Deciphering the crop-soil-enzyme C:N:P stoichiometry nexus: A 5-year study on manure-induced changes in soil phosphorus transformation and release risk

Carbon:nitrogen:phosphorus (C:N:P) stoichiometry plays a vital role in regulating P transformation in agriculture ecosystems. However, the impact of balanced C:N:P stoichiometry in paddy soil, particularly regarding relative soil P transformation, remains unknown. This study explores the response of C:N:P stoichiometry to manure substitution and its regulatory role in soil P transformation, along with the associated release risk to the environment. Based on a 5-year field study, our findings reveal that replacing 30 % of chemical P fertilizer with pig manure (equal total NPK amounts with chemical P fertilizer treatment, named CFM) increased soil total C without altering soil total P, resulting in an elevated soil C:P ratio, despite the homeostasis of crop stoichiometry. This increase promoted microbial diversity and the accumulation of organic P in the soil. The Proteobacteria and Actinobacteria produced lower C:PEEA metabolism together, and enhanced in vivo turnover of P. Additionally, by integrating high-resolution dialysis (HR-Peeper), diffusive gradients in thin films (DGT), DGT-induced fluxes in the soil (DIFS), and sediment P release risk index (SPRRI) models, we observed that, in addition to organic P, CFM simultaneously increased soil Al-P, thereby weakening the diffusion and resupply capacity of P from soil solids to the solution. Consequently, this decrease in P release risk to the environment was demonstrated. Overall, this study establishes a connection between crop-soil-enzyme C:N:P stoichiometry, soil microorganisms, and soil P biogeochemical processes. The study further evaluates the P release risk to the environment, providing a novel perspective on both the direct and indirect effects of manure substitution on soil P cycling.

Phycospheric bacteria limits the effect of nitrogen and phosphorus imbalance on diatom bloom

Human activities have caused an imbalance in the input nitrogen and phosphorus (N/P) in the biosphere. The imbalance of N/P is one of the characteristics of water eutrophication, which is the fundamental factor responsible for the blooms. The effects of the N/P imbalance on diatom and phycospheric bacteria in blooms are poorly understood. In this study, the N/P molar ratio in real water (14:1) and the predicted N/P molar ratio in future water (65:1) were simulated to analyze the response of Cyclotella sp. and phycospheric bacteria to the N/P imbalance. The results showed that the N/P imbalance inhibited the growth of Cyclotella sp., but prolonged diatom bloom duration. The resistance of Cyclotella sp. to the N/P imbalance is related to phycospheric bacteria, and there are dynamic regulatory mechanisms within the phycospheric bacteria community to resist the N/P imbalance: (1) the increase of HNA bacterial density, the decrease of LNA bacterial density, (2) the increase of phycospheric bacterial diversity and eutrophic bacteria abundance, and the change of denitrifying bacteria abundance, (3) the activity of nitrogen and phosphorus metabolism of HNA bacteria enhanced, while that of LNA bacteria decreased. And the gene hosts of nitrogen and phosphorus metabolism were most enriched in Proteobacteria, indicating that Proteobacteria played an important role in maintaining the stability of phycospheric bacteria and was the dominant phylum resistant to the N/P imbalance. This study clarified that the algal-bacteria system was resistant to the N/P imbalance and implied that the N/P imbalance had little effect on the occurrence of diatom bloom events due to the presence of phycospheric bacteria.

Nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf, South China Sea: Implication for the severe impact of human disturbance

This study examined the nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf (CNBG). Nutrient concentrations varied spatially and seasonally among the different bays. High nutrient levels were found in the regions with high riverine inputs and intensive mariculture. Using a three end-member mixing model, nutrient biogeochemistry within the ecosystem was estimated separately from complex physical mixing effects. Nutrient consumption dominated in most bays in summer, whereas nutrient regeneration dominated in winter, likely due to phytoplankton decomposition, vertical mixing and desorption. Through the Land-Ocean Interaction Coastal Zone (LOICZ) model, the robust nutrient budgets were constructed, indicating that the CNBG behaved as a sink of dissolved inorganic nitrogen, phosphorus and silicon. River-borne nutrient inputs were the dominant nutrient source, while residual flows and water exchange flows transported nutrient off the estuaries. This study could help us better understand nutrient cycles and nutrient sources/sinks in the CNBG.

How nutrient retention and TN:TP ratios depend on ecosystem state in thousands of Chinese lakes

Worldwide, anthropogenic activities threaten surface water quality by aggravating eutrophication and increasing total nitrogen to total phosphorus (TN:TP) ratios. In hydrologically connected systems, water quality management may benefit from in-ecosystem nutrient retention by preventing nutrient transport to downstream systems. However, nutrient retention may also alter TN:TP ratios with unforeseen consequences for downstream water quality. Here, we aim to increase understanding of how nutrient retention may influence nutrient transport to downstream systems to improve long-term water quality management. We analyzed lake ecosystem state, in-lake nutrient retention, and nutrient transport (ratios) for 3482 Chinese lakes using the lake process-based ecosystem model PCLake+. We compared a low climate change and sustainability-, and a high climate change and economy-focused scenario for 2050 against 2012. In both scenarios, the effect of nutrient input reduction outweighs that of temperature rise, resulting in more lakes with good ecological water quality (i.e., macrophyte-dominated) than in 2012. Generally, the sustainability-focused scenario shows a more promising future for water quality than the economy-focused scenario. Nevertheless, most lakes remain phytoplankton-dominated. The shift to more macrophyte-dominated lakes in 2050 is accompanied by higher nutrient retention fractions and less nutrient transport to downstream waterbodies. In-lake nutrient retention also alters the water's TN:TP ratio, depending on the inflow TN:TP ratio and the ecosystem state. In 2050 higher TN:TP ratios are expected in the outflows of lakes than in 2012, especially for the sustainability-focused scenario with strong TP loading reduction. However, the downstream impact of increased TN:TP ratios depends on actual nutrient loadings and the limiting nutrient in the receiving system. We conclude that nutrient input reductions, improved water quality, higher in-lake nutrient retention fractions, and lower nutrient transport to downstream waterbodies go hand in hand. Therefore, water quality management could benefit even more from nutrient pollution reduction than one would expect at first sight.

Temporal patterns and driving factors of sediment carbon, nitrogen, and phosphorus stoichiometry in a eutrophication plateau lake

Stoichiometry determines the key characteristics of organisms and ecosystems on a global scale and provides strong instructions on the fate of sediment carbon, nitrogen, and phosphorus (C-N-P) during the sedimentation process, contributing to the Earth's C-N-P balance. However, the mechanisms underlying C-N-P stoichiometry in response to intensive human activity and organic matter sources remain underexplored, especially in freshwater ecosystems. This study identifies the temporal patterns of C-N-P stoichiometry, reveals the inner driving factors, and clarifies its impact path, especially in eutrophication (the late 1970s). The results revealed that sediment RCP and RNP increased significantly and were controlled by TCAR and TNAR, respectively, indicating the direct impact of burial rate on C-N-P stoichiometry. Based on redundancy analysis and the STM model, autochthonous origin, GDP, and population had positive effects on sediment TCAR, TNAR, and TPAR, which, in turn, affected RCN, RCP, and RNP. Organic matter sources and human activities have a significant influence on RCN, RCP, and RNP, possibly regulated by the variation of TCAR and TNAR. Autochthonous origin had an indirect positive impact on RCN and RCP through the mediating effect of TCAR. Similarly, through the mediating effect of TNAR, it had an indirect negative impact on RCN and an indirect positive impact on RNP. This study showed that TCAR, TNAR, TPAR, GDP, autochthonous, allochthonous and population better explained the changes in RCN, RCP, and RNP over a-hundred-year deposition, highlighting an in-depth understanding of the dynamic change mechanism of sediment C-N-P stoichiometry during the lake deposition process.

Temperature and nutrients alter the relative importance of stochastic and deterministic processes in the coastal macroinvertebrates biodiversity assembly on long-time scales

Macroinvertebrates play a vital role in coastal ecosystems and are an important indicator of ecosystem quality. Both anthropogenic activity and environmental changes may lead to significant changes in the marine macroinvertebrate community. However, the assembly process of benthic biodiversity and its mechanism driven by environmental factors at large scales remains unclear. Here, using the benthic field survey data of 15 years at large spatial and temporal scales from the Yellow Sea Large Marine Ecosystem, we investigated the relative importance of environmental selection, dispersal processes, random-deterministic processes of macroinvertebrates community diversity assembly, and the responses of this relative importance driven by temperature and nutrients. Results showed that the macroinvertebrates community diversity is mainly affected by dispersal. Nitrogen and phosphorus are the most important negative factors among environmental variables, while geographical distance is the main limiting factor of β diversity. Within the range of 0.35-0.70 mg/L of nutrients, increasing nutrient concentration can significantly facilitate the contribution of the decay effect to β diversity. Within the temperature range studied (15.0-18.0°C), both warming and cooling can lead to a greater tendency for species diversity assembly processes to be dominated by deterministic processes. The analysis contributes to a better understanding of the assembly process of the diversity of coastal marine macroinvertebrates communities and how they adapt to global biogeochemical processes.

Dominance of net autotrophy in arid landscape low relief polar lakes, Nunavut, Canada

The Arctic is the fastest warming biome on the planet, and environmental changes are having striking effects on freshwater ecosystems that may impact the regional carbon cycle. The metabolic state of Arctic lakes is often considered net heterotrophic, due to an assumed supply of allochthonous organic matter that supports ecosystem respiration and carbon mineralization in excess of rates of primary production. However, lake metabolic patterns vary according to regional climatic characteristics, hydrological connectivity, organic matter sources and intrinsic lake properties, and the metabolism of most Arctic lakes is unknown. We sampled 35 waterbodies along a connectivity gradient from headwater to downstream lakes, on southern Victoria Island, Nunavut, in an area characterized by low precipitation, organic-poor soils, and high evaporation rates. We evaluated whether lakes were net autotrophic or heterotrophic during the open water period using an oxygen isotopic mass balance approach. Most of the waterbodies were autotrophic and sites of net organic matter production or close to metabolic equilibrium. Autotrophy was associated with higher benthic primary production, as compared to its pelagic counterpart, due to the high irradiance reaching the bottom and efficient internal carbon and nutrient cycling. Highly connected midstream and downstream lakes showed efficient organic matter cycling, as evidenced by the strong coupling between gross primary production (GPP) and ecosystem respiration, while decoupling was observed in some headwater lakes with significantly higher GPP. The shallow nature of lakes in the flat, arid region of southern Victoria Island supports net autotrophy in most lakes during the open water season. Ongoing climate changes that lengthen the ice-free irradiance period and increase rates of nutrient evapoconcentration may further promote net autotrophy, with uncertain long-term effects for lake functioning.

Increased nitrous oxide emissions from global lakes and reservoirs since the pre-industrial era

Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N2O), a potent greenhouse gas; however, this has not been well quantified yet. Here we examine how multiple environmental forcings have affected N2O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N2O-N yr-1 in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N2O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N2O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr-1. This suggests a global emission factor of 0.051% for inland water N2O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories.

Relationship between Nitrogen Dynamics and Key Microbial Nitrogen-Cycling Genes in an Intensive Freshwater Aquaculture Pond

Intensive aquaculture in high-density hybrid snakehead [Channa maculata (♀) × Channa argus (♂)] fishponds can lead to toxic conditions for fish. This study investigated nitrogen migration and transformation in these fishponds during different cultivation periods. Using qPCR technology, we analyzed the abundance variation of nitrogen-cycling microorganisms in water and sediment to reveal the nitrogen metabolism characteristics of hybrid snakehead fishponds. The results showed that fish biomass significantly impacts suspended particulate matter (SPM) flux. At the sediment-water interface, inorganic nitrogen fluxes showed predominant NO3--N absorption by sediments and NH4+-N and NO2--N release, especially in later cultivation stages. Sediments were rich in nirS and AMX 16S rRNA genes (ranging from 4.04 × 109 to 1.01 × 1010 and 1.19 × 108 to 2.62 × 108 copies/g, respectively) with nirS-type denitrifiers potentially dominating the denitrification process. Ammonia-oxidizing bacteria (AOB) were found to dominate the ammonia oxidation process over ammonia-oxidizing archaea (AOA) in both water and sediment. Redundancy analysis revealed a positive correlation between SPM flux, Chlorophyll a (Chl-a), and denitrification genes in the water, and between nitrogen-cycling genes and NH4+/NO2- fluxes at the interface. These findings provide a scientific basis for nitrogen control in hybrid snakehead fishponds.

Nutrient transport following water transfer through the world's largest water diversion channel

Nutrient levels in the artificial channel constructed for the Middle Route Project are significant indicators of water quality safety and aquatic ecological integrity for this large, inter-basin scheme. However, the distribution and transport of nutrients along the channel were poorly understood. Based on a time-series dataset as well as mass balance and material flow analysis methods, the water and nutrient transport fluxes in the Middle Route of the South-to-North Water Diversion Project were identified in this study. The results indicate that the nutrient concentrations varied considerably with time, but there was no significant difference among the 30 stations of the main channel. Seasonal temperature difference was the major factor in the large fluctuations of water quality indicators over time. The nutrient loadings varied with the water volume outputs from the main channel to the water-receiving cities. Atmospheric deposition was an important source of nutrients in the main channel, accounting for 9.13%, 20.6%, and 0.635% of the nitrogen, phosphorus, and sulfur input from the Danjiangkou Reservoir, respectively. In 2021, a net accumulation of 988 tons of N, 29 tons of P, and 2,540 tons of S, respectively, were present in the main channel. The increase of these external and internal nutrient loadings would cause water quality fluctuation and deterioration in some local sections of the main channel. Our study quantified the spatial and temporal patterns of nutrient transport in the Middle Route and revealed the ecological effects on the aquatic environment, assisting authorities on the project to develop effective water conservation strategies.

Responses of a common tropical epiphyte, Asplenium nidus, to changes in water and nutrient availability

Epiphytes are highly dependent on atmospheric inputs of water and nutrients. Reductions in water availability associated with warming and climate change and continual atmospheric nitrogen (N) deposition can affect plant growth but few studies have evaluated the effects of changes in both water and nutrient availabilities on epiphytes. We experimentally tested whether epiphyte growth is more water- or nutrient-limited, if nutrient limitation was stronger for nitrogen or phosphorus, and whether nutrient limitation interacts with water availability. We applied watering (high and low) and nutrient addition (control, +N, +P, +N+P) treatments to greenhouse-grown Asplenium nidus, a common epiphytic fern found in many tropical and subtropical wet forests. We measured leaf area production and leaf elemental concentrations to assess how A. nidus growth and physiology respond to changes in water and nutrient availabilities. We found that leaf growth of A. nidus was more affected by water availability than nutrient addition and the effect of adding nutrients was not fully realized under low-water availability. Among the different nutrient treatments, +N+P had the greatest effects on A. nidus growth and physiology in both watering treatments. Watering treatment changed leaf elemental concentrations but not their ratios (i.e. C:N and N:P). Nutrient addition altered C:N and N:P ratios and increased the concentration of the added elements in leaves, with more pronounced increases in the high-watering treatment. We conclude that the growth of A. nidus is more water- than nutrient-limited. When nutrient limitation occurs (i.e. under high-water availability), nutrient co-limitation is stronger than limitation by N or P alone. This result taken together with studies of other epiphytes suggests greater water than nutrient limitation is likely widespread among epiphytic plants. The limited effects of nutrient addition in the low-water treatment suggest that the effect of atmospheric N deposition on epiphyte growth will be limited when water availability is low.

Growth of Coal Mining Operations in the Elk River Valley (Canada) Linked to Increasing Solute Transport of Se, NO3-, and SO42- into the Transboundary Koocanusa Reservoir (USA-Canada)

Koocanusa Reservoir (KOC) is a waterbody that spans the United States (U.S.) and Canadian border. Increasing concentrations of total selenium (Se), nitrate + nitrite (NO3-, nitrite is insignificant or not present), and sulfate (SO42-) in KOC and downstream in the Kootenai River (Kootenay River in Canada) are tied to expanding coal mining operations in the Elk River Watershed, Canada. Using a paired watershed approach, trends in flow-normalized concentrations and loads were evaluated for Se, NO3-, and SO42- for the two largest tributaries, the Kootenay and Elk Rivers, Canada. Increases in concentration (SO42- 120%, Se 581%, NO3- 784%) and load (SO42- 129%, Se 443%, NO3- 697%) in the Elk River (1979-2022 for NO3-, 1984-2022 for Se and SO42-) are among the largest documented increases in the primary literature, while only a small magnitude increase in SO42- (7.7% concentration) and decreases in Se (-10%) and NO3- (-8.5%) were observed in the Kootenay River. Between 2009 and 2019, the Elk River contributed, on average, 29% of the combined flow, 95% of the Se, 76% of the NO3-, and 38% of the SO42- entering the reservoir from these two major tributaries. The largest increase in solute concentrations occurred during baseflows, indicating a change in solute transport and delivery dynamics in the Elk River Watershed, which may be attributable to altered landscapes from coal mining operations including altered groundwater flow paths and increased chemical weathering in waste rock dumps. More recently there is evidence of surface water treatment operations providing some reduction in concentrations during low flow times of year; however, these appear to have a limited effect on annual loads entering KOC. These findings imply that current mine water treatment, which is focused on surface waters, may not sufficiently reduce the influence of mine-waste-derived solutes in the Elk River to allow constituent concentrations in KOC to meet U.S. water-quality standards.

Towards a mechanistic understanding of the impacts of nitrogen deposition on producer-consumer interactions

Nitrogen (N) deposition has increased substantially since the second half of the 20th century due to human activities. This increase of reactive N into the biosphere has major implications for ecosystem functioning, including primary production, soil and water chemistry and producer community structure and diversity. Increased N deposition is also linked to the decline of insects observed over recent decades. However, we currently lack a mechanistic understanding of the effects of high N deposition on individual fitness, species richness and community structure of both invertebrate and vertebrate consumers. Here, we review the effects of N deposition on producer-consumer interactions, focusing on five existing ecological frameworks: C:N:P ecological stoichiometry, trace element ecological stoichiometry, nutritional geometry, essential micronutrients and allelochemicals. We link reported N deposition-mediated changes in producer quality to life-history strategies and traits of consumers, to gain a mechanistic understanding of the direction of response in consumers. We conclude that high N deposition influences producer quality via eutrophication and acidification pathways. This makes oligotrophic poorly buffered ecosystems most vulnerable to significant changes in producer quality. Changes in producer quality between the reviewed frameworks are often interlinked, complicating predictions of the effects of high N deposition on producer quality. The degree and direction of fitness responses of consumers to changes in producer quality varies among species but can be explained by differences in life-history traits and strategies, particularly those affecting species nutrient intake regulation, mobility, relative growth rate, host-plant specialisation, ontogeny and physiology. To increase our understanding of the effects of N deposition on these complex mechanisms, the inclusion of life-history traits of consumer species in future study designs is pivotal. Based on the reviewed literature, we formulate five hypotheses on the mechanisms underlying the effects of high N deposition on consumers, by linking effects of nutritional ecological frameworks to life-history strategies. Importantly, we expect that N-deposition-mediated changes in producer quality will result in a net decrease in consumer community as well as functional diversity. Moreover, we anticipate an increased risk of outbreak events of a small subset of generalist species, with concomitant declines in a multitude of specialist species. Overall, linking ecological frameworks with consumer life-history strategies provides a mechanistic understanding of the impacts of high N deposition on producer-consumer interactions, which can inform management towards more effective mitigation strategies.

The harmful cyanobacterium Microcystis aeruginosa PCC7806 is more resistant to hydrogen peroxide at elevated CO2

Rising atmospheric CO2 can intensify harmful cyanobacterial blooms in eutrophic lakes. Worldwide, these blooms are an increasing environmental concern. Low concentrations of hydrogen peroxide (H2O2) have been proposed as a short-term but eco-friendly approach to selectively mitigate cyanobacterial blooms. However, sensitivity of cyanobacteria to H2O2 can vary depending on the available resources. To find out how cyanobacteria respond to H2O2 under elevated CO2, Microcystis aeruginosa PCC 7806 was cultured in chemostats with nutrient-replete medium under C-limiting and C-replete conditions (150 ppm and 1500 ppm CO2, respectively). Microcystis chemostats exposed to high CO2 showed higher cell densities, biovolumes, and microcystin contents, but a lower photosynthetic efficiency and pH compared to the cultures grown under low CO2. Subsamples of the chemostats were treated with different concentrations of H2O2 (0-10 mg·L-1 H2O2) in batch cultures under two different light intensities (15 and 100 μmol photons m-2·s-1) and the response in photosynthetic vitality was monitored during 24 h. Results showed that Microcystis was more resistant to H2O2 at elevated CO2 than under carbon-limited conditions. Both low and high CO2-adapted cells were more sensitive to H2O2 at high light than at low light. Microcystins (MCs) leaked out of the cells of cultures exposed to 2-10 mg·L-1 H2O2, while the sum of intra- and extracellular MCs decreased. Although both H2O2 and CO2 concentrations in lakes vary in response to many factors, these results imply that it may become more difficult to suppress cyanobacterial blooms in eutrophic lakes when atmospheric CO2 concentrations continue to rise.

Global crop-specific nitrogen fertilization dataset in 1961-2020

Nitrogen (N) is an important nutrient for crop growth. However, the overuse of N fertilizers has led to a series of devastating global environmental issues. Recent studies show that multiple datasets have been created for agricultural N fertilizer application with varied temporal or spatial resolutions, nevertheless, how to synchronize and use these datasets becomes problematic due to the inconsistent temporal coverages, spatial resolutions, and crop-specific allocations. Here we reconstructed a comprehensive dataset for crop-specific N fertilization at 5-arc-min resolution (~10 km by 10 km) during 1961-2020, including N application rate, types, and placements. The N fertilization data was segmented by 21 crop groups, 13 fertilizer types, and 2 fertilization placements. Comparison analysis showed that our dataset is aligned with previous estimates. Our spatiotemporal N fertilization dataset could be used for the land surface models to quantify the effects of agricultural N fertilization practices on food security, climate change, and environmental sustainability.

Water availability and seasonality shape elemental stoichiometry across space and time

The interaction of climate change and increasing anthropogenic water withdrawals is anticipated to alter surface water availability and the transport of carbon (C), nitrogen (N), and phosphorus (P) in river networks. But how changes to river flow will alter the balance, or stoichiometry, of these fluxes is unknown. The Lower Flint River Basin (LFRB) is part of an interstate watershed relied upon by several million people for diverse ecosystem services, including seasonal crop irrigation, municipal drinking water access, and public recreation. Recently, increased water demand compounded with intensified droughts have caused historically perennial streams in the LFRB to cease flowing, increasing ecosystem vulnerability. Our objectives were to quantify how riverine dissolved C:N:P varies spatially and seasonally and determine how monthly stoichiometric fluxes varied with overall water availability in a major tributary of LFRB. We used a long-term record (21-29 years) of solute water chemistry (dissolved organic carbon, nitrate/nitrite, ammonia, and soluble reactive phosphorus) paired with long-term stream discharge data across six sites within a single LFRB watershed. We found spatial and seasonal differences in soluble nutrient concentrations and stoichiometry attributable to groundwater connections, the presence of a major floodplain wetland, and flow conditions. Further, we showed that water availability, as indicated by the Palmer Drought Severity Index (PDSI), strongly predicted stoichiometry with generally lower C:N and C:P and higher N:P fluxes during periods of low water availability (PDSI < -4). These patterns suggest there may be long-term and significant changes to stream ecosystem function as water availability is being dramatically altered by human demand with consequential impacts on solute transport, in-stream processing, and stoichiometric ratios.

The evolution of a typical plateau lake from macrophyte to algae leads to the imbalance of nutrient retention

Long-term anthropogenic nitrogen (N) and phosphorus (P) inputs have led to lake eutrophication and decreased environmental quality. However, the imbalance in nutrient cycling caused by ecosystem transformation during lake eutrophication is still unclear. The N, P, organic matter (OM) and their extractable forms in the sediment core of Dianchi Lake were investigated. Combining ecological data and geochronological techniques, a coupling relationship between the evolution of lake ecosystems and nutrient retention was established. The results show that the evolution of lake ecosystems promotes the accumulation and mobilization of N and P in sediments, leading to an imbalance in nutrient cycling in the lake system. From the "macrophyte-dominated" period to the "algae-dominated" period, the accumulation rates of potential mobile N and P (PMN, PMP) in sediments have significantly increased, and the retention efficiency of total N and P (TN, TP) has decreased. The increased TN/TP ratio (5.38 ± 1.52 ‒ 10.19 ± 2.94) and PMN/PMP ratio (4.34 ± 0.41 ‒ 8.85 ± 4.16), as well as the reduced humic-like/protein-like ratio (H/P, 11.18 ± 4.43 ‒ 5.97 ± 3.67), indicated an imbalance in nutrient retention during sedimentary diagenesis. Our results show that eutrophication has resulted in the potential mobilization of N in sediments exceeding P, providing new insights for further understanding the nutrient cycle in the lake system and strengthening lake management.

Atmospheric Reactive Nitrogen Deposition from 2010 to 2021 in Lake Taihu and the Effects on Phytoplankton

The effects of nitrogen deposition reduction on nutrient loading in freshwaters have been widely studied, especially in remote regions. However, understanding of the ecological effects is still rather limited. Herein, we re-estimated nitrogen deposition, both of wet and dry deposition, in Lake Taihu with monthly monitoring data from 2010 to 2021. Our results showed that the atmospheric deposition of reactive nitrogen (namely NH₄⁺ and NO₃^-) in Lake Taihu was 4.94-11.49 kton/yr, which equaled 13.9%-27.3% of the riverine loading. Dry deposition of NH₄⁺ and NO₃^- contributed 53.1% of the bulk deposition in Lake Taihu. Ammonium was the main component of both wet and dry deposition, which may have been due to the strong agriculture-related activities around Lake Taihu. Nitrogen deposition explained 24.9% of the variation in phytoplankton community succession from 2010 to 2021 and was the highest among all the environmental factors. Atmospheric deposition offset the effects of external nitrogen reduction during the early years and delayed the emergence of nitrogen-fixing cyanobacterial dominance in Lake Taihu. Our results implied that a decrease in nitrogen deposition due to a reduction in fertilizer use, especially a decrease in NH₄⁺ deposition, could limit diatoms and promote non-nitrogen-fixing cyanobacterial dominance, followed by nitrogen-fixing taxa. This result was also applied to other shallow eutrophic lakes around the middle and lower reaches of the Yangtze River, where significant reduction of fertilizer use recorded during the last decades.

Anthropogenic atmospheric deposition caused the nutrient and toxic metal enrichment of the enclosed lakes in North China

Anthropogenic emissions have resulted in increases in the atmospheric fluxes of both nutrient and toxic elements. However, the long-term geochemical impacts on lake sediments of deposition activities have not been clearly clarified. We selected two small enclosed lakes in northern China-Gonghai, strongly influenced by anthropogenic activities, and Yueliang lake, relatively weakly influenced by anthropogenic activities-to reconstruct historical trends of atmospheric deposition on the geochemistry of the recent sediments. The results showed an abrupt rise in the nutrient levels in Gonghai and the enrichment of toxic metal elements from 1950 (the Anthropocene) onwards. While, at Yueliang lake, the rise on TN was from 1990 onwards. These consequences are attributable to the aggravation of anthropogenic atmospheric deposition in N, P and toxic metals, from fertilizer consumption, mining and coal combustion. The intensity of anthropogenic deposition is considerable, which leave a significant stratigraphic signal of the Anthropocene in lake sediments.

The Effects of Intraguild Predation on Phytoplankton Assemblage Composition and Diversity: A Mesocosm Experiment

Intraguild predation (IGP) can have a significant impact on phytoplankton biomass, but its effects on their diversity and assemblage composition are not well understood. In this study, we constructed an IGP model based on the common three-trophic food chain of "fish (or shrimp)-large branchiopods (Daphnia)-phytoplankton", and investigated the effects of IGP on phytoplankton assemblage composition and diversity in outdoor mesocosms using environmental DNA high-throughput sequencing. Our results indicated that the alpha diversities (number of amplicon sequence variants and Faith's phylogenetic diversity) of phytoplankton and the relative abundance of Chlorophyceae increased with the addition of Pelteobagrus fulvidraco, while similar trends were found in alpha diversities but with a decrease in the relative abundance of Chlorophyceae in the Exopalaemon modestus treatment. When both predators were added to the community, the strength of collective cascading effects on phytoplankton alpha diversities and assemblage composition were weaker than the sum of the individual predator effects. Network analysis further showed that this IGP effect also decreased the strength of collective cascading effects in reducing the complexity and stability of the phytoplankton assemblages. These findings contribute to a better understanding of the mechanisms underlying the impacts of IGP on lake biodiversity, and provide further knowledge relevant to lake management and conservation.

Nitrogen forms and concentration influence the impact of titanium dioxide nanoparticles on the biomass and antioxidant enzyme activities of Microcystis aeruginosa

Nanoparticles (NPs) are becoming more widely produced, used, and released into the aquatic environment. In aquatic ecosystems, these NPs affect different populations of photosynthesizing organisms, such as cyanobacteria. This study aimed to evaluate the effects of titanium dioxide (TiO₂) NPs (48 mg l^-1) combined with low (0.04 mM) and high (9 mM) concentrations of urea and nitrate on Microcystis aeruginosa. Microcystins (MCs) production and release were monitored in the cyanobacterium. The results showed that high urea concentration (9 mM) combined with TiO₂ NPs inhibited growth, pigment, and malondialdehyde (MDA) content by 82%, 63%, and 47%, respectively. The treatment also increased the reactive oxygen species (ROS) and glutathione S-transferase (GST) activity by 40.7% and 67.7%, respectively. Similarly, low nitrate (0.04 mM) combined with TiO₂ NPs inhibited growth by 40.3% and GST activity by 36.3% but stimulated pigment production and ROS concentration in M. aeruginosa. These responses suggest that high urea combined with TiO₂.NPs and high nitrate combined with TiO₂ NPs induced oxidative stress in cyanobacteria. The peroxidase (POD) activity of M. aeruginosa decreased by 17.7% with increasing urea concentrations. Our findings suggest that TiO₂ NPs combined with changing nutrient (urea and nitrate) concentrations may adversely affect cyanobacterial development and antioxidant defense enzymes.

Ecosystem-level effects of re-oligotrophication and N:P imbalances in rivers and estuaries on a global scale

Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re-oligotrophication is a dominant trend in rivers and estuaries of high-income countries in the last two-three decades, while in low-income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re-oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re-oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re-oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high-order rivers and shallow estuaries.

Distribution, source identification and ecological effects of aerosol dissolved nutrients in the Bohai Bay

The atmospheric aerosols around the Bohai Bay are affected intensively by the surrounding industrial, shipping and other human activities. Although atmospheric dry deposition is an important way for nutrients to enter the Bohai Bay, few studies explore the distribution patterns, source and deposition fluxes of typical nutrients in aerosols and their impacts on the marine ecosystem. This paper explored the spatial-temporal distribution of typical aerosol nutrients in summer and autumn, and their source and ecological effects were illustrated further. The mean concentration of dissolved total phosphorus (DTP), dissolved total nitrogen (DTN), dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN), ammonium (NH₄-N), nitrate (NO₃-N), nitrite (NO₂-N), silicate (SiO₃-Si), phosphate (PO₄-P), and dissolved organic phosphorus (DOP) were 31.22, 847.22, 288.19, 559.77, 288.19, 304.00, 253.65, 2.12, 15.74 and 15.48 nmol/m³, respectively, while their fluxes were corresponding to 0.61, 8.36, 2.52, 4.90, 1.41, 2.49, 0.02, 0.04, 0.19 and 0.26 mmol/(m² month). Typical aerosol nutrient concentrations in autumn were mostly higher than those in summer, with high values occurring mainly in the central region. The potential sources of pollution were mainly concentrated in Shandong and Mongolia, and the sources of pollution were mainly agriculture, dust and industry. The large N:P and N:Si ratios in the dry deposition likely exacerbated Si and P limitation in the water column. These results provided the data basis for evaluating the pollution status and revealed that the dry deposition of aerosol nutrients should not be neglected by the ecological environment in the Bohai Bay.

Short-term responses of soil nutrients and enzyme activities to nitrogen addition in a Larix principis-rupprechtii plantation in North China

Atmospheric nitrogen (N) deposition is among the main manifestations of global change and has profoundly affected forest biogeochemical cycles. However, the threshold of N deposition to soil nutrient contents and enzyme activities has rarely been studied in a forest. In this study, we explored the effects of N deposition on soil nutrients and enzyme activities in a Larix principis-rupprechtii plantation on the northern Yanshan Mountain through multigradient N addition experiments (0, 5, 10, 20, 40, 80, and 160 kg N ha−1 year−1) after fertilization for 2 years. Compared with the controls, N addition first led to a decrease in soil NH4+-N and NO3--N, which then increased significantly. N addition had no significant effects on other soil nutrients. N addition overall elevated soil β-glucosidase activity. N application of &gt;40 kg N ha−1 year−1 significantly reduced soil leucine aminopeptidase activity but had no significant effects on soil acid phosphatase, N-acetyl-β-D-glucosidase, and urease activities. N addition increased the overall stoichiometry ratio of EEA C:N and EEA C:P, but EEA N:P started decreasing after N application of 40 kg N ha−1 year−1. The ratios of C, N, and P acquisition activities changed from 1:1.2:1 under the control conditions to 1:1.1:1 under the N application of 160 kg N ha−1 year−1. N addition increased the overall vector length and had no significant effects on the vector angle. Correlation and redundancy analyses revealed that N addition-induced change in available soil N was the main factor affecting soil enzyme activity and stoichiometry. In general, different enzyme activities had different sensitivities to N addition. Moderate N addition or atmospheric N deposition (e.g., &lt;40 kg N ha−1 year−1) had beneficial effects on soil nutrient cycling and microorganisms in a Larix principis-rupprechtii plantation.

Efficiency of reactive nitrogen removal in a model Mediterranean high-mountain lake and its downwater river ecosystem: Biotic and abiotic controls

High-mountain lakes and rivers are usually oligotrophic and strongly influenced by atmospheric transport processes. Thus, wet deposition of reactive N species (Nr), mainly in the form of nitrate (NO₃^-), is a major source of N input in these high-mountain ecosystems. Bacterial denitrifiers are thought to be largely responsible for reduction of NO₃^- to nitrous oxide (N₂O) and molecular dinitrogen (N₂) as main biological pathway of N removal in these ecosystems. Nitrifiers, through the oxidation of ammonium to NO₃^-, can also be a source of NO₃^- and N₂O. However, there is uncertainty regarding the abiotic and biotic factors controlling Nr elimination from aquatic ecosystems at different altitudes and seasons. We examined the efficiency of Nr removal as N₂O and N₂ (total removal) or N₂ only (clean removal) in a model lake and its downwater river ecosystem (Sierra Nevada, Spain) representative of Mediterranean high-mountain freshwater ecosystems along an altitudinal gradient during the warm period of the year. Denitrification activity and the abundance of nitrifiers and denitrifiers in sediments were measured at thaw, mid ice-free and late ice-free periods. We found the efficiency of total and clean removal of Nr increased from the downwater river to the high-mountain lake. Regardless of the location, the efficiency of total removal of Nr decreased over the ice-free period whereas that of clean removal of Nr peaked at mid ice-free period. The efficiency of total removal of Nr was mainly controlled by the abundance of archaeal nitrifiers and bacterial denitrifiers. Abiotic (ammonium and NO₃^- concentration) and biotic (mainly nosZI-type denitrifiers) factors drove changes in the efficiency of clean removal of Nr. Our results suggest that abiotic and biotic factors can control the efficiencies of Nr removal in Mediterranean high-mountain lakes and their downwater rivers, and that these efficiencies increase with altitude and vary over the ice-free period.

Decline of insects and arachnids driven by nutrient enrichment: A meta-analysis

Recent studies have documented global declines in insects and their relatives, but the exact mechanisms explaining these patterns are not fully understood. A potential driver underlying arthropod population declines is increases in anthropogenic inputs of nitrogen (N) and phosphorus (P). Here, we synthesize the effects of N, P, and combined N + P enrichment on the abundance of hexapods (insects and collembola) and arachnids from 901 experiments reported in 84 studies. We found that N and combined N + P enrichment caused significant decreases in the abundance of these groups overall. While arthropod responses to nutrient enrichment across aquatic and terrestrial habitats and in temperate as well as tropical climatic zones differed in magnitude, our results suggest that arthropods are decreasing similarly in response to nitrogen and phosphorus enrichment. Further, despite previously shown differences in the nutrient demands of different insect metamorphosis groups, we found consistent negative effects of N + P enrichment on all groups. Our results also showed that the negative effects of nutrient additions are stronger for aquatic insects that are considered more sensitive to changes in physical-chemical parameters in their environments, Ephemeroptera, Plecoptera, and Trichoptera (EPT), compared with other aquatic insects. In addition, N + P enrichment reduced the abundance of above-ground and below-ground arthropods, suggesting that a similar mechanism driving arthropod community change is acting on both groups. These findings suggest that changes in elemental cycles are a potential cause of the ongoing global decline of arthropods and underscore the serious effects of nutrient enrichment on ecological systems.

Decoupling of N and P aggravated upward along food chains in an urban river ecosystem

Anthropogenic input of nutrient has profoundly influenced water quality and aquatic organisms, however, large and unbalanced nitrogen (N) and phosphorus (P) inputs (decoupling) can lead to a range of ecological health problems such as eutrophication. Whether and how the decoupling varies along the aquatic food chain remains poorly addressed. Here we chose an urban river ecosystem in the cosmopolis region of Beijing, with reclaimed water as the entire replenishment water source over 20 years, to demonstrate the decoupling pattern of N vs P across trophic levels. Results showed that organism C, N and P concentration increased, but N:P ratio decreased upward along the food chains, suggesting that this decoupling of N and P increased as trophic level ascends. Compared with natural freshwater ecosystem, the decoupling of N and P was aggravated in the reclaimed water river. Moreover, the homeostasis of N and P were higher at higher relative to lower trophic levels, and higher in macro-food chain relative to planktonic food chain. This study, for the first time, revealed the increasing decoupling of N vs P upward along the major food chains in an urban aquatic ecosystem, and could improve the understanding of nutrient cycling at the food chain level under human disturbance, and provide useful information for ecological restoration and eutrophication control of urban wetlands replenished with reclaimed water.

Improvement in water pollution control alters nutrient stoichiometry of Guanting Reservoir near Beijing, North China

Significant improvement in wastewater treatment is the most effective way for eutrophication control, especially in semiarid regions. However, its effect on the nutrient status and stoichiometry of the receiving water body has remained poorly considered and understood at broad temporal scales. Taking Guanting Reservoir (GR) in Hebei-Beijing (P. R. China) as an example, we present a study that links a continuous monitoring dataset for GR with corresponding estimates of human-induced nutrient discharges in its watershed from the year 2006 to 2019. We find that current GR belongs to strict P limitation and the faster decrease of TP than TN concentrations and continuous increase of TN/TP mass ratios in GR are attributed to the water restoration investment-induced declining of nutrient loadings. The improved municipal wastewater treatment capacity is mainly responsible for these significant changes, due to the higher removal efficiency of TP than TN in municipal wastewater. Given the potential ecological impact on aquatic biodiversity as well as ecosystem function of changes in TN/TP ratios and higher retention rate of TP (97.4%) compared with TN (93.1%) in GR, our findings highlight that future strategy for water pollution control should not only concentrate on more nutrient reduction efficiencies but attach importance to their stoichiometric balance to reduce the potential risk of phytoplankton blooms and toxin production during the water quality recovery of lakes or reservoirs.

An experimental test of the growth rate hypothesis as a predictive framework for microevolutionary adaptation

The growth rate hypothesis (GRH) posits that the relative body phosphorus content of an organism is positively related to somatic growth rate, as protein synthesis, which is necessary for growth, requires P-rich rRNA. This hypothesis has strong support at the interspecific level. Here, we explore the use of the GRH to predict microevolutionary responses in consumer body stoichiometry. For this, we subjected populations of the rotifer Brachionus calyciflorus to selection for fast population growth rate (PGR) in P-rich (HPF) and P-poor (LPF) food environments. With common garden transplant experiments, we demonstrate that in HP populations evolution toward increased PGR was concomitant with an increase in relative phosphorus content. In contrast, LP populations evolved higher PGR without an increase in relative phosphorus content. We conclude that the GRH has the potential to predict microevolutionary change, but that its application is contingent on the environmental context. Our results highlight the potential of cryptic evolution in determining the performance response of populations to elemental limitation of their food resources.

Browning affects pelagic productivity in northern lakes by surface water warming and carbon fertilization

Global change impacts important environmental drivers for pelagic gross primary production (GPP) in northern lakes, such as temperature, light, nutrient, and inorganic carbon availability. Separate and/or synergistic impacts of these environmental drivers on pelagic GPP remain largely unresolved. Here, we assess key drivers of pelagic GPP by combining detailed depth profiles of summer pelagic GPP with environmental and climatic data across 45 small and shallow lakes across northern Sweden (20 boreal, 6 subarctic, and 19 arctic lakes). We found that across lakes summer pelagic GPP was strongest associated with lake water temperatures, lake carbon dioxide (CO₂ ) concentrations impacted by lake water pH, and further moderated by dissolved organic carbon (DOC) concentrations influencing light and nutrient conditions. We further used this dataset to assess the extent of additional DOC-induced warming of epilimnia (here named internal warming), which was especially pronounced in shallow lakes (decreasing 0.96°C for every decreasing m in average lake depth) and increased with higher concentrations of DOC. Additionally, the total pools and relative proportion of dissolved inorganic carbon and DOC, further influenced pelagic GPP with drivers differing slightly among the boreal, subarctic and Arctic biomes. Our study provides novel insights in that global change affects pelagic GPP in northern lakes not only by modifying the organic carbon cycle and light and nutrient conditions, but also through modifications of inorganic carbon supply and temperature. Considering the large-scale impacts and similarities of global warming, browning and recovery from acidification of lakes at higher latitudes throughout the northern hemisphere, these changes are likely to operate on a global scale.

Influence of social and environmental drivers on nutrient concentrations and ratios in lakes: A comparison between China and Europe

Global anthropogenic flows of nitrogen (N) and phosphorus (P) have exceeded planetary boundaries with significant implications for pollution of the freshwater resources in the world. Understanding the global patterns and drivers of N and P concentrations and their ratios in the lakes could help design more effective management and remediation strategies to mitigate the impacts of eutrophication. While a suite of drivers are associated with the sources of nutrients, their transport and internal processes that control concentrations of N and P in the lakes, much less is known about the relative importance of different drivers in explaining spatial variations of lake nutrients and ratios. In this study, we compared N and P concentrations and their ratios in the lakes across China and Europe and examined the differences of dominant environmental and social drivers on lake nutrients. Our comparison showed that total nitrogen (TN) and total phosphorus (TP) concentrations were much higher in the lakes in China compared to those in Europe (i.e., TN: 1.13 mg/L in China vs. 0.64 mg/L in Europe; TP: 35.83 μg/L in China vs. 19.38 μg/L in Europe, the median value). However, lake N/P ratios for both regions were not statistically different. Concentrations of TN and TP showed decoupling in both regions, with the majority of lakes having high N/P mass ratios when evaluated by the commonly accepted threshold of 23 (i.e., 61% in China and 68% in Europe), indicating that phytoplankton are more P limited relative to N. Agricultural activity in the lake catchment is an important predictor for both nutrient concentrations and their ratio in Europe. This reflects successful investments in infrastructure and policy prescriptions in addressing point sources of pollution. In comparison, lake depth and water residence time are important in the decoupling of N and P concentrations in China. The regional difference between the dominant drivers can provide important insights into development of effective water pollution control measures. It is necessary for policy makers and water resource managers to be aware of large-scale imbalance of nutrients in lake due to the potential environmental consequences. A set of spatially flexible policies for water quality controls would be beneficial for sustaining the ecological integrity and future health of lakes.

Drivers and variability of CO2:O2 saturation along a gradient from boreal to Arctic lakes

Lakes are significant players for the global climate since they sequester terrestrially derived dissolved organic carbon (DOC), and emit greenhouse gases like CO₂ to the atmosphere. However, the differences in environmental drivers of CO₂ concentrations are not well constrained along latitudinal and thus climate gradients. Our aim here is to provide a better understanding of net heterotrophy and gas balance at the catchment scale in a set of boreal, sub-Arctic and high-Arctic lakes. We assessed water chemistry and concentrations of dissolved O₂ and CO₂, as well as the CO₂:O₂ ratio in three groups of lakes separated by steps of approximately 10 degrees latitude in South-Eastern Norway (near 60° N), sub-Arctic lakes in the northernmost part of the Norwegian mainland (near 70° N) and high-Arctic lakes on Svalbard (near 80° N). Across all regions, CO₂ saturation levels varied more (6-1374%) than O₂ saturation levels (85-148%) and hence CO₂ saturation governed the CO₂:O₂ ratio. The boreal lakes were generally undersaturated with O₂, while the sub-Arctic and high-Arctic lakes ranged from O₂ saturated to oversaturated. Regardless of location, the majority of the lakes were CO₂ supersaturated. In the boreal lakes the CO₂:O₂ ratio was mainly related to DOC concentration, in contrast to the sub-Arctic and high-Arctic localities, where conductivity was the major statistical determinant. While the southern part is dominated by granitic and metamorphic bedrock, the sub-Arctic sites are scattered across a range of granitic to sedimentary bed rocks, and the majority of the high-Arctic lakes are situated on limestone, resulting in contrasting lake alkalinities between the regions. DOC dependency of the CO₂:O₂ ratio in the boreal region together with low alkalinity suggests that in-lake heterotrophic respiration was a major source of lake CO₂. Contrastingly, the conductivity dependency indicates that CO₂ saturation in the sub-Arctic and high-Arctic lakes was to a large part explained by DIC input from catchment respiration and carbonate weathering.

Assessment of physico-chemical parameters of surface waters of a tropical brackish water lake in South Asia

Brackish lake systems and estuaries are unique aquatic systems that support diversified life forms and strongly influence a region's economy. Major chemical water quality parameters of India's second-largest brackish water lake, Pulicat were assessed. Physico-chemical parameters like pH, temperature, suspended solid concentrates, total dissolved solids, salinity, nitrogenous nutrients, phosphate, silicate, and chlorophyll a were analysed. The results obtained for different parameters were compared and interpreted with statistical software SPSS version 20 and images were plotted using the Arc GIS spatial analyst tool. During the summer months, the nitrogen to phosphorus ratio ranges from a minimum of 1.96 to a maximum of 16.64 (9.55 ± 4.01) while it ranges from a minimum of 7.98 to a maximum of 15.52 (12.47 ± 2) during the pre-monsoon. In the monsoon season, the nitrogen to phosphorus ratio of surface water suggests a range from a minimum of 8.64 to a maximum of 17.58 (13.87 ± 2.14). During the post-monsoon season, the nitrogen to phosphorus ratio ranges from 4.98 to 17.34 (11.77 ± 3.68). The average nitrogen to phosphorus ratios were 9.6, 12.5, 13.9 and 11.8 in summer, pre-monsoon, monsoon, and post-monsoon respectively. The nitrogen to phosphorus ratio was lower than the Redfield ratio for all the seasons. The average concentration of chlorophyll a was 14.9, 13.4, 12.8 and 11.8 in summer, pre-monsoon, monsoon, and post-monsoon respectively. As per the Pearson Correlation Coefficient, there was no significant correlation among nitrogen, phosphorus, and chlorophyll a. This suggests the influence of suspended solid concentrates, and nitrogen and phosphorus flux in the sediment-water interface might be interfering with the nutrient cycles and primary productivity.

Multiple sources of aerobic methane production in aquatic ecosystems include bacterial photosynthesis

Aquatic ecosystems are globally significant sources of the greenhouse gas methane to the atmosphere. Until recently, methane production was thought to be a strictly anaerobic process confined primarily to anoxic sediments. However, supersaturation of methane in oxygenated waters has been consistently observed in lakes and the ocean (termed the 'methane paradox'), indicating that methane can be produced under oxic conditions through unclear mechanisms. Here we show aerobic methane production from multiple sources in freshwater incubation experiments under different treatments and based on biogeochemical, metagenomic, and metatranscriptomic data. We find that aerobic methane production appears to be associated with (bacterio)chlorophyll metabolism and photosynthesis, as well as with Proteobacterial degradation of methylphosphonate. Genes encoding pathways for putative photosynthetic- and methylphosphonate-based methane production also co-occur in Proteobacterial metagenome-assembled genomes. Our findings provide insight into known mechanisms of aerobic methane production, and suggest a potential co-occurring mechanism associated with bacterial photosynthesis in aquatic ecosystems.

Why do red tides occur frequently in some oligotrophic waters? Analysis of red tide evolution history in Mirs Bay, China and its implications

The process and management of red tide in oligotrophic waters are poorly understood as most studies on red tide were focused on eutrophic areas. In this study, 404 red tide events together with the historical water quality dynamics during 1991-2020 were investigated in an anthropogenically influenced bay in China - Mirs Bay, whose most region is oligotrophic except small inshore areas. Red tides of oligotrophic offshore accounted for 20 % of all. With the effective governmental management on inshore areas, concentration of PO₄ and DIN has been decreased to a low level (PO₄ <0.01 mg/L while DIN <0.1 mg/L) in the bay since about 2000. However, the reduction of nutrients was still accompanied by the frequent outbreaks of red tides, as well as a shift of dominant algae from diatoms to dinoflagellates, which might be due to the unbalanced nutrient reduction, such as N:P ratio fluctuation and organic nutrient increase. This shift might trigger more red tide events and even some super ones (long-duration or large-scale) in oligotrophic areas. Detailed analysis on red tide events combined with model simulation proved that the outbreak of red tide in Mirs Bay was caused by the joint contribution of nutrients and hydrodynamics. Nutrients of inshore area supported the red tides there, and with the help of physical conditions, red tides inshore could be transferred to offshore areas and then were likely to bloom again or be preyed to support blooms of other organisms. This study acknowledged that the reduction of both N and P either inorganic or organic nutrients was essential to control red tides, even in oligotrophic waters, but a balanced strategy considering the dual reduction of both nitrogen and phosphorus was of pivotal role to restore the health of coastal water systems disturbed by human.

Nitrogen to phosphorus ratio shapes the bacterial communities involved in cellulose decomposition and copper contamination alters their stoichiometric demands

All living organisms theoretically have an optimal stoichiometric nitrogen: phosphorus (N: P) ratio, below and beyond which their growth is affected, but data remain scarce for microbial decomposers. Here, we evaluated optimal N: P ratios of microbial communities involved in cellulose decomposition and assessed their stability when exposed to copper Cu(II). We hypothesized that (1) cellulose decomposition is maximized for an optimal N: P ratio; (2) copper exposure reduces cellulose decomposition and (3) increases microbial optimal N: P ratio; and (4) N: P ratio and copper modify the structure of microbial decomposer communities. We measured cellulose disc decomposition by a natural inoculum in microcosms exposed to a gradient of N: P ratios at three copper concentrations (0, 1 and 15 µM). Bacteria were most probably the main decomposers. Without copper, cellulose decomposition was maximized at an N: P molar ratio of 4.7. Contrary to expectations, at high copper concentration, the optimal N: P ratio (2.8) and the range of N: P ratios allowing decomposition were significantly reduced and accompanied by a reduction of bacterial diversity. Copper contamination led to the development of tolerant taxa probably less efficient in decomposing cellulose. Our results shed new light on the understanding of multiple stressor effects on microbial decomposition in an increasingly stoichiometrically imbalanced world.

Revisiting the growth rate hypothesis: Towards a holistic stoichiometric understanding of growth

The growth rate hypothesis (GRH) posits that variation in organismal stoichiometry (C:P and N:P ratios) is driven by growth-dependent allocation of P to ribosomal RNA. The GRH has found broad but not uniform support in studies across diverse biota and habitats. We synthesise information on how and why the tripartite growth-RNA-P relationship predicted by the GRH may be uncoupled and outline paths for both theoretical and empirical work needed to broaden the working domain of the GRH. We found strong support for growth to RNA (r²  = 0.59) and RNA-P to P (r²  = 0.63) relationships across taxa, but growth to P relationships were relatively weaker (r²  = 0.09). Together, the GRH was supported in ~50% of studies. Mechanisms behind GRH uncoupling were diverse but could generally be attributed to physiological (P accumulation in non-RNA pools, inactive ribosomes, translation elongation rates and protein turnover rates), ecological (limitation by resources other than P), and evolutionary (adaptation to different nutrient supply regimes) causes. These factors should be accounted for in empirical tests of the GRH and formalised mathematically to facilitate a predictive understanding of growth.

Response of atmospheric deposition and surface water chemistry to the COVID-19 lockdown in an alpine area

The effects of the COVID-19 lockdown on deposition and surface water chemistry were investigated in an area south of the Alps. Long-term data provided by the monitoring networks revealed that the deposition of sulfur and nitrogen compounds in this area has stabilized since around 2010; in 2020, however, both concentrations and deposition were significantly below the average values of the previous decade for SO₄ and NO₃. Less evident changes were observed for NH₄ and base cation. The estimated decrease of deposition in 2020 with respect to the previous decade was on average - 54% and - 46% for SO₄ and NO₃, respectively. The lower deposition of SO₄ and NO₃ recorded in 2020 was caused by the sharp decrease of SO₂ and particularly of NO_x air concentrations mainly due to the mobility restrictions consequent to the COVID-19 lockdown. The limited effects on NH₄ deposition can be explained by the fact that NH₃ emission was not affected by the lockdown, being mainly related to agricultural activities. A widespread response to the decreased deposition of S and N compounds was observed in a group of pristine freshwater sites, with NO₃ concentrations in 2020 clearly below the long-term average. The rapid chemical recovery observed at freshwater sites in response to the sharp decrease of deposition put in evidence the high resilience potential of freshwater ecosystems in pristine regions and demonstrated the great potential of emission reduction policy in producing further substantial ameliorations of the water quality at sensitive sites.

The influence of nutrients on the composition and quantity of buried organic carbon in a eutrophic plateau lake, Southwest China

The regulation of lacustrine organic carbon (OC) burial by nutrient is an outstanding knowledge gap in the current understanding of lake carbon cycles. In this study, we determined how nutrients quantitatively correspond with OC burial using the parallel factor analysis (PARAFAC) method in Dianchi Lake, southwest China. Factors were classified into three types according to their historical sedimentation characteristics: the background factor (BF), response factor (RF), and contingency factor (CF). The BF represented the original OC input combination in the lake and was insensitive to nutrient changes. The RF represented the OC input combination that was induced or promoted by nutrient changes in the lake. The CF represented short-term discontinuous factors in sedimentary history, which may be related to unique historical events. The results indicate that changes in the total nitrogen (TN) to total phosphorus (TP) ratio correlated with changes in the BF contribution; whereas the quantity of OC was mainly correlated with TN. The >90% of OC buried in sediment was quantitatively simulated by BF and RF; the driving effect of RF on OC burial was approximately 13 times higher than that of BF. It was observed that a 1 mg kg^-1 increase in TN led to approximately 2.2 units increase in RF contribution in Dianchi Lake, while the BF was insensitive to changes in TN. Thus, changes in lake nutrients may effectively change the composition and quantity of OC buried in lake sediment.

Muddy (silty-sand) beaches in semi-arid regions attenuate the contaminants flowing into the sea as a submarine groundwater discharge

Urbanized coastal areas are well-recognized hotspots for the contaminant-enriched groundwater discharge, influencing sensitive coastal ecosystems. The present study investigates how muddy beaches in the semi-arid region alter the contaminant flux flowing into the sea using submarine groundwater discharge (SGD) estimation and hydrogeochemical analysis of coastal waters (groundwater, porewater, and seawater). Fresh SGD carries contaminants such as nutrients and trace metals in the coastal ecosystem, causing increased vulnerability towards eutrophication, harmful algal blooms, and human health. We found that SGD reaching the coast carries immense nutrient flux (155.6 mmol NO₃^- · day^-1; 35 mmol P · day^-1 and 12.4 mmol DSi · day^-1) and trace metal load ranging from 0.1 to 14.9 mmol · day^-1. The nutrient fluxes were higher in the upper saline plume compared to the lower plume. The muddy beach attenuates the nutrients in varying percentages of 9.7 to 22% of NO₃^-, 1.9 to 25.5% of P due to denitrification and phosphorus absorption, and also caused 19.6% reduction of SO₄^2-. The reduction in SO₄^2- leads to the formation of sulfide (HS^-) that promotes the metal precipitation, resulting in the removal of Pb and Cu. This attenuation of nutrients leads to a change in the nutrient ratio (N/P = 7-11) approaching the Redfield ratio, implying the vulnerability of algal bloom at the Dehri beach. Overall, the muddy beach can serve as a natural biogeochemical reactor as it attenuates the nutrient and serves as a source for certain trace metals (Fe, Mn, Zn, and Ni), altering the composition of SGD. Probably this is the first study that emphasizes the attenuation of trace metals in the muddy beaches of a semi-arid region.

The amounts and ratio of nitrogen and phosphorus addition drive the rate of litter decomposition in a subtropical forest

Nitrogen (N) and phosphorus (P) control biogeochemical cycling in terrestrial ecosystems. However, N and P addition effects on litter decomposition, especially biological pathways in subtropical forests, remain unclear. Here, a two-year field litterbag experiment was employed in a subtropical forest in southwestern China to examine N and P addition effects on litter biological decomposition with nine treatments: low and high N- and P-only addition (LN, HN, LP, and HP), NP coaddition (LNLP, LNHP, HNLP, and HNHP), and a control (CK). The results showed that the decomposition coefficient (k) was higher in NP coaddition treatments (P < 0.05), and lower in N- and P-only addition treatments than in CK (P < 0.05). The highest k was observed with LNLP (P < 0.05). The N- and P-only addition treatments decreased the losses of litter mass, lignin, cellulose, and condensed tannins, litter microbial biomass carbon (MBC), litter cellulase, and soil pH (P < 0.05). The NP coaddition treatments increased the losses of litter mass, lignin, and cellulose, MBC concentration, litter invertase, urease, cellulase, and catalase activities, soil arthropod diversity (S) in litterbags, and soil pH (P < 0.05). Litter acid phosphatase activity and N:P ratio were lower in N-only addition treatments but higher in P-only addition and NP coaddition treatments than in CK (P < 0.05). Structural equation model showed that litter MBC, S, cellulase, acid phosphatase, and polyphenol oxidase contributed to the loss of litter mass (P < 0.05). The litter N:P ratio was negatively logarithmically correlated with mass loss (P < 0.01). In conclusion, the negative effect of N addition on litter decomposition was reversed when P was added by increasing decomposed litter soil arthropod diversity, MBC concentration, and invertase and cellulase activities. Finally, the results highlighted the important role of the N:P ratio in litter decomposition.

Sustained stoichiometric imbalance and its ecological consequences in a large oligotrophic lake

Both nitrogen (N) and phosphorus (P) drive water quality and are heavily altered by human activities that amplify their supplies to lakes, rivers, and oceans. Considerable attention is given to management of absolute nutrient levels but less to their relative abundances, their N:P stoichiometry. This study documents high N:P ratios in low-nutrient Flathead Lake that persist despite considerably lower N:P ratios in river nutrient inputs. The lake’s elevated N:P ratios are associated with phytoplankton P limitation, impaired food quality for zooplankton, and potential production of the greenhouse gas methane by P-limited microbes. These findings highlight the need to consider not only absolute levels of N and P in aquatic ecosystems, but also their stoichiometric balance.

Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans, but less is paid to their relative concentrations, their nitrogen:phosphorus (N:P) stoichiometry, and the consequences of imbalanced stoichiometry. Here, we report 38 y of nutrient dynamics in Flathead Lake, a large oligotrophic lake in Montana, and its inflows. While nutrient levels were low, the lake had sustained high total N: total P ratios (TN:TP: 60 to 90:1 molar) throughout the observation period. N and P loading to the lake as well as loading N:P ratios varied considerably among years but showed no systematic long-term trend. Surprisingly, TN:TP ratios in river inflows were consistently lower than in the lake, suggesting that forms of P in riverine loading are removed preferentially to N. In-lake processes, such as differential sedimentation of P relative to N or accumulation of fixed N in excess of denitrification, likely also operate to maintain the lake’s high TN:TP ratios. Regardless of causes, the lake’s stoichiometric imbalance is manifested in P limitation of phytoplankton growth during early and midsummer, resulting in high C:P and N:P ratios in suspended particulate matter that propagate P limitation to zooplankton. Finally, the lake’s imbalanced N:P stoichiometry appears to raise the potential for aerobic methane production via metabolism of phosphonate compounds by P-limited microbes. These data highlight the importance of not only absolute N and P levels in aquatic ecosystems, but also their stoichiometric balance, and they call attention to potential management implications of high N:P ratios.

The interaction of physical structure and nutrient loading drives ecosystem change in a large tropical lake over 40 years

Many lakes across the world are entering novel states and experiencing altered biogeochemical cycling due to local anthropogenic stressors. In the tropics, understanding the drivers of these changes can be difficult due to a lack of documented historic conditions or an absence of continuous monitoring that can distinguish between intra- and inter-annual variation. Over the last forty years (1980-2020), Lake Yojoa (Honduras) has experienced increased watershed development as well as the introduction of a large net-pen Tilapia farm, resulting in a dramatic reduction in seasonal water clarity, increased trophic state and altered nutrient dynamics, shifting Lake Yojoa from an oligotrophic (low productivity) to mesotrophic (moderate productivity) ecosystem. To assess the changes that have occurred in Lake Yojoa as well as putative drivers for those changes, we compared Secchi depth (water clarity), dissolved inorganic nitrogen (DIN), and total phosphorus (TP) concentrations at continuous semi-monthly intervals for the three years between 1979 and 1983 and again at continuous 16-day intervals for 2018-2020. Between those two periods we observed the loss of a clear water phase that previously occurred in the months when the water column was fully mixed. Seasonal peaks in DIN coincident with mixing suggest that an enhanced accumulation of ammonium in the hypolimnion (the bottom layer of a stratified lake) during stratification, and release to the epilimnion (the top layer of a stratified lake) with mixing maintains high algal abundance and subsequently low Secchi depth during what was previously the clear water phase. This interaction of nutrient loading and Lake Yojoa's monomictic stratification regime illustrates a key phenomenon in how physical water column structure and nutrients interact in tropical monomictic lakes. This work highlights the need to consider nutrient dynamics of warm anoxic hypolimnions, not just surface water nutrient concentrations, to understand environmental change in these societally important but understudied ecosystems.

Evidence for multiple potential drivers of increased phosphorus in high-elevation lakes

Total phosphorus (TP) concentrations have increased in many remote mountain waterbodies across the western United States, and reports of algal blooms in these systems have increased in frequency. Explanations for observed TP increases are uncertain, and typical landscape drivers, such as agricultural/urban runoff, are implausible. We investigated multiple atmospheric and terrestrial-P loading mechanisms to explain the observed decadal increase in TP, including a novel hypothesis that warming soils may lead to elevated P fluxes to receiving water bodies. Using northern Utah mountains ranges as a case study, we measured prospective inputs of total and bioavailable P via dust deposition. Terrestrial loading was evaluated through soil leaching experiments designed to simulate soil acidification and recovery, as well as observed decadal increases in soil temperatures and extended growing season. In the Uinta Mountains, dust-P flux appears to be one of the most plausible mechanisms for P increases where we estimated bioavailable dust-P loading ranged from 1.6 to 23.1 mg P m^-2 yr^-1. However, our results revealed that an increase of soil pH by 0.5 units could lead to a rise in leached P, ranging from 4.7 to 65 mg P m^-2. Rising temperatures also showed the potential to increase soil P leaching; Observed average historical (~ +3 °C) and future (+2 °C) increases in temperature led to a prospective increase in leached P from 2 to 264 mg SRP m^-2. While we found that pH shifts can mobilize significant amounts of P in some locations, we found no evidence of pH changes through time in the Uinta Mountains. However, summer soil temperatures increased at most locations. The mechanisms evaluated in this study can help explain the widespread observed increases in P across Western US lakes, but the mechanisms that dominate in any given region are likely to vary based on local to regional factors.

Unexpected response of nitrogen deposition to nitrogen oxide controls and implications for land carbon sink

Terrestrial ecosystems in China receive the world’s largest amount of reactive nitrogen (N) deposition. Recent controls on nitrogen oxides (NO_x = NO + NO₂) emissions in China to tackle air pollution are expected to decrease N deposition, yet the observed N deposition fluxes remain almost stagnant. Here we show that the effectiveness of NO_x emission controls for reducing oxidized N (NO_y = NO_x + its oxidation products) deposition is unforeseen in Eastern China, with one-unit reduction in NO_x emission leading to only 55‒76% reductions in NO_y-N deposition, as opposed to the high effectiveness (around 100%) in both Southern China and the United States. Using an atmospheric chemical transport model, we demonstrate that this unexpected weakened response of N deposition is attributable to the enhanced atmospheric oxidizing capacity by NO_x emissions reductions. The decline in N deposition could bear a penalty on terrestrial carbon sinks and should be taken into account when developing pathways for China’s carbon neutrality.

Recent vigorous controls in anthropogenic nitrogen oxide emissions in China cannot result in proportionate decreases in regional atmospheric nitrogen deposition. Enhanced atmospheric oxidizing capacity offsets those reductions of precursor emissions.

The Light-to-Nutrient Ratio in Alpine Lakes: Different Scenarios of Bacterial Nutrient Limitation and Community Structure in Lakes Above and Below the Treeline

The light-to-nutrient hypothesis proposes that under high light-to-nutrient conditions, bacteria tend to be limited by phosphorus (P), while under relatively low light-to-nutrient conditions, bacteria are likely driven towards carbon (C) limitation. Exploring whether this light-to-nutrient hypothesis is fitting for alpine lakes has profound implications for predicting the impacts of climatic and environmental changes on the structures and processes of aquatic ecosystems in climate-sensitive regions. We investigated the environmental conditions and bacterioplankton community compositions of 15 high-elevation lakes (7 above and 8 below treeline). High light-to-nutrient conditions (denoted by the reciprocal value of the attenuation coefficient (1/K) to total phosphorus (TP)), high chlorophyll a (Chl a) concentrations, low TP concentrations and low ratios of the dissolved organic carbon concentration to the dissolved total nitrogen concentration (DOC:DTN) were detected in above-treeline lakes. Significant positive correlations between the bacterioplankton community compositions with 1/K:TP ratios and Chl a concentrations indicated that not only high light energy but also nutrient competition between phytoplankton and bacteria could induce P limitation for bacteria. In contrast, low light-to-nutrient conditions and high allochthonous DOC input in below-treeline lakes lessen P limitation and C limitation. The most abundant genus, Polynucleobacter, was significantly enriched, and more diverse oligotypes of Polynucleobacter operational taxonomic units were identified in the below-treeline lakes, indicating the divergence of niche adaptations among Polynucleobacter oligotypes. The discrepancies in the light-to-P ratio and the components of organic matter between the above-treeline and below-treeline lakes have important implications for the nutrient limitation of bacterioplankton and their community compositions.

Nitrogen concentration response to the decline in atmospheric nitrogen deposition in a hypereutrophic lake

Atmospheric nitrogen (N) deposition is becoming an increasingly important factor affecting the nutrient level of lakes, especially considering the long-term control measures for external N inputs in developed regions. However, few studies have investigated the effects of atmospheric N deposition and the respective ecological significance in eutrophic waters. In this study, bulk and wet deposition rates of all N species and water N concentrations in Lake Taihu were determined based on the long-term (2010-2018) high-resolution (weekly or monthly) systematic observations. The results indicated that the decline in wind speed and change in land-use type likely decreased the N deposition rate. The bulk N deposition rates decreased from 45.77 kg N ha^-1 yr^-1 in 2012 to 22.06 kg N ha^-1 yr^-1 in 2018, which could account for decrease of 1.01 mg N L^-1 in the lake N concentrations via a rough estimation, and this value was close to the actual variation in N concentration in Lake Taihu. The correlation between N concentrations and atmospheric deposition fluxes was stronger than that between N concentrations and riverine N inputs or lake storage, which further indicated that change in atmospheric N deposition was the key reason for the variation in N concentrations. The direct bulk N deposition into Lake Taihu accounted for 17.5% and 51.4% of the riverine N inputs and lake N inventory, respectively. Moreover, atmospheric N deposition was concentrated in summer, which was dominated by reduced N, and it may be important for the duration of algal blooms. Therefore, external N inputs, including atmospheric N deposition, should be further controlled for an effective mitigation of eutrophication and algal blooms in Lake Taihu.

Soil Microbe-Mediated N:P Stoichiometric Effects on Solidago canadensis Performance Depend on Nutrient Levels

Both soil microbes and soil N:P ratios can affect plant growth, but it is unclear whether they can interact to alter plant growth and whether such an interactive effect depends on nutrient levels. Here, we tested the hypothesis that soil microbes can ameliorate the negative effects of nutrient imbalance caused by low or high N:P ratios on plant growth and that such an ameliorative effect of soil microbes depends on nutrient supply levels. We grew individuals of six populations of the clonal plant Solidago canadensis at three N:P ratios (low (1.7), intermediate (15), and high (135)), under two nutrient levels (low versus high) and in the presence versus absence of soil microbes. The presence of soil microbes significantly increased biomass of S. canadensis at all three N:P ratios and under both nutrient levels. Under the low-nutrient level, biomass, height, and leaf number of S. canadensis did not differ significantly among the three N:P ratio treatments in the absence of soil microbes, but they were higher at the high than at the low and the intermediate N:P ratio in the presence of soil microbes. Under the high-nutrient level, by contrast, biomass, height, and leaf number of S. canadensis were significantly higher at the low than at the high and the intermediate N:P ratio in the absence of soil microbes, but increased with increasing the N:P ratio in the presence of soil microbes. In the presence of soil microbes, number of ramets (asexual individuals) and the accumulation of N and P in plants were significantly higher at the high than at the low and the intermediate N:P ratio under both nutrient levels, whereas in the absence of soil microbes, they did not differ significantly among the three N:P ratio regardless of the nutrient levels. Our results provide empirical evidence that soil microbes can alter effects of N:P ratios on plant performance and that such an effect depends on nutrient availability. Soil microbes may, therefore, play a role in modulating ecosystem functions such as productivity and carbon and nutrient cycling via modulating nutrient imbalance caused by low and high N:P ratios.

Allometric releases of nitrogen and phosphorus from sediments mediated by bacteria determines water eutrophication in coastal river basins of Bohai Bay

Although the Chinese government has conducted much work in recent years to reduce land-based pollutant discharge, eutrophication continues to occur frequently in many rivers, estuaries, and coastal waters. This may indicate that sediment is a major source rather than a sink for nitrogen (N) and phosphorus (P). To clarify the endogenous mechanisms of eutrophication in coastal river basins, the eutrophication status, physicochemical properties, and bacterial parameters of overlying waters and sediments in the catchment (CA), estuarine (EA), and offshore (OA) areas in the Duliujian River Basin of Bohai Bay were investigated. The results showed that the eutrophication index (EI) of CA, EA, and OA were 62.71, 57.86, and 36.51, respectively. The EI was more sensitive to increases in P (slope = 3.887) than to increases in N (slope = 0.734) of the overlying water, indicating that P is the main factor driving eutrophication in the coastal river basin. However, a nonlinear relationship was found between P in sediments and overlying waters, suggesting that bacterial mediation may occur during P release. As speculated in this study, P in the overlying water increased more quickly than N with increasing bacterial diversity and metabolic abundance, indicating that the allometric release of N and P mediated by bacteria increases the risk of eutrophication. Redundancy analysis showed that organic matter and total N in sediment have positive effects on bacterial communities, which explains 21.8% and 23.7% variation in bacterial diversity, and explains 31.3% and 7.1% variation in bacterial metabolism. This also suggests that the accumulation of N in the sediment promotes the release of P and further aggravates the eutrophication of water. Therefore, simultaneous control of N and P is necessary to control water eutrophication in coastal river basins.