Environmental management implications of wastewater discharge from seaweed processing into coastal waters

The expansion of the seaweed-processing industry has increased the need for evidence-based management of wastewater discharged to coastal receiving waters. We assessed impacts from a large laver (Pyropia spp.) processing complex in Mokpo, South Korea, using four field campaigns between November 2022 and March 2024. Water-column responses were evaluated using a station-level Before-After analysis supported by operational-reference diagnostics. Dissolved organic carbon (DOC) and particulate organic carbon (POC) showed the clearest discharge-season responses, increasing by 59.6% and 242%, respectively, with the strongest concurrent enrichment near the Outfall. A total organic carbon (TOC)-based modified Organic Pollution Index (OPI_mod) and an intake-normalized eutrophication index (EI_intake) distinguished pollution intensity relative to regional reference conditions from local amplification above Intake conditions. Their joint elevation at the Outfall during discharge supported identification of laver-processing wastewater as an important source of localized organic-matter and nutrient pollution. Sediment trace metals showed no coherent area-wide discharge-period enrichment, although localized Zn enrichment at C03 indicated that site-specific accumulation could not be excluded. Organic carbon, particularly TOC, emerged as the principal target for receiving-water management, while dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) remain useful supporting variables. Because the study was limited to one industrial complex, four water-column campaigns, one operational reference, and two sediment surveys, the data were insufficient to establish regulatory thresholds. Longer-term monitoring across multiple reference sites and processing facilities is needed to develop robust TOC-based management criteria. These findings support receiving-water-based management of the expanding laver-processing industry.