Although dams have impounded the majority of the world's altered watercourses, there is a growing awareness of the importance of mitigating or reversing some of the negative effects on aquatic ecosystems and the related services they provide. We used an ecosystem approach, including detailed studies on hydrology, geomorphology, invertebrates, fish, and food web dynamics on a river altered by waterpower production and a natural flowing river to assess system responses to a change in the altered flow regime (specifically the ramping rate or rate of change of flow). Although there was significant alteration in the flow and sediment regimes under the original restricted ramping rate regime, differences in many biotic variables in the two rivers were not significant including total invertebrate abundance and diversity, fish biomass, fish condition, and food web length. However, significant differences in the abundance and distribution of some sensitive invertebrate taxa and fish diversity were observed between the altered and natural flowing rivers as was the energy base of the food web, measured with stable isotopes. The altered river had lower overall abundance of Odonata, Ephemeroptera and Plecoptera, and Diptera, Trichoptera, Ephemeroptera, and Coleoptera increase in abundance towards the deeper and higher velocity thalweg. On average, υ 13 C values were lighter in altered sites compared to unaltered sites, likely due to carbon export from the upstream reservoir. Results will inform Canadian federal and provincial policy concerning the efficacy of ramping rate restrictions as a tool to mitigate the environmental impacts associated with peaking waterpower dam operations.

To accurately assess the flux of mass and energy to higher trophic levels in a food web using stable isotopes, the isotopic signature of basal sources is required. When studying aquatic food webs, it is difficult to obtain a signature for algae because of challenges associated with isolating small organisms from a bulk sample. In this study, we compared freshwater algal δ13C values obtained using five approaches from the literature. Results indicated that the signatures derived from a primary consumer such as Daphnia sp., from particulate organic carbon with a correction for algal biomass, and from isolated algal samples were comparable. By contrast, algal δ13C values based on the signature of carbon dioxide and algal carbon fractionation were significantly lower than those of the other approaches. The inconsistent values produced by this method were likely due to problems in determining fractionation values based on current models and were potentially related to bicarbonate uptake by algae.

A BACI (before-after-control-impact) sampling design was applied to determine the possible effects of ramping rate (RR) regulation on food webs structure and function in a regulated boreal river. We used carbon and nitrogen stable isotope signatures of primary producers, macroinvertebrates and fish to determine variations in the source of carbon fuelling the food web as well as changes in the food web structure under variable RR flow regime. We hypothesized that unrestricted RR would (1) increase the connectivity between terrestrial and aquatic environments allowing for a higher contribution of terrestrial carbon to support the food web and (2) decrease food web length because of frequent disturbances. Unrestricted RR had little influence on d 13 C values for the overall food web with most of the differences found between impacted sites compared and control sites, indicating that the proportion of various carbon sources entering the diet of consumers remained unchanged under unrestricted RR. In contrast, significantly higher d 15 N values were measured in impacted sites (invertebrates and fish) and as well as under unrestricted ramping flow regime (invertebrates). Further, unrestricted RR was associated to a significant decrease in the difference between macroinvertebrates and fish d 15 N signatures, equivalent to a reduction of the length of the food web by at least one trophic level. Results from this study indicate that RR should be taken into consideration in the regulation of operating regimes on rivers. 964 J. MARTY ET AL.Figure 2. Impacted (top) and control (bottom) sites hydrographs in relation to BACI experimental design. Black bars indicate stable isotope sampling dates and letters refer to month of year

Since 2006, the known distribution of Hemimysis anomala has greatly expanded in the Great Lakes ecosystem, with, to date, 45 sites of occurrence among 91 monitored sites, located in four of the Great Lakes and the upper St. Lawrence River. By means of carbon and nitrogen stable isotopes, a first assessment of the feeding ecology of Hemimysis was completed. The d 13 C values of 18 individuals collected in Lake Erie (Port Mainland) on a single date (Sept. 23, 2008) ranged from -30.2 to -24.5%, indicating that Hemimysis could feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. In Lake Erie, variation in d 13 C was related to d 15 N, indicating the importance of food source for determining the trophic position of Hemimysis. The d 15 N signatures of individuals were strongly related to their C/N ratios, suggesting that variations in the nutritional value of Hemimysis may depend on trophic position. Isotopic variation among individuals in Lake Erie was complemented by temporal variation in Lake Ontario. Monthly changes (from June to December 2008) in carbon isotope signatures were observed and related to changes in water temperature, highlighting the variations in the baseline prey signatures that fuel Hemimysis diets. The observed variation in stable isotope signatures occurring among individuals within a localized Hemimysis assemblage and temporally should be considered as a key design feature in further studies attempting to identify the possible effects of Hemimysis on nearshore food webs in the Great Lakes.