A research team led by Prof. XIAO Kai from the Yantai Institute of Coastal Zone Research of the Chinese Academy of Sciences, has systematically elucidated the transformation and transport processes of nutrients in intertidal groundwater. The team employed a combined methodological approach, including multi-depth groundwater sampling, dynamic monitoring, stable isotope tracing, and multivariate statistical analysis. Their findings provide scientific evidence to deepen understanding of the terrestrial drivers of nearshore eutrophication.

Sandy beaches and mudflats account for 31% and 14% of the world's ice-free coastlines, respectively. Their intertidal aquifers act as critical interfaces regulating the composition and flux of nutrient inputs into marine environments. Previous research often treated intertidal sediments as a homogeneous system. However, a pervasive vertical stratification structure—characterized by fine-grained surface sediments overlying coarse-grained layers—is commonly observed in natural settings. How this stratification influences the migration, transformation, and seaward transport fluxes of nutrients in groundwater, along with its underlying mechanisms, remains poorly understood.

To address this knowledge gap, the research team selected typical sandy beach and mudflat intertidal profiles and conducted multi-depth groundwater sampling. They systematically analyzed the spatial distribution characteristics of dissolved nitrogen, phosphorus, silicon, and carbon, while simultaneously monitoring groundwater level fluctuations to identify the key driving mechanisms of nutrient migration and transformation in intertidal groundwater.

Results reveal distinct spatial distribution differences in nutrients and carbon within the groundwater between the fine-grained and coarse-grained layers in both sandy and muddy beaches. In sandy beaches, groundwater chemistry shifts from NO₃^--dominated in deep coarse-grained sediments (attenuated via denitrification) to NH₄⁺-dominated in surface fine-grained sediments (primarily from organic matter mineralization). In muddy beaches, fine-grained sediments also accumulated high concentrations of NH₄⁺, PO₄^3-, and dissolved silicon. Integrating cluster analysis, Mantel analysis, end-member mixing models, and isotope tracing (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), the study shows these patterns arise from synergistic regulation by hydrological dynamics and biogeochemical reactions.

Additionally, sandy beaches function as hotspots for groundwater discharge, with dissolved nutrient and carbon fluxes transported being two orders of magnitude higher than those from muddy beaches. The presence of fine-grained surface sediments enhances groundwater-borne fluxes of dissolved inorganic carbon (DIC) and NH₄⁺, potentially exacerbating nearshore eutrophication and acidification risks. These findings indicate that vertical stratification in intertidal sediments is a critical factor regulating the composition and flux of nutrient and carbon delivery to coastal waters via groundwater. The researchers proposed that controlling sediment structure could enhance the natural purification capacity of tidal flats for terrestrial nutrients.

The study was recently published in Journal of Hydrology. This work was supported by the National Natural Science Foundation of China, the Taishan Scholars Program, and other funding sources.

Schematic diagram of nitrogen cycling in intertidal groundwater. (Image by Prof. XIAO Kai's team)