Dispersal limitations on fish community recovery following long-term water quality remediation

In-stream barriers may impose constraints on the ecological effectiveness of restoration strategies by limiting colonization. We assessed the importance of dispersal limitations to fish community recovery following long-term pollution abatement, water quality remediation, and species introductions within the White Oak Creek watershed near Oak Ridge, Tennessee (USA). Long-term (26 years) responses in fish species richness, biomass, and community composition to water quality remediation were evaluated in light of physical barriers (culverts and weirs). We found that barriers to dispersal were potentially limiting fish community recovery by preventing colonization by introduced species and seasonal migrants. Changes in richness were negatively related to barrier index, a measure of the degree of isolation by barriers. Following introductions, upstream passage for six fish species above non-passable barriers was not observed. Highly isolated sites were dominated by a few equilibrium species, whereas less isolated sites showed more variation in life history strategies with increasing periodic and opportunistic strategists. The importance of barriers on community dynamics decreased over time—an indication of increasing community stability, homogenization of fauna, and improved water quality. However, isolating the role of dispersal limitation was complicated by multiple interacting stressors, such as the compounding effects of barriers and pervasive water quality conditions.