Obviously not a tasty treat from the bakery, a lake turnover is an event that occurs twice a year, in the spring and in the fall. It is triggered when the water temperatures at the top and the bottom of the lake are approximately the same. Any wind mixes the water up and causes the material on the lake bottom to rise to the surface.
Depending on the nature of the material on the lake bottom, this turnover can mean trouble. According to Joe Primozich, a biologist on the Pentwater Lake Association Board, the lake turnover can lead to toxic blue-green algae blooms. He warns, “These toxic algae blooms can be harmful to people who come into contact with them and can be fatal to dogs who ingest them while in the water. The blooms have an iridescent appearance similar to an oil slick on the surface of the water and have a distinctive blue-green color, so are easy to identify.” Suspected specimens should be carefully scooped up with underlying water in a bucket and brought to the Health Department for confirmation and appropriate warnings. Toxic blooms were confirmed at Hamlin Lake and Bass Lake in August. Currently Pentwater Lake has seen only single-cell green algae, but it is in high enough concentration to produce poor Secchi (visibility) readings of three feet instead of the six to seven-foot depth of visibility normally seen on the lake. That makes it challenging for fishing enthusiasts to see their lures in the water.
Primozich explains that toxic blue-green algae needs two conditions to grow: warmth and nutrients. “Although we cannot control the water temperature, we can keep nutrients out of the lake. Phosphorus enters the lake through leaves and grass clippings, and from fertilizer runoff during rain events. It settles in the muck on the lake bottom and then rises to the surface during the lake turnover.” Primozich said that in addition to NOT putting their leaves and grass clippings in the lake, riparians can also help to keep leaves from blowing into the lake and help to prevent fertilizer runoff from entering the lake by not mowing their lawns to the water’s edge. “A 12- to 15-inch-wide band of tall grass or other vegetation in front of the seawall or waterline will trap wind-blown leaves and rain runoff before it enters the water.”
Pentwater Lake has had another strong deterrent to toxic blue-green algae, and that is its mechanical harvesting program. Primozich documented that the invasive species starry stonewort grew from 2 inches to three feet thick in the lake in just three weeks this summer. The invasive fed that incredible growth with phosphorus and calcium it drew out of the water. When starry stonewort was removed from the lake by the mechanical harvesting machine, it also removed huge amounts of nutrients that would otherwise be available to feed toxic algae growth. When the starry stonewort dried out and was transported to local farmers, it provided a nutrient-rich fertilizer for crops.
Now if only there were not enough nutrients in the lake to feed the growth of starry stonewort…..
