Dissolved Inorganic Nitrogen Inputs Research Articles

A spatially explicit interpretable machine-learning method to track dissolved inorganic nitrogen pollution in a coastal watershed

Sustainable water management requires an in-depth understanding of dissolved inorganic nitrogen (DIN) in watersheds. However, it remains challenging in the context of intensifying human-induced and naturally impacted stresses in a changing environment. A spatially explicit interpretable machine-learning method was proposed based on the annual variation in land use/cover, Catchment Land Surface Model, Noah land surface model, population dynamics, and nightlight in a coastal Chinese watershed. The proposed model was effective for delineating DIN processes and sources in watersheds. The d (index of agreement), R2 (coefficient of determination), |PBIAS| (percent bias), and KGE (Kling-Gupta efficiency)during the training period were 0.91–0.98, 0.76–0.96, 0.38–3.12, and 0.73–0.87, respectively. The d, R2, |PBIAS|, and KGE during the testing period were 0.85–0.94, 0.56–0.79, 3.59–6.18, and 0.65–0.86, respectively. NH4+-N in the watersheds may be strongly related to the effects of urbanization within the watersheds while the agricultural activities may modify the patterns of NO3–-N in the watersheds. The NH4+-N was highly related to the urbanization effects, which contributed 20 %–30 % of the riverine NH4+-N in the North River. In contrast, agricultural activities may modify patterns of NO3–-N in watersheds, and agricultural activities in the West River contributed 50 %–70 % riverine NO3–-N in the watershed. Moreover, urbanization may alter the water content, soil properties, and regional climate patterns within the watersheds, while repeated DIN input through agricultural activities in the agricultural watersheds may change the nitrogen processes in the soil and groundwater. Paired with agricultural activities, dams or reservoirs may amplify NH4+-N to watersheds with modifying the advection and diffusion of DIN from agricultural activities, and increasing sediments in the watersheds. This study demonstrated the potentials of the proposed method in tracking DIN pollution and provided new insights into nutrient management in the watersheds under intensifying human-nature interactions.

Effect of submarine groundwater discharge on nutrient distribution and eutrophication in Liaodong Bay, China

Driven by the anthropogenic activities associated with coastal settlements, eutrophication has become a global issue. Submarine groundwater discharge (SGD) is a significant continuous pathway for transporting nutrients from land to coastal waters, but its influence on eutrophication in Liaodong Bay (LDB) has received limited attention. In this study, radium isotopes and nutrient data from coastal waters were analyzed to evaluate the SGD flux and its implications for potential eutrophication in LDB. We found that the mean concentrations of dissolved inorganic nitrogen (DIN), phosphorous (DIP), and silicate (DSi) in groundwater were higher than those of seawater and river water. Based on 223Ra and 228Ra mass balance models, the SGD fluxes were estimated to be (0.53–2.03) × 109 m3/d, of which the fresh SGD accounted for 4 %–15 %. SGD is a vital invisible source of nutrients, contributing more than 79 % of the total inputs of DIN, DIP, and DSi into LDB. With high DIN/DIP ratios (average=85.8) and large nutrient inputs, SGD may significantly drive the phosphorus limitation and eutrophication in LDB. This study shows that SGD-derived nutrient fluxes should be considered in the assessment of water eutrophication for the formulation of future environmental management protocols in coastal systems.

Nitrogen in the Yangtze River Basin: Pollution Reduction through Coupling Crop and Livestock Production.

Livestock production poses a threat to water quality worldwide. A better understanding of the contribution of individual livestock species to nitrogen (N) pollution in rivers is essential to improve water quality. This paper aims to quantify inputs of dissolved inorganic nitrogen (DIN) to the Yangtze River from different livestock species at multiple scales and explore ways for reducing these inputs through coupling crop and livestock production. We extended the previously developed model MARINA (Model to Assess River Input of Nutrient to seAs) with the NUFER (Nutrient flows in Food chains, Environment, and Resource use) approach for livestock. Results show that DIN inputs to the Yangtze River vary across basins, sub-basins, and 0.5° grids, as well as across livestock species. In 2012, livestock production resulted in 2000 Gg of DIN inputs to the Yangtze River. Pig production was responsible for 55-85% of manure-related DIN inputs. Rivers in the downstream sub-basin received higher manure-related DIN inputs than rivers in the other sub-basins. Around 20% of the Yangtze basin is considered as a manure-related hotspot of river pollution. Recycling manure on cropland can avoid direct discharges of manure from pig production and thus reduce river pollution. The potential for recycling manure is larger in cereal production than in other crop species. Our results can help to identify effective solutions for coupling crop and livestock production in the Yangtze basin.

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.

Pelagic denitrification and methane oxidation in oxygen-depleted waters of the Louisiana shelf

Anthropogenic nutrient inputs fuel eutrophication and hypoxia ([O2] < 2 mg L−1), threatening coastal and near shore environments across the globe. The world’s second largest anthropogenic coastal hypoxic zone occurs annually along the Louisiana (LA) shelf. Springtime loading of dissolved inorganic nitrogen (DIN) from the Mississippi River, combined with summertime stratification and increased water residence time on the shelf, promotes establishment of an extensive hypoxic zone that persists throughout the summer. We investigated the patterns of pelagic denitrification and methane (CH4) oxidation along the LA shelf. Microbial activity rates were determined along with concentrations of dissolved nutrients and greenhouse gases, nitrous oxide (N2O) and CH4, during summer in 2013, 2015, and 2016. We documented denitrification rates up to 1900 nmol N L−1 d−1 and CH4 oxidation rates as high as 192 nmol L−1 d−1 in hypoxic waters characterized by high concentrations of N2O (range: 1 to 102 nM) and CH4 (range: 3 to 641 nM). Ecosystem scaling estimates suggest that pelagic denitrification could remove between 0.1 and 47% of the DIN input from the Mississippi River, whereas CH4 oxidation does not function as an effective removal process with CH4 escaping to the atmosphere. Denitrification and CH4 oxidizing bacteria within the LA shelf hypoxic zone were largely unable to keep up with the DIN and CH4 inputs to the water column. Rates were variable and physiochemical dynamics appeared to regulate the microbial removal capacity for both nitrate/nitrite and CH4 in this ecosystem.

Long-term assessment of nutrient budgets for the four reservoirs of the Seine Basin (France).

Artificial reservoirs represent one of the most significant human disturbances of water flows and associated water quality, including nutrients and SM (suspended matter). However, most of the previous studies were only focused on few years or even single year, and the long-term dynamics of nutrient retention in reservoir are under explored. In this study, we present the long-term (1998-2018) hydrological characteristics and water quality in four reservoirs (Marne, Aube, Seine, and Pannecière reservoirs) and their related rivers (Marne, Aube, Seine, and Yonne rivers) of the Seine Basin, France. Based on the hydrology and water quality data, the long-term budgets of nutrients and SM were evaluated in these reservoirs according to mass balance calculation. The results indicated that the four reservoirs play important roles in the retention/elimination of nutrients and SM, and the retention/elimination rates may be affected by hydrophysical and biogeochemical processes. The mean annual retention rates accounted for 16-53% of the inputs of DIN (dissolved inorganic nitrogen), 26-48% of PO43--P (orthophosphates), 22-40% of Si (dissolved silicon), and 36-76% of SM in the four reservoirs during the 1998-2018 period. Further analysis suggested that the annual residence time and the percentage of water released from reservoirs during the filling period significantly correlated with DIN retention rates in the four reservoirs (p<0.01), which highlights the importance of reservoir water management strategies for the DIN concentrations in the downstream rivers. Interestingly, the Wilcoxon test results also revealed that the three diverted reservoirs (Marne, Aube, and Seine reservoirs) indeed lowered the nutrient concentrations in their downstream rivers during the emptying period, thereby modifying the biogeochemical functioning in the downstream river networks. Finally, these results emphasized the importance of hydrological characteristics in better understanding nutrient retention in reservoirs.

Tracing external sources of nutrients in the East China Sea and evaluating their contributions to primary production

Nutrients (dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP)) in the East China Sea (ECS) have four external sources, namely the Kuroshio, Taiwan Strait, rivers, and the atmosphere. In order to evaluate the contribution of each source of nutrients to the nutrient inventory and primary production over the ECS shelf, a tracking technique was applied to all the state variables in a low-trophic ecosystem model. Each source of nutrients has strong seasonal variations in the spatial distribution that depends closely on circulation, mixing, and stratification. The primary production supported by each source of nutrient is under the control of a combination of nutrients, temperature, and light. As a mean state over the entire ECS shelf, the Kuroshio contributes 72% of DIN input and 84% of DIP input, 57% of DIN inventory and 78% of DIP inventory, 50% DIN-based primary production and 61% DIP-based primary production and therefore is the dominant one among four sources. However, the contributions of four external nutrients have strong spatial dependence: the riverine nutrients and the atmospheric DIN dominate the inner shelf (0–50 m), the nutrients from the Taiwan Strait dominate the southern part of the middle shelf (50–100 m), and the nutrients from the Kuroshio dominate the outer shelf (100–200 m). The production efficiencies of the nutrients from the Kuroshio are low, while those from the atmosphere and Taiwan Strait are high. The riverine DIN is inefficient but DIP is efficient. The nutrient limitation, light and water temperature at the location of the nutrients determine the production efficiency of each specific sources.

Groundwater discharge to coastal streams – A significant pathway for nitrogen inputs to a hypertrophic Mediterranean coastal lagoon

Near-shore and direct groundwater inputs are frequently omitted from nutrient budgets of coastal lagoons. This study investigated groundwater-driven dissolved inorganic nitrogen (DIN) inputs from an alluvial aquifer to the hypertrophic Or lagoon, with a focus on the Salaison River. Piezometric contours revealed that the Salaison hydrogeological catchment is 42% bigger than the surface watershed and hydraulic gradients suggest significant groundwater discharge all along the stream. Hydrograph separation of the water flow at a gauging station located 3 km upstream from the Or lagoon combined with DIN historical data enabled to estimate that groundwater-driven DIN inputs account for 81–87% of the annual total DIN inputs to the stream upstream from the gauging station. A radon mass balance was performed for the hydrological cycle 2017–2018 to estimate groundwater inflow into the downstream part of the stream. Results showed that (1) DIN fluxes increased by a factor 1.1 to 2.3 between the gauging station and the Salaison outlet, (2) the increase in DIN was due to two groundwater-fed canals and to groundwater discharge along the stream, the latter represented 63–78% of the water flow. This study thus highlights the significance of groundwater driven DIN inputs into the Salaison River, which account for 90% of the annual DIN inputs. This is particularly true in the downstream part of the river, which, on averages, supplies 48% of total DIN inputs to the river. These downstream DIN inputs into the Or lagoon were previously not taken into account in the management of this and other Mediterranean lagoons. The inputs will probably affect restoration processes for many years due to their residence time in the aquifer. This study throws light on a rarely documented source of ‘very-nearshore’ groundwater discharge to coastal streams in water and nutrient budgets of coastal zone ecosystems.

Evaluation of the total maximum allocated load of dissolved inorganic nitrogen using a watershed–coastal ocean coupled model

Due to the recent rapid increase in human activity and economic development, many coastal areas have recently experienced a high degree of land-based pollution. Evaluating the total maximum allocated load (TMAL) of dissolved inorganic nitrogen (DIN) nutrients and the remaining capacity is of importance for improving water quality. A considerable amount of nutrients derived from the coastal watershed can be found in wet seasons, which is non-negligible for the estimation of remaining capacity. Therefore, we use a watershed–coastal ocean coupled model combined with an optimization algorithm to tackle this issue. In contrast with previous studies, this study provides a method to estimate the spatiotemporal variations in TMALs and we then compare it to the current DIN nutrient load, including both point sources and non-point sources. Our results suggest that the TMAL of Daya Bay (DB), which is located in the northern part of the South China Sea, is about 7976 metric tons per year (t/yr) and ranges from 191 metric tons per month (t/month) to 1072 t/month. The increase of non-point source (NPS) DIN input also plays an important role in daily overload events during wet seasons. Moreover, the TMALs show an inverse exponential correlation with the water age, but only about 65% of the variance is explained. This suggests that the variations from the optimization algorithm and from local water function zoning plans are also important. According to our prediction of the DIN input, the TMAL of DB will soon be exhausted in the next several years. Consequently, prompt actions are necessary to consider the distribution of TMALs in urban developments and to decelerate the rapid growth of DIN input. Therefore, the results of this study will be helpful for both local pollution control and future urban planning.

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.

Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon

Coastal lagoons are dynamic aquatic systems that are susceptible to eutrophication due to long residence times and high inputs of nutrients. We hypothesise that groundwater-derived nutrients make a significant contribution to primary production, eutrophication and carbon cycling in these systems. Here, we report results from seasonal (pre-bloom, bloom and post bloom), nutrient, pCO2 and 222Rn (a natural groundwater tracer) surveys in a coastal lagoon that frequently experiences macroalgae blooms (Avoca Lagoon, Australia). Groundwater inputs of DIN and DIP deliver the nutrient load equivalent to the entire lagoon inventory in 1.6 to 3.5 days and 1.7 and 6 days respectively, indicating groundwater was a major source of inorganic nutrients to the system. Lagoon pCO2 displayed significant spatial and temporal variability, with the average pCO2 shifting from 1717 μatm during pre-bloom, to 137 μatm during the bloom and 3056 μatm post bloom. Radon-222 displayed a significant positive relationship with dissolved inorganic nitrogen (DIN) and pCO2 during the pre-bloom period. This suggests a hydrological dominance of surface water DIN and pCO2 during the pre-bloom period. During the bloom period, DIN displayed a positive relationship with pCO2 and negative relationship with dissolved oxygen, indicating a strong biological control over the nutrient pool. Phosphate did not appear to be controlled by either groundwater inputs, or ecosystem metabolism throughout the study. While groundwater discharge stimulated primary production through nutrient inputs (thus reducing surface water CO2), it also directly delivered significant quantities of CO2 to surface waters. The net effect of groundwater inputs of nutrients and dissolved carbon into the lagoon was a stimulation of CO2 fluxes to the atmosphere.

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.

Green tide development associated with submarine groundwater discharge in a coastal harbor, Jeju, Korea

We measured the magnitude of submarine fresh groundwater discharge (SFGD) and associated nutrient inputs to Jocheon harbor, on Jeju Island, Korea, during four sampling periods, in order to determine the link between SFGD and Ulva sp. green tide development. Good correlations among salinity, 222Rn, and dissolved inorganic nitrogen (DIN) in harbor seawater suggest that SFGD is the major source of DIN and fresh water since there are no surface runoffs. Using a 222Rn mass balance model, SFGD to the harbor was estimated to be 5.8 ± 2.3 × 104 m3 d−1. The DIN inputs through SFGD enhanced DIN concentrations in harbor seawater approximately 10-fold of those in the open-ocean (outer harbor) seawater. Results from mesocosm experiments showed that the growth rate of U. pertusa increased by 160% on average due to the enhanced DIN concentrations (from 1 to 24 µM) through SFGD in this harbor. Thus, we conclude that DIN inputs through SFGD cause the green tide development in Jocheon harbor and perhaps in other green tide regions where river inputs are absent.

Large temporal changes in contributions of groundwater-borne nutrients to coastal waters off a volcanic island

We examined the contribution of submarine groundwater discharge (SGD) to nutrient budgets in Hwasun Bay, Jeju Island, Korea in August 2009, October 2014, and May 2015. The concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in fresh groundwater were in the range of 285−716 μM and 2.3−3.2 μM, respectively, which were each 1−2 orders of magnitude higher than those in the bay seawater. The outer-bay seawater flowing into the bay was oligotrophic (2.9 ± 1.9 μM for DIN and 0.2 ± 0.3 μM for DIP). Nutrient budget calculations were performed for each season by accounting for submarine fresh groundwater discharge (SFGD) and water residence times. In August 2009 (DIN = 1.8 μM and DIN:DIP ratio = 4.6 for the outerbay water), DIN inputs from SFGD accounted for approximately 40% of the DIN inventory in the bay seawater. In October 2014 (DIN = 1.1 μM and DIP < 0.05 μM for the outer-bay water), DIP from SFGD accounted for approximately 100% of the DIP inventory in the bay seawater. In May 2015, mean concentrations of DIN and DIP in the bay seawater were 8.6 ± 12 μM and 0.11 ± 0.04 μM, respectively, with conservative behaviors in the bay seawater in association with excessive groundwater inputs. These results imply that SGD plays a critical but different role in nutrient budgets and stoichiometry in coastal waters off a volcanic island depending on open-ocean nutrient conditions.

Low‐level addition of dissolved organic carbon increases basal ecosystem function in a boreal headwater stream

AbstractComprehension of basic stream ecosystem function relies on an understanding of aquatic–terrestrial linkages. One major component of such linkages is the incorporation of landscape‐derived energy and nutrients into the aquatic food web via microbes. In many boreal streams, wetlands and alder are known to be primary sources of dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN), respectively. To simulate the influence of the highly labile portion of wetland‐derived DOC subsidies on microbial production and ecosystem processes in a stream with high landscape‐derived nutrient inputs, we enriched a boreal headwater stream situated in a high‐alder, low‐wetland cover catchment (i.e., high DIN, low DOC) with low levels (~0.25 mg/L) of labile DOC (as acetate‐C) for 9 weeks. We compared nutrient uptake, bacterial biomass production, and photosynthesis of periphyton and ecosystem metabolism in physicochemically similar upstream (reference) and downstream (treatment) reaches. DIN uptake was greater in the treatment than in reference reach on six out of nine dates during the dosing period. Bacterial biomass production positively responded to C enrichment. Ecosystem respiration increased up to ~50% after dosing began. Gross primary production responded positively to DOC enrichment early in the study when riparian vegetation did not limit light availability, but negatively later on in the growing season. We conclude that even low levels of labile DOC may act as a strong subsidy to headwater stream ecosystems, particularly those with high levels of DIN inputs from alder. Headwater streams influenced by high contributions of both alder and wetlands may represent biogeochemical hotspots, and these landscape features should be viewed as vital and complementary in their roles for ecosystem function.

Evolution of the sink and source of dissolved inorganic nitrogen with salinity as a tracer during summer in the Pearl River Estuary.

In this study, we evaluated the sink and source of the surface water along the PRE using a mixing model method with salinity as tracer. The observational data showed that the decreasing of dissolved inorganic nitrogen (DIN) did not closely follow the physical mixing lines of freshwater and modified seawater. In the western part, DIN consumption by phytoplankton and bacteria uptake (ΔDIN)varied from 15.81 μmol L−1 to 88.53 μmol L−1. On the contrary, in the eastern part, ΔDIN varied from −63.66 μmol L−1 to −10.45 μmol L−1. DIN source in the eastern part may be mainly caused by organic matter decomposition, while DIN remove is strongly associated with phytoplankton growth and bacteria consumption. These differential behaviors of the estuary with respect to DIN are largely due to varying degrees of hydrodynamics due to different topography in the two areas. Sensitivity analysis indicated reduction strategies of DIN inputs to coastal waters may improve environment quality in the PRE, due to DIN changes in the freshwater end-member having a determined influence on biological activities (R). Our results indicate that the model may be a valuable way to address the sources and sink of DIN in the river-dominated estuaries.

The role of seasonal and hydrological conditions in regulating dissolved inorganic nitrogen budgets in a forested catchment in SW Slovenia

During two consecutive years the monitoring of rainfall nitrate (NO3−) and ammonium (NH4+) concentrations, combined with high-frequency measurements of streamwater NO3− concentrations, provided insight into the mechanisms controlling the dissolved inorganic nitrogen (DIN) budgets of the forested Padež stream catchment in SW Slovenia. During both years, the catchment's annual wet atmospheric DIN deposition (12.6 and 13.8kg-N/ha) exceeded the annual DIN export (10.7kg-N/ha and 8.8kg-N/ha). The analysis of the monthly DIN budgets discloses seasonal patterns. In winter and early spring, the catchment behaves as a net source of DIN, whereas during late spring, summer, and early fall the catchment generally behaves as a net sink of the atmospheric DIN. Due to large evapotranspiration in the growth period, most of the DIN remains in the catchment and is flushed during the dormant season. Despite high evapotranspiration and consequently low runoff coefficients characteristic for the Mediterranean climate, large rainfall events in the growth period can cause intensive washout of DIN from the catchment. At a monthly basis, the DIN flushing might exceed the catchment's DIN retention capacity and the catchment might behave as a net source of DIN. Therefore, the hydrological conditions have the ability to shift the catchment's role in regulating the DIN budgets. The high responsiveness of the streamwater NO3− concentration in the growth period indicates a large pool of DIN in the forest soils which can become mobile by runoff formation. Wet atmospheric DIN deposition at the Padež catchment is considerable; highest DIN inputs can be expected when air masses approach the catchment from the southwest to southeast where there are extensive urban areas along the Italian, Slovenian and Croatian coast. However, the Padež catchment does not appear to be approaching N saturation, presumably due to strong internal N cycle in the forest soils.

Impacts of human activities and climate change on the water environment of Lake Poyang Basin, China

The global mean temperature has risen by 0.85 °C from 1880 to 2012, and this increase may even accelerate in the future. The surface temperatures of large inland water bodies worldwide have been rapidly warming since 1985 with an average rate of 0.045 ± 0.011 °C yr−1 and rates as high as 0.10 ± 0.01 °C yr−1. The global climate change was responsible for an increased contribution of diffuse nutrient losses to the total nutrient loads in water bodies. More and more algae blooms were reported in inland water bodies. Lake Poyang, the largest freshwater lake in China, is experiencing a deterioration of water quality. Ganjiang River, as the longest river in the Lake Poyang catchment, plays an important role in linking between terrestrial and aquatic ecosystems. A large amount of N pollutants is transported through the Ganjiang River to Lake Poyang. It is important to study quantitatively human enhanced N (nitrogen) inputs and DIN (Dissolved Inorganic Nitrogen) exports through rivers in the background of global warming. In this paper, we estimate the inputs of nitrogen (N) and exports of dissolved inorganic nitrogen (DIN) from the Ganjiang River to Lake Poyang for the period 1995-2010 by using the Global NEWS-DIN model. Modeled DIN yields range from 1038 kg N km−2 yr−1 in 1995 to 150 kg N km−2 yr−1 in 2010, showing a decreasing trend. The study demonstrates a varied contribution of different N inputs to river DIN yields during the period 1995-2010. Biological N fixation contributes about 73.3 % of DIN yields before 2005, but the contribution decreases substantially to 28.4 % in 2010. Chemical N fertilizer application and animal manure N inputs together contribute 12.6 % of the river DIN yields, while atmospheric N deposition contributes an average of 5.5 % of DIN yields in the period 1995-2010. Sewage N inputs contributes an average of 5.2 % of DIN yields over the period 1995-2005, while the contribution increases remarkably to 69.7 % in 2010. The trophic state index (TSI) of Lake Poyang agrees well with modeled DIN yields from the Ganjiang River over the whole study period. N inputs from nonpoint sources are approximately 10-54 times that from point sources. Almost all the diffuse DIN inputs were retained in the watershed before entering the lake Poyang. While about half of sewage N inputs were input to Lake Poyang as DIN from the Ganjiang River. Modeled DIN yield from Ganjiang River decreased after 2005 significantly, providing evidence for the recovering capacity of watersheds to retain DIN as inputs decrease as a direct result of human activities. The study also demonstrates a varied contribution of different N inputs to river DIN yields during the study period. The authors also point out that more attention should be kept on reducing sewage N discharge to Lake Poyang in the background of global warming.

Dissolved inorganic nitrogen uptake kinetics and δ15N of Zostera marina L. (eelgrass) in a coastal lagoon with oyster aquaculture and upwelling influence

Differences in dissolved inorganic nitrogen (DIN) availability can lead to intraspecific physiological differences in the DIN incorporation capacities of seagrasses. The objective of this study was to evaluate these physiological differences in Zostera marina (eelgrass) exposed to anthropogenic (oyster aquaculture) and natural (upwelling events) inputs of DIN in San Quintín Bay (Baja California, México). To this end, shoots from meadows in different sites within the bay (from the bay mouth to the inner parts of the bay), and differently exposed to nitrogen effluents from oyster aquaculture and upwelling events, were incubated with 15N-labeled tracers (15NH4+ and 15NO3−) under laboratory controlled conditions. DIN uptake kinetics and δ15N isotopic signals were assessed in months without upwelling influence (February) and with recurrent upwelling events (June). Results showed that eelgrass is highly efficient in acquiring DIN (mostly NH4+ by leaves and roots) from the water column and sediment pore-water to support their vegetative productivity. However, noticeable intraspecific differences in uptake kinetics strategies were observed. In both months, eelgrass shoots exposed to oyster aquaculture showed higher capacities (maximum uptake rate, Vmax, and/or affinity coefficient, α) to acquire NH4+ compared to shoots from other meadows. This suggests that these plants are efficient in exploiting a valuable source of NH4+ from oyster excretion or biodeposit remineralization. Shoots growing in the bay mouth which are the most exposed to upwelled waters, exhibited relatively low leaf Vmax and Km for NO3− in June. These properties allow these plants to restrict the uptake of NO3− from upwelling events, probably to avoid the potential toxicity resulting from its uptake in excess. Intraspecific differences in δ15N were also evident, probably related to the isotopic signal of the DIN source and the relative incorporation of NH4+ and NO3− by leaves and roots. Generally, higher values of δ15N were found in leaves and rhizomes of plants exposed to oyster aquaculture than those growing in the bay mouth. This indicated the incorporation of more 15N-enriched DIN (mainly NH4+) by the shoots from the meadow near oysters, possibly resulting from biogeochemical processes enhanced by oyster biodeposition. The lower δ15N in plants growing in the bay mouth likely resulted from the higher uptake of more isotopically-depleted NO3− from oceanic waters relative to the total DIN incorporation.

Modeling ocean circulation and biogeochemical variability in the Gulf of Mexico

Abstract. A three-dimensional coupled physical-biogeochemical model is applied to simulate and examine temporal and spatial variability of circulation and biogeochemical cycling in the Gulf of Mexico (GoM). The model is driven by realistic atmospheric forcing, open boundary conditions from a data assimilative global ocean circulation model, and observed freshwater and terrestrial nitrogen input from major rivers. A 7 yr model hindcast (2004–2010) was performed, and validated against satellite observed sea surface height, surface chlorophyll, and in situ observations including coastal sea level, ocean temperature, salinity, and dissolved inorganic nitrogen (DIN) concentration. The model hindcast revealed clear seasonality in DIN, phytoplankton and zooplankton distributions in the GoM. An empirical orthogonal function analysis indicated a phase-locked pattern among DIN, phytoplankton and zooplankton concentrations. The GoM shelf nitrogen budget was also quantified, revealing that on an annual basis the DIN input is largely balanced by the removal through denitrification (an equivalent of ~ 80% of DIN input) and offshore exports to the deep ocean (an equivalent of ~ 17% of DIN input).