SENSITIVITY TO AERIAL EXPOSURE - POTENTIAL OF SYSTEMWIDE DRAWDOWNS TOMANAGE ZEBRA MUSSELS IN THE MISSISSIPPI RIVER

We report the results of five experiments performed during periods of lowered river water level in Pool 26 of the Mississippi River. Four ex periments compared survivorship of zebra mussels (Dreissena polymorpha ) and unionid mussels (Unionidae). Under mild spring conditions (March ), survivorship of zebra mussels was similar to that of unionid mussel s, but during summer (July) survivorship of zebra mussels was lower th an that of unionid mussels. Survivorship of zebra mussels was greatest when attached to native unionids, compared with detached zebra mussel s and zebra mussels attached rocks. A fifth experiment compared surviv orship of 10 species of unionid mussels after 24 hours of aerial expos ure. In general, survivorship of thick-shelled species, such as Amblem a plicata, was greater than survivorship of thin-shelled species, such as Potamilus ohiensis. The experiments conducted suggest that drawdow ns during warm summer conditions could have a profound, negative influ ence on zebra mussel demography and distribution. In contrast, unionid mussel survival was unaffected by aerial exposure of up to 24 hours d uring a midsummer drawdown. (C) 1997 John Wiley & Sons, Ltd.