More Like this

1. Water chemistry of Alaskan glacial rivers is affected by glacier coverage, bedrock type, and groundwater inputs

   https://doi.org/10.3389/frwa.2025.1569267  

   Coombs, Miaja; Carling, Gregory T; Munk, Lee Ann; Jenckes, Jordan; Bickmore, Barry R; Rey, Kevin A; Thompson, Alyssa N; Fernandez, Diego P; Bergstrom, Anna; Gomez, Teresa (April 2025, Frontiers in Water)

   Glacial meltwater contributions to streams depend on watershed characteristics that impact water quantity and quality, with potential changes as glaciers continue to recede. The purpose of our study was to investigate the influence of glacier and bedrock controls on water chemistry in glacial streams, focusing on a range of small to large watersheds in Alaska. Southcentral Alaska provides an ideal study area due to diverse geologic characteristics and varying amounts of glacial coverage across watersheds. To investigate spatial and temporal variability due to glacial coverage and bedrock type, we analyzed water samples (n= 343) from seven watersheds over 2 years for major and trace element concentrations and water stable isotopes. We found variable water chemistry across the glacial rivers related to glacial coverage and the relative amount of metamorphic, sedimentary, and igneous bedrock. Some sites had elevated concentrations of harmful trace elements like As and U from glacier melt or groundwater. Longitudinal (upstream to downstream) variability was apparent within each river, with increasing inputs from tributaries, and groundwater altering the water chemistry relative to glacier meltwater contributions. The water chemistry and isotopic composition of river samples compared with endmember sources suggested a range from glacier-dominated to groundwater-dominated sites along stream transects. For example, water chemistry in the Knik and Matanuska rivers (with large contributing glaciers) was more influenced by glacier meltwater, while water chemistry in the Little Susitna River (with small glaciers) was more influenced by groundwater. Across all rivers, stream chemistry was controlled by glacier inputs near the headwaters and groundwater inputs downstream, with the water chemistry reflecting bedrock type. Our study provides a greater understanding of geochemical and hydrological processes controlling water resources in rapidly changing glacial watersheds. 

   more » « less
2. Where the past meets the present: connecting nitrogen from watersheds to streams through groundwater flowpaths

   https://doi.org/10.1088/1748-9326/ad0c86  

   Moore, Eric M; Barclay, Janet R; Haynes, Adam B; Jackson, Kevin E; Bisson, Alaina M; Briggs, Martin A; Helton, Ashley M (November 2023, Environmental Research Letters)

   Abstract Groundwater discharge to streams is a nonpoint source of nitrogen (N) that confounds N mitigation efforts and represents a significant portion of the annual N loading to watersheds. However, we lack an understanding of where and how much groundwater N enters streams and watersheds. Nitrogen concentrations at the end of groundwater flowpaths are the culmination of biogeochemical and physical processes from the contributing land area where groundwater recharges, within the aquifer system, and in the near-stream riparian area where groundwater discharges to streams. Our research objectives were to quantify the spatial distribution of N concentrations at groundwater discharges throughout a mixed land-use watershed and to evaluate how relationships among contributing and riparian land cover, modeled aquifer characteristics, and groundwater discharge biogeochemistry explain the spatial variation in groundwater discharge N concentrations. We accomplished this by integrating high-resolution thermal infrared surveys to locate groundwater discharge, biogeochemical sampling of groundwater, and a particle tracking model that links groundwater discharge locations to their contributing area land cover. Groundwater N loading from groundwater discharges within the watershed varied substantially between and within streambank groundwater discharge features. Groundwater nitrate concentrations were spatially heterogeneous ranging from below 0.03–11.45 mg-N/L, varying up to 20-fold within meters. When combined with the particle tracking model results and land cover metrics, we found that groundwater discharge nitrate concentrations were best predicted by a linear mixed-effect model that explained over 60% of the variation in nitrate concentrations, including aquifer chemistry (dissolved oxygen, Cl⁻, SO₄²⁻), riparian area forested land cover, and modeled physical aquifer characteristics (discharge, Euclidean distance). Our work highlights the significant spatial variability in groundwater discharge nitrate concentrations within mixed land-use watersheds and the need to understand groundwater N processing across the many spatiotemporal scales within groundwater cycling. 

   more » « less
3. DOM in the long arc of environmental science: looking back and thinking ahead

   https://doi.org/10.1007/s10533-022-00924-w  

   McDowell, William H. (March 2022, Biogeochemistry)

   Abstract Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds that is produced through both microbial degradation and abiotic leaching of solid phase organic matter, and by a wide range of metabolic processes in algae and higher plants. DOM is ubiquitous throughout the hydrologic cycle and plays an important role in watershed management for drinking water supply as well as many aspects of aquatic ecology and geochemistry. Due to its wide-ranging effects in natural waters and analytical challenges, the focal research questions regarding DOM have varied since the 1920s. A standard catchment-scale model has emerged to describe the environmental controls on DOM concentrations. Modest concentrations of DOM are found in atmospheric deposition, large increases occur in throughfall and shallow soil flow paths, and variable concentrations in surface waters occur largely as a result of the extent to which hydrologic flow paths encounter deeper mineral soils, wetlands or shallow organic-rich riparian soils. Both production and consumption of DOM occur in surface waters but appear to frequently balance, resulting in relatively constant concentrations with distance downstream in most streams and rivers. Across biomes the concentration and composition of DOM in flowing waters is driven largely by soil processes or direct inputs to channels, but high levels can be found in streams and rivers from the tropics to the poles. Seven central challenges and opportunities in the study of DOM should frame ongoing research. These include maintaining or establishing long-term records of changes in concentrations and fluxes over time, capitalizing on the use of sensors to describe short-term DOM dynamics in aquatic systems, integrating the full carbon cycle into understanding of watershed and aquatic DOM dynamics, understanding the role of DOM in evasion of greenhouse gases from inland waters, unraveling the enigma of dissolved organic nitrogen, documenting gross versus net DOM fluxes, and moving beyond an emphasis on functional ecological significance to understanding the evolutionary significance of DOM in a wide range of environments. 

   more » « less
4. Limited progress in nutrient pollution in the U.S. caused by spatially persistent nutrient sources

   https://doi.org/10.1371/journal.pone.0258952  

   Frei, Rebecca J.; Lawson, Gabriella M.; Norris, Adam J.; Cano, Gabriel; Vargas, Maria Camila; Kujanpää, Elizabeth; Hopkins, Austin; Brown, Brian; Sabo, Robert; Brahney, Janice; et al (November 2021, PLOS ONE)

   Toor, Gurpal S. (Ed.)

   Human agriculture, wastewater, and use of fossil fuels have saturated ecosystems with nitrogen and phosphorus, threatening biodiversity and human water security at a global scale. Despite efforts to reduce nutrient pollution, carbon and nutrient concentrations have increased or remained high in many regions. Here, we applied a new ecohydrological framework to ~12,000 water samples collected by the U.S. Environmental Protection Agency from streams and lakes across the contiguous U.S. to identify spatial and temporal patterns in nutrient concentrations and leverage (an indicator of flux). For the contiguous U.S. and within ecoregions, we quantified trends for sites sampled repeatedly from 2000 to 2019, the persistence of spatial patterns over that period, and the patch size of nutrient sources and sinks. While we observed various temporal trends across ecoregions, the spatial patterns of nutrient and carbon concentrations in streams were persistent across and within ecoregions, potentially because of historical nutrient legacies, consistent nutrient sources, and inherent differences in nutrient removal capacity for various ecosystems. Watersheds showed strong critical source area dynamics in that 2–8% of the land area accounted for 75% of the estimated flux. Variability in nutrient contribution was greatest in catchments smaller than 250 km 2 for most parameters. An ensemble of four machine learning models confirmed previously observed relationships between nutrient concentrations and a combination of land use and land cover, demonstrating how human activity and inherent nutrient removal capacity interactively determine nutrient balance. These findings suggest that targeted nutrient interventions in a small portion of the landscape could substantially improve water quality at continental scales. We recommend a dual approach of first prioritizing the reduction of nutrient inputs in catchments that exert disproportionate influence on downstream water chemistry, and second, enhancing nutrient removal capacity by restoring hydrological connectivity both laterally and vertically in stream networks. 

   more » « less
5. Continental‐scale seston stoichiometry reveals fundamental constraints on the elemental composition of particles transported by streams

   https://doi.org/10.1002/lno.70084  

   Manning, David_W_P; Shogren, Arial_J; Benstead, Jonathan_P; Loveless, Zacharie_T (May 2025, Limnology and Oceanography)

   Abstract Suspended particulate matter, or seston, represents an understudied flux of carbon (C), nitrogen (N), and phosphorus (P) in river networks. Here, we summarize riverine seston C : N : P stoichiometry data from 27 streams and rivers sampled regularly from 2014 to 2022 across the United States by the National Ecological Observatory Network (NEON). We examine relationships among seston C, N, and P content using standardized major‐axis (SMA) and ordinary least squares slopes to test congruence with a constant‐ratio model (scaling coefficient = 1), and hierarchical models to identify watershed‐level covariates of seston C : nutrient stoichiometric allometry. At the continental scale, C and N were tightly coupled and conformed to the constant‐ratio model, while seston C : P and N : P indicated weaker coupling and inconstant ratios across the range of C vs. P and N vs. P values. At the stream‐site scale, C : N, C : P, and N : P often exhibited slopes < 1, indicating that within individual streams seston becomes more nutrient‐rich as seston concentration increases. Watershed forest cover, season, and discharge helped explain stoichiometric allometry across streams, where forested sites in wetter climates had lower scaling slopes, and slopes decreased with low flows. Our study underscores the importance of suspended particles as a material flux in river networks and highlights the interplay between biotic and abiotic factors that drive the relative consistency of its C : nutrient stoichiometry during transport from local to continental scales. 

   more » « less