Variations in Surface-Water Nitrogen and Phosphorus Concentrations Across Land- and Water-Use Settings in a Plain River Network

Excess nitrogen and phosphorus in surface water can degrade water quality and increase eutrophication risk. However, how surface-water nitrogen and phosphorus concentrations vary across mixed land- and water-use settings remains poorly understood in highly connected plain river networks, particularly under emerging water uses such as floating photovoltaic (FPV) installations. We investigated surface-water nitrogen and phosphorus concentrations across different land- and water-use settings in the Yangtze River Delta, focusing on seasonal variation and environmental controls. Field measurements were combined with Sentinel-2-derived waterbody metrics within 500 m buffers around sampling plots. Aquaculture ponds had the highest overall nutrient concentrations, industrial land showed relatively high nitrate–nitrogen (NO3−-N) and total nitrogen (TN), and aquatic solar farms exhibited a distinct nutrient pattern with relatively high total phosphorus (TP) but low TN. Nutrient forms showed contrasting seasonal patterns, with higher NO3−-N in the dry season and higher TP in the wet season. Water-surface proportion was weakly related to nutrient concentrations, whereas edge density was significantly correlated with them. These findings indicate that nutrient patterns were shaped by direct inputs, particle-associated transport, ecological and physicochemical conditions, and land–water interface effects. Nutrient management should therefore consider seasonal hydrology, local waterbody configuration, and emerging water-use types such as aquatic solar farms.