Dissolved N pollution and its biogeochemical constraints along a river-sea continuum of a typical dense oyster mariculture coastal water, northwest South China Sea

• Published in: Journal of hazardous materials 2024-12, Vol.480, p.136207, Article 136207
• Main Authors: Felix Dan, Solomon, Xiaolian, Zeng, Tang, Jianhui
• Format: Article
• Language: English
• Subjects: Animals; anthropogenic activities; Aquaculture; Biogeochemistry; China; coastal water; denitrification; Environmental Monitoring; Eutrophication; isotope fractionation; N isotopes; N pollution; nitrates; Nitrates - analysis; nitrification; nitrogen; Nitrogen - analysis; Nutrient stoichiometry; Ostreidae - chemistry; oyster culture; Oyster mariculture; phytoplankton; pollution; riparian areas; rivers; Rivers - chemistry; seawater; Seawater - chemistry; sewage; South China Sea; stoichiometry; summer; surface water; wastewater; Water Pollutants, Chemical - analysis; winter
• ISSN: 0304-3894; 1873-3336; 1873-3336
• DOI: 10.1016/j.jhazmat.2024.136207

Dissolved nutrients, including nitrate (NO3–-N) and its dual isotopes (δ15N-NO3– and δ18O-NO3–) were systematically studied along a river-sea continuum, wherein dense oyster mariculture is implemented, to constrain the pollution sources and biogeochemical cycling mechanisms of nitrogen (N). Total dissolved N, mainly composed of inorganic N, showed strong anthropogenic influence. Based on MixSIAR model results, N pollution was predominantly sourced from sewage/wastewater (55.9–64.3 %). Nutrient stoichiometry revealed DIP and DSi stress, and surface water in the riverine region was severely eutrophic. The occurrences of eutrophication and changes in nutrient stoichiometry were significantly related to N pollution sources in both summer and winter. N dynamics were controlled by anthropogenic activities and physical mixing. However, due to the insignificance of biological processes such as denitrification, phytoplankton assimilation, N2 fixation, and nitrification, including the lack of significant isotopic fractionation associated with these processes, and the poor fit of both the Rayleigh Model and Open system Model to the measured data, it is speculated that the several-fold reduction in N load and eutrophication along the river-sea continuum could be attributed to a combination of significant N removal by dense oyster mariculture and nutrient dilution due to physical mixing of river and seawater during winter and summer. [Display omitted] •Nitrate dual isotopes were used to constrain N pollution sources and biogeochemistry.•N pollution was dominated by sewage/wastewater discharge in summer and winter.•Eutrophication, nutrient ratios are significantly related to N pollution sources.•N biogeochemical cycling is significantly influenced by pollution sources.•Dense oyster mariculture and physical mixing significantly reduces riverine N load.