The benthic fluxes of nutrients and the potential influences of sediment on the eutrophication in Daya Bay, South China

Pore-water nutrient concentrations variability under different oxygen regimes: A case study in Elefsis Bay, Greece

Anthropogenic pressures considerably affect coastal areas, increasing nitrogen and phosphorous loads that lead to eutrophication. Eutrophication sometimes results in hypoxic and/or anoxic conditions near the bottom water. Dissolved oxygen (DO) concentrations influence redox-sensitive nutrients, which can alter the benthic flux of nutrients. We retrieved sediment cores from two sites in the eastern and western parts of Elefsis Bay, a semi-enclosed area of the Eastern Mediterranean, Greece, during winter and summer. In the western part, seasonally hypoxic or anoxic conditions occurred. We analysed pore-water samples under normoxic, hypoxic and anoxic bottom water conditions to study the pore-water nutrient concentrations variability under different oxygen regimes. Ex situ incubation experiments were conducted at the site experiencing oxygen deficiency by manipulating the DO concentrations. The pore-water nutrient concentrations showed higher variability at the site experiencing oxygen deficiency. Notably, elevated ammonium concentrations were observed in the pore water during anoxic conditions, in the 2-20-cm sediment layer. However, the benthic fluxes of ammonium and phosphate at the 0-2-cm sediment layer were comparable under hypoxic and anoxic conditions. The results of the incubation experiments demonstrate a direct decrease in nitrate concentrations as the DO concentrations diminished in the overlying water. The incubations after re-oxygenating the overlying water show that phosphate was more efficiently scavenged when anoxic conditions prevailed in the bottom water. The incubation experiments indicate the rapid response of the seafloor to oxygen availability, particularly concerning processes that influence nitrate and phosphate concentrations. These observations highlight the dynamic nature of nutrient cycling in shallow, seasonally anoxic environments, such as Elefsis Bay, and emphasise the sensitivity of the seafloor ecosystem to changes in bottom water oxygen availability.

Weekly variations of nutrients and their associations with phytoplankton blooms in the urban coastal waters of Andaman Sea coast: A case study in Patong Bay, Phuket, Thailand

Nutrient inputs to coastal waters are among the main contributors to phytoplankton blooms that can damage coastal ecosystems. To understand the main causal factors and timing of phytoplankton blooms in Patong Bay, where phytoplankton blooms have frequent occurred for the last decade, variations in phytoplankton abundance and the dissolved inorganic nutrients (nitrogen (DIN), phosphorus (DIP), and dissolved silica (DSi)) were monitored weekly from December 2021 to December 2022. The results revealed that ratios of DIP and DSi to DIN in seawater had increased rapidly in approximately 1-7 days prior to the blooms of Chaetoceros and Eunotogramma. This suggests that the diatom blooms in this area are significantly controlled by an excess of DIP and DSi, in otherwise appropriate environmental conditions. Our findings provide a thorough understanding of the role of excess nutrients on phytoplankton blooms in urban coastal waters, supporting informed coastal management actions.

Sources, burial flux and mass inventory of black carbon in surface sediments of the Daya Bay, a typical mariculture bay of China

The contents of chemothermal oxidation (CTO)-derived black carbon (BC) and organic carbon (OC) and their stable isotopes (δ¹³C_BC and δ¹³C_OC), including major elemental oxides, and grain sizes were measured to constrain the sources, burial flux, and mass inventory of BC in surface sediments of the Daya Bay. Surface sediments were mainly clayey silt (>90%) and contained 0.28-1.18% OC and 0.05-0.18% BC. Fossil fuel emission and physical erosion contributed to the sedimentary BC sources. High BC/OC ratio (6-30%), burial flux (154.88-922.67 μg cm^-2 y^-1), and mass inventory (22-34 Gg y^-1) of BC in the upper 5 cm of surface sediments indicated that the Daya Bay is a significant sink of BC. The high accumulation of BC in sediments is attributed to a strong affinity to fine-grained sediments due to the enrichment of muddy biodeposits excrements from the cultured species in the bay.

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.

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.

Fractions and mineralization potential of the sediment organic nitrogen in Daya Bay, South China Sea: Anthropogenic influence and ecological implications

Nitrogen mineralization is a critical biogeochemical process that transfers organic nitrogen into inorganic forms using heterotrophic microorganisms. However, few studies have focused on this potential nutrient supplier. In this study, the composition of sediment organic nitrogen (SON) was studied, and nitrogen mineralization flux entering the water column was quantified. The results indicate that acid-hydrolyzable nitrogen (AHN) accounts for more than 40% of the SON, especially in the riverine input and marine aquaculture areas, which had significantly higher concentrations than the bay mouth area. Similar results were found for the ammonium nitrogen (AN), amino-sugar nitrogen (ASN), the total hydrolyzable amino acid (THAA), and unidentified hydrolyzable nitrogen (HUN). The mineralization rate in the marine aquaculture area was as high as 9.03 ± 1.33 mg·kg^-1·d^-1, while those of the riverine input (4.77 ± 1.55 mg·kg^-1·d^-1) and bay mouth (5.12 ± 1.42 mg·kg^-1·d^-1) areas were lower. The SON fractions, including the AHN, AN, ASN, and AAN, could obviously affect the mineralization of the SON. However, the extracellular enzymes, including proteinase and urease, are the predominant factors controlling the SON mineralization process. Anthropogenic activities, including riverine input and marine aquaculture, exert significant influences on the fractions and mineralization of the SON, and thus, they may increase the amount of dissolved inorganic nitrogen in the bottom of the water column in Daya Bay.

Distribution of Geochemical Fractions of Phosphorus in Surface Sediment in Daya Bay, China

Surface sediment samples were collected from 19 sites throughout Daya Bay, China, to study the concentrations, and spatial distributions of different fractions of phosphorus through sequential extraction methods. Like many coastal and marine areas, De-P was the dominant form of P, contributing 47.5% of TP, followed by O-P, contributing 25.5% of TP. Ex-P and Fe-P contribute the lowest to TP. The concentration of sedimentary TP ranged from 290.3~525.1 µg/g, with the average of 395.3 µg/g, which was a similar range to other estuaries and coastal areas. Based on the spatial distribution, Pearson correlation and Principal component analysis, different fractions of phosphorus showed different spatial distributions due to different sources. The molar ratio of organic carbon to phosphorus (TOC/O-P) ranged from 199 to 609, with the average of 413, which was much higher than the Redfield ratio, suggesting terrestrial sources of organic matter in Daya Bay surface sediment. The average bioavailable phosphorus was 149.6 µg/g and contributed 37.8% (24.6~56.0%) of TP, indicating that the surface sediments of Day Bay act as an important internal source of P.