Diverse environmental drivers influence food web energy and nutrient flows, a key ecosystem function, yet their relative importance remains poorly known. We compiled thousands of bulk stable isotope measurements (carbon and nitrogen) of brown trout (Salmo trutta, n = 2854) and Arctic charr (Salvelinus alpinus, n = 3062) from 120 cold-water lakes across northern Europe, spanning 0.02 to 1090 km(2) in size, to quantify the relative importance of nine potential drivers of energy and nutrient flows. Fish community type and individual body size matched the well-described ecologies of the two species: charr shifted from littoral to pelagic sources with interspecific competition, and trout opportunistically increased their trophic position with piscivorous prey availability and increasing body size. The anthropogenic impacts of water-level regulation and catchment modification were mirrored in the two species, resulting in decreased delta C-13 values, implying increased relative pelagic production in regulated lakes, and increased delta N-15 values, possibly due to increasing cross-boundary nutrient flows. Lake size and climatic drivers (summer precipitation and winter temperature) elicited strong species-specific changes in isotope values, suggesting that predator-driven ecological responses can dominate over ecosystem-wide changes in energy and nutrient flows within lake food webs. No isotopic trends were observed with catchment productivity or shoreline complexity at the regional scale, contrasting with insights from local scale studies. This spatial mismatch suggests that environmental drivers of ecosystem processes playing out at the regional scale cannot necessarily be extrapolated from localized studies, despite correlated random lake effects capturing unexplained local baseline shifts in isotopes.

iversity of Jyväskylä (JYU), Finland Location of study lakes: Fennoscandia, including Norway, Sweden and Finland More detailed description of the dataset and its variables can be found from attached metadata document.

The Norwegian electrical energy supply system is based on hydropower. The now deregulated energy market has led to increased use of hydropeaking production, leading to greater fluctuations in discharge and water levels below hydropower stations. The power station HOL 1, with an outlet to the Storåne River, is a large hydropeaking facility. With over 300 rapid flow increases and decreases per year since 2012, it is a river subjected to frequent hydropeaking. To quantify the stranding risk downstream of the power plant, the effect of a series of different turbine shutdown scenarios was simulated in an earlier study. The residual flow of 6 m3·s−1 and a full production of 66 m3·s−1 were considered as the baselines for the calculation of dewatered areas. A three-year study of juvenile fish density both upstream as a reference and downstream of the power plant was undertaken. There were very low densities or even an absence of brown trout (Salmo trutta) older than young-of-the-year (YoY) below the outlet of the power station, despite high densities of YoY in previous years. This is probably due to the large and rapid changes in flow below the power station. Hydropeaking has less impact on the earliest life stages of brown trout during spring and summer, as well as on spawning and egg development during winter. This is attributed spawning in late autumn occurring at a low flow seldom reached during hydropeaking. The high survival of YoY during the first summer and early autumn is likely due to a lower frequency of hydropeaking and higher residual flows, leaving a larger wetted area.

The Norwegian river Suldalslågen, known for its population of large‐sized Atlantic salmon (Salmo salar), has been regulated for hydropower in 1966–1967 and in 1980. The initial regulation increased winter flows and reduced summer flows and major floods. The second regulation, involving abstraction of water to a power station in an adjacent fjord, led to a strong reduction in flow. In addition to implementing different flow regimes, many remedial actions have been taken, often concurrently, making it almost impossible to detect the effect of single measures. In addition, the monitoring data have not always been consistent as regards methods and scope, and also, few data are available for preregulation conditions. This highlights major challenges in the long‐term management of regulated rivers. The absence of major floods after regulation led to increased sedimentation and encouraged carpet mosses. This reduced interstitial spaces, creating a poor habitat for salmon fry and benthic invertebrates. The knowledge gained from the wide‐ranging studies of the different flow regimes have enabled the environmental authorities to devise a final regulation regime from 2012. The final flow regime focused on biological values and functions to sustain the strain of wild, large adult salmon. The catch of wild salmon >7 kg has in fact increased since 2010 and stabilized between 1 and 2 metric tons, although the yield of large salmon prior to 1994 is unknown. In addition, the increase in the catch of large salmon is based on hatchery fish. Hatchery fish have also to a large extent contributed to the increase in the total salmon catch in recent years. Thus, that the catches in Suldalslågen are now at an all‐time high is not due to improved conditions in the river but likely to hatchery fish.

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.

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

During their early life stages (egg maturation, hatching, alevin development), between late autumn and early spring, young Atlantic salmon are exposed to surface-groundwater interactions in the hyporheic zone and may depend on influx of subsurface water during periods of regulated low discharge for survival. Two studies, one in a seasonally regulated river and one in a river exposed to hydropeaking, displayed unexpectedly high survival of eggs in surface de-watered areas because of the influx of oxygen-rich subsurface water. Field observations of newly hatched alevins in these two rivers showed them to be more sensitive (i.e. suffered higher mortality from) to surface de-watering than were eggs. Exposure to dry conditions in drawdown areas was highlighted as the main cause for alevin mortality. Therefore, shorter periods of surface de-watering in the river with hydropeaking resulted in higher alevin survival than the seasonally regulated river when still permanently drained after egg hatching. Greater consideration should be given to all early life-history stages when implementing discharge release strategies, and the extent of groundwater influence and the potential for flexible hydropower operations should be taken into account.

Groundwater may create refuges for Atlantic salmon egg survival during low flows in regulated rivers and thus play an important role for survival during winter. To investigate the links between the survival of salmon embryos and hyporheic hydrological processes during permanent winter drawdown, a 100-m-long and 50-m-wide gravel area in a regulated river, the River Suldalslågen, was used for an experimental study. Surface and subsurface water levels were monitored with 2-min time resolution by means of water pressure sensors placed in pipes. Temperature, conductivity and dissolved oxygen were also measured. Eight cylindrical boxes, each with two compartments (at 10- and 30-cm depth, respectively) containing 50 Atlantic salmon eggs, were placed in the river bed substrate of both the drawdown zone and the permanently wetted area as a reference. They were regularly checked for survival during winter from January to May, coinciding with egg development period for this river. Survival rates in boxes in the dewatered river bed were between 8 and 78% during winter, compared to 80 to 99% in the reference wetted area. The main driver for egg survival in the dewatered area was groundwater with sufficient oxygen levels.

The genetic effects on Atlantic salmon, Salmo salar L., populations from potential bottleneck situations caused by human activities in two Norwegian rivers, Laerdalselva and Batnfjordelva, were studied by analysing DNA from fish scales collected before and after the populations had been exposed to human-induced changes: river regulation, Gyrodactylus salaris infection and rotenone treatment. Using 15 microsatellites, no significant changes were found in the genetic structure and diversity of four population samples from Laerdalselva collected over 34years. However, salmon from Laerdalselva were significantly differentiated from nearby (angstrom rOyelva) and more distant (Batnfjordelva and Suldalslagen) populations, testifying to the power of the marker system to detect small genetic differences. Furthermore, two population samples from Batnfjordelva, collected 20years apart, showed low but significant differentiation. The lack of effects on neutral genetic composition in Laerdalselva, despite several potentially severe bottleneck events, indicates that stocking and sea cohorts maintain the status quo of this population.

Many west coastal and northern Norwegian rivers run through deep, confined valleys with permeable layers of glacial and alluvial deposits. Groundwater flows through these permeable layers and enter lakes and rivers as underwater seepage and springs. Groundwater inflow to inland Norwegian rivers may constitute 40-100% of total water discharge during low flow periods in late summer and winter. Juvenile salmonids may take advantage of groundwater upwellings and actively seek out such patches. In regulated rivers groundwater influx may create refuges during low flow or hydropeaking episodes. The importance of groundwater for salmon redd site selection and egg survival is also clear, although less known and documented in regulated rivers.Eggs of Atlantic salmon (Salmo salar) are deposited in redds in river bed gravels lacking fine sediments and with high oxygen levels. Egg development is therefore dependent on the interaction of a number of environmental factors such as groundwater influx, oxygen and temperature. Atlantic salmon in the regulated River Suldalslagen, Western Norway, spawn relatively late compared to other Norwegian rivers, with a peak in early January. Newly emerged fry are found from the end of May to the beginning of June, i.e. "swim up" one month earlier than expected using models for egg and alevin development and river water temperatures. The most plausible explanation is that groundwater has a higher and more stable temperature than surface river water. In field experiments, fertilized salmon eggs were placed in boxes close to natural spawning redds in the river bed at sites influenced and those not influenced by groundwater. A difference of up to 40 days in 50% hatching was found, and "swim up" occurred at the end of May in boxes influenced by groundwater.Preliminary studies have revealed that groundwater also plays an important role in survival of salmon eggs in the River Suldalslagen when dewatered in winter. Eggs placed in boxes in groundwater seepage areas during winter in the dewatered river bed survived even when covered by ice and snow. The survival from fertilization until 30 April, one month before hatching, was 91%, the same survival as found for eggs placed in boxes in the wetted river bed. However, mortality from fertilization to hatching was higher compared to the eggs placed in wetted river bed, 57 and 91% respectively.Groundwater creates a horizontal and vertical mosaic of temperatures in spawning redd areas leading to potentially greater variation in spawning sites, time of hatching and "swim up". This is likely to increase egg survival during low flow periods in regulated rivers. In conclusion, the interaction between groundwater and surface river water should therefore be considered when managing fish populations in regulated rivers. (C) 2013 Elsevier GmbH. All rights reserved.

The introduction of coregonids, mainly whitefish (Coregonus lavaretus (L.)), into new localities together with the enhancement of existing stocks was actively pursued in southern Norway from around 1850 until the 1960s, with 1875 to 1925 being the most intensive period. According to the official records of the relevant management authorities and other documentation, whitefish were released into approximately 150 localities, mostly in south-east and central Norway. Introductions often led to subsequent (secondary) spreading to downstream localities, and in some cases or at a later stage to other lakes or reservoirs through tunnels constructed in association with hydropower development. The estimated number of native whitefish populations in Norway was approximately 360, while at present time there are approximately 900 populations. Some recipient localities were fishless lakes, while many contained other species such as brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)). Whitefish were relatively successful in establishing viable and abundant populations in many lakes, sometimes with a quite devastating impact on native fish populations, mainly planktivorous species such as Arctic charr. Vendace (C. albula (L.)) was also introduced into around 16 lakes during the period 1859 to 1890, but only in one of these cases was a viable population established. In this paper we summarize the available information about coregonid introductions in Norwegian lakes, and discuss how it has changed fish diversity. We discuss the factors leading to population establishment or failure and how these introductions have impacted native fish populations, as well as the value of fisheries in some of the lakes. The management policies behind these fish cultivation practices and the historical development of management principles are also discussed.

ABSTRACT In order to understand the factors giving rise to a stable and annual outbreak of the pest blackfly species Simulium truncatum (Lundström, 1911) (Simuliidae), the oviposition habitat has been localized and the egg density quantified at different contour levels in the studied regulated river channel bank. Larvae and adults of 12 blackfly species were identified to species based on morphology. As reference library for subsequent species identification of eggs and small larvae, these specimens were subsequently DNA sequenced for the barcode gene cytochrome c oxidase subunit I. Interspecific distance was large between species or species complexes (average nearest neighbour distance: 0.14; range: 0.09–0.20), while intraspecific distance was comparatively low except for the Simulium ornatum and Simulium tuberosum species complexes. S. truncatum was the only species located high up in the channel bank. The core oviposition habitat was a steep moist erosion edge with moss and dead roots and with a continuous supply of groundwater. Egg densities were estimated to 42 773–50 274 eggs cm –2 . Humid oviposition areas high up on the riverbank, but within the annual spring flood levels, seem to be the basis for annual outbreaks of S . truncatum . The mass occurrence of S . truncatum is a phenomenon probably created by man, directly related to the river regulation regime and the construction of a dam in 1936, which gave rise to the formation of the channel and the erosion edge. Copyright © 2013 John Wiley & Sons, Ltd.

Summary The hyporheic zone may be a habitat for fish, depending on habitat patch characteristics and interactions with fish behaviour, such as movements. Theory highlights the vertical connection and potentially significant functional role of the hyporheic zone, but actual use of the hyporheic zone by fish is understudied. In replicated factorial field experiments, we investigated movements of (i) young Atlantic salmon of various size ( S almo salar , mean total lengths, 28.3, 51.7, 52.1 and 84.6 mm) through substrata of (ii) different particle size (16–22, 24–60 mm) and (iii) thickness (20, 40, 60 cm), depending on (iv) current direction (horizontal up‐ to downstream, vertical ‘upwelling’) and (v) temperature/season (summer/autumn). In nine experiments, each with different treatments, three replicated 14‐cm‐diameter translucent PVC tubes were used as a proxy for the hyporheic zone and placed in parallel on the bottom in a natural salmon stream. Each tube consisted of five separable 20‐cm segments, with a no‐substratum segment in each end and three midsegments filled with substratum. Young salmon were placed in the upstream or/and downstream free segment, localised by a snorkeler every 15 min, and after 135 min, the segments were separated and the fish per segment counted. Results indicated that (i) a significant number of fish moved into the substratum, (ii) small Atlantic salmon (28 mm) in June rapidly moved into, and even 60 cm through, both substratum particle sizes regardless of flow direction, but fewer fish into the fine substratum, and (iii) significant numbers of larger fish (52 mm, 85 mm) in September and November also moved into the 24–60 mm substratum, but not so far and relatively fewer remained in the substratum. Horizontal or vertical flow did not affect results. We conclude that young Atlantic salmon can easily move into and through the interstitial spaces in the substratum, depending on fish size and substratum coarseness. The active use by Atlantic salmon of the substratum as part of their habitat remains to be studied, but is likely to be more prevalent than commonly assumed.