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1. Catchment concentration–discharge relationships across temporal scales: A review

   https://doi.org/10.1002/wat2.1702  

   Speir, Shannon L.; Rose, Lucy A.; Blaszczak, Joanna R.; Kincaid, Dustin W.; Fazekas, Hannah M.; Webster, Alex J.; Wolford, Michelle A.; Shogren, Arial J.; Wymore, Adam S. (December 2023, WIREs Water)

   Abstract Processes that drive variability in catchment solute sourcing, transformation, and transport can be investigated using concentration–discharge (C–Q) relationships. These relationships reflect catchment and in‐stream processes operating across nested temporal scales, incorporating both short and long‐term patterns. Scientists can therefore leverage catchment‐scale C–Q datasets to identify and distinguish among the underlying meteorological, biological, and geological processes that drive solute export patterns from catchments and influence the shape of their respective C–Q relationships. We have synthesized current knowledge regarding the influence of biological, geological, and meteorological processes on C–Q patterns for various solute types across diel to decadal time scales. We identify cross‐scale linkages and tools researchers can use to explore these interactions across time scales. Finally, we identify knowledge gaps in our understanding of C–Q temporal dynamics as reflections of catchment and in‐stream processes. We also lay the foundation for developing an integrated approach to investigate cross‐scale linkages in the temporal dynamics of C–Q relationships, reflecting catchment biogeochemical processes and the effects of environmental change on water quality. This article is categorized under:Science of Water > Hydrological ProcessesScience of Water > Water QualityScience of Water > Water and Environmental Change 

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2. Environmental DNA ‐based detection of pathogens in trade and captive settings: Best practices and validation for Batrachochytrium salamandrivorans

   https://doi.org/10.1111/2041-210X.14217  

   Brunner, Jesse L.; Yarber, Christian M.; Pearhill, Robert A. I.; Goldberg, Caren S. (September 2023, Methods in Ecology and Evolution)

   Abstract Detecting pathogens in the live animal trade is critical for tracking and preventing their movement, introduction and spillover into susceptible fauna. However, the scale of the live animal trade makes individually testing animals infeasible for all but the most economically important taxa. For instance, while the fungal pathogen,Batrachochytrium salamandrivorans(Bsal), threatens amphibian, particularly caudate diversity, in Europe and the Americas, screening even a fraction of the millions of live amphibians imported into the United States, alone, is impractically laborious and expensive. A promising alternative to individual‐level sampling (e.g. swabbing the skin of salamanders) is to instead collect DNA from the animals' environment (e.g. housing container or water) which allows us to screen a whole group of animals at a time.We used a series of experiments withBsal‐spiked water and substrates and experimentally infected rough‐skinned newts (Taricha granulosa) to determine which methods yield the mostBsalenvironmental DNA (eDNA) and evaluate the capacity of these methods to detectBsal‐infected animals in conditions found in captive settings and trade.We found that filtering water housing infected animals for even an hour can consistently recover detectable levels ofBsaleDNA, that there is little evidence ofBsaleDNA being clumped in housing containers or swamped or inhibited by dirty housing containers, and that eDNA‐based methods achieves an equivalent or higher chance of detectingBsalinfections in a (virtual) population of co‐housed newts with fewer samples than individual swabs.By sampling the genetic materials accumulated from a whole group of animals, eDNA‐based methods are a powerful means of detecting pathogens, such asBsal, in shipments and captive populations. These methods bring routine pathogen surveillance into reach in many more contexts and can thus be an important tool in conservation and disease control. 

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3. Toward catchment hydro‐biogeochemical theories

   https://doi.org/10.1002/wat2.1495  

   Li, Li; Sullivan, Pamela L.; Benettin, Paolo; Cirpka, Olaf A.; Bishop, Kevin; Brantley, Susan L.; Knapp, Julia L. A.; van Meerveld, Ilja; Rinaldo, Andrea; Seibert, Jan; et al (December 2020, WIREs Water)

   Abstract Headwater catchments are the fundamental units that connect the land to the ocean. Hydrological flow and biogeochemical processes are intricately coupled, yet their respective sciences have progressed without much integration. Reaction kinetic theories that prescribe rate dependence on environmental variables (e.g., temperature and water content) have advanced substantially, mostly in well‐mixed reactors, columns, and warming experiments without considering the characteristics of hydrological flow at the catchment scale. These theories have shown significant divergence from observations in natural systems. On the other hand, hydrological theories, including transit time theory, have progressed substantially yet have not been incorporated into understanding reactions at the catchment scale. Here we advocate for the development of integrated hydro‐biogeochemical theories across gradients of climate, vegetation, and geology conditions. The lack of such theories presents barriers for understanding mechanisms and forecasting the future of the Critical Zone under human‐ and climate‐induced perturbations. Although integration has started and co‐located measurements are well under way, tremendous challenges remain. In particular, even in this era of “big data,” we are still limited by data and will need to (1) intensify measurements beyond river channels and characterize the vertical connectivity and broadly the shallow and deep subsurface; (2) expand to older water dating beyond the time scales reflected in stable water isotopes; (3) combine the use of reactive solutes, nonreactive tracers, and isotopes; and (4) augment measurements in environments that are undergoing rapid changes. To develop integrated theories, it is essential to (1) engage models at all stages to develop model‐informed data collection strategies and to maximize data usage; (2) adopt a “simple but not simplistic,” or fit‐for‐purpose approach to include essential processes in process‐based models; (3) blend the use of process‐based and data‐driven models in the framework of “theory‐guided data science.” Within the framework of hypothesis testing, model‐data fusion can advance integrated theories that mechanistically link catchments' internal structures and external drivers to their functioning. It can not only advance the field of hydro‐biogeochemistry, but also enable hind‐ and fore‐casting and serve the society at large. Broadly, future education will need to cultivate thinkers at the intersections of traditional disciplines with hollistic approaches for understanding interacting processes in complex earth systems. This article is categorized under:Engineering Water > Methods 

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4. Geophysics as a hypothesis‐testing tool for critical zone hydrogeology

   https://doi.org/10.1002/wat2.1732  

   Dumont, Marc; Singha, Kamini (May 2024, WIREs Water)

   Abstract Geophysical methods have long been used in earth and environmental science for the characterization of subsurface properties. While imaging the subsurface opens the “black box” of subsurface heterogeneity, we argue here that these tools can be used in a more powerful way than characterization, which is to develop and test hypotheses. Critical zone science has opened new questions and hypotheses in the hydrologic sciences holistically around controls on water fluxes between surface, biological, and underground compartments. While groundwater flows can be monitored in boreholes, water fluxes from the atmosphere to the aquifer through the soil and the root system are more complex to study than boreholes can inform upon. Here, we focus on the successful application of various geophysical tools to explore hypotheses in critical zone hydrogeology and highlight areas where future contributions could be made. Specifically, we look at questions around subsurface structural controls on flow, the dimensionality and partitioning of those flows in the subsurface, plant water uptake, and how geophysics may be used to constrain reactive transport. We also outline areas of future research that may push the boundaries of how geophysical methods are used to quantify critical zone complexity. This article is categorized under:Water and Life > Nature of Freshwater EcosystemsScience of Water > Hydrological ProcessesWater and Life > Methods 

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5. Variation in riparian and stream assemblages across the primary succession landscape of Mount St. Helens, U.S.A.

   https://doi.org/10.1111/fwb.13694  

   Claeson, Shannon M.; LeRoy, Carri J.; Finn, Debra S.; Stancheva, Rosalina H.; Wolfe, Emily R. (March 2021, Freshwater Biology)

   Abstract Although most lotic ecosystems experience frequent and sometimes large disturbances, opportunities are uncommon to study primary succession in streams. Exceptions include new stream channels arising from events such as glacial retreat, volcanism, and catastrophic landslides. In 1980, the eruption and massive landslide at Mount St. Helens (WA, U.S.A.) created an entire landscape with five new catchments undergoing primary succession. We asked if riparian and lotic assemblages at early successional stages (36 years after the eruption) showed predictable change along longitudinal gradients within catchments, and whether assemblages were similar among five replicate catchments.In July 2016, we collected environmental data and characterised riparian, algal, and benthic macroinvertebrate assemblages at 21 stream reaches distributed within and among five neighbouring catchments. We evaluated patterns of richness, abundance, biomass, multivariate taxonomic community structure, and functional traits both longitudinally and among catchments.We found minimal evidence that longitudinal gradients had developed within catchments at 36 years post‐eruption. Increases in diatom and macroinvertebrate richness with downstream distance were the only biological responses with longitudinal trends. Conversely, we documented substantial variation in community structure of riparian plants, soft‐bodied algae, diatoms, and macroinvertebrates at the among‐catchment scale. Among‐catchment differences consistently separated two eastern catchments from three western catchments, and these two groups also differed in stream water chemistry, water temperature, and geomorphology.Overall, we documented greater diversity in the young catchments than predicted by ecologists in the years immediately following the eruption, yet functional traits indicate that these catchments are still in relatively early stages of succession. Variation at the among‐catchment scale is likely to be driven in part by hydrological source variation, with the two eastern catchments showing environmental signatures associated with glacial ice‐melt and the three western catchments probably fed primarily by springs from groundwater aquifers. Contemporary flow disturbance regimes also varied among catchments and successional trajectories were probably reset repeatedly in streams experiencing more frequent disturbance.Similar to new stream channels formed following glacial retreat, our results support a tolerance model of succession in streams. However, contrasting abiotic templates among Mount St. Helens catchments appear to be driving different successional trajectories of riparian plant, algal, and macroinvertebrate assemblages among neighbouring small catchments sharing the same catastrophic disturbance history. 

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