in the fall reading. A year ago, the Secchi depth was 8 meters in the spring and 7 meters in the fall. In 2020, the Secchi depths were 6 meters and 3.5 meters. This is the one measure (mentioned above) that slipped to a mesotrophic level. We are still ok as generally Secchi readings greater than 5 meters are considered oligotrophic and those between 3 and 4.9 meters mesotrophic.
Last year, I made a suggestion (plea) that we all consider the fertilizers we put on our lawns. Eventually, they mostly end up in the lake. The phosphorous level inched up in the spring from 10 micrograms per liter last year to 13 in 2020. The accompanying rating from PLM also inched up from “slightly phosphorous enriched” in 2019 to “moderately phosphorous enriched” in 2020. The good news is that by fall of this year the level had returned to last year’s levels. The middle number on a bag of fertilizer represents the amount of phosphorous, the bigger the number the greater the amount of phosphorous. The three numbers represent the amounts of nitrogen, phosphorous, and potassium. Phosphorous free fertilizers are available for use on lakeside lawns.
So, in summary, our lake is quite healthy. It’s not quite perfect (but very good) in its chemistry, and still perfect in all of our hearts.
To review PLM’s report for both spring and fall 2020, please click here.