Dams, dredging, and darters: Effects of anthropogenic disturbances on fish assemblages

Anthropogenic disturbances such as riverine gravel dredging and the construction of dams can alter hydrodynamics and nutrient flow within rivers, but their potential roles in shaping fish communities are not fully understood.  We used species richness and diversity matrices as well as stable isotope analysis of δ¹³C and δ¹⁵N to quantify differences in fish assemblages in the lower Allegheny River in Pennsylvania.  Dredged sites had the lowest species diversity, while undredged sites had the highest diversity.  Stable isotope analysis revealed shifts from periphyton-derived nutrients at free-flowing and undredged sites to primary reliance on phytoplankton and terrestrial detritus at deep-water dredged sites, suggesting a loss of benthic nutrient pathways.  These results are coincident with the loss of habitat heterogeneity associated with a decrease in the natural riffle-pool-run sequences and the increase of long continuous pools resulting from dam construction.  As a consequence, lotic species are displaced by lentic species, while generalist and invasive species displace native habitat specialists.  Sediment and organic detritus accumulate in dredged reaches and behind dams, disrupting nutrient flow and destroying critical habitat for lithophilic species.  Understanding the interaction of these processes is essential for informing sound management and conservation decisions.