Does stream current modify crayfish impacts on a benthic community?

Abstract

Non-native crayfish can have impacts on stream communities that are highly variable. A possible explanation for this variability is that crayfish are responding in a context-dependent manner to variation in environmental factors. Among the most critical environmental factors in streams is current velocity. Moving water defines lotic ecosystems and many benthic organisms, including crayfish, respond to its influence. To examine how current velocity affected the impacts of non-native Orconectes rusticus crayfish on benthic communities, we conducted a cage-enclosure experiment in a Wisconsin stream. We hypothesized that current velocities > 50 cm s⁻¹ would decrease the effects that O. rusticus had on community composition (macroinvertebrate richness and numbers) and physical structure (sediment accumulation). Three cage enclosure treatments (open, closed crayfish and closed control) were grouped into eight replicate blocks divided equally among streambed regions of slow (< 50 cm s⁻¹) and fast (> 50 cm s⁻¹) current. Open cages allowed resident crayfish access, closed crayfish cages contained one O. rusticus, and closed controls had no crayfish. After three weeks, crayfish cages had lower macroinvertebrate abundance and richness and less sediment accumulation. Current velocity showed a positive relationship to macroinvertebrates in general, and a negative relationship to sediment build-up. The most common macroinvertebrates were hydropsychid caddisflies that responded to O. rusticus in a context-dependent manner such that cages containing crayfish had decreased hydropsychid numbers in fast current, but not in slow. Open cage treatments showed a similar pattern, suggesting that natural crayfish densities in the stream were also sufficient for lowering hydropsychid numbers in fast current. Our study demonstrates that current velocity may provide an important context for understanding the effects that O. rusticus have on benthic communities, not just in terms of what these crayfish do, but how the benthic communities respond to their presence.

Keywords:

• crayfish
• Hydropsyche larva
• Orconectes rusticus
• near-bed current
• Wisconsin stream

Disclosure statement

No potential conflict of interest was reported by the authors.

Acknowledgments

We wish to thank Andrew Dyson, Daniel Schnick and Lee Vang for their help in the lab and the field, and Lois Helland for allowing us access to Lowes Creek.

Notes on contributors

Dr. Todd Wellnitz is a professor of biology at the University of Wisconsin – Eau Claire where he teaches courses in field biology, aquatic ecology and invertebrate zoology. He studies how current and flow structure stream communities and the impact that humans have on wilderness areas.

Drew Frase studied chemistry and biology at the University of Wisconsin – Eau Claire and is currently a research assistant in the Program in Gene Expression and Regulation at the Wistar Institute's Center for Chemical Biology and Translational Medicine.

Margaret Gapinski is a high school teacher at Rufus King International Baccalaureate High School in the Milwaukee Public School District. She teaches Biology and Environmental Health with a focus on environmental toxins and water quality.

Hayley Emerson Haggerty studied Ecology, Evolutionary Biology and Psychology at the University of Wisconsin – Eau Claire.

Additional information

Funding

This project was supported by BluGold Commitment funding provided by the University of Wisconsin – Eau Claire’s Office of Research and Sponsored Programs and Division of Environmental Biology. Funding was also provided by NSF DEB-0642512 to TW.