Chemical fragmentation of stream systems and fish species richness: The upstream effects of water pollution

The loading of chemical pollutants into stream systems can result in isolation of aquatic communities upstream of the discharge. Ecological theory suggests that isolation will result in species richness losses, but this potential indirect effect of stream pollution is not widely studied. Here, I present a study of fish assemblage structure in watersheds that have been longitudinally fragmented by abandoned mine drainage. A number of headwater streams in the Clarion River watershed of the upper Ohio River Basin, Pennsylvania, USA, have escaped any direct effects of abandoned mine drainage but have been isolated by severe pollution lower in the watershed. I evaluated the effects of watershed fragmentation on fish species richness, composition, and abundance by comparing the fish assemblages of 22 headwater streams embedded in watersheds receiving mine effluent with the assemblages of 34 reference streams with no apparent blockages to dispersal. I used a general linear model to evaluate the effect of isolation on species richness while accounting for stream size and water quality. I also used non-metric multidimensional scaling and permutational analysis of variance to evaluate differences in fish species composition between isolated and reference streams. Controlling for the effect of stream size, reference streams had a mean richness of 3.63 species (±0.27 SE), but isolated streams had a mean species richness of just 1.02 (±0.33), a 3.5× difference. Species occupying isolated stream systems were a nested subset of fish species in reference streams. Species composition also differed between isolated and reference streams, with higher relative abundance of Creek Chubs (Semotilus atromaculatus [Mitchill, 1818]) and lower relative abundances of Mottled Sculpin (Cottus bairdii Girard, 1850) and Brook Trout (Salvelinus fontinalis [Mitchill, 1814]) in isolated streams. These results illustrate an important indirect effect of water pollution in dendritic watersheds and provide evidence for the importance of connectivity in maintaining aquatic biodiversity.