Physicochemical Controls on Depth-Dependent Nutrient Mobility in the Intertidal Flat of a Coastal Lagoon

In this study, we investigated how porewater salinity, temperature, ionic strength, and nutrient behavior vary with depth in the intertidal flats of Lake Komuke, a coastal lagoon in northern Japan. A central feature of this work is the use of nutrient activity and activity coefficients—thermodynamic parameters that more directly represent ion mobility—rather than concentrations alone. Statistical analyses showed that salinity exhibited clear depth-dependent variation and was the primary factor associated with changes in nutrient behavior, whereas temperature showed minimal variation and no detectable effect. Physicochemical modeling using the Pitzer approach demonstrated that increases in salinity and ionic strength with depth led to reductions in the activity coefficients of NO3−, NH4+, and PO43−, with PO43− showing the greatest sensitivity due to its trivalent charge. Nutrient activities displayed contrasting vertical patterns: NO3− and NH4+ tended to increase with depth, whereas PO43− exhibited a peak at −20 cm followed by lower values at deeper, more saline layers. These results indicate that subsurface nutrient mobility in coastal tidal flats is shaped primarily by ionic strength-driven non-ideal behavior and associated geochemical gradients. The findings provide baseline information for understanding nutrient dynamics in brackish sediments and support the improved assessment of subsurface biogeochemical processes in intertidal ecosystems.