Population increase and associated effects of zebra mussels Dreissena polymorpha in Lake Mille Lacs, Minnesota, U.S.A.
Zebra mussels Dreissena polymorpha were first discovered at very low densities in Lake Mille Lacs, Minnesota in 2005. This provided a unique opportunity to document the rate of growth of a zebra mussel population and examine impacts on water chemistry and other biota. Adult densities determined through dive surveys were fitted to a logistic growth model. From 2006 to 2012, average adult density increased from 0.0015/m2 to 13,651/m2, and demonstrated an exponential growth rate with 30-fold annual increase for the first few years before density dependence slowed growth. Population growth rate and maximum density were higher in sites with hard substrates than in sites that included significant amounts of sand. Veliger densities increased with adult density early in the invasion, but did not correlate well after adult density exceeded 150/m2. Water clarity did not improve as mussel density increased, nor were changes observed in chlorophyll-a, total phosphorus, or total dissolved solids. Conductivity and total alkalinity both decreased significantly after adult densities reached 10,000/m2. Direct colonization by zebra mussels greatly reduced the abundance of native unionids as well as introduced Chinese mystery snails Cipangopaludina chinensis, while other gastropod species showed little colonization by zebra mussels. Continued monitoring of this invasive population will provide a unique long-term look at zebra mussel impacts in central North America outside of the Great Lakes.