This study aims at determining the phosphorus loads to Lake Uluabat. Following the calibration, SWAT was used to model the phosphorus and sediment transport. For the landuse and conditions assumed, it was concluded that a load of 4 grP/m /year reached to the lake. The amount of fertilizer applied, the loads from the Emet ve Orhaneli watersheds and presence of agricultural lands around the lake were the factors effecting the phosphorus input. Improvements in these conditions decreased the phosphorus load to the lake by half. But still, this value was higher than the target load of 1.0 gP/m /year. The phosphorus index developed for the watershed showed that there was a high risk of phosphorus transport through the agricultural lands in the watershed except the south region. Rainfall amount and fertilization method were among the factors that enhanced the risk of phosphorus transport. The measurements in the lake showed that algae, dissolved oxygen, and nutrient concentrations were increasing in the summer months. A small number of measurements indicated phosphorus limitation in the lake, however, this limitation was not strong as the total nitrogen to total phosphorus ratio was at the borderline. The results obtained with WASP showed that a 50% reduction in the phosphorus loads might significantly decrease the phosphorus concentrations in the lake. However, in order to have sound conclusions, sediment activities should be modeled as well.