Warm Stream Research Articles

Warming reduces trophic diversity in high-latitude food webs.

The physical effects of climate warming have been well documented, but the biological responses are far less well known, especially at the ecosystem level and at large (intercontinental) scales. Global warming over the next century is generally predicted to reduce food web complexity, but this is rarely tested empirically due to the dearth of studies isolating the effects of temperature on complex natural food webs. To overcome this obstacle, we used 'natural experiments' across 14 streams in Iceland and Russia, with natural warming of up to 20°C above the coldest stream in each high-latitude region, where anthropogenic warming is predicted to be especially rapid. Using biomass-weighted stable isotope data, we found that community isotopic divergence (a universal, taxon-free measure of trophic diversity) was consistently lower in warmer streams. We also found a clear shift towards greater assimilation of autochthonous carbon, which was driven by increasing dominance of herbivores but without a concomitant increase in algal stocks. Overall, our results support the prediction that higher temperatures will simplify high-latitude freshwater ecosystems and provide the first mechanistic glimpses of how warming alters energy transfer through food webs at intercontinental scales.

Is the Regime Shift in Gulf Stream Warm Core Rings Detected by Satellite Altimetry? An Inter‐Comparison of Eddy Identification and Tracking Products

AbstractDownstream of Cape Hatteras, the vigorously meandering Gulf Stream forms anticyclonic warm core rings (WCRs) that carry warm Gulf Stream and Sargasso Sea waters into the cooler, fresher Slope Sea, and forms cyclonic cold core rings (CCRs) that carry Slope Sea waters into the Sargasso Sea. The Northwest Atlantic shelf and open ocean off the U.S. East Coast have experienced dramatic changes in ocean circulation and water properties in recent years, with significant consequences for marine ecosystems and coastal communities. Some of these changes may be related to a reported regime shift in the number of WCRs formed annually, with a doubling of WCRs shed after 2000. Since the regime shift was detected using a regional eddy‐tracking product, primarily based on sea surface temperatures and relies on analyst skill, we examine three global eddy‐tracking products as an automated and potentially more objective way to detect changes in Gulf Stream rings. Currently, global products rely on altimeter‐measured sea surface height (SSH), with WCRs registering as sea surface highs and CCRs as lows. To identify eddies, these products use either SSH contours or a Lagrangian approach, with particles seeded in satellite‐based surface geostrophic velocity fields. This study confirms the three global products are not well suited for statistical analysis of Gulf Stream rings and suggests that automated WCR identification and tracking comes at the price of accurate identification and tracking. Furthermore, a shift to a higher energy state is detected in the Northwest Atlantic, which coincides with the regime shift in WCRs.

Morphodynamics of melting ice over turbulent warm water streams

We investigate the morphodynamics of an ice layer over a turbulent stream of warm water using numerical simulations. At low water speeds, characteristic streamwise undulations appear, which can be explained by the Reynolds analogy between heat and momentum transfer. As the water speed increases, these undulations combine with spanwise ripples of a much greater length scale. These ripples are generated by a melting mechanism controlled by the instability originating from the ice–water interactions, and, through a melting/freezing process, they evolve downstream with a migration velocity much slower than the turbulence characteristic velocity.

Bridging the spawner-recruit disconnect II: Revealing basin-scale correlations between zooplankton and lobster settlement dynamics in the Gulf of Maine

In recent decades, recruitment of young-of-year lobsters to benthic nursery habitats in the Gulf of Maine was regionally synchronized and exhibited correlative links with changes in the abundance of the copepod Calanus finmarchicus, a foundational zooplankton species of the pelagic food web. The spatial scale at which recruitment dynamics were correlated indicated that recruitment processes were not as strongly coupled to trends in spawner abundance as might be expected, but, rather, were influenced by common, ecosystem-scale processes. Here we explored how local- and basin-scale zooplankton dynamics and oceanographic indicators in the Gulf of Maine correlated with lobster settlement indices and each other since the late 1980s. Our analysis indicates that lobster settlement trends in southwestern Gulf of Maine study areas, from Midcoast Maine to Cape Cod Bay, tend to be significantly correlated with basin-wide C. finmarchicus dynamics and the composition of waters entering the Gulf of Maine through the Northeast Channel. In contrast, lobster settlement in the northeastern Gulf, from Penobscot Bay to the Bay of Fundy, tended to correlate more strongly to C. finmarchicus variability in the Bay of Fundy region, which was distinct in earlier years but converged with the broader basin-scale processes in the latter years. Our results are consistent with the hypothesis that the combined effect of climate-related declines in abundance and phenological shifts of C. finmarchicus have contributed to declines in lobster settlement over the past decade, and justify further research into the mechanisms of this interaction. These changes also align with the weakening influence of cold Labrador Slope Water and strengthening effects of warm Gulf Stream waters that precipitated an ecosystem-wide regime shift in the Gulf of Maine over the past decade and may have greater implications for lobster recruitment than previously suspected.

Environmental warming increases the importance of high-turnover energy channels in stream food webs.

Warming temperatures are altering communities and trophic networks across Earth's ecosystems. While the overall influence of warming on food webs is often context-dependent, increasing temperatures are predicted to change communities in two fundamental ways: (1) by reducing average body size and (2) by increasing individual metabolic rates. These warming-induced changes have the potential to influence the distribution of food web fluxes, food web stability, and the relative importance of deterministic and stochastic ecological processes shaping community assembly. Here, we quantified patterns and the relative distribution of organic matter fluxes through stream food webs spanning a broad natural temperature gradient (5-27°C). We then related these patterns to species and community trait distributions of mean body size and population biomass turnover (P:B) within and across streams. We predicted that (1) communities in warmer streams would exhibit smaller body size and higher P:B and (2) organic matter fluxes within warmer communities would increasingly skew toward smaller, higher P:B populations. Across the temperature gradient, warmer communities were characterized by smaller body size (~9% per °C) and higher P:B (~7% faster turnover per °C) populations on average. Additionally, organic matter fluxes within warmer streams were increasingly skewed toward higher P:B populations, demonstrating that warming can restructure organic matter fluxes in both an absolute and relative sense. With warming, the relative distribution of organic matter fluxes was decreasingly likely to arise through the random sorting of species, suggesting stronger selection for traits driving high turnover with increasing temperature. Our study suggests that a warming world will favor energy fluxes through "smaller and faster" populations, and that these changes may be more predictable than previously thought.

Empirical stream thermal sensitivity cluster on the landscape according to geology and climate

Abstract. Climate change is modifying river temperature regimes across the world. To apply management interventions in an effective and efficient fashion, it is critical to both understand the underlying processes causing stream warming and identify the streams most and least sensitive to environmental change. Empirical stream thermal sensitivity, defined as the change in water temperature with a single degree change in air temperature, is a useful tool to characterize historical stream temperature conditions and to predict how streams might respond to future climate warming. We measured air and stream temperature across the Snoqualmie and Wenatchee basins, Washington, during the hydrologic years 2015–2021. We used ordinary least squares regression to calculate seasonal summary metrics of thermal sensitivity and time-varying coefficient models to derive continuous estimates of thermal sensitivity for each site. We then applied classification approaches to determine unique thermal sensitivity regimes and, further, to establish a link between environmental covariates and thermal sensitivity regimes. We found a diversity of thermal sensitivity responses across our basins that differed in both timing and magnitude of sensitivity. We also found that covariates describing underlying geology and snowmelt were the most important in differentiating clusters. Our findings and our approach can be used to inform strategies for river basin restoration and conservation in the context of climate change, such as identifying climate-insensitive areas of the basin that should be preserved and protected.

Heat Transfer and Pressure Drop in Plate Fin Heat Exchangers: A Numerical Approach

An apparatus for transferring thermal energy across an impermeable barrier between two or more fluids is a heat exchanger. The effective transmission of heat from a hot fluid to a cold fluid is its primary goal. The temperature differential between the two fluids directly affects this heat transfer. Using CFD study with FLUENT 2020 R1, the performance characteristics of the compact plate fin heat exchanger's improved surfaces are examined. The simulation of heat transfer and flow encompassed a variety of velocities including completely turbulent regions. The pressure, temperature, and velocity contours were taken from the Fluent simulations. To determine the heat exchanger's flow and heat transfer performance, numerical analysis has been done. Since boundary conditions are crucial to CFD, they have been carefully chosen. The flow field, heat transfer, thickness, and wavelength were all predicted using the software code for a range of air to water velocities. Three distinct wavelengths of numerical simulations for a plate-fin heat exchanger were used in the study (10, 20, and 30). There were changes to the fin thickness, entry locations, and air and water entry velocities. Solid Works and Ansys were used to create the geometry for the three-level fin models that were placed inside the heat exchanger, both with and without a cover. For determining the heat exchanger's geometry and fin isometry, three levels measuring 19.6 mm in height and 53.64 mm in width were constructed. The fins have a thickness of 0.2 to 0.4 mm. As the air and water velocities vary, so does the temperature inside the heat exchanger. Because the topmost layer is a warm stream's flow channel, the temperature rises there, whereas the opposite effect is seen close to the bottom layer. Convection heat transmission to the surroundings and conduction heat transfer through the heat exchanger's side plates are the causes of fluid temperature fluctuations in the depth direction.

Chronic exposure to environmental temperature attenuates the thermal sensitivity of salmonids

Metabolism, the biological processing of energy and materials, scales predictably with temperature and body size. Temperature effects on metabolism are normally studied via acute exposures, which overlooks the capacity for organisms to moderate their metabolism following chronic exposure to warming. Here, we conduct respirometry assays in situ and after transplanting salmonid fish among different streams to disentangle the effects of chronic and acute thermal exposure. We find a clear temperature dependence of metabolism for the transplants, but not the in-situ assays, indicating that chronic exposure to warming can attenuate salmonid thermal sensitivity. A bioenergetic model accurately captures the presence of fish in warmer streams when accounting for chronic exposure, whereas it incorrectly predicts their local extinction with warming when incorporating the acute temperature dependence of metabolism. This highlights the need to incorporate the potential for thermal acclimation or adaptation when forecasting the consequences of global warming on ecosystems.

Thermal tolerance and vulnerability to warming differ between populations of wild Oncorhynchus mykiss near the species’ southern range limit

Fish habitat temperatures are increasing due to human impacts including climate change. For broadly distributed species, thermal tolerance can vary at the population level, making it challenging to predict which populations are most vulnerable to warming. Populations inhabiting warm range boundaries may be more resilient to these changes due to adaptation or acclimatization to warmer temperatures, or they may be more vulnerable as temperatures may already approach their physiological limits. We tested functional and critical thermal tolerance of two populations of wild Oncorhynchus mykiss near the species’ southern range limit and, as predicted, found population-specific responses to temperature. Specifically, the population inhabiting the warmer stream, Piru Creek, had higher critical thermal maxima and higher functional thermal tolerance compared to the population from the cooler stream, Arroyo Seco. Arroyo Seco O. mykiss are more likely to experience a limitation of aerobic scope with warming. Piru Creek O. mykiss, however, had higher resting metabolic rates and prolonged exercise recovery, meaning that they could be more vulnerable to warming if prey or dissolved oxygen become limited. Temperature varies widely between streams near the O. mykiss southern range limit and populations will likely have unique responses to warming based on their thermal tolerances and metabolic requirements.

Thermodynamic evaluation of mixed refrigerant selection in dual mixed refrigerant NG liquefaction process with respect to 3E's (Energy, Exergy, Economics)

The dual mixed-refrigeration process makes it possible to achieve higher liquefaction of natural gas with the advancement in both the refrigeration cycles and refrigerant combinations of components. This is made possible by the fact that the process uses multiple mixed refrigerants. This has a significant impact on the overall improved performance of natural gas liquefaction by having a negative influence on the quantity of energy that is consumed. This study proposes a methodology for selecting the MR components based on their thermodynamic behavior in both warm and cold refrigerant streams. As a result, eighteen alternative scenarios are simulated for this study, each based on (i) fixing the warm loop components or (ii) fixing the cold loop components. The procedure was investigated from the point of view of process engineering, with the 3E model of energy, exergy, and economics serving as the decision-making factor. The findings indicate that increasing the number of components for pre-cooling and subcooling cycles from three to five results in specific energy consumption of 0.49 kW.kgLNG−1, which seems to be a reduction of 54% in terms of the amount of energy that is consumed in comparison to the process that is based on three components. The irreversibilities of the process were uncovered by doing an exergy analysis. It identified the cases based on five refrigerant components providing reduced exergy destruction of 3505.02 kW with 59% exergy efficiency. The viability of the proposed process is assessed even further through economic analysis. It was observed that five MR-based processes save 22.93% of the total capital cost, 43.56% of the overall operating cost, and 33.61 %of the total annualized cost.

Enhanced Performance of Structurally Optimized Plate-Fin Heat Exchangers Through Numerical Modeling of Heat Transfer and Pressure Drop

This study aims to optimize the structure of compact Plate-Fin Heat Exchangers (PFHE) by incorporating corrugated fins and validating their improved performance through numerical modeling and simulation. The results provide valuable insights for refining application-specific design guidelines and enhancing the performance of PFHEs. Using Computational Fluid Dynamics (CFD), the PFHE geometry was created in SolidWorks and Ansys Fluent, with fins modeled in three layers inside the heat exchanger both with and without a cover. To investigate the fins' performance, flow field, and heat transfer, fin thickness, entry velocities, and locations of water and air were varied across three wavelengths (10, 20, and 30) during the numerical investigation. The analysis focused on the variations in pressure, temperature, and fluid velocity within the heat exchanger. Key findings include the observation that temperature distribution is influenced by the velocities of both water and air, with the upper layer experiencing a temperature increase due to the warm fluid stream, while the opposite effect is observed near the bottom layer. Furthermore, fluid temperature variation in the depth direction is attributed to conductive heat transfer through side plates and convective heat transfer to the surroundings. The outcomes of this study have the potential to reduce the pressure difference generated during heat exchange and increase the thermal efficiency of PFHEs.

Dam removal enables diverse juvenile life histories to emerge in threatened salmonids repopulating a heterogeneous landscape

Human stressors block, eliminate, and simplify habitat mosaics, eroding landscapes’ life history diversity and thus biological resilience. One goal of restoration is to alleviate human stressors that suppress life history diversity, but life history responses to these efforts are still coming into focus. Here, we report life history diversity emerging in threatened salmonids (Oncorhynchus spp.) repopulating the recently undammed Elwha River (WA, United States) in adjacent but environmentally distinct tributaries. The ~20 km tributaries entered the Elwha River <1 km apart, but one had a colder stream temperature regime and swifter waters due to its high, snow-dominated elevation and steep valley gradient (~3%), while the other had a warmer stream temperature regime and slower waters because it drained a lake, was at lower elevation, and had a lower stream gradient (~1.5%). Following the 2012 removal of Elwha Dam, the tributaries’ salmonids generally became more abundant and expressed diverse life histories within and among species. The warmer, low-gradient tributary produced more age-1+ coho salmon while the colder, steeper tributary produced a notably high abundance of steelhead smolts in 2020. Additionally, salmonids exiting the warmer tributary were older and possibly larger for their age class, emigrated ~25 days earlier, and included age-0 Chinook salmon that were larger. Also, assemblage composition varied among years, with the most abundant species shifting between Chinook salmon and coho salmon, while steelhead abundances generally increased but were patchy. These patterns are consistent with a newly accessible, heterogeneous landscape generating life history diversity against the backdrop of patchy recruitment as salmonids—some with considerable hatchery-origin ancestry—repopulate an extirpated landscape. Overall, dam removal appears to have promoted life history diversity, which may bolster resilience during an era of rapid environmental change and portend positive outcomes for upcoming dam removals with similar goals.

Temperature and winter duration requirements for reproductive success in johnny darter Etheostoma nigrum in the South Platte River basin, Colorado

Abstract Changes in water temperature and its seasonal timing influences the physiological processes of many aquatic ectotherms. Wastewater treatment plants (WWTP) contribute to warmer and more consistent water temperatures in streams draining the North American Great Plains, particularly during winter months. Reduced variation in seasonal water temperature may adversely affect fishes that rely on temperature fluctuation or specific overwintering temperatures for reproductive cues. We used johnny darter Etheostoma nigrum (Family Percidae), a thermally sensitive species native to the South Platte River Basin, as a focal species to assess the effects of winter water temperature regulations in Colorado. Our objective was to evaluate the effects of winter stream temperature and winter duration on reproductive success of johnny darter in the laboratory to inform development of water quality criteria for the South Platte River Basin. Winter duration (60, 90 or 120 days) and temperature (4 or 12°C) were crossed in a factorial design of six treatments that simulated warmed effluent‐impacted streams as well as streams with a more natural thermal regime. We defined reproductive success in johnny darter by examining four reproductive endpoints: (1) spawning timing, (2) egg and larvae production, (3) egg success, and (4) egg development. Across the four endpoints, seven main response variables were measured: spawning initiation date, eggs per gram of female per week, live larvae per gram of female per week, fertilisation success, hatching success, egg‐to‐larva survival and days to hatch. Winter temperature and duration influenced spawning timing and egg development. Earlier spawning initiation dates were observed in fish exposed to 12°C winters and was associated with a higher number of days to hatch. Eggs per gram of female per week, live larvae per gram of female per week, fertilisation success, hatching success, and egg‐to‐larva survival were similar among winter treatments. Although the State of Colorado chronic winter water temperature standard of 12°C appears to be adequate for egg and larvae production in johnny darter, our results showed that fish exposed to the standard began spawning significantly earlier than fish exposed to 4°C winters. Evaluating reproductive output in the context of seasonal timing is valid because spawning timing has the potential to affect overall production and egg development, and consequently recruitment and population sustainability. Therefore, spawning timing should be considered when making management decisions and revising water temperature standards. The warming of streams and rivers from WWTP effluent is a global issue, especially in areas that are undergoing rapid urbanisation. Fishes are particularly sensitive to water temperature and many fishes worldwide are likely to be experiencing changes in important physiological processes, affecting both reproduction and survival. Our study indicates that reproductive timing could be influenced by stream warming and stresses the need for further studies on the influence of anthropogenic effects on stream warming.

The Use of Warm Air for Solvent Evaporation in Adhesive Dentistry: A Meta-Analysis of In Vitro Studies

Any excess solvent from dental adhesive systems must be eliminated prior to material photopolymerization. For this purpose, numerous approaches have been proposed, including the use of a warm air stream. This study aimed to investigate the effect of different temperatures of warm air blowing used for solvent evaporation on the bond strength of resin-based materials to dental and nondental substrates. Two different reviewers screened the literature in diverse electronic databases. In vitro studies recording the effect of warm air blowing to evaporate solvents of adhesive systems on the bond strength of resin-based materials to direct and indirect substrates were included. A total of 6626 articles were retrieved from all databases. From this, 28 articles were included in the qualitative analysis, and 27 remained for the quantitative analysis. The results of the meta-analysis for etch-and-rinse adhesives revealed that the use of warm air for solvent evaporation was statistically significantly higher (p = 0.005). For self-etch adhesives and silane-based materials, this effect was observed too (p < 0.001). The use of a warm air stream for solvent evaporation enhanced the bonding performance of alcohol-/water-based adhesive systems for dentin. This effect seems to be similar when a silane coupling agent is submitted to a heat treatment before the cementation of a glass-based ceramic.

Impact of the extremely warm Gulf Stream on heavy precipitation induced by Hurricane Sandy (2012) during its extratropical transition

This study investigated the role of the extremely warm sea surface temperature (EWSST) over the Gulf Stream (GS) in heavy precipitation induced by Hurricane Sandy in October 2012 during its extratropical transition (ET). A control (CTL) run with observed SST and a climate (CLM) run with climatological SST over the GS were performed using a high-resolution regional atmospheric model. Sandy-induced precipitation over the Mid-Atlantic coastal region was larger by approximately 40% in the CTL run than in the CLM run during its ET phase. The western sector of Sandy in the CTL run exhibited the westward-tilting lower-to-middle tropospheric front and the pronounced convectively unstable layer created by large surface fluxes of sensible and latent heat from the EWSST region. The intensified convective instability was released by the frontal deep updrafts, enhancing frontal precipitation in the western part of Sandy. A backward trajectory analysis demonstrated that the abundant moisture evaporated from the GS was imported into the western frontal rainbands by the low-level easterlies passing the northern sector of Sandy. The EWSST facilitated the moistening process of air parcels transported by the easterlies due to the active heat and moisture supply from the underlying current, contributing to the increase in moisture influx toward the western front. Another trajectory analysis indicated that the formation of low-level easterlies north of Sandy originated from the combination of Sandy and high-latitude synoptic-scale systems such as an anomalous high over the Labrador Sea and an extratropical cyclone east of Sandy. These results corroborate the importance of the extremely warm midlatitude ocean and complex high-latitude weather systems to the enhancement of Sandy-induced heavy precipitation.

Implication of middle Eocene to early Miocene ostracodes from the N. El Faras-1X Well, Qattara Depression, Egypt, for paleobathymetry and paleobiogeographic reconstruction

The subsurface middle Eocene - early Miocene succession in the N. El Faras - 1X Well, drilled in the Qattara Depression at the northern part of the Western Desert, Egypt (Southern Tethyan Province, STP), was studied for its ostracode content to infer changes in paleobathymetry as well as paleobiogeographic reconstructions. This succession is composed mainly of three lithostratigraphic units, as the upper part of the Apollonia Formation (middle to late Eocene), the Dabaa Formation (late Eocene and Oligocene) and the lower part of the Moghra Formation (early Miocene). The stratigraphical distribution, paleoenvironmental and paleobiogeographical significance of the ostracode faunas from the Oligocene interval of the study area are documented for the first time. Fifty seven ostracode species belonging to 36 genera were identified in this succession. Among them, two new species were described. The vertical distribution of these ostracode species enabled the recognition of seven local biozones. Qualitative and quantitative statistical analysis of ostracode associations revealed a depositional depth ranging from the inner to outer neritic setting. Considering the CONISS cluster analysis, six ecozones were established. During the middle and late Eocene, the paleobiogeographical affinities of the Egyptian ostracodes from N. El Faras-1X× Well succession with that of the Southern Tethyan countries suggest a direct connection between Egypt and the countries of the Southern Tethyan Province. This connection facilitated the dispersal and exchange within this province. The low ostracode similarity between the STP and the Northern Tethyan Province (NTP) suggests a slight connection and exchange between them. The in common occurrence of these ostracodes with that of Somalia is attributed to the connection between the STP and Somalia via the Tethyan Seaway, which facilitated the dispersal and exchange of the middle Eocene ostracodes. The similarity between the Egyptian ostracodes (STP) and that of South America and the Caribbean Sub-Province indicates a direct connection between those provinces across the Gibraltar Strait and through the North Equatorial Oceanic Water Current during that time. During the Oligocene, the low ostracode similarity between the STP and the NTP may be attributed to a more or less restricted connection. Also, the low similarity indicates a continuation of the mild connection between the STP and the South America and Caribbean Sub-Province across the Gibraltar Strait and through the North Equatorial Current. During the early Miocene, the similarity of the recorded ostracodes of the STP with that of the NTP suggests a continuous connection between the two provinces. Also, there was a connection between the South America and Caribbean Sub-Province and the North American Province through the North Equatorial Current and the warm Gulf Stream within the Caribbean Sea.

Special Interest Tourism (SIT) in Murmansk (Arctic NE Scandinavia): Touristic Route around the City to Explore the Oldest Rocks in Europe

The city of Murmansk together with the neighboring town of Kola is an agglomeration in the Arctic, in the northern part of the Kola Peninsula on the Barents Sea fjord. Some of its roots date back to the 16th century when the foundations of Russian civilization were built in this region. Rock paintings and labyrinths indicate that there were peoples living in this area before then: the Saami were here much earlier. This historic heritage is superimposed on the extraordinary environment of the far north, with a relatively mild climate associated with the warm Norwegian stream. An important and inseparable element of the city’s landscape is a non-freezing port on the coast, which offers a window to the world, and numerous hills forming an interesting city landscape built of Archean gneisses as old as 3.75 billion years. These are among the oldest rocks in Europe. Murmansk, with its wealth of tourist features and as a center of science, industry, and trade, also aspires to be the capital of the entire Arctic. Walking the streets of this city, which is just over a century old, past its neoclassical buildings, one can observe several inanimate natural forms that show visitors the unusual nature of the city’s topography. Efforts to promote these have been partly implemented around the Monument to the Unknown Soldier, where a small ecological route has been marked out. However, tourist interest in the city is increasing, and this article attempts to answer this interest by proposing a loop of tourist routes displaying many interesting features of the city.

Warming undermines emergence success in a threatened alpine stonefly: A multi‐trait perspective on vulnerability to climate change

AbstractVulnerability to warming is often assessed using short‐term metrics such as the critical thermal maximum (CTMAX), which represents an organism's ability to survive extreme heat. However, the long‐term effects of sub‐lethal warming are an essential link to fitness in the wild, and these effects are not adequately captured by metrics like CTMAX.The meltwater stonefly,Lednia tumana, is endemic to high‐elevation streams of Glacier National Park, MT, USA, and has long been considered acutely vulnerable to climate‐change‐associated stream warming. As a result, in 2019, it was listed as Threatened under the U.S. Endangered Species Act. This presumed vulnerability to warming was challenged by a recent study showing that nymphs can withstand short‐term exposure to temperatures as high as ~27°C. But whether they also tolerate exposure to chronic, long‐term warming remained unclear.By measuring fitness‐related traits at several ecologically relevant temperatures over several weeks, we show thatL. tumanacannot complete its life‐cycle at temperatures only a few degrees above what some populations currently experience.The temperature at which growth rate was maximized appears to have a detrimental impact on other key traits (survival, emergence success and wing development), thus extending our understanding ofL. tumana's vulnerability to climate change.Our results call into question the use of CTMAXas a sole metric of thermal sensitivity for a species, while highlighting the power and complexity of multi‐trait approaches to assessing vulnerability.Read the freePlain Language Summaryfor this article on the Journal blog.

Elevation-dependent warming of streams in mountainous regions: implications for temperature modeling and headwater climate refugia

Climate change is warming stream temperatures with significant implications for species that require cold temperatures to persist. These species often rely on headwater habitats in mountainous regions where elevation gradients in hydroclimatic conditions may induce differential patterns of long-term warming that affect the resistance of refugia. Forecasts from mechanistic and statistical stream temperature models diverge regarding whether this elevation dependence will cause above- or below-average warming in headwaters during warm summer periods, so we examined monitoring records for stream temperature (n = 271), air temperature (n = 690), and stream discharge (n = 131) across broad elevation gradients in a mountainous region of western North America to better understand potential future trends. Over a 40-year period characterized by rapid climate change from 1976–2015, air temperature stations exhibited below-average warming rates at high elevations while stream discharge declined at above average rates. Between climatically extreme years that involved summer air temperature increases >5 °C and discharge declines >70%, temperatures in high-elevation streams exhibited below average increases but otherwise showed negligible elevation dependence during intermediate climate years. In a subsequent example, it was demonstrated that elevation dependent stream warming has a minor effect on the amount of thermal habitat loss relative to the average water temperature increase within a mountain river network. We conclude that predictions of above average warming effects on headwater organisms for this region may be overly pessimistic and discuss reasons why different types of temperature models make divergent forecasts. Several research areas warrant greater attention, including descriptions of elevation-dependent patterns in other regions for comparative purposes, examination of long-term stream temperature records to understand how sensitivity to climate forcing may be evolving, use of new data sources to better represent key processes in temperature models across broad areas, and development of hybrid models that integrate the best attributes of mechanistic and statistical approaches.

Glacier contribution to lowland streamflow: A multi-year, daily geochemical hydrograph separation study in subarctic Alaska

Glacier melt water is a critical fresh water contribution of stream discharge for many regions and in high latitudes glacier extent is decreasing with climate warming. However, characteristics of glacierized contributions to subarctic watershed-scale geochemistry and runoff have attracted limited attention. Tracer studies of glacier dominated watersheds are also commonly limited to a few days. We conducted a six-year geochemical hydrograph separation study of melt season daily streamflow in a glacierized watershed in Interior Alaska to estimate glacier, rain, snow, and baseflow contributions. Bulk streamflow water samples were collected daily from the lowlands of Jarvis Creek (634 km2, 3.3% glacier cover) throughout the flow season (late April through September) during 2011–2016. Synoptic sampling of source waters included rain, snow, headwater late-winter streamflow and late-summer glacier terminus discharge. All 1227 source water and bulk streamflow samples were analyzed for stable water isotopes (δ18O and δD) and dissolved ion concentrations (SO42−:Cl− ratio). Bulk streamflow water samples exhibited large seasonal and inter-annual geochemical variation. Considerable inter-annual differences in stable water isotopes within each source water (δ18O values of −15.8 to −20.9‰ for rain, for example) emphasize the importance of informing the mixing model with source waters sampled during the same season as bulk streamflow collection. We estimated glacier discharge to be the largest seasonal contributor to total warm season lowland streamflow (average 35%, ranging from 20% in year 2016 to 44% in 2012) with daily variability from 2 (June 2, 2011) to 80% (September 10, 2013). If the glacier contribution were to cease completely, total warm season bulk stream discharge would be reduced by between 22 and 48% during an above or below average total seasonal rainfall, respectively, with up to an 80% reduction in peak flows. Results from this study suggest that high variability in source contribution of glacierized watersheds, extending across intra- and interannual timescales, challenge any generalization of hydrologic function derived from short-term studies.