Does global change increase the risk of maladaptation of Atlantic salmon migration through joint modifications of river temperature and discharge?

In freshwater ecosystems, water temperature and discharge are two intrinsically associated triggers of key events in the life cycle of aquatic organisms such as the migration of diadromous fishes. However, global changes have already profoundly altered the thermal and hydrological regimes of rivers, affecting the timing of fish migration as well as the environmental conditions under which it occurs. In this study, we focused on Atlantic salmon ( Salmo salar ), an iconic diadromous species whose individuals migrate between marine nursery areas and continental spawning grounds. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and both salmon juvenile downstream and adult upstream migrations in three French rivers (the Bresle, Oir and Nivelle rivers). While all three rivers have gradually warmed over the last 35 years, changes in discharge have been very heterogeneous. Juveniles more frequently used warmer temperatures to migrate. Adults migrating a few weeks before spawning more frequently used warm temperatures associated with high discharges. This has already led to modifications in preferential niches of both life stages and suggests a potential mismatch between these populations' ecological preference and changes in their local environment due to global change.

Effects of Adult Biomass and Environmental Conditions on Bigheaded Carp Reproductive Output

Bighead Carp Hypothalmichthys nobilis and Silver Carp Hypothalmichthys moltrix (hereafter collectively referred to as Bigheaded Carp) have spread throughout the majority of the Mississippi River since the 1970s. The current northern invasion edge of Bigheaded Carp in the Upper Mississippi River (UMR) spans between Pools 14 and 20 because of limited passage at Lock and Dam (LD) 19. Mechanisms limiting adult Bigheaded Carp abundance above LD19 are unknown but may be due in part to lack of reproductive success influenced by adult abundance and environmental factors. Our objective was to investigate how relative adult biomass and river temperature and discharge affect maximum annual Bigheaded Carp larval production in the UMR using a Ricker stock-recruitment model. Adult Bigheaded Carp relative biomass (kg/h) was estimated annually with boat electrofishing and larvae were collected every 10 d between May and August 2014–2017 in Pools 14–20 in the UMR. Adult relative biomass ranged from 0.0 to 880.9 kg/h, whereas maximum annual larval densities ranged from 0.0 to 2,869.4 larvae/m3. After accounting for variability among pools and years, the most supported linear Ricker stock-recruitment model indicated the number of recruits per spawner decreased with increasing adult relative biomass and increased with mean discharge. Our results highlight the importance of adult biomass and river discharge conditions for reproduction of Bigheaded Carp along leading edges of invasion. Management strategies that aim to maintain low adult abundance where reproduction is not yet occurring could help limit population increases via reproduction, whereas reducing high adult biomass (e.g., commercial harvest, barriers) may result in greater Bigheaded Carp reproductive output in the UMR.

Use of statistical methods to characterise the influence of groundwater on the thermal regime of rivers in Normandy (France): comparison between the highly permeable, chalk catchment of the Touques river and the low permeability, crystalline rock catchment of the Orne river

The study of the temperature of two rivers in Normandy (France), the Orne and the Touques, between 2013 to 2018, allowed the main controlling factors regulating their thermal regime to be determined. The analysis was conducted by coupling different statistical treatments: linear regression between water and air temperatures, Independent Component Analysis (ICA), Principal Component Analysis (PCA) and a multiple linear regression model. The temperature of the two rivers is mainly controlled by climatic factors but secondary regulation factors are demonstrated to play important roles: runoff for the two rivers, and groundwater for the Touques. The influence of the chalk aquifer on river temperature appears to vary seasonally throughout the year, reaching its maximum in the early spring and increasing from upstream to downstream. The coupled use of the different statistical complex methods showed its validity in understanding both the temporal and spatial variations in water temperature and its correlation with the secondary factors, that could not be inferred from a simpler approach based on linear regression. These techniques could be valuable in other areas with rivers sufficiently monitored to determine the controlling water temperature factors and thus their sensitivity to climate change.

MAPPING THE DISTRIBUTION OF SALTWATER CROCODILE (Crocodylus porosus) AND RISKS OF HUMAN-CROCODILE CONFLICTS IN SETTLEMENTS AROUND KUTAI NATIONAL PARK, EAST KALIMANTAN

Human-crocodile conflicts (HCC) are problems affecting crocodile conservation. Scientific publications on crocodile attack cases in Indonesia are few with low validation which hinder optimal conflict mitigation efforts. The estuarine river of Kutai National Park is a natural habitat for saltwater crocodiles and mostly nearby dense settlements. This study aims to map the distribution of saltwater crocodiles and potential conflicts in the Kutai National Park area. To predict the distribution of saltwater crocodiles, we used Maximum Entropy MAXENT with its environmental predictors i.e. slope, altitude, distance from shore, distance from river, temperature, and habitat types (mangrove forest, freshwater swamp, and shrubs). MAXENT prediction showed that elevation was the most influential variable with AUC (Average Under Curve) value of 0.952. Settlements with activities occurring within one kilometer from the river and those adjacent to coastal areas proved to be the highest in human conflicts with crocodiles. Key words: conflict, crocodile, human, MAXENT

Future water temperature of rivers in Switzerland under climate change investigated with physics-based models

Rivers are ecosystems highly sensitive to climate change and projected future increase in air temperature is expected to increase the stress for these ecosystems. Rivers are also an important socio-economical factor. In addition to changes in water availability, climate change will impact the temperature of rivers. This study presents a detailed analysis of river temperature and discharge evolution over the 21st century in Switzerland, a country covering a wide range of Alpine and lowland hydrological regimes. In total, 12 catchments are studied. They are situated both in the lowland Swiss Plateau and the Alpine regions and cover overall 10 % of the country’s area. This represents the so far largest study of climate change impacts on river temperature in Switzerland. The impact of climate change is assessed using a chain of physics-based models forced with the most recent climate change scenarios for Switzerland including low, mid, and high emissions pathways. A clear warming of river water is modelled during the 21st century, more pronounced for the high emission scenarios and toward the end of the century. For the period 2030–2040, median warming in river temperature of +1.1 °C for Swiss Plateau catchments and of +0.8 °C for Alpine catchments are expected compared to the reference period 1990–2000 (similar for all emission scenarios). At the end of the century (2080–2090), the median annual river temperature increase ranges between +0.9 °C for low emission and +3.5 °C for high emission scenarios for both Swiss Plateau and Alpine catchments. At the seasonal scale, the warming on the Swiss Plateau and in the Alpine regions exhibits different patterns. For the Swiss Plateau, the spring and fall warming is comparable to the warming in winter, while the summer warming is stronger but still moderate. In Alpine catchments, only a very limited warming is expected in winter. A marked discharge increase in winter and spring is expected in these catchments due to enhanced snowmelt and a larger fraction of liquid precipitation. Accordingly, the period of maximum discharge in Alpine catchments, currently occurring during mid-summer, will shift to earlier in the year by a few weeks (low emission) or almost two months (high emission) by the end of the century. In summer, the marked discharge reduction in Alpine catchments for high emission scenarios leads to an increase in sensitivity of water temperature to low discharge, which is not observed in the Swiss Plateau catchments. In addition, an important soil warming is expected due to glacier and snow cover decrease. These effects combined lead to a summertime river warming of +6.0 °C in Alpine catchments by the end of the century for high emission scenarios. Two metrics are used to show the adverse effects of river temperature increase both on natural and human systems. All results of this study along with the necessary source code are provided with this manuscript.

How daily groundwater table drawdown affects the diel rhythm of hyporheic exchange

Groundwater table dynamics extensively modify the volume of the hyporheic zone and the rate of hyporheic exchange processes. Understanding the effects of daily groundwater table fluctuations on the tightly coupled flow and heat transport within hyporheic zones is crucial for water resources management. With this aim in mind, a physically based model is used to explore hyporheic responses to varying groundwater table fluctuation scenarios. The effects of different timing and amplitude of groundwater table daily drawdowns under gaining and losing conditions are explored in hyporheic zones influenced by natural flood events and diel river temperature fluctuations. We find that both diel river temperature fluctuations and daily groundwater table drawdowns play important roles in determining the spatiotemporal variability of hyporheic exchange rates, temperature of exfiltrating hyporheic fluxes, mean residence times, and hyporheic denitrification potentials. Groundwater table dynamics present substantially distinct impacts on hyporheic exchange under gaining or losing conditions. The timing of groundwater table drawdown has a direct influence on hyporheic exchange rates and hyporheic buffering capacity on thermal disturbances. Consequently, the selection of aquifer pumping regimes has significant impacts on the dispersal of pollutants in the aquifer and thermal heterogeneity in the sediment.

River Temperature Evolution in Switzerland over the 21st Century

<p>Climate change has and will have many impacts on natural and human systems, and many of these impacts are already well described in the literature. One impact of climate change that received less attention is the increase in river temperature, even though it is recognized as a key variable controlling the water quality of freshwater ecosystems. It influences both the metabolic activity of aquatic organisms and biochemical cycles. It is also a key variable for many industrial sectors, and favorable for the spreading of certain diseases affecting fish.</p><p>In a previous study (Michel et al., 2020) we showed a clear increase of +0.33 ± 0.03 °C per decade in water temperature over the last four decades in Switzerland. Important differences between lowland and alpine catchment were identified. Indeed, the warming rate in alpine catchments is only half of that observed in lowlands rivers. This difference is attributed mainly to the contribution of cold water from snow and glacier melt in mountainous area during summer, mitigating the impact of air temperature warming.</p><p>As a follow up, the response of selected Swiss catchments in lowland and alpine regions to the future forcing is numerically assessed using the CH2018 climate change scenarios for Switzerland. This is done using a sequence of physics-based models. The CH2018 climate change scenarios have been extended to a new set of alpine meteorological stations and downscaled to hourly resolution (Michel et al., 2021).</p><p>The results show an increase in water temperature for any of the RCP (2.6, 4.5, and 8.5) scenarios and a strong impact of climate change on alpine catchments caused by changes in snowfall/melt, glacier melt, and surface albedo. Indeed, we see a rapid acceleration of the warming in alpine catchments which “catch-up” with the warming already observed in lowland catchments. This can lead to a warming of up to +7 °C by the end of the century in some alpine rivers with the RCP8.5 scenario. An important shift in the hydrological regime is also observed, particularly in high-altitude rivers.</p><p>As a result, river ecosystems will be severely impacted. In addition, the combined changes in water temperature and discharge have an important impact on the groundwater temperature annual cycle, as we discussed in Epting et al. (2021). Seasonal shifts in rivers water infiltration associated with increased groundwater recharge during high runoff periods could be an important factor affecting future groundwater temperatures.</p><p><strong>REFERENCES</strong></p><p>Epting, J., Michel, A., Affolter, A., & Huggenberger, P.: Climate change effects on groundwater recharge and temperatures in Swiss alluvial aquifers, Journal of Hydrology X, 11, 100071, 2021, doi:10.1016/j.hydroa.2020.100071.</p><p>Michel, A., Brauchli, T., Lehning, M., Schaefli, B., & Huwald, H.: Stream temperature and discharge evolution in Switzerland over the last 50 years: annual and seasonal behaviour, Hydrological and Earth System Science, 24, 115–142, 2020, doi:10.5194/hess-24-115-2020.</p><p>Michel, A., Sharma, V., Lehning, M., & Huwald, H.: Climate change scenarios at hourly time-step over Switzerland from an enhanced temporal downscaling approach, International Journal of Climatology, under review</p>