CONSIDERATIONS IN MODELING THE SEDIMENT WATER EXCHANGE OF PHOSPHORUS

The potential to release accumulated phosphorus from sediments has bee n the major motive to study and to model the fate of this nutrient in sediments. For the dynamics of the sediment-water interaction the size s of the pools involved and the rates of conversion/transport from one pool to another are of primary interest. As the sediment pools for ph osphate are generally much larger than the pools in the water column, a rather slow adjustment of the sediment to management measures will o ccur. For the analysis of management measures it is obvious that the g radual change in sediment composition must be taken into account. Only for rather short periods the sediment composition can be assumed to b e constant; this may be appropriate for studies of e.g. the annual cyc le. The sediment-water interaction is a complex resultant of physical, chemical and biological processes, including: physical processes: adv ection due to seepage or consolidation, pore-water diffusion, transpor t and mixing of solids by resuspension, sedimentation and bioturbation . chemical processes: adsorption and desorption, dissolution and (co)p recipitation, inclusion. biological processes: mineralization of a wid e range of organic compounds by various (micro)organisms, each with th eir own nutrient requirements and electron acceptors. Aspects which ar e discussed and need to be considered in application of a model in res earch or management are the level of aggregation and detail that is re quired and may still be practical, the spatial and temporal scales whi ch are applicable for the processes mentioned and their influence upon the numerical dispersion and model stability, the availability of dat a for calibration/validation and the resolution of the analytical tech niques. These aspects are not independent however. Frequently models a re not functional because they contain details which are either unnece ssary or suggest a feigned accuracy which is not justified by analytic al and experimental resolution of system characteristics.