Legacy Phosphorus Implications of a large-scale Mississippi River Diversion into an Oligohaline Estuary Abstract The opening of the Bonnet Carré Spillway (BCS) provides a flood release valve for the Mississippi River protecting downstream New Orleans. The spillway discharges high nutrient and sediment loads into Lake Pontchartrain. Nutrient rich floodwaters could potentially trigger formation of algal blooms throughout the lake. In addition, sediments bound P released later in the summer was correlated to N-fixing harmful algae blooms. The total phosphorus pool in the 0-10cm was 10,368 mt before the BCS opening and 13,293 mt after the opening. Sediment total phosphorus concentrations prior to the opening were 525 and 515 mg/kg in the 0-5 and 5-10cm interval, respectively. After the closure of the BCS, the TP concentration increased to 677 and 656 mg/kg at 0-5 and 5-10cm intervals, respectively. The operation of the spillway led to a significant increase of ~28% of sediment TP concentration in the 0-10cm interval. The most labile P fraction increased by ~24% after the BCS opening. From core measurements on P sediment flux, it is estimated that it will take 5.66 years before the surficial sediment returns to the pre-opening TP level. This increase in P loading can therefore trigger algal blooms for several years post closure.