Quagga mussels

The Quagga mussel is a freshwater bivalve mollusk in the family Dreissenidae with an average life span of 3 to 5 years. It colonizes on surfaces such as water intake pipes, docks, boat hulls, commercial fishing nets, and native shellfish.

To prevent this colonization on its intake system in Lake Michigan, the Milwaukee Water Works feeds a small amount of chlorine at the intake cribs for each of the two water treatment plants that draw water from near the bottom of the lake. The Linnwood Water Treatment Plant intake is 6,565 feet from the shore at a depth of about 60 feet. The Texas Avenue intake, which supplies the Howard Avenue Water Treatment Plant, is 13,200 feet from shore at a depth of about 60 feet. 

These photos from the July 2018 dive inspection of the two intake cribs shows the mussels have colonized on the outside of the crib. The low dose of chlorine fed prevents the mussels from colonizing inside the crib and intake pipe.

The chlorine feed system was originally put in place in the 1990s to manage zebra mussels. Senior Scientist Dr. Russell Cuhel of the University of Wisconsin-Milwaukee School of Freshwater Sciences says the quaggas have out-competed the zebras and now are the predominant species and cover much of the bottom of Lake Michigan. Zebra mussels are larger, they need to attach to hard surfaces (e.g., rocks), and are less efficient filter feeders. Quagga mussels can attach to any kind of surface (e.g., mud, rocks, and even Zebra mussels); they’re physically streamlined and more efficient. They are evolutionarily dominant.

Read more about quaggas at the University of Wisconsin Sea Grant Institute.