Research Mississippi River Flood of 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 † Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, Department of Earth Sciences, Boston University, Boston, Massachusetts 02215, College of Marine Sciences, Shanghai Ocean University, Shanghai, China Received February 3, 2009. Revised manuscript received May 26, 2009. Accepted June 23, 2009. 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 10000 t of NO 3 -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 phosphorus (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, phytoplankton biomass, evidenced by chl a 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 of 7.6% of the N load from the Mississippi River during the one month of flood level flow may have been a contributing factor in the lower than predicted hypoxia zone off the Louisiana coast during the summer of 2008. Introduction Historically, seasonal flooding along the Mississippi River brought restorative water, sediment, and nutrients to the adjacent wetland systems. The construction of flood control levees in the early 1900s resulted in the Mississippi River becoming hydrologically disconnected from these riparian wetlands (1). Levees are designed to hold back seasonal flooding and thus prevent the transport of sediment and nutrients to the surrounding wetlands and water bodies (2). The loss of the seasonal pulse of water, sediment, and nutrients has been hypothesized to be a significant driving force in the decline and loss of the wetlands in Louisiana (3, 4). As part of a comprehensive restoration effort, the State of Louisiana has developed a plan for diverting freshwater from the Mississippi River to its surrounding wetland systems on a seasonal basis, thus also delivering sediment and nutrients. Currently there are four major freshwater diversion structures in the Lower Mississippi Delta that can control the delivery of the Mississippi River to specific coastal basins: Wax Lake Outlet delivers river water to western Atchafalaya Bay; Davis Pond delivers river water to Barataria Estuary; Caernarvon delivers river water to Breton Sound; and Bonnet Carre ´ Spillway delivers river water into Lake Pontchartrain. While the other three diversions are operated on an annual basis, the Bonnet Carre ´ Spillway is opened only under extremely high Mississippi River flood stage predictions. After the Great Flood of 1927, the Bonnet Carre ´ Spillway was constructed primarily to provide an outlet for the Mississippi River in order to prevent massive flooding of New Orleans (5). The Bonnet Carre ´ Spillway, located just upstream and 19 km west of the city, can alleviate pressure from dangerous flood stage river heights by directing up to 7080 m 3 s -1 (250000 ft 3 s -1 ) of water (∼ 20% of the flood level Mississippi River capacity) into Lake Pontchartrain and into the Gulf of Mexico on the eastern side of the bird’s foot delta, away from the area of hypoxia found on the Louisiana-Texas shelf. The Bonnet Carre ´ Spillway, therefore, has the potential of diverting significant nutrients loads from the bird’s foot delta, while the Caernarvon and Davis Pond diversions can divert considerably less water at maximum flow rates of 226 and 283 m 3 s -1 (8000 and 10000 ft 3 s -1 ), respectively. Since its construction, the spillway has been opened nine times (6), most recently in the spring of 2008. We monitored the physical, chemical, and biological char- acteristics of the Mississippi River plume as it interacted with the lake as part of a larger study on the effects of the 2008 river pulse on Lake Pontchartrain. Over a one month period (4/11-5/8/08) the Bonnet Carre ´ Spillway discharged a total of ∼ 7.96 km 3 into Lake Pontchartrain with a peak discharge of 4785 m 3 s -1 (169000 ft 3 s -1 ) on April 22, 2008, approximately 13.5% of the lower Mississippi River hydraulic load and nitrate load at the peak of diversion opening (7). The goal of this paper is to describe the physical, chemical, and biological characteristics of the Mississippi River flood plume in Lake Pontchartrain using satellite remote sensing and measures of water quality collected from areas within and outside the plume and to calculate the nutrient loads diverted from the river. Materials and Methods Study Area. Lake Pontchartrain (Figure 1) is a quasi-enclosed brackish body of water oriented east-west along its major axis, spanning 66 km in the east-west direction and 40 km in the north-south direction. The water is shallow in most parts of the lake with an average depth of 3.7 m, and the lake occupies an area of 1630 km 2 , with a volume of roughly 6 km 3 located just north of New Orleans, LA (8). Under normal conditions, the lake water exchanges with the Gulf of Mexico through three restricted outlets located to the east and southeast (Figure 1). There are also several small rivers entering the lake along the north rim. The residence time of the lake water is ∼57 days (8). Salinity in the lake varies * Corresponding author phone: (225) 578-8792; fax: (225) 578- 6423; e-mail: jrwhite@lsu.edu. † Louisiana State University. ‡ Boston University. § Shanghai Ocean University. 10.1021/es900318t CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 15, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5599 Published on Web 07/08/2009 Downloaded by LOUISIANA SCHOOLS on July 31, 2009 Published on July 8, 2009 on http://pubs.acs.org | doi: 10.1021/es900318t