Climate change impacts on eutrophication in the Po River (Italy): Temperature-mediated reduction in nitrogen export but no effect on phosphorus

Rivers worldwide are under stress from eutrophication and nitrate pollution, but the ecological consequences overlap with climate change, and the resulting interactions may be unexpected and still unexplored. The Po River basin (northern Italy) is one of the most agriculturally productive and densely populated areas in Europe. It remains unclear whether the climate change impacts on the thermal and hydrological regimes are already affecting nutrient dynamics and transport to coastal areas. The present work addresses the long-term trends (1992-2020) of nitrogen and phosphorus export by investigating both the annual magnitude and the seasonal patterns and their relationship with water temperature and discharge trajectories. Despite the constant diffuse and point sources in the basin, a marked decrease (-20%) in nitrogen export, mostly as nitrate, was recorded in the last decade compared to the 1990s, while no significant downward trend was observed for phosphorus. The water temperature of the Po River has warmed, with the most pronounced signals in summer (+0.13°C/year) and autumn (+0.16°C/year), together with the strongest increase in the number of warm days (+70%-80%). An extended seasonal window of warm temperatures and the persistence of low flow periods are likely to create favorable conditions for permanent nitrate removal via denitrification, resulting in a lower delivery of reactive nitrogen to the sea. The present results show that climate change-driven warming may enhance nitrogen processing by increasing respiratory river metabolism, thereby reducing export from spring to early autumn, when the risk of eutrophication in coastal zones is higher.

Nitrate losses from forest during snowmelt: An underestimated source in mid-high latitude watershed

The forest nitrate cycle is a crucial part of the watershed nitrate load but has received limited attention compared to that of agricultural and residential land. Here, we analyzed the status and sources of riverine nitrate fluxes and identified the characteristics and contribution of forest nitrate loss to the riverine system in a mid-high latitude forested watershed using monthly field sampling and a modified Soil and Water Assessment Tool (SWAT) with enhanced forest nutrient cycle representation. The results indicate that nitrate losses in the headwater stream and downstream exhibit different seasonal characteristics. The nitrate losses in the headwater stream show a bimodal pattern due to lower temperatures and snowmelt runoff. Redundancy analysis (RDA) revealed that, unlike nitrogen (N) fertilizer-induced nitrate loss in the rainy season, forest loss has a positive effect on headwater stream nitrate concentration during the snowmelt season. The modified SWAT was then utilized to simulate nitrate losses in forest lands. The forest nitrate export per unit area of the headwater stream (1.58 ± 1.78 kg/ha/yr) was observed to be higher than that of the downstream (0.67 ± 0.74 kg/ha/yr) due to high snowmelt and mineralization of active organic N. At watershed scale, forest lands contributed 8.18 ± 3.94 % of the total nitrate losses to the water system in the headwater watersheds during the snowmelt season, representing the highest level within the entire basin. A comparison with forest streams in similar low-temperature conditions worldwide revealed that increasing nitrate loss occurred after extreme cold weather or soil freezing events, with an average increment of 6.32 kg/ha/yr. Therefore, forest nitrate losses should be better characterized and included in future watershed N budgets in low-temperature regions, which might help to reduce the N budget uncertainty and improve watershed management.

Climate-altered Precipitation is more Important than Land Use when Modeling Ecosystem Services Associated with Surficial Processes

Ecosystem services (ESs) associated with surficial processes may change according to shifts in land use, land cover, and climate parameters. Estimating these shifts can be important for land development planning, as urbanization alters soil processes that can manifest legacy effects. We employed the InVEST suite of models for sediment retention, nutrient delivery, and carbon storage to postulate how these ESs will change in the Upstate of South Carolina under future precipitation and land use and land cover (LULC) scenarios. We used the average precipitation from 1981-2010 and WorldClim precipitation projections for 2021-2040 and 2041-2060 to embody climatic precipitation shifts. For our LULC scenarios, we used 2011 and 2016 NLCD landscapes, then projected future LULC to hypothesize four future scenarios. We found that for the ES models that included both precipitation and LULC as inputs, precipitation dictated ES delivery far more heavily than land use or land cover. LULC scenarios produced consistent changes in ES delivery for all models except sediment export. Phosphorus and sediment exports increased between 2011 and 2016 due to LULC change, while nitrogen export stayed the same and carbon storage decreased. Land development that prioritizes forest cover will cause the least change in ESs, but allowing for continued forest loss to low-density development will have the most intense implications for ESs. Prioritization of land uses that preserve ESs associated with surficial processes will be critical to the longevity of agriculture and ecosystem integrity in this rapidly developing region. Land development planners should integrate consideration of ESs associated with surficial processes into future regional planning.

A multiscale analysis of the spatially heterogeneous relationships between non-point source pollution-related processes and their main drivers in Chaohu Lake watershed, China

A better understanding of the relationships between non-point source (NPS) pollution-related processes and their drivers will help to develop scientific watershed management measures. Although various studies have explored the drivers' impact on NPS pollution-related processes, quantitative knowledge of the properties within these relationships is still needed. This study uses the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) model to produce three related processes of NPS pollution, quick flow (QF), nitrogen export (NE), and sediment export (SE), in the upstream watershed of Chaohu Lake, China. The spatial distributions of QF, NE, and SE and their responses to multiple natural-socioeconomic drivers at nine spatial scales (1 km2, 10 km2, 20 km2, 30 km2, 50 km2, 75 km2, 100 km2, 200 km2, and town) were compared. The results showed that the spatial scale has little impact on the spatial distributions of NPS pollution-related processes. Across the nine scales, the socioeconomic drivers related to agricultural activities, area proportions of cultivated land (cultivated) and paddy field (paddy), have dominant impacts on NE, while the topographical drivers, the connectivity index (IC) and slope, have dominant impacts on both SE and QF. The magnitudes of single and paired natural-socioeconomic drivers' impacts on NPS pollution-related processes increase logarithmically or linearly with increasing spatial scale, but they tend to reach a stable threshold at a certain coarse scale. Our results emphasized the necessity and importance of embracing spatial scale effects in watershed water environmental management.

Streamflow duration curve to explain nutrient export in Midwestern USA watersheds: Implication for water quality achievements

As part of federal programs to reduce nutrient pollution, states across the Midwest have developed nutrient reduction strategies, which focus on implementation of agricultural conservation practices (ACPs) or best management practices (BMPs). Despite several decades of federal investment in implementing ACPs/BMPs for reducing nutrient pollution, nutrient pollution is a continuing and growing challenge with profound implications for water quality and public health as well as ecological functions. Pollutant transport depends on water and sediment fluxes, which are governed by local hydrology. Therefore, knowing how flow conditions affect nutrients export is critical to develop effective nutrient reduction strategies. The objective of this study was to investigate the role of streamflow duration curve in controlling nutrient export in the western Lake Erie Basin and the Mississippi River Basin. To achieve this goal, we used long-term monitoring data collected by the National Center for Water Quality Research. We focused on the percentage of the annual pollutant load (nitrate-NO₃-N, dissolved reactive phosphorus-DRP, total phosphorus-TP, and total suspended solids-TSS) exported during five flow intervals that spanned the flow duration curve: High Flows (0-10th percentile), Moist Conditions (10-40th percentile), Mid-Range Flows (40-60th percentile), Dry Conditions (60-90th percentile), and Low Flows (90-100th percentile). The results show that the top 10% of flows (i.e., high flows) transported more than 50% of the annual nutrient loads in most of the studying watersheds. Meanwhile, the top 40% of flows transported 54-98% of the annual NO₃-N loads, 55-99% of the annual DRP loads, 79-99% of the annual TP loads, and 86-100% of the annual TSS loads across the studying watersheds. The percentage of the annual loads released during high flows increased as the percentage of the agricultural land use in the watershed increased, but it decreased as the watershed area increased across different watersheds. Finally, flow condition/nutrient export relationships were consistent over studying period. Therefore, reducing nutrient loads during high flow condition is the key for effective nutrient reduction.

Improving the representation of stream water sources in surrogate nutrient models with water isotope data

An important part of meeting nutrient reduction goals in the lower Great Lakes basin and assessing the success of different land management strategies is modeling nutrient losses from agricultural land. This study aimed to improve the representation of water source contributions to streamflow in generalized additive models for predicting nutrient fluxes from three headwater agricultural streams in southern Ontario monitored during the Multi-Watershed Nutrient Study (MWNS). The previous development of these models represented baseflow contributions to streamflow using the baseflow proportion derived using an uncalibrated recursive digital filter. Recursive digital filters are commonly used to partition stream discharge into separate components from slower and faster pathways. In this study, we calibrated the recursive digital filter using stream water source information from stable isotopes of oxygen in water. Across sites, optimization of the filter parameters reduced bias in baseflow estimates by as much as 68 %. In most cases, calibrating the filter also improved agreement between filter-derived baseflow and baseflow calculated from isotope and streamflow data: the average Kling-Gupta Efficiencies using default and calibrated parameters were 0.44 and 0.82, respectively. When incorporated into the generalized additive models, the revised baseflow proportion predictor was more often statistically significant, improved model parsimony, and reduced prediction uncertainty. Moreover, this information allowed for a more rigorous interpretation of how different stream water sources influence nutrient losses from the agricultural MWNS watersheds.

Diffuse nutrient export dynamics from accumulated litterfall in forested watersheds with remote sensing data coupled model

Nutrients exported from forest litterfall significantly contribute to the global cycling of elements and the water quality in watersheds. Simulating the watershed discharge load is challenging because of the combined effects of the decomposing litterfall and topographic heterogeneity. We quantified the contribution of diffuse nutrient export from forest litterfall in a low temperature watershed using artificial rainfall experiments and watershed territorial modeling with remote sensing data, and therefore, the critical spatial factors and corresponding nutrient export dynamics were identified. Rainfall intensity and terrain slope were found to be the key factors for nutrient export under different litterfall decomposition conditions. Based on the moderate resolution imaging spectroradiometer data and field observations, the temporal patterns of litterfall biomass of two types of dominant forests (broad-leaved and mixed) were interpreted. The spatial patterns of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) exports from watershed litterfall were simulated by coupling the observed discharge parameters under artificial rainfall conditions and watershed surface flow modeling with the hydrological characteristics of the forested areas. The average watershed TOC, TN, and TP loads exported from the litterfall were approximately 58.22, 7.89, and 0.37 kg ha-1 a-1, respectively. The exported loads of TOC, TN, and TP varied with the forest types, and the loads from the litterfall of deciduous broad-leaved forest were found to be ∼50-70% of loads from the litterfall of mixed forest. A comparison with similar studies worldwide also indicated that low temperature decreased the litterfall decomposition rate and diffuse nutrient export. This study indicated that litterfall nutrients were a key contributor to watershed water pollution, and their spatial discharge trend varied intensively with the terrestrial conditions. The modified simulation methods were found to accurately assess the cycling of nutrients from the forest litterfall on a watershed scale.

Nutrient and sediment fluxes in microbasins with different conservation states in the northeastern Brazil

The implications of land use change in small watersheds through the conversion of forests to agropastoral areas have altered the natural nutrient cycle, intensifying exports under freshwater ecosystems. This study aimed to investigate the land use effects on nutrient and sediment exports in two small watersheds in northeastern Brazil to understand if anthropogenic disturbance alters the structure end functioning of these systems. Thus, land use mapping and hydrological treatment of a digital elevation model were made to characterize the basins. Water samples were collected monthly from Aug. 2016 to Jan. 2017 to evaluate suspended sediments and dissolved nutrient fluxes ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and dissolved organic nitrogen and phosphorus). The results indicated that land use change had a greater influence on exports from the most disturbed basin, where the nutrient and sediment increments were respectively an average 6.61 and 5.81 times higher than the most preserved basin. Thus, the conservation status of the forest cover has influenced the assimilation capacity of diffuse loads, highlighting the differences between the microbasins of this study.

Investigation of watershed nutrient export affected by extreme events and the corresponding sampling frequency

Although the real-time monitoring technique has been widely applied due to the improvement of sensors, development of traditional sampling methods is still worth of being discussed due to the economically feasibility. Currently, extreme events (e.g. extreme rainfall caused by climate change) play a relatively important role in nutrient export. However, impacts of extreme events on the optimization of sampling strategy is still not well addressed despite the uncertainty of different frequency sampling programs has been sufficiently discussed in previous studies. Therefore, the corresponding impact of extreme events impact on the optimization of sampling strategy was investigated by examining temporal (i.e., inter-annual and seasonal) variations of available data. Uncertainty of nutrient flux estimates under different sampling frequencies was explored by subsampling daily monitoring data. Results showed that uncertainty in flux estimates differed between nitrogen and phosphorus. The relative error (RE) in annual TN flux estimates ranged from -4.2% to 2.4% (once per three-day) to -21.4-31.1% (monthly sampling), while the RE in annual TP flux estimates varied from -14.1% to 8.2% (once per three-day) to -65.9%-163.4% (monthly sampling). Biweekly and weekly sampling routines are considered the optimal sampling program for total nitrogen (TN) and for total phosphorus (TP) when the extreme events impact were not been considered. The uncertainty of flux estimates with different sampling frequencies increased with the increasing extreme events. High level of uncertainty occurred in years with the most extreme events in 2012 (RE: 21.4-69.0% for TN, 33.3-96.6% for TP), while the lowest can be found in 2011 (RE: 0-20.7% for TN, 0-48.3% for TP) (with the fewest extreme events). In addition, uncertainty in TN and TP flux estimates was generally greater during summer season than during other seasons. These results highlighted the important role of extreme events in nutrient export. Approximately half of the annual TN and TP flux occurred in some extreme days that only accounted for less than 20% in the same year. The onset of these extremes of nutrient export was likely due to the stormflow with addition of external fertilizer and the direct discharge of surface ponding water from paddy fields during special periods of time. These results would be helpful for the optimization of sampling strategy.

Climate change will pose challenges to water quality management in the st. Croix River basin

Responses of streamflow and nutrient export to changing climate conditions should be investigated for effective water quality management and pollution control. Using downscaled climate projections and the Soil and Water Assessment Tool (SWAT), we projected future streamflow, sediment export, and riverine nutrient export in the St. Croix River Basin (SCRB) during 2020-2099. Results show substantial increases in riverine water, sediment, and nutrient load under future climate conditions, particularly under the high greenhouse gas emission scenario. Intensified water cycling and enhanced nutrient export will pose challenges to water quality management and affect multiple Best Management Practices (BMPs) efforts, which are aimed at reducing nutrient loads in SCRB. In addition to the physical impacts of climate change on terrestrial hydrology, our analyses demonstrate significant reductions in ET under elevated atmospheric CO₂ concentrations. Changes in plant physiology induced by climate change may markedly affect water cycling and associated sediment and nutrient export. Results of this study highlight the importance of examining climate change impacts on water and nutrient delivery for effective watershed management.

Exploring spatiotemporal changes of the Yangtze River (Changjiang) nitrogen and phosphorus sources, retention and export to the East China Sea and Yellow Sea

Nitrogen (N) and phosphorus (P) flows from land to sea in the Yangtze River basin were simulated for the period 1900-2010, by combining models for hydrology, nutrient input to surface water, and an in-stream retention. This study reveals that the basin-wide nutrient budget, delivery to surface water, and in-stream retention increased during this period. Since 2004, the Three Gorges Reservoir has contributed 5% and 7% of N and P basin-wide retention, respectively. With the dramatic rise in nutrient delivery, even this additional retention was insufficient to prevent an increase of riverine export from 337 Gg N yr^-1 and 58 Gg P yr^-1 (N:P molar ratio = 13) to 5896 Gg N yr^-1 and 381 Gg P yr^-1 (N:P molar ratio = 35) to the East China Sea and Yellow Sea (ECSYS). The midstream and upstream subbasins dominate the N and P exports to the ECSYS, respectively, due to various human activities along the river. Our spatially explicit nutrient source allocation can aid in the strategic targeting of nutrient reduction policies. We posit that these should focus on improving the agricultural fertilizer and manure use efficiency in the upstream and midstream and better urban wastewater management in the downstream subbasin.

Decadal and seasonal trends of nutrient concentration and export from highly managed coastal catchments

Understanding anthropogenic and hydro-climatic influences on nutrient concentrations and export from highly managed catchments often necessitates trend detection using long-term monitoring data. This study analyzed the temporal trend (1979-2014) of total nitrogen (TN) and total phosphorus (TP) concentrations and export from four adjacent coastal basins in south Florida where land and water resources are highly managed through an intricate canal network. The method of integrated seasonal-trend decomposition using LOESS (LOcally weighted regrESSion) was employed for trend detection. The results indicated that long-term trends in TN and TP concentrations (increasing/decreasing) varied with basins and nutrient species, reflecting the influence of basin specific land and water management practices. These long-term trends were intervened by short-term highs driven by high rainfall and discharges and lows associated with regional droughts. Seasonal variations in TP were more apparent than for TN. Nutrient export exhibited a chemostatic behavior for TN from all the basins, largely due to the biogenic nature of organic N associated with the ubiquity of organic materials in the managed canal network. Varying degrees of chemodynamic export was present for TP, reflecting complex biogeochemical responses to the legacy of long-term fertilization, low soil P holding capacity, and intensive stormwater management. The anthropogenic and hydro-climatic influences on nutrient concentration and export behavior had great implications in nutrient loading abatement strategies for aquatic ecosystem restoration of the downstream receiving waterbody.