Profiles of sediment porewater nutrient concentrations on tidal flat of turbidity maximum zone in Changjiang (Yangtze River) estuary

Profiles of nutrients (i.e NH₄⁺, NO₂^-, NO₃^-, PO₄^-, and SiO₃^2-) in porewater of sediment were investigated monthly in the intertidal flat of Chongming Dongtan in Changjiang (Yangtze River) estuary during March 2005 to February 2006. The results indicate that concentrations of NH₄⁺ and SiO₃^2- in porewater are always in the range of 200 μmol/L to 500 μmol/L and show different patterns of distribution profile among high, middle, and low marshes. Compared to high and middle marsh, low marsh, without vegetation coverage and of coarser grain size, generally contains higher NH₄ and lower SiO₃^2- concentrations. SiO₃^2- concentrations are predominantly determined by seasonal temperature change, but variations of NH₄⁺ are more complicated and prone to exhibit lower values in spring and summer when the vegetation above show relatively high growth rate. Concentrations of NO₂^- + NO₃^- and PO₄^3- in porewater are commonly much lower than NH₄⁺ and SiO₃^2- by 2 to 3 orders of magnitude, however, due to factors such as bioturbation, the concentrations in top sediment may be higher than 10 μmol/L. The relatively high NO₃^- concentration in top sediment, together with the rapid transfer from oxygenic to anoxic environment, make it possible that significant denitrification happens, which may draw its remarkable influence on the biogeochemical cycles of nutrient elements in the land-sea interaction area in Changjiang (Yangtze River) estuary.