LAUSR

Abstract The phosphorus in precipitation provides a significant fraction of the bioavailable phosphorus to fresh and marine waters. To identify the possible sources and interpret the atmospheric deposition processes of phosphorus, we measured the phosphate oxygen isotopes (δ18OP) in the precipitation monthly from October 2015 to March 2017 in Daya Bay, South China Sea. The isotopic results show that the δ18OP values in the dry season (+19.60‰ to +25.80‰, +23.10‰ average) were significantly higher than those in the wet season (+12.52‰ to +20.11‰, +16.73‰ average), indicating a variable contribution from different phosphorus sources to the precipitation between seasons. The δ18OP value was primarily influenced by the phosphorus concentrations, rainfall, and air temperature. The exchange of phosphate molecules among phosphorus pools caused isotope dilution (wet season) or enrichment effects (dry season) during the seasonal shifts. The δ18OP value of the precipitation and the local end-members indicate that coal combustion (39% ± 16%), biomass burning (22% ± 18%), and soil (17% ± 13%) sources dominate in the dry season, whereas marine sources (28% ± 19%) and coal combustion (24% ± 12%) account for the highest proportion in the wet season using a MixSIAR model. The atmospheric wet deposition flux of phosphorus in Daya Bay was 379.0 μmol m−2·yr−1. The wet deposition of phosphorus exhibits a significant seasonal trend with higher deposition fluxes in the wet season than in the dry season as a result of higher rainfall and source shifts (local sources to marine source) between seasons.