The duality of shell deposition: how zebra mussel shells introduce a novel substrate in urban and degrade habitat in rural streams by Darrin Hunt with the University of the Ozarks in Clarksville, Arkansas

Presentation Description: Zebra mussels (Dreissena polymorpha) have invaded surface waters throughout North America and Western Europe, including urban and rural streams. Their proliferation has resulted in the deposition of sharp, jagged shells in benthic environments, which have physically altered the stream’s pre-invasion structure. This research details how zebra mussel shell deposits can obscure stream substrates, causing changes for native benthos, and explores how these impacts differ between urban and rural ecosystems. In-situ mesocosms composed of varying combinations of zebra mussel shells and natural river cobble were used to evaluate zebra mussel shells as a novel colonization substrate for macroinvertebrate communities in urban and rural reaches of three temperate streams in Southeastern Michigan. We found that macroinvertebrate communities in urban streams preferred high-density dreissenid shell substrates rather than those with fewer shells and cobble. However, sensitive gilled organisms (Ephemeroptera, Plecoptera, and Trichoptera) favored natural cobble substrates in rural streams. Rural streams commonly feature naturally heterogeneous benthic habitats, which can become homogenized by high-density shell deposits. Contrarily, shells may provide increased habitat structure and benthic complexity in urban systems, which may have been lost through channelization, sedimentation, and fragmentation. Thus, macroinvertebrates may benefit from additional habitat introduced by D. polymorpha shell deposits as a novel substrate in urban streams. Despite a consensus that invasive species negatively impact ecosystems, the legacy of dreissenid shell deposition in highly degraded urban systems yields unexpected positive outcomes.