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.

Seasonality of Organic Matter Degradation Regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach

“…Consequently, there were zones in the sediment where pore waters had low DIN and low O 2 (Figures , , and S3). A similar coupling of nitrification and denitrification has been found in intertidal sands from Tolo Harbor, Hongkong (Liu et al, ) and in nearby subtidal sediments of the North Sea where N‐loss is high. In these permeable sediments, denitrification appears to be favored over anammox due to high organic matter input and rapid fluctuations in O 2 concentrations (Marchant et al, ; Marchant et al, ).…”

“…Further, tidal dynamics have been shown to even affect pore waters down to 3 mbsf. The observed tidal changes in DIN species were in the range of lower μM, and the overall species distributions did not change significantly (Liu et al, ). We cannot exclude that tidal variabilities might have overprinted our results.…”

Using Tidal Fluctuation‐Induced Dynamics of Radium Isotopes (224Ra, 223Ra, and 228Ra) to Trace the Hydrodynamics and Geochemical Reactions in a Coastal Groundwater Mixing Zone

“…Therefore, saline water at L2‐1, L2‐2, L2–3, and L3‐1 is from the USP driven by tidal fluctuation whereas saline water at L3–5 is from the SW driven by density differences. This can also be supported by different stable isotopic compositions (δ 2 H and δ 18 O) of the two groups of saline waters (Liu et al, ). δ 2 H and δ 18 O of groundwater from the USP (L2‐1: −14.47‰ and −3.06‰) are less negative than those of groundwater from the SW (L3–5: −27.69‰ and −4.53‰) even though they have similar degrees of mixing with seawater (salinity: 10.12 versus 9.69 psu).…”

“…Four water patterns, i.e., surficial fresh groundwater (FGW), deep FGW, saline water, and NSW were identified in this study according to stable isotopic compositions (δ 2 H and δ 18 O) and salinity as described in Liu et al () while redox conditions in these water patterns were presented and discussed in Liu et al (). As shown in Figure , a brief introduction to water patterns and redox conditions is given.…”

“…Total alkalinity and pH were applied to the program CO2SYS (Cao et al, ) to calculate carbonate components such as dissolved inorganic carbon (DIC), and p CO 2 . The detailed calibration and analysis procedures of nutrients ( , , , and ) via a flow injection analyzer (FIA) were described in Liu et al () and analytical errors are <10% for , <8% for , <3% for , and <5% for . Trace metals (Fe 2+ , Mn 2+ , and Sr 2+ ) were analyzed by a inductively coupled plasma optical emission spectrometer (ICP‐OES) with accuracies and precisions better than 4.7% and 4.8% for Fe 2+ , 7.9% and 2.1% for Mn 2+ , and 5% and 4.5% for Sr 2+ , respectively.…”