Nutrient enrichment, flow regulation, and their combined effects on the structure and function of riverine communities in the Saint John River, NB, Canada.

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University of New Brunswick


Nutrient enrichment and flow regulation are common stressors that alter the structure and function of riverine communities globally, yet their combined effects, particularly in large rivers, are rarely studied. The effects of these stressors on the ecosystem integrity of the 7th order, Saint John River, NB, Canada, were studied by examining various measures of community structure and function including decomposition rates, whole ecosystem metabolism, benthic community assemblages, and trophic structure (i.e., carbon and nitrogen stable isotopes) during summer and autumn 2004-2007. Trophic status of the river was greatly affected by nutrient enrichment as mean total phosphorus (TP) concentrations ranged from mesotrophic to eutrophic and hyper-eutrophic in unregulated and regulated reaches, respectively. The regulated reach experienced extreme daily changes of water depth (⁓70 cm) that intermittently exposed 50% of the river bottom during the hottest and driest portions of the day (noon until late evening). Complex interactive effects between nutrient enrichment and flow regulation affected functional and structural community metrics. The interaction between nutrient enrichment and flow regulation significantly affected decomposition rates, which were faster in the regulated reach, particularly at nutrient enriched sites. Flow regulation was the significant stressor affecting whole ecosystem metabolism as measured by Production/Respiration (P/R) ratios. P/R ratios indicated the river was respiration-dominated (P/R <1) and more so in the regulated reach (P/R=0.31) versus in the unregulated reach (P/R=0.44). Fungal species and spore number and δ13C and δ15N values of benthic primary producers and benthic macroinvertebrate (BMI) consumers were greater in the regulated reach, except primary producer δ13C values during the summer, which supported the trends observe using functional metrics. Reduced water depth during the afternoon in the regulated reach caused increased water temperature and reduced dilution capacity for municipal and industrial effluents. The resulting increased concentrations of nitrogen (N), phosphorus (P), and carbon (C) may enter the microbial loop, enhancing decomposition rates and the heterotrophic state (decreasing the P /R ratio) in the regulated reach relative to the unregulated reach. Finally, differences in algal and BMI community assemblages corresponded most closely to differences in flow regulation, and a common group of taxa, comprised mostly of pollution tolerant organisms, contributed to the differences in community assemblage. Structural and functional metrics responded differently to particular stressors, yet both indicated that the ecosystem integrity of the Saint John River is impaired by nutrient enrichment and flow regulation. The prevalence of interactive effects between nutrient enrichment and flow regulation indicates that both factors need to be considered when setting nutrient guidelines and flow regulation regimes that protect ecological integrity of the river.