Stream Nutrient Concentrations Research Articles

Inferences based on diatom compositions improve estimates of nutrient concentrations in streams

Nutrient concentrations in streams vary strongly with flow conditions, and routinely gathered field measurements of nutrients reflect this variability. Diatom assemblage composition has been used in previous studies to infer nutrient concentrations, and because diatoms integrate nutrient concentrations over longer periods of time, diatom inferences may be less susceptible to fluctuations in streamflow. We tested this hypothesis by leveraging differences in the flashiness of streams across a large continental data set. More specifically, we tested whether the variabilities of direct measurements and diatom inferences of dissolved phosphorus and nitrate were greater in flashy versus non-flashy streams. We further considered whether models linking landscape predictor variables to nutrient concentrations yielded consistent results across flashy and non-flashy streams. Our analysis indicated that measured nutrient concentrations were more variable in flashy compared to non-flashy streams and that landscape models identified different important predictors of nutrient concentrations when fit using data from flashy vs. non-flashy streams. In contrast, variabilities of diatom-inferred nutrient concentrations were similar among stream types, as were the important predictor variables (e.g., manure application rates for nitrate and number of wet days for dissolved phosphorus). These analyses indicate that use of diatom-inferred nutrient concentrations can potentially improve efforts to quantify stream nutrient concentrations.

Causal inference approaches reveal both positive and negative unintended effects of agricultural and urban management practices on instream biological condition

Agricultural and urban management practices (MPs) are primarily designed and implemented to reduce nutrient and sediment concentrations in streams. However, there is growing interest in determining if MPs produce any unintended positive effects, or co-benefits, to instream biological and habitat conditions. Identifying co-benefits is challenging though because of confounding variables (i.e., those that affect both where MPs are applied and stream biota), which can be accounted for in novel causal inference approaches. Here, we used two causal inference approaches, propensity score matching (PSM) and Bayesian network learning (BNL), to identify potential MP co-benefits in the Chesapeake Bay watershed portion of Maryland, USA. Specifically, we examined how MPs may modify instream conditions that impact fish and macroinvertebrate indices of biotic integrity (IBI) and functional and taxonomic endpoints. We found evidence of positive unintended effects of MPs for both benthic macroinvertebrates and fish indicated by higher IBI scores and specific endpoints like the number of scraper macroinvertebrate taxa and lithophilic spawning fish taxa in a subset of regions. However, our results also suggest MPs have negative unintended effects, especially on sensitive benthic macroinvertebrate taxa and key instream habitat and water quality metrics like specific conductivity. Overall, our results suggest MPs offer co-benefits in some regions and catchments with largely degraded conditions but can have negative unintended effects in some regions, especially in catchments with good biological conditions. We suggest the number and types of MPs drove these mixed results and highlight carefully designed MP implementation that incorporates instream biological data at the catchment scale could facilitate co-benefits to instream biological conditions. Our study underscores the need for more research on identifying effects of individual MP types on instream biological and habitat conditions.

Effects of a forested state park on stream nutrient concentrations in an agriculturally dominated watershed in the U.S. Midwest

AbstractAgricultural land cover in the U.S. Midwest is a major source of nutrient pollution that has led to impairment of stream water quality. This study examines the impact of a forested state park on nutrient concentrations within an agriculturally dominated watershed. Water samples were collected over a 2‐year study period from eight stream sampling sites along four creeks and processed for total nitrogen (TN), nitrate (), total phosphorus (TP), and orthophosphate (). Hydrology, channel morphology, and remotely sensed land cover and vegetation data were also collected and analyzed within the study area. Results indicate that water quality responses to a forested state park vary between TN, , TP, and , and water quality variables are uniquely influenced by watershed and stream characteristics. The greatest water quality benefits most frequently occurred within the two smallest study streams with the greatest residence times and proportion of watershed areas within the forested state park. Overall, the greatest improvements to water quality occurred during periods of low stream discharge and when riparian vegetation was greenest. The results of this study suggest that conservation of forested areas within agriculturally dominated watersheds can provide water quality improvements in the U.S. Midwest. Targeting watersheds that drain small streams with long residence times for conservation may be most beneficial to improving water quality.

A Random Forest in the Great Lakes: Stream Nutrient Concentrations Across the Transboundary Great Lakes Basin

AbstractExcess nutrient inputs from agricultural and urban sources have accelerated eutrophication and increased the incidence of algal blooms in the Great Lakes Basin (GLB). Lake basin management to address these threats relies on understanding the key drivers of pollution. Here, we use a random forest machine learning model to leverage information from 202 monitored streams in the GLB to predict seasonal and annual flow‐weighted concentrations of nitrogen and phosphorus, as well as nutrient ratios across the GLB. Land use (agricultural and urban land) and land management (tile drainage and wetland density) emerge as the two most important predictors for dissolved inorganic nitrogen (DIN; NO3− + NO2−) and soluble reactive phosphorus (SRP; PO43), while soil type and wetland density are more important for particulate P (PP). Partial dependence plots demonstrate increasing nutrient concentrations with increasing tile density and decreasing wetland density. In addition, increasing tile and livestock densities and decreasing forest cover correspond to higher SRP:Total Phosphorus (TP) ratios. Seasonally, the highest proportions of SRP occur in summer and fall. Higher livestock densities are also correlated with increasing N:P (DIN:TP) ratios. Livestock operations can contribute to the buildup of soil nutrients from excess manure application, while increasing subsurface drainage can provide transport pathways for dissolved nutrients. Given that both SRP:TP and the N:P ratios are strong predictors of harmful algal blooms, our study highlights the importance of livestock management, drainage management, and wetland restoration in efforts to address eutrophication in intensively managed landscapes.

Influence of wetlands on nutrients in headwaters of agricultural catchments

AbstractWe investigated the influence of land cover on nutrient concentrations (295 samples) in headwater streams over a 2‐year period in 10 agriculture‐dominated subcatchments (163–8373 ha) in southern Ontario Canada. In this region, monitoring and research on nutrient dynamics in headwater wetlands is sparse. Our results indicated a significant positive correlation (Pearson coefficient ρ = 0.320) between soluble reactive phosphorus (SRP) in the headwater streams and the percentage of wetlands in these agriculture‐dominated catchments. This result suggests that headwater wetlands, and other wet riparian zones, are key sources of SRP in the headwater streams. Nitrate concentrations were positively correlated with % agricultural land cover (ρ = 0.316), consistent with previous studies, while SRP concentrations were negatively correlated with % agricultural land cover (ρ = −0.325). There was a significant positive correlation between SRP concentrations and discharge in some of the streams. Seasonal SRP trends appear to be closely related to temperature‐dependent seasonal changes in redox conditions, including levels of dissolved O2. Surficial geology had some influence on nitrate concentrations in the streams, which tended to be higher in catchments dominated by glacial till (till terrain), compared to catchments with extensive areas of outwash sand, in addition to till terrain.

Evaluating the Effects of Riparian Habitat Type on Nutrient Concentrations in Agricultural Headwater Streams

ABSTRACTGrass filter strips and other riparian buffer types are frequently used in agricultural watersheds to reduce nonpoint source pollution, but there is only limited information available on their long‐term watershed scale effects. Our research questions were as follows: (1) does nutrient concentrations differ among agricultural headwater streams with different riparian habitat types? and (2) does the effect of riparian habitat type differ annually or seasonally? Eight streams in central Ohio were selected based on watershed size, proportion of watershed agricultural land use, and riparian habitat type. Three streams possessed unplanted riparian habitats, three streams had grass filter strips, and two streams had forested riparian habitats. Water samples for the measurement of ammonia, nitrate + nitrite, total nitrogen, dissolved reactive phosphorus, total phosphorus, and dissolved organic carbon concentrations were collected weekly from two sites in each stream from March 2007 to February 2017. Mean nitrogen and phosphorus concentrations did not differ (p > 0.05) among riparian habitat types. Mean dissolved organic carbon concentrations exhibited different (p < 0.05) annual and seasonal trends among riparian habitat types. Our results indicated that planting grass filter strips alone may not decrease nitrogen and phosphorus concentrations and highlighted the influence of riparian woody vegetation on the temporal dynamics of dissolved organic carbon concentrations.

Use of geostatistical models to evaluate landscape and stream network controls on post‐fire stream nitrate concentrations

AbstractForested watersheds provide many ecosystem services, such as the filtration of sediment, pollutants, and nutrients, which are increasingly threatened by wildfire. For example, stream nutrient concentrations often increase following wildfire and can remain elevated for decades, making downstream waters susceptible to eutrophication. We investigated the drivers of persistent elevated stream nutrients, specifically nitrate (NO3−), in nine watersheds that were burned 16 years prior by the Hayman fire, Colorado, USA. We evaluated the ability of multiple linear regression and spatial stream network modeling approaches to predict observed concentrations of the biologically active solute NO3− and the conservative solute sodium (Na+) which serves as a partial control. Specifically, we quantified the degree to which landscape and stream network characteristics predict stream solute concentrations. Stream Na+ exhibited strong spatial autocorrelation that was primarily controlled by topography and hydrology. In contrast, stream NO3− had higher spatial variability and was inversely correlated to vegetation cover, measured as mean normalized differenced moisture index (NDMI). Spatially heterogeneous wildfire behaviour left intact forest patches interspersed with high burn severity patches dominated by shrubs and grasses which contributes to the spatial variability in stream NO3− concentrations. Post‐fire vegetation also interacts with watershed structure to influence stream NO3− patterns. For example, severely burned convergent hillslopes in headwaters positions were associated with the highest stream NO3− concentrations due to the high proportional influence of hillslope water in these locations. Our findings suggest that targeted reforestation in severely burned convergent hillslopes in headwater positions may enhance the recovery of stream NO3− concentrations to pre‐fire levels.

Drivers of multi-decadal nitrate regime shifts in a large European catchment

Long-term monitoring shows evidence of persistent changes in the magnitude and timing of the seasonal pattern of nitrate concentrations in streams, with possibly grave effects on aquatic ecosystems. Seasonal patterns of stream nutrient concentrations are determined by a complex interplay of inputs, transport, and turnover. Over multi-decadal periods, each of these factors may change due to socio-economic factors such as consumption patterns, governance regimes, or technological control measures. Here we test the hypothesis that observed multi-decadal changes in stream nitrate seasonality could be explained by changes in the relative importance of catchment nutrient sources over time. We analyze 66 years of shifting nitrate seasonality in a large, central-European river (Elbe) during a period of significant socio-political changes (1954–2019), with correspondingly significant changes in the sources of anthropogenic nitrate emissions. We develop a mixed-source succession model to test how the multi-decadal evolution of the composition of nitrate sources (point and diffuse) influences in-stream seasonality. We show that the in-stream nitrate seasonality of the River Elbe changed significantly from a weak seasonal pattern with peak concentrations during summer in the 1950s to a strong seasonal pattern with peak concentrations during winter in the 1990s. We link these shifts to a succession of technical and political developments which influence the contribution of point and diffuse sources over time. Such shifts in seasonal concentration patterns can significantly impact the macronutrient (carbon, nitrogen, phosphorus) ratios in rivers, which in turn highly affect the health of aquatic ecosystems.

Nutrient fluxes from an Arctic seabird colony to the adjacent coastal marine ecosystem

Seabirds are important vectors for nutrient transfer across ecosystem boundaries. In this seasonal study, we evaluate the impact of an Arctic colony (Alkhornet, Svalbard) of Black-legged Kittiwakes (Rissa tridactyla) and Brünnich’s Guillemots (Uria lomvia) on stream nutrient concentrations and fluxes, as well as utilization by coastal biota. Water samples from seabird-impacted and control streams were collected regularly throughout the melt season (June–September) for nutrient and organic carbon analysis. Stable carbon and nitrogen isotope analysis (δ13C and δ15N) was used to assess whether seabird-derived nitrogen (N) could be traced into filamentous stream algae and marine algae as well as consumers (amphipods). Concentrations of nitrate (NO3−) and nitrite (NO2−) peaked in July at 9200 µg N L−1 in seabird-impacted streams, 70 times higher than for control streams. Mean concentrations of phosphate (PO43−) in seabird-impacted streams were 21.9 µg P L−1, tenfold higher than in controls. Areal fluxes from seabird-impacted study catchments of NO3− + NO2− and PO43− had estimated ranges of 400–2100 kg N km−2 and 15–70 kg P km−2, respectively. Higher δ15N was found in all biota collected from seabird-impacted sites, indicating utilization of seabird-derived nitrogen. Acrosiphonia sp. from seabird-impacted sites had higher δ15N values (20–23‰ vs. 3–6‰) and lower C:N ratios (10.9 vs. 14.3) than specimens collected from control sites, indicating reliance on seabird-derived nitrogen sources and potentially higher N-availability at seabird-impacted nearshore sites. Our study demonstrates how marine nutrients brought onshore by seabirds also can return to the ocean and be utilized by nearshore primary producers and consumers.

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

Human agriculture, wastewater, and use of fossil fuels have saturated ecosystems with nitrogen and phosphorus, threatening biodiversity and human water security at a global scale. Despite efforts to reduce nutrient pollution, carbon and nutrient concentrations have increased or remained high in many regions. Here, we applied a new ecohydrological framework to ~12,000 water samples collected by the U.S. Environmental Protection Agency from streams and lakes across the contiguous U.S. to identify spatial and temporal patterns in nutrient concentrations and leverage (an indicator of flux). For the contiguous U.S. and within ecoregions, we quantified trends for sites sampled repeatedly from 2000 to 2019, the persistence of spatial patterns over that period, and the patch size of nutrient sources and sinks. While we observed various temporal trends across ecoregions, the spatial patterns of nutrient and carbon concentrations in streams were persistent across and within ecoregions, potentially because of historical nutrient legacies, consistent nutrient sources, and inherent differences in nutrient removal capacity for various ecosystems. Watersheds showed strong critical source area dynamics in that 2-8% of the land area accounted for 75% of the estimated flux. Variability in nutrient contribution was greatest in catchments smaller than 250 km2 for most parameters. An ensemble of four machine learning models confirmed previously observed relationships between nutrient concentrations and a combination of land use and land cover, demonstrating how human activity and inherent nutrient removal capacity interactively determine nutrient balance. These findings suggest that targeted nutrient interventions in a small portion of the landscape could substantially improve water quality at continental scales. We recommend a dual approach of first prioritizing the reduction of nutrient inputs in catchments that exert disproportionate influence on downstream water chemistry, and second, enhancing nutrient removal capacity by restoring hydrological connectivity both laterally and vertically in stream networks.

Land use, geology and soil properties control nutrient concentrations in headwater streams

Nutrient losses from headwater catchments (<50 km2) cause eutrophication problems downstream. Catchment properties are strongly reflected in the levels of nutrient concentrations in headwater streams. Based on measurements of total and dissolved nitrogen (TN, DN) and phosphorus (TP, DP) in 235 small headwater streams, we showed that proportion of arable land in a catchment had the strongest positive effect on nutrient concentrations, with coefficient of determination (R2) of 0.54, 0.64, 0.45, and 0.51 for TN, DN, TP, and DP, respectively. In contrast, increased proportion of forest and wetland led to lower nutrient concentrations in streams. The geological composition of catchments had a major influence on the soil properties. In turn, certain soil properties, such as clay content and content of aluminum (Al), an important binding agent of P, influenced losses of particulate P (PP) and DP, respectively. Consequently, by using soil properties as a link between geology and water quality, areas potentially sensitive to nutrient losses were identified by classifying bedrock categories into three geological groups. Approximately 25% of Swedish arable land was identified as potentially sensitive. Sensitive catchments were found in regions with sedimentary bedrock and showed higher concentrations of dissolved nutrient fractions even when the proportion of agricultural land was small, indicating higher background concentrations.

Invasion of temperate deciduous broadleaf forests by N-fixing tree species - consequences for stream ecosystems.

Biological invasions are a major threat to biodiversity and ecosystem functioning. Forest invasion by alien woody species can have cross-ecosystem effects. This is especially relevant in the case of stream-riparian forest meta-ecosystems as forest streams depend strongly on riparian vegetation for carbon, nutrients and energy. Forest invasion by woody species with dissimilar characteristics from native species may be particularly troublesome. The invasion of temperate deciduous broadleaf forests with low representation of nitrogen (N)-fixing species by N-fixers has the potential to induce ecosystem changes at the stream level. Although effects of tree invasion on stream ecosystems have been under assessed, knowledge of native and invasive tree characteristics allows prediction of invasion effects on streams. Here we present a conceptual model to predict the effects of forest invasion by alien N-fixing species on streams, using as a background the invasion of temperate deciduous broadleaf forests by leguminous Acacia species, which are among the most aggressive invaders worldwide. Effects are discussed using a trait-based approach to allow the model to be applied to other pairs of invaded ecosystem-invasive species, taking into account differences in species traits and environmental conditions. Anticipated effects of N-fixing species invasions include changes in water quality (increase in N concentration) and quantity (decrease in flow) and changes in litter input characteristics (altered diversity, seasonality, typology, quantity and quality). The magnitude of these changes will depend on the magnitude of differences in species traits, the extent and duration of the invasion and stream characteristics (e.g. basal nutrient concentration). The extensive literature on effects of nutrient enrichment of stream water, water scarcity and changes in litter input characteristics on aquatic communities and processes allows prediction of invasion effects on stream structure and function. The magnitude of invasion effects on aquatic communities and processes may, however, depend on interactions among different pathways (e.g. effects mediated by increases in stream nutrient concentration may contrast with those mediated by decreases in water availability or by decreases in litter nutritional quality). A review of the literature addressing effects of increasing cover of N-fixing species on streams suggests a wide application of the model, while it highlights the need to consider differences in the type of system and species when making generalizations. Changes induced by N-fixing species invasion on streams can jeopardize multiple ecosystem services (e.g. good quality water, hydroelectricity, leisure activities), with relevant social and economic consequences.

Ongoing anthropogenic eutrophication of the catchment area threatens the Doñana World Heritage Site (South-west Spain)

Eutrophication is a major cause of wetland degradation worldwide. In recent decades, reductions in nutrient inputs have led to improvements in water quality in many rivers and lakes in central and northern Europe, but long-term trends are less clear in southern Europe. We conducted the first comprehensive study of water quality in Doñana (SW Spain), one of the most important wetland complexes in Europe and the Mediterranean region. The core area of Doñana is a large shallow, seasonal marsh (UNESCO World Heritage Site—WHS) that floods during rainy, cool winter months, then dries out during the summer. The marsh is fed by three main streams whose catchments are within a Biosphere Reserve but are impacted by greenhouses (for cultivating fruit, irrigated with groundwater), poorly treated urban wastewaters and tourism. From 2013 to 2016, we monitored nutrient (Total P, Total N, PO4−3, NH4+, NO3−, NO2−) and phytoplankton chlorophyll-a (chla) concentrations in surface waters of the Doñana marsh and the three main streams. We quantified changes in greenhouse cover since 1995 using satellite images. Nutrient concentrations in streams were consistently higher than in the marsh, particularly in the Partido and Rocina streams that regularly reached concentrations equivalent to a “bad physico-chemical status” under the EU Water Framework Directive (WFD), and whose catchments suffered a fivefold expansion of greenhouses from 1995 to 2016. The Partido was the most polluted stream, and the most affected by wastewater effluents, and had particularly high concentrations of NH4+ (Geometric Mean = 0.3 mg L−1) and NO2− (GM = 0.52 mg L−1) across seasons. Patterns in chla concentrations were less consistent, but streams (GM = 6.78 µg L−1) generally had higher concentrations than the marsh (GM = 4.27 µg L−1). Nutrient concentrations in spot samples within the marsh largely depended on a combination of evaporation (as revealed by higher stable isotope δ2H values in the water column) and spatial processes (concentrations increase close to stream entry points, where conductivity is lower). Anthropogenic nutrient pollution in entry streams is a serious problem in Doñana, with extensive stretches too toxic for fish. Reinforcement of policies aimed at reducing nutrient inputs to Doñana are urgently required to meet the biodiversity conservation and environmental objectives for the WHS and WFD, respectively. Paradoxically, the marsh is currently relied upon to purify the water entering from streams.

Groundwater-surface water interactions and agricultural nutrient transport in a Great Lakes clay plain system

Nutrient export from agricultural land to surface waters is a significant environmental concern within the Great Lakes Basin (GLB). A field-based watershed-scale study was completed to investigate spatial and temporal variations of phosphorus and nitrate to assess nutrient transport pathways and groundwater-surface water interactions in an agriculturally dominated clay plain system. This was conducted in the 127 km2 Upper Parkhill Watershed, near Lake Huron in southwestern Ontario, Canada. Data collection occurred from June 2018 to May 2019 via continuous sensor deployment and discrete sampling of stream water, groundwater, hyporheic zone, and tile drainage water. Samples were analyzed for various nutrient species (total, total dissolved, soluble reactive, and particulate phosphorus, and nitrate-N) to examine the hydrological dynamics of principal transport pathways of agriculturally-derived nutrients. Total phosphorus and nitrate concentrations in stream water ranged from 0.007 to 0.324 mg/L and 0.32 to 13.13 mg NO3−-N/L, respectively. Tile drainage water total phosphorous concentrations varied from 0.006 to 0.066 mg/L. Groundwater total dissolved phosphorus concentrations ranged from <0.003 to 0.085 mg/L. Transport of phosphorus through tile drainage was observed to be greater than through groundwater over the study period. No distinct relationship was observed between nutrient concentrations in the hyporheic zone and the vertical hydraulic gradient within this zone in the studied stream reach. Preliminary correlations were discerned between water quality observations and recognized land management practices. Given the elevated stream nutrient concentrations, these results are consequential for the continual improvement of strategies and programs devised to conserve water resources within the GLB.

Differential effects of land use on nutrient concentrations in streams of Pennsylvania

Nutrient pollution of surface waters is a widespread problem, calling for regional assessments of water quality conditions. In this study, we quantified long-term median nutrient concentrations of total nitrogen (TN) and total phosphorus (TP) in streams and rivers of Pennsylvania and explored relationships between stream nutrient concentrations and the land use of their watersheds. Our analysis is based on a synthesis of monitoring data from multiple agencies that included records of nutrient concentrations observed between 2000–2019. Across the state, stream nutrient concentrations observed in predominantly undeveloped areas (e.g., forests, shrubs, and grasslands) have median concentration values of 0.42 mg l−1 for TN and 0.011 mg l−1 for TP, reflecting background concentrations for minimally impacted watersheds. Median stream concentrations of TN in agricultural areas are about eleven times higher than in undeveloped areas; and are about five times higher in developed areas than in undeveloped areas. Median stream concentrations of TP in developed areas have about eight times higher concentrations than undeveloped areas; and are about four times higher in agricultural areas than in undeveloped areas. Concentrations of TN and TP increased substantially as the combined percentage of agricultural and developed land use increased. Fragmented data storage practices (e.g. incomplete metadata, ambiguous site names, and missing coordinates) and inconsistencies in monitoring protocols (e.g., differences in constituents measured, parameter names, and measurement methods) made leveraging the secondary use of multiple sources of data challenging. Nonetheless, our integrated dataset is robust, represents the best data available, and provides a new window into the nutrient status of Pennsylvania’s surface waters. The long-term median nutrient concentrations reveal the magnitude of variability in TN and TP concentrations across the state’s diverse environmental settings of land use, physiography, and geology. This information is useful for interpreting additional monitoring data, informing evaluation of water quality conditions, and guiding watershed management.

Prediction of stream nitrogen and phosphorus concentrations from high-frequency sensors using Random Forests Regression

Stream nutrient concentrations exhibit marked temporal variation due to hydrology and other factors such as the seasonality of biological processes. Many water quality monitoring programs sample too infrequently (i.e., weekly or monthly) to fully characterize lotic nutrient conditions and to accurately estimate nutrient loadings. A popular solution to this problem is the surrogate-regression approach, a method by which nutrient concentrations are estimated from related parameters (e.g., conductivity or turbidity) that can easily be measured in situ at high frequency using sensors. However, stream water quality data often exhibit skewed distributions, nonlinear relationships, and multicollinearity, all of which can be problematic for linear-regression models. Here, we use a flexible and robust machine learning technique, Random Forests Regression (RFR), to estimate stream nitrogen (N) and phosphorus (P) concentrations from sensor data within a forested, mountainous drainage area in upstate New York. When compared to actual nutrient data from samples tested in the laboratory, this approach explained much of the variation in nitrate (89%), total N (85%), particulate P (76%), and total P (74%). The models were less accurate for total soluble P (47%) and soluble reactive P (32%), though concentrations of these latter parameters were in a relatively low range. Although soil moisture and fluorescent dissolved organic matter are not commonly used as surrogates in nutrient-regression models, they were important predictors in this study. We conclude that RFR shows great promise as a tool for modeling instantaneous stream nutrient concentrations from high-frequency sensor data, and encourage others to evaluate this approach for supplementing traditional (laboratory-determined) nutrient datasets.

Effects of invasive N2-fixing Acacia mearnsii on sediment nutrient concentrations in mountain streams: Implications of sediment geochemistry for ecosystem recovery

Widespread invasive nitrogen-fixing plant species pose major threats to water resources, biodiversity and nutrient dynamics of river catchments around the world. However, the impacts of invasive N2-fixing plants on in-stream sediment nutrient dynamics remain poorly understood. Here, we quantified the impacts of invasive N2-fixing Acacia mearnsii and A. mearnsii clearing on two mountain streams in South Africa’s Cape Floristic Region. Nutrients were measured in fine sediment that had infiltrated into the gravel bed and surface water in three reaches associated with natural fynbos shrubs, A. mearnsii invaded and cleared riparian ecotones. Results showed that the impacts of A. mearnsii invasions and Acacia clearing in riparian areas on in-stream sediment-associated nutrient concentrations are context-dependent. The difference in channel morphometry and sediment geochemical attributes at reach-scale contributed to the contrasting spatial patterns of sediment-adsorbed nutrients within these rivers. There was also an indication of a long-lasting effect of invasion on total phosphorus that could persist in river sediments ≥10 years after the removal of dense A. mearnsii stands in close proximity to streams. This could be a result of the complexation between phosphorus and immobile iron in sediment, resulting in the retention of ‘legacy phosphorus’. This underlines the importance of fine sediments in capturing nutrients and ultimately regulating nutrient concentrations in the water column.

Granular measures of agricultural land use influence lake nitrogen and phosphorus differently at macroscales.

Agricultural land use is typically associated with high stream nutrient concentrations and increased nutrient loading to lakes. For lakes, evidence for these associations mostly comes from studies on individual lakes or watersheds that relate concentrations of nitrogen (N) or phosphorus (P) to aggregate measures of agricultural land use, such as the proportion of land used for agriculture in a lake's watershed. However, at macroscales (i.e., in hundreds to thousands of lakes across large spatial extents), there is high variability around such relationships and it is unclear whether considering more granular (or detailed) agricultural data, such as fertilizer application, planting of specific crops, or the extent of near-stream cropping, would improve prediction and inform understanding of lake nutrient drivers. Furthermore, it is unclear whether lake N and P would have different relationships to such measures and whether these relationships would vary by region, since regional variation has been observed in prior studies using aggregate measures of agriculture. To address these knowledge gaps, we examined relationships between granular measures of agricultural activity and lake total phosphorus (TP) and total nitrogen (TN) concentrations in 928 lakes and their watersheds in the Northeastern and Midwest U.S. using a Bayesian hierarchical modeling approach. We found that both lake TN and TP concentrations were related to these measures of agriculture, especially near-stream agriculture. The relationships between measures of agriculture and lake TN concentrations were more regionally variable than those for TP. Conversely, TP concentrations were more strongly related to lake-specific measures like depth and watershed hydrology relative to TN. Our finding that lake TN and TP concentrations have different relationships with granular measures of agricultural activity has implications for the design of effective and efficient policy approaches to maintain and improve water quality.

Water quality and land cover in the Coastal Plain Little River watershed, Georgia, United States

Despite efforts to improve water quality in US watersheds, recent assessments of water quality trends in these watersheds indicate concentrations of nitrogen (N) and phosphorus (P) show minimal changes in the majority of studied streams across the nation. Moreover, results of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) indicate that nutrient concentrations in streams and groundwater in basins with significant agriculture or urban development are substantially greater than naturally occurring or background levels. Long-term hydrologic and water quality data from watersheds are a key component to building an understanding that will dictate what and where changes in watershed management need to be made to achieve improvements in water quality. Forty-one years of hydrologic and water quality data from the Little River Experimental Watershed (LREW) were assembled to evaluate trends in streamflow quantity and quality from the LREW and relationships to changes in land cover and management. Concentrations and loads of chloride (Cl), ammonium-N (NH₄-N), nitrate plus nitrite-N (NO₃-N), total kjeldahl N (TKN), total P (TP), and dissolved molybdate reactive P (DMRP) from 1974 to 2014 were determined. In general, concentrations of N and P have remained low and stable, with some increases observed from 1980 to 1999. In contrast, Cl concentration appears to be steadily increasing, possibly due to increased fertilization in the LREW. Flow-weighted mean concentrations were 0.21 mg L⁻¹ y⁻¹ for NO₃-N, 0.08 mg L⁻¹ y⁻¹ for NH₄-N, 1.41 mg L⁻¹ y⁻¹ for TKN, 9.30 mg L⁻¹ y⁻¹ for Cl, 0.15 mg L⁻¹ y⁻¹ for TP, and 0.03 mg L⁻¹ y⁻¹ for DMRP. Average annual loads were 0.64 kg ha⁻¹ y⁻¹ for NO₃-N, 0.25 kg ha⁻¹ y⁻¹ for NH₄-N, 4.47 kg ha⁻¹ y⁻¹ for TKN, 29.54 kg ha⁻¹ y⁻¹ for Cl, 0.46 kg ha⁻¹ y⁻¹ for TP, and 0.10 kg ha⁻¹ y⁻¹ for DMRP. The low nutrient loading in the watershed is attributed to the dense riparian buffers within the LREW, which have been shown to be effective for reducing N loading to the stream. Due to (1) small variations in nutrient concentrations and loads, (2) large variability in precipitation, (3) large variability in streamflow, and (4) small variations in land cover, no clear relationships among changes in land cover and management and nutrient loads were found. Loading rates of N and P, although somewhat influenced by changes in concentration, are largely dictated by changes in streamflow volume in the LREW.

Nutrient loss from three small-size watersheds in the southern Baltic Sea in relation to agricultural practices and policy

Agriculture is the major contributor of waterborne nutrient fluxes into the Baltic Sea, one of the world's most eutrophication-sensitive areas. Poland, as a large, densely populated state ohf the Baltic Region, with dominating agricultural land use, largely contributes to riverborne loads of N and P. The aim of our study was to examine the input of nutrients from three small first-order agricultural watersheds (Bladzikowski Stream, Gizdepka river and Mrzezino canal) in the Pomerania region, into the Bay of Puck, inner part of the Gulf of Gdansk. This study attempts to give a partial answer as to the question if inputs of nutrients from the 3 analysed watersheds comply with the targets of the Baltic Sea Action Plan (BSAP) and Country Allocated Reduction Targets (CART). The impact of agricultural practices was assessed on the basis of farm questionnaires and calculations of nutrient balances for the examined farms. The nutrient concentrations in the soil and drainage ditches were examined, followed by an assessment of nutrient concentrations in the watercourses at the sampling points located close to the estuaries. The average mineral N fertiliser consumption (109 kg N/ha) in the analysed watersheds was higher than Poland's average. The average N and P surpluses for surveyed farms (96.4 kg/ha and 4.4 kg/ha, respectively) were higher than the EU mean in case of N and markedly lower in case of P. We used Principal Component Analysis which confirmed that there were correlations between nutrient surpluses and nutrient concentrations in streams and/or drainage ditches. The N–NO3 and Pmin concentrations were also correlated to precipitation. The average N concentrations in the analysed watercourses were equal to 1.53 mg/L for Gizdepka, 1.88 mg/L for Mrzezino canal and 3.52 mg/L for Bładzikowski Stream. The mean P concentrations observed in the investigated watercourses were markedly higher than 0.1 mg/L. With regard to BSAP objectives, as well as CART set for Poland, the average nutrient concentrations in rivers should be approximately at the level of 2.5 mg N/L and 0.07 mg P/L.