Linking riverine discharge to shifts in temperate estuarine planktonic food web dynamics

Riverine discharge plays a crucial role in shaping planktonic food web dynamics in coastal systems, though its effects can vary across estuaries. This study investigated the influence of riverine discharge on key biological variables-primary productivity, nitrogen uptake (nitrate and ammonium), biomass, community composition, and mesozooplankton feeding rates-across estuarine gradients in Gwangyang Bay, a low-turbidity temperate estuary in Korea. Principal component analysis identified riverine discharge as the main driver of environmental variation, influencing salinity, light availability, and nutrient concentrations. Microphytoplankton, particularly diatoms, dominated across the estuarine gradient, with diatom peaks in the polyhaline zone and nanophytoplankton in the oligohaline zone, reflecting size-based phytoplankton responses. Riverine nutrient inputs and low turbidity supported high nitrate uptake and primary productivity in the oligohaline zone. Chlorophyll peaks in the polyhaline zone indicated diatom transport and accumulation from upstream. Mesozooplankton feeding rates were highest in the polyhaline zone, suggesting selective grazing on diatoms, while lower feeding rates on smaller phytoplankton was likely due to trophic cascades via predation on microzooplankton. An interplay of selective grazing and trophic cascades across the estuarine-marine continuum drove spatial variations in mesozooplankton feeding. These findings demonstrate how riverine discharge drives planktonic food web dynamics shifts, affecting energy transfer and ecosystem productivity.