Mississippi River Flood 2008: Observations of a Large Freshwater Diversion on Physical, Chemical, and Biological Characteristics of a Shallow Estuarine Lake

J.R. White, R.W. Fulweiler, C.Y. Li, S. Bargu, N.D. Walker, R.R.Twilley, and S.E. Green.

High nitrogen (N) loading to coastal aquatic systems can be expressed as increased algal production and subsequent low dissolved oxygen. In April, 2008, predictions for extreme flood stage for the Lower Mississippi River triggered the opening of the Bonnet Carre Spillway, a major release valve for the river. The spillway diverted ~8 km^3 of water over one month of operation into Lake Pontchartrain with a concomitant 10000t of NO3-N. Satellite imagery, physical, water quality, and chlorophyll a (chl a) measurements show that the Mississippi River plume mixed with <40% of the lake during this time and much of the nutrient load was transported to the coastal ocean. Nitrate, dissolved reactive phosphorous (P), and dissolved silica (Si) concentrations were 4.8, 5.0, and 3.2 times higher, respectively, within the river plume when compared with those of the lake water. Despite the high nutrient concentrations within the river plume, a phytoplankton biomass, evidenced by cha l concentrations, was low. Much of the nutrient load appeared to bypass the lake and was transported to the coastal ocean during the opening of the diversion. The potential removal of a total 7.6% of the N load from the Mississippi Ricer during the one month of flood level flow may have been a contribting factor in the lower than predicted hypoxia zone off the Louisiana coast during the summer of 2008.

Ref: Environmental Science & Technology, Vol. 43, No. 15, June 23, 2009

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