Dissolved Inorganic Nitrogen Load Research Articles

The bioavailability of particulate nitrogen in eroded sediment: Catchment sources and processes

PurposeAnthropogenic land use change has caused an increase in particulate nutrient loads from catchments draining to the Great Barrier Reef (GBR). The research in GBR catchments has indicated that particulate nutrients are bioavailable to both freshwater and marine phytoplankton, but relative importance of this source of nutrients to the GBR is unknown. We quantified the contribution of this source of bioavailable nitrogen in a dry-tropics grazing and a wet-tropics fertilized mixed land use catchment of the GBR.Materials and methodsThe different bioavailable nitrogen pools and associated processes through which dissolved inorganic nitrogen (DIN) is generated from eroded sediment (mass of DIN generated per mass of sediment) were identified. These pools and processes were quantified from a range of representative sediment sources (e.g. surface and subsurface soil and different land uses). We collected 17 sediment source samples in the wet tropics and 41 in the dry tropics. We combined the N pool concentration data with spatial and hydrological fine sediment modelling to estimate the contribution from different sources and processes/pools to the end-of-catchment DIN load.Results and discussionThe modelled load of DIN generated from sediment accounted for all the monitored DIN load in the grazing-dominated catchment but was insignificant in the fertilized mixed land use catchment. Sediment from surface erosion (hillslope erosion) and some soil types contributed disproportionally to the modelled DIN generation. Fast solubilisation of DIN was the main process in the catchments studied. The importance of mineralisation of the organic fraction increased with the time the sediment was in suspension.ConclusionParticulate nutrients in sediment are a significant source of bioavailable nitrogen in eroding grazing catchments. The processes that drive this bioavailability are complex, vary with sediment source and operate at different timeframes and spatial scales.

Groundwater nutrient loading into the northern Indian River lagoon: measurements and modeling

The Indian River Lagoon System (IRLS) has been impacted by the surrounding development, leading to excessive nutrient loads that have resulted in frequent and prolonged phytoplankton blooms in the northern reaches. Our study focused on estimating terrestrial groundwater discharge (TGD) and associated nutrient loads by combining field measurements and hydrogeologic modeling at four transects: Eau Gallie (EGT), River Walk (RWT), Banana River (BRT), and Mosquito Lagoon (MLT) across the IRLS. Multiple monitoring stations were installed to collect groundwater and surface water levels, salinity, and nutrient concentrations during 2014-2015. Samples were analyzed for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). Numerical modeling was accomplished using SEAWAT to simulate TGD rates, whereas nutrient loads were calculated by multiplying simulated TGD by measured concentrations. TGD rates and nutrient loads were also estimated specifically for the “near-shore zone” along each transect. The effect of recharge from underlying Hawthorn Formation was also evaluated by incorporating estimated recharge rates into the models. Porewater and lagoon water samples showed that ammonium predominated over (NO2+NO3) and PO4 at all sites, resulting in DIN/DIP ratio surpassing the Redfield ratio. Low nitrite/nitrate, coupled with elevated ammonium concentrations at RWT, BRT, and MLT, may be attributed to biogeochemical transformations catalyzed by mangroves and wetlands. Simulated TGD showed mild temporal but significant spatial variation, especially between EGT and RWT compared to BRT and MLT. The highest average TGD of 0.73 and 0.77 m3/d.m occurred at RWT and EGT, respectively, whereas the lowest rates were predicted at BRT and MLT. The highest estimated average DIN loads of 507 and 428 g/yr.m were received at EGT and RWT, respectively, whereas MLT and BRT exhibited lower loads. The DIP loads were remarkably lower than the DIN loads and were significantly different in space and time between sites. Elevated DIN combined with reduced DIP resulted in DIN/DIP exceeding the Redfield ratio, thereby encouraging the blooming of harmful algae. Although the majority of seepage occurs through the near-shore zone, small amounts are received along the entire transect at all sites. The Hawthorn Formation does not contribute significant recharge to the aquifer at the transect locations.

The Amazon shelf sediments, a reactor that fuels intense nitrogen cycling at the seabed

AbstractLarge river plumes such as the Amazon River Plume are known as regions of intense biological, chemical, and physical forcing affecting the delivery and speciation of nutrients. On the shelf, sediments may act as a net source or sink of dissolved inorganic nitrogen (DIN) for the water column depending on the preservation of organic matter (OM) and its remineralization pathways in benthic environments. Previous studies on sediments of the Amazon shelf suggested that the seabed mainly acted as a sink of DIN, with denitrification as the main loss process. However, the Amazon Basin faced drastic changes over the past decades, with climate change, deforestation, and damming potentially impacting the DIN and OM loads to the estuary. These perturbations might have led to changes in benthic remineralization, calling for updates of previously studied benthic remineralization processes on the Amazon shelf. Our analysis of eight short cores confirmed the occurrence of intense recycling of mostly terrestrial material (> 60%) in the river mouth, and little organic carbon was remaining in shelf sediments. We measured a sediment oxygen demand of ~ 23 mmol m−2 d−1, which can explain the ammonification and nitrification of all the organic nitrogen (N) reaching the sediment, underlining the importance of the seabed as a sink for organic N species. Therefore, in spite of the changes in the Amazon Basin, nitrification still seems to be balanced by denitrification, as, similarly to previous results, no DIN diffused out of the sediment.

Co-occurrence patterns and environmental factors associated with rapid onset of Microcystis aeruginosa bloom in a tropical coastal lagoon

The environmental factors contributing to the Microcystis aeruginosa bloom (hereafter referred to as Microcystis bloom) are still debatable as they vary with season and geographic settings. We examined the environmental factors that triggered Microcystis bloom outbreak in India's largest brackish water coastal lagoon, Chilika. The warmer water temperature (25.31–32.48 °C), higher dissolved inorganic nitrogen (DIN) loading (10.15–13.53 μmol L−1), strong P−limitation (N:P ratio 138.47–246.86), higher water transparency (46.62–73.38 cm), and low-salinity (5.45–9.15) exerted a strong positive influence on blooming process. During the bloom outbreak, M. aeruginosa proliferated, replaced diatoms, and constituted 70–88% of the total phytoplankton population. The abundances of M. aeruginosa increased from 0.89 × 104 cells L−1 in September to 1.85 × 104 cells L−1 in November and reduced drastically during bloom collapse (6.22 × 103 cells L−1) by the late November of year 2017. The decrease in M. aeruginosa during bloom collapse was associated with a decline in DIN loading (2.97 μmol L−1) and N:P ratio (73.95). Sentinel-3 OLCI-based satellite monitoring corroborated the field observations showing Cyanophyta Index (CI) > 0.01 in September, indicative of intense bloom and CI < 0.0001 during late November, suggesting bloom collapse. The presence of M. aeruginosa altered the phytoplankton community composition. Furthermore, co-occurrence network indicated that bloom resulted in a less stable community with low diversity, inter-connectedness, and prominence of a negative association between phytoplankton taxa. Variance partitioning analysis revealed that TSM (16.63%), salinity (6.99%), DIN (5.21%), and transparency (5.15%) were the most influential environmental factors controlling the phytoplankton composition. This study provides new insight into the phytoplankton co-occurrences and combination of environmental factors triggering the rapid onset of Microcystis bloom and influencing the phytoplankton composition dynamics of a large coastal lagoon. These findings would be valuable for future bloom forecast modeling and aid in the management of the lagoon.

Nutrients across Time: Relationships with Climate, Hydrology, and Land Use in Four Rivers of the Pacific Northwest

AbstractUnderstanding the temporal dynamics and drivers of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) provides a critical link to better management of nutrient‐related impacts such as eutrophication and harmful algal blooms (HABs). DIN and DIP are a primary control on persistent eutrophication and HABs in the Pacific Northwest (PNW). An understudied phenomenon, this paper examines multi‐decadal trends in DIN and DIP concentrations and loads, and their relationships to climatic and hydrologic factors (e.g., stream and air temperature, discharge, precipitation) in the PNW. Dissolved constituents act as a broad sentinel of linkages between watershed and in‐stream mechanisms such as nitrification, denitrification, nutrient use efficiency, evapotranspiration, hydrologic connectivity, groundwater extraction, irrigation, and land uses. As opposed to the total N and P often used in individual, autochthonous, lentic systems, DIN and DIP are used here as measures of multiscaled processes in allochthonous lotic systems with diverse flow paths. Time‐series data from public agencies were used for up to 20 years in river outlets from the Willamette, Salmon, Spokane, and Yakima watersheds. Seasonal Mann Kendall (SMK) tests suggest significant decreasing multi‐decadal trends in DIN and DIP loads for three out of four watersheds (for DIN, SMK = −0.104; for DIP, SMK = −0.081, −0.181, and −0.213), significant decreasing trends in DIN concentrations for one of the four watersheds (SMK = −0.144), and significant decreasing trends in DIP concentrations in three of the four watersheds (SMK = −0.120, −0.135, and −0.157). Multivariate regressions found significant relationships for concentrations, loads, and ratios when regressed against stream and air temperatures, precipitation, and discharge (16 significant regressions, with adjusted R2 values between 0.016 and 0.65). Highlights of these regression results are as follows: (1) precipitation, discharge, and water and air temperatures help to explain DIN and DIP concentrations and loads, (2) changes in DIN concentrations are sensitive to more hydroclimatic variables than DIP concentrations, and (3) DIP concentrations are positively correlated with stream temperature while DIP loads are negatively correlated with stream temperature. Furthermore, seasonal changes in nutrients — and their potential to alter aquatic productivity during a year — has received little attention in the literature. Regressions established significant seasonality or monthly variation of DIN and DIP concentrations and loads in all four watersheds (20 significant regressions, with adjusted R2 values between 0.038–0.65). Nutrient thresholds of DIN (0.3–0.5 mg/L) and DIP (0.05–0.005 mg/L) concentrations were used to analyze N‐ and P‐limitation. P‐limitation is known to occur in lakes, and N‐limitation is known to occur in rivers. Surprisingly, except for one watershed (Salmon), nutrient concentrations for both DIN and DIP in all watersheds were shown to be above the limitation thresholds across multiple seasons. In certain situations, such as where significant decreasing trends continue, the DIN:DIP ratio suggests seasonal switching between N‐ and P‐limited could create ideal conditions for HABs. The findings of this study have important implications for water resource management issues such as agriculture, land use development, fish populations, timber harvests, water quality, and public health.

Attributing Controlling Factors of Acidification and Hypoxia in a Deep, Nutrient-Enriched Estuarine Embayment

Measuring and attributing controlling factors of acidification and hypoxia are essential for management of coastal ecosystems affected by those stressors. We address this using surveys in the Firth of Thames, a deep, seasonally stratified estuarine embayment adjoing the Hauraki Gulf in northern Aotearoa/New Zealand. The Firth’s catchment has undergone historic land-use intensification transforming it from native forest cover to dominance by pastoral use, increasing its riverine total nitrogen loading by ∼82% over natural levels and switching it’s predominate loading source from offshore to the catchment. We hypothesised that seasonal variation in net ecosystem metabolism [NEM: dissolved inorganic carbon (DIC) uptake/release] will be a primary factor determining carbonate and oxic responses in the Firth, and that organic matter involved in the metabolism will originate primarily by fixation within the Firth system and be driven by catchment dissolved inorganic nitrogen (DIN) loading. Seasonal ship-based and biophysical mooring surveys across the Hauraki Gulf and Firth showed depressed pH and O2 reaching pH ∼7.8 and O2 ∼4.8 mg L–1 in autumn in the inner Firth, matched by shoreward increasing nutrient loading, phytoplankton, organic matter, gross primary production (GPP) and apparent O2 utilization. A carbonate system deconvolution of the ship survey data, combined with other ship survey and mooring results, showed how CO2 partial pressure responded to seasonal shifts in temperature, NEM, phytoplankton sinking and mineralisation and water column stratification, that underlay the late-season expression of acidification and hypoxia. This aligned with seasonal shifts in net DIC fluxes determined in a coincident nutrient mass-balance analysis, showing near-neutral fluxes from spring to summer, but respiratory NEM from summer to autumn. Particulate C:N and ratios of organic C fixed by Firth GPP to that from river inputs (∼29- to 100-fold in summer and autumn) showed that the dominant source of organic matter fuelling heterotrophy in autumn was autochthonous GPP, driven by riverine DIN loading. The results signified the sensitivity of deep, long-residence time, seasonally stratifying estuaries to acidification and hypoxia, and are important for coastal resource management, including aquaculture developments and catchment runoff limit-setting for maintenance of ecosystem health.

Modelled estimates of dissolved inorganic nitrogen exported to the Great Barrier Reef lagoon

Measuring stream pollutant loads across the Great Barrier Reef (GBR) catchment area (GBRCA) is challenging due to the spatial extent, climate variability, changing land use and evolving land management practices, and cost. Thus, models are used to estimate baseline pollutant loads. The eWater Source modelling framework is coupled with agricultural paddock scale models and the GBR Dynamic SedNet plugin to simulate dissolved inorganic nitrogen (DIN) generation and transport processes across the GBRCA. Catchment scale monitoring of flow and loads are used to calibrate the models, and performance is assessed qualitatively and quantitatively. Modelling indicates almost half (47%) of the total modelled DIN load exported to the GBR lagoon is from the Wet Tropics, and almost half of the total modelled DIN load is from sugarcane areas. We demonstrate that using locally developed, customised models coupled with a complementary monitoring program can produce reliable estimates of pollutant loads.

Submarine groundwater discharge and associated nutrient fluxes in the Greater Bay Area, China revealed by radium and stable isotopes

The estuary-bay system is a common and complex coastal environment. However, quantifying submarine groundwater discharge (SGD) and associated nutrient fluxes in the complex coastal environment is challenging due to more dynamic and complicated riverine discharge, ocean processes and human activities. In this study, SGD and SFGD (submarine fresh groundwater discharge) fluxes were evaluated by combining stable and radium isotopes in the Guangdong-Hong Kong-Macau Greater Bay Area (GBA), a typical estuary-bay system. We first built a spatially distributed radium mass balance model to quantify SGD fluxes in coastal areas of GBA integrating the Pearl River Estuary (PRE), bays and shelf. We then used the stable water isotope (δ2H and δ18O) end-member mixing model to distinguish submarine fresh groundwater discharge (SFGD) from SGD. Based on the 228Ra mass balance, the estimated SGD fluxes in the PRE, adjacent bay, and shelf areas were (6.14 ± 2.74) × 108 m3 d-1, (3.00 ± 1.11) × 107 m3 d-1, and (5.00 ± 5.64) × 108 m3 d-1, respectively. Results showed that the largest area-averaged SGD was in the PRE, followed by that in the adjacent shelf and the bay. These differences may be mainly influenced by ocean forces, urbanization and benthic topographies controlling the variability of groundwater pathways. Further, the three end-member mixing model of 228Ra and salinity was developed to confirm the validity of the estimated SGD using the Ra mass balance model. In the two models, groundwater end-member and water apparent age estimation were the main sources of uncertainty in SGD. The estimated SFGD flux was (1.39 ± 0.76) × 108 m3 d-1, which accounted for approximately 12% of the total SGD. Combining stable and radium isotopes was a useful method to estimate groundwater discharge. Moreover, the estimated SGD associated dissolved inorganic nitrogen (DIN) flux was one order of magnitude higher than other DIN sources. SGD was considered to be a significant contributor to the DIN loading to the GBA. The findings of this study are expected to provide valuable information on coastal groundwater management and environmental protection of the GBA and similar coastal areas elsewhere.

Increasing nutrient inputs risk a surge of nitrous oxide emissions from global mangrove ecosystems

<h2>Summary</h2> We document a substantial increase in global N₂O emissions from mangroves. Based on our analysis of two decades of mangrove N₂O emission studies, we estimate N₂O emission of 0.023 Tg N year⁻¹ from global mangrove ecosystems. N₂O fluxes from mangrove ecosystems are strongly increased by sediment dissolved inorganic nitrogen (DIN) concentration transported from river catchments to coastal waters. Continuing growth of nutrient inputs from anthropogenic sources, i.e., agricultural intensification, excessive fertilizer use and waste water discharge, will appreciably increase DIN loading and consequently global N₂O emission from mangroves. Based on the Millennium Ecosystem Assessment scenarios of riverine DIN inputs into mangrove ecosystems coupled with our estimates of DIN-controlled emissions rates, we expect N₂O emission to increase by 20%–51% by 2030 and 27%–74% by 2050 compared with estimated emissions in the year 2000. These forecasts underline the urgency of improvements in catchment-scale nitrogen management strategies.

Reconstructing primary production in a changing estuary: A mass balance modeling approach

AbstractEstuarine primary production (PP) is a critical rate process for understanding ecosystem function and response to environmental change. PP is fundamentally linked to estuarine eutrophication, and as such should respond to ongoing efforts to reduce nutrient inputs to estuaries globally. However, concurrent changes including warming, altered hydrology, reduced input of sediments, and emergence of harmful algal blooms (HABs) could interact with nutrient management to produce unexpected changes in PP. Despite its fundamental importance, estuarine PP is rarely measured. We reconstructed PP in the York River Estuary with a novel mass balance model based on dissolved inorganic nitrogen (DIN) for the period 1994–2018. Modeled PP compared well to previous estimates and demonstrated a long‐term increase and down‐estuary shift over the study period. This increase occurred despite reductions in discharge, flushing time, DIN loading, and DIN standing stock over the same period. Increased PP corresponded to increased water temperature, decreased turbidity and light attenuation, and increased photic depth and assimilation ratio, suggesting that phytoplankton in the York River Estuary have become more efficient at converting nutrients into biomass primarily due to a release from light limitation. The increase in PP also coincided with the increasing occurrence of late summer HABs in the lower York River Estuary, including the emergence of a second bloom‐forming dinoflagellate in 2007. Results demonstrate how changes concurrent with nutrient management could alter expected system responses and illustrate the utility of the mass balance approach for estimating critical rate processes like PP in the absence of observations.

Intra- and Inter-Annual Variability in the Dissolved Inorganic Nitrogen in an Urbanized River before and after Wastewater Treatment Plant Upgrades: Case Study in the Grand River (Southwestern Ontario)

External nitrogen (N) inputs originating from human activities act as essential nutrients accumulation in aquatic ecosystems or it is exported elsewhere, where the assimilation capacity is surpassed. This research presents a multi-annual case study of the dissolved inorganic nitrogen (DIN) in an urban river in Ontario (Canada), assessed changes in N downstream of the largest wastewater treatment plant (WTP) in the watershed. Changes in the DIN effluent discharge, in-river concentrations and loads were observed comparing the intra- and inter-annual variability (2010–2013) before, during and after WTP upgrades. These upgrades reduced the ammonium concentration in the river from 0.44 to 0.11 mg N-NH4+/L (year average), but the N load in the effluent increased. In the river, nitrate and ammonium concentrations responded to seasonal variability, being higher during the low temperature (&gt;10 °C) and high flow seasons (spring and spring melt). Among years, changes in the DIN concentration are likely controlled by the effluent to river dilution ratio, which variability resides on the differences in river discharge between years. This suggest that the increasing trend in the DIN concentration and loads are the result of agricultural and urban additions, together with reduced N assimilation, in addition to N loads responding to variable river discharge. Finally, we propose monitoring both concentrations and loads, as they provide answers to different questions for regulatory agencies and water managers, allowing tailored strategies for different purposes, objectives and users.

Low-technology recirculating aquaculture system integrating milkfish Chanos chanos, sea cucumber Holothuria scabra and sea purslane Sesuvium portulacastrum

Closed recirculation aquaculture systems (RAS) in combination with integrated multi-trophic aquaculture (IMTA) are considered best management practices, but high material costs and difficult maintenance still hinder their implementation, especially in developing countries and the tropics. Few case studies of such systems with tropical species exist. For the first time, an extremely low-budget system was tested combining the halophyte sea purslane Sesuvium portulacastrum and a detritivore, sandfish Holothuria scabra, with finfish milkfish Chanos chanos over 8 wk on Zanzibar, Tanzania. In a 2 m3 RAS, milkfish and sea purslane showed good growth, producing an average (±SD) of 1147 ± 79 g fish and 1261 ± 95 g plant biomass, while sea cucumber growth was variable at 92 ± 68 g. The system operated without filter units and did not discharge any solid or dissolved waste. Water quality remained tolerable and ammonia levels were reliably decreased to &lt;1 mg l-1. A NO2- peak occurred within the first 30 d, indicating good biofilter performance of the different system compartments. Changes in dissolved inorganic nitrogen (DIN) species support the notion that the sea cucumber tank was the main site of nitrification, while the hydroponic halophyte tank acted as a net sink of NO3-. A nitrogen budget accounted for 63.7 ± 5.3% of the nitrogen added to the system as fish feed. Increasing the plant to fish biomass ratio to 5:1 would fully treat the DIN load. The experiment provides proof-of-concept of a simple pilot-scale RAS, integrating tropical species at 3 trophic levels.

Ridge to Reef Management Implications for the Development of an Open-Source Dissolved Inorganic Nitrogen-Loading Model in American Samoa.

Excessive nutrient discharge to tropical island coastlines drives eutrophication and algal blooms with significant implications for reef ecosystem condition and provision of ecosystem services. Management actions to address nutrient pollution in coastal ecosystems include setting water-quality standards for surface waters discharging to the coast. However, these standards do not account for the effects of groundwater discharge, variability in flow, or dilution, all of which may influence the assessment of true nutrient impacts on nearshore reef habitats. We developed a method to estimate dissolved inorganic nitrogen (DIN) loads to coastal zones by integrating commonly available datasets within a geospatial modeling framework for Tutuila, American Samoa. The DIN-loading model integrated an open-source water budget model, water-sampling results, and publicly available streamflow data to predict watershed-scale DIN loading to the island's entire coastline. Submarine groundwater discharge (SGD) was found to deliver more terrigenous DIN to the coastal zone than surface water pathways, supporting findings from other tropical islands. On-site wastewater disposal systems were also found to be the primary anthropogenic sources of DIN to coastal waters. Our island-wide DIN-loading model provides a simple and robust metric to define spatially explicit sources and delivery mechanisms of nutrient pollution to nearshore reef habitats. Understanding the sources and primary transport modes of inorganic nitrogen to nearshore reef ecosystems can help coastal resource managers target the most impactful human activities in the most vulnerable locations, thereby increasing the adaptive capacity of unique island ecosystems to environmental variation and disturbances.

Total Maximum Allocated Loads on Stoichiometry of Nitrogen and Identification of Critical Form in Jiaozhou Bay, China

Total pollutant load control management for total dissolved nitrogen (TDN) is an urgent task required to gain a good water quality status in Jiaozhou Bay (JZB), China. In this paper, the stoichiometry of multiform TDN on land-ocean interactions associated with marine biogeochemical reaction (LOIMBR) was studied by modeling the load-response relationship based on a three-dimensional water quality model of nitrogen in JZB. The results showed that the stoichiometry on LOIMBR of dissolved organic nitrogen (DON), NO3-N and NH4-N was 3:1:1, with one-third of the contribution on the concentration of dissolved inorganic nitrogen (DIN) in JZB for the land-based DON loads to DIN loads. Based on the stoichiometric relationship of nitrogen forms, the total maximum allocated load (TMAL) of equivalent TDN (ETDN) was approximately 5300 t a−1 in JZB, equivalent to the TMAL of 5700, 5800 and 15600 t a−1 for NH4-N, NO3-N and DON, respectively. According to the loads of ETDN, there were four outfalls overloaded in JZB in 2015, which lie in the head of the bay. In the four overloaded outfalls, besides NO3-N, NH4-N was the critical nitrogen control form for Moshui River, while DON for Dagu River and Haibo River. The results of numerical experiments further showed that JZB will achieve good water quality after 7 years by implementation of the ‘different emission reduction’ based on TMAL of ETDN, which is significantly better than ‘equal percent removal’.

Reduced phosphorus loads from the Loire and Vilaine rivers were accompanied by increasing eutrophication in the Vilaine Bay (south Brittany, France)

Abstract. The evolution of eutrophication parameters (i.e., nutrients and phytoplankton biomass) during recent decades was examined in coastal waters of the Vilaine Bay (VB, France) in relation to changes in the Loire and Vilaine rivers. Dynamic linear models were used to study long-term trends and seasonality of dissolved inorganic nutrient and chlorophyll a concentrations (Chl a) in rivers and coastal waters. For the period 1997–2013, the reduction in dissolved riverine inorganic phosphorus (DIP) concentrations led to the decrease in their Chl a levels. However, while dissolved inorganic nitrogen (DIN) concentrations decreased only slightly in the Vilaine, they increased in the Loire, specifically in summer. Simultaneously, phytoplankton in the VB underwent profound changes with increase in biomass and change in the timing of the annual peak from spring to summer. The increase in phytoplankton biomass in the VB, manifested particularly by increased summer diatom abundances, was due to enhanced summer DIN loads from the Loire, sustained by internal regeneration of DIP and dissolved silicate (DSi) from sediments. The long-term trajectories of this case study evidence that significant reduction of P inputs without simultaneous N abatement was not yet sufficient to control eutrophication all along the Loire–Vilaine–VB continuum. Upstream rivers reveal indices of recoveries following the significant diminution of P, while eutrophication continues to increase downstream, especially when N is the limiting factor. More N input reduction, paying particular attention to diffuse N sources, is required to control eutrophication in receiving VB coastal waters. Internal benthic DIP and DSi recycling appears to have contributed to the worsening of summer VB water quality, augmenting the effects of anthropogenic DIN inputs. For this coastal ecosystem, nutrient management strategies should consider the role played by internal nutrient loads to tackle eutrophication processes.

Interannual Improvement in Sea Lettuce Blooms in an Agricultural Catchment

Riverine nutrient loading from agriculture is one of the most prominent pressures in the second cycle of river basin management planning for the European Union (EU) Water Framework Directive (WFD). Better farmyard nutrient management planning is the measure most likely to reduce agricultural nutrient loading to catchment watercourses and coastal receiving waters. The adjoining Argideen Estuary and Courtmacsherry Bay in the south west of Ireland drain a 150km2 catchment comprising mainly agricultural land. The receiving waters were allocated Poor ecological status under the WFD at the most recent appraisal. Subhourly water quality monitoring in the Timoleague River has been carried out by the Teagasc Agricultural Catchments Programme to track the changes in nutrient loading to the Argideen Estuary in response to improved farming practice. A biophysical model of the adjoining Argideen Estuary and Courtmacsherry Bay was calibrated and Six nutrient load scenarios were simulated to determine their impact upon macroalgae and phytoplankton bloom magnitude. In addition, nutrient flow-load relationships were derived for summers 2010 and 2016 to elucidate the improvements induced by better catchment management practice. Flow-load relationships for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) for each year were applied to the flow data for the other year, to query the outcome if 1) there had been no change in farm management practice between 2010 and 2016, or 2) the improved farm management practice in place by 2016 had been implemented by 2010. The difference between expediting and delaying improvement in farm nutrient management practice was a 5% increase in DIN loading and a 233% increase in DIP load. Application of this higher estimated load to the calibrated bio-physical model projected an increase in 2016 Ulva bloom magnitude from 381t to 1,391t. Although phosphorus retention within the catchment has improved in recent years, with an attendant improvement in Ulva bloom magnitudes, flow connectivity in the catchment still facilitates a higher phosphorus transfer during large rainfall events. A high amount of phosphorus is stored within the catchment, while point source pressures continue to contribute to phosphorus transfer to streams during periods of low flow.

Land-derived inorganic nutrient loading to coastal waters and potential implications for nearshore plankton dynamics

Algoa Bay represents a complex suite of aquatic ecosystems that support high biodiversity and provide significant socio-economic benefits. Since land-ocean interactions are central to shaping the structure and functioning of these ecosystems, the aim of this study was to quantify land-derived inorganic nutrient loads entering the coastal waters of Algoa Bay. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) data together with flow discharge volumes were sourced from long-term monitoring programmes and existing literature to determine annual and seasonal nutrient loads. The annual loads of DIN and DIP entering the coastal waters of Algoa Bay were estimated at 8.7 × 105 and 1.4 × 105 kg, respectively. Seasonal peaks were observed in winter for DIN and in spring for DIP. Wastewater discharges were the dominant source of inorganic nutrients to the nearshore environment, representing 71.3% and 62.0% of total DIN and DIP loads, respectively. The DIN and DIP inputs entering estuarine waters of Algoa Bay accounted for 21.9% and 38.0% of total loads, respectively. Inshore zooplankton biomass in Algoa Bay has increased significantly (p < 0.01) since 2013 concomitant with elevated DIN inputs from WWTPs. Additionally, anthropogenic nutrient loading to the estuarine and coastal waters of Algoa Bay has facilitated, in part, the increased observations of eutrophic symptoms, including harmful algal blooms (e.g. Heterosigma akashiwo and Lingulodinium polyedra) and hypoxia (<2 mg l−1). Effective ecosystem-based management is key to preventing the exacerbation of these undesirable disturbances, and thus maintaining ecosystem functionality.

Salt Marsh Aboveground Production in New England Estuaries in Relation to Nitrogen Loading and Environmental Factors

Aboveground production responses of Spartina alterniflora and S. patens in estuaries in Massachusetts, USA were assessed in relation to temporal (date) and physical (elevation and distance from creek edge) factors as well as nitrogen loading using stem δ15N, water column dissolved inorganic nitrogen (DIN), and upland nitrogen loading as nitrogen input proxies. All nitrogen input proxies had negative relationships with S. alterniflora stem density while stem height and biomass increased or were unaffected. Nitrogen content of S. alterniflora increased with stem δ15N but was not related to DIN or upland nitrogen loading proxies. For S. patens, stem density, biomass, and height all increased with stem δ15N while nitrogen content decreased. Stem density and biomass also varied with elevation. For S. alterniflora, this relationship was parabolic for stem density and declined linearly for biomass. Both stem density and biomass increased linearly for S. patens. Across the growing season, S. alterniflora stem density decreased, S. patens biomass increased, and nitrogen content declined for both Spartina species. S. alterniflora stem height also decreased with distance from the creek edge. Results show different responses for Spartina species to upland and water column nitrogen inputs and provide complementary information to results from controlled fertilization experiments.

Impacts of Atmospheric Nitrogen Deposition and Coastal Nitrogen Fluxes on Oxygen Concentrations in Chesapeake Bay

AbstractAlthough rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and coastal DIN concentrations on the continental shelf can also significantly influence hypoxia; however, the relative impact of these additional sources of DIN on Chesapeake Bay hypoxia has not previously been quantified. In this study, the estuarine‐carbon‐biogeochemistry model embedded in the Regional‐Ocean‐Modeling‐System (ChesROMS‐ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN from the atmosphere has roughly the same impact on hypoxia as the same gram‐for‐gram change in riverine DIN loading, although their spatial and temporal distributions are distinct. DIN concentrations on the continental shelf have a similar overall impact on hypoxia as DIN from the atmosphere (~0.2 mg L−1); however, atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, whereas coastal DIN concentrations primarily impact DO via the decomposition of allochthonous organic matter entering the Bay mouth from the shelf. The impacts of atmospheric DIN deposition and coastal DIN concentrations on hypoxia are greatest in summer and occur farther downstream (southern mesohaline) in wet years than in dry years (northern mesohaline). Integrated analyses of the relative contributions of all three DIN sources on summer bottom DO indicate that impacts of atmospheric deposition are largest in the eastern mesohaline shoals, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN concentrations are most influential in the polyhaline region.

Fertiliser management effects on dissolved inorganic nitrogen in runoff from Australian sugarcane farms

Dissolved inorganic nitrogen (DIN) movement from Australian sugarcane farms is believed to be a major cause of crown-of-thorns starfish outbreaks which have reduced the Great Barrier Reef coral cover by ~21% (1985–2012). We develop a daily model of DIN concentration in runoff based on >200 field monitored runoff events. Runoff DIN concentrations were related to nitrogen fertiliser application rates and decreased after application with time and cumulative rainfall. Runoff after liquid fertiliser applications had higher initial DIN concentrations, though these concentrations diminished more rapidly in comparison to granular fertiliser applications. The model was validated using an independent field dataset and provided reasonable estimates of runoff DIN concentrations based on a number of modelling efficiency score results. The runoff DIN concentration model was combined with a water balance cropping model to investigate temporal aspects of sugarcane fertiliser management. Nitrogen fertiliser application in December (start of wet season) had the highest risk of DIN movement, and this was further exacerbated in years with a climate forecast for ‘wet’ seasonal conditions. The potential utility of a climate forecasting system to predict forthcoming wet months and hence DIN loss risk is demonstrated. Earlier fertiliser application or reducing fertiliser application rates in seasons with a wet climate forecast may markedly reduce runoff DIN loads; however, it is recommended that these findings be tested at a broader scale.