Elements needed to obtain mass balance of a nutrient in a coastal sea by direct methods are: movement of water masses (current field), estimates of location and intensity of all major nutrient sources, estimate of nutrient inflow from the neighboring sea and a reliable estimate of the nutrient extinction in the water column. In practice, the intensity of major sources and extinction of nutrient water column are seldom known. Therefore inverse methods for the transport equation are employed to obtain mass balance.

Mass balance of total phosphorus in the Izmit Bay for March 1994 suggests that total source from the coast is about 1.3 t P/day. The extinction constant is 12 days. The majority of TP originating from the land-based sources of the bay proper ends up in the lower layer of the bay. A small part is transported into the Marmara sea.