Distribution and sources of organic carbon, nitrogen and their isotopes in surface sediments from the largest mariculture zone of the eastern Guangdong coast, South China

The increasing influence of oyster farming on sedimentary organic matter in a semi-closed subtropical bay

Oyster farming activities play a pivotal role in the biogeochemical cycles of coastal marine ecosystems, particularly in terms of sedimentary carbon cycling. To gain deep insights into the influence of expanding oyster culture on the sedimentary carbon cycle, surface sediments were collected from the Maowei Sea, which is the largest oyster farming bay in south China, based on six filed surveys between July 2010 and December 2022. The sediment samples were analyzed for total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ13C and δ15N) to evaluate the inter-annual variations in the source contribution to sedimentary organic matter (SOM). The results revealed that the average contents of sedimentary TOC and TN were 0.67 ± 0.41 % and 0.06 ± 0.03 %, respectively. Fluctuations in the C/N molar ratios ranged from 5.8 to 23.6, with an average of 12.6 ± 2.9, indicating a significant terrestrial input contribution to SOM in the study area. Furthermore, the integration of stable isotope analysis and Bayesian mixing model demonstrated a gradual increase in the mean proportion of shellfish biodeposition to SOM, from 12.0 ± 5.6 % in July 2010 to 21.1 ± 7.3 % in December 2022, consistent with the progressive expansion of oyster aquaculture along this coastal area, thereby emphasizing the substantial influence of oyster farming on SOM composition. With the anticipated expansion of oyster farming scale and production in the future, shellfish biodeposition is expected to assume a more important role in shaping SOM dynamics and sedimentary organic carbon cycling in coastal waters. Overall, this study provided an important perspective for better assessing the impact of expanding mariculture scale on coastal biogeochemical cycles, thereby making valuable contributions to future policy formulation concerning mariculture and ecological conservation.

Sources and transport of organic matter in the Ganges-Brahmaputra-Meghna River system of Bengal Basin, Bangladesh

This study aimed to understand the sources and transport mechanism of organic matter (OM) in the Ganges-Brahmaputra-Meghna (GBM) river system in Bangladesh. We conducted analyses of total organic carbon (TOC), total nitrogen (TN), their stable isotopes (δ13C and δ15N), and sediment grain size. The results reveal a heterogeneous mixture of OM derived from terrestrial plants, aquatic environments, and anthropogenic sources. The Brahmaputra River exhibited higher concentrations of TOC and TN, with δ13C and δ15N values indicating that the OM is primarily sourced from C3 plants. Conversely, the Ganges River demonstrated lower TOC levels and higher isotopic values, reflecting significant anthropogenic inputs. The Lower Meghna showed a mixture of terrestrial and marine sources. Variations in the TOC/TN ratios across the river system underscore the complex interplay between natural and anthropogenic factors. Additionally, sediment grain size plays a crucial role, with finer sediments in the Brahmaputra River associated with increased OM concentrations, while coarser sediments in the Ganges River correlate with lower TOC and TN levels.

Exploring the impact of ocean warming and nutrient overload on macroalgal blooms and carbon sequestration in deep-sea sediments of the subtropical western North Pacific

The role of macroalgae as blue carbon (BC) under changing climate was investigated in the subtropical western North Pacific. Sea surface temperatures (SSTs) and nutrient influx increased over the past two decades (2001-2021). The proliferation of climate-resilient macroalgae was facilitated. Using Pterocladiella capillacea and Turbinaria ornata, outdoor laboratory experiments and elemental assays underscored the influence of nutrient enrichment on their resilience under ocean warming and low salinity. Macroalgal incorporation into marine sediments, indicated by environmental DNA barcoding, total organic carbon (TOC), and stable isotope analysis. Over time, an increase in δ13C and δ15N values, particularly at greater depths, suggests a tendency of carbon signature towards macroalgaeand nitrogen pollution or high tropic levels. eDNA analysis revealed selective deposition of these species. The species-dependent nature of macroalgae in deep-sea sediments highlights the role of nutrients on climate-resilient macroalgal blooms as carbon sinks in the western North Pacific.

Anthropogenic influences on the sources and distribution of organic carbon, black carbon, and heavy metals in Daya Bay's surface sediments

The total organic carbon (TOC), total nitrogen (TN), black carbon (BC), δ13CTOC, δ15N, δ13CBC, grain size, and heavy metals of surface sediments collected from Daya Bay were determined to investigate the spatial distributions of these parameters and to evaluate the influences of human activities. Marine organic matter was found to constitute approximately 84.41 ± 7.70 % of these sediments on average. The western and northern regions of Daya Bay exhibited relatively fine grain sizes, weak hydrodynamic conditions, and high sedimentation rates, which favored the burial and preservation of organic matter. The high concentration of organic matter could be attributed to the influence of petroleum and aquaculture industries. Fossil fuels were the main source of BC. The enrichment factor (EF) and geo-accumulation index (Igeo) were used to evaluate the sources and pollution levels of heavy metals. The results revealed that the source and distribution of heavy metals were strongly influenced by human activities, resulting in moderate pollution levels across most regions of Daya Bay. A strong correlation was observed between the Igeo values of heavy metals and BC, TOC, TN, and mean particle grain size (Mz). This suggests that the ability of sediments in Daya Bay to enrich and adsorb heavy metals depends on the sediment grain size, the content and type of organic matter. Importantly, sediments in the inner bay of Daya Bay exhibited a greater capacity to impede the migration of heavy metals compared to those in the outer bay.

The anthropogenic effects on organic matter in sediment core based on Bayesian mixing model: a case study of Daya Bay

Sediment is an important carrier of evidence about environmental evolution which receives huge volumes of organic material originated from both anthropogenic and natural sources. In this study, based on sedimentary chronology, the vertical trends of particle size distribution, total organic carbon (TOC), total nitrogen (TN), and their stable isotopes (δ13C, δ15N) in the sediment core of the nuclear power sea in southwest Daya Bay were analyzed, and the distribution characteristics and contribution ratios of different sources of organic matter in the sedimentary environment over the past 70 years were resolved using a Bayesian mixing model (MixSIAR). TOC, TN, δ13C, and δ15N ranged from 0.89 to 1.56%, 0.09 to 0.2%, - 22.3 to - 20.6‰, and 4.38 to 6.51‰, respectively. The organic matter in the sediment is controlled by a mixture of terrestrial input and marine autochthonous, the proportion of organic matter from terrestrial sources increases, while that from marine sources decreases in the sediment core, which persists from 1960 to 2000, yet organic matter from marine sources still dominates. The first signs of increased primary productivity occurred in 1960, and it was primarily due to agricultural activity. After the 1980s, the rapid increase in population around Daya Bay, the construction of nuclear power plants, the rise of aquaculture, and the quick expansion of industrial bases were all major factors that changed the ecological environment of Daya Bay.

Plutonium in sediments of the Eastern Guangdong coast-its sources and their contribution

The ^239+240Pu activities and ²⁴⁰Pu/²³⁹Pu atom ratios of surface sediments from the Eastern Guangdong coast (EGDC) were determined by sector field ICP-MS in order to examine the sources of plutonium (Pu) and quantify their contributions. The ^239+240Pu activities in the EGDC ranged from 0.113 to 0.451 Bq kg^-1, with an average of 0.225 ± 0.090 Bq kg^-1 (n = 17). Consistently high ²⁴⁰Pu/²³⁹Pu atom ratios, ranging from 0.218 to 0.274 (average = 0.254 ± 0.014, n = 17), indicate a non-global fallout Pu source in the EGDC. The horizontal distribution of the ²⁴⁰Pu/²³⁹Pu atom ratios in the EGDC sediment suggests the non-global fallout Pu is sourced from close-in fallout from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we calculated the relative proportions of Pu from the PPG and global fallout in the EGDC to be 57 ± 9 % and 43 ± 9 %, respectively. Moreover, from the well-defined relationship between ^239+240Pu activity and total organic carbon content in sediments and a two end-member mixing model using δ¹³C, we further calculated the Terr-global fallout (riverine input) and Mar-global fallout (direct atmospheric deposition) to be 11 ± 2 % and 32 ± 6 %, respectively. Finally, from the activity levels and atom ratios of Pu isotopes in the EGDC, we established a baseline for future use in environmental risk assessment related to nuclear power plant operations.

Sediment mercury concentration of a subtropical mangrove wetland responded to Hong Kong-Shenzhen industrial development since the 1960s

Mercury (Hg) in coastal wetlands is of great concern due to its acute toxicity. We measured the total Hg content (THg) from a ²¹⁰Pb-dated sediment core obtained from the Futian mangrove wetland in Shenzhen Bay, South China to explore the historical variation and possible sources. Our results extend the sediment THg record back to 1960 and reveal three distinct intervals. Interval I (1960-1974) has low and increasing THg values, averaging 83.0 μg/kg; Interval II (1975-1984) witnesses a remarkably increase, peaking in 1980 (261.6 μg/kg) then remaining elevated; Interval III (1985-2014) shows a steady reduction, averaging 118.4 μg/kg. The good correlation among THg, TOC, and Hg/TOC, and the downstream decrease in monitoring sediment THg consistently suggest that the bulk THg are mainly sourced from the Shenzhen River discharge. The different timing in industrial development attributes the elevated THg concentrations during 1975-1984 to Hong Kong industrial sewage pollution.

Comprehensive analysis of the migration and transformation of nutrients between sediment and overlying water in complex habitat systems

Under the influence of environmental change, disturbance and other external conditions, sediments release internal nutrients to the overlying water and become a contamination source in the lake. Complex habitat systems provide a unique opportunity for determining the influences of environmental changes in lakes. In this study, Baiyangdian Lake (BYDL) was divided into different habitat systems (connected water areas, river courses, reed fields, lotus ponds, fishponds, farmland, and thorps) based on the influence of natural and artificial activities. The physical and chemical properties of overlying water and sediment in different habitat systems were investigated. In addition, statistical analytical methods were used to analyze the relationship between sediment characteristics and overlying water parameters in different habitat systems. The results showed that nitrogen and phosphorus in the overlying water could accumulate in the sediments, while disturbance was one of the main factors affecting the release of nutrients from sediments. Disturbance promoted the suspension of sediments and increased the oxygen content, thereby facilitating the internal release of nutrients. However, there were also some differences in the process of internal release of nutrients between the habitat systems. Nitrogen in the overlying water was closely related to the source of organic matter (r > 0.950), especially in the ponds (including lotus ponds, reed fields, and fishponds), and phosphorus was mainly influenced by turbidity (r > 0.870). In the river course (p = 0.198, n = 26), the disturbance and increase in pH promoted the internal release of nutrients from the sediments (contributions of 35.2 % and 25.1 %, respectively). In the ponds, the aquatic macrophytes reduced the release of nitrogen and phosphorus in sediments. Overall, this study provides more information on the migration and transformation of nutrients between sediment and overlying water in lakes with multiple habitats.

Risk assessment of heavy metal and pesticide mixtures in aquatic biota using the DGT technique in sediments

Heavy metals and pesticides (HMPs) are common contaminants due to their extensive use worldwide. Diffusive gradients in thin films (DGT) are a good method for measuring the bioavailable concentration of pollutants. This study represents the first evaluation of HMP toxicity in aquatic biota using the DGT technique in sediments. Zhelin Bay was selected as the case study site because it has been contaminated by pollutants. Nonmetric multidimensional scaling (NMS) analysis reveals that a diverse range of pollutants (V, Cr, Ni, Cu, Zn, As, Se, InHg, Mo, Cd, Sb, W, Pb, CLP, PYR) are mainly influenced by sediment characteristics. Assessment of single HMP toxicity found that the risk quotient (RQ) values for Mn, Cu, inorganic Hg (InHg), chlorpyrifos (CLP) and diuron (DIU) are significantly higher than 1, indicating that the adverse effects of these single HMPs should not be ignored. The combined toxicity of HMP mixtures based on probabilistic ecotoxicological risk assessment shows that Zhelin Bay surface sediments had a medium probability (54.6%) of toxic effects to aquatic biota.

Tracing the organic matter source of cage culture sediments based on stable carbon and nitrogen isotopes in Poyang Lake, China

Collected sediment samples from the cage fish farm were measured to determine carbon and nitrogen stable isotope compositions and to understand the influence of the aquaculture waste on the sediment. The average δ¹³C of the sediment organic matter was -27.2 ‰ and -26.5 ‰, and the average δ¹⁵N value was 5.6 ‰ and 6.2 ‰ in October 2017 and November 2018, respectively. A linear mixing model was used to calculate the contribution ratios of the aquaculture waste in sediment organic matter. The contribution ratio of fish feces was 53.9 % and 25.5 %, and the contribution ratio of waste feed was 18.4 % and 52.6 % in October 2017 and November 2018, respectively. The sediment in the "cage fish area" was characterized by high waste feed ratio in sediment organic matter. The sediment organic matter was affected by the aquaculture waste even at sites 1500 m away from the cage fish farm.

Enrichment differences and source apportionment of nutrients, stable isotopes, and trace metal elements in sediments of complex and fragmented wetland systems

Anthropogenic activities significantly influence the lake environment and are reflected by the element contents in sediments/soils. The lake fragmentation provides a unique opportunity for comparing the influences of natural/anthropogenic activities of different wetlands systems. In this study, a complex and fragmented lake was investigated, and sediment/soil samples were collected from different systems. The nutrient contents (C, N, and P), stable isotopic compositions (δ¹³C and δ¹⁵N), and trace metal contents (As, Cd, Cr, Cu, Ni, Pb, and Zn) in the sediments/soils were measured to determine the natural and anthropogenic influences and pollution sources. Lake fragmentation was caused by insufficient water input and long-term agricultural and aquacultural activities of local residents. Due to the effect of anthropogenic activities, the enrichment conditions of various elements differed significantly for different wetland systems. Industrial, agricultural, and biological sources significantly influenced the element enrichment in different systems. The results demonstrated that the anthropogenic activities significantly influenced the sediments/soils in wetland systems, and the lake fragmentation reduced the diffusion of the contaminants. These results provide accurate reference information for pollution control, lake management, and ecological restoration.

δ13C, δ15N and TOC/TN as indicators of the origin of organic matter in sediment samples from the estuary of a tropical river

The present study aimed to determine the total organic carbon (TOC), total nitrogen (TN), the carbon‑nitrogen ratio (TOC/TN), carbon isotope (δ¹³C), and nitrogen isotope (δ¹⁵N) in five sediment cores collected from upstream to downstream of the Rio Serinhaem estuary, State of Bahia, Northeast Brazil, in order to investigate the origin of the deposited organic matter (OM).Significant positive correlation was found between TOC and NT (r_s = 0.75); TOC/TN and TOC (r_s = 0.64); δ¹⁵N and TOC (r_s = 0.72); and δ¹⁵N and TOC/TN (r_s = 0.63). The values of δ¹³C and δ¹⁵N found are characteristic of terrestrial sources. The TOC/TN ratio confirmed the data found for δ¹³C and δ¹⁵N, which recorded the origin of organic matter from terrestrial C3 type plants. Upstream of the estuary, the highest means of TOC/TN were found (T1 = 36.9 and T2 = 24.4), as reflected by the increase in TOC content or reduction in TN. The OM along the estuary is predominantly from plants with a C3 photosynthetic pattern, indicating that the Serinhaem River estuary is considered a relatively well-preserved environment.

Influence of sedimentary organic matter sources on the distribution characteristics and preservation status of organic carbon, nitrogen, phosphorus, and biogenic silica in the Daya Bay, northern South China Sea

Surface sediment samples were collected from Daya Bay in October 2018, and analyzed for total organic carbon (OC), total nitrogen (TN) and their stable isotopes (δ¹³C and δ¹⁵N), total phosphorus (TP), biogenic silica (BSi), sediment textures and specific surface area (SSA). The primary objective was to evaluate the influence of mariculture/aquaculture on the distribution characteristics of organic matter (OM), and preservation status of OC, TN, TP, and BSi in sediments. The average δ¹³C and δ¹⁵N values, and OC/TN ratios were -21.27‰, 6.74‰, and 8.90, respectively. Monte Carlo simulation results revealed that mariculture/aquaculture biodeposits accounted for >40% of the buried OM at sites where the breeding rafts and cages are located, whereas marine OM increased gradually to the open sea. Terrestrial OM was generally low accounting for 17% by average. The contents and distribution characteristics of biogenic elements were more influenced by mariculture/aquaculture and primary productivity than sediment textures. Lower OC/SSA (0.3-1.2 mg OC/m²), TN/SSA (~0.05-0.18 mg TN/m²), and TP/SSA (0.02-0.04 mg TP/m²) loadings indicated that increased sequestration of labile OM in a coastal bay could contribute to significant degradation of recalcitrant OM in sediments with significant loss of P relative to OC. Nitrogen contamination in surface sediments was due to increased injection of aquaculture biodeposits, and may pose a detrimental effect on the ecological sustainability of the bay. Higher BSi/SSA loadings (0.9-1.7 mg BSi/m²) revealed that BSi was more preserved, and that BSi-based proxy could be used for paleo-productivity studies. However, such preservation may induce adverse dissolved silicate limitation in a bay perturbed by eutrophication. Fine-grained sediments (clay and silt) accounted for >77% of the sediment texture types with higher SSA, and while controlling the contents of biogenic elements under given depositional conditions were not the main determining factors of OC, TN, TP, and BSi preservation.

Morphological characteristics and ecological risk assessment of nitrogen and phosphorus in the sediments of Futunxi watershed in Fujian province

The total amount and morphology of nitrogen and phosphorus in sediments have important environmental significance. The study of the ecological risk assessment of nitrogen and phosphorus in sediments is important to understand the environmental quality of water body in the basin. In this study, taking the Futunxi Basin of Fujian Province as an example, the single factor index and bioavailability coefficient method were used to evaluate the ecological risk of nitrogen and phosphorus in the sediments, and reveal the spatial change and environmental significance of the nitrogen and phosphorus forms. The results showed that different morphological components of the bio-available nitrogen were distributed as organic sulfide bound nitrogen (SOEF-N) > iron-manganese oxidized nitrogen (SAEF-N) > weak acid leached nitrogen (WAEF-N) > ion exchange nitrogen (IEF-N). The inorganic phosphorus included the highest proportion of metal oxide bound phosphorus (NaOH-P), followed by calcium bound phosphorus (HCl-P). The proportion of reduced phosphorus (BD-P) was even lower, and the proportion of weakly adsorbed phosphorus (NH4Cl-P) was the lowest. The results of single factor pollution index showed that the sediment nitrogen in the study area mainly caused moderate pollution and phosphorus mainly caused light pollution. By contrast, the results of the bioavailability index method indicated that nitrogen mainly caused light pollution, and phosphorus mainly caused clean pollution in sediments. Combined with the characteristics of social economy and environment in the study area, it can be inferred that the bioavailability index method based on comprehensive evaluation of total amount and morphology can better characterize the spatial change and ecological risk of nitrogen and phosphorus in sediments.

Carbon and Nitrogen Stable Isotopes Evidence for the Environmental Impact of the Cage Fish Farm in Poyang Lake, China

The carbon and nitrogen stable isotope compositions of the sediment organic matter, fish feed and fish feces were measured to assess the impact of the aquaculture waste from the cage fish farm in Poyang Lake. The results provide evidence of the non-negligible effect of aquaculture waste on the sediment organic matter. The δ¹³C and δ¹⁵N of sediments varied from - 27.62 to - 25.66‰ and 4.83 to 6.92‰, respectively. The sediment organic matter had a mixed source of waste feed, fish feces and plankton. The average contribution ratio of waste feed and fish feces was 53.1% and 24.6%, respectively. The aquaculture derived organic matter ratio was high with ranging from 57.6 to 95.6%. The aquaculture waste seemed to disperse widely enough to influence the study site about 2.5 km distance from the cage. The dispersion and resuspension were the main two factors which can explain the great effect area found in this study.

Glomalin-related soil protein enriched in δ13C and δ15N excels at storing blue carbon in mangrove wetlands

Glomalin-related soil protein (GRSP) derived from arbuscular mycorrhizal fungi can be transported from land to sea and captured in mangrove wetlands, thereby contributing to soil C and N pools. However, the stable isotope signatures of GRSP and the key influencing factors that affect its isotope values in coastal wetlands remain unknown. In this study, the results showed that total-GRSP (T-GRSP) was a significant contributor of C and N content to mangrove soil. We first compared stable isotope (δ¹³C and δ¹⁵N) values and C/N ratios of GRSP with those of other blue carbon sources in a typical mangrove wetland. The isotope fingerprints of T-GRSP, mangrove soils, mangrove plants, and tidal waters were identified. Unlike those of the conventional sources, the δ¹³C and δ¹⁵N values of T-GRSP were -25.04‰ to -22.83‰ and 3.22‰ to 7.24‰, respectively, and the mean C/N ratio was 12.95 in the mangrove cover sites. These findings indicated that T-GRSP is a novel blue carbon source mainly originating from terrestrial ecosystems. Moreover, the δ¹³C and δ¹⁵N values of T-GRSP in mangrove wetlands were affected by vegetation interception and soil properties. Redundancy analysis results indicated that pH, moisture, depth, and salinity were key factors influencing the T-GRSP isotope fingerprints in mangrove wetlands. Additionally, the simultaneous changes in T-GRSP content, isotope values, and C/N ratios among mangrove cover sites, a mudflat, and tidal waters suggested that this protein is a sensitive tracer between land and sea. Overall, the isotope signatures of GRSP captured by mangroves were identified for the first time, which will have important implications for the estimation of the blue carbon budget and identification of the blue carbon sources in global coastal regions.

Spatiotemporal variations of biogenic elements and sources of sedimentary organic matter in the largest oyster mariculture bay (Maowei Sea), Southwest China

China is the largest mariculture producer in the world, but detailed information on the spatiotemporal variations of biogenic elements and sources of sedimentary organic matter (SOM) via mariculture is limited. The primary objective of this study was to assess the influence of mariculture on the origin of SOM in relation with biogenic elements and geochemical paramaters due to the importance of SOM as a potential source of nutrients and energy in coastal marine environments. Surface sediments from the Maowei Sea were collected in August (summer) and December (winter), 2016 for grain size, total organic carbon (TOC), total nitrogen (TN), organic phosphorus (OP), biogenic silica (BSi), δ¹³C and δ¹⁵N analyses. Significant correlation (p < 0.01) was observed between TOC and TN in summer and winter respectively, indicating that they have common source in both seasons. The spatiotemporal distributions of TOC, TN, OP and BSi were influenced by the sources and distribution of SOM, grain sizes and hydrodynamic conditions in the Maowei Sea. The overall ranges of δ¹³C (-26.86‰ to -23.01‰) and δ¹⁵N (2.54‰ to 9.82‰) and C/N ratio (5.83 to 18.67) showed that SOM is derived from mixed sources. The δ¹³C and δ¹⁵N-based three-end-member mixing model results revealed that >40% of the deposited SOM originates from terrestrial source during two seasons. The SOM from shellfish mariculture was seasonal, mainly deposited in the intensive mariculture areas, and its proportions were only higher than contributions from marine plankton in summer. Generally, this study indicates that shellfish biodepositions can significantly influence the cycle of carbon and other biogenic elements in the intensive mariculture areas. Nevertheless, the overall dominance of terrestrial and marine SOM suggests that the sources of SOM and factors influencing carbon cycling in the Maowei Sea do not exclusively depend on the intensity of mariculture activities.

Qualitative and quantitative analysis of source for organic carbon and nitrogen in sediments of rivers and lakes based on stable isotopes

Sediment is the most dominant reservoir of organic pollutants in the aquatic environment. Understanding carbon and nitrogen sources in sediments and factors that controls distribution enhances our understanding of biogeochemical cycles of carbon and nitrogen. Different end-members and surface sediments of rivers and sediments profiles of lakes were collected. The concentrations of TOC and TON and their δ¹³C and δ¹⁵N were studied for qualitative and quantitative analysis of natural and anthropogenic sources. The results show that TOC and TON concentrations of the sediments from rivers range from 0.63% to 10.83% and 0.06%-0.86%, respectively, indicating substantial great environmental risks in these rivers. The concentrations of TOC and TON for the four sediment profiles below the 5 cm, increase in the order of Miyun < Chuidiao < Qunming < Houhai, as influenced by their respective environment condition. Moreover, water quality was quite good and there was no risk of eutrophication in Miyun reservoir. δ¹³C_org and δ¹⁵N_org in surface sediments of the studied 18 rivers range from -27.2‰ to -24.9‰ and -2.2‰ to +10.9‰, respectively. Based on a simple δ¹³C-based end-member mixing and a C/N ratio model, organic matter in the surface sediments of these rivers were mainly derived from sewage and C3 plant. In addition, the sources of organic matter differed in each layer of the four sediment profiles. This study provides a reliable method for qualitative and quantitative identification of the source of organic matter in sediments, and offers theoretical basis for better management of rivers and lakes.

Spatial and Seasonal Variations of Sedimentary Organic Matter in a Subtropical Bay: Implication for Human Interventions

Elemental (total organic carbon (TOC) and total nitrogen (TN)) and stable carbon and nitrogen isotope compositions (δ¹³C and δ¹⁵N, respectively) in the surface sediment of Zhanjiang Bay (ZJB) in spring and summer were measured to study the spatial and seasonal changes of organic matter (OM) and assess the human-induced and environment-induced changes in the area. The OM in the surface sediment of ZJB was a mixture of terrestrial and marine sources, and was dominated by marine OM (54.9% ± 15.2%). Compared to the central ZJB, the channel and coastal ZJB areas had higher δ¹³C and δ¹⁵N values, higher TOC and TN concentrations, and lower TOC/TN ratios, indicating higher primary productivity and higher percentages of marine OM in the latter two subregions. Mariculture activities, sewage inputs, and dredging were responsible for these phenomena. Clear seasonal variations in OM were observed in ZJB. The average proportions of terrestrial OM in summer increased by 10.2% in the ZJB channel and 26.0% in the coastal ZJB area compared with those in spring. Heavy rainfall brought a large amount of terrestrial OM into the channel and coastal ZJB areas, leading to the increase of the terrestrial OM fraction in these two subregions in summer. In summary, anthropogenic influences had a significant influence on the spatial and seasonal variations of sedimentary OM in ZJB.

Variations in stable carbon and nitrogen isotopes of particulate organic matter in surface waters of water-receiving area of Eastern Route of South-to-North Water Transfer Project, China

The purpose of this study was to assess the water quality and variations in stable carbon and nitrogen isotopes of particulate organic matter (δ¹³C_POM and δ¹⁵N_POM), as well as to evaluate the sources of carbon and nitrogen that contribute to the POM pools in lakes and reservoirs located in the water-receiving area of the Eastern Route of South-to-North Water Transfer Project (SNWTP) in Northern China. During each season from October 2013 to July 2014, samples of POM from 14 lakes and reservoirs in Northern China were collected. The lakes and reservoirs were meso-eutrophic with considerably high brackish ions (SO₄^2-, 173 mg/L; Cl^-, 296 mg/L) in Yangtze River lake, and high total nitrogen: total phosphorus ratio (averaged with 772) or dissolved inorganic nitrogen: soluble reactive phosphorus molar ratios (averaged with 1077) in mountainous reservoirs. The δ¹³C_POM, δ¹⁵N_POM, carbon to nitrogen (C/N) ratios showed significant seasonal variation, with ranges of - 32.5 to - 17.4‰, - 3.6 to 13.5‰, and 5.1-13.2, respectively, while they were hard to be distinguished among types of water sources. Principal component analysis (PCA) indicated that brackish ions, nutrients, and their molar ratios were the main factors influencing variations in δ¹³C_POM and δ¹⁵N_POM. δ¹³C_POM and C/N ratios suggested autochthonous primary production mainly contributed to POM during from April to October, while exogenous organic matter might mainly contribute these carbon pools in January. The low values of δ¹⁵N_POM (< 0‰) and negative correlation between δ¹⁵N_POM and TN suggested discharge of agricultural waste water (e.g., fertilizers, irrigation tailwater) in Bailanghe, Xinan, and Taihe Reservoir during the fertilization season, while higher values indicated domestic sewage input to waterbodies (e.g., Mishan, Gengjing, Donghai Reservoir). Our results suggested that the aquatic ecosystem in water-receiving area of SNWTP would be potentially affected by the inter-basin water diversion, and thus, ecosystem-based strategies were also presented accordingly.

Risk Assessment Based on Nitrogen and Phosphorus Forms in Watershed Sediments: A Case Study of the Upper Reaches of the Minjiang Watershed

In order to achieve effective eutrophication control and ecosystem restoration, it is of great significance to investigate the distribution characteristics of nutrient elements in sediments, and to perform ecological risk assessments. In the current grading criteria for nutrient elements in sediments, only the overall or organic components of carbon, nitrogen and phosphorus are considered, while the specific species distributions and bioavailability characteristics are rarely taken into account. Hence, using the current grading criteria, the differences in the release, migration and biological activity of nutrient elements in sediments cannot be accurately reflected. Taking the upper reaches of the Minjiang River watershed as an example, we analyzed the overall distributions and the ratio of nutrient elements in sediments, the spatial changes of nitrogen and phosphorus forms, the bioavailability, and the environmental significance. The ecological risk of nitrogen and phosphorus in sediments was assessed using an evaluation method based upon the biological effective parameter. The results were compared with the results of the evaluation methods based on the single pollution index, and then these evaluation methods were confirmed accordingly. From the results, the following conclusions can be obtained: (1) The spatial distributions of nutrient elements in sediments in the upper reaches of the Minjiang River Watershed (including the Jianxi Basin, Futunxi Basin, and Shaxi Basin) were significantly affected by the local ecology and the urban sewage discharge system. (2) The maximum average contents of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) in sediments were observed in the Jianxi Basin, the Futunxi Basin and the Shaxi Basin, respectively. (3) According to the contents of nitrogen and phosphorus in sediments, the bioavailable nitrogen (TTN) accounted for 35.49% of the total contents of TN. The components of TTN can be sorted from high to low as follows: Nitrogen in organic sulfide form (SOEF-N) > nitrogen in iron-manganese oxide form (SAEF-N) > nitrogen in ion exchange form (IEF-N) > nitrogen in weak acid leaching form (WAEF-N). Inorganic phosphorus (IP) was the main component of TP. The components of IP can be sorted from high to low as follows: Metal oxide bound phosphorus (NaOH-P) > calcium bound phosphorus (HCl-P) > reduced phosphorus (BD-P) > weakly adsorbed phosphorus (NH4Cl-P). Meanwhile, bioavailable phosphorus (BAP, BAP = NH4Cl-P + BD-P + NaOH-P) accounted for 36.94% of TP. According to the results of the single pollution index method, the risk level of TOC pollution in the sediments was relatively low in the whole area, while the risk level of TN pollution was low or moderate in most zones, and severe in certain ones. The risk level of TP pollution was low to moderate. (4) From the results of the bioavailability index evaluation method, based on the total amounts and forms of N and P, the risk level of N pollution was moderate, while the risk of P pollution was negligible. In addition, the results of the bioavailability index evaluation method were more consistent with the actual situation and reflected the overall environmental effects of nitrogen and phosphorus.

Geochemical discrimination of bulk organic matter in surface sediments of the Cross River estuary system and adjacent shelf, South East Nigeria (West Africa)

Knowledge of the sources, distribution and fate of organic matter (OM) in estuarine and adjacent shelf sediments are important for the understanding of the global biogeochemical cycles. Bulk organic carbon (C-org), total nitrogen (TN), biogenic silica (BSi), stable carbon (δ¹³C-org) and nitrogen (δ¹⁵N) isotopes, and sediment grain sizes were measured to study the spatial distributions and sources of sediment OM in the Cross River estuary system (CRES) and adjacent shelf. Surface sediments in the CRES were composed of clayey silt and sandy silt, while the adjacent shelf sediments were mainly silty sand. The range of the studied parameters was -28.79‰ to -22.20‰ for δ¹³C-org, -1.32‰-6.31‰ for δ¹⁵N, 6.7-29.2 for C-org/N ratios, 0.08%-0.33% for TN, 0.24‰-0.74‰ for BSi, and 0.47%-5.28% for C-org, and their spatial distributions showed a general decreasing trend in both the terrestrial and estuarine OM from the riverine regions to the adjacent shelf. Based on the three-end-member mixing model using the δ¹³C and δ¹⁵N isotopic values, ~58.01 ± 15.32% of sediment OM are derived from terrestrial sources dominated by C₃ vascular plants, while ~26.34 ± 9.71% are attributed to estuarine sources dominated by aquatic macrophytes, and ~15.65 ± 12.37% for marine plankton source. Other sources of OM identified included soils underlain C₃ vascular plants and agricultural farms enriched with N, sewage, and petroleum hydrocarbons. The relationship between C-org vs. BSi, and the atomic BSi/Corg ratios suggested that diatoms also play an important role in OM sequestration in surface sediments of the CRES and adjacent shelf. The correlations of the δ¹³C-org and δ¹⁵N isotopic values vs. C-org/N ratios resulted in scatter plots, indicating that the distributions of sediment OM in the CRES and adjacent shelf are influenced by post depositional processes, fixed inorganic N adsorbed on fine-grained sediments, microbial degradation, as well as sediment grain size.

Land use-induced change in trophic state of Shenzhen Bay (South China) over the past half-century

Mangroves connect terrestrial and marine ecosystems and are sensitive to environmental change, and the sediments within faithfully record long-term changes in the aquatic environment and supplement the generally short monitoring archives. Here we present a sediment-core record of TN, TP, TOC, and C/N ratios from the Futian mangrove wetland, Shenzhen Bay (South China), and compare with our remote sensing-derived land use change and the nearby seawater monitoring records. Our reconstructed record extends the history of tropic state change back to the Year 1958, supplementing the monitoring activity initiated in 1987. More importantly, the results reveal two distinct periods. The pre-1983 period (Year 1958-1982) is characterized by increased nutrient due to agricultural activities, while the post-1983 period (Year 1983-2014) is marked by reduced TP but relatively stable TN values, during which the simultaneous increase in construction lands and C/N ratios suggests enhanced soil erosion due to the rapid urbanization of Shenzhen City.

Stable isotopes and geochemical indicators in marine sediments as proxies for anthropogenic impact: A baseline for coastal environments of central Chile (33°S)

Stable isotopes and geochemical proxies (TOC/TN and Chla/TOC) in sediments can be used to distinguish organic matter sources (anthropogenic, terrestrial, or marine). This study aims to characterize organic matter (OM) in superficial sediments from three sectors of central Chile (33°S) that are highly influenced by urban and industrial development. Our results show that a substantial fraction of these OM in Quintero Bay is anthropogenic, from industrial and domestic wastewater sources. In contrast, a mixture of terrestrial and anthropogenic OM dominates the isotopic signal of surface sediments from Concón, derived from non-point industrial and agriculture sources associated with the Aconcagua River basin, while Ritoque exhibited a mixture of different OM sources. However, deposition of allochthonous OM in our study area depends on coastal topography, which influences the local currents and well-ventilated waters, promoting the dispersion and assimilation of OM and thus providing an environment with a high capacity for natural remediation.

Carbon dioxide dynamics from sediment, sediment-water interface and overlying water in the aquaculture shrimp ponds in subtropical estuaries, southeast China

Aquaculture ponds can emit a large amount carbon dioxide (CO₂), with the consequence of exacerbating global climate change. Many studies about CO₂ dynamics across the water-air interface, but CO₂ in sediment and overlying water received relative less attention. In this study, CO₂ concentration in sediment porewater, the diffusive CO₂ fluxes across the sediment-water interface (SWI), and the CO₂ production rates in the overlying water (CO_{2_WP}) were determined in the shrimp ponds in the Min River Estuary (MRE) and Jiulong River Estuary (JRE), southeast China, to analyze the dynamics of CO₂ among different growth stages of shrimps. Our results showed large variations in porewater CO₂ concentrations, CO₂ diffusive fluxes and CO_{2_WP} rates among different growth stages, with markedly larger values in the middle stage of shrimp growth. The temporal variation of CO₂ in both estuarine ponds followed closely the seasonal change of temperature. The internal CO₂ production (CO_{2_IP}) in these ponds was dominated by sediments. A significantly larger mean porewater CO₂ concentrations, diffusive fluxes and production rate were observed in the MRE ponds than those in the JRE ponds, which could be attributed to the lower water salinity and a larger source of carbon substrates in the former estuary. Considering a total surface area of 6.63 × 10³ km² across the mariculture ponds in subtropical estuaries, it is estimated conservatively that approximately 100 Gigagram (Gg) of dissolved organic carbon and 190 Gg of dissolved inorganic carbon were transported annually from the mariculture ponds into China's coastal areas. Because of the substantial supply of dissolved carbon, the adjacent coastal waters receiving effluent discharge from the mariculture ponds could become "hotspots" of CO₂ emissions. Our results highlight the role of aquaculture pond as a major CO₂ source in China's coastal areas, and effective actions are needed to alleviate the greenhouse gas (GHG) emissions from these ponds.

Effects of abalone (Haliotis discus hannai Ino) and kelp (Saccharina japonica) mariculture on sources, distribution, and preservation of sedimentary organic carbon in Ailian Bay, China: Identified by coupling stable isotopes (δ13C and δ15N) with C/N ratio analyses

To investigate the effects of mariculture on the sources, distribution and preservation of sedimentary organic carbon (SOC), sediments from an mariculture area in Ailian Bay, China, and a control area were analyzed for grain size composition, total organic carbon (TOC), total nitrogen (TN), carbon/nitrogen (C/N) ratio, and stable carbon and nitrogen isotopic composition (δ13C and δ15N). The sedimentary type of sediments in study area was clay silt. TOC, TN, C/N, δ13C, and δ15N ranged from 0.58 to 1.21%, 0.06-0.17%, 6.29-9.82, -23.20 to -18.50‰, and 6.17-7.38‰, respectively, and followed similar spatial patterns. TOC, TN and δ13C were higher in mariculture area than in control area. Biodeposit and kelp OC contributions greater of SOC in mariculture area than the control area (biodeposits: 20.10 ± 4.84 to 6.2 ± 1.3%; kelp: 15.3 ± 6.63 to 5.2 ± 0.84%). Overall, mariculture activities significantly influence the sources, distribution and preservation of SOC.

Adsorption-desorption of hydrophilic contaminants rhodamine B with/without Cd2+ on a coastal soil: implications for mariculture and seafood safety

An experimental study on adsorption and desorption behaviors of rhodamine B (RhB) with/without cadmium ion (Cd²⁺) on a coastal soil was carried out in the face of the scarce studies on its environmental behaviors, especially its combined pollution with metal ions. The implications on mariculture and seafood safety, rarely concerned either, were also analyzed for providing a new insight between azo dyes and them. It showed that the adsorption of RhB on the tested soil was affected by Cd²⁺ and could be better fitted by the Freundlich model and dominantly identified as chemical adsorption according to the threshold of ΔG⁰ (40 kJ/mol). There is a positive hysteresis on the desorption of RhB, which was generally strengthened with the increasing concentration of RhB but generally weakened in the presence of Cd²⁺. The azo dyes including RhB and heavy metals such as Cd, as the major class of typical pollutants in coastal environment, are closely related to the mariculture and seafood safety. In the consideration of three aspects including potential pollutants for the mariculture, potential risks for seafood safety, and potential relationships between dyes and mariculture and seafood safety, their implications were further clarified.

Heavy metal fractionation and ecological risk implications in the intertidal surface sediments of Zhelin Bay, South China

Intertidal surface sediments collected from Zhelin Bay, the largest mariculture base of eastern Guangdong Province of China, were analyzed for total metal concentrations and chemical speciation. Average total metal concentrations (mg/kg) were 0.063 (Cd), 35.69 (Pb), 23.07 (Cr), 7.50 (Ni), 7.95 (Cu), 74.95 (Zn), and 751.32 (Mn). Concentrations of Cd, Cu, Zn, and Mn were significantly higher than the corresponding background values of Zhelin Bay. All studied metals were dominated by residual fractions, whereas the second relatively higher average portions of Cd (24.10%) and Mn (15.17%) were strongly associated with the acid-soluble fraction. Overall, the intertidal surface sediments of Zhelin Bay were only slightly polluted based on the pollution load index (PLI), with a 21% probability of toxicity based on the mean effects range-median quotient. The metals Cd and Mn posed medium to high risk levels based on the method of risk assessment code (RAC).