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Objective To assess contribution and seasonal variation in groundwater discharge (LGD) to lake water and nutrient budgets, Methods this study investigated Changhu Lake in the middle reaches of the Yangtze River. Field sampling was conducted during both the wet and dry seasons using multiple tracing techniques, including electrical conductivity (EC), stable isotope (2H and 18O), hydrochemical element (Ca2+ and Mg2+), and 222Rn isotope data. The 222Rn mass balance model was employed to quantify the LGD and associated nutrient fluxes in different seasons. Results Results show that LGD rates during the wet and dry seasons were 64.52 mm/d and 14.95 mm/d, respectively, with significant differences between these seasons. Furthermore, during the wet and dry seasons, groundwater carried TN inputs of approximately 25.68×106 g/d and 5.58×106 g/d, respectively. The TP inputs were approximately 8.14×106 g/d and 0.17×106 g/d in the wet and dry seasons, respectively. The differences in the LGD rates between the wet and dry seasons lead to differences in groundwater carrying TN and TP inputs, and inputs of TP during the wet season is also influenced by agricultural activities during that period. Sronger precipitation and evaporation during the wet season drive greater LGD intensity and their carrying TN and TP fluxes. Conclusion The research can provide scientific reference for water resource management and aquatic ecosystem preservation efforts in the Changhu area.

Abstract Global change drives multiple facets of biodiversity including interaction diversity, which is fundamental for ecosystem functioning. However, studying trophic interactions is challenging in meta‐ecosystems, that is ecosystems connected by spatial flows of energy, materials and organisms across ecosystem boundaries. While analytical methods based on abundances of polyunsaturated fatty acids (PUFAs) and stable isotopes of amino acids (AAs) are being increasingly used, it has never been explored if both approaches could be: (i) combined in mixing models to enhance precision in dietary inference (ii) compared to disentangle transfers of various PUFAs and proteins in food webs in the wild. We explore the utility of analytical approaches based on PUFA abundances and AA isotopes to resolve resource transfers in a natural riparian food web. We focus on spiders and their potential prey from blue, green and brown sources to address three important and persisting methodological issues in food‐web ecology, namely whether (i) essential AA carbon isotopes can resolve protein origin from blue, green and brown resources, (ii) PUFA relative abundance and AA isotopes can be combined in a mixing model to provide higher precision estimates (i.e. narrower intervals) and (iii) combining the two approaches can unveil the coupling of protein and PUFA transfers in food webs. Our research demonstrates the power of AA isotopes and PUFAs to distinguish blue, green, and brown sources and their transfer up to consumers. We show that combining PUFA relative abundance and AA isotopes in a mixing model provides overall estimates similar to the individual estimates but significantly increases precision. In addition, we showcase how combining approaches unveil the coupling of protein and PUFA transfers. For instance, we show that most PUFAs are less concentrated from prey to predators, relative to proteins, highlighting uncoupling of PUFAs and protein transfer along food chains. We show for the first time the effectiveness of combining AA isotopes and PUFA abundances, particularly relevant for complex trophic interactions in a meta‐ecosystem context. Our study illustrates the trophic uncoupling of proteins and PUFAs, highlighting the necessity in combining both approaches.

Global change drives multiple facets of biodiversity including interaction diversity, which is fundamental for ecosystem functioning. However, studying trophic interactions is challenging in meta‐ecosystems, that is ecosystems connected by spatial flows of energy, materials and organisms across ecosystem boundaries. While analytical methods based on abundances of polyunsaturated fatty acids (PUFAs) and stable isotopes of amino acids (AAs) are being increasingly used, it has never been explored if both approaches could be: (i) combined in mixing models to enhance precision in dietary inference (ii) compared to disentangle transfers of various PUFAs and proteins in food webs in the wild.We explore the utility of analytical approaches based on PUFA abundances and AA isotopes to resolve resource transfers in a natural riparian food web. We focus on spiders and their potential prey from blue, green and brown sources to address three important and persisting methodological issues in food‐web ecology, namely whether (i) essential AA carbon isotopes can resolve protein origin from blue, green and brown resources, (ii) PUFA relative abundance and AA isotopes can be combined in a mixing model to provide higher precision estimates (i.e. narrower intervals) and (iii) combining the two approaches can unveil the coupling of protein and PUFA transfers in food webs.Our research demonstrates the power of AA isotopes and PUFAs to distinguish blue, green, and brown sources and their transfer up to consumers. We show that combining PUFA relative abundance and AA isotopes in a mixing model provides overall estimates similar to the individual estimates but significantly increases precision. In addition, we showcase how combining approaches unveil the coupling of protein and PUFA transfers. For instance, we show that most PUFAs are less concentrated from prey to predators, relative to proteins, highlighting uncoupling of PUFAs and protein transfer along food chains.We show for the first time the effectiveness of combining AA isotopes and PUFA abundances, particularly relevant for complex trophic interactions in a meta‐ecosystem context. Our study illustrates the trophic uncoupling of proteins and PUFAs, highlighting the necessity in combining both approaches. [ABSTRACT FROM AUTHOR]

Aquaculture is increasing globally, providing protein to a growing population, but little is known regarding the nutrient budgets in aquaculture. To obtain insight into how management practices impact nutrient fluxes in freshwater aquaculture earthen ponds worldwide, we conducted a literature review. Our literature search yielded 23 papers in which nutrient budgets were reported. Our results showed that the main nutrient inputs are inlet water, feed, and fertilizers, but they varied according to location and management practices. Overall, feed and fertilizers constituted the predominant nutrient sources, accounting for up to 99% of the total inputs. The most quantified outputs were outlet water, fish, and sedimentation. Our findings indicate that only up to 20% of carbon, 45% of nitrogen, and 60% of phosphorus are assimilated by the fish. In some systems, up to 80% of carbon, 80% of nitrogen, and 60% of phosphorus accumulate in pond bottoms; in other systems, the outflow represents substantial losses of up to 16% of carbon, 76% of nitrogen, and 55% of phosphorus. More insight into nutrients' fate in fishponds is crucial from a sustainability perspective, as feed and fertilizer use can likely be optimized, reducing operational costs and the potential impact on the surrounding environment and atmospheric greenhouse gas concentrations. [ABSTRACT FROM AUTHOR]

This is a review of studies related with the effects generated by the fast-growing exotic plantations (PECR) of Pinus radiata and Eucalyptus spp. on water quality of watersheds in south-central Chile, at different spatial and temporal scales. The analyzed data comes from a region between 36° - 40° S in the Central Valley and includes both mountain ranges, the Coastal and the Andes. Studies on the watershed's capacity to export nutrients indicate that the conversion from native forests to forest plantations implies a net loss in nitrogen (N) and phosphorus (P) retention capacity. Consequently, watersheds with plantations coverage experience a net loss of these nutrients. Similarly, in watersheds dominated by Eucalyptus globulus, nitrogen retention decreases, especially NO3-N, and is partially reversed as a wider riparian vegetation is maintained. Studies of soil erosion and sediment transport losses show that clearcut harvesting has significant impacts on water quality, increasing sediment transport rates in streams. However, when soil conservation measures are practiced, erosion rates decrease significantly during forest harvesting. Some of the physical-chemical characteristics of the water, such as pH and temperature, are similar between streams with native forests and forest plantations. There is also a tendency for a decrease of richness and abundance of the macrobenthic invertebrates community due to the replacement of riparian vegetation by Eucalyptus spp. [ABSTRACT FROM AUTHOR]

Eutrophication and hypoxia represent an ever-growing stressor to estuaries and coastal ecosystems due to population growth and climate change. Understanding water quality dynamics in shallow water systems is particularly challenging due to the complex physical and biogeochemical dynamics and interactions among them. Within shallow waters, benthic microalgae can significantly contribute to autotrophic primary production, generate organic matter, increase dissolved oxygen consumption, and alter nutrient fluxes at the sediment-water interface, yet they have received little attention in modeling applications. A state-of-the-art modeling system, the Semi-Implicit Cross-Scale Hydroscience Integrated System Model (SCHISM), coupled with the Integrated Compartment Model (ICM) of water quality and benthic microalgae, has been implemented in the Corsica River estuary, a tributary to Chesapeake Bay, to study benthic microalgal impact on water quality in shallow water systems. The model simulation has revealed a broad impact of benthic microalgae, ranging from sediment-water interface fluxes to water column dynamics, and the effects are observed from near-field to far-field monitoring stations. High-frequency variability and non-linearity dominate benthic microalgal dynamics, sediment oxygen demand, and nutrient fluxes at the sediment-water interface. Resource competition and supply determine the spatial scope of benthic microalgal impacts on far-field stations and the whole estuary system. Our study shows that benthic microalgae are a significant factor in shallow water dynamics that needs adequate attention in future observation and modeling applications. [ABSTRACT FROM AUTHOR]

Soil nutrient distribution is heterogeneous in space and time, potentially altering nutrient acquisition by trees and microorganisms. Ecologists have distinguished "hot spots" (HSs) as areas with enhanced and sustained rates of nutrient fluxes relative to the surrounding soil matrix. We evaluated the spatial and temporal patterns in nutrient flux HSs in two mixed-conifer forest soils by repeatedly sampling the soil solution at the same spatial locations (horizontally and vertically) over multiple seasons and years using ion exchange resins incubated in situ. The climate of these forests is Mediterranean, with intense fall rains occurring following summers with little precipitation, and highly variable winter snowfall. Hot spots formed most often for NO3− and Na+. Although nutrient HSs often occurred in the same spatial location multiple times, HSs persisted more often for PO43− NH4+, and NO3−, and were more transient for Ca2+, Mg2+, and Na+. Sampling year (annual precipitation ranged from 558 to 1223 mm) impacted the occurrence of HSs for most nutrients, but season was only significant for PO43−, NH4+, NO3−, and Na+, with HSs forming more often after fall rains than after spring snowmelt. The frequency of HSs significantly decreased with soil depth for all nutrients, forming most commonly immediately below the surficial organic horizon. Although HSs accounted for less than 17% of the sampling volume, they were responsible for 56–88% of PO43−, NH4+, and NO3− resin fluxes. Our results suggest that macronutrient HSs have a disproportional contribution to soil biogeochemical structure, with implications for vegetation nutrient acquisition strategies and biogeochemical models. [ABSTRACT FROM AUTHOR]

Aquaculture is increasing globally, providing protein to a growing population, but little is known regarding the nutrient budgets in aquaculture. To obtain insight into how management practices impact nutrient fluxes in freshwater aquaculture earthen ponds worldwide, we conducted a literature review. Our literature search yielded 23 papers in which nutrient budgets were reported. Our results showed that the main nutrient inputs are inlet water, feed, and fertilizers, but they varied according to location and management practices. Overall, feed and fertilizers constituted the predominant nutrient sources, accounting for up to 99% of the total inputs. The most quantified outputs were outlet water, fish, and sedimentation. Our findings indicate that only up to 20% of carbon, 45% of nitrogen, and 60% of phosphorus are assimilated by the fish. In some systems, up to 80% of carbon, 80% of nitrogen, and 60% of phosphorus accumulate in pond bottoms; in other systems, the outflow represents substantial losses of up to 16% of carbon, 76% of nitrogen, and 55% of phosphorus. More insight into nutrients’ fate in fishponds is crucial from a sustainability perspective, as feed and fertilizer use can likely be optimized, reducing operational costs and the potential impact on the surrounding environment and atmospheric greenhouse gas concentrations.

Eutrophication and hypoxia represent an ever-growing stressor to estuaries and coastal ecosystems due to population growth and climate change. Understanding water quality dynamics in shallow water systems is particularly challenging due to the complex physical and biogeochemical dynamics and interactions among them. Within shallow waters, benthic microalgae can significantly contribute to autotrophic primary production, generate organic matter, increase dissolved oxygen consumption, and alter nutrient fluxes at the sediment–water interface, yet they have received little attention in modeling applications. A state-of-the-art modeling system, the Semi-Implicit Cross-Scale Hydroscience Integrated System Model (SCHISM), coupled with the Integrated Compartment Model (ICM) of water quality and benthic microalgae, has been implemented in the Corsica River estuary, a tributary to Chesapeake Bay, to study benthic microalgal impact on water quality in shallow water systems. The model simulation has revealed a broad impact of benthic microalgae, ranging from sediment–water interface fluxes to water column dynamics, and the effects are observed from near-field to far-field monitoring stations. High-frequency variability and non-linearity dominate benthic microalgal dynamics, sediment oxygen demand, and nutrient fluxes at the sediment–water interface. Resource competition and supply determine the spatial scope of benthic microalgal impacts on far-field stations and the whole estuary system. Our study shows that benthic microalgae are a significant factor in shallow water dynamics that needs adequate attention in future observation and modeling applications.

The Quaternary red soil widely distributed in China is an important arable land resource. A quantitative understanding of nutrient changes of Quaternary red soils under different land-use patterns is the necessary premise for effective regulation, management, and sustainable utilization. In this study, five typical Quaternary red soil profiles under different land-use patterns were taken as the research object in Chaoyang City, Liaoning Province, China. The results showed that: (1) Buried Quaternary red soils were minimally affected by external disturbances. The contents of nitrogen (around 0.02%), phosphorus (ranging from 0.06% to 0.07%), and potassium (ranging from 3.12% to 3.50%) were at relatively low levels and homogeneously distributed with depth. (2) The total nitrogen content of red soils under each land-use pattern showed an increasing trend in the upper part of the profile (A and B horizons), and a sequence of woodland (CL-04) > grassland (CL-03) > arable land (CL-05) = sparse forest–grassland (CL-02). The nitrogen content in the lower part of different land-use patterns was about 0.02%. The phosphorus content of the topsoil layer remained unchanged (ranging from 0.05% to 0.06%), while the subsoil phosphorus decreased to varying extents. The potassium experienced leaching in both topsoil and subsoil layers, with the topsoil losses being lower than that in the subsoil. The range of total potassium content in the grassland (CL-03) ranged from 2.64% to 4.21%, from 3.91% to 4.44% for sparse forest–grassland (CL-02), from 2.41% to 2.63% for woodland (CL-04), and 2.85% to 2.92% for arable land (CL-05), respectively. The variation in nutrient content was related to the vegetation type, coverage rate, artificial fertilization method and species, etc. The accumulative mass change in the sparse forest–grassland increased by 384.16 g/100 cm2, and the other land-use patterns showed a decreasing trend of arable land (83.71 g/100 cm2) > woodland (83.71 g/100 cm2) > grassland (83.71 g/100 cm2), with the topsoil leaching losses being smaller than those in the subsoil layer. The characteristics of windbreak, sand fixation, and soil and water conservation of the sparse forest–grassland could well hold the nutrient-rich loess sediments, resulting in increased nutrients in the Quaternary red soil, which is a reasonable land-use pattern for the Chaoyang area. [ABSTRACT FROM AUTHOR]

Coastal eutrophication is a global environmental concern, primarily driven by nutrient contamination from human and natural sources entering the sea via river mouths. The Bang Pakong River, situated on the eastern coast of Thailand's Upper Gulf, exemplifies this issue. Its watershed, largely dedicated to agriculture, has witnessed rapid urban and industrial growth, contributing to water eutrophication in the Upper Gulf. To address this, a study conducted in 2016 and 2017 analyzed the exchange of dissolved nutrients and total suspended solids at the Bang Pakong River mouth across different seasons. During the rainy season, the concentration of nutrients and suspended solids was high. Hypoxia in near-bottom water occurred. The net flux of these materials depended on seasonal variations in runoff, with higher amounts (71.2×106 m³·d-1) being washed into the sea during high runoff periods. In contrast, during the dry season in February, material accumulated in the river due to seawater intrusion, with net water flux (11.6×106 m³·d-1) reversing. Significantly, the study found a strong correlation between the net flux of studied materials and average monthly rainfall in the Bang Pakong Basin. This correlation can be valuable in predicting substance transport from the river to the Upper Gulf, aiding in the assessment of nutrient enrichment in the region. Addressing eutrophication is critical for preserving the ecological balance of coastal ecosystems and ensuring the sustainability of these vital environments. [ABSTRACT FROM AUTHOR]

The climate is changing. The average temperature in Wuhan, China, is forecast to increase by at least 4.5 °C over the next century. Shallow lakes are important components of the biosphere, but they are sensitive to climate change and nutrient pollution. We hypothesized that nutrient concentration is the key determinant of nutrient fluxes at the water-sediment interface, and that increased temperature increases nutrient movement to the water column because warming stimulates shifts in microbial composition and function. Here, twenty-four mesocosms, mimicking shallow lake ecosystems, were used to study the effects of warming by 4.5 °C above ambient temperature at two levels of nutrients relevant to current degrees of lake eutrophication levels. This study lasted for 7 months (April–October) under conditions of near-natural light. Intact sediments from two different trophic lakes (hypertrophic and mesotrophic) were used, separately. Environmental factors and bacterial community compositions of overlying water and sediment were measured at monthly intervals (including nutrient fluxes, chlorophyll a [chl a], water conductivity, pH, sediment characteristics, and sediment-water et al.). In low nutrient treatment, warming significantly increased chl a in the overlying waters and bottom water conductivity, it also drives a shift in microbial functional composition towards more conducive sediment carbon and nitrogen emissions. In addition, summer warming significantly accelerates the release of inorganic nutrients from the sediment, to which microorganisms make an important contribution. In high nutrient treatment, by contrast, the chl a was significantly decreased by warming, and the nutrient fluxes of sediment were significantly enhanced, warming had considerably smaller effects on benthic nutrient fluxes. Our results suggest that the process of eutrophication could be significantly accelerated in current projections of global warming, especially in shallow unstratified clear-water lakes dominated by macrophytes. [ABSTRACT FROM AUTHOR]

In this study, sediment diffusive and total nutrient fluxes were calculated for the Mersin Bay, Northeastern (NE) Mediterranean Sea. Positive fluxes of nutrients from the sediments indicated that sediment acts as a source for phosphate, nitrate, ammonium and reactive silicate in the NE Mediterranean Sea. Total nutrient fluxes from sediments were calculated as 9.1 mmol m–2 year–1 for phosphate, 278 mmol m–2 year–1 for nitrate, 32.5 mmol m–2 year–1 for ammonium and 315 mmol m–2 year–1 for reactive silicate, respectively. These nutrient fluxes from the sediment would further support primary production in the NE Mediterranean Sea displaying oligotrophic conditions in the offshore waters. [ABSTRACT FROM AUTHOR]

Understanding the behavior of past upwelling cells is paramount when assessing f 17 uture climate changes. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling cell is located along the Oman Margin in the Western Arabian Sea (WAS). Driven by cross-hemispheral winds, the WAS upwelling zone's intense productivity led to the formation of one of the most extensive oxygen minimum zones known today. In this study covering the Middle to Late Miocene at ODP Site 722, we investigate the inception of upwelling derived primary productivity. We combine novel data with existing model- and data-based evidence, constraining the tectonic and atmospheric boundary conditions for an upwelling cell to exist in the region. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the Ocean Drilling Project (ODP) LEG 117. We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ~14 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) ~13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established by ~12 Ma. Rare occurrences of diatoms frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6‰) and high organic carbon accumulation also confirm high productivity and beginning denitrification simultaneously. Our multi-proxy-based evaluation of Site 722B primary producers thus indicates a stepwise evolution of productivity in the western Arabian Sea related to the intensity of upwelling and forcing SAM dynamics throughout the Middle to Late Miocene. The absence of full correspondence with existing deep marine climate records also suggests that local processes, such as lateral nutrient transport, likely played an important role in modulating productivity in the western Arabian Sea. Finally, we show that using a multi-proxy record provides novel insights into how fossil plankton responded to changing nutrient conditions through time in a monsoon wind-driven upwelling zone. [ABSTRACT FROM AUTHOR]

Coastal wetlands are an important hotspot for nutrient cycling and transport from the land to the ocean. Silicon (Si) as a vital biogenic element affects plant growth and health of coastal ecosystems. The understanding of key factors and processes controlling dissolved silicate (DSi) exchange between the wetlands and coastal water has been limited due to the lack of measured data. We carried out intensive investigations of time-series DSi concentrations and porewater exchange across the Sediment-Water Interface (SWI) along a tidal creek with a mangrove-salt marsh gradient during neap and spring tides in 2020. Seasonal observations of surface water in a tidal creek and Zhangjiang Estuary (Fujian Province, China) were conducted from 2017 to 2020. The results showed that there was a net export of DSi from the mangroves to tidal creek with rates of 2.11 and 2.40 mmol m-2 d-1 in neap and spring tides respectively, suggesting the mangroves served as the source of DSi. However, the salt marshes had a net DSi import with one or two orders of magnitude lower than the export from the mangroves. DSi export across the wetland‐estuarine continuum was largely controlled by porewater exchange, groundwater geochemistry (pH, temperature) and plant root uptake. Groundwater in the mangroves has larger ratios of DSi : DIN (dissolved inorganic nitrogen) (2.5 ± 0.6) and DSi : DRP (dissolved reactive phosphorus) (1257 ± 35) compared with surface water. The net export of DSi from mangroves would modify the nutrient stoichiometry and mitigate the effects of reduced river DSi flux caused by damming on coastal ecosystem. This study provides new insights into the wetland Si cycling for sustaining coastal ecosystem health.

High-intensity hurricane disturbances have severe consequences on forest structure and functioning. Through wind force and heavy rainfall, they cause extensive canopy removal and an input of fine litter and woody debris well above normal levels. We examined litterfall N and P concentrations and fluxes before and after Hurricane Patricia (category 4) landfall in October 2015 in the seasonally dry tropical forest of the Chamela region, Jalisco, Mexico. Additionally, we compared the forest response to Patricia with those to Hurricane Jova (2011), from the Eastern North Pacific basin and to Hurricane Dean (2007) from the Atlantic basin. Nutrient concentrations in hurricane-induced litterfall in October 2015 were 2.2 times higher in leaf litter than in the woody fraction. Both litterfall N and P concentrations during the period November–February following the hurricane were generally higher than in similar periods in years previous to and after Patricia. Nutrient fluxes in October 2015 (75.9 kgN/ha and 3.6 kgP/ha) represented 55% (N) and 52% (P) of the total fluxes that year, which were much higher than those in any of the three years following the hurricane. These results suggest that forest biogeochemical resilience has changed in the short term. The annual litterfall N and P fluxes during the year of Hurricane Dean were lower than in 2011 and 2015, but similar to non-hurricane years. After Patricia, the annual N flux was higher, but the annual P flux lower than after Jova, and the former represents the largest annual N flux in our more than 25 years record. [ABSTRACT FROM AUTHOR]

In recent decades, global crop production is being threatened by contamination of arable lands with vanadium (V). Among many stress-relief substances, melatonin (ME) is a widely studied biomolecule acting as an antioxidant under stress conditions. The current study was aimed to investigate the response of tomato seedlings towards vanadium stress, along with the circumventing role of ME by promoting V stress tolerance in tomato seedlings. Our results revealed that accentuated inhibition of growth and biomass were caused by V (40 mg/L) stress, mainly due to impairments of photosynthetic systems, root traits, and mineral homeostasis. Conversely, notable reinforcement of plant growth parameters was seen with ME (100 µM) application, with improved chlorophyll content, root morphology, mineral nutrient homeostasis, and gas exchange parameters, along with reduced V accumulation. Further, ME efficiently triggered the antioxidant enzymes activities, by restoring cellular integrity [reduced electrolyte leakage (EL) and malondialdehyde] and restricted production of superoxide (O2•−) and hydrogen peroxide (H2O2) radicals, mainly through regulation of antioxidant enzymes. The present study highlighted the potential role of ME in tomato, for circumventing V-induced phytotoxicity, mainly by boosting photosynthesis, biomass production, redox balance, nutrient uptake, and root traits. In conclusion, ME application restricted the V availability in plant, improved plant growth and, thus, provided an improved V stress tolerance. [ABSTRACT FROM AUTHOR]

The Quaternary red soil widely distributed in China is an important arable land resource. A quantitative understanding of nutrient changes of Quaternary red soils under different land-use patterns is the necessary premise for effective regulation, management, and sustainable utilization. In this study, five typical Quaternary red soil profiles under different land-use patterns were taken as the research object in Chaoyang City, Liaoning Province, China. The results showed that: (1) Buried Quaternary red soils were minimally affected by external disturbances. The contents of nitrogen (around 0.02%), phosphorus (ranging from 0.06% to 0.07%), and potassium (ranging from 3.12% to 3.50%) were at relatively low levels and homogeneously distributed with depth. (2) The total nitrogen content of red soils under each land-use pattern showed an increasing trend in the upper part of the profile (A and B horizons), and a sequence of woodland (CL-04) > grassland (CL-03) > arable land (CL-05) = sparse forest–grassland (CL-02). The nitrogen content in the lower part of different land-use patterns was about 0.02%. The phosphorus content of the topsoil layer remained unchanged (ranging from 0.05% to 0.06%), while the subsoil phosphorus decreased to varying extents. The potassium experienced leaching in both topsoil and subsoil layers, with the topsoil losses being lower than that in the subsoil. The range of total potassium content in the grassland (CL-03) ranged from 2.64% to 4.21%, from 3.91% to 4.44% for sparse forest–grassland (CL-02), from 2.41% to 2.63% for woodland (CL-04), and 2.85% to 2.92% for arable land (CL-05), respectively. The variation in nutrient content was related to the vegetation type, coverage rate, artificial fertilization method and species, etc. The accumulative mass change in the sparse forest–grassland increased by 384.16 g/100 cm2, and the other land-use patterns showed a decreasing trend of arable land (83.71 g/100 cm2) > woodland (83.71 g/100 cm2) > grassland (83.71 g/100 cm2), with the topsoil leaching losses being smaller than those in the subsoil layer. The characteristics of windbreak, sand fixation, and soil and water conservation of the sparse forest–grassland could well hold the nutrient-rich loess sediments, resulting in increased nutrients in the Quaternary red soil, which is a reasonable land-use pattern for the Chaoyang area.

Mixed layer eddies resulting from baroclinic instability of fronts convert horizontal buoyancy gradients into vertical stratification, shoaling the mixed layer. In light-limited regimes – high-latitudes – this process can initiate phytoplankton blooms prior to the springtime warming. The question is whether mixed layer eddies can enhance the spring bloom by delivering nutrients from beneath the mixed layer. We couple a submesoscale-resolving model (SUB) with a simple ecosystem model and examine the role of mixed layer eddies on the development of the spring bloom. We compare the SUB simulation to two coarser resolution (10 km) simulations, one that includes a mixed layer eddy parameterization (MLE) and another that prescribes the restratification from SUB and advects the biogeochemical tracers using geostrophic velocities (NVF). The MLE simulates restratification of the mixed layer and bloom onset, but the spring bloom has a deficit of 10–13% in the new production compared to SUB. The NVF has the same restratification as SUB, and with no vertical flux of nutrients, leads to a spring bloom with a 32–40% new production deficit compared to SUB. Submesoscale processes lead to exchange across the mixed layer base, which is not represented in coarse resolution model simulations, even with mixed layer eddy parameterizations. Our results show that nutrients supplied by mixed layer eddies are important to enhance the spring bloom.

Diffusion flux is an essential tool to estimate the contribution of internal nitrogen and phosphorus in eutrophic lakes. There are mainly two methods, i.e., model calculation based on in-situ porewater sampling and water quality monitoring in laboratory incubation. The results obtained by the two methods are rarely compared, decreasing the validity of internal contribution and following management strategies. In this study, sediment samples were collected from a lake in China, then the fluxes were estimated by model calculation and laboratory incubation. The results show that there is an order of magnitude difference in the fluxes measured by these two methods. The mean values of ammonia (NH4+-N) and soluble reactive phosphate (SRP) obtained from the model calculations were 24.4 and 1.30, respectively. The mean values of NH4+-N and SRP obtained in the undisturbed group of sediment incubation were 7.84 and 5.47, respectively, and in the disturbed group of sediment incubation were 16.2 and 4.06, respectively. Sediment incubation is a combination of multiple influencing factors to obtain fluxes, while porewater model is based on molecular diffusion as the theoretical basis for obtaining fluxes. According to the different approaches of the two methods, sediment incubation is recommended as a research tool in lake autochthonous release management when the main objective is to remove pollution, while the porewater model is recommended as a research tool when the main objective is to control pollution. When assessing the diffusive flux of nitrogen, it is recommended to choose the stable form of total dissolved nitrogen to discuss the flux results. [ABSTRACT FROM AUTHOR]

Seagrasses are gaining attention thanks to their metabolism and potential major role as carbon sinks, with further implications as nature-based solutions against climate change. Despite their recognized importance and the growing number of studies published, there is still a striking paucity of information on seagrass metabolism and contribution to biogeochemical cycles for some seagrass species and ocean areas. In this study we assessed the metabolic balance and nutrient cycling contribution of seagrasses to the benthic compartment of a tropical reef lagoon in Reunion Island, providing original information on a barely studied seagrass species (Syringodium isoetifolium) and a poorly studied ocean region (West Indian Ocean). We measured the net productivity, respiration and the metabolic balance in different components of the lagoon benthic compartment (i.e. seagrass, sediment, and benthic community) and the water-sediment nutrient benthic fluxes at differently impacted sites within the lagoon. The biogeochemical environmental variability, including inorganic and organic indicators of anthropogenic contamination, was also assessed at each site.Large spatial variability was detected in the metabolic balance of each benthic component assessed, also associated with the natural and/or anthropic-driven environmental variability found in the lagoon. The seagrass S. isoetifolium was net autotrophic across the lagoon and contributed to the lagoon benthic metabolism with net plant productivity exceeding by one order of magnitude the plant respiration. The lowest seagrass metabolism was detected at the impacted site. The metabolic balance of the sediment was heterotrophic but the high productivity of S. isoetifolium contributed to reducing the heterotrophy of the whole benthic community. The lagoon-wide benthic metabolic balance was slightly heterotrophic, but the associated uncertainty ranged from autotrophy to heterotrophy. Nutrient concentrations in the lagoon were low and the benthic community capacity for nutrient retention (uptake) and removal (denitrification and anammox) indicated potential for buffering moderate nutrient inputs into the lagoon. Organic contaminants of emerging concern (CECs) were low but detectable in the lagoon, especially in highly frequented beach areas, arising as an environmental quality indicator of interest.

The climate is changing. The average temperature in Wuhan, China, is forecast to increase by at least 4.5 °C over the next century. Shallow lakes are important components of the biosphere, but they are sensitive to climate change and nutrient pollution. We hypothesized that nutrient concentration is the key determinant of nutrient fluxes at the water-sediment interface, and that increased temperature increases nutrient movement to the water column because warming stimulates shifts in microbial composition and function. Here, twenty-four mesocosms, mimicking shallow lake ecosystems, were used to study the effects of warming by 4.5 °C above ambient temperature at two levels of nutrients relevant to current degrees of lake eutrophication levels. This study lasted for 7 months (April–October) under conditions of near-natural light. Intact sediments from two different trophic lakes (hypertrophic and mesotrophic) were used, separately. Environmental factors and bacterial community compositions of overlying water and sediment were measured at monthly intervals (including nutrient fluxes, chlorophyll a [chl a], water conductivity, pH, sediment characteristics, and sediment-water et al.). In low nutrient treatment, warming significantly increased chl a in the overlying waters and bottom water conductivity, it also drives a shift in microbial functional composition towards more conducive sediment carbon and nitrogen emissions. In addition, summer warming significantly accelerates the release of inorganic nutrients from the sediment, to which microorganisms make an important contribution. In high nutrient treatment, by contrast, the chl a was significantly decreased by warming, and the nutrient fluxes of sediment were significantly enhanced, warming had considerably smaller effects on benthic nutrient fluxes. Our results suggest that the process of eutrophication could be significantly accelerated in current projections of global warming, especially in shallow unstratified clear-water lakes dominated by macrophytes.

Monitoring in remote areas can represent a real challenge in environmental studies. Numerous techniques have been developed over the last decades to monitor nutrients and other elements in different systems. However, not all of them are suitable for field applications, particularly when the locations are difficult to access or its accessibility depends on seasonal climate conditions. This study was aimed to test the applicability and efficiency of resin samplers and resin bags to monitor nitrates fluxes (NO3-N) in two small semi-arid catchments in Northwestern Mexico. Resin samplers were installed in the hyporheic zone below the river bed in order to monitor the vertical fluxes of NO3-N and remained there for 5 months (during the summer rains). Resin bags were anchored in rock outcrops upstream of the resin samplers before the onset of the summer rainfall season and replaced every 2 weeks during 4 months to capture pulses of NO3-N in ephemeral streams. NO3-N pulses in the stream are a potential source of NO3-N that can infiltrate into the soil. Results of the resin samplers found a difference of up to 12 kg ha−1 season−1 between the two catchments. The resin bags showed a higher accumulation of NO3-N in the catchment with lower vegetation cover (160.3 mg L−1 season−1) compared to the one with higher vegetation (67.8 mg L−1 season−1). Measured nitrate fluxes at both sites responded to rainfall pulses recorded during the monitoring period. Resin samplers and resin bags can be used together, to assess nutrient fluxes on the surface and in the soil and can be tested in any type of ecosystem. In this particular case, these methods demonstrated an efficient way of determining spatio-temporal nitrate fluxes in semi-arid ecosystems in remote areas that are difficult to access, monitor, and collect data. [ABSTRACT FROM AUTHOR]

Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan‐Arctic vertical nitrate supply is comparable with the basin‐wide transport, and is predicted to increase with declining sea ice cover in the future. Plain Language Summary: Microscopic algae, growing in the sunlit surface layer of the ocean, provide food for other species and form the basis of the ecosystem. In the Arctic Ocean, their growth is limited by the availability of nutrients. The main source of these nutrients are waters entering from the Atlantic and Pacific Oceans. These nutrient‐rich waters reside far below the sunlit zone, and vertical mixing is required to bring them upwards to support algal growth. With rapidly declining summer sea ice and changes in the ocean layering, these mixing processes might substantially change. Changes are considered most likely in the region of the steep slopes in the Siberian Seas. To investigate this, we analyze nutrient and mixing measurements in this region from 2007, 2008, and 2018. In 2018, we observed strong mixing, which is connected to ice free conditions and a process that has only recently been described. This strong mixing only happens at the narrow, steep slope region, but might supply the same amount of nutrients to the surface zone as the weak mixing over the much larger area of the deep basins. Key Points: Nutrient fluxes are higher in the western compared with the eastern Laptev SeaEpisodic mixing above the slope might significantly contribute to Pan‐Arctic vertical nitrate supplyIn the central Laptev Sea, a thinning halocline and shoaling Atlantic Water indicate an emerging regime shift in line with Atlantification [ABSTRACT FROM AUTHOR]

Nutrients play an important role as biogenic elements in modulating marine productivity, and water mixing usually facilitates the transportation of nutrients in the coastal ocean. In this study, the distributions of naturally occurring radioisotopes 226Ra and 228Ra in the surface and water column of the northern South China Sea (NSCS) have been investigated to estimate oceanic mixing and nutrient supplies. We identified three masses of the South China Sea Warm Current (SCSWC), the South China Sea Branch of the Kuroshio (SCSBK), and shelf water in the summer of June 2015, but only SCSWC and SCSBK were observed in the spring of March 2017. The fraction of the SCSBK in summer was estimated to be an average of 0.25 ± 0.16, which was lower than that in the spring of 0.57 ± 0.32 in our study area. The horizontal mixing from the Pearl River plume revealed eddy diffusion of (1.2 ± 0.79) × 105 cm2/s and advection velocity ω of 0.25 ± 0.16 cm/s in the slope region. In the water column, the best-fit exponential curve gradient of 228Ra led to a vertical diffusion coefficient of 0.43 ± 0.33 cm2/s that went down to the subsurface of the upper 1,000 m, and an upward vertical diffusion coefficient was revealed as 18 ± 9.9 cm2/s from the near-bottom. Combining the nutrient distributions, horizontal mixing from the Pearl River plume carried (5.6 ± 4.9) × 102 mmol N/m2/d, 2.2 ± 2.0 mmol P/m2/d, and (4.1 ± 3.9) × 102 mmol Si/m2/d in the very surface layer, suggesting that shelf water plays a significant role in the nutrient sources of the slope of the NSCS during June 2015. The upward vertical mixing supplied 2.7 ± 1.6 mmol N/m2/d, 0.18 ± 0.11 mmol P/m2/d, and 15 ± 8.4 mmol Si/m2/d to the upper layer, which appeared more important than atmospheric deposition and rivaled submarine groundwater discharge.

Recent efforts to quantify biogeochemical and ecological processes in oyster habitats have focused on provision of habitat and regulation of the nitrogen cycle. However, it is unclear how these two processes may interact. In this study, seasonal patterns of habitat use and nitrogen removal from natural oyster beds were quantified for comparison with nearby bare sediment in Green Hill Pond, a temperate coastal lagoon in RI, USA. Relationships were tested between benthic macrofaunal abundance and nitrogen removal via denitrification and burial in sediments. Nitrogen removal by oyster bio-assimilation was quantified and compared with nearby oyster aquaculture. Despite limited differences in habitat use by macrofauna, there were fewer non-oyster benthic organisms (e.g., filter-feeders, detritivores) where oysters were present, possibly due to competition for resources. Additionally, low rugosity of the native oyster beds provided little refuge value for prey. There was a shift from net N removal via denitrification in bare sediments to nitrogen fixation beneath oysters, though this change was not statistically significant (t(96) = 1.201; p = 0.233). Sediments contained low concentrations of N; however sediments beneath oysters contained almost twice as much N (0.07%) as bare sediments (0.04%; p < 0.001). There was no difference in tissue N content between wild oysters and those raised in aquaculture nearby, though caged oysters had more tissue per shell mass and length, and therefore removed more N on a shell length basis. These oyster beds lacked the complex structure of three-dimensional oyster reefs which may have diminished their ability to provide habitat for refugia, foraging sites for macrofauna, and conditions known to stimulate denitrification. [ABSTRACT FROM AUTHOR]

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. • The CNBG behaved as a sink of dissolved inorganic nitrogen, phosphorus and silicon. • Riverine inputs was the dominant nutrient source, water exchange flows was the main nutrient output. • Nutrient consumption dominated in summer, nutrient regeneration dominated in winter. • Microbial nitrogen removal exceeded nitrogen fixation in the CNBG. [ABSTRACT FROM AUTHOR]

Sedimentary processes are expected to play a crucial role in macronutrient cycling of the shallow Arabian Gulf. To investigate this aspect, sediment cores were collected from the shallow intertidal and subtidal expanses of the first Marine Protected Area (MPA) of Kuwait in the Northwestern Arabian Gulf (NAG). Porewater nutrient profiling and whole core incubation experiments were conducted to measure the nutrient fluxes, both with and without the addition of the nitrification inhibitor allylthiourea (ATU). The porewater data confirmed the potential of sediments to host multiple aerobic and anaerobic pathways of nutrient regeneration. The average (±SD) of net nutrient fluxes from several incubation experiments indicated that ammonium (NH 4 +) predominantly fluxed out of the sediment (3.81 ± 2.53 mmol m−2 d−1), followed by SiO 4 4− (3.07 ± 1.21 mmol m−2 d−1). In contrast, the average PO 4 3− flux was minimal, at only 0.06 ± 0.05 mmol m−2 d−1. Fluxes of NO 3 − (ranged from 0.07 ± 0.005 to 1.16 ± 0.35 mmol m−2 d−1) and NO 2 − (0.03 ± 0.003 to 0.71 ± 0.21 mmol m−2 d−1) were moderate, which either reduced or reversed in the presence of ATU (−0.001 ± 0.0001 to 0.01 ± 0.0001 mmol m−2 d−1 and -0.001 ± 0.0003 to 0.006 ± 0.001 mmol m−2 d−1 for NO 3 − and NO 2 − respectively). Thus, this study provides preliminary experimental evidence that nitrification can act as a source of NO 3 − and NO 2 − as well as contribute towards the relatively high concentrations of NO 2 − (>1 in the gulf waters. [ABSTRACT FROM AUTHOR]

Diffusion flux is an essential tool to estimate the contribution of internal nitrogen and phosphorus in eutrophic lakes. There are mainly two methods, i.e., model calculation based on in-situ porewater sampling and water quality monitoring in laboratory incubation. The results obtained by the two methods are rarely compared, decreasing the validity of internal contribution and following management strategies. In this study, sediment samples were collected from a lake in China, then the fluxes were estimated by model calculation and laboratory incubation. The results show that there is an order of magnitude difference in the fluxes measured by these two methods. The mean values of ammonia (NH4+-N) and soluble reactive phosphate (SRP) obtained from the model calculations were 24.4 and 1.30, respectively. The mean values of NH4+-N and SRP obtained in the undisturbed group of sediment incubation were 7.84 and 5.47, respectively, and in the disturbed group of sediment incubation were 16.2 and 4.06, respectively. Sediment incubation is a combination of multiple influencing factors to obtain fluxes, while porewater model is based on molecular diffusion as the theoretical basis for obtaining fluxes. According to the different approaches of the two methods, sediment incubation is recommended as a research tool in lake autochthonous release management when the main objective is to remove pollution, while the porewater model is recommended as a research tool when the main objective is to control pollution. When assessing the diffusive flux of nitrogen, it is recommended to choose the stable form of total dissolved nitrogen to discuss the flux results.

The variability of rainfall‐dependent streamflow at catchment scale modulates many ecosystem processes in wet temperate forests. Runoff in small mountain catchments is characterized by a quick response to rainfall pulses which affects biogeochemical fluxes to all downstream systems. In wet‐temperate climates, water erosion is the most important natural factor driving downstream soil and nutrient losses from upland ecosystems. Most hydrochemical studies have focused on water flux measurements at hourly scales, along with weekly or monthly samples for water chemistry. Here, we assessed how water and element flows from broad‐leaved, evergreen forested catchments in southwestern South America, are influenced by different successional stages, quantifying runoff, sediment transport and nutrient fluxes during hourly rainfall events of different intensities. Hydrograph comparisons among different successional stages indicated that forested catchments differed in their responses to high intensity rainfall, with greater runoff in areas covered by secondary forests (SF), compared to old‐growth forest cover (OG) and dense scrub vegetation (CH). Further, throughfall water was greatly nutrient enriched for all forest types. Suspended sediment loads varied between successional stages. SF catchments exported 455 kg of sediments per ha, followed by OG with 91 kg/ha and CH with 14 kg/ha, corresponding to 11 rainfall events measured from December 2013 to April 2014. Total nitrogen (TN) and phosphorus (TP) concentrations in stream water also varied with rainfall intensity. In seven rainfall events sampled during the study period, CH catchments exported less nutrients (46 kg/ha TN and 7 kg/ha TP) than SF catchments (718 kg/ha TN and 107 kg/ha TP), while OG catchments exported intermediate sediment loads (201 kg/ha TN and 23 kg/ha TP). Further, we found significant effects of successional stage attributes (vegetation structure and soil physical properties) and catchment morphometry on runoff and sediment concentrations, and greater nutrients retention in OG and CH catchments. We conclude that in these southern hemisphere, broad‐leaved evergreen temperate forests, hydrological processes are driven by multiple interacting phenomena, including climate, vegetation, soils, topography, and disturbance history. [ABSTRACT FROM AUTHOR]

Coastal marine soft-sediment ecosystems profoundly influence nutrient cycling, carbon flows and primary productivity; both the resident animal communities and environmental characteristics can drive the direction and magnitude of fluxes across the sediment–water interface. Coastal and estuarine sedimentary ecosystems are spatially heterogeneous, and many are degrading due to environmental change. How ecosystem function is affected by spatial variation in both habitats and associated animals is critical for understanding the role of specific habitats in ecosystem function and defining the consequence of the functional extinction of species and changes in habitat spatial distributions. In a multi-site field experiment, we tested the effects of spatial variation in benthic communities and environmental characteristics along natural gradients in sediment grain size. We experimentally simulated the depletion of large benthic macrofauna to assess the impact of these functionally important taxa on key ecosystem functions through ammonium and oxygen solute fluxes across the sediment–water interface. The magnitude of change in solute fluxes varied across sites and depletion treatments and was explained by changes in both sediment physico-chemical characteristics and the functional attributes of the benthic community. Quantifying variation in fluxes across multiple habitats provides insight into the trajectory of change and the consequences for ecosystem function at broad spatial scales. [ABSTRACT FROM AUTHOR]

This study examined the ability of a Mediterranean demosponge Chondrosia reniformis to oxidize exogenous ammonium, simulating N-rich conditions that occur near finfish farms. We hypothesized that as the concentration of ammonium increases in the surrounding seawater, nitrification mediated by microbes associated with C. reniformis will lead to enhancement of ammonium uptake, nitrate excretion and oxygen consumption by the sponge holobiont. To test this hypothesis, we conducted laboratory experiments with C. reniformis explants exposed to ammonium enrichments (300–6667 nM) and to ambient seawater (45–1511 nM ammonium). We analyzed inhaled (IN) and exhaled (EX) water samples for dissolved oxygen, ammonium, nitrates and retention of picoplankton cells. We observed ammonium uptake in nearly half the cases and excretion of nitrate in most experimental outcomes. Yet, the consumption of ammonium and oxygen, as well as the excretion of nitrate by C. reniformis were not related to the concentration of inhaled ammonium, which suggests that the nitrification activity of sponge-associated microbes is not necessarily related to the concentration of ammonium in the surrounding seawater. Further research is required to reveal the sources of nitrate released from sponges and the fate of this nitrate in natural and manipulated ecosystems.

Small mountainous coastal basins display steeper gradients, suggesting intensified river transport. However, there is little information about downstream trends in rivers with low discharge and material fluxes across coastal plains and its influence on land‐sea material transport. This study examined 2 years of river discharge, suspended sediment and nutrient concentrations, fluxes and yields from upstream to the upper estuary of a typical South‐east Atlantic Basin. This study provides data about the material retention capacity of the coastal plain. The results indicated that the coastal plain did not affect nutrient concentrations, but reduced turbidity, inducing chlorophyll a and consequently primary productivity in the lower river basin and upper estuary. The coastal plain attenuated the increasing downstream material fluxes, but also curbed suspended sediment and TN fluxes, across the lower river/upper estuary. In contrast, TP and PO43− fluxes increased sharply across the coastal plain. This was influenced by natural and anthropogenic P inputs from soils and run‐off to the river channel. These exceeded the retention capacity of the coastal plain and result in a high export efficiency of this nutrient to the sea. [ABSTRACT FROM AUTHOR]

In this project, scientists describe the processes by which a subsurface nearshore algal bloom is carried to the surface, swept closer to the shore and ultimately blocked from entering the surf zone at Huntington Beach in Orange County, California.

The relationships between infaunal diversity and ecosystem function of biogenic structures in the Eastern Canadian Arctic remain poorly documented. Our study investigated the influence of sponge gardens at the Frobisher Bay site (137 m) and bamboo corals at the Baffin Bay site (1007 m) on the infaunal community structure and benthic ecosystem functioning. The occurrence of both types of biogenic structure type enhanced particular taxa and/or feeding guilds. A large density of suspension filter feeders was observed in bamboo coral sediment, whereas bare sediment exhibited a large proportion of nematodes and deposit-detritus feeders. Sponge gardens’ sediment showed a high proportion of isopods, Paraonidae polychaetes and up/down conveyors whereas bare sediment exhibited a large density of filter feeders. Through incubation cores, we measured ex situ benthic nutrient and oxygen fluxes at the sediment-water interface in each habitat and site. Biogeochemical fluxes varied significantly between habitats in the Baffin Bay site with a significant impact of bamboo coral habitat on nutrient fluxes (nitrate, ammonium, and silicate). Surprisingly, the sediment hosting bamboo corals acted as a source of nitrate and ammonium reaching values similar or higher to the Frobisher site despite the difference in water depth, and thus food supply between the two sites. These significant releases could derive from (i) a high organic matter deposition in bamboo coral habitat, allowed by their erected structure, (ii) a high efficiency of bioturbators (surficial modifiers and burrowers) mixing the surface layer of the sediment, and (iii) the difference in sediment type. Our study highlighted that, compared to its adjacent habitat, the presence of bamboo corals appeared to enhance the infaunal density and nutrient release of its sediment. In contrast, the impact of sponge gardens was not as clear as for bamboo coral habitat, likely due to the relatively significant presence of megabiota in the sponge garden adjacent habitat. Thus, our results based on a relatively small sample size, indicate that the bamboo coral habitat seems to increase the efficiency of deep-benthic ecosystem functioning, while that of sponge garden on the shallow ecosystem functioning remains uncertain.

Determination of submarine groundwater discharge (SGD) from karstic coastal aquifers is important to constrain hydrological and biogeochemical cycles. However, SGD quantification using commonly employed geochemical methods can be difficult to constrain under the presence of large riverine inputs, and is further complicated by the determination of the karstic groundwater endmember. Here, we investigated a coastal region where groundwater discharges from a karstic aquifer system using airborne thermal infrared mapping and geochemical sampling conducted along offshore transects. We report radium data (223Ra, 224Ra, 228Ra) that we used to derive fluxes (water, nutrients) associated with terrestrial groundwater discharge and/or seawater circulation through the nearshore permeable sediments and coastal aquifer. Field work was conducted at different periods of the year to study the temporal variability of the chemical fluxes. Offshore transects of 223Ra and 224Ra were used to derive horizontal eddy diffusivity coefficients that were subsequently combined with surface water nutrient gradients (NO2− + NO3−, DSi) to determine the net nutrient fluxes from SGD. The estimated DSi and (NO2− + NO3−) fluxes are 6.2 ± 5.0 *103 and 4.0 ± 2.0 *103 mol d−1 per km of coastline, respectively. We attempted to further constrain these SGD fluxes by combining horizontal eddy diffusivity and 228Ra gradients. However, SGD endmember selection in this area (terrestrial groundwater discharge vs. porewater) adds further uncertainty to the flux calculation and thus prevented us from calculating a reliable flux using this latter method. Additionally, the relatively long half-life of 228Ra (5.75 y) makes it sensitive to specific circulation patterns in this coastal region, including sporadic intrusions of Rhône river waters that impact both the 228Ra and nutrient surface water distributions. We show that SGD nutrient fluxes locally reach up to 20 times the nutrient fluxes from a small river (Huveaune River). On a regional scale, DSi fluxes driven by SGD vary between 0.1 and 1.4% of the DSi inputs of the Rhône River, while the (NO2− + NO3−) fluxes driven by SGD on this 22 km long coastline are between 0.1 and 0.3% of the Rhône River inputs, the largest river that discharges into the Mediterranean Sea. Interestingly, the nutrient fluxes reported here are similar in magnitude compared with the fluxes quantified along the sandy beach of La Franqui, in the western Gulf of Lions (Tamborski et al., 2018), despite the different lithology of the two areas (karst systems vs. unconsolidated sediment).

Tagoro, the shallow submarine volcano that erupted south of El Hierro (Canary Islands, Spain) in October 2011, has been intensely monitored for over 7 years, from the early eruptive stage to the current degassing stage characterized by moderate hydrothermal activity. Here, we present a detailed study of the emissions of inorganic macronutrients (NO2– + NO3–, PO4, and Si(OH)4) comprising a dataset of over 3300 samples collected through three different sampling methodologies. Our results show a significant nutrient enrichment throughout the whole studied period, up to 8.8-fold (nitrate), 4.0-fold (phosphate), and 16.3-fold (silicate) in the water column, and larger enrichments of phosphate (10.5-fold) and silicate (325.4-fold), but not of nitrate, in the samples collected directly from the vents. We also provide some preliminary results showing ammonium (NH4+) concentrations up to 1.97 μM in the vent fluids as compared to 0.02 μM in the surrounding waters. Nutrient fluxes from the volcano during the degassing stage were estimated as 3.19 ± 1.17 mol m–2 year–1 (NO2– + NO3–), 0.02 ± 0.01 mol m–2 year–1 (PO4), and 0.60 ± 1.35 mol m–2 year–1 (Si(OH)4), comparable to other important nutrient sources in the region such as fluxes from the NW-African upwelling. Nutrient ratios were affected, with a minimum (NO3– + NO2–):PO4 ratio of 2.36:1; moreover, a linear correlation between silicate and temperature enabled the use of this nutrient as a mixing tracer. This study sheds light on how shallow hydrothermal systems impact the nutrient-poor upper waters of the ocean.

Measurement of the spatial and temporal scales of nearshore chlorophyll-a (chl) is critical to our ability to understand or predict dense, and potentially harmful blooms of phytoplankton along the California Coast. Patchiness in chl may result from patchiness in nutrient availability, swimming or sinking, predation, or transport and mixing by breaking waves, currents, internal waves and tides. We performed a study during the Fall 2006 Huntington Beach (HB06) experiment to investigate the biological and physical factors that affect phytoplankton patchiness. Autotrophic phytoplankton require light and nutrients to survive. However, the nearshore euphotic zone (the critical depth for photosynthesis) is often nutrient-depleted due to uptake and planktonic growth. In order to sustain populations, phytoplankton rely on delivery of new nutrients from below the euphotic zone through upwellling or vertical mixing. or 2) terrestrial sources (from rain, ground water, tidal flushing or river outflow) that feed directly into the nearshore.

The iconic picture of Arctic marine ecosystems shows an intense pulse of biological productivity around the spring bloom that is sustained while fresh organic matter (OM) is available, after which ecosystem activity declines to basal levels in autumn and winter. We investigated seasonality in benthic biogeochemical cycling at three stations in a high Arctic fjord that has recently lost much of its seasonal ice-cover. Unlike observations from other Arctic locations, we find little seasonality in sediment community respiration and bioturbation rates, although different sediment reworking modes varied through the year. Nutrient fluxes did vary, suggesting that, although OM was processed at similar rates, seasonality in its quality led to spring/summer peaks in inorganic nitrogen and silicate fluxes. These patterns correspond to published information on seasonality in vertical flux at the stations. Largely ice-free Kongsfjorden has a considerable detrital pool in soft sediments which sustain benthic communities over the year. Sources of this include macroalgae and terrestrial runoff. Climate change leading to less ice cover, higher light availability and expanded benthic habitat may lead to more detrital carbon in the system, dampening the quantitative importance of seasonal pulses of phytodetritus to seafloor communities in some areas of the Arctic. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'. [ABSTRACT FROM AUTHOR]

The Barents Sea is experiencing long-term climate-driven changes, e.g. modification in oceanographic conditions and extensive sea ice loss, which can lead to large, yet unquantified disruptions to ecosystem functioning. This key region hosts a large fraction of Arctic primary productivity. However, processes governing benthic and pelagic coupling are not mechanistically understood, limiting our ability to predict the impacts of future perturbations. We combine field observations with a reaction-transport model approach to quantify organic matter (OM) processing and disentangle its drivers. Sedimentary OM reactivity patterns show no gradients relative to sea ice extent, being mostly driven by seafloor spatial heterogeneity. Burial of high reactivity, marine-derived OM is evident at sites influenced by Atlantic Water (AW), whereas low reactivity material is linked to terrestrial inputs on the central shelf. Degradation rates are mainly driven by aerobic respiration (40–75%), being greater at sites where highly reactive material is buried. Similarly, ammonium and phosphate fluxes are greater at those sites. The present-day AW-dominated shelf might represent the future scenario for the entire Barents Sea. Our results represent a baseline systematic understanding of seafloor geochemistry, allowing us to anticipate changes that could be imposed on the pan-Arctic in the future if climate-driven perturbations persist. This article is part of the theme issue 'The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning'. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) live in symbiosis with plant roots, facilitating mineral nutrient transfer from soil to hosts through large networks of extraradical hyphae. Limited data are available on the fungal structures (appressoria) connecting soil- to root-based mycelium, in relation to plant nutrition. Two in vivo systems were set up using three AMF, Funneliformis mosseae, Funneliformis coronatus and Rhizoglomus irregulare, grown in symbiosis with Cichorium intybus. The assessment of plant P content, number of appressoria, diameter of their subtending hyphae and length of colonized roots allowed calculation of the total cross-section area of appressorium-subtending hyphae, which differed among the three AMF and was correlated with plant P contents and with extraradical mycelium density. A conservative evaluation of P fluxes from soil- to plant-based hyphae occurring through appressoria gave values ranging from 1.7 to 4.2 × 10−8 mol cm−2 s−1 (moles per total cross-section area of the appressorium subtending hyphae per time elapsed), depending on AMF identity. This work suggests that, beyond intraradical colonization and extraradical mycelium extent, connections between extraradical and intraradical fungal mycelium through appressoria are important for mycorrhizal plant nutrition, as appressorium structural traits and density can be related to P transfer mediated by AMF. [ABSTRACT FROM AUTHOR]

Wu, B.; Jin, H.; Gao, S.; Xu, J., and Chen, J., 2020. Nutrient budgets and recent decadal variations in a highly eutrophic estuary: Hangzhou Bay, China. Journal of Coastal Research, 36(1), 63–71. Coconut Creek (Florida), ISSN 0749-0208. Hangzhou Bay is one of China's most eutrophic waterbodies, yet water quality observations within the bay are few, and the estuary's nutrient dynamics are poorly understood. Five cruises (summer, autumn, winter, spring) were conducted in Hangzhou Bay and adjacent waters between July 2006 and November 2007 and in August 2018. A steady-state box model was also constructed to estimate summer and winter nutrient fluxes. The results show that the spatial distributions and temporal variations of nutrients within the bay are significantly influenced by not only direct inputs from rivers around the bay but also the Changjiang River plume to the north. Bay concentrations of nitrate (), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) were quite high throughout the year, especially in the head of the bay. Relative to concentrations measured in 1981–1982, estuarine concentrations of and DIP had increased, whereas DSi had been stable. The model results indicate that riverine discharge is the major source of nutrients to Hangzhou Bay. In both summer and winter, nutrients were exported from the bay to the East China Sea. The fluxes of , dissolved inorganic nitrogen (DIN), DIP, and DSi were higher in summer than winter, but the flux of ammonium was higher in winter. Nutrient fluxes to Hangzhou Bay observed here during 2006–2007 are much higher than those observed in the 1980s. Additionally, nutrient abundances in Hangzhou Bay are significantly higher than those reported for other coastal embayments in China. The difference in algal blooms inside and outside the bay is possibly due to the difference in turbidity or the hydrological conditions. [ABSTRACT FROM AUTHOR]

To face the difficulties involved in supplying freshwater to Abidjan (Côte d'Ivoire), the Aghien Lagoon connected by a channel to the Potou-Ébrié lagoon is a potential resource for production of drinking water. A hydro-meteorological network was installed with the objective of assessing the sustainability of the lagoon in terms of both quantity and quality, and 10 sampling campaigns were conducted between June 2015 and December 2016 to evaluate the water quality. Even if the annual flow of the two peri-urban tributaries is just equivalent to the volume of the lagoon, the large daily inflow by the channel can secure the future quantitative demand. Unfortunately, the main tributaries in the peri-urban zones have lacked sanitation services and present numerous markers of pollution. Even if the water quality status of the lagoon remains quite good with respect to total nitrogen and total phosphorus, it nevertheless presents a high potential risk of eutrophication. [ABSTRACT FROM AUTHOR]

Integrated dissolved inorganic nutrient fluxes (nitrite, nitrate, ammonium, phosphate and silicate) between Patos Lagoon estuary (PLE), Brazil, and the inner continental shelf of the southwest Atlantic Ocean, were estimated from averaged water column nutrient concentrations and water fluxes (method 1 – M1). Daily surveys were carried out for 30 days, spanning from Feb-22 to Mar-23 of 2021 during late austral summer drought, at two stations — one next to the estuary mouth, the other in the inner estuary, at the interface between the estuary main channel and a shallow embayment called the Arraial bight. On average, PLE delivered dissolved inorganic phosphorus (DIP), nitrogen (DIN) and silicate outfluxes to the adjacent continental shelf at rates of 22.4 ± 596.1, 89.8 ± 207.3 and 1715.9 ± 3283.2 × 103 mmol m−2 d−1, respectively. Budgets for DIP and DIN were computed by the LOICZ box model approach (method 2 – M2) considering Patos Lagoon divided into its limnic (fresh, mean salinity 2 psu) and estuarine (brackish, mean salinity 18.5 psu) regions. The residual fluxes for DIP and DIN from PLE to the inner continental shelf were similar to those of M1, but the complete budget (M2) revealed that PLE also imported DIP, at a positive exchange rate between PLE and coastal waters. There is evidence that DIP is supplied by Plata Plume Water. Net ecosystem metabolism (NEM) and N-fixation – denitrification were assessed through the LOICZ budgets for DIP and DIN, respectively. Denitrification prevailed in both limnic and estuarine regions. Estimated NEM indicated weak autotrophy in the limnic region, and net heterotrophy in the estuarine region. The net heterotrophy in the estuarine region was mainly attributed to nitrogen limitation due to low allochthonous supply by river discharge, and N-loss by denitrification during austral summer drought conditions. [Display omitted] • Patos Lagoon act as a sink of DIN, and as a source of DIP to the southwest Atlantic Ocean. • There is evidence of phosphorus supply from Plata Plume Water into Patos Lagoon. • The net ecosystem metabolism was weakly heterotrophic. • Denitrification prevails in both the lagoon and estuarine regions. • N-loss by denitrification contributes to primary production limitation and net heterotrophy in the estuary. [ABSTRACT FROM AUTHOR]