Fuels for modern internal combustion engines contain nitrogen and phosphorus compounds as additives (e.g., alkyl phosphate, amine phosphate, etc.). In addition, nitrogen oxides are formed from atmospheric gases during combustion. In the case of marine engines, exhaust gases and incompletely burned fuel and oil are discharged directly into the water. In situ enclosure experiments were conducted to quantify nitrogen and phosphorus additions from two- and four-cycle outboard engines per liter of fuel consumed. The modern two-cycle outboard engine added to test tanks 8,600 mg carbon dioxide, 300 mg total nitrogen, and 1.0 mg total phosphorus per liter fuel consumed. A motorboat census was conducted on Twin Lakes, Kootenai County, Idaho, throughout the 1986 boating season and annual fuel consumption was estimated. Phosphorus and nitrogen loading to Twin Lakes from motorboat engine discharges were calculated and are low when compared to loading from other sources.