Porewater-derived nutrient biogeochemistry in a saltmarsh-fringed estuary under intensive human activity

Saltmarshes in estuaries with intensive human activity regulate coastal nutrient biogeochemistry, however, limited studies have quantified porewater-derived nutrient fluxes and identified the implication on coastal nutrients cycling. Here, we hypothesize that porewater-derived nutrient fluxes may contribute to coastal water quality issues. Spatial investigation was conducted in a saltmarsh-fringed estuary with multi-species saltmarshes. Based on a radon mass balance model, porewater-derived fluxes of nitrogen (including total nitrogen, nitrate, nitrite, and ammonium), phosphorus (total phosphorus and orthophosphate) and silicate were estimated. While nitrate was absorbed by saltmarshes, ammonium was export from porewater to surface water. Nitrate flux was supposed to provide nitrogen for saltmarsh species and ammonium flux was considered as the production of organic nitrogen mineralization. Furthermore, obvious inorganic phosphorus sink and silicate source were observed in the saltmarsh. According to nutrient fluxes and related N/P ratio, porewater-derived nitrogen and phosphorus fluxes can significantly mitigate the eutrophication in nearby water body.