No abstract

Water level fluctuations are expected to deteriorate the littoral zone in heavily regulated hydropower reservoirs, but there is limited empirical evidence of how food webs and fish populations are affected. We contrasted the size, growth, condition, niche use (i.e., habitat and diet), and parasite infection of allopatric Arctic charr (Salvelinus alpinus) populations in two neighboring and comparable Norwegian mountain lakes. We hypothesized that the presumed abiotic and biotic deterioration of the littoral zone would lead to reduced abundance and growth as well as to increased pelagic niche use and reduced niche width of the charr in the heavily regulated Govdajavri (maximum regulation amplitude 24 m) as compared with the unregulated Cazajavri. Our stable isotope and parasite data showed that charr had a slightly narrower and more pelagic feeding niche in the regulated than in the unregulated lake. The relative abundance of charr was lower in the regulated lake, but no between‐lake differences were observed in charr condition, and the charr grew slightly faster in the regulated than in the unregulated lake. Our study suggests that impaired littoral production can alter food webs in alpine hydropower reservoirs and induce a pelagic niche shift by top predators. These results argue for further investigations of hydropower impacts on lake food webs along with other factors that influence the abundance and niche use of fish, such as intraspecific interactions and compensatory growth, which may partly mask the potential impacts.

Long-term monitoring of the food web of a regulated hydropeaking river was conducted to assess if previously documented effects of changing ramping rates (RRs) were maintained with the addition of 6 years of data. Using carbon and nitrogen stable isotope analyses, we hypothesized that: (1) macroinvertebrates and fish inhabiting areas below peaking hydrodams would be higher in δ 15 N and lower in δ 13 C due to increased flow velocity and the influence of light respired dissolved inorganic carbon, relative to those sampled from areas with a natural flow regime; (2) the increase in δ 15 N of macroinvertebrates would lead a shorter food web length in the regulated river, but δ 13 C and niche width would be similar between the restricted and unrestricted RR periods (i.e., the BACI analysis); and (3) isotopic metrics (e.g., δ 13 C, δ 15 N, niche width [SEA B ], and food chain length [Δ 15 N]) would correlate with variations in flow characteristics through time. Consistent with previous analysis conducted over a shorter time period, a shift toward higher δ 15 N values was observedfor both fish and invertebrates, but, contrarily, only invertebrates (not fish) had a lower δ 13 C value downstream of the dam. Over the long term, the before-aftercontrol-impact analysis found no effect of RRs on any of the food web metrics, implying that the change in operation did not affect the river food web. However, analysis of the time series data indicated that flow metrics and trophic metrics were often correlated, including a negative effect of RR (invertebrates) and discharge (fish) on food chain length. This study illustrates the difficulty in detecting changes in food web structure and function under changing flow regime influenced by natural and anthropogenic effects. As such, this study highlights the need for considering large spatial and temporal scales to differentiate between confounding effects of climate, natural variability, and altered flow regimes on food webs in regulated rivers. KEYWORDScarbon and nitrogen isotopic niche, flow, hydropeaking, regulated rivers, stable isotopes

Low‐flow events can reduce food availability and decrease the feeding niche of consumers within rivers. Stable carbon (δ13C) and nitrogen (δ15N) isotope and stomach content analyses were employed to evaluate resource use and overlap between fish species in a natural and regulated river in normal and low‐flow years, with the use of multiple methodological approaches providing the best means of understanding short‐term and long‐term observations on fish feeding and resource overlap under changing flow conditions. Diet analyses generally indicated significant inter‐specific differences in the diets of key fish species within rivers and similarities in resource use between rivers. In comparison with fish from the natural river, fish from the regulated river had lower and less inter‐annually variable δ13C values. In the natural river, there was a significant reduction and increase, respectively, in δ13C and δ15N variation in the low‐flow year. Intra‐annual or inter‐annual differences in trophic niche area were not apparent in the regulated river, whereas within the natural river, intra‐annual and inter‐annual differences in trophic niche were found. Resource overlap between key fish species was also higher in the low‐flow year and lower in the spring and higher in the summer as a result of differences in flow. Resource overlap was also higher between rivers in the low‐flow year. High resource overlap between rivers during decreased summer flow indicates a strong effect of flow on river organisms, where both fish and their invertebrate prey resources are concerned. Copyright © 2014 John Wiley & Sons, Ltd.

Few studies have examined the temporal variability of fish feeding niche in response to variable flows and temperature or the temporal consistency of spatial differences in fish feeding niche within natural rivers. Using a 10-year dataset from the boreal Batchawana River in Northern Ontario, we found that fish feeding niche was temporally invariant in the lower sampled river reaches but increased over time in the upper reaches of the river. A significant relationship between the standard deviations of mean δ 15 N and mean daily summer flow was found. No other significant relationships between measures of flow or temperature variability and variability in δ 13 C or δ 15 N were observed. Fish feeding niche was significantly larger in the lower than in the upper Batchawana River, but there were no significant differences in mean fish community δ 13 C or δ 15 N between reaches. Fish assemblage δ 13 C, δ 15 N and standard ellipse area were consistent among years within river reaches despite flow and temperature variability over the same years. Results highlight that in natural undisturbed rivers, fish feeding niche appears to be temporally invariant in the face of naturally imposed environmental variability.