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

Dissolved nutrients, including nitrate (NO ₃ ^- -N) and its dual isotopes (δ ¹⁵ N-NO ₃ ^- and δ ¹⁸ O-NO ₃ ^- ) 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, N ₂ 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.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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