Effect of zebra mussels, Dreissena polymorpha, on macroinvertebrates in a lake-outlet stream

Zebra mussels, Dreissena polymorpha, increase macroinvertebrate abundance o n hard substrata in lakes and large rivers because they enhance surface are a, substratum heterogeneity and the accumulation of benthic organic matter (BOM). We tested the hypothesis that zebra mussel colonization of hard subs trata in a lake-outlet stream would similarly increase macroinvertebrate ab undance. Zebra mussels were attached to artificial rocks (0.05 m(2)) in a c rossed factorial design with two states of mussels (live or dead) and three densities [low (200.m(-2)), medium (500.m(-2)) or high (1000.m(-2))] plus a noncrossed control (no attached mussels). Rocks were placed in an erosion al zone of a lake-outlet stream in southwestern Michigan that contained low densities of zebra mussels. After 28 d total macroinvertebrate abundance w as significantly higher on rocks with high densities of zebra mussels, bur live or dead treatment had no effect on macroinvertebrate abundance. Family richness differed significantly among density treatments (high = medium > low = control), but Simpson's diversity index did not differ among treatmen ts. We conclude that changes in macroinvertebrate community structure proba bly were related to the increased substrate complexity provided by zebra mu ssels.