In the fall of 2014 the DNR treated the Christmas lake infestation of zebra mussels intensively with copper sulfate, potassium chloride, and Zequanox (dead Pseudomonas bacteria).
It was thought at first that all zebra mussels had been killed.
UPDATE 5-22-2015 The Minnesota DNR announced today that 10 more adult zebra mussels were just found in Christmas Lake. The mussels were found outside the previously treated area.
No zebra mussels were found in the treated area.
So what happens now? It appears that further treatment will be given to a selective area (or areas). Exactly what the treatments will be is still being considered.
The Background on Christmas Lake: Christmas Lake in Hennepin County, only 267 acres in size, was found in 2014 to have a very limited invasion of zebra mussels. DNR officials treated a small part of Christmas Lake last fall and winter using three different treatments.
In September 2014 they treated a swimming pool size area adjacent to the access with Zequanox, a natural product which destroys the gut of zebra mussels.
In November 2014 a larger area, less than acre, was treated with a copper compound. Copper in sufficient amounts can kill a wide range of animals and plants.
In December 2014, they added potash (potassium chloride) to an area of water less than an acre through a hole drilled in the ice.
Unfortunately, it did not work. Zebra mussels were found in the spring of 2015.
The solution is easy to state, but so far impossible to obtain: get the chemical to every zebra mussel in the lake in a concentration sufficient to kill it without harming other species. A high concentration would be better at getting to hard to reach sites, but might kill desirable species.
Copper and potassium chloride can kill zebra mussels, but have the potential to kill everything else in the lake–animal and plant. So far it would appear that it has not been possible to get concentrations necessary to kill zebra mussels to every zebra mussel in the lake without affecting other species. So far, Zequanox is too expensive to use large-scale on lakes.