In situ survival and growth of zebra mussels (Dreissena polymorpha) under chronic hypoxia in a stratified lake

Experiments and field surveys were conducted in Hargus Lake (Ohio, U.S.) to investigate the effect of lake stratification on the survival, growth and distribution of zebra mussels. During the lake stratification period, relat ively stable temperature and dissolved oxygen (DO) gradients persisted acro ss the water column, allowing us to examine the chronic effect of hypoxia o n zebra mussels. Zebra mussels were incubated in cages and suspended at dif ferent depths in the water column at both pelagic (max. depth = 12 m) and l ittoral (max. depth = 3.5 m) sites from April 18 to September 28, 1994. No mussel survived to the end of the experiment in cages greater than or equal to 5.5 m, whereas the highest survival rate (76%) occurred at 5 m depth wh ere temperature and DO remained fairly stable for at least 3 months. The th reshold oxygen level for survival was between 1.0-1.7 mg l(-1) when water t emperature was at about 17-18 degrees C. While zebra mussels' survival rate was not affected under the sublethal hypoxic conditions, their growth was greatly retarded by poor water quality. The field survey showed that the ze bra mussels and macrophytes had about the same distribution and their bioma sses were positively related. The percentage of mussels in aggregates incre ased towards their maximum distribution depth. The maximum distribution dep th of the naturally occurring zebra mussels was only 2.8 m, whereas the adu lt mussels could survive the entire stratification period when being artifi cially placed on the 3.5 m bottom, and young mussels could colonize the 3.5 m bottom if solid substrates were provided. We conclude that lack of subst rate, rather than hypoxia, was the limiting factor of zebra mussel distribu tion above 5 m depth in Hargus Lake.