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In the seasonal climates of northern Europe and North America, water temperature variations are ecologically challenging for aquatic organisms in natural streams. In winter, low water temperature and ice conditions becall biological responses by aquatic species, whereas in summer water temperatures rise and may, in particular during daytime low flows, approach lethal levels. In ectothermic animals, like the cold-water fish Atlantic salmon and brown trout, and the benthic fauna, biochemical reactions and behaviors depend on water temperature. As many northern rivers are impacted by hydropower regulation, natural water temperature regimes are potentially altered. Typically, low head (run-of-the-river) regulation has limited impacts on water temperatures, but depending on the, usually limited, water storage capacity. In contrast, high head hydropower regulation systems have considerable water storage capacity in reservoirs. This alters downstream water temperatures, typically with an increase in winter (tapped from warm bottom water (below thermocline) in reservoir), and a decrease in summer (bottom water being colder than natural surface water). Thus, the naturally stable, ice-covered mid-winter periods may be replaced by repeated unstable transition periods and changing (or no) ice and flood conditions, whereas in summer, (downstream) water temperatures may be lowered when the hydropower plant is in operation. Conversely, by-pass reaches and reaches with reduced residual or environmental minimum flows during water storage, are more prone to fluctuate with environmental temperatures. In summer, this may lead to higher day-night and seasonal water temperatures and amplitudes, and lower and more frequent sub-zero temperatures in winter. However, the effects of alternative regulation regimes and reservoir intakes/outlets on downstream ambient temperatures are not well publicized. Hydro-peaked systems may aggravate high-low temperature effects, depending on flow regime (ramping rate and amplitude, frequency of flow fluctuations, prior flow conditions, timing of pulse). Atlantic salmon and brown trout feeding and growth performance in summer (and transient periods) depend primarily on water temperatures (above ≈ 3-4°C, optimum ≈ 13-16°C). At high temperatures, and approaching tolerance limits (≈ 22-24°C), trout and salmon reduce and cease feeding, and may move into cooler water in the substrate or groundwater inflow. Larger fish may move more and aggregate in restricted suitable deep-slow or cool refuge habitats, both in summer and winter. In winter, feeding and growth in typically absent, and energy storage is the basic fish low temperature survival strategy. Additional strategies may be risk reduction effected by behavioral changes, and reduced metabolism, tolerance and starvation effected by quiescence. Amount of energy stored may depend on local conditions, but otherwise there is hitherto little indication of adaptation to local thermal climates. However, phenotypic plasticity is important and substantial. The main behavioral strategy at low water temperatures (less than 6-8°C) is risk-reducing daytime sheltering in the substrate and primarily nocturnal activity. In winter, mechanical ice breakups may still reduce survival. Moreover, sudden drops in water flow due to hydropeaking may result in fish stranding, in particular at low temperatures in the daytime when fish are less mobile and seek shelter. Local movements between daytime refuges and nighttime activity areas occur both at low and high temperatures. Water temperature and/or flow may control more local spawning/smolt migrations, particularly in smaller streams. Fish egg development is also closely linked to water temperature. Emerging alevins and young fry are sensitive and vulnerable, often incurring high mortalities. Survival may depend on a delicate balance between temperature, suitable low flows and available drifting food during and after the ‘swim-up’ stage. Benthic macroinvertebrates display various strategies for dealing with high summer temperatures and low winter temperatures coupled with ice formation. Growth temperatures tend to be species specific, and as a group benthic macroinvertebrates exhibit growth across a wide temperature range. Some species are adapted to high summer temperatures, while others undergo quiescence or diapause, most commonly in the egg stage, in order to avoid such unfavorable periods. Likewise, other species avoid unfavorable periods during winter by quiescence or diapause. Another strategy, which can be coupled with diapause/quiescence, is to move down into the substrate, even into the hyporheic, especially in groundwater-fed areas. A further strategy is to undertake seasonal movements, either into tributaries or into deeper ice-free river waters. However, certain species, notably Diptera, but also other orders, have been shown to tolerate freezing – a stay-put strategy. Modified water temperatures may have major effects on life cycles, and even lead to extinction of specialist species, as many facets of growth and emergence are affected by or cued by temperature. Benthic species that have a rigid synchronous one year or univoltine life cycles are most at risk, while species with flexible and asynchronous life cycles will be favored. A reduction in the temperature range between winter and summer will lead to a reduction in species richness, again favoring the widespread, ubiquitous species. Nevertheless, most benthic organisms have high fecundity which may be offset high mortality during the different life cycle stages. Knowledge about thermal changes caused by hydropower regulation, and associated biological impacts on aquatic species, is a prerequisite for sustainable management of regulated rivers. In ongoing revisions of environmental guidelines and terms for hydropower regulated river systems, thermal changes and impacts have gained central focus. Thermal considerations will continue to be a key for setting future environmental flows and mitigation strategies. However, quantitative and observational knowledge is rather limited, whereas theoretical assumptions and modelling are more developed. Therefore, in situ target-oriented investigations in combination with more general research programs on water temperature changes, is essential, both from an operators (reduce costs and production losses) and managers perspective (fact-based knowledge and environmental guidelines). Based on this review, we recommend some main topics for further knowledge development: i) general quantitative knowledge on thermal changes due to hydropower regulation; ii) longitudinal water temperature changes from power outlet and in minimum flow reaches; iii) biological effects of reduced winter snow and ice cover, and increased frazil ice production, iv) invertebrate production in winter-warm rivers, and v) thermal adaptations to changed thermal regimes. With respect to mitigating measures, more knowledge is needed on i) effects of restored/increased minimum flows on invertebrate and fish composition, survival and production, ii) effects of changed/restored thermal regime, e. g. by installing multiple intakes in reservoirs, iii) empirical studies to test and validate the predictions of the at present quite well-developed hydrohydraulic models. Because of the substantial background variation in river systems, long-term studies are of particular importance.
