Effects of water level fluctuations on zebra mussel distribution in a Lake Erie coastal wetland

In May-August 2002, we examined how seasonal water level changes and frequent wind-driven seiches limited the amount of habitat for zebra mussels (Dreissena polymorpha) in a Lake Erie coastal wetland. We counted numbers of veligers that colonized artificial substrates at different depths (1-17 cm, 18-34 cm and >34 cm below the water surface). Water levels decreased during the summer, and by August colonization plates set at 1- 17 cm were exposed 85% of the time, plates set at 18-34 cm were exposed 15% of the time, but plates set at >34 cm were rarely exposed. The highest densities (numbers/m(2)) of colonization occurred in June (7176. 1) and August (3 119.5), and this bimodal pattern has been observed in other Great Lake habitats. Densities were highest at >34 cm depths (5552.7), intermediate at 18-34 cm (2802.1) and lowest at 1-17 cm (410.8). Veliger numbers on some plates in deep water levels were as high as 10,000/m(2), which are similar to densities in the Great Lakes. We also measured survivorship of adult mussels in wire mesh cages from 28 July to 25 August. Most (similar to 90%) adult zebra mussels survived in both 18-34 cm and >34 cm, water levels, but only similar to 2% survived in 1- 17 cm water levels. We determined the percent of Crane Creek Marsh in each of the three water depth zones and estimated that water level changes prevent zebra mussels from inhabiting 62% of this coastal wetland. The low survival of zebra mussels may be a reason why abundant unionids have recently been found in this and some other Great Lake coastal wetlands.