Stable carbon and nitrogen isotopic characterization and tracing nutrient sources of Ulva blooms around Jeju coastal areas

A new set of N isotopic reference values for monitoring Ulva green tides in coral reef ecosystems

Green tides occurrence has increased in coral reefs, yet few reference values have been documented to support bloom management in these ecosystems. Here, we took advantage of recent Ulva green tides that occurred in New Caledonia to (i) identify the elements limiting the growth of Ulva spp. during these blooms; and (ii) validate the use of isotopic markers for identifying sources of nutrients that generated blooms. N/P ratios highlighted a stronger limitation of algae by phosphorus than by nitrogen on sites under oceanic influence, while the proportions of N and P were optimal for algal growth at sites where green tides occurred. Macroalgae highly exposed to sewage water was characterized by higher δ15N than macroalgae collected in areas exposed to synthetic inorganic fertilizers. From these results, we established a new set of threshold values for using δ15N in Ulva species as an indicator of nitrogen source type in coral reefs.

Effects of wastewater effluent-borne nutrients on phytoplankton off the coast of Jeju Island

The spatiotemporal distributions of nutrients in coastal waters surrounding eight wastewater treatment plants (WWTPs) in four seasons were investigated to determine the effects of WWTP effluents on seawater off Jeju Island, Korea. The highest concentrations of nutrients were observed in the outlets of WWTPs with relatively high ammonium concentrations among dissolved inorganic nitrogen (DIN). The reduced DIN (NO₂^- and NH₄⁺)/total DIN ratios are used as a potential short-term index for marine environmental conditions. In seawater surrounding the WWTPs, relatively low nutrient concentrations were observed in spring and fall, due to enhanced biological production, which is closely linked to decreased N/P ratios. Because the highest WWTP effluent fluxes of ammonium in this study were similar to the fluxes of nutrients from submarine groundwater discharge, diffusion from bottom sediments, and discharge from land-based fish farm wastewater, WWTP effluent-derived nutrients are potentially important in oligotrophic environments and can be readily utilized by phytoplankton.

Microbial Fuel Cell Based on Nitrogen-Fixing Rhizobium anhuiense Bacteria

In this study, the nitrogen-fixing, Gram-negative soil bacteria Rhizobium anhuiense was successfully utilized as the main biocatalyst in a bacteria-based microbial fuel cell (MFC) device. This research investigates the double-chambered, H-type R. anhuiense-based MFC that was operated in modified Norris medium (pH = 7) under ambient conditions using potassium ferricyanide as an electron acceptor in the cathodic compartment. The designed MFC exhibited an open-circuit voltage (OCV) of 635 mV and a power output of 1.07 mW m-2 with its maximum power registered at 245 mV. These values were further enhanced by re-feeding the anode bath with 25 mM glucose, which has been utilized herein as the main carbon source. This substrate addition led to better performance of the constructed MFC with a power output of 2.59 mW m-2 estimated at an operating voltage of 281 mV. The R. anhuiense-based MFC was further developed by improving the charge transfer through the bacterial cell membrane by applying 2-methyl-1,4-naphthoquinone (menadione, MD) as a soluble redox mediator. The MD-mediated MFC device showed better performance, resulting in a slightly higher OCV value of 683 mV and an almost five-fold increase in power density to 4.93 mW cm-2. The influence of different concentrations of MD on the viability of R. anhuiense bacteria was investigated by estimating the optical density at 600 nm (OD600) and comparing the obtained results with the control aliquot. The results show that lower concentrations of MD, ranging from 1 to 10 μM, can be successfully used in an anode compartment in which R. anhuiense bacteria cells remain viable and act as a main biocatalyst for MFC applications.

Impact of aquaculture on distribution of dissolved organic matter in coastal Jeju Island, Korea, based on absorption and fluorescence spectroscopy

In Jeju Island, multiple land-based aquafarms were fully operational along most coastal region. However, the effect of effluent on distribution and behaviours of dissolved organic matter (DOM) in the coastal water are still unknown. To decipher characteristics of organic pollution, we compared physicochemical parameters with spectral optical properties near the coastal aquafarms in Jeju Island. Absorption spectra were measured to calculate the absorption coefficient, spectral slope coefficient, and specific UV absorbance. Fluorescent DOM was analysed using fluorescence spectroscopy coupled with parallel factor analysis. Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) were measured using high-temperature catalytic oxidation. The DOC concentration near the discharge outlet was twice higher than that in natural groundwater, and the TDN concentration exponentially increased close to the outlet. These distribution patterns indicate that aquafarms are a significant source of DOM. Herein, principal component analysis was applied to categorise the DOM origins. There were two distinct groups, namely, aquaculture activity for TDN with humic-like and high molecular weights DOM (PC1: 48.1%) and natural biological activity in the coastal water for DOC enrichment and protein-like DOM (PC2: 18.8%). We conclude that the aquafarms significantly discharge organic nitrogen pollutants and provoke in situ production of organic carbon. Furthermore, these findings indicate the potential of optical techniques for the efficient monitoring of anthropogenic organic pollutants from aquafarms worldwide.

Sources, transformations of suspended particulate organic matter and their linkage with landscape patterns in the urbanized Beiyun river Watershed of Beijing, China

This study explored the sources, transformations of suspended particulate organic matter (POM), and the influence of landscape patterns on POM within the Beiyun River Watershed by applying the stable carbon and nitrogen isotope technique combined with multiple statistical analyses. The POM variables showed great spatial fluctuations under different urban development gradients. Analysis of multiple isotopes revealed that assimilation of phytoplankton might exist in the rainy season, while nitrification occurs in the dry season. SIAR modeling results indicated that the sewage debris and phytoplankton were the main sources of POM in both seasons, accounting for 52.58% and 38.39% in the rainy season, 33.17% and 31.95% in the dry season, respectively. Spatiotemporal variations of POM sources existed in the study watershed, probably due to urbanization and human disturbance. The multiple linear stepwise regression and redundant analysis results indicated that landscape metrics reflecting contagion and fragmentation at the class level correlated well with the POM variables over seasons. Interspersion and juxtaposition indices of grassland and water were negatively related to POM variables in the rainy season, whereas the landscape division index of buildup land showed negative correlations with POM parameters in the dry season. Increasing the adjacency of grassland and water to other land uses, while reducing the aggregation of buildup lands would be an efficient way for urban river water quality improvement.

Nitrate contamination of coastal groundwater: Sources and transport mechanisms along a volcanic aquifer

Groundwater is the sole water supply source on Jeju volcanic island in Korea and increasing levels of nitrate contamination have raised serious public concerns. This study was objected to understand the nitrate contamination mechanisms of the coastal groundwater overlain by a low-permeability layer in the western part of the island. The 2-yrs of quarterly groundwater monitoring from Sep. 2016 to Oct. 2018 revealed that NO₃-N concentrations of 60.7% of sampled wells exceeded 10 mg/L, and those of 17.8% in the range of 3-10 mg/L, indicating those wells are already contaminated or under influence of potential sources. The spatio-temporal variation of groundwater chemistry and stable isotopic signatures suggested that nitrate originated mainly from chemical fertilizers due to agricultural activities and in part from liquefied manure affected by local livestock industries. Two transport processes appeared to be responsible for the nitrate in the confined coastal aquifer: 1) the direct penetration from the land surface through the wellbore leakage at incompletely grouted wells, and 2) the addition of nitrate-contaminated groundwater from upgradient areas where the confining layer is pinched out. Presently the potential nitrate sources are distributed up to 600 m above mean sea level(amsl), and the isotopic signatures implies that the provenance of coastal groundwater could be upland areas of up to 1280 m amsl. Subsequently, nitrate sources at upland areas could be picked up and carried down to the coastal groundwater under the confined condition of the basaltic aquifer that the regional groundwater recharge occurs. Consequently, groundwater contamination in the coastal aquifer should be considered in the big picture of groundwater system accommodating both regional and local flows.

Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (²²³Ra, ²²⁴Ra, and ²²⁸Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 10⁸ m³/d and (3.77 ± 0.52) × 10⁷ mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO₃^- dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH₄⁺) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.