Abstract:

Understanding nutrient flux in aquaculture systems is critical for sustainable coastal environmental management. This study comprehensively investigated nitrogen and phosphorus dynamics in land-based mariculture across four coastal cities of Zhejiang province: Ningbo, Taizhou, Wenzhou, and Zhoushan, utilizing data from 1 018 sampling points at marine outfalls. Elemental concentration analyses revealed total nitrogen levels ranging from 0.015 to 36.000 mg/L, with total phosphorus concentrations between 0.005 and 2.860 mg/L. Notably, over 90% of the samples remained within Category II of the Zhejiang Province Mariculture Tailwater Discharge Standards, indicating relatively controlled nutrient emissions. Employing both chemical analysis and pollution discharge coefficient methodologies, we estimated the annual nutrient flux from land-based mariculture tailwaters. Chemical analysis methods indicated annual total nitrogen and phosphorus fluxes of 2 969.2 t and 83.6 t, respectively. Significantly, the third and fourth quarters contributed 84.4% of the annual nutrient flux, coinciding with peak harvesting periods and influenced by complex hydrodynamic factors including oceanic currents and Yangtze River dilution waters. Comparative analysis between methodological approaches revealed substantial discrepancies, with pollution discharge coefficient methods estimating higher annual fluxes(4 634.3 t nitrogen and 801.4 t phosphorus). These variations suggest considerable nutrient sequestration within bottom sediments, presenting potential long-term environmental implications. The continuous accumulation of nitrogen and phosphorus in sedimentary environments raises critical concerns about secondary nutrient release mechanisms and escalating eutrophication risks in adjacent marine ecosystems. These findings underscore the necessity for sophisticated nutrient management strategies in coastal mariculture systems.

Key words: chemical analysis method, total nitrogen and total phosphorus, fluxes, land-based mariculture, coastal Zhejiang