Using Functional Diversity to Examine the Long-Term Effects of Improved Water Quality on Fish Assemblages

For over 100 years The Illinois River Waterway (IRW) has been subjected to municipal and industrial effluents emanating from the Chicago Metropolitan Area. Electrofishing surveys found the upper-IRW extremely depauperate until the early 1980s when researchers began detecting previously extirpated fishes. Researchers hypothesized these changes were due to improved water quality resulting from the 1972 Clean Water Act and implementation of the Chicago Area Tunnel and Reservoir Plan, but lacked comparable water quality data to test this hypothesis in a rigorous manner. Following an extensive search, the Metropolitan Water Reclamation District of Greater Chicago provided a spatially comparable water quality dataset for 200km of the upper-IRW (1983-2010). Changes in fish assemblages were characterized using four functional dispersion indices weighted with either abundance or biomass. We analyzed how these whole assemblage depictions responded to changes in 12 water quality parameters and 16 weather variables (included to reduce inter-annual “noise”) using random forests regressions (0 ≤ pseudo-R² ≤ 0.65). Models strongly indicated that improvements in ammonia, phenols, and water clarity were critical to fish assemblage recovery. Our long-term study demonstrates the importance of water quality to fishes, the advantages of biomass-based functional dispersion, and highlights the need for inter-agency cooperation.