Sediment TP Research Articles

Gully regulates snowmelt runoff, sediment and nutrient loss processes in Mollisols region of Northeast China

Gully is a prominent indicator of land degradation in agroecosystems, functioning as a crucial pathway connecting upslopes to downstream channels. However, little is known about how gully regulates runoff, sediment, and nutrient loss processes in the catchment during snowmelt. In this study, we monitored these processes in situ at both the gully head (the upslope accumulated catchment of the gully head, CGH) and outlet of two representative and typical gully-dominated catchments (F1 and F2) during snowmelt in Mollisols region of Northeast China. Our results showed that runoff discharge of CGH and outlet exhibited a multi-peak trend during snowmelt, driven by the transition from snow melting to soil thawing. This transition resulted in distinct runoff patterns in both CGH and outlet, with significant differences in their response to air temperature. The total runoff yield of CGH accounted for 57.8 % in F1 and 40.6 % in F2 of the total runoff yield of the outlet. Notably, the peak sediment concentration displayed a marked lag compared to the peak runoff discharge, primarily dominated by the increased sensitivity of gully erosion after the thawing of gully slopes. Gully erosion was the main source of sediment yield in the catchment, contributing 98.2 % in F1 and 96.6 % in F2. Furthermore, nutrient concentrations exhibited a decreasing trend during snowmelt. The comparison of high nutrient concentrations in CGH and relatively low nutrient concentrations in outlet highlighted the gully's role in intercepting and diluting runoff nutrients. Hysteresis analysis confirmed the differential contribution of CGH and gully to nutrient sources. CGH accounting for 50.9 % and 93.3 % of runoff TN and runoff TP loss, while contributing only 8.3 % and 5.8 % to sediment TN and sediment TP loss, respectively. These findings offer valuable insights for effective erosion control and nonpoint source pollution management in gully-dominated agroecosystems during snowmelt.

Profiling sediment bacterial communities and the response to pattern-driven variations of total nitrogen and phosphorus in long-term polyculture ponds

Sediment bacterial communities are decisive drivers of nutrient cycling processes in aquaculture ecosystems and are readily affected by surrounding environmental factors. However, the knowledge of sediment nutrient accumulations and bacterial community structure is limited in the emerging polyculture systems. Herein, we investigated the profiles of sediment properties and bacterial communities in six typical polyculture ponds and primarily explored the influence of total nitrogen and phosphorus on the bacterial species and diversity. In almost all sediment samples, Proteobacteria, Chloroflexi, and Bacteroides were the dominant species at the phylum level, and the five most abundant bacterial genera were Sulfurovum, Woeseia, Ilumatobacter, Robiginitalea, and Cyanobium_PCC-6307. A clear different bacterial community was observed with the most dominant bacterial phylum Firmicutes and the lowest bacterial diversity in TZ1 pond sediment; meanwhile, the TZ1 pond also showed the highest TN and TP concentrations. Notably, sediments from WZ1 and WZ2 ponds in low-latitude regions exhibited higher bacterial richness and diversity. Based on Pearson’s correlation analysis, bacterial α-diversity indices showed significant negative correlation with sediment TP content, and TN content contributed the most to the abundance of sediment dominant bacterial genus, indicating that the bacterial community is highly associated with sediment nutrient concentrations. Moreover, co-occurrence network analysis further revealed some keystone taxa that exhibited high correlations with other bacterial species, especially the high-abundance genus Robiginitalea bridging a large number of connections. Compared to traditional mono-mariculture pattern, our study provided direct evidence of lower nutrient loadings and different bacterial communities in the polyculture ponds. This could assist polyculture practitioners in developing effective strategies for detailed nutritional management.

Linking centennial scale anthropogenic changes and sedimentary records as lessons for urban coastal management

Coastal eutrophication and urban flooding are increasingly important components of global change. Although increased seawater renewal by barrier openings and channelizing are common mitigation measures in coastal lagoons worldwide, their effects on these ecosystems are not fully understood. Here, we evaluated the relationships between human interventions in the watershed, artificial connections to the sea, and the sediment burial rates in an urban coastal lagoon (Maricá lagoon, Southeastern Brazil). Sediment accretion along with nutrient and carbon burial rates were determined in two sediment cores representing the past ∼120 years (210Pb dating) and associated with anthropogenic changes as indicated by historical records and geoinformation analyses. Lagoon infilling and eutrophication, expressed by the average sediment accretion, TP, TN, and OC burial rates, respectively, increased ∼9–18, 13–15, 11–14 and 11–12-fold from the earliest (<1950) to the most recent (2000–2017) period. These multi-proxy records confirm mechanistic links between deforestation, urbanization, and untreated sewage discharges. In addition, our findings reveal artificial connections to the sea may contribute to lagoonal eutrophication and infilling, particularly when not integrated with sewage treatment and forest conservation or reforestation in the watershed. Therefore, increased seawater renewal by physical interventions commonly considered as mitigation measures may in contrast cause severe degradation in coastal lagoons, causing harmful consequences that should be not neglected when implementing management practices.

Variability of sedimentary phosphorus composition across Canadian lakes

Phosphorus (P) in lake sediments is stored within diverse forms, often associated with metals, minerals, and organic matter. Sediment P can be remobilized to the water column, but the environmental conditions influencing the P retention-release balance depend upon the sediment chemistry and forms of P present. Sequential fractionation approaches can be used to help understand forms of P present in the sediments, and their vulnerability to release. We assessed P composition in surficial sediments (as an assemblage of six P-fractions) and its relationship with watershed, and lake-specific explanatory variables from 236 lakes across Canada. Sediment P composition varied widely across the 12 sampled Canadian ecozones. The dominant P-fractions were the residual-P and the labile organic P, while the loosely bound P corresponded to the smallest proportion of sediment TP. Notable contrasts in sediment P composition were apparent across select regions – with the most significant differences between sediment P in lakes from the mid-West Canada region (Prairies and Boreal Plains ecozones) and both Eastern coastal (Atlantic Maritime and Atlantic Highlands) and Western coastal (Pacific Maritime) ecozones. The ecozone attributes most critical to sediment P speciation across Canadian lakes were related to soil types in the watershed (e.g., podzols, chernozems, and Luvisols) and the chemical composition of lake water and sediments, such as dissolved Ca in lake water, bulk sedimentary Ca, Al, and Fe, dissolved SO4 in lake water, lake pH, and salinity. Understanding predictors of the forms of P stored in surficial sediments helps advance our knowledge of in-lake P retention and remobilization processes across the millions of unstudied lakes and can help our understanding of controls on internal P loading.

Sediment Pollution and Dredging Effect of Waiqinhuai River

The Waiqinhuai River is an important urban landscape flood channel in Nanjing, which has been seriously polluted by industrial and domestic sewage for many years. To fully understand the characteristics of Waiqinhuai river sediment pollution and provide a decision-making basis for dredging, the sediment interface microenvironment and nutrient content of river sediment collected from method-typical sections in the upper, middle, and lower reaches were determined, and the organic index and pollution index methods were used to evaluate the sediment pollution condition. We also simulated the effect of desilting on the reduction of endogenous release in sediment according to pollutant characteristics of vertical distribution. The results showed that the average dissolved oxygen concentrations at the upper, middle, and lower sediment interfaces were 4.62, 3.25, and 3.41 mg·L-1, respectively; the concentrations were exhausted at 4.4, 3.5, and 5.5 mm, respectively, which were typical characteristics of urban river pollution. The average contents of TN, TP, and OM in the surface sediment of the investigated reach were 1734 mg·kg-1, 1337 mg·kg-1, and 4.82%, respectively. The organic pollution index of TN and OM in the sediment was 0.48 on average, which was at the clean level, whereas the individual pollution index of TP was 3.18 on average, which was at the severe pollution level. The results of simulating the dredging depth of 30 cm showed that the release rates of SRP and iron divalent were reduced by 42%-82% and 88%-96%, respectively, whereas the release rate of ammonia nitrogen was increased. The results of DGT determination and phosphorus speciation analysis showed that the phosphorus activity of surface sediment decreased significantly after desilting, and DGT-P and Mobile-P decreased by 9%-11% and 1%-35%, respectively, compared with those in the control treatment. These results indicated that the sediment of Waiqinhuai River was seriously polluted by endogenous phosphorus, and desilting could reduce the release of endogenous phosphorus to a certain extent, which may be an important method for improving the water quality of the Waiqinhuai River.

CAPABILITY OF SEA CUCUMBER Holothuria scabra TO REMOVE NITROGEN AND PHOSPHOR WASTE FROM SHRIMP PONDS CULTURE

Solid organic waste (PSW) in shrimp ponds contains relatively high levels of nitrogen and phosphorus and can endanger the ecological balance of the waters. This study evaluates the ability of sea cucumber Holothuria scabra to remove nitrogen and phosphorus loads from shrimp pond sediment waste in water and sediment. Sea cucumbers were reared for 40 days with a density of 20 individuals/m2 (average body weight 2.65±0.09 g) and a double-bottom recirculation system. Five levels of PSW accumulation were inserted into the aquarium substrate and were the sole source of nutrition for sea cucumbers without additional feeding: 10%, 20%, 30%, 40%, and 50% (with three replications). The results showed that increasing the PSW content in the aquarium research substrate significantly increased the substrate's TOC, TN, and TP content and increased the concentrations of TOM, DOC, NH3, NO2, and PO4 in the water column. The activity of sea cucumbers in utilizing PSW nutrients in all treatments up to a PSW level of 50% significantly reduced TOC, TN, and TP in sediments. This activity also substantially removes the concentration of TOM, DOC, NH3, NO2, and PO4 in the water. It is estimated that every kilogram of H. scabra can remove up to 12.65-12.73 g of nitrogen/day and 2.57-2.60 g of phosphorus/day contained in the solid organic waste of shrimp ponds. Therefore, this study concluded that H. scabra has great potential to be used as integrated multi-trophic aquaculture (IMTA) species, especially to remove nitrogen and phosphorus loads from shrimp pond sediment waste in waters.

Ecological and environmental risks of heavy metals in sediments in Dingzi Bay, South Yellow Sea

As a special geographical location between rivers and oceans, coastal estuaries always face severe heavy metal contaminations, especially in semi-closed bay. In this study, the spatial distribution, chemical fraction, ecological risks, and potential sources of heavy metals (Pb, Cr, Cu, As, Cd, Zn, and Ni) in surface sediments and sediment cores were investigated in Dingzi Bay, Shandong Peninsula. The Igeo values and modified potential ecological risk index (MRI) indicated that Cd and As presented high environmental risks in the surface and sediment cores. The high concentration sites were mainly located in the middle and the mouth of the Dingzi Bay. The source identification indicated that most heavy metals in surface sediments originated from shipping and aquaculture, while As and Ni from industrial pollution. The correlation coefficients showed that high proportion of fine particle, TN, TOC, TP, and AVS in surface sediments could significantly elevate the bioavailability of most heavy metals.

Long-term study of ecological restoration in a typical shallow urban lake

We investigated the long-term effects (6 years) of sediment improvement and submerged plant restoration of a subtropical shallow urban lake, Hangzhou West Lake China. To reveal the lake ecosystems variations, we analyzed the sediment properties, submerged macrophyte characteristics, sediment microorganisms, and benthic macroinvertebrate communities from 2015 to 2020. The ecological restoration project decreased sediment TP and OM, increased submerged macrophyte biomass and sediment microbial diversity, and improved the benthic macroinvertebrate communities in the restored area. The sediment TP decreased from 2.94 mg/g in 2015 to 1.33 mg/g in 2020. The sediment OM of the restored area decreased from 27.44 % in 2015 to 8.08 % in 2020. Principal component analysis (PCA) confirmed that the restoration improved the sediment conditions, making it suitable for the growth of submerged macrophytes, and then sped up the restoration and reconstruction of the lake ecosystem. These results have significant implications on the ecological management of shallow lakes.

Research On the Characteristics of Sediment and the Release Law of Nitrogen and Phosphorus Pollutants in Landscape Lake

Under the premise that the external input of nitrogen and phosphorus is controlled, the endogenous release has become the dominant factor of lake eutrophication. In order to understand the degree of eutrophication of the landscape lake and the release of nitrogen and phosphorus in the sediment, a sediment lake in Jinan was used as a test object,three sampling points were set near the inlet, outlet and reed bushes of the landscape lake to collect water samples. And sediment samples, the three indicators of TN, NH4+-N and TP in the water body and TN, NH4+-N, TP, pH, DW, LOI in the sediment were analyzed. Release of TN, NH4+-N and TP in sediments under different temperature, pH, dissolved oxygen (DO), and disturbance conditions. The results show that both elevated temperature and low DO can accelerate the release of nitrogen and phosphorus in the sediment, while high DO can inhibit the release of N and P in the sediment; acidic conditions are favorable for the release of TN and NH4+-N, and alkaline conditions are favorable for TP release; disturbance can accelerate the release of nitrogen in the sediment, but has little effect on total phosphorus release.

Spatial–Temporal Distribution of Phosphorus Fractions and Their Relationship in Water–Sediment Phases in the Tuojiang River, China

Understanding the distribution and transportation of phosphorus is vital for the sustainable development of aquatic environmental protection and ecological security. The spatial–temporal distribution of phosphorus fractions in water–sediment phases in river systems and their relationships remain unclear in Southwest China. In this study, the spatial and temporal distribution of phosphorus fractions in water–sediment phases in the Tuojiang River, a primary tributary of the Yangtze River which plays important roles in the economy and ecology of the region, and the relationships among the different phosphorus fractions were analyzed. These fractions were soluble reactive phosphorus (SRP), dissolved organic phosphorus (DOP), and total particulate phosphorus (PP) fractions in water, and exchangeable phosphorus (Ex-P), organic phosphorus (Or-P), phosphorus bound by Fe oxides (Fe-P), authigenic phosphorus (Ca-P), detrital phosphorus (De-P), and refractory phosphorus (Res-P) fractions in sediment. The SPR and Fe-P were the dominant phosphorus fractions in the water and sediment, respectively. The TP content was greater in the lower reaches than in the middle and upper reaches. The average abundances of most phosphorus fractions in water–sediment phases showed significant seasonal variations. The Fe-P, Org-P, and TP in sediments were released to the water interface, resulting in the increase of phosphorus in the overlying water. Redundancy analysis (RDA) indicated that the physicochemical properties of water and sediment have a certain influence on the spatial–temporal distribution of the phosphorus fractions. Principal component analysis (PCA) revealed that the main phosphorus source was anthropogenic activities. These results provide primary data regarding phosphorus fractions and contribute to understanding phosphorus cycling and controlling phosphorus pollution in the Tuojiang River.

Assessment of eutrophication and nitrogen and phosphorus carrying capacity before and after removing pen culture (2013-2018) in Lake Changhu, China.

In this study, the eutrophication levels and nitrogen and phosphorus carrying capacities of Lake Changhu in Jingzhou, Hubei Province, China, were measured using the trophic level index (TLI) and Dillon model for the first time. The measurements were taken before (2013 and 2015) and after (2017 and 2018) the removal of pen aquaculture from the lake. The lake was divided into three districts: Lake Haizihu, Mahongtai Channel, and Lake Dahu. The results showed total nitrogen (TN), chemical oxygen demand (COD), chlorophyll a (Chl-a), and total suspended solid values were significantly higher in 2017 than in the other years. The Lake Haizihu district was predicted to be more seriously polluted than the other districts. In the sediment, the organic matter, STN (TN in sediment) and STP (TP in sediment) contents increased from 2013 to 2018. The mean TLI values ranged from 62.99 to 78.93 in the studied years, and the eutrophication level was highest in 2017. According to the Dillon model, when the target water quality was level III (GB 3838-2002, Ministry of Environmental Protection of China, 2002), the remaining TN and TP loading capacities were -1470.72 t/a and -182.74 t/a, respectively, in 2015, and 320.03 t/a and -111.14 t/a, respectively, in 2018. Our results provide valuable and integrated information about the water conditions of Lake Changhu, thus laying a foundation for the theoretical study of water eutrophication process in lakes and paving the way for informed decision-making for managing water environments to ensure the safety of ecology.

Bacterial Communities and Enzymatic Activities in Sediments of Long-Term Fish and Crab Aquaculture Ponds.

Aquaculture is among the most important and fastest growing agriculture sectors worldwide; however, it generates environmental impacts by introducing nutrient accumulations in ponds, which are possibly different and further result in complex biological processes in the sediments based on diverse farming practices. In this study, we investigated the effects of long-term farming practices of representative aquatic animals dominated by grass carp (GC, Ctenopharyngodon idella) or Chinese mitten crab (CMC, Eriocheir sinensis) on the bacterial community and enzyme activity of sediments from more than 15 years of aquaculture ponds, and the differences associated with sediment properties were explored in the two farming practices. Compared to CMC ponds, GC ponds had lower contents of TC, TN, and TP in sediments, and similar trends for sediment pH and moisture content. Sediment bacterial communities were significantly different between GC and CMC ponds, with higher bacterial richness and diversity in GC ponds. The bacterial communities among the pond sediments were closely associated with sediment pH, TC, and TN. Additionally, the results showed profoundly lower activities of β-1,4-glucosidase, leucine aminopeptidase, and phosphatase in the sediments of GC ponds than CMC ponds. Pearson’s correlation analysis further revealed strong positive correlations between the hydrolytic enzyme activities and nutrient concentrations among the aquaculture ponds, indicating microbial enzyme regulation response to sediment nutrient dynamics. Our study herein reveals that farming practices of fish and crab differently affect bacterial communities and enzymatic activities in pond sediments, suggesting nutrient-driven sediment biological processes in aquaculture ponds for different farming practices.

Growth of Zostera japonica in different sediment habitats of the Yellow River estuary in China.

The estuarine delta of the Yellow River is a region of strong land-ocean-human interactions that undergoes a unique evolutionary process. The delta is formed by deposition of large quantities of sediments carried by the Yellow River, especially during the annual water and sediment regulation period; more than one-third of the total annual sediments is deposited to the estuary area. The seagrass Zostera japonica is located at the forefront of the Yellow River delta. To study the impact of the different sediment environments on the Z. japonica growth, its growth and water quality and sediment parameters were measured on the northern and southern sides of the estuary from April to October in 2019. The action of wind and tides have re-suspended and dispersed sediments over time, producing shores on the southern delta characterized by nutrient-enriched clays and shores on the northern delta characterized by coarser sands and silts with poor nutrients. During the monitoring period, the concentrations of TC, TN, and TP in the root-zone sediments at the southern site were 1.56%, 0.04%, and 0.06%, respectively, whereas they were 0.69%, 0.007%, and 0.06%, respectively, at the northern site. Sufficient nutrients supported the growth of Z. japonica at the southern site, while poor nutrition limited the continuous growth of Z. japonica at the northern site. In July, the plant height, biomass, and shoot density of Z. japonica at the southern site reached the maximum values of 23.6 cm, 0.14 g/shoot, and 3245 shoots/m2, respectively, whereas they were 16.4 cm, 0.06 g/shoot, and 2740 shoots/m2, respectively, at the northern site. The sediment grain size and their nutrients contributed to different growth patterns of Z. japonica at the southern and northern sites. Our research could provide important implication for the conservation of Z. japonica habitats in the Yellow River estuary in China.

The Relationship of Different COD Concentration, Sediment Pollutant Content with Hydrodynamic in Black Bloom Water Based on Stepwise Regression Method

To analyze the relationship of different COD concentration, sediment pollutants with hydrodynamic. Taking a polluted river as the research object, the data of flow velocity, water temperature, COD concentration, sediment OM content, sediment TN content and sediment TP content at nine sampling points were analyzed by stepwise regression and SPSS 25 data analysis software. Construct linear regression model of COD concentration Y, river flow velocity X1, water temperature X2, sediment OM content X3, sediment TN content X4, sediment TP content X5. The regression equation Y=106.172-64.304X4 + 0.065X5 with correlation coefficient R=0.954 and determination coefficient R2=0.91 was obtained by stepwise regression method. The results showed that the concentration of COD in water was correlated with the content of nitrogen and phosphorus in river sediments, and negatively correlated with the content of TN and positively correlated with the content of TP. The model can be used to estimate COD load of water body by analyzing the nitrogen and phosphorus content in river sediments.

Occurrence of total phosphorus in surface sediments of Chinese lakes and its driving factors and implications

In contrast to instantaneous concentration of phosphorus in water columns, sediments record valuable information on occurrence and sources of phosphorus for a long period associated with human activities and environmental protection. However, much less attention has been paid to phosphorus in sediments. To date the relative contributions of driving factors for TP concentration in sediments has not been elucidated. We investigated the occurrence and driving factors for TP in 270 surface sediments of 83 lakes in the catchments of two large rivers (Yangtze River and Huaihe River) in China, identified the relative contributions of the driving factors, and discussed the implication. The average TP concentration in the surface sediments of these lakes was 0.82 ± 0.04 mg gdw−1. The average TP concentration in the surface sediments of the urban lakes was larger than that in the rural lakes. Mitigation measures, human activities, and lake characteristics contributed 59.0%, 20.5%, and 20.5% to the variation in sediment TP concentration. The average TP concentration in the surface sediments significantly decreased with the percentage of annual investment in pollution control to GDP of the cities where the lakes are located. For the first time we find mitigation measures have exceeded human activities, and lake characteristics to dominate the variation in TP concentration in surface sediments of Chinese lakes. Given the annual investment in pollution control only accounted for 1.14% ± 0.06% of GDP in China from 1991 to 2018, higher percentage would be beneficial to reduce TP concentration in sediments of Chinese lakes.

Distribution and Bioavability of Phosphorus in Surface Sediments in Megalopolis: A Case Study of Chengdu

A total of 33 surface sediments were collected from rivers in Chengdu. The content of phosphorus species was measured with the chemical continuous extraction method (SMT) and in situ monitoring techniques (DGT). Multivariate statistical analysis was used to analyze the spatial distribution of phosphorus species in sediments. The release flux of DGT-P in sediments and their pore water was calculated in this study. It is helpful to understand the influence of sediment endogenous pollution and human activities on the environment. The results show that the phosphorus species have a spatial variability. The average content of TP in the surface sediments is 1132.41 mg·kg-1, which is higher than the background value of 365.00 mg·kg-1 in Chengdu. The Ca-P is the most dominant species, accounting for 70.58% of the TP on average. The study area is divided into three groups based on spatial clustering. Groups 1 and 3 show large differences of phosphorus morphological structures, while Group 2 is small. The contents of phosphorus in the surface sediment of Groups 1 and 3 are generally higher than those of Group 2. The DGT-P concentration has a good correlation with the soluble DTP concentration, bioavailable Fe/Al-P, and OP concentration, respectively. The DGT technology can be used as a fast, in situ, reliable method for measurements of the bioavailable content of sediments. The higher release fluxes of bioavailable phosphorus are N8, W11, and N2, which are 20.05, 17.13, and 14.79 mg·(m2·d)-1 respectively. The available phosphorus release capacity is closely related to human activities.

Reclamation Culture Alters Sediment Phosphorus Speciation and Ecological Risk in Coastal Zone of Southeastern China

Sediment phosphorus (P) speciation plays a critical role in coastal wetland ecosystems, while the response of P speciation to reclamation culture remains unclear. In this study, sediment P species (total P [TP], organic P [Org‐P], inorganic P [IP], iron‐ and aluminum‐bound P [Fe/Al‐P], calcium‐bound P [Ca‐P], loosely adsorbed P [Lsor‐P], P sorption index [PSI], and P adsorption saturation [DPS] are investigated in the natural marsh wetland, 3‐year and 15‐year reclamation culture ponds. Sediment P pollution level and eutrophication risk are evaluated based on the single‐factor standard index (Si) and eutrophication risk index (ERI). Furthermore, a simulation experiment is conducted to reveal the effects of baits input and overlying water aeration on sediment P species and ecological risk. Measured sediment TP, Org‐P, IP, Fe/Al‐P, Ca‐P, and Lsor‐P concentrations are significantly higher in 15‐year culture ponds than in 3‐year culture ponds and natural marsh wetland. Sediment PSI is lower in reclamation culture ponds than in natural marsh wetland, while DPS is just opposite. The Si and ERI are higher in 15‐year culture ponds than in natural marsh wetland, suggesting that reclamation culture significantly increases sediment P pollution level and eutrophication risk. A short‐term simulation experiment further indicates that baits input and overlying water aeration can alter sediment P dynamics and increase sediment P loading. Overall, these results confirm that reclamation culture can make more available P accumulation and thus increase eutrophication risk of adjacent coastal water.

Spatial distribution of sediment nitrogen and phosphorus in Lake Taihu from a hydrodynamics-induced transport perspective

Hydrodynamics play an important role in sediment nutrient dynamics in large shallow eutrophic lakes. In this study, the spatial patterns of sediment nitrogen and phosphorus in Lake Taihu were compared from a hydrodynamics-induced transport perspective based on high-resolution investigation of sediment, field observations, numerical simulations and long-term ecological data analysis. The results showed that sediments were primarily distributed in the west and southeast portions of the lake. Additionally, the total nitrogen (TN) and phosphorus (TP) stored in the active sediments was 166,329 t and 67,112.4 t, respectively. The sediment TN content was 319.4–3123.8 mg kg−1, with high content areas being primarily located in the Zhushan, Meiliang and East Taihu bays. The external nitrogen-containing nutrients in the overlying water, which is mostly dissolved nitrogen, can be horizontally transported by lake currents to the water areas with high biomass levels and weak vertical hydrodynamic disturbance where sediment nitrogen enrichment primarily occurs via bio-deposition. The sediment TP content ranged between 382.6 and 1314.1 mg kg−1, and the high content areas were primarily distributed near the inflowing river mouths. Sediment phosphorus enrichment primarily occurred via physical and chemical deposition. Surface waves caused vertical phosphorus transport from sediments to the overlying water but had a limited effect on its spatial distribution. Although the horizontal transport of phosphorus was found to be weaker than that of nitrogen, short-distance vertical transport of sediment phosphorus may relieve nutrient limitations, leading to maintenance of cyanobacterial blooms found in Lake Taihu.

Phosphorus Forms and Distribution Characteristics in the Sediment and Soil of the Water-Level-fluctuating Zone in the Main Stream of the Three Gorges Reservoir

Hydrological regimes changed after the Three Gorges Reservoir (TGR) became fully operational, with the water level fluctuating between 145 m and 175 m. This has altered phosphorus (P) distribution within sediments and soils of the water-level-fluctuating zone (WLFZ). Eleven field investigations within the main stream were carried out in June 2016. The aim of the study was to determine lateral (sediment and WLFZ soil) and longitudinal variations (from the end of the backwater area to the Three Gorges Dam) of phosphorus species. P fractions were analyzed using a Standards, Measurements, and Testing (SMT) protocol. Results showed that both TP and NaOH-P content in sediment exhibited a clear increasing trend from the end of the backwater area to the Three Gorges Dam, while HCl-P declined. Average of totals P, OP (organic P), HCl-extracted P (HCl-P, calcium-bond P), and NaOH-extracted P (NaOH-P, metal oxide-bound P) in sediment were (859.6±106.8), (224.6±113.9), (435.3±77.7), and (101.5±31.6) mg·kg-1, respectively. The concentration average of P species in sediment was higher than in WLFZ soil. In both sediment and soil, HCl-P was the main form of P, accounting for 51.3% and 58.2% of TP, while the ratio of NaOH-P to TP was 11.7% and 8.1%, respectively. P fractions with a higher coefficient of variation had greater spatial heterogeneity.

Temporal and Spatial Changes in Sediment Nutrients and Heavy Metals of the Danjiangkou Reservoir Before and After Water Division of the Mid-route Project

Reservoir/lake sediments are potential pollutant acceptors and sources of pollution. The Danjiangkou Reservoir, as the largest drinking water source in China, is the source water area for the Mid-route Project of the South-North Water Diversion. To illustrate the temporal and spatial changes in the environmental risks of both nutrients and heavy metals in the surface sediments of Danjiangkou Reservoir, the levels of pH, OM, TP, TN, and six kinds of metal elements during the period 2011-2016 were measured at five ecological sites. Canadian freshwater sediment quality guidelines for aquatic environments (from the CCME) were used to assess the potential ecological risks. The results showed that the contents of OM, TP, and TN in the surface sediments were 25.85, 0.57, and 1.34 g·kg-1, respectively, and those of Cr, Mn, Cu, Zn, Cd, and Pb were 57.96, 521.78, 13.91, 195.74, 0.37, and 12.92 mg·kg-1, respectively. The contents of both Zn and Cd were under the corresponding Ⅱ-level standards of soil environmental quality, but the contents of the other four heavy metals were lower than their corresponding I-level standards. After water diversion of the Mid-route Project, the OM, TN, Mn, and Zn contents increased, and the TP and other heavy metal contents displayed downward trends. Excluding the pH value, the distribution of measured nutrients and heavy metals in the surface sediments of the Danjiangkou Reservoir reached the extremely significant level (P<0.01). OM in the sediments was negatively correlated with TN and Pb, but had positive correlations with the other indicators measured. The results from the potential ecological risk assessment showed that:①Cd, Cu, and Pb had no harmful effects on organisms; ② OM had low ecological risk in most years; ③ the risk of TP was between zero and low ecological risk; and ④Cr, Zn, and TN had low ecological risk in most cases. The K and H sites had higher ecological risk for Cr, Zn, and TN, thus attention should be paid to those cases. These results illustrate the effects of human activities on nutrients and heavy metals in surface sediments.