River Nutrient Research Articles

Dissolved silicon and nitrogen in glacial rivers and water of Blago bay (Russian Arctic, Novaya Zemlya): origin, variability and spreading

Hydrochemical studies of watercourses and the water area of Blagopoluchia bay (Novaya Zemlya, Arctic, Russia) have been carried out. The concentrations of nutrients in rivers and streams are higher than those in the water area of Blagopoluchia bay. It is shown that the concentration of silicon in constantly flowing rivers is 1–13 μM, the concentration of NO3 — 0.5–8, for small and temporary streams these values are higher and are in the range of 18–46 μM Si, 1–11 μM NO3– . The influence of streams and rivers flowing into Blagopoluchia Bay on the water area of the bay is local and extends to 1 km from the mouth, and does not influence the Kara Sea nutrient content.

Spatial scale effects of integrated landscape indicators on river water quality in Chaohu Lake basin, China.

Climate change and rapid urbanization have changed the characteristics of basin landscapes. Non-point-source (NPS) pollution affects river water quality. Exploring the impact of landscapes on river water quality is crucial for the control of water pollution in a basin. Current researchers focus on the impact of landscape pattern change on NPS pollution in the basin, but few consider climate, terrain, soil, and other geographical factors. In this study, we selected a subtropical agricultural basin in China named Chaohu Lake basin as the study area, added precipitation, soil erosion resistance, and slope to the original landscape pattern indicators. We quantified the spatial scale effect and seasonal dependence of integrated landscape indicators on water quality and comprehensively analyzed the optimal spatial scale and key landscape indicators. According to the nonlinear relationship between the key landscape indicators and river nutrients, we also determined the Type-1 threshold values of key landscape indicators for water quality protection in the basin. The results showed that the rivers in Chaohu Lake basin were mainly polluted by nitrogen and phosphorus. The strength of interpretation of the integrated landscape indicators of river water quality increased with riparian zone width. We determined the subbasin scale to be the optimal spatial scale. The key landscape indicators affecting water quality in the wet season at the optimal scale were precipitation and aggregation index of construction land (AIbul), whereas those in the dry season were AIbul and COHESION. The interpretation of the key landscape indicators in the wet season was slightly higher than that in the dry season. The above conclusions provide a scientific reference for NPS pollution control and water quality protection in subtropical agricultural basins.

Silicon composition and stoichiometric ratios with other nutrients in the lower Minjiang River, Southeast China.

Over the past decades, rivers have delivered imbalanced nutrient loads to coastal marine ecosystems due to human activities, which leads to serious regional or global eutrophication problems. The Minjiang River is heavily influenced by human activities. To understand the changing characteristics of nutrient transport ratios in the Minjiang River waters, we measured the seasonal variations of carbon, nitrogen, phosphorus and silicon nutrients in the lower surface waters of the Minjiang River between July 2019 and July 2020. The results showed that the annual average contents of dissolved silicon (DSi), lithogenic silicon (LSi) and biogenic silicon (BSi) in the surface waters of the lower Minjiang River were 5.30, 4.58 and 2.37 mg·L-1, respectively. There were large seasonal differences among these parameters, with higher content of DSi than LSi and BSi in summer, higher content of DSi than BSi and LSi in autumn and higher content of LSi than DSi and BSi in winter. The proportions of DSi in total silicon tended to decrease gradually from land to sea, while the proportion of BSi was on the contrary. In term of stoichiometric ratios, the Minjiang River mostly presented carbon or phosphorus limitation and was unlimited by silicon or nitrogen. About 1.03×1010 mol DSi and 0.46 ×1010 mol BSi were delivered via the Minjiang River to the ocean yearly, showing a decreasing trend year by year. Based on the data in recent years, the nutrient loads of carbon, nitrogen, and phosphorus transported by Minjiang River showed an increasing trend. The imbalanced nutrient loads may lead to changes in the structure and function of the river, estuary, and offshore ecosystems. The study of nutrient stoichiometric ratios can provide a theoretical basis for solving the problems in structural balance of nutrients and eutrophication in Minjiang River estuary and adjacent marine waters.

Metagenomic and isotopic signatures of the Amazon River plume into the southern red snapper fish (Lutjanus purpureus)

The Amazon reef system harbors a large proportion of the fish diversity known for the SW Atlantic, including some critical fishery resources, such as the Southern Red Snapper, Lutjanus purpureus, that thrives in the Great Amazon Reef System (GARS). Brazilian exports of L. purpureus result in millions of dollars every year, being socio-economically relevant at northern Brazil. Its capture is largely associated with the GARS. On the other hand, its regional abundance is attributed to the productivity of the area, which is, in turn, the result of the Amazon River plume (ARP). Thus, its trophodynamics is rather complex and important to understand the ARP-GARS linkage. The aim of the present study was to evaluate the trophic ecology of this fish and the possible influences of the nutrients from the ARP. To this end, gut microbiome and stable isotopic profiling of the gut and muscles of L. purpureus were performed. Fish samples were collected at two different stations (S8, S10) at different distances from the coast. The isotopic composition (δ13C and δ15N) of the fish tissues shows the influence of the river plume (particulate and dissolved), suggesting a substantial contribution of continental organic matter to L. purpureus food items. A total of ten 16S rRNA illumina libraries were obtained from the gut contents (546,301 sequences). The presence of picoplanktonic plume microbes (Synechococcus, Cyanobium, SAR324, Blastopirellula, Vibrio) in the gut microbiome of L. purpureus reinforces the influence of the river plume on the diet of this fish. Fluxes from the Amazon Forest and river nutrients which reach the ocean may indirectly contribute to the nutrition of L. purpureus through the food web by fueling its invertebrates and fish preys.

Restoration of Qaraoun Lake aquatic life based on wetland treatment concept

Qaraoun Lake, the largest artificial lake in Lebanon, suffered severe environmental issues due to discharging untreated domestic and industrial wastewater into it, throwing garbage, which transformed this lake into waste storage instead of using the water for agricultural purposes and making the surrounding places attractive for tourists as was before. Moreover, the violations on Litany River, Lebanon's main artery, also affected Qaraoun Lake. Therefore, this main reservoir suffers from annual blooms of potentially toxic cyanobacteria. Recently, tons of fish are washed up at the surface of the water, agricultural areas are irrigated with polluted water and the Qaraoun Lake is no longer an attractive touristic place. Besides, the climate change represented in lower precipitation and higher evaporation rates in the past few years in addition to the increase in the water demand due to the growth in the local population and the refugees from nearby countries have affected the vulnerability of the water sector in Lebanon. All these issues have resulted in the deterioration of the water quality, generating environmental issues, and seriously affecting the ecosystem. The purpose of this research is to investigate possible remediation strategies, which could help in the restoration of the Qaraoun reservoir. For this purpose, the Litani River water quality and hydrological data are collected from the Litani River Authority (LRA). Moreover, a hydrodynamic water quality model has been developed using Mike21 in order to restore the lake's aquatic life by eliminating the Litani River nutrients through constructed wetland concept, which reasonably simulated the water quality parameters of Qaraoun Lake. Consequently, the wetland could remarkably reduce the Litani River pollutants by 85%, 43.7%, 57%, and 56% for BOD, Phosphorous, Nitrate, and Ammonia, respectively. The resulted treated water, after passing the wetland, successfully improved the lake water quality and may lead to re-originate its ecosystem.

Trace metals and nutrients in tropical volcanic island rivers: Insights on chemical weathering and anthropogenic influences of Pohnpei, Micronesia

River systems in tropical islands play a vital role in maintaining the ecosystem function of surrounding oceans by delivering land-derived materials. Researches on tropical river systems frequently focused on continents and peninsulas, while little research has been conducted in tropical islands. In the present study, we conducted field surveys in a Pacific volcanic island, Pohnpei, Micronesia, and determined the substance composition of river waters (major ions, trace metals, nutrients, etc.) to quantify terrestrial material loading and biogeochemical influence on the surrounding oceans. In Pohnpei, total dissolved solutes were observed to be 71.4 mg/L; the NH4+, NO3−, PO4−, and SiO32- concentrations were 0.46, 3.9, 0.52, 221.7 μmol/L, respectively; and NO2− was lower than the detection limit (0.14 μmol/L). The trace metals concentrations of rivers in Pohnpei Island were much lower than records in tropical rivers of the world. The variation in concentration levels of major ions and nutrients among rivers was relatively limited. A similar pattern was observed in the distribution of Al and V. Trace elements, including As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, Sr, U and Zn in rivers were most enriched in Urban Area (human activity dominated region), compared with the Mixing Area (Multiple effect region) to Natural Area (natural weathering region). Exchangeable trace metals concentrations from suspended particulate matter (SPM) were lower than in dissolved state in river waters, while water-soluble, while acid-soluble metals concentrations in the sediment were much higher than in dissolved state in rivers. In our results, trace metals in Pohnpei rivers may mainly be influenced by domestic sewage and agricultural activity—these data can be used in biogeochemical models to estimate the contribution of domestic sewage. Furthermore, we have quantified the land-derived input of trace metals from tropical volcanic islands in Pacific (TVIP) into the surrounding oceans. Though the land area of TVIP only accounts for 0.1 % of global land area, the terrigenous inputs of some trace metals from TVIP account for approximately 1 %—and even more than 2 % of those in global riverine flux. Therefore, the nutrients and trace metals inputs from TVIP would be significant for the cycle of biogenic elements in the receiving ocean.

The different responses of planktonic bacteria and archaea to water temperature maintain the stability of their community diversity in dammed rivers

BackgroundPlanktonic bacteria and archaea play a key role in river nutrient biogeochemical cycling; however, their respective community assembly and how to maintain their diversity are not well known in dammed rivers. Therefore, a seasonal survey of planktonic bacterial and archaeal community compositions and related environmental factors was conducted in 16 cascade reservoirs and corresponding river waters on the Wujiang River and the Pearl River in southwest China to understand the above mechanisms.ResultsDeterministic processes dominated bacterial and archaeal community assembly. The structural equation models showed that water temperature can directly or indirectly affect the microbial diversity. Interestingly, planktonic bacterial diversity increased with increasing water temperature, while archaea showed the opposite trend; the overall diversity of bacteria and archaea was no significant changes with changeable water temperature. Abundant microbes had a stronger distance–decay relationship than middle and rare ones, and the relationship was stronger in winter and spring than in summer and autumn.ConclusionsPlanktonic bacteria and archaea in dammed rivers had different biogeographic distributions, and water temperature was a key controlling factor. The different responses of planktonic bacterial and archaeal diversity to water temperature could be due to their different phylogenetic diversity. This ultimately maintained the stability of total microbial community diversity. This study reveals the different responses of planktonic bacteria and archaea to water temperature and perfects the theoretical framework for planktonic microbial biogeography in dammed rivers.

Determining the main contributing factors to nutrient concentration in rivers in arid northwest China using partial least squares structural equation modeling

Understanding the main driving factors of oasis river nutrients in arid areas is important to identify the sources of water pollution and protect water resources. Twenty-seven sub-watersheds were selected in the lower oasis irrigated agricultural reaches of the Kaidu River watershed in arid Northwest China, divided into the site, riparian, and catchment buffer zones. Data on four sets of explanatory variables (topographic, soil, meteorological elements, and land use types) were collected. The relationships between explanatory variables and response variables (total phosphorus, TP and total nitrogen, TN) were analyzed by redundancy analysis (RDA). Partial least squares structural equation modeling (PLS-SEM) was used to quantify the relationship between explanatory as well as response variables and fit the path relationship among factors. The results showed that there were significant differences in the TP and TN concentrations at each sampling point. The catchment buffer exhibited the best explanatory power of the relationship between explanatory and response variables based on PLS-SEM. The effects of various land use types, meteorological elements (ME), soil, and topography in the catchment buffer were responsible for 54.3% of TP changes and for 68.5% of TN changes. Land use types, ME and soil were the main factors driving TP and TN changes, accounting for 95.56% and 94.84% of the total effects, respectively. The study provides a reference for river nutrients management in arid oases with irrigated agriculture and a scientific and targeted basis to mitigate water pollution and eutrophication of rivers in arid lands.

Effects of rainfall and land use on nutrient responses in rivers in the Brazilian semiarid region.

This paper investigated whether rainfall promotes dilution or increase in nutrient concentrations and which land use indicators are the main predictors of nutrients in intermittent rivers in a large Brazilian semiarid region. The total phosphorus (TP) and total inorganic nitrogen (TIN) were monitored between 2013 and 2018 at 92 river water quality monitoring sites. The monthly rainfall (Rn) was obtained from 575 rain gauges. Pearson's correlation (R) between Rn and nutrient concentration was performed. The correlation patterns were also analysed based on land use data: urban area (%), agricultural field area (%), demographic density (inhabitants/km2), sewer system coverage (%), and reservoir density (reservoir/km2). Backward stepwise regression was performed to identify predictors of nutrient concentrations. The results revealed a marginal effect of rainfall on nutrients when the effects of urbanisation outweigh all other aspects. However, in regions with greater accumulated rainfall and lower reservoir density, the rainfall was related to a linear increase in nutrient concentrations (R > 0.8). Contrastingly, in the basins with less accumulated rainfall and greater inter-basin hydrological disconnection, there was a linear reduction in nutrient concentration (R < - 0.5). In the backward stepwise regression, sewer system coverage and Rn had the greatest influence for TP, and the urban area was the strongest predictor for TIN. Importantly, our results demonstrated that in semiarid rivers in densely populated regions, there is no single pattern of variability in nutrient concentration, on a wide scale of assessment. Therefore, adaptative and decentralised management can be more effective in improving water quality in these regions.

Nutrients in tropical and temperate rivers and floodplains – comparison of the Rivers Songkhram (Thailand) and Narew (Poland)

Ecological processes in floodplains may function differently across climate regions. We compared the river discharge, water chemistry, and nutrient budget and balance of floodplain vegetation in a temperate climate (River Narew, Poland) with those in a tropical climate (River Songkhram, Thailand). Both rivers show a discharge regime with a flood pulse, following snowmelt (Narew) or monsoon rainfall (Songkhram), with peak discharges roughly 25 times higher in the River Songkhram. Electrical Conductivity (EC) values of both rivers are generally comparable, while nutrient concentrations are somewhat higher in the temperate River Narew (with total phosphorus (TP) approximately 1.5 and total inorganic nitrogen (TIN) approximately 2.2 times higher than in Songkhram). A comparison of the nutrient budget of floodplain vegetation suggests that soil is the most important source of nutrients for most vegetation types, i.e., Narew sedge (N), Narew forest floor (N and P), Songkhram bamboo (N and P), and Songkhram grass (P). Additionally, floodwater is the main input source of P for the Narew sedge and a secondary input source for Songkhram grass. Vegetation close to the river tends to have higher productivity, emphasizing the nutrient-filtering function of floodplain vegetation. For both rivers, nutrient input into the floodplains by floodwater is higher than nutrient export from the floodplains, indicating that both floodplains have a nutrient sink function. These findings demonstrate that the floodwater pulse is a source of nutrient input for floodplain vegetation in both temperate and tropical climates, with the soil playing a vital role in the nutrient budgets and balance.

Nutrient inputs from subarctic rivers into Hudson Bay

Hudson Bay (HB), a large subarctic inland sea, is impacted by rapid climate change and anthropogenic disturbance. HB plays crucial roles in supporting resident and migratory species of birds and marine mammals, providing subsistence to coastal communities, and exporting nutrients into the western Labrador Sea. To better constrain the impact of river nutrients on the HB ecosystem and to obtain a contemporary reference point by which future change can be evaluated, we estimated fluxes of nitrate plus nitrite (N), phosphate (P), and silicate using contemporary and historical nutrient data in conjunction with discharge estimates produced by three global climate models. Concentrations and molar ratios of the different nutrients exhibited large contrasts between different sectors of HB, which is attributed to the diversity of geological settings across distinct watersheds. With respect to the needs of primary producers, river waters were characterized by a shortage of P during winter and spring (N:P molar ratios in dissolved nutrients &amp;gt;16), nearly balanced N:P ratios in summer, and a shortage of N during fall (N:P &amp;lt; 16). Southwestern rivers made the largest regional contribution to the total annual delivery of all nutrients, followed by modest contributions from southern and eastern rivers, and minor ones from northwestern rivers. While the regulation of river flow in the Nelson and La Grande rivers had no discernible impact on nutrient concentrations and ratios, it clearly shifted nutrient transports toward the winter when biological activity in the estuaries is reduced. Finally, the potential amount of new production supported by riverine N inputs was nearly two orders of magnitude (1.8 × 1011 g C yr−1) lower than the new production supported by marine nutrients (7.3 × 1012 g C yr−1). Although the potential contribution of river nutrients to new primary production is small (2.4%) at the bay-wide scale, it can be significant locally.

Temporal patterns and causal drivers of aboveground plant biomass in a coastal wetland: Insights from time-series analyses

Salt marshes play a crucial role in coastal biogeochemical cycles and provide unique ecosystem services. Salt marsh biomass, which can strongly influence such services, varies over time in response to hydrologic conditions and other environmental drivers. We used gap-filled monthly observations of Spartina alterniflora aboveground biomass derived from Landsat 5 and Landsat 8 satellite imagery from 1984-2018 to analyze temporal patterns in biomass in comparison to air temperature, precipitation, river discharge, nutrient input, sea level, and drought index for a southeastern US salt marsh. Wavelet analysis and ensemble empirical mode decomposition identified month to multi-year periodicities in both plant biomass and environmental drivers. Wavelet coherence detected cross-correlations between annual biomass cycles and precipitation, temperature, river discharge, nutrient concentrations (NOx and PO43–) and sea level. At longer periods we detected coherence between biomass and all variables except precipitation. Through empirical dynamic modeling we showed that temperature, river discharge, drought, sea level, and river nutrient concentrations were causally connected to salt marsh biomass and exceeded the confounding effect of seasonality. This study demonstrated the insights into biomass dynamics and causal connections that can be gained through the analysis of long-term data.

Summer phytoplankton photosynthetic characteristics in the Changjiang River Estuary and the adjacent East China Sea

IntroductionThe Changjiang (Yangtze) River is one of the largest rivers in the world, and its estuary and offshore plume create a diversity of ecological habitats for the phytoplankton community. The phytoplankton community has to balance between light limitation in the sediment-laden inshore waters and nutrient limitation in the offshore waters. Active fluorescence measurements can provide rapid, non-intrusive estimates of photosynthetic characteristics at high spatial and temporal resolution.MethodsIn the summer of 2020, a field survey of hydrodynamic characteristics, availability of nutrients, the maximum quantum efficiency of photosystem II (Fv/Fm), and rapid light curves across the Changjiang River Estuary and its adjacent sea was conducted, assessing relationships between photosynthetic physiology and biomass accumulation.ResultsThe photosynthetic activities significantly differed among the turbid river water, the stratified river plume water, and the oceanic East China Sea Water. The photosynthetic physiology of phytoplankton was the most active near the front of Changjiang Diluted Water, where the Fv/Fm was over 0.5.DiscussionPhytoplankton photosynthesis was alleviated from light limitation downstream of the river mouth, and benefited from phosphorus supply via tidal mixing and upwelling. The relatively suitable light and nutrients led to high photosynthetic activities, supporting increased productivity and biomass in this water. The phytoplankton in the Changjiang estuary rivermouth were under intense stress, suggested by the Fv/Fm values under 0.3. Also, the strong vertical mixing process diluted the river nutrients before the phytoplankton consumed them. Nutrients further limited the phytoplankton offshore in the East China Sea.

Trends in nutrients in the Changjiang River

Better documentation and understanding of long-term temporal dynamics of nutrients in watersheds are necessary to support effective water quality management. We examined the hypothesis that the recent management of fertilizer use and pollution control in the Changjiang River Basin could govern the fluxes of nutrients from the river to the sea. Results based on historical data since 1962 and surveys in recent years show that concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the mid- and downstream reaches were higher than those in the upper reaches due to intensive anthropogenic activities, while dissolved silicate (DSi) was distributed evenly from the up- to downstream reaches. Fluxes of DIN and DIP increased rapidly, and DSi declined during the 1962–1980 and 1980–2000. After the 2000s, concentrations and fluxes of DIN and DSi remained almost unchanged; those of DIP remained stable until the 2010s and slightly decreased afterward. The decline in fertilizer use explains 45 % of the variance in the decline of DIP flux, followed by pollution control, groundwater and water discharge. As a result, the molar ratio of DIN:DIP, DSi:DIP and ammonia:nitrate varied largely during 1962–2020, and the excess DIN relative to DIP and DSi lead to increased limitations of silicon and phosphorus. A turning point probably occurred for nutrient fluxes in the Changjiang River in the 2010s, with the pattern of DIN from continuous increase to stability and DIP from increase to decrease. This decline in phosphorus in the Changjiang River has many similarities with the rivers worldwide. The continued basin nutrient management is likely to have a major influence on river nutrient delivery and therefore may control coastal nutrient budget and ecosystem stability.

Human-driven long-term disconnect of nutrient inputs to the Yellow River basin and river export to the Bohai Sea

Combining long-term measurements of nitrogen (N) and phosphorus (P) concentrations in the Yellow River (YR) (1981–2018) and river discharge (1981–2018) to the Bohai Sea (BS) with model-based estimates of nutrient delivery shows a disconnect between nutrient delivery and export fluxes to the BS. Basin-wide nutrient delivery to surface water, primarily from agriculture and sewage, exhibited an increasing trend over the whole period. However the nutrient export fluxes to BS declined from 1981 to 2002, followed by a period of large inter-annual variability. Nutrient export is strongly controlled by water discharge, the Water and Sediment Regulation Scheme (WSRS), and the large volume of water containing nutrients that is extracted for primarily irrigation. Dissolved organic N (average 15% in the period 2001–2018) and dissolved organic P (68%) accounted for an important share of total dissolved N and P, which is due to agricultural inputs and preferential retention of inorganic forms in the YR basin. River nutrient export fluxes alone cannot explain the changes in the frequency and area of red tides in the BS due to impacts of the WSRS, the long water residence time in the BS and climate change. This calls for the application of mechanistic models for describing the impact of combined long-term changes in the BS ecosystem.

Phosphorus – The main limiting factor in riverine ecosystems in China

River receive substantial nutrient inputs, and serve as the main channel for nitrogen and phosphorus to enter the lake, their nutrient control is of great significance to the alleviation of lake eutrophication. While nutrient limitation affects the primary productivity of water ecosystems and the biodiversity of aquatic communities, identifying the limiting factors in riverine ecosystems across China remains elusive. Here, we explore which nutrients have a stronger effect on nutritional balance and aquatic ecosystems in China's rivers based on the total nitrogen (TN) and total phosphorus (TP) observations from 1412 sampling sites in 2018. This study supports the following three main conclusions. Though the percentages of the sites with TN or TP exceeding the limits varied as per different mesotrophic targets, and TP (53.7 %) contributed more to nutrient enrichment than TN (46.3 %). In addition, the spatial distribution characteristics of river nutrients were high in the north (arid zone) and low in the south (humid zone) in China. According to four classification criteria of N:P ratio, 70.8 % of the sampling sites were attributed to phosphorus limiting, much higher than the sites with nitrogen limiting (4.1 %). TN and TP have a synergistic effect on river nutrients, while TP has a stronger regulation framework. Our results reveal that the nutrients in China's rivers are mainly phosphorus limiting, which implies that phosphorus-oriented best management practices are more likely to maintain the nutrient balance of rivers towards healthy aquatic ecosystems.Synopsis: Phosphorus is the key factor that affecting the stability and nutrient balance of riverine ecosystem.

Nutrients and Environmental Factors Cross Wavelet Analysis of River Yi in East China: A Multi-Scale Approach.

The accumulation of nutrients in rivers is a major cause of eutrophication, and the change in nutrient content is affected by a variety of factors. Taking the River Yi as an example, this study used wavelet analysis tools to examine the periodic changes in nutrients and environmental factors, as well as the relationship between nutrients and environmental factors. The results revealed that total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH4+-N) exhibit multiscale oscillation features, with the dominating periods of 16-17, 26, and 57-60 months. The continuous wavelet transform revealed periodic fluctuation laws on multiple scales between nutrients and several environmental factors. Wavelet transform coherence (WTC) was performed on nutrients and environmental factors, and the results showed that temperature and dissolved oxygen (DO) have a strong influence on nutrient concentration fluctuation. The WTC revealed a weak correlation between pH and TP. On a longer period, however, pH was positively correlated with TN. The flow was found to be positively correct with N and P, while N and P were found to be negatively correct with DO and electrical conductance (EC) at different scales. In most cases, TP was negatively correlated with 5-day biochemical oxygen demand (BOD5) and permanganate index (CODMn). The correlation between TN and CODMn and BOD5 was limited, and no clear dominant phase emerged. In a nutshell, wavelet analysis revealed that water temperature, pH, DO, flow, EC, CODMn, and BOD5 had a pronounced influence on nutrient concentration in the River Yi at different time scales. In the case of the combination of environmental factors, pH and DO play the largest role in determining nutrient concentration.

Seasonal Variation of Long‐Chain Alkyl Diols Proxies in Suspended Particulate Matter of the Changjiang River Estuary

AbstractLong‐chain alkyl diols (LCDs) were applied as indicators of salinity, sea surface temperature (SST), upwelling, and nutrient conditions in marine settings. While increasing the body of most applications of LCDs proxies reconstructed paleo‐environment information, few studies reported seasonal variation in LCDs and evaluated their applicability as environmental indicators in modern marine environments. Here, the composition and distribution of LCDs in suspended particulate matter (SPM) in three seasons in the Changjiang River Estuary (CRE) were analyzed to evaluate seasonal influence on the application of LCDs as environmental indicators. We found that the reconstructed temperature based on the long‐chain diol index (LDI; LDI = C30 1,15‐diol/(C28 1,13‐diol + C30 1,13‐diol + C30 1,15‐diol)) was not consistent with satellite‐measured SST, likely due to vertical mixing of LCDs in this region. The 1,14‐diols previously used to indicate upwelling conditions, did not reflect the nutrient distribution of this region due to the Changjiang River (CR) nutrients discharge. Our results showed that the mean relative proportions of FC32 1,15‐diol (FC32 1,15‐diol = C32 1,15‐diol × 100%/(C28 1,13‐diol + C30 1,13‐diol + C30 1,15‐diol + C32 1,15‐diol)) gradually decreased from nearshore to offshore (71.1%, 37.4%, and 16.5%, respectively), suggesting that FC32 1,15‐diol was a reliable proxy for riverine input to the CRE. We found that the diol index (DI; DI = C30 1,15‐diol × 100/(C30 1,15‐diol + C32 1,15‐diol)) values were correlated strongly with salinities in spring (R2 = 0.82) and summer (R2 = 0.94), indicating that DI can be considered as a useful indicator for salinity construction in this estuary.

River damming enhances ecological functional stability of planktonic microorganisms.

Planktonic microorganisms play an important role in maintaining the ecological functions in aquatic ecosystems, but how their structure and function interrelate and respond to environmental changes is still not very clear. Damming interrupts the river continuum and alters river nutrient biogeochemical cycling and biological succession. Considering that river damming decreases the irregular hydrological fluctuation, we hypothesized that it can enhance the ecological functional stability (EFS) of planktonic microorganisms. Therefore, the community composition of planktonic bacteria and archaea, functional genes related to carbon, nitrogen, sulfur, and phosphorus cycling, and relevant environmental factors of four cascade reservoirs in the Pearl River, Southern China, were investigated to understand the impact of damming on microbial community structure and function and verify the above hypothesis. Here, the ratio of function to taxa (F:T) based on Euclidean distance matrix analysis was first proposed to characterize the microbial EFS; the smaller the ratio, the more stable the ecological functions. The results showed that the reservoirs created by river damming had seasonal thermal and chemical stratifications with an increasing hydraulic retention time, which significantly changed the microbial structure and function. The river microbial F:T was significantly higher than that of the reservoirs, indicating that river damming enhances the EFS of the planktonic microorganisms. Structural equation modeling demonstrated that water temperature was an important factor influencing the relationship between the microbial structure and function and thus affected their EFS. In addition, reservoir hydraulic load was found a main factor regulating the seasonal difference in microbial EFS among the reservoirs. This study will help to deepen the understanding of the relationship between microbial structure and function and provide a theoretical basis of assessing the ecological function change after the construction of river damming.

Future projections of river nutrient export to the global coastal ocean show persisting nitrogen and phosphorus distortion

Nitrogen (N) and phosphorus (P) from anthropogenic sources are needed to produce food for the growing world population. As a result, these nutrients can be found in nearly every water body across the globe. Not only nutrient loading is important but also the molar ratio and its deviation from the “natural” Redfield ratio. Here we show that rivers, which have more than 50% anthropogenic sources and at the same time elevated N:P ratios (&amp;gt; 25) contributed 36% to the total global N export to coastal waters in 2015. The five Shared Socioeconomic Pathways (SSP) were used in combination with the Representative Concentration Pathways climate scenarios to project river nutrient loadings for 2050. Future nutrient export is projected to decline in high-income countries (with N:P ratios exceeding Redfield). In Brazil, India and China, however, a decline of N:P is only the case in a scenario oriented toward sustainable development (SSP1). The human-dominated river N and P export with elevated N:P ratios will increase in all SSPs, except in SSP1 where it stabilizes. Integrated strategies for both N and P considering all relevant trade-offs and societal sectors are urgently needed to reduce the nutrient pressure on surface waters.