Contrasting roles of nitrogen and phosphorus in modulating salinization effects on phytoplankton biomass and resource use efficiency in a coastal urban river.

Nitrogen and phosphorus play contrasting roles in shaping the ecological effects of salinization on phytoplankton, with implications for managing freshwater salinization syndrome (FSS) and water quality in coastal urban rivers. Using a two-year spatiotemporal dataset from a subtropical coastal urban river spanning a freshwater-to-polyhaline salinity gradient, we combined random forest model, generalized additive mixed model (GAMM), and piecewise structural equation model (SEM) to examine how salinity and nutrient gradients jointly shape phytoplankton biomass and resource use efficiency (RUE), defined as chlorophyll a per unit of dissolved nutrients (RUEChl-N or RUEChl-P). Contrary to classical estuarine expectations, biomass and RUE were not directly related to the longitudinal salinity gradient and instead tracked nutrient concentrations and stoichiometry. In the GAMMs, higher total nitrogen (TN) and total phosphorus (TP) were each associated with a weaker negative effect of salinity on biomass and RUEChl-N, whereas a higher TN:TP ratio was associated with a stronger negative effect on RUEChl-P. The biomass-RUE relationship shifted from weak, non-significant coupling in freshwater to significant coupling in brackish waters, indicating tighter resource use under saline conditions. The piecewise SEMs identified biomass as the main correlate of RUE, with salinity related to RUE through nutrient concentrations. These results suggest that the global rise in nitrogen relative to phosphorus could increase the sensitivity of coastal phytoplankton to salinization by increasing the stoichiometric ratio. Effective FSS management may therefore need to consider TN:TP balance alongside absolute nutrient reduction, a target currently absent from water quality frameworks.