Spatial distribution and groundwater discharge fluxes of nutrients in a mangrove tidal creek

Tidal creeks serve as critical conduits for surface water-groundwater exchange and material transport in mangrove wetlands. However, the spatial distribution patterns of nutrients in groundwater and the underlying driving mechanisms remain poorly understood. In this study, two representative cross-sectional profiles (upstream and downstream) along a typical tidal creek system in the Hailing Island mangrove wetland, Guangdong Province, were investigated. Groundwater level monitoring, stratified groundwater sampling, and multivariate statistical analysis were combined to characterize nutrient distributions, quantify surface water-groundwater exchange rates and associated nutrient fluxes, and place the results in a global context through a comparative synthesis of the multi-scale patterns of submarine groundwater discharge (SGD) associated nutrient fluxes across mangrove regions. The groundwater discharge rate was higher downstream than upstream, and the fluxes of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSi) carried by groundwater discharge were also higher downstream than upstream, following the order DSi > DIN > DIP. Additionally, physicochemical parameters and nutrients exhibited clear spatial differentiation between the tidal creek bottom and its banks. This spatial pattern was mainly driven by tidally driven groundwater discharge, salinity-induced density flow, and biogeochemical reactions. Global data comparison indicated that mangrove areas were hotspots for SGD (median = 9.0 cm/d) and associated DSi median = 36.74 mmol/(m²·d) and DIP fluxes median = 0.21 mmol/(m²·d), while DIN flux median = 1.95 mmol/(m²·d) was relatively low. Additionally, significant differences existed in SGD rates and associated nutrient fluxes across different scales in mangrove areas. This study advances the understanding of nutrient transport and transformation processes in tidal creek systems, and provides a scientific basis for mangrove wetland conservation and coastal environmental management.