Nitrogen Load Reduction Research Articles

Patterns of microbial abundance and heterotrophic activity along nitrogen and salinity gradients in coastal wetlands

Coastal wetlands are valuable aquatic ecosystems with high biological productivity, which provide services such as a reduction in nitrogen loading into coastal waters and storage of organic carbon acting as carbon dioxide sinks. The predicted rise of sea level or freshwater extractions, particularly in the arid Mediterranean biome, will salinize many coastal wetlands. However, there is considerable uncertainty about how salinization will affect microbial communities and biogeochemical processes. We determined the abundance of total prokaryotes, cyanobacteria, and viruses and quantified the heterotrophic production of prokaryotes sensitive- (predominantly Bacteria) and resistant- (predominantly Archaea) to erythromycin in 112 ponds from nine coastal wetlands. We explored the main drivers of prokaryotic abundance and heterotrophic production using generalized linear models (GLMs). The best GLM, including all the wetlands, indicated that the concentration of total dissolved nitrogen (TDN) positively affected the total abundance of prokaryotes and the heterotrophic erythromycin-resistant (ery-R) production. In contrast, heterotrophic erythromycin-sensitive (ery-S) production was negatively related to TDN. This negative relationship appeared to be mediated by salinity and virus abundance. Heterotrophic ery-S production declined as salinity and virus abundance increased. Consequently, we observed a switch from heterotrophic ery-S production towards ery-R production as salinity and virus abundance increased. Our results imply that microbial activity will change from heterotrophic bacterial-dominated processes to archaeal-dominated processes with anthropogenic nitrogen and salinization increases. However, more studies are required to link the mineralization rates of dissolved nitrogen and organic carbon with specific archaeal taxa to enable more accurate predictions on future scenarios in coastal wetlands.

Economic Impact of Climate Change on the Implementation of Best Management Practices in the Fort Cobb Watershed

AbstractThis study determines the most cost‐effective selection and location of Best Management Practice (BMP) scenarios for farmlands to reduce soil erosion and the delivery of sediment and phosphorus to the Fort Cobb Reservoir watershed under current and future climate scenarios. Using the Soil and Water Assessment Tool, detailed conservation practices (conservation tillage, contour farming, no‐tillage, and terracing) are simulated to determine crop yields, sediment, nitrogen, and phosphorus loss of three crops (winter wheat, irrigated cotton, and pasture) by each land use unit (hydrologic response units) and location in the watershed. An economic model is used to determine the cost‐minimizing choice of BMP scenarios for each land‐use unit that meets 50%, 75%, and 90% reduction of sediment, nitrogen, and phosphorus loads and the impact of climate change on the net return to farmers. Results indicate that the effective spatial placement of BMP scenarios is not the same under different climatic conditions. The average optimal net return to farmers ranges from 170.39 to 316.32 $/acre for all the scenarios. The shadow prices vary from 2.36 to 17.78 $/ton/acre of sediment abated. Although individual BMP scenario is effective in reducing sediment loads, combined BMP scenarios achieve higher performance. This work will help guide policymakers and farmers in decision‐making for the future regarding water quantity used and water quality downstream in the recreation area of the Fort Cobb Reservoir.

Nitrogen flow through waste management systems in an academic campus

In the current study, N flows are modelled for an academic campus and the major flows are identified to examine alternate management scenarios and discover environmental degradation areas. Substance Flow Analysis (SFA) is carried out using STAN software to quantify N flows. The primary sources of information were the official records, literature, and interviews with officials and staff. After modelling the present scenario, futuristic scenarios were created and modelled to investigate the potential of reducing nitrogen loadings to the environment. The alternative scenario involving onsite urine collection and treatmenthas the potential to reduce the nitrogen loadings to the environment as compared to the scenario with nutrient recovery from food waste.

Evaluation of nitrogen loading in the last 80 years in an urbanized Asian coastal catchment through the reconstruction of severe contamination period

Most semi-enclosed seas have experienced severe eutrophication owing to high nutrient loading from rivers during rapid population growth periods. In Japan, the coastal areas of some megacities (e.g. Tokyo and Osaka) experienced considerable economic growth during the 1960s–1970s. Therefore, determining the amount of nutrient loading during this period is essential to undertake measures for the conservation of coastal environments. However, determining the nutrient loading that occurred several decades ago is generally difficult owing to lacking water quality records. In this study, the nitrogen loading in the Yamato River catchment, an urbanized coastal catchment in Asia, for 80 years from the 1940s to the 2010s is reconstructed using the Soil and Water Assessment Tool. We considered factors such as population growth, wastewater treatment plant (WWTP) construction, and changes in land and fertilizer usage in different urbanization stages. Results show that the total nitrogen loading in the catchment peaked in the 1970s at 6616 tons yr−1 owing to untreated wastewater discharge and rapid increase in population growth. By reducing 57% of the nitrogen loading in the 2010s from the catchment, WWTPs have been instrumental in improving the water environment. The decrease in and integration of agricultural land has reduced nitrogen loading attributed to nonpoint sources; however, this reduction was not obvious because of the high fertilizer usage before the 2000s. Overall, the findings of this study provide a comprehensive understanding of the impact of rapid urbanization in an Asian coastal catchment on nitrogen loading during the high economic growth period in the past. This study will be useful for the long-term assessment of nutrient loading in other.

Targeted regulation of nitrogen loads: A national, cross-sectoral analysis

In Denmark, eutrophication of coastal areas is one of the major challenges in meeting the requirements from the European Water Framework Directive (WFD). This eutrophication is mainly a cause of excessive nitrogen loads from agricultural production, which can be reduced by implementing abatement measures. The cost-effectiveness of a land use policy may vary depending on the abatement measures used and where they are implemented. Taking account of the spatial heterogeneity of costs and effects in the choice of abatement measures significantly reduces the cost of meeting WFD targets. Targeting of regulation to identify least cost options to reduce nitrogen has for many years focused on agricultural abatement measures. However, the marginal costs of reductions in agriculture have now increased to an extent, where it has become relevant also to reconsider other sectors. We set out to compare nitrogen abatement measures across agriculture and waste water treatment at a national level, acknowledging heterogeneity in marginal costs across spatial location as well as differences in reduction requirements across catchments. In the analysis, we combine data on costs and effects of abatement measures at a fine spatial scale to estimate marginal costs within the two sectors at a national level for Denmark. The model minimizes the costs of meeting the specific required nitrogen load reduction targets for all catchments and finds the composition and spatial location of the optimal abatement effort. We find that waste water treatment abatement measures are only relevant in two coastal catchments, where they mainly serve as a supplement due to insufficient potential for agricultural land in rotation to provide all the N load reductions. Furthermore, we find that the pressure on agricultural land to reduce nutrient loads is very high in some catchments, implying that abatement measures such as land retirement where abatement costs are high enters the optimal solution due to their high level of effectiveness. Recommendations and average costs vary across catchments, indicating that results are not easily translated to generic national-level policy design.

Gains or losses? A quantitative estimation of environmental and economic effects of an ecological compensation policy.

Ecological compensation is an innovative and effective tool to explore the coordinated development of socioeconomic prosperity and ecological protection, especially for a watershed crossing different regions. It converts the externalities of ecosystem services into practical financial incentives for local stakeholders. This empirical study applies a quantitative policy evaluation approach to evaluate the environmental and economic effects of an ecological compensation policy, using the paddy land-to-dry land (PLDL) program implemented in China's Miyun Reservoir watershed as an example. The study is based on responses to a 2017 questionnaire regarding agricultural production inputs and outputs administered to 269 households in Hebei Province, where the PLDL program has been operational for over 10 yr. The results show that the program has reduced nitrogen usage by 24% on average in 2017 and decreased the total nitrogen emission load by 16.98 tons for the entire case area, which accounts for approximately 18.6% of the total nitrogen load reduction of the Miyun Reservoir basin. However, the upstream households involved in this program have experienced agricultural income losses higher than that allowed for by the current compensation criterion. Therefore, this paper discusses the factors that should be considered in the process of determining ecological compensation criteria. In particular, the paper proposes a differential compensation scheme based on the environmental effect at the individual level to avoid a standard payment for all households irrespective of their different contributions. This differential compensation payment scheme facilitates the fair treatment of environmental contributors and maximizes environmental benefits through an equitable allocation of limited ecological compensation funds. This study serves as a theoretical and practical reference for further improvement of the current ecological compensation policy in China. The study also sheds light on practices for estimating ecological compensation criteria and formulating ecological compensation policies for other regions or countries in the future.

Modelling the effects of changing loads of nitrogen discharged from a sewage treatment plant on plankton dynamics in a large, urbanised bay

The input of nutrients to the coastal environment is essential to support biological productivity but an excess of nutrients can lead to eutrophication. Here we use a pelagic ecosystem model to investigate the effects of nutrient inputs from a sewage treatment plant on the plankton dynamics of Port Phillip Bay, southern Australia. The effects of a 50% increase or decrease in nitrogen concentration in the Western Treatment Plant (WTP) outfall was examined relative to the 2016–17 base case. Increasing the nitrogen concentration in the WTP discharge had only a small effect on the summer diatom bloom. This can be explained by the fact that summer diatom blooms are typically silicate rather than nitrogen limited and WTP discharges are dominated more by N than Si compared with flows from the catchment. Consequently, increasing N had a relatively greater effect on flagellates, that in turn led to an increase in abundance of zooplankton (Paracalanus) at the beginning of the spawning window for snapper, Chrysophrys auratus. Paracalanus are the preferred prey for snapper larvae and the abundance of this zooplankter can strongly influence the larval survival and recruitment of snapper. Decreasing the nitrogen load from the WTP led to a marked reduction in the planktonic production in the bay, reflecting the potential effects of factors such as drought that can lead to a reduction in nitrogen load. Overall, the effect of changing nutrient load was sensitive to differences in relative nutrient concentrations from the treatment plant versus catchment sources.

Do cost-share programs increase cover crop use? Empirical evidence from Iowa

AbstractCover crops can generate both on-farm and water-quality benefits. However, their use in Iowa remains subdued, partly due to implementation costs faced by farmers. We tested the hypothesis that monetary incentives through cost-share programs are effective at increasing the area of farmland planted to cover crops in Iowa, as opposed to the alternative in which the participants of cost-share programs would have planted the same cover-crop acreage in the absence of payment. We found that cost-share payments induced a 15 percentage-point expansion in cover-crop acreage beyond what would have been planted in the absence of payment, among farmers who participated in cost-share programs. The estimated additionality rate was 54%, suggesting at least half of cost-share expenditures funded cover-crop acreage that would not have been planted without payment. Furthermore, we estimated the public cost to reduce nitrogen loads to Iowa waterways via cover crop, beyond what would have occurred in the absence of cost-share programs, to be $1.72–$4.70 lb−1N ($3.79–$10.36 kg−1N). Farmers absorbed about 70% of those costs as private losses, and cost-share payments offset the remaining 30%. Although the additionality rate estimated in this study is less than what has been found in other states, the cost-share programs in Iowa have been relatively cost-effective, due to their lower payment rate.

Case study for analyzing nutrient-management technologies at three scales within a sewershed

ABSTRACT Centralized wastewater treatment plants remove nutrients prior to discharge. Recently, technologies have become available to manage nutrients at different scales (individual buildings, neighborhoods/communities, or large centralized plants). Nutrient-recovery technologies applied upstream of the centralized treatment plant may affect both the wastewater flow rates and the nutrient fluxes reaching the plant. This study evaluates how the introduction of such upstream technologies affects the treatment efficiency and economics of nutrient management across a sewershed. Employing treatment technologies upstream from the centralized plant that receives highly loaded municipal wastewater could reduce nitrogen loading to the mainstream treatment train by over 50%, which generally reduces treatment plant effluent concentrations and saves money. In extreme cases, excessive upstream removal of nutrients can negatively affect biological treatment processes at the centralized treatment plant. Treatment of sidestreams at the centralized plant would likely be effective in reducing phosphorus loading, but not nitrogen loading, to the mainstream process.

Alternative Management Systems of Beef Cattle Manure for Reducing Nitrogen Loadings: A Case-Study Approach.

Simple SummaryLivestock manure is one of the primary sources of agricultural nonpoint source pollution and poses a great threat to the environment and human health. Sustainable management of manure via recycling is an effective means to tackle the problem. Based on field interviews in China, four alternative manure management systems were investigated: Compost-based systems, product-based systems, substrate-based systems, and biogas-based systems. For each system, the reasons of emergence, success factors, risk factors, operation mechanism, scalability, key elements, and environmental effects were discussed. Results showed that the adoption of a system is driven by various factors and market-oriented operation is the dominant operation mechanism of all the manure management systems. Compared to direct application of manure to croplands, all the manure management systems can reduce nitrogen loadings from livestock farms and lower their environmental effects. Specifically, biogas-based systems can reduce nitrogen loadings to the greatest extent, followed by product-based systems and substrate-based systems, and then by compost-based systems. Integrated management of manure with mixed recycling systems is imperative for reducing its environmental effects, which can benefit from the increasing role of third-party entities in manure recycling. Policy implications were also discussed.Livestock manure is one of the main sources of agricultural nonpoint source pollution and poses a great threat to the environment and human health. Sustainable management of manure via recycling is an effective means to tackle the problem. Based on field interviews in China, multiple case studies were employed to investigate alternative manure management systems. Four conclusions arose. First, compost-based systems, product-based systems, substrate-based systems, and biogas-based systems were identified as four main types of manure management systems, with each possessing its success factors and risk factors. The adoption of a system was driven by various factors. Second, market-oriented operation was the dominant operation mechanism of all the manure management systems. Third, compared to direct application of manure to croplands, all the four manure management systems could reduce nitrogen loadings from livestock farms and lower their environmental effects. Among the systems, biogas-based systems could reduce nitrogen loadings to the greatest extent, followed by product-based systems and substrate-based systems, and then by compost-based systems. Lastly, integrated management of manure with mixed recycling systems is imperative for reducing its environmental effects, which can benefit from the increasing role of third-party entities in manure recycling. Policy implications were also discussed.

Successful mitigation of stormwater-driven nutrient, fecal bacteria and suspended solids loading in a recreational beach community

Marine and estuarine waterways adjacent to urban areas are often the final recipient of polluted stormwater runoff. Microbial degradation of coastal water quality is a direct threat to human health through fecal contamination of bathing waters and shellfish, as well as distressing local economies through the loss of waterways to commercial (shellfishing) and recreational use. In coastal waters reduction of nitrogen loading is a key strategy for prevention of noxious and toxic algal blooms. Best management practices (BMPs) can be successful tools for mitigating such pollutants in runoff, but BMPs must be tailored to individual situations for maximum effectiveness. This study examines the efficacy of a set of BMPs installed in the coastal resort Town of Wrightsville Beach, North Carolina, USA. The BMPs targeted the highly-impervious (90%+) drainage area of two stormwater outfall pipes emptying into recreationally used Banks Channel. Mitigation measures included replacement of impervious pavement with pervious concrete and construction of an infiltration chamber in the parking lot of a local recreational seaside club. Significant reductions were achieved in total stormwater discharge (62%), as well as loading of the fecal indicator bacteria Enterococcus (76%) and total nitrogen (TN – 87% decrease). Additionally, there were reductions in loading of total phosphorous (TP) and total suspended solids (TSS) to estuarine waters following BMP installment. The set of BMPs applied here have wide management applicability to coastal ecosystems, as well as freshwater riparian areas characterized by sandy, porous soils.

Reducing Hypoxia in an Urban Estuary Despite Climate Warming.

Seasonal hypoxia is a serious threat to coastal ecosystems. This study on hypoxia in Long Island Sound (LIS), a large urbanized estuary, focuses on responses to managed nitrogen load reductions and climate change. At the analyzed station in western LIS, warming in bottom waters (0.8 °C per decade) favors hypoxia. Total nitrogen concentrations have decreased (0.06 mg L-1 per decade) with load reductions, but no linear temporal trend in chlorophyll is discernible. Bottom dissolved oxygen has increased (0.48 mg L-1 per decade), despite warming-induced solubility decreases (0.13 mg L-1 per decade). Decreasing trends in hypoxic area and volume (100 km2 and 1 km3 per decade) reflect improved conditions and are coincident with reducing loads. Regressions link hypoxic extent to nitrogen loads, chlorophyll, salinity, and winds. Though mitigation has reduced hypoxia, these improvements will not be sustained in the warming climate without continued intervention. The warming-induced oxygen solubility decrease forecasted for 2099 (0.4 mg L-1) would erode 35% of the observed oxygen gains. Implementing a nitrogen load reduction of 1.2 × 106 kg year-1 before the century's end would offset the oxygen solubility decline. This overall approach is applicable to areas experiencing warming and continued development that complicate efforts to reign in hypoxia.

Reduction of nitrogen and phosphorus loading from polluted sediment by electrolysis

The objective of the research presented here was to determine the effectiveness of applying electrolysis in polluted sediment to enhance ammonia nitrogen (NH3−N) and phosphate phosphorous (PO43−-P) removal from sediment and overlying water. A magnesium‑aluminum (MgAl) alloy was used as the anode, and a titanium (Ti) mesh served as the cathode with a low-current of 40 mA (5 V) in polluted river sediment and overlying water. The total nitrogen (TN), NH3-N and nitrate nitrogen (NO3−-N) were mainly removed through the electro-oxidation and electro-reduction process and the highly pH and DO value caused by electrolytic reaction which could also enhance the removal of NH3-N through nitrifying bacteria. There was significant difference (p = 0.021) during the removal between electrolyzed sediment and sediment without electrolysis control. Approximately 253.47 ± 1.82 mg TN were removed using an electrolysis treatment on sediment since the initial TN concentration was about 1312 ± 104.37 mg/kg in the polluted sediment and about 14.91 mg TN per day was removed which was 4.62 times greater than that removed by non-electrolysis controls. As for P in the sediment, the results showed that the Mg-Al alloy anode mostly released Mg2+ ions to promote the removal of PO43−-P in the sediment and overlying water; meanwhile, the electrodeposition of PO43−-P on the Ti mesh cathode was used to enhance PO43−-P removal and totally about 106.65 ± 0.23 mg TP was removed since the TP concentration was 607.68 ± 32.27 mg/kg. Notably, electrolysis promoted the transformation of sodium hydroxide-P (NaOH-P) fractions to ammonia chloride-P (NH4Cl-P), which improved the bioavailability of P. Through energy dispersive spectrometry (EDS) and Fourier transform infrared (FTIR) analysis, the precipitates of the Ti mesh cathode and the precipitates produced between the sediment and overlying water both contain Mg, Ca, Al and P. Electrolysis had little effect on the removal of organic carbon in the sediment (p = 0.64). Taken as a whole, this study suggested that electrolysis-driven N and P removal could be a sustainable technology for the management of polluted river sediments.

Biogeochemical evidence of anaerobic methane oxidation and anaerobic ammonium oxidation in a stratified lake using stable isotopes

Abstract. Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The n-damo (nitrite-dependent anaerobic methane oxidation), nitrate-dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) represent two microbially mediated processes that can reduce nitrogen loading of aquatic ecosystems and associated methane emissions to the atmosphere. Here, we report vertical concentration and stable-isotope profiles of CH4, NO3-, NO2-, and NH4+ in the water column of Fohnsee (lake in southern Bavaria, Germany) that may indicate linkages between denitrification, anaerobic oxidation of methane (AOM), and anammox. At a water depth from 12 to 20 m, a methane–nitrate transition zone (NMTZ) was observed, where δ13C values of methane and δ15N and δ18O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with the results of a simple 1-D diffusion model linked with a degradation term, show that the nonlinear methane concentration profile cannot be explained by diffusion and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most parsimonious explanation for these data trends. In the methane zone at the bottom of the NMTZ (20 to 22 m) δ15N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occurring together with denitrification. The conversion of nitrite to N2 and nitrate during anammox is associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δδ15N) of 26 ‰ between δ15N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in the NMTZ at a water depth between 16 and 18 m. The associated methane concentration and stable-isotope profiles indicate that some of the denitrification may be coupled to AOM, an observation supported by an increased concentration of bacteria known to be involved in n-damo/denitrification with AOM (NC10 and Crenothrix) and anammox (“Candidatus Anammoximicrobium”) whose concentrations were highest in the methane and ammonium oxidation zones, respectively. This study shows the potential for a coupling of microbially mediated nitrate-dependent methane oxidation with anammox in stratified freshwater ecosystems, which may be important for affecting both methane emissions and nitrogen concentrations in lakes.

Toxic cyanobacteria and microcystin dynamics in a tropical reservoir: assessing the influence of environmental variables.

Toxic cyanobacterial blooms (TCBs) have become a growing concern worldwide. The present study investigated the dynamic of toxic cyanobacteria and microcystin (MC) concentrations in the Tri An Reservoir (TAR), a tropical system in Vietnam, with quantitative real-time polymerase chain reaction (qPCR) and high-performance liquid chromatography (HPLC), respectively. The results of the qPCR quantification revealed that Microcystis was the dominant group and the primary MC producer in the TAR. Potentially toxigenic cyanobacteria varied from 1.2 × 104 to 1.58 × 107 cells/mL, and the mean proportion of toxic Microcystis to that of the total toxic cyanobacteria varied from 21 to 88%. Microcystin concentrations in raw water and sediment samples often peaked during June to October as blooms occurred and varied from 0.27 to 6.59μg/L and from 1.79 to 544.9ng/g in wet weight, respectively. The results of this study indicated that conditions favoring Microcystis proliferation lead to the selection of more toxic genotypes. Water temperature and light availability were not driving factor in the formation of TCBs in the TAR. However, the high loads of total nitrogen (TN), phosphate, and total phosphorus (TP) into the water via rainfall runoff in combination with a high total suspended solid (TSS) and decreased water level during the early months of the rainy seasons did lead to a shift in Microcystis blooms and higher proportions of toxic genotypes of Microcystis in the TAR. This research may provide more insight into the occurrence mechanism of TCBs in tropical waters. The strategy to control TCB problems in tropical regions should be focused on these limnological and hydrological parameters, in addition to a reduction in nitrogen and phosphorus loading.

The potential contribution of oyster management to water quality goals in the Chesapeake Bay

Excess nutrients have led to eutrophication of the Chesapeake Bay, USA. It has been suggested that oyster restoration can play an important role in achieving water quality goals in the Bay. An optimal control bioeconomic model is applied to the management of oysters in the Chesapeake Bay, taking into account nutrient removal by the oysters. Optimal management of oyster harvests in the Bay reduces the cost of attaining water quality goals by 4–6% relative to an open access fishery. A “naïve” management optimization that maximizes discounted net revenues from oyster harvests but that does not take into account their nitrogen impacts performs almost as well as the fully optimal solution. Sensitivity analyses show that the optimal oyster harvest depends on the cost of reducing nitrogen loadings from point and nonpoint sources through best management practices. Further, denitrification by living oysters is a much more important process than nutrient removal through harvest.

Biomass Production with Conservation Practices for Two Iowa Watersheds

AbstractHydrologic modeling was used to estimate potential changes in nutrients, suspended sediment, and streamflow in various biomass production scenarios with conservation practices under different landscape designs. Two major corn and soybean croplands were selected for study: the South Fork of the Iowa River watershed and the headwater of the Raccoon River watershed. A physically based model, the Soil and Water Assessment Tool, was used to simulate hydrology and water quality under different scenarios with conservation practices and biomass production. Scenarios are based on conservation practices and biomass production; riparian buffer (RB), saturated buffer, and grassed waterways; various stover harvest rates of 30%, 45%, and 70% with and without winter cover crops; and conversion of marginal land to switchgrass. Conservation practices and landscape design with different biomass feedstocks were shown to significantly improve water quality while supporting sustainable biomass production. Model results for nitrogen, phosphorus, and suspended sediments were analyzed temporally at spatial scales that varied from hydrologic response units to the entire watershed. With conservation practices, water quality could potentially improve by reducing nitrogen loads by up to 20%–30% (stover harvest with cover crop), phosphorus loads by 20%–40% (RB), and sediment loads by 30%–70% (stover harvest with cover crop and RB).

Growth Rates for Quahogs (Mercenaria mercenaria) in a Reduced Nitrogen Environment in Narragansett Bay, RI

Nitrogen is essential in sustaining estuarine ecosystems, particularly lower trophic levels and filter-feeding organisms. However, excess nitrogen is an often-cited cause of coastal ecosystem decline. One of the largest sources of nitrogen in Narragansett Bay, RI, is effluent from wastewater treatment facilities discharging into the head of the Bay. Beginning in 2005, nitrogen reductions at wastewater treatment facilities were implemented to minimize eutrophication and improve water quality. These reductions in nitrogen have led to speculation as to how the Narragansett Bay ecosystem will respond. The filter-feeding clam Mercenaria mercenaria (Quahog) is a prominent component of the Narragansett Bay ecosystem and an economically important resource in Rhode Island. In the northeastern United States, studies of several estuaries have linked nitrogen increases with increased phytoplankton concentrations and faster growth of Quahogs. We evaluated Quahog growth rates to assess how this species has responded to nutrient reductions in the bay. In 2016 and 2017, we collected and aged Quahogs from 6 sites in Narragansett Bay along a gradient of historical nitrogen loading and primary production. We found that Quahog growth rates still vary through space, as previously documented, but despite the recent 50% reduction in nitrogen loading to Narragansett Bay, rates were not significantly different from rates reported for Quahogs collected from 1984 and 2005. Our findings suggest that the reduced nitrogen has not had adverse impacts to Quahog growth, and this information can be used to inform the population's dynamics. However, ecosystem responses to the nutrient reductions are ongoing and the ecology of Quahogs should continue to be monitored to ensure the sustainability of this highly valued commercial and recreational fishery in Narragansett Bay.

Discrimination of Nitrogen Sources in Karst Spring Contributing Areas Using a Bayesian Isotope Mixing Model and Wastewater Tracers (Florida, USA)

ABSTRACT Many springs in Florida have experienced a proliferation of nuisance algae and alteration of trophic structure in response to increases in nitrate concentration concurrent with rapid population growth and land use intensification beginning in the mid-20th century. While loading targets and remediation plans have been developed by state agencies to address excess nitrogen inputs, further confirmation of the relative contribution of nitrogen sources to groundwater is necessary to optimize the use of resources when implementing projects to reduce nitrogen loads. In the present study, stable isotopes of nitrate and wastewater indicators were used to discriminate sources of nitrogen in wells and springs in central Florida. Sampling was performed in 50 wells at 38 sites and at 10 springs with varying levels of agriculture and urban development. Nitrate isotope values were used to develop Bayesian mixing models to estimate the probability distribution of the contributions of nitrate sources in wells. Prior probabilities for the fractional contribution of each source were adjusted based on land use and density of septic tanks. Sucralose and the Cl:Br mass ratio were used as confirmatory indicators of wastewater sources. In residential areas, mixing model results indicated that fertilizer or mixed fertilizer and wastewater (septic tank effluent and reuse water) were the primary sources, with sucralose detections corresponding to wells with elevated contributions from wastewater. Sources of nitrogen in pasture and field crop areas were primarily fertilizer and manure; however, model posterior distributions of δ15N indicated that manure sources may have been overpredicted. The present study demonstrates the utility of a multi-tracer approach to build multiple lines of evidence to develop locally relevant remediation strategies for nitrogen sources in groundwater.

Tracking denitrification in green stormwater infrastructure with dual nitrate stable isotopes

Strategies to mitigate watershed nitrogen export are critical in managing water resources. Green infrastructure (GI) has shown the ability to remove nitrogen from stormwater, but the removal mechanism is unclear. Denitrification removes nitrate from water permanently, making it the most desirable removal mechanism. The year-round field performance of a roadside infiltration GI practice (bioretention) in Northern Virginia was monitored to investigate the transport of nitrogen and the occurrence and contribution of denitrification. Stormwater runoff volumes, nitrogen concentrations, and nitrate isotope ratios (δ15N-NO3− and δ18O-NO3−) were measured at the inlet and outlet of the bioretention during 24 storm events over 14 months. Nitrate concentration reductions (inlet vs. outlet) displayed seasonal trends, with higher reductions happening during warmer events and lower reductions or increases occurring during colder events. Cumulative bioretention nitrate and total dissolved nitrogen load reductions were 73% and 70%, respectively. Two out of 24 monitored events displayed denitrification isotope trends, indicating that although bioretention has denitrification potential, it is infrequent and other nitrogen removal mechanisms (i.e. infiltration and plant uptake) are primarily responsible for nitrogen surface effluent reductions. Only approximately 1.4% of the total reduced nitrate surface effluent load over the monitoring period was attributable to denitrification. Denitrification occurred during two of the largest monitored events, suggesting increased hydraulic retention time (HRT) promotes denitrification. Future GI designs should consider increasing HRT to encourage the important ecosystem service denitrification provides.