Saltmarshes as selective nutrient filters: Insights from groundwater-derived nutrient exchange

Saltmarshes are commonly regarded as coastal filters for removing terrestrial nutrients. However, the behavior of different nutrient species in saltmarshes, in terms of removal or production, can vary greatly and is still poorly understood. In this study, we quantified groundwater-derived nutrient fluxes and examined their sources and pathways in a saltmarsh using multiple isotopes (²²²Rn, δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻ and δ¹⁵N-NH₄⁺). Our findings reveal that tidal-driven groundwater flow significantly facilitates the removal of nitrate and phosphorus from saltmarshes. However, it also leads to the release of substantial amounts of ammonium and dissolved silicate into coastal waters. This suggests that saltmarshes function more specifically as nitrate filters rather than general nutrient filters when it comes to removing anthropogenic solutes. As phytoplankton preferentially use ammonium, groundwater-derived large ammonium flux with high N/P ratio (∼158) in saltmarshes would significantly affect the nutrient structure and phytoplankton biomass of coastal seawater dominated by nitrate, suggesting that groundwater-derived ammonium export is probably a key driving force in controlling local algal blooms.