Hydrologic modification and global climate change have resulted in rapid and dramatic changes in community structure across a wide range of ecosystems. Freshwater ecosystems are particularly sensitive to environmental change and anthropogenic modification which are known to exhibit multiple regime shifts and alternative stable states. Large rivers are susceptible to anthropogenic modification and identifying how communities are responding to these changes is imperative for active restoration. We evaluated changes in fish community structure and identified both the environmental and hydrological drivers of these changes across six study reaches within the Upper Mississippi River System (UMRS). Catch-per-unit effort was estimated for every species and standardized across multiple gear types using the multigear mean standardization method to form a community composition matrix. Water quality variables from the Long-Term Resource Monitoring Program were compiled along with hydrological variables from local water gages. A similarity percentage index (SIMPER) was used to identify changes in fish community structure and the species responsible for these changes. Principal Components Analysis (PCA) then identified temporal patterns in both water quality and hydrology, and a Mantel test was used to evaluate whether the observed changes in fish community structure were driven by changes in water quality or hydrology.
Results/Conclusions
SIMPER analysis detected an average 38.56% +/- 2.11 change for Pool 4, 48.62% +/- 2.41 for Pool 8, 47.94% +/- 1.54 for Pool 13. The southern three reaches indicated an average 45.10% +/- 1.27 change for Pool 26, 34.60% +/- 1.31 for the Open River, and 34.81% +/- 1.18 for the La Grange reach from 1994 to 2018. The most common species to contribute to community shifts among the six study reaches were Bluegill (Lepomis macrochirus) and Common Carp (Cyprinus carpio). Results indicated that these species contributed between 6.68-13.23% and 6.83-32.98% of the differences respectively for five of the six study reaches. The recently introduced Silver Carp (Hypophthalmichthys nobilis) resulted in >5% of the dissimilarity for two of the three southern reaches. PCA revealed temporal trends in both water quality and hydrology that were driven by changes in primary productivity and the duration of floods respectively. Changes in the fish community were driven primarily by water quality at all study reaches except Pool 13. Although increases in primary productivity and water quality are associated with flood pulses, our results indicate that the fish communities are responding indirectly to flood pulses via changes in water quality.