Tributary phosphorus loading to Lake Ontario

► Phosphorus (P) loading was estimated for 145 streams entering Lake Ontario. ► U.S. and Canadian tributaries contributed 1411 and 1195 mT P/year. ► Tributary P load to Lake Ontario was 234% higher than that of WWTPs. ► Tributary P load was 70.6% of the 2004 Niagara River P load. ► Tributary P loading is a driver of nearshore lake conditions.