Large river fish functional diversity responses to improved water quality over a 28 year period

Abstract

The enactment of environmental legislation and subsequent improvements in wastewater treatment during the late 20th century resulted in improved water quality for many large rivers in developed countries. Yet, little is known about how these improvements affected the functional diversity of aquatic assemblages, or of its relationship with species diversity during recovery. We examined species richness (SR), functional dispersion (FDis), and functional evenness (FEve) changes in four reaches of the Illinois River, USA, during a recovery period (1983–2010). The functional indices were based on fish habitat and foraging traits and weighted with either abundance or biomass. Though most correlations of the indices with SR were weak, additional species tended to increase FDis and decrease FEve. Biomass-weighted functional dispersion (FDisw) and SR consistently increased during the recovery, while the responses of FEve and abundance weighted FDis were much more variable. FDisw was better explained by improvements in water quality in all but the least impacted reach, where SR was best explained. Un-ionized ammonia, phosphorus, phenols, and water clarity were among the most important predictors of biodiversity changes. Increases in habitat and foraging specialized species appeared to contribute equally to functional changes in the three more degraded reaches, whereas increases in habitat specialization had a greater impact on FDisw changes in the least impacted reach. Later stage recovery of habitat specialist fishes in the absence of major habitat alterations indicates habitat improvement projects should be undertaken following water quality remediation. Functional diversity and species richness are complementary and both should be assessed to detect the responses of fish assemblages to environmental alterations. Biomass may better reflect functional changes in response to disturbance than abundance and should be assessed when estimating functional diversity.