Increased Soluble Phosphorus Loads to Lake Erie: Unintended Consequences of Conservation Practices?

Difficulties in using land use pressure and soil quality indicators to predict water quality

Intensive agriculture can impair river water quality. Soil quality monitoring has been used to measure the effect of land use intensification on water quality at a point and field scales but not at the catchment scale. Other farm scale land use pressures, like stocking rate and the value of land, which relate to land use intensity are now publicly available, nationally. We therefore tested whether point scale soil quality measures, together with newly available farm scale land use pressures (land valuation and stocking rate) and existing catchment and climatic characteristics could help predict the behaviour of water quality data across 192 catchments in New Zealand. We used a generalised additive model to make predictions of the change in nitrogen fractions (r2 = 0.65-0.71), phosphorus fractions (r2 = 0.51-0.70), clarity and turbidity (r2 = 0.42-0.46), and E. coli (r2 = 0.35) over 15 years. The state and trend of water quality was strongly related to a refined farm scale land use classification, and to catchment and climatic characteristics (e.g. slope, elevation, and rainfall). Relationships with point scale soil quality measures and the land use pressures were weak. The weak relationship with land use pressures may be caused by using a single snapshot in time (2022), which cannot account for lag times in water quality response but leaves room for additional temporal data to improve predictive power. The weak relationship to soil quality measures was probably caused by limited data points (n = 667 sites) that were unrepresentative of land use, and areas of catchment subject to processes like runoff or leaching. While national soil quality measures might be useful for evaluating environmental risk at the field or farm scale, without a large increase in sampling, they were not relevant at the catchment scale. Additional analyses should be performed to determine how many samples would be needed to detect a change using an environmentally focused soil test that can guide water quality management.

A regional examination of the footprint of agriculture and urban cover on stream water quality

Freshwater systems in cold regions, including the Laurentian Great Lakes, are threatened by both eutrophication and salinization, due to excess nitrogen (N), phosphorus (P) and chloride (Cl-) delivered in agricultural and urban runoff. However, identifying the relative contribution of urban vs. agricultural development to water quality impairment is challenging in watersheds with mixed land cover, which typify most developed regions. In this study, a self-organizing map (SOM) analysis was used to evaluate the contributions of various forms of land cover to water quality impairment in southern Ontario, a population-dense, yet highly agricultural region in the Laurentian Great Lakes basin where urban expansion and agricultural intensification have been associated with continued water quality impairment. Watersheds were classified into eight spatial clusters, representing four categories of agriculture, one urban, one natural, and two mixed land use clusters. All four agricultural clusters had high nitrate-N concentrations, but levels were especially high in watersheds with extensive corn and soybean cultivation, where exceedances of the 3 mg L-1 water quality objective dramatically increased above a threshold of ‍∼30 % watershed row crop cover. Maximum P concentrations also occurred in the most heavily tile-drained cash crop watersheds, but associations between P and land use were not as clear as for N. The most urbanized watersheds had the highest Cl- concentrations and expansions in urban area were mostly at the expense of surrounding agricultural land cover, which may drive intensification of remaining agricultural lands. Expansions in tile-drained corn and soybean area, often at the expense of mixed, lower intensity agriculture are not unique to this area and suggest that river nitrate-N levels will continue to increase in the future. The SOM approach provides a powerful means of simplifying heterogeneous land cover characteristics that can be associated with water quality patterns and identify problem areas to target management.

Spatiotemporal investigation of geochemical and hydrological controls on release of soluble reactive phosphorus from the shallow aquifer of a riparian zone

Phosphorus (P) that accumulates in agricultural riparian zones can be released under certain hydrological and biogeochemical conditions, thereby limiting the effectiveness of these zones in reducing P loads from field to stream. The study objective was to explore factors that may be contributing to, or limiting, high soluble reactive phosphorus (SRP) concentrations in the shallow aquifer of an alluvial upland riparian zone located in a continental climate. Field investigations including porewater sampling from six vertical nests, soil sampling, and continuous soil moisture, groundwater table, and redox measurements were conducted over 19 months. Porewater SRP concentrations were generally low in the aquifer considering all sampling times (median = 14.7 µg/L; interquartile range [IQR] = 11.1 µg/L, 287 samples). The overall low SRP may be due to low reducible labile soil P (median = 21.1 µgP/g dw, IQR = 10.9 µgP/g dw, 21 samples). However, high SRP concentrations (>52 µg/L, 95% quartile) did occur intermittently in space and time with no clear spatial or temporal patterns. Analyses indicate that most high concentrations were likely not associated with factors previously reported to influence SRP release in riparian aquifers, including redox conditions, pH, and soil drying and wetting. Further, data indicate that internally released or externally supplied SRP may undergo rapid (re-) sequestration within the aquifer, limiting its vertical or horizontal transport. The study findings highlight the complexity of P behavior in riparian zones and the need for caution when assessing the effectiveness of conservation practices and in interpreting potential impacts of subsurface water quality on stream water quality when monitoring locations are distant from the stream.

Phosphorus speciation in manure and fertilizer impacted Mid-Atlantic coastal plain soils

Historical applications of manures and fertilizers at rates exceeding crop P removal in the Mid-Atlantic region (United States) have resulted in decades of increased water quality degradation from P losses in agricultural runoff. As such, many growers in this region face restrictions on future P applications. An improved understanding of the fate, transformations, and availability of P is needed to manage P-enriched soils. We paired chemical extractions (i.e., Mehlich-3, water extractable P, and chemical fractionation) with nondestructive methods (i.e., x-ray absorption near edge structure [XANES] spectroscopy and x-ray fluorescence [XRF]) to investigate P dynamics in eight P-enriched Mid-Atlantic soils with various management histories. Chemical fractionation and XRF data were used to support XANES linear combination fits, allowing for identification of various Al, Ca, and Fe phosphates and P sorbed phases in soils amended with fertilizer, poultry litter, or dairy manure. Management history and P speciation were used to make qualitative comparisons between the eight legacy P soils; we also speculate about how P speciation may affect future management of these soils with and without additional P applications. With continued P applications, we expect an increase in semicrystalline Al and Fe-P, P sorbed to Al (hydro)oxides, and insoluble Ca-P species in these soils for all P sources. Under drawdown scenarios, we expect plant P uptake first from semicrystalline Al and Fe phosphates followed by P sorbed phases. Our results can help guide management decisions on coastal plain soils with a history of P application.

Phosphorus fractions and speciation in an alkaline, manured soil amended with alum, gypsum, and Epsom salt

Snowmelt runoff is a dominant pathway of phosphorus (P) losses from agricultural lands in cold climatic regions. Soil amendments effectively reduce P losses from soils by converting P to less soluble forms; however, changes in P speciation in cold climatic regions with fall-applied amendments have not been investigated. This study evaluated P composition in soils from a manured field with fall-amended alum (Al2(SO4)3·18H2O), gypsum (CaSO4·2H2O), or Epsom salt (MgSO4·7H2O) using three complementary methods: sequential P fractionation, scanning electron microscopy with energy-dispersive X-rays (SEM-EDX) spectroscopy, and P K-edge X-ray absorption near-edge structure spectroscopy (XANES). Plots were established in an annual crop field in southern Manitoba, Canada, with unamended and amended (2.5 Mg ha-1) treatments having four replicates in 2020 fall. Soil samples (0-10 cm) taken from each plot soon after spring snowmelt in 2021 were subjected to P fractionation. A composite soil sample for each treatment was analyzed using SEM-EDX and XANES. Alum- and Epsom salt-treated soils had significantly greater residual P fraction with a higher proportion of apatite-like P and a correspondingly lower proportion of P sorbed to calcite (CaCO3) than unamended and gypsum-amended soils. Backscattered electron imaging of SEM-EDX revealed that alum- and Epsom salt-amended treatments had P-enriched microsites frequently associated with aluminum (Al), iron (Fe), magnesium (Mg), and calcium (Ca), which was not observed in other treatments. Induced precipitation of apatite-like species may have been responsible for reduced P loss to snowmelt previously reported with fall application of amendments.

Gypsum amendment of agricultural fields to decrease phosphorus losses - Evidence on a catchment scale

Amending agricultural fields with gypsum has been proposed as a cost-effective measure to reduce P load on coastal waters. We treated 1490 ha of clayey fields with phosphogypsum (4 t ha-1) in Southwest Finland and monitored the recipient river with online sensors and water sampling for the preceding spring and 5 years after the amendment. Gypsum immediately decreased the riverine fluxes, the effect lasting at least 5 years for particulate P (PP), total suspended solids (TSS), and dissolved organic C (DOC) and 1-2 years for dissolved reactive P (DRP). Compared with an upstream control area, the fluxes of PP, TSS, and DOC decreased by 15%, 25%, and 8.9%, respectively, as a 5-year average. Assuming the change in the fluxes occurred only due to gypsum, the amended fields showed 35%, 59%, and 64% lower losses for PP, TSS, and DOC than the unamended ones. More than half of the gypsum remained in the soil even after 5 years; thus, although the efficiency of gypsum lessened over time, its residual effect may be present. However, the difference in the erodibility between the control and treatment areas impacted the validity of the results, especially as the pre-gypsum period was short. In addition, the performance of gypsum showed spatial variation.

Evaluating the longevity of in-stream phosphorus legacies: A downstream cascade of recovery following point source remediation

In-stream phosphorus (P) legacies cause lags between upstream remediation and downstream load reductions. However, the length of these lags is largely unknown, especially for long stream distances. As a result, lag time estimates at the large-watershed scale have been abstract and sometimes understated. Here, we leverage a large area watershed model with newly improved in-stream P simulation (SWAT+P.R&R) to evaluate the magnitude, longevity, and spatial cascade of legacy P remobilization in a U.S. corn belt watershed. Our results illustrate the "spiraling recovery" of P loads after a hypothetical point source remediation, where locations further downstream take longer to recover to baseline load levels. At the watershed outlet, in-stream legacy P contributions are equivalent to 30% of the baseline average annual P loads for three years after remediation. In-stream legacies do not approach exhaustion (95% remobilized) until at least 9 years after remediation. In hypothetical weather scenarios beginning with dry years, legacy contributions persist even longer. These findings (1) suggest that in-stream legacies could impact P loads for years to decades in large river basins, (2) support explicit accounting for spatial scale in future studies of in-stream legacies, and (3) provide concerning implications for water quality recovery in large river basins.

Legacy sediment as a potential source of orthophosphate: Preliminary conceptual and geochemical models for the Susquehanna River, Chesapeake Bay watershed, USA

Nutrient pollution from agriculture and urban areas plus acid mine drainage (AMD) from legacy coal mines are primary causes of water-quality impairment in the Susquehanna River, which is the predominant source of freshwater and nutrients entering the Chesapeake Bay. Recent increases in the delivery of dissolved orthophosphate (PO4) from the river to the bay may be linked to long-term increases in pH, decreased acidity of precipitation, and decreased acidity, iron, and aluminum loading from widespread AMD. Since the 1950s, baseline pH increased from ~6.5 to ~8 in the West Branch and "North Branch" of the Susquehanna River, which drain bituminous and anthracite coalfields of Pennsylvania. A current baseline pH of ~8 and daily maxima exceeding 9 have been documented along the lower Susquehanna River. In response to improved river quality, bioavailable PO4 now may be released into solution from legacy sediment that has filled major impoundments in lower reaches of the river. At typical pH (5-8) of natural water, aqueous PO4 species tend to be adsorbed by hydrous iron, aluminum, and manganese oxides that coat soil and sediment particles; however, PO4 may be substantially desorbed at pH >8. We created a geochemical model that simulates equilibrium aqueous/solid distributions of PO4 as pH and other solution characteristics change. Considering current conditions in the lower Susquehanna River, the model demonstrates potential for extensive release of adsorbed PO4 at pH >8. Empirical data from laboratory experiments corroborate model results. The transfer of PO4 into the water column may increase algae growth, which removes CO2 and drives pH to higher values, facilitating additional PO4 release and exacerbating the potential for harmful algal blooms. Thus, legacy sediment is a currently unquantified source of PO4 that warrants consideration by resource managers and programs collaborating to reduce phosphorus loads to the bay and similar settings worldwide.

The importance of consensus science to managing phosphorus in the environment: SERA-17 and the legacy of Andrew Sharpley

Phosphorus (P) loss from agricultural systems to surface waters, and ultimately, eutrophication, presents a wicked problem requiring transdisciplinary solutions. The mission of SERA-17 (Southern Extension and Research Advisory Information Exchange Group-17) has been to address this problem by developing "Innovative Solutions to Minimize Phosphorus Losses from Agriculture." Over the course of his career, Dr. Andrew Sharpley demonstrated a rare ability to collaboratively achieve consensus around issues related to the science and management of P. The SERA-17 organization served as the central community of experts and stakeholders where that consensus was built and applied. The consensus-based approach, demonstrated by Sharpley and at the core of the SERA-17 organization, was routinely applied to key areas of P science to produce applied outcomes that have been readily adopted: advance foundational science to resolve knowledge gaps and to promote innovation; promote consistency in methods to facilitate comprehensive investigations and conclusions across a diversity of systems; engage diverse stakeholders to prioritize research, and ultimately, ensure that outcomes reflect a plurality of perspectives; and deliver pragmatic solutions that reflect the best information available at a particular time. We review the history of SERA-17 in delivering new science and management recommendations for P, with an eye to elucidating Sharpley's role and legacy in this process.

Managing mineral phosphorus application with soil residual phosphorus reuse in Canada

With limited phosphorus (P) supplies, increasing P demand, and issues with P runoff and pollution, developing an ability to reuse the large amounts of residual P stored in agricultural soils is increasingly important. In this study, we investigated the potential for residual soil P to maintain crop yields while reducing P applications and losses in Canada. Using a P cycling model coupled with a soil P dynamics model, we analyzed soil P dynamics over 110 years across Canada's provinces. We found that using soil residual P may reduce mineral P demand as large as 132 Gg P year-1 (29%) in Canada, with the highest potential for reducing P applications in the Atlantic provinces, Quebec, Ontario, and British Columbia. Using residual soil P would result in a 21% increase in Canada's cropland P use efficiency. We expected that the Atlantic provinces and Quebec would have the greatest runoff P loss reduction with use of residual soil P, with the average P loss rate decreasing from 4.24 and 1.69 kg ha-1 to 3.45 and 1.38 kg ha-1 , respectively. Ontario, Manitoba, and British Columbia would experience relatively lower reductions in P loss through use of residual soil P, with the average runoff P loss rate decreasing from 0.44, 0.36, and 4.33 kg ha-1 to 0.19, 0.26, and 4.14 kg ha-1 , respectively. Our study highlights the importance of considering residual soil P as a valuable resource and its potential for reducing P pollution.

Effects of switching redox conditions on sediment phosphorus immobilization by calcium/aluminum composite capping: Performance, ecological safety and mechanisms

There many materials were used in lake restoration to immobilize phosphorus (P) and reduce the effect of eutrophication. Among them, calcium/aluminum composite (CAC) showed a good capacity of P adsorption. However, a comprehensive of its performance, ecological safety, and the mechanism of P passivation in the aluminum-bound P (Al -P) dominated sediments under varying redox conditions remains incomplete. In the current study, both unwashed CAC (UCAC) and washed CAC (WCAC) showed good P adsorption properties, and the greatest maximum capacity for P adsorption (Qmax) reached 206.8 mg/g at pH 8.5 for UCAC. The SRP and TP in the overlying water of the uncapped sediments showed a decrease-increase-decrease trend in a sequence of transition from aerobic to anaerobic to re-aerobic stages. In contrast, the SRP and TP of the two CACs-capped sediments were maintained low. Phosphorus forms in the uncapped sediment also underwent significant changes during continuous variation of dissolved oxygen (DO) levels. In particular, the decrease in iron-bound P (Fe-P) and Al-P was significantly promoted in the anaerobic phase, and the released P was reabsorbed to form mainly Fe-P in the re-aerobic phase. The CACs-capping promoted the transformation of Fe-P to residual P (Res-P), forming a thick static layer in the surface sediment, thus significantly inhibiting sediment P release. Moreover, the CACs-capping did not induce the Al3+ leaching and significant changes of the microbial community in sediments, and their performances of P immobilization could keep stable to resist the redox variation, which promised to be a good choice for P passivation in eutrophic lake sediments dominated by Al/Fe-P. These findings also confirmed that the risk of P release from Al/Fe-P (mainly Al-P)-dominated sediments was strongly influenced by continuously changing redox conditions, and was probably enhanced by the formation of Fe-P from the resorption of the released P.

Trade-offs in nutrient and sediment losses in tile drainage from no-till versus conventional conservation-till cropping systems

Nutrient and soil loss from agricultural areas impairs surface water quality globally. In the Great Lakes region, increases in the frequency and magnitude of harmful and nuisance algal blooms in freshwater lakes have been linked to elevated phosphorus (P) losses from agricultural fields, some of which are transported via tile drainage. This study examined whether concentrations and loads of P fractions, total suspended sediments (TSS), nitrate (NO3 - ), and ammonium (NH4 + ) in tile drainage in a clay soil differed between a continuous no-till system combining cover crops and surface broadcast fertilizer (no-till cover crop [NTCC]), and a more conventional tillage system with shallow tillage, fertilizer incorporation and limited use of cover crops (conventional conservation-till, CT). Both sites had modest soil fertility levels. Year-round, high-frequency observations of tile drainage flow and chemistry are described over 4 full water years and related to management practices on the associated fields. There were similar water yields in tile drainage between the two systems; however, losses of TSS, particulate P (PP), and NO3 - were consistently greater from the CT site, which received larger quantities of fertilizer. In contrast, dissolved reactive P (DRP) losses were considerably greater from the NTCC site, offsetting the lower PP losses, such that there was little difference in TP losses between sites. Approximately 60% of the DRP losses from the NTCC site over the 4 years were associated with incidental losses following surface application of fertilizer in fall. This study provides insight into trade-offs in controlling losses of different nutrient fractions using different management systems.

Efficient protection of the Baltic Sea needs a revision of phosphorus metric

Eutrophication of the Baltic Sea is driven by phosphorus and nitrogen. While the anthropogenic point source loads of both nutrients have decreased markedly, further reductions are needed. This is true particularly for phosphorus, as highlighted by its stringent abatement targets in HELCOM's Baltic Sea Action Plan. To meet the targets, more results need to be achieved in non-point source abatement, specifically from agricultural sources. The growing pressure for phosphorus abatement from agriculture may lead to environmentally and economically inefficient outcomes unless we account for the variability in how different forms of phosphorus respond to abatement measures, and how these forms contribute to eutrophication. The precautionary and efficiency improving way to advance policies is to either replace or supplement the Total Phosphorus metric with a metric more accurate in reflecting the biologically available phosphorus. This policy fix becomes more important as the relative share of agricultural emissions of total pollution increases.

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.

A slurry approach to identify nutrient critical source areas from subtropical catchment erosion

Targeting catchment nutrient critical source areas (CSAs) (areas contributing most of the nutrients in a catchment) is an efficient way to prioritize remediation sites for reducing nutrient runoff to waterways. We tested if the soil slurry approach - with particle sizes and sediment concentrations representative of those in streams during high rainfall events - can be used to identify potential CSAs within individual land use types, examine fire impacts, and identify the contribution of leaf litter in topsoil to nutrient export in subtropical catchments. We first confirmed the slurry approach met the prerequisite to identify CSAs with relatively higher nutrient contribution (not absolute load estimation) by comparing the slurry sampling with stream nutrient monitoring data. We validated that: 1) differences in slurry total nitrogen to phosphorus mass ratios from different land uses were consistent with stream monitoring data; and 2) our estimated nutrient export contribution from agricultural land, via the slurry approach, was comparable to that derived from monitoring data. Additionally, we found nutrient concentrations in slurries differed across soil types and management practice within individual land uses, correlating with nutrient concentrations in fine particles. These results indicate the slurry approach can be used to identify potential small-scale CSAs. Slurry results from burnt soils were also comparable to other studies showing increased levels of dissolved nutrient loss and higher nitrogen than phosphorus loss, than non-burnt soils. The slurry method also showed the contribution of leaf litter to slurry nutrient concentrations from topsoil was greater for dissolved nutrients than particulate nutrients, indicating different forms of nutrients need to be considered for impacts of vegetation. Our study reveals that the slurry method can be used to identify potential small-scale CSAs within the same land use from erosion and can account for impacts of vegetation and bushfires, providing timely information to guide catchment restoration actions.

Sudden eutrophication of an aluminum sulphate treated lake due to abrupt increase of internal phosphorus loading after three decades of mesotrophy

Aluminum salts are widely used to immobilize phosphorus (P) in lakes suffering from internal loading. However, longevity of treatments varies among lakes; some lakes eutrophy faster than others. We conducted biogeochemical investigations of sediments of a closed artificial Lake Barleber, Germany that was successfully remediated with aluminum sulfate in 1986. The lake became mesotrophic for almost 30 years; a rather rapid re-eutrophication took place in 2016 leading to massive cyanobacterial blooms. We quantified internal loading from sediment and analyzed two environmental factors that might have contributed to the sudden shift in trophic state. Increase in lake P concentration started in 2016, reaching 0.3 mg L^-1, and remained elevated into the spring of 2018. Reducible P fraction in the sediment was 37 - 58% of total P, indicating a high potential for mobilization of benthic P during anoxia. Estimated P release from sediments for 2017 was approximately 600 kg for the whole lake. This is consistent with sediment incubation results; higher temperature (20°C) and anoxia contributed to release of P (27.9 ± 7.1 mg m^-2 d^-1, 0.94 ± 0.23 mmol m^-2 d^-1) to the lake, triggering re-eutrophication. Loss of aluminum P adsorption capacity together with anoxia and high water temperatures (organic matter mineralization) are major drivers of re-eutrophication. Accordingly, treated lakes at some time require a repeated aluminum treatment for sustaining acceptable water quality and we recommend regular sediment monitoring in treated lakes. This is crucial given the effects of climate warming on duration of stratification in lakes which may result in the need for treatment of many lakes.

Representing soil health practice effects on soil properties and nutrient loss in a watershed-scale hydrologic model

Watershed-scale hydrologic models are commonly used to assess the water quality effects of agricultural conservation practices that improve soil health (e.g., cover crops and no-till). However, models rarely account for how these practices (i.e., soil health practices) affect soil physical and functional properties such as water holding capacity and soil aggregate stability, which may, in turn, affect water quality. We introduce a method to represent changes in soil physical and functional properties caused by soil health practices in the Soil and Water Assessment Tool (SWAT) model. We used the SWAT model's default representation of winter cover crops and no-till and modified soil descriptive parameters to depict soil health practice effects on soil properties. We assumed that the soil health practices would increase soil organic carbon (SOC), a principal indicator of soil health, by 0.01 g C g^-1 of soil and then estimated changes in other soil properties (e.g., water holding capacity) using SOC-based predictive equations and preceding literature. Results indicated that our soil property modifications had statistically significant effects on simulated hydrology and nutrient loss, though outputs were more substantially affected by the model's default representation of cover crops and no-till. Results also indicated that soil health practices can reduce nitrogen and total phosphorus loss but may increase dissolved reactive phosphorus loss. Our representation of soil health practices provides a more complete estimate of practice efficacy but underscores a need for additional observational data to verify results and guide further model improvements.

Periphyton Phosphorus Uptake in Response to Dynamic Concentrations in Streams: Assimilation and Changes to Intracellular Speciation

Effective modeling and management of phosphorus (P) losses from landscapes to receiving waterbodies requires an adequate understanding of P retention and remobilization along the terrestrial-aquatic continuum. Within aquatic ecosystems, the stream periphyton can transiently store bioavailable P through uptake and incorporation into biomass during subscouring and baseflow conditions. However, the capacity of stream periphyton to respond to dynamic P concentrations, which are ubiquitous in streams, is largely unknown. Our study used artificial streams to impose short periods (48 h) of high SRP concentration on stream periphyton acclimated to P scarcity. We examined periphyton P content and speciation through nuclear magnetic resonance spectroscopy to elucidate the intracellular storage and transformation of P taken up across a gradient of transiently elevated SRP availabilities. Our study demonstrates that the stream periphyton not only takes up significant quantities of P following a 48-h high P pulse but also sustains supplemental growth over extended periods of time (10 days), following the reestablishment of P scarcity by efficiently assimilating P stored as polyphosphates into functional biomass (i.e., phospho-monoesters and phospho-diesters). Although P uptake and intracellular storage approached an upper limit across the experimentally imposed SRP pulse gradient, our findings demonstrate the previously underappreciated extent to which the periphyton can modulate the timing and magnitude of P delivery from streams. Further elucidating these intricacies in the transient storage potential of periphyton highlights opportunities to enhance the predictive capacity of watershed nutrient models and potentially improve watershed P management.

Advancing freshwater ecological forecasts: Harmful algal blooms in Lake Erie

Ecological models help provide forecasts of ecosystem responses to natural and anthropogenic stresses. However, their ability to create reliable predictions requires forecasts with track records sufficiently long to build confidence, skill assessments, and treating uncertainty quantitatively. We use Lake Erie harmful algal blooms as a case study to help formalize ecological forecasting. Key challenges for models include uncertainty in the deterministic structure of the load-bloom relationship and the need to assess alternative drivers (e.g., biologically available phosphorus load, spring load, longer term cumulative load) with a larger dataset. We enhanced a Bayesian model considering new information and an expanded data set, test it through cross validation and blind forecasts, quantify and discuss its uncertainties, and apply it for assessing historical and future scenarios. Allowing a segmented relationship between bloom size and spring load indicates that loading above 0.15 Gg/month will have a substantially higher marginal impact on bloom size. The new model explains 84 % of interannual variability (9.09 Gg RMSE) when calibrated to the 19-year data set and 66 % of variability in cross validation (12.58 Gg RMSE). Blind forecasts explain 84 % of HAB variability between 2014 and 2020, which is substantially better than the actual forecast track record (R² = 0.32) over this same period. Because of internal phosphorus recycling, represented by the long-term cumulative load, it could take over a decade for HABs to fully respond to loading reductions, depending on the pace of those reductions. Thus, the desired speed and endpoint of the lake's recovery should be considered when updating and adaptively managing load reduction targets. Results are discussed in the context of ecological forecasting best pactices: incorporate new knowledge and data in model construction; account for multiple sources of uncertainty; evaluate predictive skill through validation and hindcasting; and answer management questions related to both short-term forecasts and long-term scenarios.

Investigating water quality sensitivity to climate variability and its influencing factors in four Lake Erie watersheds

Climate change alters weather patterns and hydrological cycle, thus potentially aggravating water quality impairment. However, the direct relationships between climate variability and water quality are complicated by a multitude of hydrological and biochemical mechanisms dominate the process. Thus, little is known regarding how water quality responds to climate variability in the context of changing meteorological conditions and human activities. Here, a longitudinal study was conducted using trend, correlation, and redundancy analyses to explore stream water quality sensitivity to temperature, precipitation, streamflow, and how the sensitivity was affected by watershed climate, land cover percentage, landscape configuration, fertilizer application, and tillage types. Specifically, daily pollutant concentration data of suspended solid (SS), total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), nitrate and nitrite (NO_x), and chloride (Cl) were used as water quality indicators in four Lake Erie watersheds from 1985 to 2017, during which the average temperature has increased 0.5 °C and the total precipitation has increased 9%. Results show that precipitation and flow were positively associated with SRP, NO_x, TKN, TP, and SS, except for SRP and NO_x in the urban basin. The rising temperatures led to increasing concentrations of SS, TKN, and TP in the urban basin. SRP and NO_x sensitivity to precipitation was higher in the years with more precipitation and higher precipitation seasonality, and the basins with more spatially aggregated cropland. No-tillage and reduced tillage management could decrease both precipitation and temperature sensitivity for most pollutants. As one of the first studies leveraging multiple watershed environmental variables with long-term historical climate and water quality data, this study can assist target land use planning and management policy to mitigate future climate change effects on surface water quality.

Removal of harmful algae by Shigella sp. H3 and Alcaligenes sp. H5: algicidal pathways and characteristics

Application of algicidal bacteria is a promising technology to control harmful algal blooms (HABs). In this study, algicidal bacteria strains Shigella sp. H3 and Alcaligenes sp. H5 were obtained via two different isolation methods from the same lake water sample, with optimal algicidal efficiencies 96% and 74% against algae mixture. The Shigella sp. H3 and Alcaligenes sp. H5 lysed algae cells through cells-to-cells direct contact and secretion of algicidal metabolites, respectively. The stronger algicidal capability of Shigella sp. H3 was also attributable to its higher efficiency for triggering reactive oxygen species, which led to broken down of the antioxidant system and more severe damage to the bacterial cells. The antioxidant enzyme activities in Alcaligenes sp. H5 group were still expressed because of its relatively weaker algicidal capability and some intact algal cells were remained. The liquid carbohydrates from algal lysis in both groups increased significantly, whereas the quantities of liquid protein decreased, which might be assimilated by algicidal bacteria. Nonetheless, the whole algicidal process resulted in the increase of total released organic matters content. This study revealed the algicidal pathways of diverse bacterial strains, and the possible secondary environmental problem caused by the algal released organic matters should be considered when applying bacteria to control HABs.

Assessing the concept of control points for dissolved reactive phosphorus losses in subsurface drainage

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept, but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85 ha) subsurface drainage plots in Illinois were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four study years. Three plot-months were identified as DRP control points (one export and two transport control points), where each plot-month contributed >10% of the annual DRP load from the field. These control points occurred on separate plots and in both the growing and nongrowing seasons but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The nongrowing season was hypothesized to be the most significant period of P loss, but this was only documented in two of the four study years. A cereal rye (Secale cereale L.) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing and thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small (<3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated a potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point).

Clean Water Act at 50: Celebrating the golden anniversary of the blue policy

Both the U.S. Clean Water Act (CWA) and the Journal of Environmental Quality (JEQ) trace back to 1972. The journal has a strong history of publishing science in support of the CWA, from basic science used to develop best management practices to address water quality problems to special sections on hot issues of the day and publishing long-term data that provide evidence of success. The objective of this article is to provide a brief overview of how JEQ has provided a publication outlet for researchers involved in efforts focused on key CWA issues and to introduce a special section celebrating the golden anniversary of this blue policy. The special section includes papers that look back historically on biosolids research that began with the advent of the CWA as well as a forward-looking paper encouraging readers to think about the implications of the ubiquitous use of lithium-ion batteries and a call to action before tangible problems are realized.

River phosphorus cycling during high flow may constrain Lake Erie cyanobacteria blooms

Cyanobacterial harmful blooms have been increasing worldwide, due in part to excessive phosphorus (P) losses from agriculture-dominated watersheds. Unfortunately, cyanobacteria bloom management is often complicated by uncertainty associated with river P cycling. River P cycling mediates P exports during low flow but has been assumed to be unimportant during high flows. Thus, we examined interactions between dissolved reactive phosphorus (DRP) and suspended sediment P during high flows in the Maumee River network, focusing on March-June Maumee River DRP exports, which fuel recurring cyanobacteria blooms in Lake Erie. We estimate that during 2003-2019 March to June high flow events, P sorption reduced DRP exports by an average of 13-27%, depending upon the colloidal-P:DRP ratio, decreasing the bioavailability of P exports, and potentially constraining cyanobacteria blooms by 13-40%. Phosphorus sorption was likely lower during 2003-2019 than 1975-2002 due to reductions in suspended sediment loads, associated with soil-erosion-minimizing agricultural practices. This unintended outcome of erosion management has likely decreased P sorption, increased DRP exports to Lake Erie, and subsequent cyanobacteria blooms. In other watersheds, DRP-sediment P interactions during high flow could have a positive or negative effect on DRP exports; therefore, P management should consider riverine P cycles, particularly during high flow events, to avoid undermining expensive P mitigation efforts.

Agricultural intensification leads to higher nitrate levels in Lake Ontario tributaries

Eutrophication remains the most widespread water quality impairment globally and is commonly associated with excess nitrogen (N) and phosphorus (P) inputs to surface waters from agricultural runoff. In southern Ontario, Canada, increases in nitrate (NO3-N) concentrations as well as declines in total phosphorus (TP) concentration have been observed over the past four decades at predominantly agricultural watersheds, where major expansions in row crop production at the expense of pasture and forage have occurred. This study used a space-for-time approach to test whether 'agricultural intensification', herein defined as increases in row crop area (primarily corn-soybean-winter wheat rotation) at the expense of mixed livestock and forage/pasture, could explain increases in NO3-N and declines in TP over time. We found a clear, positive relationship between the extent of row crop area within watersheds and NO3-N losses, such that tributary NO3-N concentrations and export were predicted to increase by ~0.4 mg/L and ~130 kg/km2 respectively, for every 10% expansion in row crop area. There was also a significant positive relationship between row crop area and total dissolved phosphorus (TDP) concentration, but not export, and TP was not correlated with any form of landcover. Instead, TP was strongly associated with storm events, and was more sensitive to hydrologic condition than to landcover. These results suggest that pervasive shifts toward tile-drained corn and soybean production could explain increases in tributary NO3-N levels in this region. The relationship between changes in agriculture and P is less clear, but the significant association between dissolved P and row crop area suggests that increased adoption of reduced tillage practices and tile drainage may enhance subsurface losses of P.

Pioneering Farmers Value Agronomic Performance of Cover Crops and Their Impacts on Soil and Environment

Cover crops (CCs) have aroused a great deal of interest as a multifunctional measure to improve the sustainability of agriculture. Understanding farmers’ views are important for future farm-scale implementation. A farmer survey was carried out in Finland in 2021 with the aims to gather farmers’ views on agronomic performance of CCs, their environmental impacts and contribution to climate smart agriculture, and understand how farmers’ views on CCs differed depending on farm/farmer characteristics. The farmers’ sample was conventional and organic farms that had selected CCs as a registered measure in 2020. 6493 farmers were invited to answer a questionnaire with 18 statements (a Likert scale, 5 answer choices), and 1130 responded (17.4%). A Cochran–Mantel–Haenszel test was used to measure the strength of the association between ten characteristics of the respondents and 18 statements. Farmers considered CCs to have wide-ranging benefits for soil conditions. Only 21% of farmers agreed that CCs increase the need for nitrogen fertilizer use. 49% of farmers agreed that CCs reduce weed problems. Farmers mostly agreed (ca. 80%) that CCs reduce nutrient leaching and erosion. They were in general more uncertain about CCs’ contribution to climate change mitigation (53% agreed), adaptation (51%), and resilience (58%). In agri-environmental schemes subsidies for use of CCs should aim large-scale implementation with two important target groups: younger farmers (≤50 years) as they were slightly more skeptical than older ones and farmers with less diverse land use as they were more doubtful of benefits provided by CCs.

The role of organic nutrients in structuring freshwater phytoplankton communities in a rapidly changing world

Carbon, nitrogen, and phosphorus are critical macroelements in freshwater systems. Historically, researchers and managers have focused on inorganic forms, based on the premise that the organic pool was not available for direct uptake by phytoplankton. We now know that phytoplankton can tap the organic nutrient pool through a number of mechanisms including direct uptake, enzymatic hydrolysis, mixotrophy, and through symbiotic relationships with microbial communities. In this review, we explore these mechanisms considering current and projected future anthropogenically-driven changes to freshwater systems. In particular, we focus on how naturally- and anthropogenically- derived organic nutrients can influence phytoplankton community structure. We also synthesize knowledge gaps regarding phytoplankton physiology and the potential challenges of nutrient management in an organically dynamic and anthropogenically modified world. Our review provides a basis for exploring these topics and suggests several avenues for future work on the relation between organic nutrients and eutrophication and their ecological implications in freshwater systems.

Phosphorus Release and Speciation in Manganese(IV) Oxide and Zeolite-Amended Flooded Soils

Phosphorus (P) losses from flooded soils and subsequent transport to waterways contribute to eutrophication of surface waters. This study evaluated the effectiveness of MnO₂ and a zeolite Y amendment in reducing P release from flooded soils and explored the underlying mechanisms controlling P release. Unamended and amended (MnO₂ or zeolite, surface-amended at 5 Mg ha^-1) soil monoliths from four clayey-alkaline soils were flooded at 22 ± 2 °C for 56 days. Soil redox potential and dissolved reactive P (DRP), pH, and concentrations of major cations and anions in porewater and floodwater were analyzed periodically. Soil P speciation was simulated using Visual MINTEQ at 1, 28, and 56 days after flooding (DAF) and P K-edge X-ray absorption near-edge structure spectroscopy and sequential fractionation at 56 DAF. Porewater DRP increased with DAF and correlated negatively with pe+pH and positively with dissolved Fe. Reductive dissolution of Fe-associated P was the dominant mechanism of flooding-induced P release. The MnO₂ amendment reduced porewater DRP by 30%-50% by favoring calcium phosphates (Ca-P) precipitation and delaying the reductive dissolution reactions. In three soils, the zeolite amendment at some DAF increased porewater and/or floodwater DRP through dissolution of Ca-P and thus was not effective in reducing P release from flooded soils.

SWAT model application for evaluating agricultural conservation practice effectiveness in reducing phosphorous loss from the Western Lake Erie Basin

Lake Erie is threatened by eutrophication and harmful algal blooms due to excess nutrient loading from agricultural sources. Agricultural conservation practices (ACPs) have been developed and implemented to reduce nutrient losses but estimating ACP effectiveness is challenging. The Soil and Water Assessment Tool (SWAT) has been used to investigate ACP effectiveness for water quality improvement. Many SWAT applications have been developed by different investigators to evaluate ACP effectiveness for reducing nutrient, particularly phosphorus (P), loading in the agriculturally-dominated Western Lake Erie Basin (WLEB). Our objective is to establish what has been achieved by past modeling research and make suggestions for future applications and improvements. We synthesized the findings of 28 SWAT modeling studies within the WLEB. Models generally performed satisfactorily against accepted criteria for streamflow and sediment, but performance for P loads, like soluble reactive P, was mostly "unsatisfactory". The "unsatisfactory" performance maybe due to imperfections and idealizations in model formulations and/or parameterization. Thus, simulations of P transport and transformation processes need improvement. In addition, model parameter selection is the key part of model set-up. Most SWAT modeling studies used default values during initial set-up, then performed calibration and validation. It was found that the calibrated P related parameter values varied widely across different studies, even within the same watershed with some values unrealistic for the study areas. The phenomena of different combinations of model parameters producing similar outputs indicates equifinality. Equifinality in the baseline model may impact results when ACPs are incorporated. Furthermore, the unrealistic values used in ACP assessment undermine the credibility of ACP effectiveness. Future model applications should try to re-examine the calibrated P parameters and make sure they are realistic for the study area as well as reduce equifinality by constraining the model with characterization of watershed conditions, better understanding of hydrologic processes, and parameter values based on real-world observations. In summary, future model applications should focus on improving P transport and transformation processes, using measured watershed characteristics for parameterization, and improving reflections of climate change, which could result in more accurate assessments of ACP effectiveness to meet targeted goals.

Extending vegetative cover with cover crops influenced phosphorus loss from an agricultural watershed

Excess phosphorus (P) from agriculture is a leading cause of harmful and nuisance algal blooms in many freshwater ecosystems. Throughout much of the midwestern United States, extensive networks of subsurface tile drains remove excess water from fields and allow for productive agriculture. This enhanced drainage also facilitates the transport of P, particularly soluble reactive phosphorus (SRP), to adjacent streams and ditches, with harmful consequences. Thus, reducing SRP loss from tile-drained cropland is a major focus of regional and national efforts to curb eutrophication and algal blooms. The planting of cover crops after crop harvest is a conservation practice that has the potential to increase retention of fertilizer nutrients in watersheds by extending the growing season and limiting bare ground in the fallow season; however, the effect of cover crops on SRP loss is inconsistent at the field-scale and unknown at the watershed-scale. In this study, we conducted a large-scale manipulation of land cover in a small, agricultural watershed by planting cover crops on >60% of croppable acres for six years and examining changes in SRP loss through tile drains and at the watershed outlet. We found reduced median SRP loss from tiles with cover crops compared to those without cover crops, particularly during periods of critical export from January to June. Variation in tile discharge influenced SRP loss, but relationships were generally weaker in tiles with cover crops (i.e., decoupled) compared to tiles without cover crops. At the watershed outlet, SRP yield was highly variable over all seasons and years, which complicated efforts to detect a significant effect of changing land cover on SRP export to downstream systems. Yet, watershed-scale planting of cover crops slowed cumulative SRP losses and reduced SRP export during extreme events. Overall, this study demonstrates the potential for cover crops to alter patterns of SRP loss at both the field- and watershed-scale.

Linking multi-media modeling with machine learning to assess and predict lake Chlorophyll a concentrations

Eutrophication and excessive algal growth pose a threat on aquatic organisms and the health of the public, environment, and the economy. Understanding what drives excessive algal growth can inform mitigation measures and aid in advance planning to minimize impacts. We demonstrate how simulated data from weather, hydrological, and agroecosystem numerical prediction models can be combined with machine learning (ML) to assess and predict Chlorophyll a (Chl a) concentrations, a proxy for lake eutrophication and algal biomass. The study area is Lake Erie for a 16-year period, 2002-2017. A total of 20 environmental variables from linked and coupled physical models are used as input features to train the ML model with Chl a observations from 16 measuring stations. Included are meteorological variables from the Weather Research and Forecasting (WRF) model, hydrological variables from the Variable Infiltration Capacity (VIC) model, and agricultural management practice variables from the Environmental Policy Integrated Climate (EPIC) agroecosystem model. The consolidation of these variables is conducive to a successful prediction of Chl a. Aside from the synergistic effects that weather, hydrology, and fertilizers have on eutrophication and excessive algal growth, we found that the application of different forms of both P and N fertilizers are highly ranked for the prediction of Chl a concentration. The developed ML model successfully predicts Chl a with a coefficient of determination of 0.81, bias of -0.12 μg/l and RMSE of 4.97 μg/l. The developed ML-based modeling approach can be used for impact assessment of agriculture practices in a changing climate that affect Chl a concentrations in Lake Erie.

Long-Term Mississippi River Trends Expose Shifts in the River Load Response to Watershed Nutrient Balances Between 1975 and 2017

Excess nutrients transported by the Mississippi River (MR) contribute to hypoxia in the Gulf of Mexico. Nutrient balances are key drivers to river nutrient loads and represent inputs (fertilizer, manure, deposition, wastewater, N-fixation, and weathering) minus outputs (nutrient uptake and removal in harvest, and N emissions). Here, we quantified annual changes in nitrogen (N) and phosphorus (P) river loads and nutrient balances at the MR Outlet and documented that the river load response to watershed nutrient balances shifted between 1975 and 2017. Annual nutrient balances and river loads were positively correlated between 1975 and 1985, but after, a disconnect between both the N and P balances and river loads emerged, and the subsequent river load patterns were different for N versus P. We evaluated the relative impacts of legacy nutrients and other latent factors, for which data were not available, on river nutrient load trends. Our analysis showed that in the case of N, latent factors were potentially just as important in explaining changes in river nutrient loads over time as N balances, and in the case of P, they were even more important. We hypothesized that these factors included implementation of best management practices, changes in watershed buffering capacity, the effects of tile drainage, or increased precipitation. Our analytical approach shows promise for the investigation of drivers of water quality trends that are not well-represented in typical national scale geospatial datasets.

Phosphorus mobilization in unamended and magnesium sulfate-amended soil monoliths under simulated snowmelt flooding

Enhanced release of phosphorus (P) from soils with snowmelt flooding poses a threat of eutrophication to waterbodies in cold climatic regions. Reductions in P losses with various soil amendments has been reported, however effectiveness of MgSO₄ has not been studied under snowmelt flooding. This study examined (a) the P release enhancement with flooding in relation to initial soil P status and (b) the effectiveness of MgSO₄ at two rates in reducing P release to floodwater under simulated snowmelt flooding. Intact soil monoliths were collected from eight agricultural fields from Southern Manitoba, Canada. Unamended and MgSO₄ surface-amended monoliths (2.5 and 5.0 Mg ha^-1) in triplicates were pre-incubated for 7 days, then flooded and incubated (4 °C) for 56 days. Pore water and floodwater samples collected at 7-day intervals were analyzed for dissolved reactive P (DRP), pH, Ca, Mg, Fe and Mn. Redox potential (Eh) was measured on each day of sampling. Representative soil samples collected from each field were analyzed for Olsen and Mehlich 3-P. Simulated snowmelt flooding enhanced the mobility of soil P with approximately 1.2-1.6 -fold increase in pore water DRP concentration from 0 to 21 days after flooding. Mehlich-3 P content showed a strong relationship with the pore water DRP concentrations suggesting its potential as a predictor of P loss risk during prolonged flooding. Surface application of MgSO₄ reduced the P release to pore water and floodwater. The 2.5 Mg ha^-1 rate was more effective than the higher rate with a 21-75% reduction in average pore water DRP, across soils. Soil monoliths amended with MgSO₄ maintained a higher Eh, and had greater pore water Ca and Mg concentrations, which may have reduced redox-induced P release and favored re-precipitation of P with Ca and Mg, thus decreasing DRP concentrations in pore water and floodwater.

Measuring the Supply of Ecosystem Services from Alternative Soil and Nutrient Management Practices: A Transdisciplinary, Field-Scale Approach

Farmers and policy makers pursue management practices that enhance water quality, increase landscape flood resiliency, and mitigate agriculture’s contribution to climate change, all while remaining economically viable. This study presents a holistic assessment of how two practices influence the supply of these ecosystem services—the use of an aerator prior to manure application in haylands, and the stacked use of manure injection, cover crops, and reduced tillage in corn silage production. Field data are contextualized by semi-structured interviews that identify influences on adoption. Causal loop diagrams then illustrate feedbacks from ecosystem services onto decision making. In our study, unseen nutrient pathways are the least understood, but potentially the most important in determining the impact of a practice on ecosystem services supply. Subsurface runoff accounted for 64% to 92% of measured hydrologic phosphorus export. Average soil surface greenhouse gas flux constituted 38% to 73% of all contributions to the equivalent CO2 footprint of practices, sometimes outweighing carbon sequestration. Farmers identified interest in better understanding unseen nutrient pathways, expressed intrinsic stewardship motivations, but highlighted financial considerations as dominating decision making. Our analysis elevates the importance of financial supports for conservation, and the need for comprehensive understandings of agroecosystem performance that include hard-to-measure pathways.

Cover crops differentially influenced nitrogen and phosphorus loss in tile drainage and surface runoff from agricultural fields in Ohio, USA

Nitrogen (N) and phosphorus (P) loss from crop production agriculture is transported to adjacent and downstream water bodies, resulting in negative environmental impacts including harmful and nuisance algal blooms. Cover crops are a conservation management practice that replaces bare soil with vegetation outside of the cash crop growing season, purportedly reducing N and P loss by increasing water and nutrient demand in agroecosystems. In this study, we compared nitrate (NO₃^--N), total N (TN), dissolved reactive P (DRP), and total P (TP) loads in subsurface (tile) drainage and surface runoff from fields with cover crop management (CC) and fields without cover crop management (NoCC) using continuous monitoring data from 40 agricultural fields located throughout northcentral Ohio, United States (US). We found that average monthly tile NO₃^--N and TN loads from CC fields were ~50% less than NoCC fields, while average monthly tile discharge, DRP, and TP loads did not differ between CC and NoCC fields. Cover crops also did not significantly influence average monthly surface metrics. Cover crops reduced monthly totals of tile NO₃^--N and TN loads by ~1.0-2.6 kg N ha^-1 from January to June (winter and spring), coinciding with critical periods of nutrient loss from agroecosystems in the midwestern US, but increased monthly totals of tile DRP (by 0.4-12.1 g DRP ha^-1) and TP (by 1.2-31.6 g TP ha^-1) loads during some months. We found similar patterns at the annual time scale whereby CC fields had lesser cumulative annual totals of tile NO₃^--N and TN but greater cumulative annual totals of tile DRP and TP. These results show that the influence of cover crops on N loads, but not P, were consistent across temporal scales of examination, demonstrating that cover crops effectively increased N demand and mitigated N losses from agricultural fields. The variable influence of cover crops on P loads underscores the need for greater understanding of the factors and mechanisms that control P loss in systems that include cover crop management. Furthermore, these findings stress the importance of identifying and selecting conservation management practices tailored to the natural resource concern.

The impacts of biofouling on automated phosphorus analysers during long-term deployment

In-situ nutrient analysers are a promising tool for improving the temporal resolution of data and filling knowledge gaps in drivers of harmful algal blooms. There are significant challenges however regarding instrument biofouling and data drift, which remain largely unquantified and unresolved. In this study the effects of biofouling on data consistency and accuracy is quantified on automated wet chemical analysers during long-term monitoring. In 2019 three fractions of phosphorus (P); total phosphorus (TP), total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP), were measured in-situ at four sites in Southern Ontario, Canada. The analysers were exposed to a wide range of P concentrations and biofouling extremes over an 8-month period. They were calibrated using chemical standards both in the field and the lab, and validated with fortnightly grab samples, and the representativeness of real-time data under a range of biofouling conditions were analysed. Results show that analysers biofouling during long-term deployment can desensitize instrument measurements, with greatest impacts on instruments operating in highly turbid environments. Temporal changes in calibration curves suggest that equilibrium P concentrations (EPC₀) of sediments accumulating inside filters can elicit a rapid exchange of dissolved P (SRP, TDP) with the water sample. Data drift increases the further from the EPC₀ an instrument is required to analyse, and thus this study demonstrates that for in-situ P monitoring, unless filters are frequently replaced or renovated, in-situ probes should ideally be dedicated to a specific waterbody type defined by similar EPC₀ values. It is recommended that in order to ensure accuracy in in-situ monitoring of TP, TDP and SRP during long-term deployment, preliminary site trials should be conducted to ascertain sediment EPC₀; the extent of biofouling should be monitored; and/or frequent grab samples taken for post-deployment validation. The findings apply to any in-situ phosphorus monitoring techniques for SRP or TDP.

Temporal Evolution of the Gravitaxis of Euglena gracilis from a Single Cell

Gravitaxis is one of the most important issues in the growth of microalgae in the water column; it determines how easily cells receive sunlight with a comfortable intensity that is below the damaging threshold. We quantitatively investigated and analyzed the gravitaxis and cell multiplication of Euglena gracilis using vertically placed microchambers containing a single cell. A temporal change in gravitaxis and cell multiplication was observed after transferring the cells to fresh culture medium for 9 days. We performed 29 individual experiments with 2.5 mm × 2.5 mm × 0.1 mm square microchambers and found that the cells showed positive, negative, and moderate gravitaxis in 8, 7, and 14 cases, respectively, after transferring to fresh culture medium. A common trend was observed for the temporal change in gravitaxis for the eight initially positive gravitaxis cases. The cells with initially positive gravitaxis showed a higher rate of cell multiplication than those with initially negative gravitaxis. We also discussed the gravitaxis mechanism of E. gracilis from the observed trend of gravitaxis change and swimming traces. In addition, bioconvection in a larger and thicker chamber was investigated at a millimeter scale and visualized.

Runoff and leaching of dissolved phosphorus in streams from a rainfed mixed cropping and grazing catchment under a Mediterranean climate in Australia

Managing phosphorus (P) is a global priority for environmental water quality due to P lost from agricultural land through leaching, runoff and subsurface flow. In Western Australia (WA), following decades of P fertiliser application to crops and pastures in low rainfall regions, questions have been raised about this region's contribution to environmental P loss. This study was conducted on the Fitzgerald River catchment in the south Western Australia (WA) with mixed cropping and grazing land uses and a Mediterranean climate with low mean rainfall (~350 mm yr^-1). Phosphorus forms were monitored continuously over a three-year period in five separate streams, each draining a defined sub-catchment. The P concentrations in streams consistently exceeded Australian and New Zealand Environment Conservation Council (ANZECC) trigger values throughout the monitoring period. Of the measured total P concentration, ~75% was dissolved P (DRP; <0.45 μm) and 80% of that fraction was in the filterable reactive form (FRP). These water quality measurements and other independent soil investigations at this site, suggest that transport of dissolved P rather than erosion of sediment-bound P was dominant in this environment. Based on extractable soil P (Colwell P) and the P buffering index (PBI), predicted concentrations of dissolved reactive P (DRP) in soil solution in topsoils (0-10 cm) across this catchment, generally exceeded ANZECC's values of 0.07 mg PL^-1. The level of exceedance was spatially variable. Streams draining areas with the lowest predicted DRP concentrations also had the lowest measured FRP concentrations. Elsewhere stream water FRP concentrations depended on both DRP concentration and the PBI of the land being drained. Our findings suggest that deployment of practices that physically filter runoff, for example riparian vegetation, would be ineffective in restricting P transport into stream in this environment. This conclusion is consistent with previous findings of the ineffectiveness of riparian buffers on coarse textured sandy soils in higher rainfall areas of southwest WA. A reduction in DRP losses without yield loss could be achieved by following evidence-based fertiliser advice from soil testing to limit losses of legacy P".

Characterizing bioavailable phosphorus concentrations in an agricultural stream during hydrologic and streambed disturbances

In freshwater ecosystems, phosphorus (P) is often considered a growth-limiting nutrient. The use of fertilizers on agricultural fields has led to runoff-driven increases in P availability in streams, and the subsequent eutrophication of downstream ecosystems. Isolated storms and periodic streambed dredging are examples of two common disturbances that contribute dissolved and particulate P to agricultural streams, which can be quantified as soluble reactive P (SRP) using the molybdate-blue method on filtered water samples, or total P (TP) measured using digestions on unfiltered water reflecting all forms of P. While SRP is often considered an approximation of bioavailable P (BAP), research has shown that this is not always the case. Current methods used to estimate BAP do not account for the role of biology (e.g., NaOH extractions) or require specialized platforms (e.g., algal bioassays). Here, in addition to routine analysis of SRP and TP, we used a novel yeast-based bioassay with unfiltered sample water to estimate BAP concentrations during two storms (top 80% and > 95% flow quantiles), and downstream of a reach where management-associated dredging disturbed the streambed. We found that the BAP concentrations were often greater than SRP, suggesting that SRP is not fully representative of P bioavailability. The SRP concentrations were similarly elevated during the two storms, but remained consistently low during streambed disturbance. In contrast, turbidity and TP were elevated during all events. The BAP concentrations were significantly related to turbidity during all disturbance events, but with TP only during storms. The novel yeast assay suggests that BAP export can exceed SRP, particularly when streams are not in equilibrium, such as the rising limb of storms or during active dredging.

One size does not fit all: Toward regional conservation practice guidance to reduce phosphorus loss risk in the Lake Erie watershed

Agricultural phosphorus (P) losses to surface water bodies remain a global eutrophication concern, despite the application of conservation practices on farm fields. Although it is generally agreed upon that the use of multiple conservation practices ("stacking") will lead to greater improvements to water quality, this may not be cost effective to farmers, reducing the likelihood of adoption. At present, wholesale recommendations of conservation practices are given; however, the application of specific conservation practices in certain environments (e.g., no-till with surface application, cover crops) may not be effective and can even lead to unintended consequences. In this paper, we present the Lake Erie watershed as a case study. The Lake Erie watershed contains regions with unique physical geographies that include differences in climate, soil, topography, and land use, which have implications for both P transport from agricultural fields and the efficacy of conservation practices in mitigating P losses. We define major regions within the Lake Erie watershed where common strategies for conservation practice implementation are appropriate, and we propose a five-step plan for bringing regionally tailored, adaptive, and cost-conscious conservation practice into watershed planning. Although this paper is specific to the Lake Erie watershed, our framework can be transferred across broader geographic regions to provide guidance for watershed planning.

Quantifying uncertainty cascading from climate, watershed, and lake models in harmful algal bloom predictions

In response to increased harmful algal blooms (HABs), hypoxia, and nearshore algae growth in Lake Erie, the United States and Canada agreed to phosphorus load reduction targets. While the load targets were guided by an ensemble of models, none of them considered the effects of climate change. Some watershed models developed to guide load reduction strategies have simulated climate effects, but without extending the resulting loads or their uncertainties to HAB projections. In this study, we integrated an ensemble of four climate models, three watershed models, and four HAB models. Nutrient loads and HAB predictions were generated for historical (1985-1999), current (2002-2017), and mid-21st-century (2051-2065) periods. For the current and historical periods, modeled loads and HABs are comparable to observations but exhibit less interannual variability. Our results show that climate impacts on watershed processes are likely to lead to reductions in future loading, assuming land use and watershed management practices are unchanged. This reduction in load should help reduce the magnitude of future HABs, although increases in lake temperature could mitigate that decrease. Using Monte-Carlo analysis to attribute sources of uncertainty from this cascade of models, we show that the uncertainty associated with each model is significant, and that improvements in all three are needed to build confidence in future projections.

Critical Review of Polyphosphate and Polyphosphate Accumulating Organisms for Agricultural Water Quality Management

Despite ongoing management efforts, phosphorus (P) loading from agricultural landscapes continues to impair water quality. Wastewater treatment research has enhanced our knowledge of microbial mechanisms influencing P cycling, especially regarding microbes known as polyphosphate accumulating organisms (PAOs) that store P as polyphosphate (polyP) under oxic conditions and release P under anoxic conditions. However, there is limited application of PAO research to reduce agricultural P loading and improve water quality. Herein, we conducted a meta-analysis to identify articles in Web of Science on polyP and its use by PAOs across five disciplines (i.e., wastewater treatment, terrestrial, freshwater, marine, and agriculture). We also summarized research that provides preliminary support for PAO-mediated P cycling in natural habitats. Terrestrial, freshwater, marine, and agriculture disciplines had fewer polyP and PAO articles compared to wastewater treatment, with agriculture consistently having the least. Most meta-analysis articles did not overlap disciplines. We found preliminary support for PAOs in natural habitats and identified several knowledge gaps and research opportunities. There is an urgent need for interdisciplinary research linking PAOs, polyP, and oxygen availability with existing knowledge of P forms and cycling mechanisms in natural and agricultural environments to improve agricultural P management strategies and achieve water quality goals.

Cyanobacterial bloom phenology in Saginaw Bay from MODIS and a comparative look with western Lake Erie

Saginaw Bay and western Lake Erie basin (WLEB) are eutrophic catchments in the Laurentian Great Lakes that experience annual, summer-time cyanobacterial blooms. Both basins share many features including similar size, shallow depths, and equivalent-sized watersheds. They are geographically close and both basins derive a preponderance of their nutrient supply from a single river. Despite these similarities, the bloom phenology in each basin is quite different. The blooms in Saginaw Bay occur at the same time and place and at the same moderate severity level each year. The WLEB, in contrast, exhibits far greater interannual variability in the timing, location, and severity of the bloom than Saginaw Bay, consistent with greater and more variable phosphorus inputs. Saginaw Bay has bloom biomass that corresponds to relatively mild blooms in WLEB, and also has equivalent phosphorus loads. This result suggests that if inputs of P into the WLEB were reduced to similarly sized loads as Saginaw Bay the most severe blooms would be abated. Above 500 t P input, which occur in WLEB, blooms increase non-linearly indicating any reduction in P-input at the highest inputs levels currently occurring in the WLEB, would yield disproportionately large reductions in cyanobacterial bloom intensity. As the maximum phosphorus loads in Saginaw Bay lie just below this inflection point, shifts in the Saginaw Bay watershed toward greater agriculture uses and less wetlands may substantially increase the risk of more intense cyanobacterial blooms than presently occur.

Evidence for the leaching of dissolved organic phosphorus to depth

Phosphorus (P) can leach from topsoil in inorganic and organic forms. While some evidence has shown inorganic P (orthophosphate) can leach to depth in some soils, less is known of dissolved organic P (DOP). This is not helped by a paucity DOP data for groundwater. We hypothesized that DOP species would leach in greater amounts to depth and at a faster rate through aquifer gravels than orthophosphate. We applied superphosphate with or without dung to a low P-sorption soil under pasture and irrigation. Between 0.7 (control) and 2.4 (dung +superphosphate) kg P ha^-1 was leached through 30 cm with a mean ratio of DRP to DOP of 1.5. At 50 cm, 0.7 and 1.3 kg P ha^-1 was leached with the DRP to DOP ratio decreasing to 1.1 due to greater DOP leaching (or DRP sorption). There was little difference in DRP losses measured at 50 and 150 cm depth. All DOP compounds except the monoester - inositol hexakisphosphate were leached at a faster rate than orthophosphate through aquifer gravels. These data suggest that where low P-sorption soils overlay similarly low P-sorption aquifers, DOP may reach groundwater at a faster rate than orthophosphate. Furthermore, as many DOP species are bioavailable to periphyton, our data suggest that DOP should be included in the assessment of the risk of P contamination of groundwater where connection to baseflow could be a long-term stimulant of periphyton growth.

A Case Study of the Effects of Management Interventions on the Phosphorus Dynamics at a Coastal, Eutrophic, Caribbean lagoon (Laguna Cartagena, Puerto Rico)

Laguna Cartagena is a coastal, eutrophic, shallow lake and freshwater wetland in southwestern Puerto Rico, managed by the US Fish and Wildlife Service. This ecosystem has been impacted by phosphorus loading from adjacent agricultural areas since the 1950s, causing eutrophication and deteriorating wildlife habitats. Herein, we describe phosphorus input and export during September 2010–September 2011 (Phase One) and October 2013–November 2014 (Phase Two). These two phases bracket a period of intensified management interventions including excavation and removal of sediment and vegetation, draining, and burning during the summers of 2012 and 2013. Results indicate that Laguna Cartagena retains a phosphorus (sink) in its sediments, and exhibits nutrient-releasing events (source, mainly total phosphorus) to the lagoon water column, which are associated with rainfall and rising water levels. External factors including water level fluctuations and rainfall influenced phosphorus export during Phase One, but after management interventions (Phase Two), internal processes influenced sink/source dynamics, releasing elevated phosphorus concentrations to the water column. When exposed sediments were re-flooded, phosphorus concentrations to the water column increased, releasing elevated P concentrations downstream to an estuarine wetlands area and the Caribbean Sea. Herein we offer management recommendations to optimize wildlife habitat without elevating phosphorus concentrations.

Nutrient Load Mitigation with Wintertime Cover as Estimated by the INCA Model

Increased nutrient loading causes deterioration of receiving surface waters in areas of intensive agriculture. While nitrate and particulate phosphorus load can be efficiently controlled by reducing tillage frequency and increasing vegetation cover, many field studies have shown simultaneously increased loading of bioavailable phosphorus. In the latest phase of the Rural Programme of EU agri-environmental measures, the highest potential to reduce the nutrient loading to receiving waters were the maximum limits for fertilization of arable crops and retaining plant cover on fields with, e.g., no-till methods and uncultivated nature management fields. Due to the latter two measures, the area of vegetation cover has increased since 1995, suggesting clear effects on nutrient loading in the catchment scale as well. We modeled the effectiveness of agri-environmental measures to reduce phosphorus and nitrogen loads to waters and additionally tested the performance of the dynamic, process-based INCA-P (Integrated Nutrients in Catchments—Phosphorus) model to simulate P dynamics in an agricultural catchment. We concluded that INCA-P was able to simulate both fast (immediate) and slow (non-immediate) processes that influence P loading from catchments. Based on our model simulations, it was also evident that no-till methods had increased bioavailable P load to receiving waters, even though total P and total N loading were reduced.

Less Agricultural Phosphorus Applied in 2019 Led to Less Dissolved Phosphorus Transported to Lake Erie

Extreme precipitation events affect water quantity and quality in various regions of the world. Heavy precipitation in 2019 resulted in a record high area of unplanted agricultural fields in the U.S. and especially in the Maumee River Watershed (MRW). March-July phosphorus (P) loads from the MRW drive harmful algal bloom (HAB) severity in Lake Erie; hence changes in management that influence P export can ultimately affect HAB severity. In this study, we found that the 2019 dissolved reactive P (DRP) load from March-July was 29% lower than predicted, while the particulate P (PP) load was similar to the predicted value. Furthermore, the reduced DRP load resulted in a less severe HAB than predicted based on discharge volume. The 29% reduction in DRP loss in the MRW occurred with a 62% reduction in applied P, emphasizing the strong influence of recently applied P and subsequent incidental P losses on watershed P loading. Other possible contributing factors to this reduced load include lower precipitation intensity, altered tillage practices, and effects of fallow soils, but more data is needed to assess their importance. We recommend conservation practices focusing on P application techniques and timing and improving resiliency against extreme precipitation events.

Influence of Soil and Manure Management Practices on Surface Runoff Phosphorus and Nitrogen Loss in a Corn Silage Production System: A Paired Watershed Approach

Best management practices (BMPs) can mitigate erosion and nutrient runoff. We evaluated runoff losses for silage corn management systems using paired watershed fields in central Wisconsin. A two-year calibration period of fall-applied liquid dairy manure incorporated with chisel plow tillage (FMT) was followed by a three and a half-year treatment period. During the treatment period FMT was continued on one field, and three different systems on the others: (a) fall-applied manure and chisel tillage plus a vegetative buffer strip (BFMT); (b) a fall rye cover crop with spring manure application and chisel tillage (RSMT), both BMPs; a common system (c) fall manure application with spring chisel tillage (FMST). Year-round runoff monitoring included flow, suspended sediment (SS), total phosphorus (TP), dissolved reactive phosphorus (DRP), ammonium (NH4+-N), nitrate, and total nitrogen (TN). Results showed BFMT reduced runoff SS, TP, and TN concentration and load compared to FMT. The RSMT system reduced concentrations of SS, TP, and TN, but not load because of increased runoff. The FMST practice increased TP, DRP, and NH4+-N loads by 39, 376, and 197%, respectively. While BMPs showed mitigation potential for SS, TN, and TP, none controlled DRP, suggesting additional practices may be needed in manured corn silage fields with high runoff potential.