Process based modelling of the nutrient dynamics in a tidal dominated coastal area
The Northern German Wadden Sea is characterized as a natural eutrophic system, which is subjected to an additional anthropogenic nutrient input deriving mainly from the discharge of the Elbe river through its seaward border. Information about the distribution and transport of the nutrients near to the coast is still insufficient, since most of the previous measurements were restricted to the tidal inlets only. The aim of the present study is to set up a nutrient dynamic model which takes into account the main Wadden Sea specific processes governing the dissolved inorganic nutrient distribution in the shallow area. Investigations were carried out in a semi-enclosed tidal flat area known as the Meldorf Bight. The hydrodynamics in the area is mainly dominated by tidal forcing with the mean tidal range of about 3.5m. During low tide the tidal flat area is dry and it is flooded during high tide. First, seasonal nutrient gradients and their transport patterns were evaluated. The seasonal cycle of dissolved inorganic nitrogen compounds corresponds to the usual pattern in temperate seas with maxima in winter and minima in spring and summer. In the case of phosphate, high concentrations prevailed also in late summer, which is a result of remobilization from the temporary iron hydroxide sediment buffer. Nutrient concentrations during ebb tide were slightly higher than the concentrations during flood tide indicating the importance of local sources near the coast. Due to a strong tidal mixing, the vertical variation of nutrient concentrations was found to be insignificant. Computed net transports of nutrients were found to be slightly different for spring, summer and winter. Export of phosphate was observed in late summer. Results from small scale spatial measurements and from tidal nutrient cycles were used as a data basis to set-up the nutrient dynamic model. The Delft3D modelling system (Delft Hydraulics, The Netherlands) was employed for the numerical analysis of the flow and nutrient dynamics. The model was set-up for the simulation of nitrate, ammonium and phosphate gradients. Measured nutrient concentrations at the model open sea boundaries were used as input data. A sensitivity analysis with respect to the model parameters such as the dispersion coefficient was carried out. Due to the complexity of the natural processes, only the seasonal dominant ones were selected for the calibration. The model results were compared to the measured data. There is a good agreement in winter when the nutrients are conservative and biological processes are minimal. During spring the influence of local freshwater discharge deriving from the hinterland drainage was found to be significant. The model is capable of capturing the impact due to the freshwater releases. Main nutrient trends are relatively well estimated, especially for nitrate and phosphate. In summer, denitrification of nitrate as well as ammonification and phosphorus remineralisation were accounted for in the model calibration. Although the trends in ammonium are well predicted by the model, some discrepancies between the measured and computed concentrations are still significant. However, the results of model validation confirm the ability of the calibrated model in predicting the short term nutrient distribution in the investigation area.