Decadal changes in nutrient fluxes and environmental effects in the Jiulong River Estuary

Phosphorus starvation response genes and function coupling: A mechanism to regulate phosphorus availability in a subtropical estuary

Phosphorus (P) plays an important role in regulating primary production in estuarine environments. However, knowledge of the P-functional gene composition of microbial communities and the mechanisms of microbial adaptation to changes in available P in estuaries remain limited. This study coupling 16 s rDNA and metagenomics sequencing was conducted to reveal the relationship between P cycling functional genes, microbial interactions, and P availability in the Jiulong River Estuary. The results showed that the relative abundance of P cycling functions genes was highest in winter, and lowest in summer. Spatially, the total relative abundance of P cycling functions genes was higher in the riverward than that in the seaward. P cycling functional microbial interactions and P cycling gene coupling were strongest in summer and in the seaward. Changes in both temperature and salinity had significant direct and indirect effects on P cycling function, and the influence of salinity on P cycling function was greater than that on the microbial community in the estuary. Salinity had significant direct negative effects on inorganic P-solubilization (IP), organic P-mineralization (OP), and P uptake and transport functions (PT). Whereas, salinity had a significant positive effect on P-starvation response regulation (PR) function. Thus, salinity and microbial communities regulate the soluble reactive phosphate concentrations in estuarine environments by strengthening internal coupling among P cycling functions, promoting PR function, and facilitating PT gene expression. PR is the most important predictors, PR, PT, and PR-PT together explained 38.56 % of the overall soluble reactive phosphorus (SRP) variation. Over 66 % of the explained SRP variations can be predicted by the PR, PT, and PR-PT functional genes. This finding improves the knowledge base of the microbial processes for P cycling and provides a foundation for eutrophication management strategies in the estuary.

Microbial interactions strengthen deterministic processes during community assembly in a subtropical estuary

Systematic studies on the assembly process and driving mechanisms of microbial communities in estuaries with diverse seasonal and spatial scales are still limited. In this study, high-throughput sequencing, and microbial network analysis were combined to decipher the impact of environmental changes and biological interactions on the maintenance of microbial diversity patterns in the Jiulong River Estuary (JRE). The results showed that overall, stochastic processes dominated the bacterioplankton community assembly in the estuary, accounting for 49.66-74.78 % of the total. Additionally, bacterioplankton community diversity varied significantly across seasons and subzones. Specifically, the concentration of soluble reactive phosphorus (SRP) in the estuary steadily reduced from winter to summer, and the corresponding bacterioplankton community interactions gradually shifted from the weakest interaction in winter to the strongest in summer. The deterministic processes contributed more than half (50.34 %) to microbial assembly in the summer, but only 25.22 % in winter. Deterministic processes prevailed in the seaward with low SRP concentrations and strong bacterioplankton community interactions, while stochastic processes contributed 70.14 % to the assembly of microbial communities riverward. Biotic and abiotic factors, such as nutrients and microbial interactions, jointly drove the seasonal and spatial patterns of bacterioplankton community assembly, but overall, nutrients played a dominant role. Nevertheless, the contributions of nutrients and microbial interactions were equivalent in spatial assembly processes, albeit nutrients were the primary seasonal driver of the bacterioplankton community assembly process. This study emphasizes the significance of microbial interactions in the bacterioplankton community assemblage. These findings provide new and comprehensive insights into the microbial communities' organization in estuaries.

Multiple stable isotopic approaches for tracing nitrate contamination sources: Implications for nitrogen management in complex watersheds

Nitrate (NO3-) contamination of surface water is a global environmental problem that has serious consequences for watershed ecosystems and endangers human health. It is crucial to identify influences of different sources of NO3-, especially the incoming water from upper reaches. A combination of hydrochemistry and multi-isotope tracers (δ11B, δ15N-NO3-, and δ18O-NO3-) were used to determine NO3- sources and their transformation the North Jiulong River (NJLR), Southeast China. The findings revealed that NO3-, which accounted for an average of 87.1% of dissolved inorganic nitrogen (DIN), was the main chemical form of nitrogen species. The integration of dual stable isotopes of NO3-, δ11B, and hydrochemistry showed that NO3- was primarily contributed by sewage, soil nitrogen (SN), and ammonium (NH4+) via precipitation or fertilizers. The contributions from the sewage and soil nitrate source were almost equivalent and much higher than those from other sources in the NJLR watershed. The contributions from diverse sources varied seasonally and spatially. Manure and sewage (M&S) were the leading sources in the summer and autumn, accounting for 60.9 ± 8.5% and 47.3 ± 7.9%, respectively. However, NO3- fertilizers were the predominant source in the spring and winter. The NO3- inflow from upper reaches was proposed as an additional end-member to identify its contribution in the midstream and downstream in this study. The contributions of NO3- from the upper reaches were significant sources in the midstream and downstream, accounting for 27.2 ± 17.8% and 42.9 ± 21.9%, respectively. The obvious decline in local NO3-contribution shares from midstream to downstream implied structural changes in pollutant sources and regional environmental responsibility. Therefore, tracing nitrate sources and quantifying their contributions is critical for clarifying environmental responsibilities for precise local nitrogen management in watersheds.

Spatial variability of nitrogen cycling in the sediments of the Danshuie River Estuary (Northern Taiwan)

Dissolved N species, TOC and total N (TN) in sediment cores (SC) collected from an eutrophic estuary were analyzed to understand the N geochemical variation in SC of the eutrophic estuary. Extremely higher concentrations of ammonium (6550 μM) and DON (2050 μM) were observed in pore water of the upper estuary and both concentrations generally accounted for 65-99 % and 1-34 % of the dissolved total N pool, respectively, in the three sediment pore waters. The DON and TN concentrations decreased with increasing depth in SC of the upper estuary, opposite the ammonium profile, suggesting that the mineralization of DON and TN provided the ammonium source to the SC. While, the TN mineralization was more profound than the DON mineralization in SC of the middle and lower estuary. The mineralization rate of DON and TN obviously differed from the different depth intervals of the three SC.

Nitrogen enrichment ameliorates the stimulatory effects of reduced salinity on photosynthesis and growth of phytoplankton assemblages in the northern South China sea

Anthropogenic activities and climate change are exacerbating the occurrence of extreme rainfall that normally brings large amounts of nutrient-rich freshwater from the land to the sea, resulting in acute salinity decrease and nutrient increase. To evaluate the effects of such changes in salinity and nutrients, we tracked the changes in photosynthetic efficiency and growth of phytoplankton assemblages from the northern South China Sea at 5 salinity levels and at an intermediate salinity level with 3 or 4 nitrogen concentrations. The results showed that the reduction of salinity reduced the maximum photochemical quantum yield (FV/FM) of photosystem II of phytoplankton within a short-term cultivation (i.e. 24-72 h), followed by a stimulatory effect. The reducing effect of reduced salinity lasted longer in the nearshore area than in the offshore area, so the stimulatory effect occurred later in the former area. Nitrogen enrichment mitigated the negative effect of reduced salinity in short-term cultivation and showed a positive effect on FV/FM in long-term cultivation. Moreover, both reduced salinity and enriched nitrogen stimulated phytoplankton growth after an acclimation period. Our results suggest that the reduced salinity stresses phytoplankton in the short term, which is mitigated by nitrogen enrichment, but benefits them in the long term. This sheds light on how phytoplankton thrive and even flourish in coastal or estuarine environments where salinity and nutrients typically covary strongly after extreme rainfall.

Soil organic carbon storages and bacterial communities along a restored mangrove soil chronosequence in the Jiulong River Estuary: From tidal flats to mangrove afforestation

Among many ecological services provided by mangrove ecosystems, soil organic carbon (SOC) storages have recently received much attention owing to the increasing atmospheric partial pressure of dissolved CO2 (pCO2). Bacteria are fundamental to ecosystem functions and strongly influence the coupling of coastal carbon, nitrogen, and sulfur cycling in soils. The SOC storage and bacterial communities along a restored mangrove soil chronosequence in the Jiulong River Estuary were explored using the 16S rDNA sequencing technique. The results showed the SOC storage in the 100 cm soil profile was 103.31 ± 5.87 kg C m-2 and 93.10 ± 11.28 kg C m-2 for mangroves with afforestation ages of 36 and 60 years, respectively. The total nitrogen (TN) and total sulfur (TS) contents exhibited significant correlations with the SOC in the mangrove soils, but only TN and SOC showed significant correlation in tidal flat soils. Although the tidal flats and mangroves occupied the contiguous intertidal zone within several kilometers, the variations in the SOC storage along the restored mangrove soil chronosequence were notably higher. The Functional Annotation of Prokaryotic Taxa (FAPROTAX) database was used to annotate the metabolic functions of the bacteria in the soils. The annotation revealed that only four metabolic functions were enriched with a higher relative abundance of the corresponding bacteria, and these enriched functions were largely associated with sulfate reduction. In addition, the specifically critical bacterial taxa that were associated with the SOC accumulation and nutrient cycling, shaped the distinct metabolic functions, and consequently facilitated the SOC accumulation in the mangrove soils with various afforestation ages. The general homogenization of the microbial community and composition along the intertidal soil chronosequence was primarily driven by the reciprocating tidal flows and geographical contiguity.

Spatio-temporal analysis of the sources and transformations of anthropogenic nitrogen in a highly degraded coastal basin in Southeast China

Nitrogen transport from terrestrial to aquatic environments could cause water quality deterioration and eutrophication. By sampling in the high- and low-flow periods in a highly disturbed coastal basin of Southeast China, hydrochemical characteristics, nitrate stable isotope composition, estimation of potential nitrogen source input fluxes, and the Bayesian mixing model were combined to determine the sources and transformation of nitrogen. Nitrate was the main form of nitrogen. Nitrification, nitrate assimilation, and NH4+ volatilization were the main nitrogen transformation processes, whereas denitrification was limited due to the high flow rate and unsuitable physicochemical properties. For both sampling periods, non-point source pollution from the upper to the middle reaches was the main source of nitrogen, especially in the high-flow period. In addition to synthetic fertilizer, atmospheric deposition and sewage and manure input were also major nitrate sources in the low-flow period. Hydrological condition was the main factor determining nitrate transformation in this coastal basin, despite the high degree of urbanization and the high volume of sewage discharge in the middle to the lower reaches. The findings of this study highlight that the control of agricultural non-point contamination sources is essential to pollution and eutrophication alleviation, especially for watersheds that receive high amounts of annual precipitation.

Nitrogen and phosphorus spatio-temporal distribution and fluxes intensifying eutrophication in three tropical rivers of Côte d'Ivoire (West Africa)

Nutrient contamination assessments in the three West African tropical Comoé, Bandama, and Bia Rivers (Côte d'Ivoire) were performed from March 2016 to March 2018. Five stations per river were sampled. Nutrients spatio-temporal distributions were mapped and showed nitrogen concentrations (nitrite 0.001 to 0.025 mg/L NO₂^--N, and nitrate 0.26 to 3.60 mg/L NO₃^--N) increased significantly with rainfall contrary to phosphorus (0.01 to 0.12 mg/L P). The Chl-a and TSI_tsr data revealed the hypereutrophic status of rivers. Moreover, N:P mass ratio suggests nitrogen as the main limiting factor of primary production during the low (March) and high flow periods (October-November), while phosphorus is the limiting factor in June, at the high flow beginning. The land uses around watersheds were the main sources of phosphorus and nitrogen enhancing the rivers' eutrophication. Phosphorus and nitrogen fluxes were related to leaching river catchments and were significant sources of nutrients to the Atlantic Ocean.

Interannual and Decadal Changes in Harmful Algal Blooms in the Coastal Waters of Fujian, China

The temporal and spatial variability of harmful algal blooms (HABs) in coastal waters of Fujian were analyzed at interannual and decadal scales based on recorded HAB events collected from 1956 to 2019. The number and impact area of HABs exhibited little change from 1959 to the 1990s, sharply increased from the 1990s to 2000s, and decreased from the 2000s to 2010s. The highest number and greatest coverage of blooms occurred in the 2000s. The proportion of HABs caused by dinoflagellates increased, while the proportion caused by diatoms decreased from the 2000s to the 2010s. Toxic HAB events caused by Karenia mikimotoi increased in frequency and spatial coverage in the 2010s, especially on the central Fujian coast. Increasing concentrations of dissolved inorganic nitrogen and dissolved inorganic phosphorus have been essential for increasing HAB occurrences since the 1980s. The combined effects of eutrophication and climate change have been suggested to be important reasons for long-term changes in HABs. Knowledge of the change patterns in and the mechanisms of HABs gained in this study will extend the current understanding of HABs along the Fujian coast and support future studies on HAB monitoring, early warning, prevention, and management.

Diel change in inorganic nitrogenous nutrient dynamics and associated oxygen stoichiometry along the Pearl River Estuary

The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using ¹⁵N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (∼250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 days), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including substrate, organics and oxygen, of coastal microbes in water column and sediments.

Spatial-temporal distribution of total organic carbon and its transportation in the Jiulong River Estuary

Spatial and temporal distributions of total organic carbon (TOC) in the Jiulong River Estuary (JRE) were determined using data collected during three cruises in summer 2010, autumn 2010, and spring 2011. The TOC concentration influencing factors were identified, and the export fluxes were calculated. TOC concentration ranges were 0.73-4.17 mg/L in summer, 0.90-5.32 mg/L in autumn, and 1.78-8.03 mg/L in spring, respectively. TOC concentrations of the surface water and nearshore area were higher than those of the bottom water and offshore area, respectively, and the maximum TOC content occurred in the JRE upper reaches. The TOC concentration decreased with increasing salinity and exhibited a significant positive correlation with petroleum and dissolved inorganic nitrogen (DIN), indicating the influence of terrestrial input. A weak relationship between TOC and chlorophyll-a indicated that phytoplankton was not the dominant source of TOC. TOC fluxes discharged into the JRE were 50.39 × 10³ t/a in 2010 and 46.08 × 10³ t/a in 2011, and those transported into the sea were 38.46 × 10³ t/a in 2010 and 33.66 × 10³ t/a in 2011, respectively, accounting for approximately 75% of the total estuary fluxes. This study elucidates the biogeochemical processes of estuarine organic carbon and provides a quantitative basis for the land-sea integration of carbon dioxide emission reduction and sink increase projects.

Differential Response of Nutrients to Seasonal Hydrological Changes and a Rain Event in a Subtropical Watershed, Southeast China

A large amount of terrestrial nutrients are discharged into the ocean through rivers. However, the impact of seasonal hydrological variations on riverine nutrient concentrations and fluxes remains unclear, especially in the medium-sized subtropical rivers that are highly influenced by human activities. In this study, we investigated the monthly changes in nutrient concentrations (soluble reactive phosphorus, SRP; dissolved silicate, DSi; and dissolved inorganic nitrogen, DIN) in the North Stream (NS) and West Stream (WS) of the Jiulong River (JLR). The results show that the concentrations of SRP and DSi in the NS and the WS displayed a similar seasonal variability, which was different from the pattern of DIN. Hydrological conditions, chemical fertilizer loss and biogeochemical processes are responsible for the seasonal changes in the nutrients in the two streams, especially during extreme rain events. Nutrient concentrations in the NS exhibited a clockwise trajectory along with river discharge during rain events, while a reverse pattern in the WS was observed since it experienced a moderately long rain event. Different rainfall features between the two main tributaries resulted in the majority of nutrients being exported at the start of the rain event in the NS and the end of rain event in the WS. Indeed, the annual high flow (Q/Qm > 3) accounts for ~17.3% of the annual nutrient flux in the JLR even though this period spans only ~4.0% of a year, which suggests the importance of rain events on nutrient export in these subtropical rivers. Although the annual fluxes of DIN and SRP in the JLR were smaller than many rivers worldwide, higher areal yields of DIN and SRP were observed, indicating that the JLR is highly influenced by human activities. Our study systematically evaluated the response of nutrient concentrations to hydrological changes in two tributaries of the JLR, which is useful in better understanding the nutrient dynamics in medium-sized subtropical rivers.

Dissolved and particulate nitrogen species partitioning and distribution in the Danshuei River estuary, northern Taiwan

Danshuei River Estuary (DRE) total and inorganic nitrogen in the dissolved (TDN, DIN) and particulate (TPN, PIN) phases were analyzed to study their distribution and partitioning. The carbon contents in particles were also analyzed. The upper estuary contained higher ammonium concentration (304-557 μM), leading to TDN completely dominating (>95%) the total N (TDN + TPN) pool within the DRE. Ammonium played a crucial role in controlling the speciation variation of DIN and partitioning between dissolved and particulate phases. Nitrification seemed to occur in the salinity >30 region where elevated percentages of nitrite and nitrate were observed. PON dominated the particulate N and contributed an average of 62% of the TPN pool. A constant organic C/N ratio (6.55) was observed in particles, indicating that POM was mainly from phytoplankton detritus. The N distribution coefficient values, log(K_D), ranged from 3 to 4, suggesting that the affinity of DIN for particles was weak.

Genomic Characteristics of a Novel Species of Ammonia-Oxidizing Archaea from the Jiulong River Estuary

Ammonia-oxidizing archaea (AOA) are ubiquitous in diverse ecosystems and play a pivotal role in global nitrogen and carbon cycling. Although AOA diversity and distribution are widely studied, mainly based on the amoA (alpha subunit of ammonia monooxygenase) genotypes, only limited investigations have addressed the relationship between AOA genetic adaptation, metabolic features, and ecological niches, especially in estuaries. Here, we describe the AOA communities along the Jiulong River estuary in southern China. Nine high-quality AOA metagenome-assembled genomes (MAGs) were obtained by metagenomics. Five of the MAGs are proposed to constitute a new species, "Candidatus Nitrosopumilus aestuariumsis" sp. nov., based on the phylogenies of the 16S and 23S rRNA genes and concatenated ribosomal proteins, as well as the average amino acid identity. Comparative genomic analysis revealed unique features of the new species, including a high number of genes related to diverse carbohydrate-active enzymes, phosphatases, heavy-metal transport systems, flagellation, and chemotaxis. These genes may be crucial for AOA adaptation to the eutrophic and heavy-metal-contaminated Jiulong River estuary. The uncovered detailed genomic characteristics of the new estuarine AOA species highlight AOA contributions to ammonia oxidation in the Jiulong River estuary.IMPORTANCE In this study, AOA communities along a river in southern China were characterized, and metagenome-assembled genomes (MAGs) of a novel AOA clade were also obtained. Based on the characterization of AOA genomes, the study suggests adaptation of the novel AOAs to estuarine environments, providing new information on the ecology of estuarine AOA and the nitrogen cycle in contaminated estuarine environments.

Metal/metalloid and phosphorus characteristics in porewater associated with manganese geochemistry: A case study in the Jiulong River Estuary, China

Sediment porewater can be an important source of contaminants in the overlying water, but the mechanisms of metal(loid) and phosphorus (P) remobilization remain to be investigated. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) samplers were used to determine the porewater dissolved iron (Fe), manganese (Mn), cobalt (Co), chromium (Cr), vanadium (V), selenium (Se), arsenic (As), P and DGT-Labile S in coastal sediments in the Jiulong River Estuary (JRE), China. The results showed that high concentrations of dissolved Mn, Se and P were present in the overlying water, indicating potential water pollution with excessive amounts of Mn, Se and P. The dissolved Mn concentrations in the porewater were higher than the dissolved Fe concentrations, especially at submerged sites, demonstrating that Mn(III/IV) reduction is the dominant diagenetic pathway for organic carbon (OC) degradation, which directly affects Fe cycling by the competitive inhibition of Fe(III) reduction and Fe(II) reoxidation. Dissolved Co, Cr, V, Se, As and P show significant positive correlations with Mn but nearly no correlations with Fe, suggesting that the mobility of these metal(loid)s and P is associated with Mn but not Fe cycling in this region. In addition, the coelevated concentrations of the metal(loid)s, P and Mn at the submerged sites are attributed to the strengthened Mn reduction coupled with OC degradation fueled by hypoxia. The higher positive diffusion fluxes of Mn, Se and P were consistent with the excess Mn, Se and P concentrations in the overlying water, together with the approximately positive fluxes of the other metal(loid)s, indicating that sediment Mn(III/IV) reduction and concomitant metal(loid) and P remobilization might be vital pathways for metal(loid) and P migration to the overlying water.

Transforming nitrogen management of the urban food system in a food-sink city

Nitrogen flows in urban food systems are attracting increasing concern. However, characteristics of nitrogen flow and systematic measures to reduce reactive nitrogen losses in the food systems of consumption-oriented cities in developing countries have not been well understood, especially in a quantitative way. This study empirically investigates the transforming nitrogen flows of an urban food system in a food-sink city in China, with a nitrogen metabolism model. Three types of nitrogen loads transfer are identified: from production to consumption side, between different environmental media, and from areas within to areas beyond the city boundary. By integrating sensitivity analysis into the metabolism model, increases in the sewage treatment rate, the sewage nitrogen removal rate, and the ratio of animal excreta returned to field are found to contribute the most to the water nitrogen load reduction, and reducing food waste at the consumer level is the most influential measure for lowering soil nitrogen loads, under the existing nitrogen flow regime. Additionally, a three-tier template framework is proposed to streamline city strategies (prevention, abatement, recycling, regional cooperation, etc.) for reducing the N loads of urban food systems, providing references for sustainable nutrient management in urban ecosystems.

Occurrence, seasonal variation and risk evaluation of selected endocrine disrupting compounds and their transformation products in Jiulong river and estuary, China

In the present study, 13 endocrine disrupting compounds (EDCs) and their transformation products (TPs) were monitored in Jiulong River and its estuary, China in different seasons. The analytes included antimicrobials [triclosan (TCS) and triclocarbon (TCC)]; estrogens [estrone (E1), estradiol (E2), estriol (E3) and ethinylestradiol (EE2)]; alkylphenol ethoxylates [4-n-nonylphenol (NP) and 4-n-octylphenol (OP)] and the TPs [methyl triclosan (MeTCS), carbanilide (NCC), dichlorocarbanilide, 4-hydroxy estrone (4-OH E1) and 4-hydroxy estradiol (4-OH E2)]. A significant seasonal variation was observed for most EDCs. Approximately, 79% of the total E2 residues were detected in the normal season in comparison with the wet season to indicate recalcitrant behavior of E2 during the normal season. Risk assessment revealed that E2 was most potent among the EDCs to cause highest risk to both vertebrate and invertebrate aquatic species, whereas, E1, EE2, MeTCS and TCS also showed relatively high risk for some surface water aquatic species.

Riverine nutrient fluxes and environmental effects on China's estuaries

An increase in riverine nutrient fluxes significantly influences the estuarine ecosystem. This study collected nutrient data in most of China's rivers from 1963 to 2015 to estimate the nutrient fluxes from major rivers and analyze interannual variability of nutrient fluxes and estuarine environmental effects. The results showed that the nutrient fluxes from the Yangtze River increased annually from 1963 to 2012. The trend of nutrient fluxes from the Yellow River was consistent with that from the Jiulong River, i.e., nutrient fluxes increased from 1998 to 2007 and then decreased. The areal nutrient fluxes from China's major rivers were higher than those from major world rivers, while the areal nutrient yield rates per capita were lower than those from major world rivers. We also found that China's estuaries were predominantly phosphorus-limited and slowly moving toward lower dissolved silica over dissolved inorganic nitrogen (DSi:DIN) ratios with time. Meanwhile, the nutrient limitation of phytoplankton growth in most of China's estuary systems was moving toward a higher incidence of phosphorus and silicon limitations as a result of increased DIN fluxes, and this would likely alter phytoplankton communities. Furthermore, the decreases in the DSi:DIN ratio and dissolved silica over dissolved inorganic phosphate (DSi:DIP) ratio, and the increases in both DIN and DIP fluxes, caused increased red tide blooms.

Stable isotopic evidence of nitrogen sources and C4 metabolism driving the world's largest macroalgal green tides in the Yellow Sea

During recent years, rapid seasonal growth of macroalgae covered extensive areas within the Yellow Sea, developing the world's most spatially extensive "green tide". The remarkably fast accumulation of macroalgal biomass is the joint result of high nitrogen supplies in Yellow Sea waters, plus ability of the macroalgae to optionally use C4 photosynthetic pathways that facilitate rapid growth. Stable isotopic evidence shows that the high nitrogen supply is derived from anthropogenic sources, conveyed from watersheds via river discharges, and by direct atmospheric deposition. Wastewater and manures supply about half the nitrogen used by the macroalgae, fertiliser and atmospheric deposition each furnish about a quarter of the nitrogen in macroalgae. The massive green tides affecting the Yellow Sea are likely to increase, with significant current and future environmental and human consequences. Addressing these changing trajectories will demand concerted investment in new basic and applied research as the basis for developing management policies.

Optimal reservoir operation using multi-objective evolutionary algorithms for potential estuarine eutrophication control

Increased nutrient loads and changed nutrient ratios in estuarine waters have enhanced the occurrence of eutrophication and harmful algae blooms. Most of these consequences are caused by the new proliferation of toxin-producing non-siliceous algae. In this study, we propose a multi-objective reservoir operation model based on 10-day time scale for estuarine eutrophication control to reduce the potential non-siliceous algae outbreak. This model takes the hydropower generation and social economy water requirement in reservoir into consideration, minimizing the ICEP (indicator of estuarine eutrophication potential) as an ecological objective. Three modern multi-objective evolutionary algorithms (MOEAs) are applied to solve the proposed reservoir operation model. The Three Gorges Reservoir and its operation effects on the Yangtze Estuary were chosen as a case study. The performances of these three algorithms were evaluated through a diagnostic assessment framework of modern MOEAs' abilities. The results showed that the multi-objective evolutionary algorithm based on decomposition with differential evolution operator (MOEA/D-DE) achieved the best performance for the operation model. It indicates that single implementation of hydrological management cannot make effective control of potential estuarine eutrophication, while combined in-estuary TP concentration control and reservoir optimal operation is a more realistic, crucial and effective strategy for controlling eutrophication potential of non-siliceous algae proliferation. Under optimized operation with controlled TP concentration and estuarine water withdrawal of 1470 m³/s, ecological satiety rate for estuarine drinking water source increased to 77.78%, 88.89% and 83.33% for wet, normal and dry years, the corresponding values in practical operation were only 72.22%, 58.33% and 55.56%, respectively. The results suggest that these operations will not negatively affect the economic and social interests. Therefore, the proposed integrated management approaches can provide guidance for water managers to reach a stable trophic control of estuarine waters.

Assessment of nitrogen reduction by constructed wetland based on InVEST: A case study of the Jiulong River Watershed, China

The Jiulong River watershed (JRW) in southeast China includes livestock breeding and agriculture, leading to large amounts of non-point source pollution. Nitrogen (N) reductions were simulated and mapped using the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) under scenarios that were built considered both constructed wetlands (CWs) and climate change, which are not common in the literature on ecosystem services assessments. The results showed that the amount of N exported from non-point sources within the JRW was 12,569 t·yr^-1. The areal N load was relatively higher in the north, while more N exported in the southeast. Constructed riparian wetlands can intercept and reduce the N loads that enter water bodies, but climate change may be a factor driving the deterioration of water quality. The methodology can be generalized to reduce other contaminants, and provides a tool for decision-makers to weigh the costs and benefits of urbanization and conservation.