Downstream Aquatic Ecosystems Research Articles

Effects of spruce budworm defoliation on in-stream algal production and carbon use by food webs

Insect defoliation is a widespread cause of forest disturbance in Canada, and it has the potential to alter terrestrial carbon contributions to stream consumers and thus affect stream ecosystems. Although defoliation by spruce budworms ( Choristoneura fumiferana) is considered a natural disturbance with importance to forest succession, the magnitude and duration of outbreaks are increasing, and there is a growing need to understand their downstream consequences. To examine this, algal productivity and autochthonous (in-stream) energy supporting macroinvertebrate and fish consumers (using δ13C, δ15N) were examined in 12 streams in the Gaspé Peninsula, Québec, Canada, that ranged in watershed defoliation from spruce budworm. Hierarchical partitioning models were used to assess the contributions of watershed and stream variables to these endpoints. Total seasonal algal production ranged fourfold among sites, and cumulative defoliation was the strongest predictor. Consumers from streams with greater defoliation (cumulative or annual) had lower reliance on autochthonous carbon, despite increased algal production, and this effect was greater for brook trout than macroinvertebrates. These findings demonstrate for the first time that insect outbreaks—which alter forest structure and function—also similarly affect downstream aquatic ecosystems by causing changes in the production and contributions of carbon sources. This suggests that insect control strategies and forest management decisions should also consider implications for downstream aquatic ecosystems.

Seasonal dynamics of water and sediment in the permafrost basin of the Qinghai‐Tibet plateau and its driving factors

AbstractClimate change has regulated cryosphere‐fed rivers and altered interannual and seasonal sediment dynamics, with significant implications for terrestrial material cycles and downstream aquatic ecosystems. However, there has been a notable scarcity of research focusing on the patterns of water–sediment transport within these permafrost zones. Integrating 6 years (2017–2022) of in‐situ observational data from FengHuoShan basin with the partial least squares‐structural equation modelling (PLS‐SEM) method, we analyse the driving factors, characteristics and seasonal patterns of the water–sediment transport process. We observed a gradual increase in both suspended sediment flux (SSF, Mt/yr) and runoff (Q, km3/yr) within the basin, with annual growth rates of 1.34%/yr and 0.75%/yr, respectively. It is worth noting that these growth rates exhibit seasonal variations, with the highest values observed in spring (SSF: 1.76%/yr, Q: 1.71%/yr). This indicates that the response to climate change is more pronounced in spring compared with summer and autumn. Through mathematical statistics and the PLS‐SEM model, we found that temperature plays a predominant role in the dynamics of water–sediment in both spring and autumn, whereas rainfall exerts a more significant impact during the summer. Most suspended sediment concentration (SSC, kg/m3) peak events throughout the year are primarily driven by rainfall. Affected by the freeze–thaw cycle of permafrost, SSC and discharge (Q, m3/s) exhibit distinct seasonality. SSC and Q demonstrate a clockwise trend; both Q and SSC begin to increase from May and peak in August before declining. The insights gleaned from this study hold significant implications for water resource management and soil conservation strategies in the region, particularly in the face of ongoing climatic changes characterized by warming and increased humidity.

Historical gold mining increased metal(loid) concentrations in lake sediments from Nova Scotia, Canada

Historical gold mining operations between the 1860s and 1940s have left substantial quantities of arsenic- and mercury-rich tailings near abandoned mines in remote and urban areas of Nova Scotia, Canada. Large amounts of materials from the tailings have entered the surface waters of downstream aquatic ecosystems at concentrations that present a risk to benthos. We used paleolimnological approaches to examine long-term trends in sedimentary metal(loid) concentrations, assess potential sediment toxicity, and determine if geochemical recovery has occurred at four lakes located downstream of three productive gold-mining districts. During the historical mining era, sedimentary total arsenic and mercury concentrations and enrichment factors increased substantially at all downstream lakes that received inputs from tailings. Similarly, chromium, lead, and zinc concentrations increased in the sediments after mining activities began and the urbanization that followed. The calculated probable effects of concentration quotients (PEC-Qs) for sediments exceeded the probable biological effects threshold (PEC-Q > 2) during the mining era. Although sedimentary metal(loid) concentrations have decreased for most elements in recent sediments, relatively higher PEC-Q and continued exceedance of Canadian Interim Sediment Quality Guidelines suggest that complete geochemical recovery has not occurred. It is likely that surface runoff from tailing fields, urbanization, and climate-mediated changes are impacting geochemical recovery trajectories.

Urban Stormwater Quality in Arequipa, Southern Peru: An Initial Assessment

Urban stormwater quality has been sampled and chemically analyzed in the city of Arequipa, southern Peru. Stormwater samples generated from a 0.04 km2 drainage area in a downtown location were collected during two rainy seasons (2022 and 2023), including both first flush and peak flow for each storm event, analyzing physical and chemical (metals) variables. Results were compared with Peruvian agricultural irrigation standards, identifying also temporal changes and statistical correlations. Several metals (B, Cu, Fe, Mn, and Zn) were detected at concentrations above Peruvian MPLs, with B being the analyte that violated the norm more often. Most pollution occurred at the beginning of each rainy season and during the first flush stages. All vehicle-related contaminants were well correlated except for Pb and Se, which were assumed to have a different source of origin. We recommend that further investigations should focus on the effects of urban stormwater on downstream ecosystems in Peru. Similarly, we strongly recommend the creation of new regulations that ensure proper stormwater quality released from urban areas of this country, as well as preventive/treatment practices to minimize the pollution of downstream aquatic ecosystems and ensure healthy water to irrigate crops located downstream from cities.

Sediment-Nitrogen (N) connectivity: suspended sediments in streams as N exporters and reactors for denitrification and assimilatory N uptake during storms

Nitrogen (N) pollution in riverine ecosystems has substantial environmental, economic, and policy consequences. Various riverine N removal processes include permanent dissimilatory sinks such as denitrification (Uden) and temporary assimilatory sink such as microbial N uptake (Uassim). Both processes have been extensively evaluated in benthic sediments but only sparsely in the water column, particularly for storm flows producing high suspended sediment (SS) concentrations. Stormflows also increase the sediment bound N (Sed-N) export, and in turn, the overall N exports from watersheds. The balance between N removal by Uden and Uassim vs. Sed-N export has not been studied and is a key knowledge gap. We assessed the magnitude of Uden and Uassim against stormflow Sed-N exports for multiple storm events of varying magnitude and across two drainage areas (750 ha and 15,330 ha) in a mixed landuse mid-Atlantic US watershed. We asked: How do the Uden and Uassim sinks compare with Sed-N exports and how do these N fluxes vary across the drainage areas for sampled storms on the rising and falling limbs of the discharge hydrograph? Mean Uden and Uassim as % of the Sed-N exports ranged between 0.1–40% and 0.6–22%, respectively. Storm event Uassim fluxes were generally slightly lower than the corresponding Uden fluxes. Similarly, comparable but slightly higher Uden fluxes were observed for the second order vs. the fourth order stream, while Uassim fluxes were slightly higher in the fourth-order stream. Both of these N sinks were higher on the falling vs. rising limbs of the hydrograph. This suggests that while the N sinks are not trivial, sediment bound N exports during large stormflows will likely overshadow any gains in N removal by SS associated denitrification. Understanding these N source-sink dynamics for storm events is critical for accurate watershed nutrient modeling and for better pollution mitigation strategies for downstream aquatic ecosystems. These results are especially important within the context of climate change as extreme hydrological events including storms are becoming more and more frequent.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water.

Glacier melting exports a large amount of nitrate to downstream aquatic ecosystems. Glacial lakes and glacier-fed rivers in proglacial environments serve as primary recipients and distributors of glacier-derived nitrate (NO3-), yet little is known regarding the sources and cycling of nitrate in these water bodies. To address this knowledge gap, we conducted a comprehensive analysis of nitrate isotopes (δ15NNO3, δ18ONO3, and Δ17ONO3) in waters from the glacial lake and river of the Rongbuk Glacier-fed Basin (RGB) in the mountain Everest region. The concentrations of NO3- were low (0.43 ± 0.10 mg/L), similar to or even lower than those observed in glacial lakes and glacier-fed rivers in other high mountain regions, suggesting minimal anthropogenic influence. The NO3- concentration decreases upon entering the glacial lake due to sedimentation, and it increases gradually from upstream to downstream in the river as a soil source is introduced. The analysis of Δ17ONO3 revealed a substantial contribution of unprocessed atmospheric nitrate, ranging from 34.29 to 56.43%. Denitrification and nitrification processes were found to be insignificant in the proglacial water of RGB. Our study highlights the critical role of glacial lakes in capturing and redistributing glacier-derived NO3- and emphasizes the need for further investigations on NO3- transformation in the fast-changing proglacial environment over the Tibetan Plateau and other high mountain regions.

Elevated Mercury Deposition, Accumulation, and Migration in a Karst Forest.

The karst forest is one of the extremely sensitive and fragile ecosystems in southwest China, where the biogeochemical cycling of mercury (Hg) is largely unknown. In this study, we investigated the litterfall deposition, accumulation, and soil migration of Hg in an evergreen-deciduous broadleaf karst forest using high-resolution sampling and stable isotope techniques. Results show that elevated litterfall Hg concentrations and fluxes in spring are due to the longer lifespan of evergreen tree foliage exposed to atmospheric Hg0. The hillslope has 1-2 times higher litterfall Hg concentration compared to the low-lying land due to the elevated atmospheric Hg levels induced by topographical and physiological factors. The Hg isotopic model suggests that litterfall Hg depositions account for ∼80% of the Hg source contribution in surface soil. The spatial trend of litterfall Hg deposition cannot solely explain the trend of Hg accumulation in the surface soil. Indeed, soil erosion enhances Hg accumulation in soil of low-lying land, with soil Hg concentration up to 5-times greater than the concentration on the hillslope. The high level of soil Hg migration in the karst forest poses significant ecological risks to groundwater and downstream aquatic ecosystems.

Acute Ecotoxicity Potential of Untreated Tannery Wastewater Release in Arequipa, Southern Peru

The centralized Rio Seco Industrial Park (RSIP) tannery collective in the Southern Peruvian city of Arequipa releases untreated tannery wastewater into a proximal creek that is a tributary of the Chili River. As industrial leather tanning wastewater contains high concentrations of metal(loid)s, salts, dyes, and organics, this complex mixture could exert a myriad of toxicological effects on the surrounding ecosystem. The RSIP effluent was analyzed to quantify the acute ecotoxicity and ecotoxicological status of this untreated industrial wastewater at multiple trophic levels with the following bioindicators: the floating macrophyte Lemna minor, invertebrates Daphnia magna and Physa venustula, and the amphibian Xenopus laevis. A physicochemical characterization of the RSIP effluent revealed a highly contaminated waste stream. In addition to chromium (10.4 ± 0.4 mg/L) and other toxic metals, the water harbored extremely high concentrations of total dissolved solids (67,770 ± 15,600 mg/L), biochemical oxygen demand (1530 ± 290 mg/L) and total nitrogen (490 ± 10 mg/L). The toxicological responses of certain bioindicator species tested were evaluated after exposure to 0, 1.5, 3.0, and 4.5% untreated tannery wastewater blended with dechlorinated tap water. L. minor experienced a significant decrease in the number of fronds, wet weight, and dry weight at the lowest blended wastewater of 1.5%. Bioassays with D. magna showed the effect on neonatal mortality with a calculated LC50 of 1.1% for 48 h. Bioassays with P. venustula embryos showed high sensitivity to diluted effluent with complete mortality at 3.0% wastewater and above. Finally, X. laevis showed a high sensitivity to the dilutions with an LC50 of 1.6 for embryos and 1.8% for tadpoles. Although RSIP wastewater contains many potentially toxic components, chromium and total dissolved solids, with a major contribution from sodium, are best correlated with acute toxicity variables. This suggests that conductivity or analogous measurements could provide a rapid and affordable forensic tool to query acute ecosystem pressures. Collectively, the results indicate that the release of untreated tannery wastewater from RSIP can exert pronounced acute impacts across trophic levels with the need for treatment or dilution to below 1% of total flow. As the assays addressed acute toxicity, the necessary treatment and/or dilution to mitigate chronic effects is likely much lower. In conclusion, untreated RSIP tannery wastewaters represent an ecological risk to downstream aquatic ecosystems; this needs to be addressed to prevent current and future environmental consequences.

A century of nitrogen dynamics in agricultural watersheds of Denmark

Intensive agriculture has been linked to increased nitrogen loads and adverse effects on downstream aquatic ecosystems. Sustained large net nitrogen surpluses have been shown in several contexts to form legacies in soil or waters, which delay the effects of reduction measures. In this study, detailed land use and agricultural statistics were used to reconstruct the annual nitrogen surpluses in three agriculture-dominated watersheds of Denmark (600–2700 km2) with well-drained loamy soils. These surpluses and long-term hydrological records were used as inputs to the process model ELEMeNT to quantify the nitrogen stores and fluxes for 1920–2020. A multi-objective calibration using timeseries of river nitrate loads, as well as other non-conventional data sources, allowed to explore the potential of these different data to constrain the nitrogen cycling model. We found the flux-weighted nitrate concentrations in the root zone percolate below croplands, a dataset not commonly used in calibrating watershed models, to be critical in reducing parameter uncertainty. Groundwater nitrate legacies built up in all three studied watersheds during 1950–1990 corresponding to ∼2% of the surplus (or ∼1 kg N ha yr−1) before they went down at a similar rate during 1990–2015. Over the same periods active soil nitrogen legacies first accumulated by approximately 10% of the surplus (∼5 kg N ha yr−1), before undergoing a commensurate reduction. Both legacies appear to have been the drivers of hysteresis in the diffuse load at the catchments’ outlet and hindrances to reaching water quality goals. Results indicate that the low cropland surpluses enforced during 2008–2015 had a larger impact on the diffuse river loads than the European Union’s untargeted grass set-aside policy of 1993–2008. Collectively, the measures of 1990–2015 are estimated to have reset the diffuse load regimes of the watersheds back to the situation prevailing in the 1960s.

Contrasting the influence of crop rotation on phosphorus balances and losses in agricultural fields across a tile-drained landscape in Ohio, USA

Excess phosphorus (P) applied to agricultural fields accumulates in the soil profile and is susceptible to leaching, subsequently impacting adjacent and downstream aquatic ecosystems. Planting a series of crops in a field over a given period of time is considered conservation crop rotation, which may reduce nutrient losses by reducing P inputs (i.e., including crops with lower P requirements) and increasing P demand (i.e., by increasing biodiversity, vegetative land cover, and length of the growing season). Here, we calculated P balances (Inputs–Outputs) in agricultural fields with corn-soybean rotations (CS) and corn-soybean-wheat rotations (CSW) to determine P accumulation or depletion. We then compared P losses, as dissolved reactive P (DRP) and total P (TP), between the CS and CSW rotations. Results indicated annual P deficits in CS (median = −9 kg P/ha) and CSW (median = −5 kg P/ha) fields. However, DRP and TP losses persisted and were significantly greater from CSW (median = 0.5 kg DRP/ha and 2 kg TP/ha) compared to CS fields (median = 0.1 kg DRP/ha and 1 kg TP/ha), likely due to elevated soil test phosphorus (STP) concentrations in CSW fields. The fraction of applied P exported from fields as both DRP and TP was also greater in CSW (DRP = 2%; TP = 9%) compared to CS (DRP = 1%; TP = 3%), further confirming more available P susceptible to loss. Findings here highlight the need for further research between conventional and conservation crop rotation on fields with similar STP concentrations and emphasize the importance of P application in accordance with fertility recommendations.

Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.

In-drain denitrifying woodchip bioreactors for reducing nitrogen runoff from sugarcane

Denitrifying woodchip bioreactors offer an opportunity to intercept and reduce nitrogen loads between agricultural fields and downstream aquatic ecosystems. Here we assessed the performance of three in-drain bioreactor beds installed in open drains receiving runoff and shallow ground water from rain-fed sugarcane production in the Wet Tropics of Australia. Drain nitrate-N concentrations were generally low, with a mean of 0.2 mg L−1 and maximum of 3.3 mg L−1, which may have been partially due to denitrification in the contributing soils. Bioreactors reduced the concentration of nitrate-N in intercepted waters (average 41% reduction). However, removal rates were often limited by nitrate-N availability. Load reduction over the 2018/19 season was just 0.11 kg N ha−1 yr−1. This limited performance was in large part due to the dynamic nature of nitrogen loads in this system. Specifically, a high proportion of the annual nitrogen load occurring during ‘first-flush’ events immediately after fertilization (i.e. 72% during a 10-day period in 2018/19) resulting in considerable bypass flow (i.e. low interception). Our study is the first in the Australian Wet Tropics to assess the annualized performance and total load reductions of in-drain denitrifying bioreactor beds. Although showing their potential, our results highlight the fact that denitrifying bioreactor performance is determined by the hydraulic context of the catchment, and the proportion of the annual nitrate load that can be successfully intercepted.

Patterns and drivers of water quality changes associated with dams in the Tropical Andes

Abstract. The Tropical Andes is a biodiversity hotspot facing pressure from planned and ongoing hydropower development. However, the effects of dams on the region's river ecosystems, as mediated by physicochemical changes in the water quality, are poorly known. Colombia is unique among its peers in South America with respect to managing central public environmental databases, including surface water quality data sets associated with the environmental monitoring of dams. To assess the relationship between hydropower and Colombian river conditions, we analyze monitoring data associated with 15 dams, focusing on oxygen availability, thermal regimes and sediment losses because these properties are influenced directly by river damming and impose fundamental constraints on the structure of downstream aquatic ecosystems. We find that most Colombian dams (7 of 10) seasonally reduce concentrations of total suspended solids by large percentages (50 %–99 %) through sediment trapping. Most dams (8 of 15) also, via the discharge of warm reservoir surface waters, seasonally increase river temperatures by 2 to 4 ∘C with respect to upstream conditions. A subset of four dams generate downstream hypoxia (< 4 mg L−1) and water that is 2 to 5 ∘C colder than inflows, with both processes driven by the turbination and discharge of cold and anoxic hypolimnetic waters during periods of reservoir stratification. Reliance on monitoring data likely leads us to under-detect impacts: many rivers are only sampled once or twice per year, which cannot capture temporal shifts across seasons and days (i.e., in response to hydropeaking). Despite these blind spots, the monitoring data point to some opportunities for planners and hydropower companies to mitigate downstream ecological impacts. These findings highlight the importance of implementing environmental monitoring schemes associated with hydrologic infrastructure in developing countries.

Modeling multi-year phosphorus dynamics in a bioretention cell: Phosphorus partitioning, accumulation, and export

Phosphorus (P) export from urban areas via stormwater runoff contributes to eutrophication of downstream aquatic ecosystems. Bioretention cells are a Low Impact Development (LID) technology promoted as a green solution to attenuate urban peak flow discharge, as well as the export of excess nutrients and other contaminants. Despite their rapidly growing implementation worldwide, a predictive understanding of the efficiency of bioretention cells in reducing urban P loadings remains limited. Here, we present a reaction-transport model to simulate the fate and transport of P in a bioretention cell facility in the greater Toronto metropolitan area. The model incorporates a representation of the biogeochemical reaction network that controls P cycling within the cell. We used the model as a diagnostic tool to determine the relative importance of processes immobilizing P in the bioretention cell. The model predictions were compared to multi-year observational data on 1) the outflow loads of total P (TP) and soluble reactive P (SRP) during the 2012–2017 period, 2) TP depth profiles collected at 4 time points during the 2012–2019 period, and 3) sequential chemical P extractions performed on core samples from the filter media layer obtained in 2019. Results indicate that exfiltration to underlying native soil was principally responsible for decreasing the surface water discharge from the bioretention cell (63 % runoff reduction). From 2012 to 2017, the cumulative outflow export loads of TP and SRP only accounted for 1 % and 2 % of the corresponding inflow loads, respectively, hence demonstrating the extremely high P reduction efficiency of this bioretention cell. Accumulation in the filter media layer was the predominant mechanism responsible for the reduction in P outflow loading (57 % retention of TP inflow load) followed by plant uptake (21 % TP retention). Of the P retained within the filter media layer, 48 % occurred in stable, 41 % in potentially mobilizable, and 11 % in easily mobilizable forms. There were no signs that the P retention capacity of the bioretention cell was approaching saturation after 7 years of operation. The reactive transport modeling approach developed here can in principle be transferred and adapted to fit other bioretention cell designs and hydrological regimes to estimate P surface loading reductions at a range of temporal scales, from a single precipitation event to long-term (i.e., multi-year) operation.

Effects of ecological roofs in water quality: an experimental study over a humid tropical climate

Although ecological roofs (vegetated or non-vegetated) provide many benefits, it can also leach substances such as nutrients and metals that can affect downstream aquatic ecosystems. Therefore, this work aims to investigate the rainwater quality from ecological roofs in Recife, located in the Northeast Brazil, using local species in a tropical and humid climate. Using four test cells of 1 m2 (one non-vegetated filled with expanded clay aggregate, two vegetated with cactus “Coroa-de-Frade” and grass “Grama Esmeralda”, and one control roof), we analyzed thirteen water quality variables regarding irrigation parameters: pH, electrical conductivity, turbidity, nitrate, ammonia, phosphate, bicarbonate, carbonate, calcium, magnesium, sulfate, potassium, boron, sodium, and sodium adsorption ratio. We simulated rain events controlling its intensity and analyzed a sample of natural rainwater event. All roofs neutralized the pH. Control and clay roofs were source of bicarbonate and calcium, responsible for more alkaline effluents. Carbonate and ammonia were below the recommended limits for irrigation purposes for all roofs. Green roofs were source of nitrate, ammonia, and boron. Neither roofs were source or sink for sulfate and chloride for all analyzed samples. Regarding the natural rainwater experiment, only green roof with Coroa-de-Frade exceeded the recommend irrigation parameters for potassium and phosphate. A post-treatment is required for irrigation purposes. We recommend a first-flush system followed by a filter with sand and activated carbon.

Impacts of Forest Fire Ash on Aquatic Mercury Cycling.

Mercury (Hg) is a ubiquitous contaminant in the environment and its methylated form, methylmercury (MeHg), poses a worldwide health concern for humans and wildlife, primarily through fish consumption. Global production of forest fire ash, derived from wildfires and prescribed burns, is rapidly increasing due to a warming climate, but their interactions with aqueous and sedimentary Hg are poorly understood. Herein, we compared the differences of wildfire ash with activated carbon and biochar on the sorption of aqueous inorganic Hg and sedimentary Hg methylation. Sorption of aqueous inorganic Hg was greatest for wildfire ash materials (up to 0.21 μg g-1 or 2.2 μg g-1 C) among all of the solid sorbents evaluated. A similar Hg adsorption mechanism for activated carbon, biochar made of walnut, and wildfire ash was found that involves the formation of complexes between Hg and oxygen-containing functional groups, especially the -COO group. Notably, increasing dissolved organic matter from 2.4 to 70 mg C L-1 remarkably reduced Hg sorption (up to 40% reduction) and increased the time required to reach Hg-sorbent pseudo-equilibrium. Surprisingly, biochar and wildfire ash, but not activated carbon, stimulated MeHg production during anoxic sediment incubation, possibly due to the release of labile organic matter. Overall, our study indicates that while wildfire ash can sequester aqueous Hg, the leaching of its labile organic matter may promote production of toxic MeHg in anoxic sediments, which has an important implication for potential MeHg contamination in downstream aquatic ecosystems after wildfires.

Reservoir water quality deterioration due to deforestation emphasizes the indirect effects of global change

Deforestation is currently a widespread phenomenon and a growing environmental concern in the era of rapid climate change. In temperate regions, it is challenging to quantify the impacts of deforestation on the catchment dynamics and downstream aquatic ecosystems such as reservoirs and disentangle these from direct climate change impacts, let alone project future changes to inform management. Here, we tackled this issue by investigating a unique catchment-reservoir system with two reservoirs in distinct trophic states (meso‑ and eutrophic), both of which drain into the largest drinking water reservoir in Germany. Due to the prolonged droughts in 2015–2018, the catchment of the mesotrophic reservoir lost an unprecedented area of forest (exponential increase since 2015 and ca. 17.1% loss in 2020 alone). We coupled catchment nutrient exports (HYPE) and reservoir ecosystem dynamics (GOTM-WET) models using a process-based modeling approach. The coupled model was validated with datasets spanning periods of rapid deforestation, which makes our future projections highly robust. Results show that in a short-term time scale (by 2035), increasing nutrient flux from the catchment due to vast deforestation (80% loss) can turn the mesotrophic reservoir into a eutrophic state as its counterpart. Our results emphasize the more prominent impacts of deforestation than the direct impact of climate warming in impairment of water quality and ecological services to downstream aquatic ecosystems. Therefore, we propose to evaluate the impact of climate change on temperate reservoirs by incorporating a time scale-dependent context, highlighting the indirect impact of deforestation in the short-term scale. In the long-term scale (e.g. to 2100), a guiding hypothesis for future research may be that indirect effects (e.g., as mediated by catchment dynamics) are as important as the direct effects of climate warming on aquatic ecosystems.

Catchment controls of denitrification and nitrous oxide production rates in headwater remediated agricultural streams

Heavily modified headwater streams and open ditches carry high nitrogen loads from agricultural soils that sustain eutrophication and poor water quality in downstream aquatic ecosystems. To remediate agricultural streams and reduce the export of nitrate (NO3−), phosphorus and suspended sediments, two-stage ditches with constructed floodplains can be implemented as countermeasures. By extending hydrological connectivity between the stream channel and riparian corridor within constructed floodplains, these remediated ditches enhance the removal of NO3− via the microbial denitrification process. Ten remediated ditches were paired with upstream trapezoidal ditches in Sweden across different soils and land uses to measure the capacity for denitrification and nitrous oxide (N2O) production and yields under denitrifying conditions in stream and floodplain sediments. To examine the controls for denitrification, water quality was monitored monthly and flow discharge continuously along reaches. Floodplain sediments accounted for 33 % of total denitrification capacity of remediated ditches, primarily controlled by inundation and stream NO3− concentrations. Despite reductions in flow-weighted NO3− concentrations along reaches, NO3− removal in remediated ditches via denitrification can be masked by inputs of NO3−-rich groundwaters, typical of intensively managed agricultural landscapes. Although N2O production rates were 50 % lower in floodplains compared to the stream, remediated ditches emitted more N2O than conventional trapezoidal ditches. Higher denitrification rates and reductions of N2O proportions were predicted by catchments with loamy soils, higher proportions of agricultural land use and lower floodplain elevations. For realizing enhanced NO3− removal from floodplains and avoiding increased N2O emissions, soil type, land use and the design of floodplains need to be considered when implementing remediated streams. Further, we stress the need for assessing the impact of stream remediation in the context of broader catchment processes, to determine the overall potential for improving water quality.

Development of a morphometric connectivity model to mitigate sediment derived from storm-driven shallow landslides

In silvopastoral environments, landslide erosion results in loss of productive soils and pasture. Sediment delivered to streams from landslides contributes to the degradation of freshwater and marine receiving environments by smothering benthic habitats and increasing turbidity, light attenuation, and sediment-bound contaminants. Biological mitigation is an important strategy in pastoral environments to combat landslide erosion and improve the health of downstream aquatic ecosystems. Using lasso logistic regression, we investigate determinants of sediment connectivity for a landslide-triggering storm event in 1977 in the Wairarapa, New Zealand. Furthermore, we develop the first morphometric connectivity model to predict the likelihood of sediment delivery to streams following landslide initiation. We explore a range of connectivity scenarios by defining a set of sinks and simulating varying rates of sediment generation during flood events of increasing magnitude. The likelihood of sediment delivery is greatly enhanced where landslide deposits coalesce. Besides scar size variables, overland flow distance and vertical distance to sink were the most important morphometric predictors of connectivity. When scar size variables were removed from the connectivity model, median AUROC was reduced from 0.88 to 0.75.By coupling landslide susceptibility and connectivity predictions in a modular form, we quantify the cost effectiveness of targeted versus non-targeted approaches to shallow landslide mitigation. Sediment delivery ratios range from 0.21 to 0.29, equating to an event sediment yield of 3548 t km−2 to 9033 t km−2. Targeted mitigation of landslide-derived sediment is approximately an order of magnitude more cost-effective than a non-targeted approach. Compared with a pasture-only baseline, a 34% reduction in sediment delivery can be achieved by increasing slope stability through spaced tree planting on 6.5% of the pastoral land. The maximum reduction achievable through comprehensive coverage of widely spaced planting is 56%. The landslide connectivity model provides an objective method to support management decisions relating to mitigation of landslide erosion and sediment delivery to streams.

Application of swat hydrological model to assess the impacts of land use change on sediment loads

Controlling and reducing the watershed's erosion and sedimentation is essential to ensure the continuity of projects implemented to develop land and water resources and improve sustainability, performance, and longevity. Sediment control is also critical in managing the river basin in limiting the transport of solids, improving water quality, sustaining aquatic life, and preventing damage to downstream aquatic environments and ecosystems. Estimating the potential effects of land-use changes on surface runoff and soil erosion requires distributed hydrological modeling methods. In addition to naturally occurring sediments, changes in land-use types for different applications can be a primary cause for the increase in sediment rates in the watershed. This study used the Soil and Water Assessment Tool (SWAT), a rainfall-runoff model, to evaluate land use/cover changes (i.e., deforestation) and their impact on sediment load under different scenarios. For the baseline (no changes) scenario, the watershed is calibrated using the flow and sediment data measured from the rain gauge station during the time step to estimate the post-deforestation changes at the sub-catchment scale of the study area. The study results indicated that the total surface runoff and sediment yield for the selected sub-catchment in the deforestation scenario with the highest spatial distribution, due to the high erosivity (24% increase) of excessive surface runoff after deforestation, sediment yield increased 3.5-fold. In contrast, due to the removal of trees and vegetation's canopy, the evapotranspiration, leaf area index, and dissolved oxygen transported into reach showed the inverse ratios, and the values decreased by 5%, 24, and 17%, respectively, in compared with the baseline scenario. In terms of watershed management, therefore, the application of hydrological models such as SWAT rainfall-runoff and erosion models can be a helpful method for decision-makers to apply for the protection of forests from intensive impacts such as deforestation and limiting their socio-environmental effects.