Extremely high sulfate reduction, sediment oxygen demand and benthic nutrient flux associated with a large-scale artificial dyke and its implication to benthic-pelagic coupling in the Yeongsan River estuary, Yellow Sea

Metagenomic insights into comparative study of nitrogen metabolic potential and microbial community between primitive and urban river sediments

As a result of anthropogenic pollution, the nitrogen nutrients load in urban rivers has increased, potentially raising the risk of river eutrophication. Here, we studied how anthropogenic impacts alter nitrogen metabolism in river sediments by comparing the metagenomic function of microbial communities between relatively primitive and human-disturbed sediments. The contents of organic matter (OM), total nitrogen (TN), NO₃^--N and NO₂^--N were higher in primitive site than in polluted sites, which might be due to vegetation density, sediment type, hydrology, etc. Whereas, NH₄⁺-N content was higher in midstream and downstream, indicating that nitrogen loading increased in the anthropogenic regions and subsequently leading higher NH₄⁺-N. Hierarchical cluster analyses revealed significant changes in the community structure and functional potential between the primitive and human-affected sites. Metagenomic analysis demonstrated that Demequina, Streptomyces, Rubrobacter and Dechloromonas were the predominant denitrifiers. Ardenticatena and Dechloromonas species were the most important contributors to dissimilatory nitrate reduction. Furthermore, anthropogenic pollution significantly increased their abundance, and resulting in a decrease in NO₃^-, NO₂^--N and an increase in NH₄⁺-N contents. Additionally, the SOX metabolism of Dechloromonas and Sulfuritalea may involve in the sulfur-dependent autotrophic denitrification process by coupling the conversion of thiosulfate to sulfate with the reduction of NO₃^--N to N₂. From pristine to anthropogenic pollution sediments, the major nitrifying bacteria harboring Hao transitioned from Nitrospira to Nitrosomonas. This study sheds light on the consequences of anthropogenic activities on nitrogen metabolism in river sediments, allowing for better management of nitrogen pollution and eutrophication in river.

C, N, and P Mass Balances in the Bottom Seawater–Surface Sediment Interface in the Reducing Environment due to Anoxic Water of Gamak Bay, Korea

Current mass balances of C, N, and P were estimated using a model (Fluxin = Fluxout + ΔFlux) from Gamak Bay, Korea, in August 2017, where eutrophication and reducing conditions are prevalent. To examine the current fluxes of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP), sinking and re-floating sediment traps were deployed, a sediment oxygen demand (SOD) chamber experiment and ex-situ nutrient incubation experiment were conducted, and Fick’s first law of diffusion was applied. The principal component analysis and cluster analysis were performed to identify the three groups of water masses based on the characteristics of the bay, including the effects of the reducing environment due to the anoxic water mass using 14 bottom water quality parameters. In the reducing environment (sampling point GA4), the SOD20 flux was 3047.2 mg O2/m2/d. Additionally, the net sinking POC flux was 861.0 mg C/m2/d, while 131.8% of the net sinking POC flux (1134.5 mg C/m2/d) was removed toward the overlying water. This indicates that the organic matter that had been deposited was decomposed as a flux of 273.6 mg C/m2/d. The net sinking PON flux was 187.9 mg N/m2/d, whereas 15.8% of the net sinking PON flux was eluted, and 84.2% remained in the surface sediments. The dissolved inorganic nitrogen (DIN) elution flux from the surface sediments consisted of NH4+ elution (33.7 mg N/m2/d) and NOx− elution (−4.1 mg N/m2/d) fluxes. Despite the net sinking POP flux being 26.0 mg P/m2/d, the 47.7 mg P/m2/d of DIP elution flux (179.5% of the net sinking POP flux) was eluted to the overlying water. Similar to C mass balance, the additional elution flux occurred. Therefore, severe eutrophication (16.5 of the Okaichi eutrophication index) with the lowest N:P ratio (2.6) in GA4 was noted. This indicates that not only the freshly exported organic matter to the surface sediments but also the biochemical processes under anoxic conditions played an essential role as a remarkable nutrient source–particularly P–for eutrophication in Gamak Bay, Korea.

Nutrient exchanges at the sediment-water interface and the responses to environmental changes in the Yellow Sea and East China Sea

Release from the sediment is an important nutrient source to the water column of global oceans, especially for marginal seas with active biogeochemical processes. Benthic nutrient biogeochemistry and its responses to environmental changes were investigated in the eastern marginal seas of China using a two-layer diffusion-advection-reaction diagenetic model. Overall, the sediment represented the primary nutrient source with fluxes of ~-342 ± 197, -1.25 ± 0.50, and -114 ± 56 × 10⁸ mol/month for dissolved inorganic nitrogen (DIN), phosphate, and silicate, respectively. This could contribute up to ~42% of nutrients requested by primary production (PP), with a DIN/SiO₃^2-/PO₄^3- molar ratio of 273:91:1, which was higher than that in the overlying water (49:47:1). Future benthic nutrient fluxes were predicted under two environmental change scenarios (increasing and decreasing PP and biogenic silica). Our study may help rebuild nutrient budgets in the future and formulate environmental management policies in marginal seas.

Impact of finfish aquaculture on biogeochemical processes in coastal ecosystems and elemental sulfur as a relevant proxy for assessing farming condition

We conducted experiments to investigate the effects of finfish aquaculture and to propose appropriate proxies for assessing their environmental impact. Due to enhanced fish feed input, sulfate reduction (SR) and the resulting metabolic products (H₂S, NH₄⁺, PO₄^3-) were significantly greater at the farm than at the control site. Benthic release of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from farm sediment accounted for 52-837% and 926-1048%, respectively, of the potential DIN and DIP demand for phytoplankton production. The results suggest that excess organic loading in fish farms induces deleterious eutrophication and algal blooms in coastal ecosystems via benthic-pelagic coupling. Direct SR measurement provided the most useful information of all the parameters on organic contamination in fish farms. However, given its abundance, relatively lower chemical reactivity and relative ease of analysis, elemental sulfur was regarded as the most appropriate proxy for assessing the environmental impacts of finfish aquaculture.

A large artificial dyke greatly alters partitioning of sulfate and iron reduction and resultant phosphorus dynamics in sediments of the Yeongsan River estuary, Yellow Sea

We investigated sediment geochemistry, partitioning of organic carbon (C_org) oxidation by iron reduction (FeR) and sulfate reduction (SR), and benthic phosphorus (P) release, together with the P speciation in the sediments to elucidate the P dynamics in two contrasting sediments (i.e., estuarine vs. limnetic) separated by a large dyke in the Yeongsan River estuary of the Yellow Sea. In the sediments of the Yeongsan River estuary (St. YE), SR dominated the C_org oxidation pathway, accounting for 81.7% of total anaerobic C_org oxidation. Under the SR-dominated condition, H₂S derived from SR reacts quickly with iron oxides to form iron sulfides, which ultimately release the P bound to Fe(III) into the pore water. The enhanced benthic P flux (0.24 mmol m^-2 d^-1) at the YE site accounted for 80% of the P required for primary production in the water column. In contrast, in the limnetic sediments of the Yeongsan Lake (St. YL), where high levels of CH₄ accumulated, most P was bound to Fe and Al, which resulted in a low benthic P flux (0.03 mmol m^-2 d^-1). The results suggest that the frequent discharge of relatively P-depleted freshwater into the estuary via the artificial dyke may result in relatively P-limiting conditions in estuarine ecosystems. As a result, benthic P release from the SR-dominated estuarine sediment is a significant internal source of P in the coastal ecosystem. Our results indicate that the construction of a large dyke at a river mouth greatly alters C_org oxidation pathways and P dynamics in coastal ecosystems.