The distribution and seasonal variations of sedimentary organic matter in the East China Sea shelf

Variations of sediment organic phosphorus and organic carbon during the outbreak and decline of algal blooms in Lake Taihu, China

In this study, sediment organic phosphorus (OP) and organic carbon (OC) in Lake Taihu, China, as well as their relationships, were analyzed during the outbreak and decline of algal blooms (ABs) over a five-month field study. The results showed synchronous temporal changes in the sediment OP and OC contents with the development of ABs. In addition, there was a significant positive correlation between the sediment OP and OC (p < 0.01), suggesting simultaneous deposition and consumption during the ABs outbreak. The sediment OP and OC contents decreased significantly at the early and last stages of the ABs outbreak and increased at the peak of the ABs outbreak and during the ABs decline. These temporal variation patterns suggest that the sediment OC and OP contents did not consistently increase during the ABs outbreak, even though algae are an important source of organic matter in sediments. The depletion or enrichment of OC and OP in sediments may also depend on the scale of the ABs outbreak. The obtained results revealed significant differences in the sediment OC and OP contents between the months (p < 0.05). In addition, OP in the sediments was dominated by orthophosphate diester (phospholipids and DNA-P) and orthophosphate monoester during the ABs outbreak and decline, respectively. The active OC contents and proportions in the sediments in the ABs outbreak were significantly lower than those observed in the ABs decline period, demonstrating the significant impacts of the ABs outbreak and decline on the sediment OC and OP in Lake Taihu.

Effects of intensive oyster farming on nitrogen speciation in surface sediments from a typical subtropical mariculture bay

The spatial-temporal distributions of various nitrogen (N) species in surface sediments were examined in a typical subtropical mariculture bay (Maowei Sea) in the northern Beibu Gulf to assess the impact of intensive oyster culture activities on sedimentary N speciation. The results indicated that the mean contents of total nitrogen (TN), extractable (labile) nitrogen (LN) and residual nitrogen (RN) in the surface sediments were 33.3 ± 15.5 μmol g-1, 13.8 ± 1.3 μmol g-1 and 19.5 ± 15.0 μmol g-1, respectively, which lacked significant seasonal variability (P > 0.05). Four forms of LN, namely ion extractable form (IEF-N), weak acid extractable form (WAEF-N), strong alkali extractable form (SAEF-N) and strong oxidant extractable form (SOEF-N) were identified based on sequential extraction. SOEF-N was the dominant form of LN, accounting for 67.8 ± 2.5 % and 63.7 ± 5.9 % in summer and winter, respectively. Spatially, the contents of sedimentary TN, LN, RN, WAEF-N and SOEF-N in intensive mariculture areas (IMA) were significantly higher than those in non-intensive mariculture areas (NIMA) during summer (P < 0.05). Stable nitrogen isotope (δ15N) mixing model revealed that shellfish biodeposition was the predominant source of sedimentary TN in IMA with a contribution of 67.8 ± 23.0 %, approximately 5.4 times that of NIMA (12.6 ± 3.3 %). Significant positive correlations (P < 0.05) were observed between most forms of N species (WAEF-N, SOEF-N, LN and RN) and shellfish-biodeposited N in the surface sediments during summer, indicating that intensive oyster farming greatly enhanced sedimentary TN accumulation.

Amino acids as indicators of seasonal variations in organic matter degradation in surface sediments from a shallow lake

Degradation of organic matter (OM) in sediments is a key link in nutrient cycling and sedimentation processes in lakes. The aim of this study was to explore the degradation of OM in surface sediments of a shallow lake (Baiyangdian Lake, China) under seasonal temperature variations. For this, we used the amino acid-based degradation index (DI) and the spatiotemporal distribution characteristics and sources of OM. Sediment OM in the lake mainly originated from freshwater aquatic plants and terrestrial C4 plants. The sediment at some sampling sites was affected by surrounding crops. The organic carbon and total nitrogen contents, and the total hydrolyzed amino acid concentrations in the sediments were highest in summer and lowest in winter. The lowest DI occurred in spring, which indicated that the OM in the surface sediment at this time was highly degraded and relatively stable, and the highest DI occurred in winter, which showed that the sediment was fresh. The water temperature was positively correlated with the organic carbon content (p < 0.01) and total hydrolyzed amino acids concentration (p < 0.05). Seasonal variations in the overlying water temperature had a large effect on OM degradation in the lake sediments. Our results will facilitate the management and restoration of lake sediments that suffer from endogenous release of OM in a warming climate.

Pollution characteristics of typical ARGs in the sediments of the sea area adjacent to the Yangtze Estuary, China

The pollution characteristics of the typical antibiotic resistance genes (ARGs) in the sediments of the sea area adjacent to the Yangtze Estuary, China were investigated with both seasonal and spatial insights. The positive rates of sulfonamides sul1, sul2, tetracycline tetW and quinolone gyrA resistance genes in the sediments of all sampling sites were 100%, indicating that the sea area adjacent to the Yangtze Estuary were extensively polluted by these ARGs. Occupying a dominance in most sampling sites, sul1 was found to be the dominant resistance gene in the sediments of the sea area adjacent to the Yangtze Estuary. In terms of seasonal variation characteristics, the absolute abundances of the ARGs in flood season were higher than those in dry season. In terms of spatial distribution characteristics, the absolute abundances of the ARGs in the southern sites were mostly higher than those in the northern sites, and the high abundance sites were mostly located near the coast. Different ARGs also showed different spatial distribution characteristics. The disclosure of the pollution characteristics of the typical ARGs in the sediments of the sea area adjacent to the Yangtze Estuary can provide valued information for protecting the environment of this area as well as the Yangtze River Basin.

Contaminant occurrence, spatiotemporal variation, and ecological risk of organophosphorus flame retardants (OPFRs) in Hangzhou Bay and east China sea ecosystem

Occurrence, spatiotemporal variation, sources, and ecological risks of 20 organophosphorus flame retardants (OPFRs) in water and sediments from Hangzhou Bay (HZB) and its adjacent East China Sea (ECS) were investigated in this study. The concentrations of OPFRs (∑OPFR) in water ranged from 0.51 ng/L to 885 ng/L, with chlorinated OPFRs having the highest value. For sediments, ∑OPFR ranged from 2.93 ng/g, dry weight (dw) to 37.8 ng/g, dw. The ∑OPFR in the water and sediments of HZB in summer was significantly (p < 0.05) higher than that in autumn. Additionally, the pollution of OPFRs in HZB was higher than that in ECS, and the high-concentration areas appeared in the north and south banks of HZB and near the coast of ECS. Principal component analysis-multiple linear regression showed that the OPFRs in this region were mainly from industrial products (e.g., polyurethane foam/paint/coating/textiles/product processing). In terms of aquatic environments, ecological risks were in a low (∑RQs<0.1) to moderate (0.1<∑RQs<1) level, with regard to median exposure levels, a moderate risk (0.1<∑RQs<1) was found in the sediments during autumn. This study can provide new insights into the OPFR pollution characteristic and ecological risk in a specific eco-environment.

Spatial distribution, compositional pattern and source apportionment of n-alkanes in surface sediments of the Bohai Sea, Yellow Sea, and East China Sea and implications of carbon sink

China's marginal seas (CMSs, including the Bohai Sea, Yellow Sea, East China Sea) are a significant sink for both terrestrial organic matter (OM) and marine OM, and they play an important role in the global biogeochemical carbon cycle. The spatial distribution and origin of organic matter based on n-alkanes in the surface sediments of CMSs and the implications of carbon sinks were comparatively analyzed. The n-alkane content in surface sediment from the Bohai Sea was higher than that of the Yellow Sea and the East China Sea. The spatial distribution trends of marine and terrestrial organic matter are obviously different in the surface sediments of China's marginal seas. The n-alkanes in the sediments of the Yellow Sea and the East China Sea were mainly derived from terrestrial higher plants, and land-based influence gradually decreased from the near shore to the open sea. Higher concentration of terrigenous OM are concentrated nearshore, especially near estuaries, such as the Yellow River Estuary, the Old Yellow River Estuary and the Yangtze River Estuary. The input of n-alkanes from woody plants in the Bohai Sea area was slightly higher than that of herbaceous plants, and the input of herbaceous plants in the Yellow Sea and East China Sea was slightly dominant. The distribution of marine organic matter is controlled by marine productivity and the sedimentary environment. Due to climate change, the decomposition and enrichment of organic matter also show the climate effect of carbon molecular combinations. As a semiclosed sea area, the Bohai Sea was beneficial to the growth and reproduction of marine phytoplankton. From the perspective of petroleum pollution, the Bohai Sea was the most serious, followed by the Yellow Sea, and the East China Sea was the lightest. The carbon burial amount of terrestrial organic matter accounts for approximately 7% of the terrestrial organic matter burial amount of global marginal sea sediments, indicating that China's marginal sea plays an important role in the global carbon cycle. The result provide a basis for further understanding the source pattern and burial preservation of sedimentary organic matter in this sea area.

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.

Occurrence and toxicity of perfluoroalkyl acids along the estuarine and coastal regions under varied environmental factors

Due to the significant economic and ecological value, the increasing pollution threat to estuarine and coastal regions is of great concern. Perfluoroalkyl acids (PFAAs) are emerging pollutants which possess adverse ecological risk. In this review, we have compiled the data on the levels of PFAAs in environmental samples, mainly in estuarine and coastal zones. A worldwide map was generated to show the distribution of PFAAs. The experimental results have also been considered, which, together with those of environmental samples, has allowed us to infer about the factors that intervene in the behavior of PFAAs. The presence of PFAAs is determined primarily by the source of pollution. Salinity is as well shown as a significant condition, dependent too on the sampling environment. The analysis of PFAAs from environmental samples constitutes a fundamental tool for the surveillance of these pollutants, but the lack of homogeneity of protocols for sampling, as well as for the results presentation, limits the comparative capacity. Laboratory studies are also an essential tool in the analysis of particular aspects related to PFAAs, but many times the conditions tested are not environmentally significant. In this way, it would not be prudent to establish "paradigms" about the behavior of the PFAAs in certain areas or organisms, instead to suggest the points that can be considered fundamental for each issue addressed. The main variables that appear to intervene in estuarine and coastal regions are mainly the proximity to the source of pollution, salinity, pH, precipitation (rain) as well as types of PFAAs. All these can synergistically lead to different impacts on the ecosystem. Therefore, the particular risks of PFAAs in estuarine and coastal regions is a set of multiple variables, dependent on each sampling condition and according to the previously named parameters.

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.

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.

Evolution of sedimentary organic matter in a small river estuary after the typhoon process: A case study of Quanzhou Bay

Extreme weather events occur frequently under global warming scenarios and have an important impact on the global carbon cycle. Compared to large rivers, small rivers are more sensitive to extreme weather events (such as typhoons). This paper reports the results of a study carried out in the Quanzhou Bay to explore the evolution of small river estuarine sedimentary organic matter after typhoon process using measurements of the grain-size, total organic carbon (TOC), total nitrogen (TN) and δ¹³C of surface sediment samples collected 2-3 days and a month, respectively, after typhoon Matmo landing in 2014. The results show that the contents of TOC and TN in the sediments, which gradually decrease from the estuary to the outer sea of Quanzhou Bay, decreased approximately 13% and 16%, respectively, a month later compared with 2-3 days after typhoon landing. The significant decrease occurred in the Jinjiang River estuary and along the South Channel of Quanzhou Bay, while the North Channel and Luoyangjiang River estuary retained high levels of TOC and TN. The results of δ¹³C values and TOC/TN ratios show that the organic matter in the sediment of the Quanzhou Bay was a mixture derived from C₃ terrestrial plants and marine algae. The terrestrial organic matter was mainly deposited in the Jinjiang River estuary 2-3 days after typhoon landing and then spread along the tidal channel to the outer sea a month later. It indicates that the hydrodynamic forces stirred sedimentary organic matters that were input and settled during typhoon, and transported later along the North and South Channel to the outer sea. Some of those organic matters were accumulated in the North Channel during the transport process. The results provide significant meaning for the carbon cycle and material flux study on the coastal and margin seas.