The lower Mississippi River Delta (MRD), inhabited by >2 million people, is a critical national resource in terms of maritime transport, fisheries, and energy. MRD wetlands and waters are degrading at alarming rates, losing wetlands equivalent to the area of Delaware since the 1930?s, and more over the next century. The health of coastal waters and the fate of wetlands are strongly governed by the flow of fresh water and accompanying sediments and nutrients, which are now largely controlled by engineered structures along the river. For these reasons, conservation and restoration of the MRD is one of the outstanding environmental and socioeconomic challenges faced by our country over the next century. Understanding and managing water, sediment, and nutrient flux and fate into the MRD coastal zone is central to this challenge. The Spring 2011 hydrograph of the Mississippi River is reaching levels exceeding the Great Flood of 1927 in many locations in Louisiana. To provide relief to river levees, water is being released through the Bonnet Carré Spillway (BCS) into Lake Ponchartrain (LP), a large mostly enclosed estuary within the MRD (Fig. 1). BCS opening on May 9 is now producing record fluxes of water (>8800 m3 s-1, 126% of design capacity) into LP, likely along with record fluxes of sediment and nutrients. Understanding the dynamics and impact of this plume in LP is important for several reasons: (1) nutrient loading to the lake will produce potentially toxic algal blooms; (2) fresh water flux will cause strong exchange of the fresher lake with the saltier coastal ocean; and (3), measurement and study of sediment supply and deposition will provide important information for use of sediment in engineered diversions for delta restoration. PIs will sample water, sediment, nutrients, and plankton over time to understand impacts to the lake, in order to better understand impacts from future manmade diversions for delta restoration. Because LP has strong similarities to other MRD estuaries, and engineered diversions of river water and sediment are primary tools planned for MRD conservation and restoration, results of this work will be directly applicable to large regions of the MRD in need of conservation and restoration.
Historic flooding on the Mississippi River in Spring 2011 triggered the opening of the Bonnet Carré Spillway for flood prevention, connecting the Mississippi River to Lake Pontchartrain over 42 days, reaching flows > 8,000 m3/s. The State of Louisiana is presently planning to use large river diversions, similar to the spillway, to provide water and sediment to the rapidly-disappearing Mississippi Delta, as a coastal restoration approach. However, few studies have documented the combined impacts of water, sediment, and nutrient discharge from a large diversion on a coastal water body. To use spillway operation as an analog for future river-sediment diversions, a nutrient and sediment budget was developed and phytoplankton populations studied by analyzing sediment cores and water samples collected across the ~1500 km2 lake before, during, and after spillway operation. Sediment measurements show that the plume rapidly dispersed across the lake and deposited a sediment mass of 2.45±1.35 Mt, accounting for approximately 100% of mud delivered from the spillway into the lake. This sediment retention rate is substantially higher than sediment retention in other diversion receiving basins (25-50%) that mostly have open boundaries with the coastal ocean, suggesting that diversions into enclosed basins represent an efficient use of muddy sediment for land building in the Mississippi River Delta. The Nitrate load from the Mississippi River exceeded 20,000 metric tons into Lake Pontchartrain after over one month of spillway operation. While some nitrate was removed by microbial processes(< 5 % of the total load) ,the vast majority of the nitrate was assimilated by phytoplankton (microscopic plants) or was shunted to the coastal ocean through two narrow connections to the Gulf of Mexico. The sediment in Lake Pontchartrain became enriched with phosphorous as a result of the spillway opening. This phosphorous was slowly released from sediments over the following three months, producing a long-lasting nutrient impact from the diversion opening. Work will continue to combine the total sediment load in the lake with specific pools of phosphorus to determine the effect of river diversions on resultant water quality in the receiving basin. No obvious algal bloom was detected during the summer of 2011. The maximum chlorophyll concentrations became elevated, but there was never a time when a single species dominated the water column. Toxins produced by blue-green algae growing in response to nutrient input concentrations were variable from the different stations and at depth. Particulate toxins (phycocyanin) levels did not exceed allowable concentrations, although some measurements of dissolved toxins did exceed allowable levels. This information was shared with the Louisiana Department of Health. Results of this study have been provided in invited testimony to three Louisiana state agencies, in order to inform public policy for coastal restoration.
