Nutrient Exchange between Sediments and Overlying Waters in the Modaomen Estuary (China) over a Complete Semidiurnal Tide Cycle: Implications of Saltwater Intrusion

Spatial and temporal variations of inorganic nutrients (NO3-, NO2-, NH4+, SiO42-, and PO43-) and some relevant environmental parameters, including temperature, salinity, pH, Eh, dissolved oxygen (DO), in surface sediments and overlying waters were investigated during the cruise held on 16 to 19 April 2016 over a complete tidal cycle (a spring and neap tide together) in the Modaomen estuary, Pearl River, China. It was found that the dominant component of dissolved inorganic nitrogen (DIN, expressed as sum of NO3-, NO2-, and NH4+) was NH4+ in sediment interstitial waters and NO3- in overlying waters. Overlying waters contained higher concentration of NO3 and lower concentrations of SiO4- and PO34- than interstitial waters. The exchange flux of inorganic nutrients between sediments and overlying waters was calculated using the Fick diffusion equation. Exchange flux calculations revealed that sediments were the sources of SiO42-, PO34-, and NH4+ but the sink of NO3- and NO2-. However, nutrimental exchange flux variations were not obvious during the present tidal cycle. Results of correlation analysis of single factor and principal component analysis revealed that nutrimental exchange fluxes mainly depend on the nutrient concentration gradient and overlying water chemical environment due to high stratification of waters, obvious homogeneity of sediments, and relative weakness of bioturbation in the Modaomen estuary