Low densities and little change in water clarity characterize zebra mussel invasions near the southern extent of their range

Abstract Invasive species are often assumed to have measurable impacts throughout their potential range, but this is not always true. This study evaluated whether zebra mussels ( Dreissena polymorpha ) were causing an increase in water clarity near the southern edge of their North American expansion using a 40+ year dataset. We looked for abrupt shifts and long-term trends in water clarity and compared these to the estimated zebra mussel invasion date for 7 reservoirs in Missouri, USA. We also looked at water clarity in 26 non-invaded reservoirs to evaluate whether zebra mussel impacts were being masked by variation in environmental factors. Collectively, our analyses provide a weight of evidence based on a lack of consistent shifts in water clarity corresponding with zebra mussel invasions. Mussel establishment did not increase water clarity, likely because densities were too low to result in a measurable impact. The highest zebra mussel density we observed was 65 mussels m −2 , an order of magnitude less than in systems where they have had a sustained impact. Low densities could be due to a combination of sublethal environmental conditions such as warm water temperatures, suspended inorganic particulates, fluctuating water levels, and reduced oxygen concentrations, as well as limited hard substrata or predation by fish. While additional study is required to understand which, if any, of these factors may be limiting zebra mussel establishment in Missouri, our study shows that dreissenids are not having an impact as elsewhere throughout their range, suggesting that how an invader alters ecosystem functions is critical to determining its impact.