Morphological divergence was evident among three sympatric morphs of Arctic charr (Salvelinus alpinus (L.)) that are ecologically diverged along the shallow-, deep-water resource axis in a subarctic postglacial lake (Norway). The two deep-water (profundal) spawning morphs, a benthivore (PB-morph) and a piscivore (PP-morph), have evolved under identical abiotic conditions with constant low light and temperature levels in their deep-water habitat, and were morphologically most similar. However, they differed in important head traits (e.g., eye and mouth size) related to their different diet specializations. The small-sized PB-morph had a paedomorphic appearance with a blunt head shape, large eyes, and a deep body shape adapted to their profundal lifestyle feeding on submerged benthos from soft, deep-water sediments. The PP-morph had a robust head, large mouth with numerous teeth, and an elongated body shape strongly related to their piscivorous behavior. The littoral spawning omnivore morph (LO-morph) predominantly utilizes the shallow benthic–pelagic habitat and food resources. Compared to the deep-water morphs, the LO-morph had smaller head relative to body size. The LO-morph exhibited traits typical for both shallow-water benthic feeding (e.g., large body depths and small eyes) and planktivorous feeding in the pelagic habitat (e.g., streamlined body shape and small mouth). The development of morphological differences within the same deep-water habitat for the PB- and PP-morphs highlights the potential of biotic factors and ecological interactions to promote further divergence in the evolution of polymorphism in a tentative incipient speciation process. The diversity of deep-water charr in this study represents a novelty in the Arctic charr polymorphism as a truly deep-water piscivore morph has to our knowledge not been described elsewhere.

Sustainable development of hydropower demands a holistic view of potential impacts of water level regulation (WLR) on reservoir ecosystems. Most environmental studies of hydropower have focused on rivers, whereas environmental effects of hydropower operations on reservoirs are less well understood. Here, we synthesize knowledge on how WLR from hydropower affects alpine lake ecosystems and highlight the fundamental factors that shape the environmental impacts of WLR. Our analysis of these impacts ranges from abiotic conditions to lower trophic levels and ultimately to fish. We conclude that the environmental effects are complex and case-specific and thus considering the operational regime of WLR (i.e., amplitude, timing, frequency, and rate of change) as well as the reservoir's morphometry, geology and biotic community are prerequisites for any reliable predictions. Finally, we indicate promising avenues for future research and argue that recording and sharing of data, views and demands among different stakeholders, including operators, researchers and the public, is necessary for the sustainable development of hydropower in alpine lakes.

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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.

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