Modest dispersal, low densities, and little change in water clarity characterize zebra mussel populations near the southern extent of their range

Abstract

AbstractInvasive, filter feeding zebra mussels(Dreissena polymorpha)typically cause an increase in water clarity shortly after their establishment. The purpose of this study is to evaluate whether this occurred in Midwest reservoirs, near the southern edge of their North American expansion, using a 40 + year dataset. We look for regime shifts and long-term trends in annual water clarity and compare these to the estimated zebra mussel invasion date for 7 invaded reservoirs in Missouri, USA. We also look at water clarity in 26 non-invaded, reference reservoirs to evaluate if zebra mussel impacts are being masked by changes in environmental factors. Collectively, our analyses provide a weight of evidence showing that zebra mussel establishment did not increase water clarity, likely because densities are too low to result in a noticeable impact. The highest zebra mussel density we observe is 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. We identified common characteristics of invaded reservoirs, including reduced particulate inorganic material and water temperatures.