1
|
Dong Y, Zhang X, Yi L. Hypoxia exerts greater impacts on shallow groundwater nitrogen cycling than seawater mixture in coastal zone. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2024; 31:43812-43821. [PMID: 38907819 DOI: 10.1007/s11356-024-34045-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/07/2024] [Accepted: 06/16/2024] [Indexed: 06/24/2024]
Abstract
There is no doubt that hypoxia and seawater mixture are profoundly affecting the global nitrogen (N) cycle. However, their mechanisms for altering N cycling patterns in shallow coastal groundwater are largely unknown. Here, we examined shallow groundwater N transformation characteristics (dissolved inorganic N and related chemical properties) in the coastal area of east and west Shenzhen City. Results showed that common hypoxic conditions exist in this study area. Ions/Cl- ratios indicated varying levels of saltwater mixture and sulfide formation across this study area. Dissolved oxygen (DO) affects the N cycle process by controlling the conditions of nitrification and the formation of sulfides. Salinity affects nitrification and denitrification processes by physiological effects, while sulfide impacts nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) processes through its own toxicity mechanism and the provision of electron donors for DNRA organisms. Redundancy analysis (RDA) results indicate that the influence magnitude is in the following order: DO > sulfide > salinity. Seawater mixture weakened the nitrification and denitrification of groundwater by changing salinity, while hypoxia and its controlled sulfide formation not only weaken nitrification and denitrification but also stimulated the DNRA process and promotes N regeneration. In this study area, hypoxia is considered to exert greater impacts on N cycling in the coastal shallow groundwater than seawater mixture. These findings greatly improve our understanding of the consequences of hypoxia and seawater mixture on coastal groundwater N cycling.
Collapse
Affiliation(s)
- Yingchun Dong
- College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, PR China
| | - Xiang Zhang
- College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, PR China
| | - Lixin Yi
- College of Environmental Science and Engineering, Nankai University, Tianjin, 300350, PR China.
| |
Collapse
|
2
|
Gao H, Yang L, Song X, Guo M, Li B, Cui X. Sources and hydrogeochemical processes of groundwater under multiple water source recharge condition. THE SCIENCE OF THE TOTAL ENVIRONMENT 2023; 903:166660. [PMID: 37657547 DOI: 10.1016/j.scitotenv.2023.166660] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/03/2023] [Revised: 08/26/2023] [Accepted: 08/26/2023] [Indexed: 09/03/2023]
Abstract
Ecological water replenishment (EWR) is an essential approach for improving the quantity and quality of regional water. The Chaobai River is a major river in Beijing that is replenished with water from multiple sources, including reclaimed water (RW), the South-North Water Transfer Project (SNTP), reservoir discharge (RD). The effects of multiple water source recharge (MWSR) on groundwater quality remain unclear. In this study, hydrochemical ions, isotopes (δ2H-H2O, δ18O-H2O, δ15N-NO3-, and δ18O-NO3-), mixing stable isotope analysis in R (MixSIAR), and hydrogeochemical modeling were used to quantify the contributions and impacts of different water sources on groundwater and to propose a conceptual model. The results showed that during the period before reservoir discharge, RW and SNTP accounted for 38 %-41 % and 54 % of the groundwater in their corresponding recharge areas, respectively. The groundwater in the RW recharge area contained high levels of Na+ and Cl- leading to the precipitation of halite, and was the main factor for the spatial variation in groundwater hydrochemical components. The surface water changed from Na·K - Cl·SO4 type to Ca·Mg - HCO3 type which was similar to groundwater after reservoir discharge. RD accounted for 30 % of the groundwater; however, it did not change the hydrochemical type of groundwater. Dual nitrate stable isotopes and MixSIAR demonstrated that RW was the primary source of NO3- in groundwater, contributing up to 76-89 %, and reservoir discharge effectively reduced the contribution of RW. δ15N-NO3- or δ18O-NO3- in relation to NO3-N suggests that denitrification is the main biogeochemical process of nitrogen in groundwater, whereas water recharge from the SNTP and RD reduces denitrification and dilutes NO3-. This study provides insights into the impact of anthropogenically controlled ecological water replenishment from different water sources on groundwater and guides the reasonable allocation of water resources.
Collapse
Affiliation(s)
- Heng Gao
- Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100049, China.
| | - Lihu Yang
- Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100049, China.
| | - Xianfang Song
- Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100049, China.
| | - Minli Guo
- Beijing Water Science and Technology Institute, Beijing Engineering Technique Research Center for Exploration and Utilization of Non-Conventional Water Resources and Water Use Efficiency, Beijing 100048, China
| | - Binghua Li
- Beijing Water Science and Technology Institute, Beijing Engineering Technique Research Center for Exploration and Utilization of Non-Conventional Water Resources and Water Use Efficiency, Beijing 100048, China
| | - Xu Cui
- Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; University of Chinese Academy of Sciences, Beijing 100049, China
| |
Collapse
|
3
|
Nauer PA, Kessler AJ, Hall P, Popa ME, ten Hietbrink S, Hutchinson T, Wong WW, Attard K, Glud RN, Greening C, Cook PLM. Pulses of labile carbon cause transient decoupling of fermentation and respiration in permeable sediments. LIMNOLOGY AND OCEANOGRAPHY 2023; 68:2141-2152. [PMID: 38516532 PMCID: PMC10952694 DOI: 10.1002/lno.12411] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 07/04/2022] [Revised: 12/07/2022] [Accepted: 07/15/2023] [Indexed: 03/23/2024]
Abstract
Dihydrogen (H2) is an important intermediate in anaerobic microbial processes, and concentrations are tightly controlled by thermodynamic limits of consumption and production. However, recent studies reported unusual H2 accumulation in permeable marine sediments under anoxic conditions, suggesting decoupling of fermentation and sulfate reduction, the dominant respiratory process in anoxic permeable marine sediments. Yet, the extent, prevalence and potential triggers for such H2 accumulation and decoupling remain unknown. We surveyed H2 concentrations in situ at different settings of permeable sand and found that H2 accumulation was only observed during a coral spawning event on the Great Barrier Reef. A flume experiment with organic matter addition to the water column showed a rapid accumulation of hydrogen within the sediment. Laboratory experiments were used to explore the effect of oxygen exposure, physical disturbance and organic matter inputs on H2 accumulation. Oxygen exposure had little effect on H2 accumulation in permeable sediments suggesting both fermenters and sulfate reducers survive and rapidly resume activity after exposure to oxygen. Mild physical disturbance mimicking sediment resuspension had little effect on H2 accumulation; however, vigorous shaking led to a transient accumulation of H2 and release of dissolved organic carbon suggesting mechanical disturbance and cell destruction led to organic matter release and transient decoupling of fermenters and sulfate reducers. In summary, the highly dynamic nature of permeable sediments and its microbial community allows for rapid but transient decoupling of fermentation and respiration after a C pulse, leading to high H2 levels in the sediment.
Collapse
Affiliation(s)
- Philipp A. Nauer
- Water StudiesSchool of Chemistry, Monash UniversityClaytonVictoriaAustralia
| | - Adam J. Kessler
- School of Earth Atmosphere and Environment, Monash UniversityClaytonVictoriaAustralia
| | | | - Maria Elena Popa
- Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht UniversityUtrechtNetherlands
| | - Sophie ten Hietbrink
- Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht UniversityUtrechtNetherlands
| | - Tess Hutchinson
- Water StudiesSchool of Chemistry, Monash UniversityClaytonVictoriaAustralia
| | - Wei Wen Wong
- Water StudiesSchool of Chemistry, Monash UniversityClaytonVictoriaAustralia
| | - Karl Attard
- Nordcee and HADAL, Department of BiologyUniversity of Southern DenmarkOdense MDenmark
- Danish Institute for Advanced Study, University of Southern DenmarkOdense MDenmark
| | - Ronnie N. Glud
- Nordcee and HADAL, Department of BiologyUniversity of Southern DenmarkOdense MDenmark
- Danish Institute for Advanced Study, University of Southern DenmarkOdense MDenmark
- Department of Ocean and Environmental SciencesTokyo University of Marine Science and TechnologyTokyoJapan
| | - Chris Greening
- Department of MicrobiologyBiomedicine Discovery InstituteClaytonVictoriaAustralia
| | - Perran L. M. Cook
- Water StudiesSchool of Chemistry, Monash UniversityClaytonVictoriaAustralia
| |
Collapse
|
4
|
Xiong G, Zhu X, Wu J, Liu M, Yang Y, Zeng X. Seawater intrusion alters nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions: Based on Bayesian isotope mixing model and microbial functional network. THE SCIENCE OF THE TOTAL ENVIRONMENT 2023; 867:161368. [PMID: 36621512 DOI: 10.1016/j.scitotenv.2022.161368] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/17/2022] [Revised: 12/07/2022] [Accepted: 12/30/2022] [Indexed: 06/17/2023]
Abstract
Seawater intrusion is a global coastal environmental issue of great concern and significantly impacts the regional biogeochemical environment and material cycles, including nitrogen cycling. To reveal the mechanism of seawater intrusion altering nitrogen cycling patterns through hydrodynamic behavior and biochemical reactions, the Bayesian mixing model (δ15N-NO3- and δ18O-NO3-) and 16S rDNA gene amplicon sequencing are used to establish nitrogen cycling pathways and microbial functional network. The results show that the nitrate in the coastal groundwater is from manure and septic waste (M&S, over 44 %), soil organic nitrogen (SON, over 20 %), and nitrogen fertilizer (FN, over 16 %). The hydrological interaction has promoted the coupling between material cycling and microbial community in the coastal groundwater systems. Among them, precipitation infiltration has caused the gradual decrease of specific microbes along the flow direction, such as Lactobacillus, Acinetobacter, Bifidobacterium, etc. And seawater intrusion has caused the mutations of specific microbes (Planktomarina, Clade_Ia, Wenyingzhuangia, Glaciecola, etc.) and convergence of microbial community at the salt-freshwater interface in the aquifer. In the coastal intruded aquifer systems, the nitrogen cycling pattern can be divided into oxidation and reduction processes. The oxidation process involves the enhancement of nitrification while the weakening of denitrification and anammox with the increase of aquifer depth. The reduction process consists of the enhancement of denitrification and anammox while the erosion of nitrification and ammonification with increased seawater intrusion. In addition, seawater intrusion can mitigate nitrate contamination by promoting denitrification and anammox in coastal areas.
Collapse
Affiliation(s)
- Guiyao Xiong
- Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
| | - Xiaobin Zhu
- Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China.
| | - Jichun Wu
- Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China.
| | - Mengwen Liu
- Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
| | - Yun Yang
- School of Earth Sciences and Engineering, Hohai University, Nanjing 211100, China
| | - Xiankui Zeng
- Key Laboratory of Surficial Geochemistry of Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
| |
Collapse
|
5
|
Marshall AJ, Phillips L, Longmore A, Hayden HL, Heidelberg KB, Tang C, Mele P. Temporal profiling resolves the drivers of microbial nitrogen cycling variability in coastal sediments. THE SCIENCE OF THE TOTAL ENVIRONMENT 2023; 856:159057. [PMID: 36174701 DOI: 10.1016/j.scitotenv.2022.159057] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/06/2022] [Revised: 09/21/2022] [Accepted: 09/22/2022] [Indexed: 06/16/2023]
Abstract
Here we describe the potential for sediment microbial nitrogen-cycling gene (DNA) and activity (RNA) abundances to spatially resolve coastal areas impacted by seasonal variability in external nutrient inputs. Three sites were chosen within a nitrogen-limited embayment, Port Phillip Bay (PPB), Australia that reflect variability in both proximity to external nutrient inputs and the dominant form of available nitrogen. At three sediment depths (0-1; 1-5; 5-10 cm) across a 2 year study key genes involved in nitrification (archaeal amoA and bacterial β-amoA), nitrite reduction (clade I nirS and cluster I nirK, archaeal nirK-a), anaerobic oxidation of ammonium (anammox 16S rRNA phylogenetic marker) and nitrogen fixation (nifH) were quantified. Sediments impacted by a dominance of organic nitrogen inputs were characterised at all time-points and to sediment depths of 10 cm by the highest transcript abundances of archaeal amoA and archaeal nirk-a. Proximity to a dominance of external nitrate inputs was associated with the highest transcript abundances of nirS which temporally co-varied with seasonal changes in sediment nitrate. Sediments isolated from external inputs displayed the greatest depth-specific decrease in quantifiable transcript abundances. In these isolated sediments bacterial β-amoA transcripts were temporally associated with increased sediment ammonium levels. Across this nitrogen limited system variability in the abundance of bacterial β-amoA, archaeal amoA, archaeal nirk-a or nirS transcripts from the sediment surface (0-1 and 5 cm) demonstrated a capacity to improve our ability to monitor coastal zones impacted by anthropogenic nitrogen inputs. Specifically, the spatial detection sensitivity of bacterial β-amoA transcripts could be developed as a metric to determine spatiotemporal impacts of large external loading events. This temporal study demonstrates a capacity for microbial activity metrics to facilitate coastal management strategies through greater spatial resolution of areas impacted by external nutrient inputs.
Collapse
Affiliation(s)
- Alexis J Marshall
- La Trobe University, AgriBio Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia; Department of Jobs, Precincts and Regions, AgriBio, Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia.
| | - Lori Phillips
- Department of Jobs, Precincts and Regions, AgriBio, Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia
| | - Andrew Longmore
- Centre for Aquatic Pollution Identification and Management, Melbourne University, Parkville, Australia
| | - Helen L Hayden
- Department of Jobs, Precincts and Regions, AgriBio, Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia
| | - Karla B Heidelberg
- The University of Southern California, Department of Biology, Los Angeles, CA 90089, United States of America
| | - Caixian Tang
- La Trobe University, AgriBio Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia
| | - Pauline Mele
- La Trobe University, AgriBio Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia; Department of Jobs, Precincts and Regions, AgriBio, Centre for AgriBiosciences, 5 Ring Road Bundoora, Australia
| |
Collapse
|
6
|
Wu K, Liu Y, Liao X, Yang X, Chen Z, Mo L, Zhong S, Zhang X. Fungal Diversity and Its Relationship with Environmental Factors in Coastal Sediments from Guangdong, China. J Fungi (Basel) 2023; 9:jof9010101. [PMID: 36675922 PMCID: PMC9866456 DOI: 10.3390/jof9010101] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2022] [Revised: 01/09/2023] [Accepted: 01/10/2023] [Indexed: 01/14/2023] Open
Abstract
As one core of the Guangdong-Hong Kong-Macao Greater Bay Area (GBA), Guangdong is facing some serious coastal environmental problems. Fungi are more vulnerable to changes in coastal environments than bacteria and archaea. This study investigated the fungal diversity and composition by high-throughput sequencing and detected basic parameters of seven environmental factors (temperature, dissolved oxygen, pH, salinity, total organic carbon, total nitrogen, and total phosphorus) at 11 sites. A total of 2056 fungal operational taxonomic units (OTUs) belonging to 147 genera in 6 phyla were recovered; Archaeorhizomyces (17.5%) and Aspergillus (14.19%) were the most dominant genera. Interestingly, a total of 14 genera represented the first reports of coastal fungi in this study. Furthermore, there were nine genera of fungi that were significantly correlated with environmental factors. FUNGuild analysis indicated that saprotrophs and pathogens were the two trophic types with the highest proportions. Saprotrophs were significantly correlated with total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP), while pathogens were significantly correlated with pH. This study provides new scientific data for the study of the diversity and composition of fungal communities in coastal ecosystems.
Collapse
Affiliation(s)
- Keyue Wu
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
- Guangdong Provincial Key Laboratory of Aquatic Product Processing and Safety, College of Food Science and Technology, Guangdong Ocean University, Zhanjiang 524088, China
| | - Yongchun Liu
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
| | - Xinyu Liao
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
| | - Xinyue Yang
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
| | - Zihui Chen
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
| | - Li Mo
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
| | - Saiyi Zhong
- Guangdong Provincial Key Laboratory of Aquatic Product Processing and Safety, College of Food Science and Technology, Guangdong Ocean University, Zhanjiang 524088, China
- Correspondence: (S.Z.); (X.Z.)
| | - Xiaoyong Zhang
- University Joint Laboratory of Guangdong Province, Hong Kong and Macao Region on Marine Bioresource Conservation and Exploitation, College of Marine Sciences, South China Agricultural University, Guangzhou 510642, China
- Guangdong Laboratory for Lingnan Modern Agriculture, Guangzhou 510642, China
- Correspondence: (S.Z.); (X.Z.)
| |
Collapse
|
7
|
Colette M, Guentas L, Gunkel-Grillon P, Callac N, Della Patrona L. Is halophyte species growing in the vicinity of the shrimp ponds a promising agri-aquaculture system for shrimp ponds remediation in New Caledonia? MARINE POLLUTION BULLETIN 2022; 177:113563. [PMID: 35325793 DOI: 10.1016/j.marpolbul.2022.113563] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/26/2021] [Revised: 03/07/2022] [Accepted: 03/11/2022] [Indexed: 06/14/2023]
Abstract
Plant culture integration within aquaculture activities is a topic of recent interest with economic and environmental benefits. Shrimp farming activities generate nutrient-rich waste trapped in the sediments of farming ponds or release in the mangrove area. Thus, we investigate if the halophytes species naturally growing around the pond can use nitrogen and carbon from shrimp farming for remediation purposes. Halophyte biomasses and sediments influenced by shrimp farm effluents, were collected in two farms in New-Caledonia. All samples were analyzed for their C and N stable isotopic composition and N content. Higher δ15N values were found in plants influenced by shrimp farm water thus evidenced their abilities to take nutrient derived from shrimp farming. Deep root species Chenopodium murale, Atriplex jubata, Suaeda australis and Enchylaena tomentosa appears more efficient for shrimp pond remediation. This work demonstrates that halophytes cultivation in shrimp ponds with sediments, could be effective for the pond's remediation.
Collapse
Affiliation(s)
- Marie Colette
- French Institute for Research in the Science of the Sea (IFREMER), Research Institute for Development (IRD), University of New Caledonia, University of Reunion, CNRS, UMR 9220 ENTROPIE, Noumea, New Caledonia; Institute of Exact and Applied Sciences (ISEA), EA 7484, University of New Caledonia, 98851 Noumea, New Caledonia.
| | - Linda Guentas
- Institute of Exact and Applied Sciences (ISEA), EA 7484, University of New Caledonia, 98851 Noumea, New Caledonia.
| | - Peggy Gunkel-Grillon
- Institute of Exact and Applied Sciences (ISEA), EA 7484, University of New Caledonia, 98851 Noumea, New Caledonia.
| | - Nolwenn Callac
- French Institute for Research in the Science of the Sea (IFREMER), Research Institute for Development (IRD), University of New Caledonia, University of Reunion, CNRS, UMR 9220 ENTROPIE, Noumea, New Caledonia.
| | - Luc Della Patrona
- French Institute for Research in the Science of the Sea (IFREMER), Research Institute for Development (IRD), University of New Caledonia, University of Reunion, CNRS, UMR 9220 ENTROPIE, Noumea, New Caledonia.
| |
Collapse
|
8
|
Seasonal Sources and Cycling of Nitrogen Revealed by Stable Isotopes in the Northeastern Beibu Gulf, China. JOURNAL OF MARINE SCIENCE AND ENGINEERING 2021. [DOI: 10.3390/jmse9101123] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Isotope measurements were performed on dissolved nitrate (NO3−) and ammonium (NH4+) in the coastal waters of the northeastern Beibu Gulf, China, to investigate the seasonal nitrate sources and their biogeochemical processes, which are due to the rapid development of local industrialisation and urbanisation. The high N/P ratio observed in the coastal bay during both fall and spring suggests that P is a limiting nutrient, which in turn indicates that increasing P causes conditions favourable for algal blooms. Higher nutrient concentrations and δ15N-NO3− and δ15N-NH4+ values were found in the nearshore area in the fall, suggesting that nutrients originated mainly from land-based pollution. A Bayesian isotope mixing model was used to calculate the contribution of potential NO3− sources and the results showed that in the nearshore area, NO3− originated mainly from manure and sewage (58%). In the spring, however, in addition to the impact of urban sewage effluents, the exchange of sediment and water was another important factor causing higher nutrient concentrations and positive NO3− isotopes in the nearshore area. There were lower concentrations of nutrients and an increase in δ15N-NO3− and δ15N-NH4+ values in the offshore area in the fall, and the NO3− loss in the surface water was mainly caused by the process of assimilation. However, the exchange of sediment and water was the dominant factor causing higher nutrient concentrations (except for NO3−) and positive dual nitrate isotopes but lower NO3− concentration in the offshore area during the spring. Overall, isotope analysis of NO3− and NH4+ helps to illustrate the major sources of the former and their biological transformation in the northeastern Beibu Gulf.
Collapse
|