Could Lake Level Impact Available Nutrients
During the informational presentation tonight as well as the May 10th meeting there were two slides that challenged the commonly held belief that more water volume equals higher dilution, and lower lake temperatures. The slides claims the actual outcome would be increased dissolved nutrients, increased phosphorus, increased sediments from erosion and septic drain field incursion. The slides also claim there would be higher temperatures with current lake levels. The reasoning included a suggestion that increased surface area and inundated shallows equal greater warming, while deeper water equals greater volume which takes longer to cool.
There was no further explanation of these claims. Most of the audience and at least one board member questioned the concepts presented. Experience has shown that generally large deep lakes seem to have cooler water than small shallow lakes. That’s a pretty subjective observation; might there be better data available? Since water temperature is a basic measurement of water quality it would seem that the District would have a long history of temperature readings in its records that predate the water right. This historical temperature information would have been a valuable data set for the board to use in your deliberations but it has not been provided.
Temperature is just one part of a fundamental set of properties governing lakes; this set includes the interactions of light, temperature and wind mixing. The absorption and attenuation of light by the water column is a major factor controlling temperature. The rate at which light decreases with depth depends upon the amount of light-absorbing dissolved substances (mostly organic carbon compounds washed in from decomposing vegetation in the watershed) and the amount of absorption and scattering caused by suspended materials (soil particles, algae and detritus). Generally, 40% of the light will reach a depth of 5 meters in clear lakes; as the lake becomes more turbid; more light is absorbed and stored in the form of heat.
The temperature of water is an expression of heat energy per unit volume. There are six processes that allow heat energy exchange between a lake and its environment: solar energy, longwave radiation, evaporation, convection, lake bed conduction and groundwater inflow/outflow. All of these energy processes act in accordance with the laws of thermodynamics with the primary heating process being solar energy and the primary cooling process being evaporation. In past 4 years these process have resulted low lake temperatures in winter ranging from 35 to 30 degrees and high lake temperatures in summertime ranging from 68 to 75 degrees.
While the overall temperature of the lake is important, especially should it rise above thresholds safe for fish, there is another thermal process that determines how the lake waters mix. Thermal stratification of lakes defines a change in the temperature at varying depths, due to the change in water’s density with temperature. Cold water at the bottom of the lake is denser than the warm water at the. The dividing line between these layers is called the thermocline. Imagine a bottle of salad dressing containing vegetable oil and vinegar. The oil floats on top and as the saying goes; oil and water don’t mix. That is true in Devils Lake as well, the water above the thermocline readily mixes while that below does not remaining relatively stagnant.
This process begins in the spring, when the surface water begins to absorb heat and warms. As the temperature rises, the water becomes lighter than the water below. For a while, winds may still mix the lake from bottom to top, but eventually the upper water becomes too warm and too buoyant to mix completely with the denser deeper water. Once temperatures reach 68 degrees, the relatively large differences in density above the thermocline are very effective at preventing mixing into the waters below. It simply takes too much energy to mix the water any deeper.
As summer progresses, the temperature (and density) differences between upper and lower water layers become more distinct. The depth of mixing depends in part on the exposure of the lake to wind, but is most closely related to the lake’s size. The mixing depth of Devils Lake was identified in the 1983 Diagnostic and Feasibility Study commissioned by the District where sampling identified that stratification exists in Devils Lake marked by a temperature variance and water quality measurements and established a mixing boundary of 2.5 meters.
There will be changes in the temperature of the lake resulting from a reduction of 330 million gallons of lake volume. With several factors at play including summer weather patterns I don’t want to try to attempt a projection on lake temperature. What is perhaps more important is the impact of lake level on the location of the thermocline. Much like pulling the plug on a bathtub when water is released through the D-River the surface of the lake will begin to lower. This is true of the thermocline as well. In this case both will drop 18 inches.
In the illustration I distributed the blue shaded area represents the lakebed below the thermocline and the yellow areas represents the lakebed above. The green areas represent the 124 acres which would cross the thermocline boundary as the lake level is lowered. This is the size of the additional nutrient laden lakebed that could be inserted into the mixing zone of Devils Lake.
What is the magnitude of these additional nutrients that will become the fuel for blue-green algae blooms? If we reference the Updated Paleolimnology Study performed by Max Depth Aquatics in 2005 we can make a good estimate. The study indicates that nitrogen represented .7 % and phosphorus .14% of the dry weight in the top foot of lakebed. We can calculate that the dry weight of the 124 acres on newly exposed lakebed is approximately 11,000 tons. With that we can determine that each inch of sediment depth in the green area contains 630 tons of nitrogen and 126 tons of phosphorus, with over 9,000 tons of nutrients in that first foot of material.
How much of that material will make it into the water column. We cannot say for sure, but we can say that quite a bit more should the District reverse its policy and abandon the use of its water right. Let us not experiment with the health of our lake.