Tidal Pumping-Induced Nutrients Dynamics and Biogeochemical Implications in an Intertidal Aquifer

Abstract

Tidal pumping is a major driving force affecting water exchange between land and sea, biogeochemical reactions in the intertidal aquifer, and nutrient loading to the sea. At a sandy beach of Tolo Harbour, Hong Kong, the nutrient (NH4+, NO2−, NO3−, and PO43−) dynamic in coastal groundwater mixing zone (CGMZ) is found to be fluctuated with tidal oscillation. Nutrient dynamic is mainly controlled by tidal pumping‐induced organic matter that serves as a reagent of remineralization in the aquifer. NH4+, NO2−, and PO43− are positively correlated with salinity. Both NH4+ and PO43− have negative correlations with oxidation/reduction potential. NH4+ is the major dissolved inorganic nitrogen species in CGMZ. The adsorption of PO43− onto iron oxides occurs at the deep transition zone with a salinity of 5–10 practical salinity unit (psu), and intensive N‐loss occurs in near‐surface area with a salinity of 10–25 psu. The biogeochemical reactions, producing PO43− and consuming NH4+, are synergistic effect of remineralization‐nitrification‐denitrification. In CGMZ, the annual NH4+ loss is estimated to be ~ 4.32 × 105 mol, while the minimum annual PO43− production is estimated to be ~ 2.55 × 104 mol. Applying these rates to the entire Tolo Harbour, the annual NH4+ input to the harbor through the remineralization of organic matters is estimated to be ~ 1.02 × 107 mol. The annual NH4+ loss via nitrification is 1.32 × 107 mol, and the annual PO43− production is ~ 7.76 × 105 mol.