Effect of zebra mussels (Dreissena polymorpha) on trophic state in northern Michigan lakes

Undergraduate Research Exper.Invasive species cause massive damage to the environment, human health, and the economy. Zebra mussels (Dreissena polymorpha) first invaded North America in 1988 and have had dramatic effects on many lake ecosystems. They are prolific filter feeders, effectively decreasing plankton densities and increasing water clarity, thereby altering the trophic state of lakes. I used Carlson’s Trophic State Index to quantify changes in lake trophic state associated with zebra mussels. I examined the effect of zebra mussels on trophic state by comparing lake TSI values pre- and post-zebra-mussel invasion and the influence lake morphometry has on the zebra mussels’ effect on trophic state. Secchi depth, chlorophyll-a, and total phosphorous data were collected at six lakes in northern lower Michigan. Sites were paired in each lake according to depth (15 m). No significant differences were found between shallow versus deep sites among any of the variables. Comparisons of trophic state variables showed no significant changes from data collected in 1973-1974. Zebra mussel density differed among lakes (0 – 3,600 m-2) but showed no correlation with trophic state variables. TSI for Secchi was significantly greater than TSI for TP or chlorophyll. These results suggest that zebra mussel influences on trophic state variables were minor at this early stage in the invasion of these inland lakes.http://deepblue.lib.umich.edu/bitstream/2027.42/57587/1/Stoermer_Danielle_2007.pd