Effects of freshwater input on nutrient loading, phytoplankton biomass, and cyanotoxin production in an oligohaline estuarine lake
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Pulsed river water events can increase nutrient levels potentially translating into enhanced primary production, phytoplankton community shifts, and bloom formation. The Bonnet Carré Spillway is a managed river diversion which can be used to redirect a significant amount of Mississippi River water into Lake Pontchartrain, reducing the risks of flood in the downstream communities during runoff seasons. We investigated nutrient enrichment and consequent changes in phytoplankton biomass, including toxic species in Lake Pontchartrain during and after a 1-month Bonne Carré Spillway opening in 2008. Water samples were collected along a 30 km transect. A freshwater plume was found to have formed by the strong river input that had limited mixing with the lake during the opening. The plume and lake water gradually mixed together after the Spillway was closed, indicated by the reduction of the horizontal salinity gradient. The river pulse increased the lake nitrate and dissolved reactive phosphorus concentrations to more than five times the lake background in the plume stations. Nutrient concentrations decreased rapidly after the Spillway closure as the plume dissipated. Diatoms and chlorophytes dominated the system during the opening. After the Spillway closure, there was a shift over time from diatom dominance to toxic cyanobacteria dominance that corresponded to more stable, warmer, and nutrient-limited water conditions. Associated toxins were present and varied over time and space. Further research on the phytoplankton assemblages on the lake is needed in subsequent, non-Spillway opening years to evaluate the impact of river water pulses on the development of these toxic cyanobacterial blooms.
KeywordsRiver diversions Lake Pontchartrain Mississippi River Nutrient enrichment Cyanobacterial blooms Microcystin
Support for this research effort was provided by a National Science Foundation Grant (Grant no DEB-0833225). The work was also partially supported by the Shanghai Ocean University Program for International Cooperation (No. A-2302-10-0003), the Program of Science and Technology Commission of Shanghai Municipality (No. 09320503700), and the Leading Academic Discipline Project of Shanghai Municipal Education Commission (Project Number: J50702). We extend our thanks to the Field Support Group of the Coastal Studies Institute, particularly Eddie Weeks and Rodney Fredericks for providing us with valuable support in the field as well as Jeremy Conkle and Brett Marks of the Wetland and Aquatic Biogeochemistry Laboratory for sample collections. Maverick and Ben Leblanc and Ross Del Rio are acknowledged for assistance in nutrient and toxin analyses. The USGS is acknowledged for providing flow number and total discharge numbers for the opening.
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