Effects of multiple stresses hydropower, acid deposition and climate change on water chemistry and salmon populations in the River Otra, Norway

Understanding the role of biodiversity in the climate, food, water, energy, transport and health nexus in Europe

Biodiversity underpins the functioning of ecosystems and the diverse benefits that nature provides to people, yet is being lost at an unprecedented rate. To halt or reverse biodiversity loss, it is critical to understand the complex interdependencies between biodiversity and key drivers and sectors to inform the development of holistic policies and actions. We conducted a literature review on the interlinkages between biodiversity and climate change, food, water, energy, transport and health ("the biodiversity nexus"). Evidence extracted from 194 peer-reviewed articles was analysed to assess how biodiversity is being influenced by and is influencing the other nexus elements. Out of the 354 interlinkages between biodiversity and the other nexus elements, 53 % were negative, 29 % were positive and 18 % contained both positive and negative influences. The majority of studies provide evidence of the negative influence of other nexus elements on biodiversity, highlighting the substantial damage being inflicted on nature from human activities. The main types of negative impacts were land or water use/change, land or water degradation, climate change, and direct species fatalities through collisions with infrastructure. Alternatively, evidence of biodiversity having a negative influence on the other nexus elements was limited to the effects of invasive alien species and vector-borne diseases. Furthermore, a range of studies provided evidence of how biodiversity and the other nexus elements can have positive influences on each other through practices that promote co-benefits. These included biodiversity-friendly management in relevant sectors, protection and restoration of ecosystems and species that provide essential ecosystem services, green and blue infrastructure including nature-based solutions, and sustainable and healthy diets that mitigate climate change. The review highlighted the complexity and context-dependency of interlinkages within the biodiversity nexus, but clearly demonstrates the importance of biodiversity in underpinning resilient ecosystems and human well-being in ensuring a sustainable future for people and the planet.

Light and temperature controls of aquatic plant photosynthesis downstream of a hydropower plant and the effect of plant removal

Many rivers worldwide are regulated, and the altered hydrology can lead to mass development of aquatic plants. Plant invasions are often seen as a nuisance for human activities leading to costly remedial actions with uncertain implications for aquatic biodiversity and ecosystem functioning. Mechanical harvesting is often used to remove aquatic plants and knowledge of plant growth rate could improve management decisions. Here, we used a simple light-temperature theoretical model to make a priori prediction of aquatic plant photosynthesis. These predictions were assessed through an open-channel diel change in O2 mass balance approach. A Michaelis-Menten type model was fitted to observed gross primary production (GPP) standardised at 10 °C using a temperature dependence from thermodynamic theory of enzyme kinetics. The model explained 87 % of the variability in GPP of a submerged aquatic plant (Juncus bulbosus L.) throughout an annual cycle in the River Otra, Norway. The annual net plant production was about 2.4 (1.0-3.8) times the standing biomass of J. bulbosus. This suggests a high continuous mass loss due to hydraulic stress and natural mechanical breakage of stems, as the biomass of J. bulbosus remained relatively constant throughout the year. J. bulbosus was predicted to be resilient to mechanical harvesting with photosynthetic capacity recovered within two years following 50-85 % plant removal. The predicted recovery was confirmed through a field experiment where 72 % of J. bulbosus biomass was mechanically removed. We emphasise the value of using a theoretical approach, like metabolic theory, over statistical models where a posteriori results are not always easy to interpret. Finally, the ability to predict ecosystem resilience of aquatic photosynthesis in response to varying management scenarios offers a valuable tool for estimating aquatic ecosystem services, such as carbon regulation. This tool can benefit the EU Biodiversity Strategy and UN Sustainable Development Goals.

No additional stress of sublethal gas supersaturation in a landlocked population of Atlantic salmon (Salmo salar) exposed to environmental acidification

The landlocked Atlantic salmon population "bleke" faces extinction due to environmental acidification (EA) and hydropower expansion in the Norwegian river Otra. Despite of restoration, unexpected mortality has been reported for this population, possibly due to gas bubble trauma (GBT) from gas supersaturation (GSS) downstream of hydroelectric plants, or EA induced aluminum toxicity. In this study, we applied the allostasis concept to investigate interactions between EA and GBT. This concept comprises additive effects of stressors, which can lead to allostatic overload. Stress coping mechanisms become maladaptive in such situations, which can be indicated by an inability to mount a proper cortisol response in fish. Fish were exposed to sublethal levels of simulated EA (SEA), GSS (a total gas pressure; TGP; of 110%) or a combination of these stressors for six days. Effects on allostatic load were subsequently investigated by assessing the cortisol response to an acute stress test. SEA increased cortisol responsiveness and GSS induced clinical signs of GBT, but no interacting effects between GSS and SEA were observed. This suggests that that 110% TGP did not have an additive effect on the allostatic load imposed by SEA.

Estimation of ecosystem respiration and photosynthesis in supersaturated stream water downstream of a hydropower plant

The estimation of whole stream metabolism, as determined by photosynthesis and respiration, is critical to our understanding of carbon cycling and carbon subsidies to aquatic food-webs. The mass development of aquatic plants is a worldwide problem for human activities and often occurs in regulated rivers, altering biodiversity and ecosystem functions. Hydropower plants supersaturate water with gases and prevent the use of common whole stream metabolism models to estimate ecosystem respiration. Here we used the inert noble gas argon to parse out biological from physical processes in stream metabolism calculations. We coupled the O2:Ar ratio determined by gas chromatography in grab samples with in-situ oxygen concentrations measured by an optode to estimate aquatic plant photosynthesis and ecosystem respiration during supersaturation events through a parsimonious approach. The results compared well with a more complicated two-station model based on O2 mass balances in non-supersatured water, and with associated changes in dissolved CO2 (or dissolved inorganic carbon). This new method provides an independent approach to evaluate alternative corrections of dissolved oxygen data (e.g. through the use of total dissolved gases) in long term studies. The use of photosynthesis-irradiance models allows the determination of light parameters such as the onset of light saturation or low light use efficiency, which could be used for inverse modelling. The use of the O2:Ar approach to correct for oversaturation may become more applicable with the emergence of portable mass inlet mass spectrometers (MIMS). Photosynthesis was modest (2.9-5.8 g O2 m2 day-1) compared to other rivers with submerged vegetation, likely indicating nutrient co-limitations (CO2, inorganic N and P). Respiration was very low (-2.1 to -3.9 g O2 m2 day-1) likely due to a lack of allochthonous carbon supply and sandy sediment.

Management strategy of the naked carp (Gymnocypris przewalskii) in the Qinghai lake using matrix population modeling

Naked carp (Gymnocypris przewalskii) is the only fish species commercially harvested in Qinghai Lake, which is the largest inland saltwater lake in China. Multiple ecological stresses such as long-term overfishing, drying-up of riverine inflows, and decreases in spawning habitat caused the naked carp population to decrease from 320,000 tons before the 1950s to only 3000 tons by the early 2000s. We used matrix projection population modeling to quantitatively simulate the dynamics of the naked carp population from the 1950s to the 2020s. Five versions of the matrix model were developed from the field and laboratory information that represented different population states (high but declining, low abundance, very low abundance, initial recovery, pristine). Equilibrium analysis was applied to density-independent versions of the matrices and population growth rate, age composition, and elasticities were compared among versions. Stochastic, density-dependent version of the most recent decade (recovering) version was used to simulate the time-dependent responses to a range of levels of artificial reproduction (addition of age-1 from hatchery) and of the pristine version to simulate combinations of fishing rate and minimum age of harvest. Results showed the major role of overfishing in the population decline and that the population growth rate was most sensitive to the survival of juveniles and the spawning success of early-age adults. Dynamic simulations showed a rapid population response to artificial reproduction when population abundance was low and that if artificial reproduction continues at its current level, then population biomass would reach 75% of its pristine biomass after 50 years. Simulations with the pristine version identified sustainable fishing levels and the importance of protecting the first few ages of maturity. Overall, modeling results showed that artificial reproduction under conditions of no fishing is an effective approach to restoring the naked carp population. Further effectiveness should consider maximizing survival in the months just after release and maintaining genetic and phenotypic diversity. More information on density-dependent growth, survival, and reproduction, as well as on the genetic diversity and growth and migratory behavior (phenotypic variation) of released and native-spawned fish, would help inform management and conservation strategies and practices going forward.

Effects of repeated short episodes of environmental acidification on Atlantic salmon (Salmo salar) from a landlocked population

Chronic or repeated exposure to environmental contaminants may result in allostatic overload, a physiological situation in which the costs of coping affect long-term survival and reproductive output. Continuous measurements in Otra, the largest river in southern Norway, show the occurrence of repeated 24-48 h episodes of acidification. This work investigates the impact of repeated short acidification episodes on a unique land-locked population of normally anadromous Atlantic salmon ("Bleke"). This was done by recording physiological measures of stress and allostatic load in fish exposed for 7 days to continuous or repeated episodes of simulated environmental acidification or untreated Otra water (controls). A standardized acute stress test was performed after these different exposure regimes, with brain and blood samples taken before (baseline) or after the stress test. Treatment effects on stress coping ability were assessed by neuroendocrine indicators, including telencephalic serotonergic activity and plasma cortisol. Continuous exposure to acidification resulted in increased baseline plasma Cl^- and Na⁺ and elevated baseline plasma cortisol compared to episodic exposed fish. However, both episodic and continuous acidification resulted in similar increase in gill Al, indicating similar impact on gill permeability of these two exposures. This suggests a lower impact on the electrolyte homeostasis in episodic compared to continuous exposure and that this effect is not directly related to the effects of Al complexes binding to the gills. Furthermore, there were no treatment induced differences on stress coping ability, suggesting that episodic exposure to the sublethal concentrations of Al in pH 5.5 in the present study do not result in higher allostatic load than in control or continuous exposed Bleke.

Neuroendocrine indicators of allostatic load reveal the impact of environmental acidification in fish

When mobilized from surrounding soils and binding to gills at moderately low pH, aluminum (Al) cations can adversely affect fish populations. Furthermore, acidification may lead to allostatic overload, a situation in which the costs of coping with chronic stress affects long-term survival and reproductive output and, ultimately, ecosystem health. The brain's serotonergic system plays a key role in neuroendocrine stress responses and allostatic processes. Here, we explored whether sublethal effects of Al in acidified water affects serotonergic neurochemistry and stress coping ability in a unique land-locked salmon population from Lake Bygelandsfjorden, in southern Norway. Fish were exposed to untreated water with pH 6.5 and 74 μg Al l^-1 or acidified (pH 5.5) water with different aluminum concentrations ([Al]; 74-148 μg l^-1) for 5-6 days. Afterward, effects on stress coping ability were investigated by analyzing plasma cortisol levels and telencephalic serotonergic neurochemistry before and after a standardized acute stress test. Before the stress test, positive dose-response relationships existed between [Al], serotonergic turnover rate and plasma cortisol. However, in acutely stressed fish, exposure to the highest [Al] resulted in reduced cortisol values compared with those exposed to lower concentrations, while the positive dose-response relationship between Al concentrations and serotonergic turnover rate persisted in baseline conditions. This suggests that fish exposed to the highest Al concentration were unable to mount a proper cortisol response to further acute stress, demonstrating that neuroendocrine indicators of allostatic load can be used to reveal sublethal effects of water acidification-and potentially, the environmental impacts of other factors.

Effects of Climate Change and Flow Regulation on the Flow Characteristics of a Low-Relief River within Southern Boreal Climate Area

We investigated how hydro-climatological changes would affect fluvial forces and inundated area during a typical high-flow situation (MHQ, mean high discharge), and how adaptive regulation could attenuate the climate change impacts in a low-relief river of the Southern Boreal climate area. We used hydrologically modeled data as input for 2D hydraulic modeling. Our results show that, even though the MHQ will increase in the future (2050–2079), the erosional power of the flow will decrease on the study area. This can be attributed to the change of timing in floods from spring to autumn and winter, when the sea levels during flood peaks is higher, causing backwater effect. Even though the mean depth will not increase notably (from 1.14 m to 1.25 m) during MHQ, compared to the control period (1985–2014), the inundated area will expand by 15% due to the flat terrain. The increase in flooding may be restrained by adaptive regulations: strategies favoring ecologically sustainable and recreationally desirable lake water levels were modeled. The demands of environment, society, and hydropower are not necessarily contradictory in terms of climate change adaptation, and regulation could provide an adaptive practice in the areas of increased flooding.

Configuration of multiple human stressors and their impacts on fish assemblages in Alpine river basins of Austria

This work addresses multiple human stressors and their impacts on fish assemblages of the Drava and Mura rivers in southern Austria. The impacts of single and multiple human stressors on riverine fish assemblages in these basins were disentangled, based on an extensive dataset. Stressor configuration, i.e. various metrics of multiple stressors belonging to stressor groups hydrology, morphology, connectivity and water quality were investigated for the first time at river basin scale in Austria. As biological response variables, the Fish Index Austria (FIA) and its related single as well as the WFD biological- and total state were investigated. Stressor-response analysis shows divergent results, but a general trend of decreasing ecological integrity with increasing number of stressors and maximum stressor is observed. Fish metrics based on age structure, fish region index and biological status responded best to single stressors and/or their combinations. The knowledge gained in this work provides a basis for advanced investigations in Alpine river basins and beyond, supports WFD implementation and helps prioritizing further actions towards multi-stressor restoration- and management.