Average residence time of matter in coastal waters in a transport system including biochemical processes

Abstract

A mechanism of transport of a matter such as nitrogen or phosphorus which is transformed between dissolved form and particulate form by biochemical processes is proposed. In coastal waters, there is often a seaward mean flow in the upper layer and a landward flow in the lower layer due to inflow of river water and nonlinear effect of tidal current. In such a flow system, nitrogen or phosphorus in the form of particulate organisms settles down to the lower layer, and is carried back to the inner region by the landward flow there. It then returns to dissolved form by decomposition, and goes back to the upper layer. Thus, such a circulation will make its average residence time larger than that of a matter which is dissolved and does not change its form. Efficiency of this mechanism is tested in the present paper using a simple transport model. The result shows critical differences between the average residence times in both cases; it is feasible in some cases that the average residence time of the matter which is transformed in such a way is larger than that of the dissolved one by an order of magnitude or more.