Fish and macroinvertebrate assemblage composition and diversity at revetted banks in the Pearl River and the response of these assemblages to a paper mill effluent spill

The armoring of river banks with riprap can have detrimental effects on lotic ecosystems due to the subsequent alteration of hydrologic regimes; however, evidence suggests that riprap can also increase aquatic diversity in degraded systems. The goal of my study was to determine what impacts riprapped banks have on fish and macroinvertebrate assemblages in the Pearl River, which has a history of anthropogenic degradation. I collected fishes with an electrofishing boat from armored and natural banks at five regions during fall 2011, winter 2012, and summer 2012. I also collected macroinvertebrates with introduced substrates in fall 2011. Richness was analyzed with rarefaction curves, whereas seasonal abundance and evenness were analyzed with ANOVA, and differences in assemblage structure were assessed with PERMANOVA. Fish and macroinvertebrate richness never varied between bank types. However, fish evenness and abundance were higher at riprapped banks during the summer, and fish assemblage composition varied during base flows in the summer and fall. Macroinvertebrate and assemblage structure also varied between bank types in the fall. My results imply that, at base flows, riprapped banks in the Pearl River support unique aquatic assemblages, possibly due to differences in heterogeneous habitat availability. On August 9, 2011 waste material from a paper mill in Bogalusa, LA was accidentally discharged into the Pearl River, causing anoxic conditions that resulted in complete fish extirpation downstream of the spill’s source. ANOVA indicated that, by October 2011, fish species richness and evenness at sites ~10 km downstream of the spill did not differ from sites in undisturbed areas, although richness and evenness at sites ~40 km downstream were still significantly lower at this time. However, by January, richness and evenness at all disturbed and undisturbed sites were similar. PERMANOVA indicated that, despite similarities in richness and evenness, fish assemblage composition at sites ~10 km downstream of the spill remained significantly different from undisturbed areas until January 2012, while sites ~40 km downstream of the spill were still significantly different by the study’s end in July 2012. These results suggest a gradual recovery, with colonization rates related to the proximity of source populations