Thompson Lake - Conclusions

This study was conducted to generate comparable data so an update and assessment of the limnological condition of Thompson Lake could be made. The usual caveats apply to this study. The data we measure are subject to variation due to "warm" vs. "colder" years, presence or absence of an algal bloom or a heavy crop of macrophytes, herbicide treatment of aquatic plants which could increase nutrient levels, and one real difference: the 1976 data were collected on 6 August, while the 2002 data were collected sometime later on 27 August. This disparity in collection times may or may not make a difference in how degraded the bottom waters would have gotten for example, later in the season. We feel that a valid, but guarded assessment is warranted.

The strongest data connected with this appraisal is the comparison of the dissolved oxygen depletion (and associated toxic chemicals) in the bottom waters and whether it got worse or better over the 26 years since the 1976 study was performed. A second bellwether of health is the change in nutrient concentrations over this period.

Lastly, the chlorides also provide a general assessment of environmental health, especially since they reflect runoff patterns and high buildups imply large inputs of this substance and by association probably other deleterious substances, such as toxic substance and nutrients. A wealth of additional measurements was made to ensure that any other drastic changes would be reflected in changes over time.

The evidence from the extent and amount of degradation of bottom waters was fairly clear, in that there was a large amount of the bottom waters at station D (the deepest spot in Thompson Lake and the best place to compare these data) depleted of dissolved oxygen in 2002, much larger than was measured there in 1976. The evidence was less compelling at the other deep stations we studied, stations E and F.

There was also strong evidence that hydrogen sulfide, another toxic chemical, was extremely high at 5 mg/L - no comparable data were available for 1976. However, there was one measurement at station E, where hydrogen sulfide was five fold higher in 2002 than in 1976. At the other deep station, hydrogen sulfide concentration was also 5 mg/L.

These levels are extremely toxic to fish and they also provide nutrients to the entire lake at spring turnover, fueling algal blooms and aquatic plant growth. In addition, the anoxic conditions (devoid of dissolved oxygen) have other detrimental consequences, since under anoxia, P is released from the sediments. Phosphorus is not released when there is dissolved oxygen present on the bottom.

The four nutrients we measured between the two periods provided a picture of how these important fertilizers responded over time. Soluble reactive phosphorus mean concentrations in 1976 were much higher (0.08 mg/L) than they were in 2002 (0.02 mg/L). Mean total phosphorus was half as high in 2002 (0.06 mg/L) as it was in 1976 (0.12 mg/L).

In addition, values were much higher in bottom waters at deep stations in 1976 than they were in 2002. Nitrate, like SRP and TP, was generally higher in 1976 (mean = 0.03 mg/L) than in 2002 (mean = 0.02 mg/L). The ammonia trends followed those of the other three nutrients; values at comparable stations were mostly higher in 1976 than in 2002 and concentrations in bottom waters were 1.5 to almost 6 fold higher than comparable 2002 values. One would conclude from these data that nutrients have actually declined or were much lower in Thompson Lake in 2002 than what was found during 1976.

Chlorides, a major indicator of runoff, increased almost 2.1 fold from 1976 to 2002, a rate of almost 2 mg/L per year. This is a detrimental change, since it indicates that a considerable amount of runoff waters are seeping or running into Thompson Lake. Along with the road salt there is undoubtedly considerable amounts of toxic substances, nutrients, and other deleterious materials, such as sediment.

Among the other parameters we measured, water clarity is also very important, but is more reliant on algal blooms, so may be less reliable. However, the mean Secchi Disc reading in 2002 of 5.4 ft was 1.5 fold higher than the 1976 mean value, which was 3.8 feet. This is a positive sign for the lake.

Like many assessments, some parameters indicated a degradation of the health of Thompson Lake; others showed a positive improvement in the lake environment.

The degraded and more extensive loss of dissolved oxygen in the lake certainly bodes poorly for northern pike, which require oxygenated cool waters for good survival, and certainly would prevent any walleyes stocked from growing well or even surviving, in addition, these conditions provide more nutrients at spring turnover from the decomposition of organic matter (nitrates and ammonia) and higher release ofP from the sediments. These nutrients in turn will fuel more algal growth and exacerbate the already heavy growth of aquatic plants, especially Eurasian milfoil in the lake.

On the other hand, the residents can take some consolation in the fact that nutrients (nitrates, SRP, TP, and ammonia) generally were lower in 2002 than what was present in 1976. The increase in chlorides shows that a considerable input of road salt or other sources of chlorides are entering the lake and along with this substance, we can confidently predict that considerable amounts of toxic substances, sediment, and nutrients are accompanying this pollutant.

The one major step to curtail nutrients was taken, installation of sewers. Now residents need to take additional steps to stop runoff from their lawns (plant greenbelts), stop paving and ruining the infiltration capacity of the land, and reduce or eliminate the use of fertilizers (studies have shown most lake residents do not need any more P in their lawns). If fertilizers must be used, use only those with nitrogen; no phosphorus fertilizers should be put on the lawns. The tributaries and culverts we tested showed occasional problems with high concentrations of nutrients and chlorides.

Some additional work needs to be done during rain events to quantify just how much runoff, especially from areas like golf courses, these culverts and tributaries that run into the lake provide.

A hypolimnectic aerator (aeration of the bottom waters only of the lake) could reduce the dissolved oxygen problem in the lake, but these are very expensive. One technique that was utilized in the past was hypolimnectic draw down (pumping water from the bottom waters through the dam). We are unaware of how this worked or what problems were associated with this technique.

What Was Done - Results - Thompson Lake