Contrasting short-term changes in nutrients and dissolved organic matter in groundwater and surface water on a coastal island following Super Typhoon Yagi.

Groundwater on coastal islands is an important water source for human activities and may influence nutrient and carbon cycling in adjacent marginal seas. However, its short-term response to extreme climate events remains poorly understood. Here, 19 groundwater and 4 surface pond-water samples were collected on Donghai Island, South China, before and within 3-5 days after Super Typhoon Yagi (No. 2411). Nutrients, dissolved organic carbon (DOC), chromophoric and fluorescent dissolved organic matter (CDOM and FDOM), together with elevation and land-use data, were analyzed using correlation, regression, and paired tests. Before the typhoon, groundwater showed high concentrations of NO3--N (633.62 ± 555.10 μmol/L) and DSi (354.20 ± 254.96 μmol/L) but low SRP (5.96 ± 15.17 μmol/L) and DOC (0.69 ± 0.48 mg/L). The associations of NO3--N with cultivated land, SRP with elevation and forest cover, and DOC and humic-like DOM with wetland-influenced lowland settings suggest that groundwater had a pre-existing hydrochemical and DOM framework shaped by landscape setting, land-use effects, and subsurface processes, against which short-term post-typhoon changes were superimposed. In contrast, surface pond water had lower NO3--N (62.67 ± 124.07 μmol/L) and DSi (150.27 ± 90.30 μmol/L), but much higher DOC (9.49 ± 4.61 mg/L), indicating a more hydrologically open and DOM-rich system. After the typhoon, groundwater showed limited paired changes in most nutrients, small increases in DOC (+13.0%) and humic-like FDOM (C3; +4.9%), and convergence of selected nutrient and protein-like FDOM high-value features. Surface pond water showed larger post-typhoon changes, including increased salinity (+111.1%) and NO3--N (+156.1%), decreased DSi (-20.5%), DOC (-19.5%), and protein-like FDOM (-36.7% and -57.1%). Meanwhile, DOM composition shifted toward relatively more aromatic and humified characteristics, as reflected by increases in specific UV absorbance at 350 nm (SUVA350; +39.1%) and the humification index (HIX; +38.3%), together with stronger DOC-CDOM-FDOM coupling. These results indicate contrasting short-term changes in hydrochemistry and DOM composition between groundwater and surface pond water.