Characterization of processes shaping shallow coastal and shelf planktonic food web dynamics in the South Sea of Korea

Abstract

Planktonic food webs support productivity, biogeochemical cycling, and fisheries in coastal seas, yet the processes that jointly regulate energy flow and nutrient cycling across estuarine–shelf continua remain incompletely resolved. This thesis characterizes how physical forcing, nutrient dynamics, and key consumers interact to shape shallow coastal and shelf planktonic food-web structure in the South Sea of Korea and adjacent East China Sea. I combined multi-year hydrographic and nutrient observations with measurements of primary production and nitrogen uptake, mesozooplankton grazing, suspension-feeding bivalve physiology, stable isotope analyses, and flexible statistical models to resolve both bottom-up and top-down controls from river-dominated estuaries to continental shelf waters. In a low-turbidity temperate estuary (Gwangyang Bay), principal component analysis identified riverine discharge as the dominant environmental gradient linking salinity, light, and nitrate supply. Along this gradient, river-borne nitrate and short residence times supported high new production in the oligohaline upper estuary, while phytoplankton biomass and mesozooplankton feeding peaked downstream in the polyhaline zone as diatoms accumulated and selective grazing intensified. Negative clearance on nano–pico phytoplankton revealed trophic cascades mediated by microzooplankton, coupling microbial and classical grazing pathways. In an intensively farmed oyster embayment (Jinhae Bay), two-year time series showed estuarine– coastal primary productivity typical of temperate bays but persistently low f-ratios, indicating regeneration- dominated production. Allometry-based scaling of oyster filtration and ammonium excretion, linked with generalized additive models, demonstrated that oysters simultaneously suppress phytoplankton biomass through grazing and sustain high productivity by recycling nitrogen, making aquaculture a central driver of internal nutrient regeneration. On the northern East China Sea shelf, three seasonal cruises across contrasting water masses revealed nano–pico phytoplankton dominance offshore, episodic microphytoplankton blooms at coastal and frontal sites, and mesozooplankton that feed selectively on microphytoplankton when available but are ultimately supported primarily by nano–pico-derived particulate organic matter, emphasizing the prevalence of microbial- loop-based pathways. Finally, stable-isotope niche and mixing-model analyses showed that Japanese anchovy undergo ontogenetic trophic niche expansion and shift from specialist to generalist feeding as they migrate across inshore spawning, offshore foraging, and wintering grounds, acting as mobile trophic links that integrate spatially separated planktonic food webs. Together, these results provide a process-based, cross-scale framework for understanding how hydrological forcing, aquaculture, and migratory fishes jointly regulate energy transfer and nitrogen cycling in temperate coastal ecosystems, with implications for managing eutrophication, shellfish culture, and small pelagic fisheries.