Table 2
Key findings on impacts from unnatural water level fluctuation in hydropower reservoirs and regulated lakes.
| Group | Impact | References |
|---|---|---|
| Riperian vegetation | Lowered diversity, more non-native species, and fewer rare shoreline herbs compared to a natural shore. | Hill et al., 1998 |
| Flood duration affects vegetation cover and composition. | Nilsson and Keddy, 1988 | |
| Flood magnitude and frequency impact recruitment and survival of plants, excluding flood-sensitive species. Initial promotion of flood-tolerant species can later be counteracted by continuous erosion. | Bejarano et al., 2020 | |
| Extended lowered water levels can cause water stress and reduced growth and survival of plants. | Bejarano et al., 2018 | |
| Erosion of fine sediment, associated with water level regulation, can hinder plant recruitment. | Bejarano et al., 2018 | |
| Increased ice chafing against the reservoir or river margins, associated with water level regulation, cause physical injury to plants through breakage and uprooting. | Bejarano et al., 2018 | |
| Macro-phytes | Overall macrophyte richness and abundance can be reduced by water level fluctuations. | Poikane et al., 2020; Keto et al., 2006 |
| Increase of more water level fluctuation tolerant species and decrease or disappearance of more sensitive species. | Cott et al., 2008a; Zohary and Ostrovsky, 2011; Mjelde et al., 2013 | |
| Large quillworts (Isoetes spp.) are particularly sensitive to water level fluctuations. | Turner et al., 2005; Keto et al., 2006; Mjelde et al., 2013 | |
| Artificially reduced water level fluctuations can cause changes in macrophyte composition, inducing the dominance of erect aquatic macrophytes. | Wilcox and Meeker, 1992 | |
| Water level fluctuations of 1-3 m promote macrophyte diversity, while winter drawdowns of >3 m has been suggested to be detrimental for the macrophytes community | Rørslett, 1991; Hellsten and Mjelde, 2009; Mjelde et al., 2013; Sutela et al., 2013 | |
| Benthic algae | Water level fluctuations reduce colonizable surface areas for benthic algae, leading to reduced contribution to the reservoir food web. | Turner et al., 2005 |
| Macro-Inverte-brates | Desiccation effects restrict many speciesto deeper, permanent wetted areas. | Cott et al., 2008a; Leira and Cantonati, 2008 |
| Community composition alterations and diversity reduction due to erosion, disconnection from arse substrates (roots, woody debris, etc.), and reduction of macrophytes; mediated direct impacts and decreased habitat and food availability and diversity. | Brauns et al., 2008; Zohary and Ostrovsky, 2011; Poikane et al., 2020; Roy et al., 2021 | |
| Increase in drought-resistant and mobile taxa, as well as organisms with short life cycles. Seasonal drawdowns lead to higher density and biomass of tolerant taxa such as chironomids, oligochaetes and nematodes. | Furey et al., 2006 | |
| Accumulation of organic material. Reduced outflow of reservoir- or lake derived plankton and coarse and particulate organic matter may starve downstream sections of nutrients. | He et al., 2024; Gerwing and Plate, 2019 | |
| Fish | Loss of access to structural complexity (roots, boulders, macrophytes) and food resources in the littoral zone associated with large artificial water level amplitude. | Hirsch et al., 2017 |
| Littoral benthic feeding and nest building fishes (e.g., common minnow, ruffe, bullheads, and juvenile burbot), are sensitive to artificial water level fluctuation. | Sutela and Vehanen, 2008; Sutela et al., 2011; Logez et al., 2016 | |
| Phytophilic juvenile fish (e.g., cyprinid larvae), can be sensitive to loss of macrophytes in the littoral zone. | Yamamoto et al., 2006 | |
| Loss of spawning habitat or desiccation of eggs (observed e.g., in kokanee salmon Oncorhynchus nerka and whitefish Coregonus sp.). | Modde et al., 1997; Sutela et al., 2002; Linløkken and Sandlund, 2016 | |
| For pelagic fish, water level fluctuations per se do not seem to be an important stressor and several studies fail to indicate notable regulation-dependent behavioral effects in habitat generalist predatory fish. | Vehanen and Lahti, 2003; Westrelin et al., 2018; Roy et al., 2021; Sutela et al., 2013 | |
| Water level fluctuations affect pelagic fish with littoral life-stages, and can cause habitat shifts and influence predator-prey interactions in the littoral. | Fischer and Öhl, 2005; Klobucar and Budy, 2016 | |
| Water level regulation, by disruption of nutrient exports from land to the aquatic environment, results in oligotrophication of reservoirs and regulated lakes, with effects on the planktonic food web and fish growth, size, and biomass. | Milbrink et al., 2011; Rydin et al., 2008 | |
| For riverine species that persist in reservoirs, changed selection pressures may cause evolutionary change in the affected populations. | Haas et al., 2010 | |
| Birds | Birds foraging for macrophytes, macroinvertebrates, or fish in the littoral zone may be affected by the impacts on these groups and associated change in feeding opportunities. | Cott et al., 2008a |
| Birds that nest on or very close to the water are typically adapted to the natural water level variations and can be sensitive to unnatural water level fluctuations that drown nests or expose chicks and eggs to terrestrial predators. | Keto et al., 2008; Leira and Cantonati, 2008; Walseng and Jerstad, 2014 | |
| Unpredictable change of water level from the time of nest establishment to the time chicks leave the nest can result in failed breeding and make the regulated river site into an ecological trap. Loons (Gaviidae) and some gulls (Laridae), for example, nest close to the water surface and are considered sensitive to water level increases of just a few dm. | Keto et al., 2008; Walseng and Jerstad, 2014 | |
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