High Dissolved Inorganic Nitrogen Research Articles

Comparison of nutrients status in Liaodong Bay and Northern Yellow Sea, China: Controlling factors and nutrient budgets

Under the dual stress of global warming and human interaction, Liaodong Bay (LDB) and northern Yellow Sea (NYS) are undergoing significant ecological changes. Little is known about the driving nutrients characteristics supporting fishery resource output in these areas. We carried out three field observations in 2019 to investigate nutrient status. Results showed that dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silica (DSi) concentrations changed seasonally, with lowest values in spring, and highest values in autumn. High DIN, DIP, and DSi concentrations were detected in LDB and NYS's estuary areas. The Yellow Sea Cold Water Mass plays a role in the distribution and seasonal variation of nutrients. Exchanges across the sediment-water interface, SFGD, atmospheric deposition, and the adjacent sea input dominated DIN dynamics of these areas. DIP primarily came from the adjacent sea input and DSi mainly originated from sediment release and the adjacent sea input. NYS seawater invasion accounted for 13.8% of DIN, 63.4% of DIP, and 35.1% of DSi in LDB. These results provide new insights to better facilitate the formulation of nitrogen and phosphorus reduction and control policies in these marginal seas.

A novel method of identifying estuary high-nutrient zones for water quality management

Coastal shallow waters are highly vulnerable to pollution, often leading to the development of intricate eutrophication zones. However, accurately determining these areas poses a significant challenge due to the complex interplay of estuarine hydrodynamics and nutrient transformation. To address such issue, a novel method was proposed to identify high-nutrient zones through calculating the continuous zonation of released tracers when their instantaneous concentrations declined to 1/e of their initial values. The method was well tested using idealized estuary models with varying shape parameters, water depths and river discharges. The results consistently revealed that the boundaries of high-nutrient zones fell within the mixed zone, characterized by salinity levels of 10– 20 psu. In Shenzhen Bay, a typical shallow bay, distinct differences were observed in the concentrations of dissolved inorganic nitrogen (DIN) and PO43−. Both the 20 psu isohaline and the proposed method effectively identified the partition boundary of high DIN and PO43− in 2001–2010, but only the newly proposed method demonstrated accuracy in delineating the actual high-nutrient zone during the continuous nutrient reduction period from 2010 to 2020. This study provides a practical and feasible approach that can serve as an auxiliary decision-making tool for managing estuarine water environments, and it has potential to facilitate the implementation of timely and effective measures for pollution control.

The Transmission Effectiveness of the Dissolved Nutrients (N, P, Si) Through a South Mediterranean Estuary (Seybouse, Algeria) Under Large Anthropogenic Forcing

The Seybouse Estuary (SE) is the downstream of the second great river in Algeria draining one of the main country’s watersheds (6471 km2, 1.5 million inhabitants). This study highlights the effect of the estuarine part on the distribution and the flux of dissolved nitrogen (N), phosphorus (P) and silicon (Si) through the SE. Surface water sampling was taken and analysed in the dry (September 2016) and wet (April 2017) seasons. Physical parameters and water discharge data were jointly recorded in parallel to sampling dates. Very low flows varying between 0.3 and 4.7 m3 s-1 were recorded at the outlet in dry and wet seasons, respectively. The SE outlet waters were 2.6 and 10 folds higher than upstream in dissolved inorganic nitrogen (DIN) and phosphates (PO4) levels, respectively. However, the silicates (SiO4) levels decreased by about 1/2 in the outlet waters. In terms of flux, the SE delivered to the coast 84 t·yr-1, 9 t·yr-1 and 56 t·yr-1 of DIN, PO4 and SiO4, respectively. The high DIN:PO4 = 137 and low SiO4:DIN = 0.52 molar ratios classify SE as one of the strong anthropised estuaries. The estuarine part amply modifies cycles and budgets of riverine nutrients introduced to the coast.

Spatiotemporal Nutrient Patterns, Stoichiometry, and Eutrophication Assessment in the Tieshan Bay Coastal Water, China

Land-source inputs into coastal water have increased remarkably in recent years, resulting in the deterioration of water quality, eutrophication, and algae blooms. However, we have limited understanding of spatiotemporal nutrient patterns, stoichiometry, and eutrophication assessment in Tieshan Bay coastal water at present. To investigate the rapid development of the coastal areas in Tieshan Bay in the South China Sea, nutrients and other physicochemical parameters were observed in Tieshan Bay during the normal season (April), wet season (July), and dry season (October) in 2021. The results showed that the average concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and chemical oxygen demand (COD) in Tieshan Bay are 0.071 ± 0.115 mg/L, 0.008 ± 0.013 mg/L, and 0.71 ± 0.219 mg/L, respectively. DIN/DIP ratio ranges from 9.1–69.3, with an average value of 19.9 ± 19.2, which exceeds the Redfield value, behaving P limitations. In addition, the mean eutrophication index (EI) was low in Tieshan Bay, with an average value of 0.5 ± 1.5. Moreover, the hotspot coastal water with high DIN, DIP, and COD concentrations was located in the upper half of Tieshan Bay in all seasons. In addition to the DIN, DIP, and COD contributions to EI, the average contribution rates of DIN, DIP, and COD are 26.6%, 8.8%, and 64.6%, respectively, which leads to the largest contribution of COD to EI. Furthermore, the average comprehensive index (CI) of organic pollution in Tieshan Bay surface seawater ranged from −1 to 5.6. The seawater near Hepu in S8 station has organic pollution in wet and dry seasons, and Tieshan Bay’s middle region also has slight organic pollution. Additionally, the DIN, DIP, and COD had significant relationships with salinity (p < 0.05), suggesting that coastal water quality is affected by land-based sources input. To achieve the seawater quality target and mitigate regional eutrophication, it is critical to implement land-based source management across the river-bay-coastal water continuum.

A Probabilistic Approach to Mapping the Contribution of Individual Riverine Discharges into Liverpool Bay Using Distance Accumulation Cost Methods on Satellite Derived Ocean-Colour Data

Assessments of the water quality in coastal zones often rely on indirect indicators from contributing river inputs and the neighbouring ocean. Using a novel combination of distance accumulation cost methods and an ocean-colour product derived from SENTINEL-3 data, we developed a probabilistic method for the assessment of dissolved inorganic nitrogen (DIN) in Liverpool Bay (UK) for the period from 2017 to 2020. Using our approach, we showed the annual and monthly likelihood of DIN exposure from its 12 major contributory rivers. Furthermore, we generated monthly risk maps showing the probability of DIN exposure from all rivers, which revealed a seasonal variation of extent and location around the bay. The highest likelihood of high DIN exposure throughout the year was in the estuarine regions of the Dee, Mersey, and Ribble, along with near-shore areas along the north Wales coast and around the mouth of the rivers Mersey and Ribble. There were seasonal changes in the risk of DIN exposure, and this risk remained high all year for the Mersey and Dee estuary regions. In contrast, for the mouth and near the coastal areas of the Ribble, the DIN exposure decreased in spring, remained low during the summer and early autumn, before displaying an increase during winter. Our approach offers the ability to assess the water quality within coastal zones without the need of complex hydrodynamic models, whilst still having the potential to apportion nutrient exposure to specific riverine inputs. This information can help to prioritise how direct mitigation strategies can be applied to specific river catchments, focusing the limited resources for coastal zone and river basin management.

Transition of carbon-nitrogen coupling under different anthropogenic disturbances in subtropical small mountainous rivers

The commonly observed inverse relationship between dissolved organic carbon (DOC) and nitrate (NO3−) concentrations in aquatic systems can be explained by stoichiometric and thermodynamic principles regulating microbial assimilation and dissimilation processes. However, the interactive effects of human activities and dissolved oxygen (DO) on the DOC and DIN (dissolved inorganic nitrogen, mainly composed of NO3−-N and NH4+-N) relations are not well identified, particularly in subtropical small mountainous rivers (SMRs). Here, we investigated the exports and relations of DOC-DIN in 42 Taiwan SMRs under different anthropogenic disturbances. Results showed that the island-wide mean concentrations of the three solutes in streams are generally low, yet the abundant rainfall and persistent supply contrarily lead to disproportional high DOC and DIN yields. The inverse DOC-NO3−-N relation does not appear under well‑oxygenated conditions, regardless of low or high human disturbance. However, a significant inverse relationship between DOC-NO3−-N would emerge in highly-disturbed watersheds under low-oxygenated conditions (mean annual DO <6.5 mg L−1), where excess N accumulates as NH4+-N rather than NO3−-N. The controlling mechanism of DOC-DIN relations would shift from energetic constraints to redox constraints in low-oxygenated conditions. Although riverine concentrations of DOC, NO3−-N, and NH4+-N could be elevated by human activities, the transition of DOC-DIN relation pattern is directly linked to DO availability. Understanding the mechanism that drives CN coupling is critical for assessing the ecosystem function in the delivery and retention of DOC and DIN in aquatic ecosystems.

Sources and long-term variation characteristics of dissolved nutrients in Maowei Sea, Beibu Gulf, China

Maowei Sea is a typical semi-enclosed subtropical bay where eutrophication has been rising due to human activities in recent years. At present, the source and long-term variability properties of nutrients are not well understood. This study investigated the dissolved inorganic nutrients in the riverine sections to the sea and the coastal areas from 2011 to 2017 in Qinzhou Bay (QZB), Beibu Gulf. Results showed that harmful blooms were frequent in the dry season due to rapid urbanization in recent years. During dry season, the concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) increased by 67% and 150%, respectively. Most of the nutrients input to the bay were supplied by the adjoining rivers, followed by groundwater discharge according to the classical box model devised by the Land-Ocean Interactions in the Coastal Zone. High DIN concentrations in rivers were mainly caused by intensified agriculture, including urban and rural effluents, due to inadequate sanitary sewer systems. Maowei Sea (the inner bay of QZB) is a sink system for DIN but a strong source system for DIP, which surpassed riverine input, probably due to the discharge of DIP-containing effluent from the industries around the Maoling River mouth. The discrepancy in DIN and DIP sources resulted in different DIN/DIP ratios between the wet and dry seasons. P-limitation generally occurs in the wet season and is relieved in the dry season. The relief of P-limitation in the dry season may be crucial for the frequent harmful algal blooms, which may be further aggravated in the future with the rapid increase in DIP concentrations in the bay. Therefore, the results of this study provide a basis for the effective improvement of the ecological environment in the Maowei Sea.

Nitrogen driven niche differentiation in bacterioplankton communities of northeast coastal Bay of Bengal

The Bay of Bengal receives nitrogen inputs from multiple sources and the potential role of nitrogen-metabolizing microbial communities in the surface water is not well understood. The nitrogen budget estimate shows a deficit of 4.7 ± 2.4 Tg N yr-1, suggesting a significant role of dissolved organic nitrogen remineralization in fuelling ecosystem processes. Unravelling the process of remineralization leading to increasing concentrations of dissolved inorganic nitrogen (DIN) in coastal ecosystems such as in mangroves require a better understanding of the composition of functional resident bacterioplankton communities. Bacterioplankton communities were elucidated from eight stations along different estuaries spanning west to east of northeast coastal Bay of Bengal to understand the influence of DIN on shaping these communities. The eight stations were differentiated into ‘low’ and ‘high’ DIN stations based on DIN concentration, with five stations with High DIN concentration (>45 μM) and three stations with Low DIN concentration (<40μM). The V3–V4 region of 16S rRNA was amplified and sequenced to elucidate resident bacterioplankton community structure from environmental DNA. Proteobacteria, Bacteroidetes, and Firmicutes were the dominant bacterioplankton phyla across all stations. Nitrogen-fixing groups such as Nitrospirae, Lentisphaerae, Chloroflexi, and Planctomycetes make up about 1% of the bacterioplankton communities. Abundances of Spirochaetes and Tenericutes showed a positive correlation with DIN. Pseudomonadales, Alteromonadales, and Desulfovibrionales were found to distinctly vary in abundance between Low and High DIN stations. Predicted metagenomic profiles from taxonomically derived community structures indicated bacterial nitrate-nitrite reductase to be negatively correlated with prevalent DIN concentration in High DIN stations but positively correlated in Low DIN stations. This trend was also consistent for genes encoding for nitrate/nitrite response regulators and transporter proteins. This indicates the need to delineate functional bacterioplankton community structures to better understand their role in influencing rates and fluxes of nitrogen within mangroves.

Monthly Variation in Flux of Inorganic Nutrients From Submarine Groundwater Discharge in a Volcanic Island: Significant Nitrogen Contamination in Groundwater

To determine nutrient fluxes derived from submarine groundwater discharge (SGD), we conducted monthly hydrological surveys on the coast of Jeju, a volcanic island located in the southern sea of Korea. The concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicon (DSi) were significantly correlated with salinity, indicating that fresh SGD (FSGD) is a major nutrient source in Jeju Island where no other coastal freshwater origins exist. Based on a DSi-mass balance model, seepage rate of FSGD was found to depend on 5-day precipitation before sampling campaigns, which immediately permeated via porous aquifers. Thus, the FSGD-driven nutrient fluxes were generally higher in rainy season (July–August) and September 2019 when typhoons occurred. However, high DIN and DIP fluxes were found during spring (March–May), even at low seepage rate, perhaps by a fertilizer input from agriculture activity. This study highlights that large variation of the SGD-driven nutrient fluxes was caused by environmental and anthropogenic factors and emphasizes on the importance of long-term investigation.

Interannual Variation in Phytoplankton Community Driven by Environmental Factors in the Northern East China Sea

The relative importance of interannual environmental changes in shaping phytoplankton community structure remains unclear in the East China Sea (ECS), which is the largest marginal sea in the western North Pacific Ocean. Based on high-performance liquid chromatography (HPLC) analysis of a variety of photosynthetic pigments, we investigated spatiotemporal variations in the phytoplankton community in the northern East China Sea (NECS) from 2018 to 2020 to understand biomass and compositional responses to environmental conditions in a complex current system. Correlation heatmaps and generalized additive models (GAMs) were used to explore the variations in major phytoplankton groups and their relationships to temperature, salinity, depth, and nutrients. The results indicated that the phytoplankton community structure was significantly different between 2018/2019 and 2020. Under the conditions of high dissolved inorganic nitrogen (DIN) and low dissolved inorganic phosphate (DIP) concentrations in winter 2020, cryptophytes replaced diatoms as the dominant species and were observed for the first time in this study area. Dinoflagellates predominated throughout the euphotic zone in spring 2020, and statistical analysis showed that dinoflagellates tended to prevail at low DIP and high DIN conditions compared with diatoms. Summer was the only season with strong water stratification that reoccurred every year, and diatoms were mainly concentrated at nutrient-enriched 1% light depths in the summers of 2018/2019. However, a large amount of Changjiang River Diluted Water (CDW) from the surface and western part of the study area affected all sampling sites eastward to 126°E in 2020, allowing diatoms to be dominant at 100% and 30% light depths in the westernmost stations in the NECS. In autumn 2020, diatoms decreased significantly because of DIP-limited environmental conditions and were replaced by small phytoplankton. Under expected warming ocean scenarios with human-induced nutrient inputs, small phytoplankton will become dominant, while spring dinoflagellate blooms will occur more frequently in the NECS. International monitoring programs for marine ecosystems are currently needed in the ECS.

Spatiotemporal nutrient patterns, composition, and implications for eutrophication mitigation in the Pearl River Estuary, China

Human activities and climate change have changed large river estuary ecosystems, which impacts nutrient enrichment in coastal waters. Enhanced nutrient levels lead to eutrophication, oxygen deficits, and frequent occurrences of harmful algal blooms. In this study, the spatiotemporal nutrient patterns and composition of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), as well as the relationships between DIN, DIP, eutrophication index (EI), and environmental factors were explored. The spatiotemporal pattern and composition of DIN and DIP in coastal water based on samples from fifteen stations in the Pearl River Estuary (PRE) were investigated during the dry (April), wet (July), and normal (October) seasons in 2018. The results showed that the average concentrations of DIN and DIP were 65.35 ± 47.72 μmol L−1 and 0.94 ± 0.70 μmol L−1 in the PRE, respectively. The concentration of DIN at most monitoring stations failed to reach the Grade IV national seawater quality standard, while the concentration of DIP did meet them. The ratio of DIN/DIP ranged from 4.33 to 242.50, with an average of 42.60 ± 47.06, exceeding the Redfield ratio. EI was high in the PRE, ranging from 0.01 to 67.19, with an average of 8.64 ± 11.94. Hotspots with high DIN and DIP concentrations were located in the upper PRE during all seasons. In addition, there were significant negative correlations among DIN, DIP, EI, and salinity (P < 0.05), except for DIP and EI in the wet water flow period, implying an impact from riverine nutrient input. Moreover, the DIN, DIP, and EI also had negative relationships with dissolved oxygen in all seasons (P < 0.05), indicating that eutrophication led to oxygen deficits in the PRE. Therefore, land–ocean integration management of PRE-associated nutrient criteria, composition, and sources should be introduced to effectively and efficiently mitigate eutrophication in the future.

Active dissolved organic nitrogen cycling hidden in large river and environmental implications

Large rivers are important terrestrial dissolved organic matter (DOM) sources to marginal seas, and dissolved organic nitrogen (DON) plays an essential role in DOM cycling. The Yellow River ranks as the fifth largest river (in length) in the world and is well-known for its high dissolved inorganic nitrogen (DIN) concentration and relatively low DON concentration, leading to extreme measuring uncertainties in DON and nitrogen isotopic composition (δ15N), consequently leaving its DON cycling as an unresolved puzzle. To fill such a knowledge gap, we analyzed 17 samples from the middle to downstream with a combination of spectroscopy, tangential flow filtration, nitrogen isotope, and DNA sequencing. DON<1kDa dominated the DON pool and significantly correlated inversely with DIN, indicating the DON<1kDa mineralized into nitrate. This finding was further supported by the observed Rayleigh fractionation in δ15NDON<1kDa and the spatial distribution pattern of ammonia-oxidizing bacteria/archaea abundance. The redundancy analysis revealed that the geographical features and the microbial community were closely related, which joined together to drive the DON cycling. In addition, we propose a rational method to quantify the flux of mineralized DON in large rivers. This study discovered the active DON cycling hidden in high DIN large river and highlighted the importance of DON mineralization as well as its role in marginal seas carbon cycling.

Land use conversion to improve water quality in high DIN risk, low-lying sugarcane areas of the Great Barrier Reef catchments

Eutrophication of coastal and nearshore receiving environments downstream of intensive agricultural production areas is a global issue. The Reef 2050 Water Quality Improvement Plan (2017–2022) sets ambitious targets for reducing pollutant loads entering the Great Barrier Reef from contributing agricultural catchments. At a regional scale, the Wet Tropics end-of-catchment target load reduction for dissolved inorganic nitrogen (DIN) is 60% from the 2012–2013 anthropogenic load level. However, not even with the combined efforts of the Reef Regulations (December 2019) mandate and adoption of best practice nutrient management on farm, is it likely that these DIN targets will be reached. Thus, there is a need for innovative and cost-effective approaches to deliver further water quality improvement. Transitioning low-lying, marginal sugarcane land to alternative land uses that require lower or no nitrogen inputs, but still provide farmers with income streams, is a potentially attractive solution. In this study, a multi-criteria analysis was conducted to identify sites suitable for such alternative land uses. The cost-effectiveness of DIN reductions from these land use changes were calculated, accounting for reductions in annuity gross margins and land conversion cost. In certain locations (where conversion costs are low and DIN reductions are high) treatment wetlands and no-input cattle grazing offer cost-effective DIN reduction in the range of 20–26$/kg DIN. This compares favourably with existing agricultural extension-based approaches (c. $50/kg DIN reduction). Ecosystem service wetlands (i.e., wetland restoration for fish production) – again when appropriately situated – offer the prospect of even more cost-effective performance (11–14 $/kg DIN reduction). These results, in conjunction with best management practices, support the premise that alternative land uses are cost-effective options for improving water quality in certain areas of low-lying, low productivity sugarcane land. On-going investments by government in addition to private market funding mechanisms could be appropriate for supporting such land use transitions. These approaches need to be tested and refined via targeted pilot projects, as part of a whole-of-landscape approach to achieve broader reef water quality targets.

Cross-basin differences in the nutrient assimilation characteristics of induced phytoplankton blooms in the subtropical Pacific waters

Abstract. To better understand the nutrient assimilation characteristics of subtropical phytoplankton, deep-water addition incubation experiments were carried out on surface waters collected at seven stations across the subtropical North and South Pacific Ocean. These deep-water additions induced phytoplankton blooms with nutrient drawdown at all stations. The drawdown ratios of dissolved inorganic nitrogen (DIN) to phosphate (PO4) varied from 14.1 to 30.7 at the PO4-replete stations in the central North Pacific (CNP) and eastern South Pacific (ESP). These ratios were similar to the range represented by the canonical Redfield ratio (16) through to typical particulate N:P ratios in the surface subtropical ocean (28). In contrast, lower DIN:PO4 drawdown ratios (7.7–13.3) were observed in induced blooms at the PO4-depleted stations in the western North Pacific (WNP). The DIN:PO4 drawdown ratios in the PO4-replete ESP were associated with eukaryote-dominated blooms, while those in the PO4-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus PO4 assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular N:P ratio and was likely due to the high PO4 uptake capability as induced by low-PO4-adapted phytoplankton. The low- and high-P* (=PO4- DIN/16) regimes geographically corresponded to the low and high DIN:PO4 drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide P* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (&lt;50 nM) to high in the ESP (&gt;100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling P* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.

Spatiotemporal variation, composition of DIN and its contribution to eutrophication in coastal waters adjacent to Hainan Island, China

The past few decades have seen a massive increase in global coastal eutrophication, for which nitrogen enrichment is usually the primary causative agent. Based on field monitoring data from coastal waters during dry, wet, and normal flow seasons in 2016, the distribution characteristics of dissolved inorganic nitrogen (DIN) and its influencing factors were studied. The level of DIN pollution was evaluated, and its contribution to the eutrophication in the coastal waters was analyzed. Furthermore, the main source and relationships of DIN were also discussed. The results showed that the concentration of DIN ranged from 0.008 mg/L to 0.384 mg/L, with an average of 0.114 ± 0.087 mg/L. Areas of high DIN concentration (>0.2mg/L) were located in coastal waters adjacent to Haikou City and Qiongzhou Strait in all seasons. The species and composition of DIN in the coastal waters showed seasonal variation. N-NO3− and N-NH4+ were the main species of DIN. The contribution of DIN to eutrophication ranged from 21.2% to 47.8%, with a mean value of 34.6% ± 4.2%. Under poor nutrient condition (EI<1), the contribution of DIN fluctuated with increasing of eutrophication index (EI). When EI was greater than 1, the contribution of DIN to the eutrophication remained stable. During the investigation period, the concentration of DIN showed a significantly negative correlation with dissolved oxygen. DIN was an important factor contributing to eutrophication, but it was not the decisive factor. With increasing EI, the contribution of chemical oxygen demand and dissolved inorganic phosphorus may become more important. The results provided a scientific basis for effectively solving the DIN pollution problem in the coastal waters of Hainan Island.

Nitrous oxide flux from a Tropical estuarine system (Cochin estuary, India)

The Cochin estuarine system (CES) has experienced rapid changes over the last few decades from extensive urbanisation, industrialisation, agricultural development activities and aquaculture practices. Intense pollution and other anthropogenic disturbances have led to an accelerated increase in organic matter (OM) and dissolved inorganic nitrogen (DIN) in the estuarine complex. This study explores nitrous oxide (N2O) fluxes and its drivers in the CES. Relatively higher N2O concentrations were observed in the inner estuary when compared to the outer estuary. Higher DIN with varying salinity gradients likely controlled the production and flux of N2O into the inner portions of the system. Hypoxia was never established in the water column indicating that conditions were favourable for nitrification, which in turn possibly led to increased N2O concentrations. It was observed particularly during the monsoon season that the inorganic nutrients and dissolved greenhouse gases from the river Periyar at the head of the estuary, contributed to the increased N2O concentrations. The N2O water to air flux ranged from 0.49 to 29.73μmol m−2 day−1, which was higher than what was documented in earlier reports. Although the CES exports a substantial amount of nutrients to the adjacent Arabian Sea, the estuary effectively processes nitrogen inputs as N2O making CES a net source of N2O emissions.

Net Submarine Groundwater‐Derived Dissolved Inorganic Nutrients and Carbon Input to the Oligotrophic Stratified Karstic Estuary of the Krka River (Adriatic Sea, Croatia)

AbstractSubmarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on 226Ra and 228Ra, the total SGD flux into the KRE surface layer was estimated to be (12.8–16.2) × 105 m3/day. A conservative estimation of the fresh groundwater flux was (5.0–8.3) × 105 m3/day, which accounts for 10–17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD‐derived dissolved inorganic nitrogen (DIN) and silicates contributed 58–90% and 24–64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD‐derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD‐derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol · m2 · day) was much higher than those in most estuaries worldwide, suggesting that the DIC‐enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD‐derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise.

Nutrient dynamics in the Changjiang and retention effect in the Three Gorges Reservoir

2009 ∼ 2010 was a critical period for the Three Gorges Reservoir (TGR) transition from staged impoundment to full operation. This study reports on the composition of nutrients in the whole Changjiang and retention effect of the reservoir during the transitional period, and the potential impact of the reservoir impoundment on the eutrophication of the TGR and estuary. Water samples were collected in the mainstream and tributaries of Changjiang from August to October 2009 and June 2010 to analyze the concentrations of dissolved and particulate nutrients, and suspended particulate matter (SPM). The Changjiang was enriched with dissolved inorganic nitrogen (DIN), dissolved silicate (DSi) and SPM, but depleted in PO43− and biogenic silica (BSi), with higher DIN:PO43− and DSi:PO43− ratios. DIN and PO43− concentrations were lower in the Jinshajiang, but increased in the Mid-downstream of the Changjiang due to high human activities and nutrients import from major tributaries. DSi concentration varied little along the Changjiang, and was relatively low in the TGR. Particulate inorganic phosphorus (PIP), total particulate phosphorus (TPP) and BSi concentrations were very high in the Jinshajiang, and then dramatically decreased with the SPM in the TGR. The northern tributaries contributed more nutrients to the Changjiang than the southern tributaries. Depending on the nutrients input-output mass balance model, the retention of nutrients in the TGR were higher for TPP and BSi, lower for dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP). Total dissolved nitrogen (TDN), total phosphorus (TP) and DSi retained in the reservoir accounted for 15%, 12% and 1%, respectively of the TDN, TP and DSi fluxes in the Datong station near the estuary. Historical data showed that the impoundment of the TGD greatly contributed to the eutrophication of the reservoir and estuary. Since 2003, algal blooms were likely to occur in the TGR, estuary and adjacent seas, but the outbreak frequency and area of red tides in the East China Sea (ECS) dramatically decreased. Therefore, further study is required to estimate the response of estuarine ecosystem on the dispatching mode of the TGR.

Temporal and spatial variations and impact factors of nutrients in Bohai Bay, China

The temporal and spatial distributions of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi), and their long-term changes were investigated in Bohai Bay (BHB) in spring, summer, and autumn (2013–2014). The high DIN values were consistently distributed in the western inshore waters, mainly determined by terrestrial factors, e.g., riverine input, while DIP and DSi were mostly distributed in the southern coastal waters, the central BHB, or near the sea port Caofeidian in northern BHB, largely related to non-terrestrial factors, e.g., sediment release. Based on the nutrient distribution, BHB could be partitioned into western and eastern parts, with −15 m depth as the separation. The long-term variations of nutrients since 2000 showed an increase in DIN and decreases in DIP and DSi. Relatively slow changes in DIN and DIP and a rapid decrease in DSi were exhibited in summer, which was associated with precipitation and sediment release.

Stable isotopes in helophytes reflect anthropogenic nitrogen pollution in entry streams at the Doñana World Heritage Site

Nitrogen (N) loading from anthropogenic activities is contributing to the eutrophication and degradation of wetlands worldwide. Doñana (southwestern Spain), includes a dynamic marshland protected as a UNESCO World Heritage Site, which has a catchment area exposed to increasing N inputs from intensive agriculture and poorly treated urban wastewaters. Identifying the sources of N entering this iconic wetland complex is vital for its conservation. To this end, we combined multiyear (2014–2016), spatially-explicit data on N concentration in water samples with measurements on the relative abundance of N stable isotopes (δ15N) in Bolboschoenus maritimus and Typha domingensis, two dominant helophytes (i.e. emergent macrophytes) in the Doñana marsh and entry streams. Overall, plant tissues from entry streams showed higher δ15N values than those from the marsh, particularly in those streams most affected by urban wastewaters. Isotopic values did not differ between plant species. Water samples affected by isotopically-enriched urban wastewaters and other diffuse organic N inputs (e.g. livestock farming) had relatively high Dissolved Inorganic Nitrogen (DIN) concentrations. In contrast, in streams mainly affected by diffuse N pollution from greenhouse crops, high DIN values were related to isotopically-depleted N sources (e.g., inorganic fertilizers). Thus, helophytes, in combination with other parameters such as N concentration in water or land cover, can be valuable indicators of anthropogenic pressures in Mediterranean wetlands. Helophytes have widespread distributions, and can be readily sampled even when water is no longer present. However, identification of specific N sources through helophyte δ15N values is limited when key potential N sources are isotopically undistinguishable (e.g. fertilizers vs. atmospheric sources).