Dust storms increase the tolerance of phytoplankton to thermal and pH changes.

Aquatic communities are increasingly subjected to multiple stressors through global change, including warming, pH shifts, and elevated nutrient concentrations. These stressors often surpass species tolerance range, leading to unpredictable consequences for aquatic communities and ecosystem functioning. Phytoplankton, as the foundation of the aquatic food web, play a crucial role in controlling water quality and the transfer of nutrients and energy to higher trophic levels. Despite the significance in understanding the effect of multiple stressors, further research is required to explore the combined impact of multiple stressors on phytoplankton. In this study, we used a combination of crossed experiment and mechanistic model to analyze the ecological and biogeochemical effects of global change on aquatic ecosystems and to forecast phytoplankton dynamics. We examined the effect of dust (0-75 mg L-1 ), temperature (19-27°C), and pH (6.3-7.3) on the growth rate of the algal species Scenedesmus obliquus. Furthermore, we carried out a geospatial analysis to identify regions of the planet where aquatic systems could be most affected by atmospheric dust deposition. Our mechanistic model and our empirical data show that dust exerts a positive effect on phytoplankton growth rate, broadening its thermal and pH tolerance range. Finally, our geospatial analysis identifies several high-risk areas including the highlands of the Tibetan Plateau, western United States, South America, central and southern Africa, central Australia as well as the Mediterranean region where dust-induced changes are expected to have the greatest impacts. Overall, our study shows that increasing dust storms associated with a more arid climate and land degradation can reverse the negative effects of high temperatures and low pH on phytoplankton growth, affecting the biogeochemistry of aquatic ecosystems and their role in the cycles of the elements and tolerance to global change.

Wildfire smoke impacts lake ecosystems.

Wildfire activity is increasing globally. The resulting smoke plumes can travel hundreds to thousands of kilometers, reflecting or scattering sunlight and depositing particles within ecosystems. Several key physical, chemical, and biological processes in lakes are controlled by factors affected by smoke. The spatial and temporal scales of lake exposure to smoke are extensive and under-recognized. We introduce the concept of the lake smoke-day, or the number of days any given lake is exposed to smoke in any given fire season, and quantify the total lake smoke-day exposure in North America from 2019 to 2021. Because smoke can be transported at continental to intercontinental scales, even regions that may not typically experience direct burning of landscapes by wildfire are at risk of smoke exposure. We found that 99.3% of North America was covered by smoke, affecting a total of 1,333,687 lakes ≥10 ha. An incredible 98.9% of lakes experienced at least 10 smoke-days a year, with 89.6% of lakes receiving over 30 lake smoke-days, and lakes in some regions experiencing up to 4 months of cumulative smoke-days. Herein we review the mechanisms through which smoke and ash can affect lakes by altering the amount and spectral composition of incoming solar radiation and depositing carbon, nutrients, or toxic compounds that could alter chemical conditions and impact biota. We develop a conceptual framework that synthesizes known and theoretical impacts of smoke on lakes to guide future research. Finally, we identify emerging research priorities that can help us better understand how lakes will be affected by smoke as wildfire activity increases due to climate change and other anthropogenic activities.

Constraining the atmospheric limb of the plastic cycle.

Plastic pollution is one of the most pressing environmental and social issues of the 21st century. Recent work has highlighted the atmosphere's role in transporting microplastics to remote locations [S. Allen et al., Nat. Geosci. 12, 339 (2019) and J. Brahney, M. Hallerud, E. Heim, M. Hahnenberger, S. Sukumaran, Science 368, 1257-1260 (2020)]. Here, we use in situ observations of microplastic deposition combined with an atmospheric transport model and optimal estimation techniques to test hypotheses of the most likely sources of atmospheric plastic. Results suggest that atmospheric microplastics in the western United States are primarily derived from secondary re-emission sources including roads (84%), the ocean (11%), and agricultural soil dust (5%). Using our best estimate of plastic sources and modeled transport pathways, most continents were net importers of plastics from the marine environment, underscoring the cumulative role of legacy pollution in the atmospheric burden of plastic. This effort uses high-resolution spatial and temporal deposition data along with several hypothesized emission sources to constrain atmospheric plastic. Akin to global biogeochemical cycles, plastics now spiral around the globe with distinct atmospheric, oceanic, cryospheric, and terrestrial residence times. Though advancements have been made in the manufacture of biodegradable polymers, our data suggest that extant nonbiodegradable polymers will continue to cycle through the earth's systems. Due to limited observations and understanding of the source processes, there remain large uncertainties in the transport, deposition, and source attribution of microplastics. Thus, we prioritize future research directions for understanding the plastic cycle.

Metal concentrations in wetland plant tissues influences transfer to terrestrial food webs.

Wetland plants tolerate potentially hazardous metals through a variety of strategies, including exclusion or accumulation. Whether plants sequester metals and where they store them in their tissues is important for understanding the potential role of plants as remediators or vectors of metals to terrestrial food webs. Here we evaluate metal sequestration in Great Salt Lake wetlands for one invasive (Phragmites australis; phragmites) and three native plant species, i.e. threesquare bulrush (Schoenoplectus americanus), hardstem bulrush (Schoenoplectus acutus), alkali bulrush (Bolboschoenus maritimus), and their terrestrial invertebrates. We observed higher concentrations of arsenic and copper than other metals in plant tissues, although high lead concentrations were observed in phragmites. All plants acted as excluders of arsenic and selenium, retaining the bulk of the metal mass in belowground tissues. In contrast, lead, copper, and cadmium were transferred to above ground tissues of hardstem bulrush and phragmites. The aboveground translocation facilitated the movement of these metals into invertebrates, with the highest concentrations in most cases found in predators. Though our results highlight the potential for metal remediation via wetland plant growth and removal, care should be taken to ensure that remediation efforts do not lead to bioaccumulation.

Sediment source fingerprinting as an aid to large-scale landscape conservation and restoration: A review for the Mississippi River Basin.

Reliable quantitative information on sediment sources to rivers is critical to mitigate contamination and target conservation and restoration actions. However, for large-scale river basins, determination of the relative importance of sediment sources is complicated by spatiotemporal variability in erosional processes and sediment sources, heterogeneity in sediment transport and deposition, and a paucity of sediment monitoring data. Sediment source fingerprinting is an increasingly adopted field-based technique that identifies the nature and relative source contribution of sediment transported in waterways. Notably, sediment source fingerprinting provides information that is independent of other field, modeling, or remotely sensed techniques. However, the diversity in sampling, analytical, and interpretive methods for sediment fingerprinting has been recognized as a problem in terms of developing standardized procedures for its application at the scale of large river basins. Accordingly, this review focuses on sediment source fingerprinting studies conducted within the Mississippi River Basin (MRB), summarizes unique information provided by sediment source fingerprinting that is distinct from traditional monitoring techniques, evaluates consistency and reliability of methodological approaches among MRB studies, and provides prospects for the use of sediment source fingerprinting as an aid to large-scale landscape conservation and restoration under current management frameworks. Most MRB studies reported credible fingerprinting results and found near-channel sources to be the dominant sediment sources in most cases, and yet a lack of standardization in procedural steps makes results difficult to compare. Findings from MRB studies demonstrated that sediment source fingerprinting is a highly valuable and reliable sediment source assessment approach to assist land and water resource management under current management frameworks, but efforts are needed to make this technique applicable in large-scale landscape conservation and restoration efforts. We summarize research needs and discuss sediment fingerprinting use for basin-scale management efforts with the aim of encouraging that this technique is robust and reliable as it moves forward.

Hf-Nd-Sr Isotopic Composition of the Tibetan Plateau Dust as a Fingerprint for Regional to Hemispherical Transport.

Large areas of arid regions in the Tibetan Plateau (TP) are undergoing desertification and subsequent aeolian emission and transport. The contribution of TP soils to the atmospheric aerosol burden in Asia and elsewhere is not known. Here, we use Hf, Nd, and Sr isotopes to distinguish the TP from other Asian dust-producing regions and compare the signatures to sediments in major dust sink regions. We found that the Hf-Nd-Sr isotopes of TP soils showed unique spatial signatures. From north to south, 87Sr/86Sr ratios gradually increased, while εNd and εHf values gradually decreased; from west to east, 87Sr/86Sr and εHf gradually increased, while εNd changed indistinctly. The Hf-Nd-Sr isotopic compositions of TP soils were controlled by four geographic isotope regions: the northern, southern, western, and eastern TP. Compared with Asian large deserts, the TP showed a unique isotopic composition, which together exhibited a significant spatial change across Asia. Compared to dust isotopes in prominent sink areas, we found that the TP is an important dust source to eastern TP glaciers, the Chinese Loess Plateau, South China Sea, Japan, and Greenland. This study provides clear isotopic evidence that the TP is a major aeolian contributor in the Northern Hemisphere and may have important implications for the global aeolian cycle.

Source or sink? Quantifying beaver pond influence on non-point source pollutant transport in the Intermountain West.

Non-point source (NPS) pollution remains high in many watersheds despite strategies aimed at reducing such pollution. Beaver (Castor canadensis) activity converts lotic systems to semi-lentic by impounding stream flow and trapping sediments, which have a high affinity for NPS pollutants such as nitrogen (N), phosphorus (P), and heavy metals. This study aimed to identify environmental conditions under which beaver ponds influence the fate and cycling of NPS pollutants. Dissolved and particulate nutrients were sampled upstream and downstream of three headwater beaver ponds differing in age and character through the summer season. Sedimentation rates and sediment concentrations of nutrients and metals were also determined. Results from this study suggest that beaver ponds can attenuate heavy metals at a rate 2 to 4 times greater than a riffle reach (p < 0.05). Metal sequestration scaled with pond age and sediment organic matter content. The oldest and youngest ponds had no significant effect on dissolved nutrients (NO3-, TDN and SRP) or total P (TP). The middle age pond was a significant TN sink in summer (0.6-0.8 g N m-2 d-1 [p = 0.03]) and influenced dissolved nutrient concentrations differently in spring (21% NO3- sink [p = 0.03], 61% SRP source [p = 0.05]) compared to summer (34% NO3- source, 7% SRP sink). This pond had little apparent effect on TP loads during the study period but accumulated a total of 146 g m-2 of phosphorus in the sediments suggesting that beaver ponds may reach their phosphorus sequestration potential within the first few years of pond development and then subsequently act as a weak SRP source. We use a theoretical relationship describing sediment-water interactions to show that biogeochemical processing in a beaver pond is optimized at intermediate levels of pond nutrient supply and residence time. If beaver ponds are to be considered as an option for landscape scale restoration, this theoretical relationship may be useful for predicting the effects of beaver ponds on water chemistry, and aid in the interpretation of variable water quality results from inherently heterogeneous environments.

Key rules of life and the fading cryosphere: Impacts in alpine lakes and streams.

Alpine regions are changing rapidly due to loss of snow and ice in response to ongoing climate change. While studies have documented ecological responses in alpine lakes and streams to these changes, our ability to predict such outcomes is limited. We propose that the application of fundamental rules of life can help develop necessary predictive frameworks. We focus on four key rules of life and their interactions: the temperature dependence of biotic processes from enzymes to evolution; the wavelength dependence of the effects of solar radiation on biological and ecological processes; the ramifications of the non-arbitrary elemental stoichiometry of life; and maximization of limiting resource use efficiency across scales. As the cryosphere melts and thaws, alpine lakes and streams will experience major changes in temperature regimes, absolute and relative inputs of solar radiation in ultraviolet and photosynthetically active radiation, and relative supplies of resources (e.g., carbon, nitrogen, and phosphorus), leading to nonlinear and interactive effects on particular biota, as well as on community and ecosystem properties. We propose that applying these key rules of life to cryosphere-influenced ecosystems will reduce uncertainties about the impacts of global change and help develop an integrated global view of rapidly changing alpine environments. However, doing so will require intensive interdisciplinary collaboration and international cooperation. More broadly, the alpine cryosphere is an example of a system where improving our understanding of mechanistic underpinnings of living systems might transform our ability to predict and mitigate the impacts of ongoing global change across the daunting scope of diversity in Earth's biota and environments.

Continental-Scale Increase in Lake and Stream Phosphorus: Are Oligotrophic Systems Disappearing in the United States?

We describe continental-scale increases in lake and stream total phosphorus (TP) concentrations, identified through periodic probability surveys of thousands of water bodies in the conterminous U.S. The increases, observed over the period 2000-2014 were most notable in sites in relatively undisturbed catchments and where TP was initially low (e.g., less than 10 µg L(-1)). Nationally, the percentage of stream length in the U.S. with TP ≤ 10 µg L(-1) decreased from 24.5 to 10.4 to 1.6% from 2004 to 2009 to 2014; the percentage of lakes with TP ≤ 10 µg L(-1) decreased from 24.9 to 6.7% between 2007 and 2012. Increasing TP concentrations appear to be ubiquitous, but their presence in undeveloped catchments suggests that they cannot be entirely attributed to either point or common non-point sources of TP.

Soil temperature and moisture as key controls of phosphorus export in mountain watersheds.

Oligotrophic mountain lakes act as sensitive indicators of landscape-scale changes in mountain regions due to their low nutrient concentration and remote, relatively undisturbed watersheds. Recent research shows that phosphorus (P) concentrations are increasing in mountain lakes around the world, creating more mesotrophic states and altering lake ecosystem structure and function. The relative importance of atmospheric deposition and climate-driven changes to local biogeochemistry in driving these shifts is not well established. In this study, we test whether increasing temperatures in watershed soils may be contributing to the observed increases in mountain lake P loading. Specifically, we test whether higher soil temperatures increase P mobilization from mountain soils by accelerating the rate of geochemical weathering and soil organic matter decomposition. We used paired soil incubation (lab) and soil transplant (field) experiments with mountain soils from around the western United States to test the effects of warming on rain-leachable P concentration, soil P mobilization, and soil respiration. Our results show that while higher temperature can increase soil P mobilization, low soil moisture can limit the effects of warming in some situations. Soils with lower bulk densities, higher pH, lower aluminum oxide contents, and lower ratios of carbon to nitrogen had much higher rain-leachable P concentration across all sites and experimental treatments. Together, these results suggest that mountain watersheds with high-P soils and relatively high soil moisture could have the largest increases in P mobilization with warming. Consequently, lakes and streams in such watersheds could become especially susceptible to soil-driven eutrophication as temperatures rise.

Satellite-based aerosol optical depth estimates over the continental U.S. during the 2020 wildfire season: Roles of smoke and land cover.

Wildfires produce smoke that can affect an area >1000 times the burn extent, with far-reaching human health, ecologic, and economic impacts. Accurately estimating aerosol load within smoke plumes is therefore crucial for understanding and mitigating these impacts. We evaluated the effectiveness of the latest Collection 6.1 MODIS Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm in estimating aerosol optical depth (AOD) across the U.S. during the historic 2020 wildfire season. We compared satellite-based MAIAC AOD to ground-based AERONET AOD measurements during no-, light-, medium-, and heavy-smoke conditions identified using the Hazard Mapping System Fire and Smoke Product. This smoke product consists of maximum extent smoke polygons digitized by analysts using visible band imagery and classified according to smoke density. We also examined the strength of the correlations between satellite- and ground-based AOD for major land cover types under various smoke density levels. MAIAC performed well in estimating AOD during smoke-affected conditions. Correlations between MAIAC and AERONET AOD were strong for medium- (r = 0.91) and heavy-smoke (r = 0.90) density, and MAIAC estimates of AOD showed little bias relative to ground-based AERONET measurements (normalized mean bias = 3 % for medium, 5 % for heavy smoke). During two high AOD, heavy smoke episodes, MAIAC underestimated ground-based AERONET AOD under mixed aerosol (i.e., smoke and dust; median bias = -0.08) and overestimated AOD under smoke-dominated (median bias = 0.02) aerosol. MAIAC most overestimated ground-based AERONET AOD over barren land (mean NMB = 48 %). Our findings indicate that MODIS MAIAC can provide robust estimates of AOD as smoke density increases in coming years. Increased frequency of mixed aerosol and expansion of developed land could affect the performance of the MAIAC algorithm in the future, however, with implications for evaluating wildfire-associated health and welfare effects and air quality standards.

Growth of grasses and forbs, nutrient concentration, and microbial activity in soil treated with microbeads.

Microplastics have emerged as an important threat to terrestrial ecosystems. To date, little research has been conducted on investigating the effects of microplastics on ecosystem functions and multifunctionality. In this study, we conducted the pot experiments containing five plant communities consisting of Phragmites australis, Cynanchum chinense, Setaria viridis, Glycine soja, Artemisia capillaris, Suaeda glauca, and Limonium sinense and added polyethylene (PE) and polystyrene (PS) microbeads to the soil (contained a mixture of 1.5 kg loam and 3 kg sand) at two concentrations of 0.15 g/kg (lower concentration, hereinafter referred to as PE-L and PS-L) and 0.5 g/kg (higher concentration, hereinafter referred to as PE-H and PS-H) to explore the effects of microplastics on total plant biomass, microbial activity, nutrient supply, and multifunctionality. The results showed that PS-L significantly decreased the total plant biomass (p = 0.034), primarily by inhibiting the growth of the roots. ß-glucosaminidase decreased with PS-L, PS-H, and PE-L (p < 0.001) while the phosphatase was noticeably augmented (p < 0.001). The observation suggests that the microplastics diminished the nitrogen requirements and increased the phosphorus requirements of the microbes. The decrease in ß-glucosaminidase diminished ammonium content (p < 0.001). Moreover, PS-L, PS-H, and PE-H reduced the soil total nitrogen content (p < 0.001), and only PS-H considerably reduced the soil total phosphorus content (p < 0.001), affecting the ratio of N/P markedly (p = 0.024). Of interest, the impacts of microplastics on total plant biomass, ß-glucosaminidase, phosphatase, and ammonium content did not become larger at the higher concentration, and it is observable that microplastics conspicuously depressed the ecosystem multifunctionality, as microplastics depreciated single functions such as total plant biomass, ß-glucosaminidase, and nutrient supply. In perspective, measures to counteract this new pollutant and eliminate its impact on ecosystem functions and multifunctionality are necessary.

Sources and fate of atmospheric microplastics revealed from inverse and dispersion modelling: From global emissions to deposition.

We combine observations from Western USA and inverse modelling to constrain global atmospheric emissions of microplastics (MPs) and microfibers (MFs). The latter are used further to model their global atmospheric dynamics. Global annual MP emissions were calculated as 9.6 ± 3.6 Tg and MF emissions as 6.5 ± 2.9 Tg. Global average monthly MP concentrations were 47 ng m-3 and 33 ng m-3 for MFs, at maximum. The largest deposition of agricultural MPs occurred close to the world's largest agricultural regions. Road MPs mostly deposited in the East Coast of USA, Central Europe, and Southeastern Asia; MPs resuspended with mineral dust near Sahara and Middle East. Only 1.8% of the emitted mass of oceanic MPs was transferred to land, and 1.4% of land MPs to ocean; the rest were deposited in the same environment. Previous studies reported that 0.74-1.9 Tg y-1 of land-based atmospheric MPs/MFs (< 5 mm) are transported to the ocean, while riverine transport is between 3.3 and 14 Tg y-1. We calculate that 0.418 ± 0.201 Tg y-1 MPs/MFs (size up to 250 and 2500 µm) were transported from the land to ocean (large particles were ignored). Model validation against observations showed that particle removal must be urgently updated in global models.

Limited progress in nutrient pollution in the U.S. caused by spatially persistent nutrient sources.

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 km2 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.

Identifying factors that affect mountain lake sensitivity to atmospheric nitrogen deposition across multiple scales.

Increased nitrogen (N) deposition rates over the past century have affected both North American and European mountain lake ecosystems. Ecological sensitivity of mountain lakes to N deposition varies, however, because chemical and biological responses are modulated by local watershed and lake properties. We evaluated predictors of mountain lake sensitivity to atmospheric N deposition across North American and European mountain ranges and included as response variables dissolved inorganic N (DIN = NNH4+ + NNO3-) concentrations and phytoplankton biomass. Predictors of these responses were evaluated at three different spatial scales (hemispheric, regional, subregional) using regression tree, random forest, and generalized additive model (GAM) analysis. Analyses agreed that Northern Hemisphere mountain lake DIN was related to N deposition rates and smaller scale spatial variability (e.g., regional variability between North American and European lakes, and subregional variability between mountain ranges). Analyses suggested that DIN, N deposition, and subregional variability were important for Northern Hemisphere mountain lake phytoplankton biomass. Together, these findings highlight the need for finer-scale, subregional analyses (by mountain range) of lake sensitivity to N deposition. Subregional analyses revealed differences in predictor variables of lake sensitivity. In addition to N deposition rates, lake and watershed features such as land cover, bedrock geology, maximum lake depth (Zmax), and elevation were common modulators of lake DIN. Subregional phytoplankton biomass was consistently positively related with total phosphorus (TP) in Europe, while North American locations showed variable relationships with N or P. This study reveals scale-dependent watershed and lake characteristics modulate mountain lake ecological responses to atmospheric N deposition and provides important context to inform empirically based management strategies.

Plastic rain in protected areas of the United States.

Eleven billion metric tons of plastic are projected to accumulate in the environment by 2025. Because plastics are persistent, they fragment into pieces that are susceptible to wind entrainment. Using high-resolution spatial and temporal data, we tested whether plastics deposited in wet versus dry conditions have distinct atmospheric life histories. Further, we report on the rates and sources of deposition to remote U.S. conservation areas. We show that urban centers and resuspension from soils or water are principal sources for wet-deposited plastics. By contrast, plastics deposited under dry conditions were smaller in size, and the rates of deposition were related to indices that suggest longer-range or global transport. Deposition rates averaged 132 plastics per square meter per day, which amounts to >1000 metric tons of plastic deposition to western U.S. protected lands annually.

Protecting endangered species in the USA requires both public and private land conservation.

Crucial to the successful conservation of endangered species is the overlap of their ranges with protected areas. We analyzed protected areas in the continental USA to assess the extent to which they covered the ranges of endangered tetrapods. We show that in 80% of ecoregions, protected areas offer equal (25%) or worse (55%) protection for species than if their locations were chosen at random. Additionally, we demonstrate that it is possible to achieve sufficient protection for 100% of the USA's endangered tetrapods through targeted protection of undeveloped public and private lands. Our results highlight that the USA is likely to fall short of its commitments to halting biodiversity loss unless more considerable investments in both public and private land conservation are made.

Relationships between borders, management agencies, and the likelihood of watershed impairment.

In the United States, the Clean Water Act (CWA) establishes water quality standards important for maintaining healthy freshwater ecosystems. Within the CWA framework, states define their own water quality criteria, leading to a potential fragmentation of standards between states. This fragmentation can influence the management of shared water resources and produce spillover effects of pollutants crossing state lines and other political boundaries. We used numerical simulations to test the null prediction of no difference in impairment between watersheds that cross political boundaries (i.e. state lines, national or coastal borders, hereafter termed "transboundary") and watersheds that cross no boundaries (hereafter "internal"). We found that transboundary watersheds are more likely to be impaired than internal watersheds. Further, we examined possible causes for this relationship based on both geographic and sociopolitical drivers. Though geographic variables such as human-modified land cover and the amount of upstream catchment area are associated with watershed impairment, the number and type of agencies managing land within a watershed better explained the different impairment levels between transboundary and internal watersheds. Watersheds primarily consisting of public lands are less impaired than watersheds consisting of private lands. Similarly, watersheds primarily managed by federal agencies are less impaired than state-managed watersheds. Our results highlight the importance of considering Integrated Watershed Management strategies for water resources within a fragmented policy framework.