Watershed Disturbance Research Articles

Natural Flow Simulation in a Baisin considering Artificial Water Cycle

This study seeks to regionalize the parameters of the grid-based rainfall-runoff model (GRM) and simulate the natural flow considering an artificial water cycle. We established a water supply and demand network for the Han River basin, derived data related to the artificial water cycle through various organizations, and performed a water budget analysis by reflecting the simulated natural flow. When comparing the results of regionalization using only the Chungju Dam parameters and those using the Chungju Dam and Soyanggang Dam parameters, both results for the simulated flow showed good agreement with the observed flow, and the fitness evaluation results were also high (pentad: NSE 0.913–0.977, KGE 0.795–0.935, CC 0.974–0.995). The natural flow simulated by the GRM and TANK indicated that both models were able to reproduce the observed data well. Water budget analyses were performed for different time intervals, and daily analysis exhibited relatively lower fitness than pentad analysis (daily NSE 0.887-0.963, KGE 0.792-0.928, CC 0.960-0.990), but no significant difference was observed. The results indicated that the simulation of natural flow through the GRM was appropriate, and the model was sufficiently applicable to a disturbed watershed.

Cumulative Effects of Watershed Disturbances and Run-of-river Dams on Mercury Cycling: Case Study and Recommendations for Environmental Managers.

Run-of-river power plants (ROR) represent the majority of hydroelectric plants worldwide. Their environmental impacts are not well documented and are believed to be limited, particularly regarding the contamination of food webs by methylmercury (MeHg), a neurotoxin. RORs are typically installed in small rivers where combined effects of watershed disturbances with dam construction can complicate environmental management. We report a multi-year case study on the Saint-Maurice River (Canada) where an unpredicted temporary increase in MeHg accumulation in predator fish was observed after the construction of two ROR plants. The associated pondages acted as sedimentation basins for mercury (Hg) and organic matter from a watershed disturbed by a forest fire and by logging. This fresh organic carbon likely fueled microbial MeHg production. Hg methylation was more associated with environmental conditions than to the presence of Hg, and main methylating microbial groups were identified. A constructed wetland was a site of significant Hg methylation but was not the main source of the fish Hg increase. Organic carbon degradation was the main driver of MeHg accumulation at the base of the food chain whereas trophic levels explained the variations at the top of the food chain. Overall, carbon cycling was a key driver of Hg dynamics in this system, and ROR plants can cause temporary (ca. 12 years) Hg increase in food webs when developed in disturbed watersheds, although this increase is smaller than for large reservoirs. Recommendations for future ROR construction are to establish a good environmental monitoring plan with initial high temporal resolution and to consider recent and potential watershed disturbances in the plan.

Influence of a protected riparian corridor on the benthic aquatic macroinvertebrate assemblages of a small northern Michigan (USA) agricultural stream

The biotic integrity of streams is frequently compromised by anthropogenic disturbance; thus, it is important to study the potential mitigating effects of undisturbed riparian zones on streams within disturbed watersheds. In this study, two first-order northern Michigan streams located 1 km apart in adjacent agricultural watersheds, but with differing riparian land use, were compared to observe the effects of intact forest within the immediate 100 m stream corridor. Both streams had high specific conductance values associated with agricultural streams. The stream with a primarily forested riparian corridor, however, had greater canopy cover, faster stream velocity, greater coarse substrate, lower percentage of organic sediment, lower water temperatures, higher dissolved oxygen, higher taxonomic richness, higher relative biomass of shredders, lower relative biomass of gathering collectors, and more pollution intolerant taxa than the stream with a primarily agricultural corridor. Moreover, the benthic macroinvertebrate assemblages were highly distinct from each other, with the riparian forested stream dominated by aquatic insects like mayflies and caddisflies, and the agricultural stream by non-emergent isopods and amphipods. These results demonstrate the effects of a buffer zone on stream substrate composition, water physicochemistry, and benthic assemblages, and indicate the importance of preserving riparian zones within agricultural watersheds.

Major fluvial erosion and a 500‐Mt sediment pulse triggered by lava‐dam failure, Río Coca, Ecuador

AbstractThe failure of a 144‐m‐high lava‐dam waterfall on the Río Coca, Ecuador, in February 2020 initiated a catastrophic watershed reset—regressive erosion upstream and a massive sediment pulse downstream—as the river evolves towards a new equilibrium grade. The evolution of this river corridor after a sudden base‐level fall embodies the “complex response” concepts long understood through laboratory experiments, numerical modelling and smaller‐scale field studies, but that have not been observed in the field before on this scale. This paper presents geomorphic and geotechnical data to characterize the evolution of the Río Coca since 2020. In the three years after the lava‐dam failure, the erosion front migrated almost 13 km upstream along the mainstem river and triggered secondary headcuts that began migrating up tributaries. Erosion of the mainstem and tributary valleys generated a sediment pulse estimated to be 277 million m3 and ~500 million tonnes (Mt) over three years, depositing sediment tens of meters thick over tens of kilometres downstream from the former waterfall. This sediment pulse is one of the largest in modern times, comparable to the annual sediment load of a major continent‐draining river but with orders‐of‐magnitude greater sediment yield. Geomorphic adjustment of the Río Coca represents a highly unusual natural disaster threatening life, property, water quality, the regional economy, major infrastructure and energy security. However, this event also provides a rare opportunity to learn how a large autogenic watershed disturbance and recovery evolve, with important lessons for interpreting the sedimentary record of volcanic landscapes.

Understanding the Challenges of Hydrological Analysis at Bridge Collapse Sites

There is a crucial need for modeling hydrological extremes in order to optimize hydraulic system safety. It is often perceived that the best-fitted distribution accurately captures the intricacies of the hydrological extremes, particularly for the least disturbed watersheds. Thirty collapse sites with the least disturbed watersheds within the Appalachian Highland region in the U.S. are identified and used to test this perception. Goodness-of-fit tests, time series analysis, and comparison of predictor variables are carried out to find out the best-fitted distribution, identify trends and seasonal variation, and assess site variability. The study results are found to be inconclusive and sometimes contradictory; sometimes even complex distribution models do not provide better results. For most sites, the historic peak flow data are best-fitted with multiple distributions, including heavy and light tails. For monthly flow data, seasonal variation and trend cannot be categorized since no definitive, distinct tendency can be identified. When comparing sites best-fitted with a single distribution to sites best-fitted with multiple distributions, significant differences in certain geospatial characteristics are identified. However, these characteristics at the watershed scale are claimed to be less important in predicting the behavior of a flood event. All of these results capture the difficulties and inconsistencies in interpreting the results of hydrologic analysis, potentially reducing the robustness of the hydrologic tools used in the design and risk assessment of bridges.

Striving for consistent bioassessment across diverse landscapes: Using land use matters for classifying reference and disturbed sites for index development

Consistency in ecological assessments is challenging across large diverse landscapes because natural geological, climatic, and hydrological factors vary greatly. As a result, large landscapes are often subdivided into ecoregions and assessments are based on ecoregion specific indices. In the present study, we developed and compared multimetric indices (MMIs) using benthic diatom data from the 2008–2009 dataset from the United States (US) National Rivers and Streams Assessment. Nationwide and separate ecoregion specific MMIs were developed with reference, moderately disturbed, and highly disturbed sites selected using criteria based on physicochemical condition of the habitat or based on watershed land use (% agriculture and % urban). Metrics were adjusted to account for variation in natural conditions when needed. We found only land use criteria selected reference sites with consistently low median % watershed disturbance (%WD) and large differences in %WD between reference and highly disturbed sites. <38 % of sites were identified as reference or highly disturbed by both physicochemical and land use criteria. All MMIs displayed substantial discrimination ability between reference and highly disturbed sites. At the national scale, MMIs based on land use outperformed MMIs based on physicochemical conditions for all performance attributes tested. When national scale MMIs were applied to ecoregions, MMIs based on land use were again better than MMIs based on physicochemical conditions for most performance attributes and even had better or comparable performance to the land use MMIs developed separately for each ecoregion. Our findings show that developing MMIs using land use criteria and adjusting metrics for natural variation could improve assessment consistency without losing MMI performance across large, diverse landscapes as in the US National Rivers and Streams Assessment.

Decision support for aquatic restoration based on species-specific responses to disturbance.

Disturbances to aquatic habitats are not uniformly distributed within the Great Lakes and acute effects can be strongest in nearshore areas where both landscape and within lake effects can have strong influence. Furthermore, different fish species respond to disturbances in different ways. A means to identify and evaluate locations and extent of disturbances that affect fish is needed throughout the Great Lakes. We used partial Canonical Correspondence Analysis to separate “natural” effects on nearshore assemblages from disturbance effects. Species‐specific quadratic models of fish abundance as functions of in‐lake disturbance or watershed‐derived disturbance were developed separately for each of 35 species and lakewide predictions mapped for Lake Erie. Most responses were unimodal and more species decreased in abundance with increasing watershed disturbance than increased. However, eight species increased in abundance with current in‐lake disturbance conditions. Optimum Yellow Perch (Perca flavescens) abundance occurred at in‐lake disturbance values less than the gradient mean, but decreased continuously from minimum watershed disturbance to higher values. Bands of optimum in‐lake conditions occurred throughout the eastern and western portions of the Lake Erie nearshore zone; some areas were less disturbed than desirable. However, watershed‐derived disturbance conditions were generally poor for Yellow Perch throughout the lake. In contrast, optimum Smallmouth Bass (Micropterus dolomieu) abundance occurred at in‐lake disturbance values greater than the gradient mean and continuously increased with increasing watershed disturbance. Smallmouth Bass responses to disturbance indicated that most of the nearshore zone was less disturbed than is desirable and were most abundant in areas that the Yellow Perch response indicated were highly disturbed. Mapping counts of species response models that agreed on the disturbance level in each spatial unit of the nearshore zone showed a fine‐scale mosaic of areas in which habitat restoration may benefit many or few species. This tool may assist managers in prioritizing conservation and restoration efforts and evaluating environmental conditions that may be improved.

Quantifying the resilience of coldwater lake habitat to climate and land use change to prioritize watershed conservation

AbstractManaging ecological systems for resilience can increase their capacity to maintain key functions even under global change. Oxygenated coldwater (oxythermal) habitat in lakes is an important ecological resource that is threatened by both climate change and eutrophication. Here, we quantify the resilience of oxythermal habitat in over 10,000 glacial lakes in the upper Midwestern United States to climate change and watershed disturbance and classify lakes for conservation prioritization based on their current conditions and resilience. Oxythermal habitat was predicted by lake morphometry, July air temperatures, and watershed land use. Temperatures are projected to increase by mid‐century, and the magnitude of warming, its effect on oxythermal habitat, and the uncertainty surrounding that effect varied among lakes. Under mid‐century climate conditions, the number of lakes containing suitable coldwater habitat was predicted to decline by 67%, while the number of lakes with unsuitable habitat was predicted to increase by over 200%. Lakes varied in the amount of temperature increase that they could sustain without a resultant change in habitat tier (i.e., their climate resilience). Median climate resilience was 4.3°C, with some lakes capable of remaining in their habitat tier even with temperature increases up to 14°C. Changing watershed land use was predicted to influence oxythermal habitat in 24% of lakes (n = 2391). We used the magnitude of increase in watershed development that a lake could sustain while remaining in its current habitat class as a measure of its resilience to watershed disturbance. Conversely, decreased watershed development may improve oxythermal habitat conditions and push a lake into an improved condition, and this value represented a lake's restoration potential. We classified lakes into seven management classes based on their current oxythermal habitat conditions and the resilience of oxythermal habitat to climate and watershed disturbance. To facilitate management on individual lakes, we also assessed the vulnerability and resilience of individual lakes and the uncertainty surrounding these estimates. By quantifying the resilience of lakes and how it is influenced by local action across a multistate region, we can prioritize conservation action across multiple scales to maintain the critical habitat and ecosystem function of glacial lakes.

Variable Streamflow Response to Forest Disturbance in the Western US: A Large‐Sample Hydrology Approach

AbstractForest cover and streamflow are generally expected to vary inversely because reduced forest cover typically leads to less transpiration and interception. However, recent studies in the western U.S. have found no change or even decreased streamflow following forest disturbance due to drought and insect epidemics. We investigated streamflow response to forest cover change using hydrologic, climatic, and forest data for 159 watersheds in the western U.S. from the CAMELS data set for the period 2000–2019. Forest change and disturbance were quantified in terms of net tree growth (total growth volume minus mortality volume) and mean annual mortality rates, respectively, from the U.S. Forest Service's Forest Inventory and Analysis database. Annual streamflow was analyzed using multiple methods: Mann‐Kendall trend analysis, time trend analysis to quantify change not attributable to annual precipitation and temperature, and multiple regression to quantify contributions of climate, mortality, and aridity. Many watersheds exhibited decreased annual streamflow even as forest cover decreased. Time trend analysis identified decreased streamflow not attributable to precipitation and temperature changes in many disturbed watersheds, yet streamflow change was not consistently related to disturbance, suggesting drivers other than disturbance, precipitation, and temperature. Multiple regression analysis indicated that although change in streamflow is significantly related to tree mortality, the direction of this effect depends on aridity. Specifically, forest disturbances in wet, energy‐limited watersheds (i.e., where annual potential evapotranspiration [PET] is less than annual precipitation) tended to increase streamflow, while post‐disturbance streamflow more frequently decreased in dry water‐limited watersheds (where the PET to precipitation ratio exceeds 2.35).

Windows into the past: lake sediment phosphorus trajectories act as integrated archives of watershed disturbance legacies over centennial scales

Historic land alterations and agricultural intensification have resulted in legacy phosphorus (P) accumulations within lakes and reservoirs. Internal loading from such legacy stores can be a major driver of future water quality degradation. Yet, little is known about the magnitude and spatial patterns of legacy P accumulation in lentic systems, and how watershed disturbance trajectories drive these patterns. Here, we used a meta-analysis of 113 paleolimnological studies across 124 lakes and four reservoirs (referred here on as lakes) in 20 countries to quantify the linkages between the 100 year trajectories of P concentrations in lake sediments, watershed inputs, and lake morphology. We find five distinct clusters for lake sediment P trajectories, with lakes in the developing and developed world showing distinctly different patterns. Lakes in the developed world (Europe and North America) with early agricultural intensification had the highest sediment P concentrations (1176–1628 mg kg−1), with a peak between the 1970–1980s and a decline since then, while lakes in the developing world, specifically China, documented monotonically increasing sediment P concentrations (857–1603 mg kg−1). Sediment P trajectories reflected watershed disturbance patterns and were driven by a combination of anthropogenic drivers (fertilizer input and population density) and lake morphology (watershed to lake area ratio). Specifically, we found the largest legacy accumulation rates to occur in shallow lakes experiencing long-term land-use disturbances. These links between land-use change and P accumulation in lentic systems can provide insights about inland water quality response and help to develop robust predictive models useful for resource managers and decision-makers.

Analyzing the Effects of Land Cover Change on the Water Balance for Case Study Watersheds in Different Forested Ecosystems in the USA

We analyzed impacts of interannual disturbance on the water balance of watersheds in different forested ecosystem case studies across the United States from 1985 to 2016 using a remotely sensed long-term land cover monitoring record (U.S. Geological Survey Land Change Monitoring, Assessment, and Projection (LCMAP) Collection 1.0 Science products), gridded precipitation and evaporation data, and streamgaging data using paired watersheds (high and low disturbance). LCMAP products were used to quantify the timing and degree of interannual disturbance and to gain a better understanding of how land cover change affects the water balance of disturbed watersheds. In this paper, we present how LCMAP science products can be used to improve knowledge for hydrologic modeling, climate research, and forest management. Anthropogenic influences (e.g., dams and irrigation diversions) often minimize the impacts of land cover change on water balance dynamics when compared to interannual fluctuations of hydroclimatic events (e.g., drought and flooding). Our findings show that each watershed exhibits a complex suite of influences involving climate variables and other factors that affect each of their water balances differently when land cover change occurs. In this study, forests within arid to semi-arid climates experience greater water balance effects from land cover change than watersheds where water is less limited.

Drivers of zooplankton community composition in a novel ecosystem: Hawai'i mangroves as a case study

Abstract Management of established non‐native plants is challenging because removal is expensive and can produce negative consequences, yet establishment can create novel ecosystems. Red mangrove propagules were introduced to Moloka'i, Hawai'i, in 1902 to mitigate the effects of soil erosion and have since spread along the coast and to adjacent islands creating novel habitat. We compared zooplankton communities between novel mangrove and historical non‐mangrove habitat both within fishponds and along open coastline to examine environmental factors, including mangrove presence, affecting zooplankton community composition. Community composition patterns were driven by lunar cycle and site characteristics, including fishpond structure, mangrove and open‐coast shoreline length, percent of mangrove shoreline length, total percent mangrove leaf carbon and upstream watershed disturbance. Our findings indicate that during the tropical summer reproductive season, non‐native mangroves support diversity, richness and community composition similar to non‐mangrove areas, though some widespread taxa have lower abundance, and some rare taxa are more abundant in mangroves. Additionally, fishpond zooplankton community structure is significantly different from open‐coast areas, indicating fishponds, themselves, create novel habitat. Synthesis and application. In the face of declining fisheries, threatened reef habitat and changing climatic conditions, non‐native mangroves may provide, rather than impede, zooplankton habitat availability in novel locations.

Connecting the Age and Reactivity of Organic Carbon to Watershed Geology and Land Use in Tributaries of the Hudson River

AbstractWe characterized the dissolved organic matter (DOM) under baseflow conditions from a set of rivers in the Mohawk and Hudson River watersheds. The rivers in this study drain a range of bedrock geologies and land cover. We identify how those factors influence riverine DOM reactivity and the source, age, and composition of the biolabile DOM. We performed laboratory incubation experiments to characterize each river's reactive and non‐reactive DOM pools. Measurements of dissolved organic carbon concentration, radiocarbon, Ultraviolet‐visible spectroscopy absorbance, and Fourier‐transform ion cyclotron resonance mass spectrometry (FTICR‐MS) analysis were performed at each incubation start and end, allowing us to determine the quantity, age, and composition of the reactive and nonreactive DOM pools. We find that lithology controls bulk DOM ages, with watersheds underlain by shale/limestone having the most aged DOM and crystalline/metasedimentary watersheds having the youngest DOM. We observe that for a given lithology, bulk DOM age increases with the proportion of agricultural land in the watershed–suggesting agricultural practices mobilize aged DOM. FTICR‐MS analysis reveals that both lithology and land cover influence DOM composition. Shale/limestone watersheds showed DOM compositions distinct from other watershed lithologies, and the percentage of nitrogen‐containing DOM correlated with agricultural influence. In two of the studied rivers we find that the biolabile DOM fraction is older than the bulk DOM (upwards of 7 kyr) revealing that aged DOM may be preferentially consumed in these rivers. Our findings provide insight into how riverine carbon cycles may respond to watershed disturbances that influence DOM inputs to rivers.

Potential sediment supply fluxes associated with greenfield residential construction

• City-scale land disturbance was concentrated in outer suburban growth areas. • New construction sites followed consistent land disturbance patterns. • Land disturbance tended to last around seven years in each site. • Bare soil cover peaked near 100 % and on average, remained above 50 % for over a year. • Both development and lot-scale activities were major potential sediment producers. Residential construction is among the most intensive land disturbance activities in cities, increasing runoff and sediment export. In growing cities, large-scale residential construction areas are ubiquitous on the city fringes, but their relative contribution to city-wide land disturbance patterns is unknown. So is the relative sediment supply potential of different phases of construction. Such insights are required to guide prioritisation of sediment management measures at both city and development scales. A multi-scale assessment was therefore undertaken of land development spatial and temporal patterns. The assessment used remotely-sensed elevation data to characterise earthworks intensity and extent across different zones of a large city (Melbourne, Australia), and fine-scale aerial imagery surveys of bare soil and impervious cover to characterise watershed disturbance over time in new construction areas. Earthworks extent and intensity were greatest in outer growth areas where they were dominated by residential construction. Within residential developments, soil disturbance and drainage alteration proceeded in a consistent temporal sequence, lasting around 4–9 years in total. Bare soil cover approached 100 % during bulk earthworks. A conceptual model of changes in potential runoff and sediment supply over a series of construction phases suggests that potential sediment supply peaks with bare soil cover, but then remains high during subsequent road and house construction as imperviousness and drainage connection increase. Policies to reduce sediment loads to streams and receiving waters should place greater priority on improving construction erosion and sediment control in growth areas, and should target both development-scale and lot-scale activities. The conceptual model developed here provides a generalised hypothesis which could guide monitoring studies to target relevant construction phases at a fine temporal resolution and spatial scale.

Effect of watershed disturbance and river-tributary confluences on watershed sedimentation dynamics in the Western Allegheny Plateau

Harmful algal blooms (HABs) have increased in the Ohio River Basin over the past decade. Fine sediment dynamics are now recognized to play an important role in the proliferation and toxicity of HABs. As a result, the fate of sediment at confluences of tributaries and regulated river systems may be important zones for sediment retention and burial. The objective of this study was to improve understanding of watershed sediment loading dynamics and backwater inundation impacts on sedimentation within confluence watersheds. The study site selected for this work was the Fourpole Creek watershed in the Western Allegheny Plateau ecoregion of Appalachia, which is a mixed-use watershed with predominantly forested and urban landcover with a backwater floodplain wetland at the confluence with the Ohio River. Approximately 16 months of high-frequency (15-minute) turbidity data was used as a surrogate for total suspended solids at upstream and downstream nodes of the backwater confluence feature. Continuous sediment yield estimates suggested an annual sediment yield of 82 t/km2 and was found to be higher than sediment yields reported elsewhere in Appalachian watersheds without mining disturbances. A modified sediment rating curve using both seasonal and annual regression approaches were found to underpredict sediment yield by 29% and 80%, respectively. A hysteresis-based source unmixing method and hysteresis index were used to quantify sediment source and flow pathways during events and suggested high variability in sediment source contributions both seasonally and within-seasons, suggesting three likely sediment sources in the watershed including in-channel/gully erosion, resuspension of recently deposited sediment in the fluvial network, and erosion of subsurface macropores from forested uplands. Upstream-downstream monitoring of the confluence floodplain suggested 34% of the annual sediment inputs were retained during the continuously monitored 2019 calendar year. Our results suggest that confluence floodplains play an important role in intercepting and storing sediment inputs. Efforts to limit downstream sediment loads should ensure the function of confluence floodplains is maintained or enhanced when considering watershed management strategies in these landscapes.

Riparian forests buffer the negative effects of cropland on macroinvertebrate diversity in lowland Amazonian streams

Riparian forests regulate stream ecosystems and biodiversity. Therefore, changes to riparian structure may threaten stream ecosystem function by triggering taxonomic and functional changes to aquatic communities. Because macroinvertebrate assemblages are sensitive to environmental changes, they can be effective indicators of stream integrity in disturbed landscapes. To assess the role of riparian forests in maintaining tropical stream communities in areas experiencing large-scale watershed disturbance, we quantified the taxonomic and functional response of stream macroinvertebrate communities to forest clearing in the southeastern Amazon’s agricultural frontier, a region experiencing widespread deforestation. Our results show that watershed deforestation can lead to significant changes in macroinvertebrate richness and community composition. We found a predominance of shredders in forested watersheds; scrapers in cropland watersheds with riparian forests; and collector-filterers in cropland watersheds without riparian forest buffers. Taxonomic composition was controlled by available organic material in forested watersheds and by periphyton in cropland sites regardless of whether they had a riparian buffer. Our results show that the clearing of riparian forests alters food sources supporting aquatic food webs, leading to ecosystem-level shifts through changes in light and temperature dynamics that affect aquatic communities in areas with intense land-use change such as the southeastern Amazon.

Shenandoah Watershed Study‐Virginia Trout Stream Sensitivity Study (SWAS‐VTSSS): Stream water quality and hydrologic monitoring data for mid‐Appalachian headwater streams

AbstractThe Shenandoah Watershed Study (established in 1979) and the Virginia Trout Stream Sensitivity Study (established in 1987) serve to increase understanding of hydrological and biogeochemical changes in western Virginia mountain streams that occur in response to acidic deposition and other ecosystem stressors. The SWAS‐VTSSS program has evolved over its 40+ year history to consist of a temporally robust and spatially stratified monitoring framework. Currently stream water is sampled for water quality bi‐hourly during high‐flow events at three sites and weekly at four sites within Shenandoah National Park (SHEN), and quarterly at 72 sites and on an approximately decadal frequency at ~450 sites within the wider western Virginia Appalachian region. Stream water is evaluated for pH, acid neutralizing capacity (ANC), base cations (calcium, magnesium, sodium and potassium ion), acid anions (sulphate, nitrate and chloride), silica, ammonium, and conductivity with a subset of samples evaluated for monomeric aluminium and dissolved organic carbon. Hourly stream discharge (four sites) and in‐situ measurements of conductivity, water and air temperature (three sites) are also measured within SHEN. Here we provide an overview and timeline of the SWAS‐VTSSS stream water monitoring program, summarize the field and laboratory methods, describe the water chemistry and hydrologic data sets, and document major watershed disturbances that have occurred during the program history. Website links and instructions are provided to access the stream chemistry and time‐series monitoring data in open‐access federal databases. The purpose of this publication is to promote awareness of these unique, long‐term data sets for wider use in catchment studies. The water chemistry and hydrologic data can be used to investigate a wide range of biogeochemical research questions and provide key inputs for models of these headwater stream ecosystems. SWAS‐VTSSS is an ongoing program and quality assured data sets are uploaded to the databases annually.

Role of organic matter and microbial communities in mercury retention and methylation in sediments near run-of-river hydroelectric dams

Run-of-river power plants (RoRs) are expected to triple in number over the next decades in Canada. These structures are not anticipated to considerably promote the mobilization and transport of mercury (Hg) and its subsequent microbial transformation to methylmercury (MeHg), a neurotoxin able to biomagnify in food webs up to humans. To test whether construction of RoRs had an effect on Hg transport and transformation, we studied Hg and MeHg concentrations, organic matter contents and methylating microbial community abundance and composition in the sediments of a section of the St. Maurice River (Quebec, Canada). This river section has been affected by the construction of two RoR dams and its watershed has been disturbed by a forest fire, logging, and the construction of wetlands. Higher total Hg (THg) and MeHg concentrations were observed in the surface sediments of the flooded sites upstream of the RoRs. These peaks in THg and MeHg were correlated with organic matter proportions in the sediments (r2 = 0.87 and 0.82, respectively). In contrast, the proportion of MeHg, a proxy for methylation potential, was best explained by the carbon to nitrogen ratio suggesting the importance of terrigenous organic matter as labile substrate for Hg methylation in this system. Metagenomic analysis of Hg-methylating communities based on the hgcA functional gene marker indicated an abundance of methanogens, sulfate reducers and fermenters, suggesting that these metabolic guilds may be primary Hg methylators in these surface sediments. We propose that RoR pondages act as traps for sediments, organic matter and Hg, and that this retention can be amplified by other disturbances of the watershed such as forest fire and logging. RoR flooded sites can be conducive to Hg methylation in sediments and may act as gateways for bioaccumulation and biomagnification of MeHg along food webs, particularly in disturbed watersheds.

Understanding Food Web Mercury Accumulation Through Trophic Transfer and Carbon Processing along a River Affected by Recent Run-of-river Dams.

Unlike large dams which favor methylation of Hg in flooded soils over long periods, run-of-river dams are designed to flood a limited area of soils and are therefore not expected to significantly affect mercury (Hg) cycling or carbon processing. We studied the Hg and carbon cycles within food webs from several sectors along the Saint-Maurice River, Quebec, Canada, that differ in how they are influenced by two run-of-river dams and other watershed disturbances. We observed peak Hg concentrations in fish five-year postimpoundment, but these levels were reduced three years after this peak. Methylmercury concentrations in low trophic level fish and invertebrates were related to their carbon source (δ13C) rather than their trophic positions (δ15N). Biomagnification, measured by trophic magnification slopes, was driven mainly by methylmercury concentrations in low-trophic level organisms and environmental factors related to organic matter degradation and Hg-methylation. River sectors, δ13C and δ15N, predicted up to 80% of the variability in food web methylmercury concentrations. The installation of run-of-river dams and the related pondages, in association with other watershed disturbances, altered carbon processing, promoted Hg-methylation and its accumulation at the base of the food web, and led to a temporary increase in Hg levels in fish.

Interaction among Controlling Factors on Riverine DIN Export in Small Mountainous Rivers of Taiwan: Inseparable Human-Landscape System

Increasing anthropogenic nitrogen (N) emission via different pathways has shown prominent impact on aquatic ecosystems for decades, but the effects of interaction among climate-, landscape- and human-associated variables on riverine DIN (dissolved inorganic nitrogen, mainly NO3− and NH4+) export are unclear. In this study, the data of 43 watersheds with a wide range of climate-, landscape- and human-associated gradients across Taiwan were evaluated with partial redundancy analysis (pRDA) to examine their interactive controls on riverine DIN export. Results show that the annual riverine DIN export in Taiwan is approximately 3100 kg-N km−2 yr−1, spanning from 230 kg-N km−2 yr−1 in less disturbed watersheds (eastern and central Taiwan) to 10,000 kg-N km−2 yr−1 in watersheds with intensive human intervention (southwestern and northern Taiwan). NO3− is generally the single dominant form of DIN, while NH4+ renders significance in disturbed watersheds. Nearly all environmental variables display a positive correlation with DIN export, except for landscape setting variables (e.g., slope, area, channel length), which show a negative relationship. In terms of seasonal pattern, climate and human-landscape variables are related to NO3− export independently in the wet season, yet in the dry season climate-human variables jointly dominate NO3− export. Meanwhile, human-landscape (LH) variables (λ1 of LH &gt; 0.60) control NH4+ exports in both seasons, and human-associated (H) variables (λ1 of H = 0.13) have a minor effect on NH4+ exports in dry season. Precisely, the contribution of controlling variables on DIN export vary with species and seasons, indicating water quality management could be time-dependent, which should be taken into consideration for designing mitigation strategies.