By Jim Steele —
Lakes sometimes are divided into age categories depending on when they were formed, their levels of nutrient enrichment, sediment and plant material.
The theory is that eventually a lake will become a marsh and then a meadow and finally a valley as it grows older.
Natural lakes can take thousands or even millions of years to go through this process.
Clear Lake is a shallow lake that has taken an extraordinary amount of time to fill in because of the constant tilting and sinking of the land form under it because of geological processes.
Runoff over rich volcanic soils in the surrounding hills has naturally endowed Clear Lake with a high nutrient level of phosphorus, nitrogen and minerals.
This condition is called eutrophication or “well-fed.” More than most places, these nutrients have resulted in a rich production of algae, rooted aquatic plants, single cell organisms, insects, fish and wildlife. Ordinarily, this level of nutrient loading and sedimentation can signify a lake nearing the older stages of its life as a lake.
In the past the stability of Clear Lake”s depth has helped to hold the aging process in place. Other stabilizers are wetlands surrounding the lake that filter out much of the nutrients and sediment before they enter the lake.
Porous volcanic soils of surrounding hills also tend to soak up rain before it can form runoff concentrations that create erosion. A combination of slowly sinking lakebed, undisturbed hill slope soils and slowed runoff kept the lake in a limbo state of highly productive balance for thousands of years.
Unfortunately, the 90 percent loss of wetlands and shoreline vegetation from development beginning in the 1800s has allowed increases of sediment and nutrients into the main lake. Lake nutrient levels also increased from septic treatment and impervious surface runoff such as from highways, paving and roofs.
The high use of fertilizers, pesticides, livestock production, phosphate rich soaps and extensive ground disturbance added to the total amount. This culturally caused eutrophication is also a problem for more than 50 percent of U.S. lakes.
Winter rainwater runoff brings the greatest amounts of nutrients to the lake and westerly breezes create a tractor effect of surface water flowing east and bottom water flowing west. This water movement coupled with boat waves stirring up the shallow shoreline bottom creates a well-mixed nutrient rich water column; even in the summer. Winter rains bring summer blooms.
One more ingredient adds to the mix. As plants die back each fall, they form a decomposing mat on the bottom, which decreases oxygen levels. This changes the chemistry in the sediments and releases phosphorus back to the water column. This nutrient feedback mechanism can slow phosphorus loss to bottom sediments and increase algae blooms.
Because of this, a lake”s nutrient balance can change if nutrients and sediment are added at only a slightly faster rate. The spectrum of biological responses shifts to one that can utilize the increased nutrients, such as extreme blooms of rooted aquatic plants, algae and cyanobacteria.
Some years more than others, this runaway productivity happens to Clear Lake.
But all of this can be reversed by re-establishing the upslope and lakeside mechanisms normally stabilizing the lake and by reducing the human contributions.
Reconstructed wetlands along with reduced inputs can change the lake”s biological response back to an earlier time; once the stored nutrients are used or trapped.
A knowledgeable public is a major factor toward both reversing the effects of the last 150 years and establishing the techniques for living in harmony next to a highly productive eutrophic lake.
Next time, Techniques for harmony.
Jim Steele is a retired Cal Fish and Game scientist, registered professional forester, part-time consultant and full-time Lake County resident-volunteer.
