Hydrologic variation is a core driver of river ecosystem structure and function.  Many studies have focused on how hydrology and river regulation affects discrete, short-lived, and often well-studied taxa over relatively brief ecological time periods.  We have focused on how hydrology and river regulation affects long-lived, resistant taxa by studying long-term growth trends for fishes (>20 y), bivalve mussels (>50 y), and riparian trees (>250 y) in an unregulated (Sipsey River, AL) and a nearby regulated (Pearl River, MS) floodplain river of similar discharge. 

Results/Conclusions

Mean annual growth rate (mm y^-1) was significantly lower for 11 of 14 fishes and was never significantly higher at the regulated site.  Mean annual growth of all riparian tree species at the regulated site was significantly higher pre-dam (pre-regulation) than post-dam; in contrast, tree growth at the unregulated sites showed no such pattern for any species during the same time period.  In all the unregulated river reaches, interannual growth variations were synchronous within and among species and taxa and correlated to one or more major components of the annual hydrograph. Patterns included: growth of fishes was positively correlated to Spring streamflows, bivalve mussel growth was negatively correlated to 90-day minimum flows, and riparian tree growth was positively correlated to 90-day minimum flows and negatively correlated to hydrographic fall rate.  In contrast, synchronicity of annual growth in the regulated reach was lost; growth of fishes and mussels was not correlated to any hydrologic variables, and growth of trees was either not or negatively correlated to 90-day minimum flows (this correlation was uniformly positive at unregulated reaches).  These results demonstrate that multiple aspects of the natural flow regime are critical to the overall productivity of floodplain ecosystems and the fitness of its constituent organisms.  We suggest that management of floodplain river ecosystems is optimized when the entirety of the unregulated river hydrograph is conserved as opposed to focusing on selected factors perceived as important.