Legacy Phosphorus Implications in the Lake Pontchartrain Estuary Sediment Due to the 2011 Bonnet Carre Spillway Opening

Abstract

Phosphorus (P) is an essential nutrient for life and excess P in aquatic systems can trigger algal blooms. Nutrient-rich Mississippi River water is diverted into the Lake Pontchartrain estuary (LPE) periodically through the Bonnet Carré Spillway (BCS) to avoid downstream flooding to the city of New Orleans, Louisiana and can significantly increase the internal P load in the sediment. A sequential P fractionation procedure was performed on sediments collected before the opening and after the closing of the 2011 BCS operation to understand the role of these large river diversions on P dynamics. Before the 2011 BCS opening, 10,368 Mt of P were found in the 0-10 cm sediment interval of the LPE. After the closure of the spillway, 13,293 Mt of P were measured in the 0-10 cm sediment interval. Total P significantly increased by 28% and a mass loading of 2,925 Mt of P was a consequence of the 42 days BCS opening. Sediment grain size analyzes revealed that majority of the finer sand and silt fractions were deposited near the BCS entrance, while the lighter clay fraction were transported tens of km towards the center of the LPE. Calculating the time for all newly added P from the 2011 BCS operation to flux out of the water column is important to understanding the impact on water quality of the LPE. Using a previously determined linear flux rate of ~517 Mt yr-1, it was estimated that ~6 years are needed to flux out the newly added sediment TP. While assuming a nonlinear model of flux rate, it is suggested that it would take a longer period of time to flux out all the newly P loaded from the sediments in the LPE. If the operations of these BCS opening are closely spaced in time (< 6 years), there will be an increase of P in the sediments which could lead to changes in the trophic status of the LPE. This increase of TP in sediments can lead to an increase in the frequency and persistence of harmful algal blooms for many years after the spillway opening.