Spectroscopic measurements of estuarine dissolved organic matter dynamics during a large-scale Mississippi River flood diversion

Abstract

The Mississippi River Flood of 2011 prompted the opening of the Bonnet Carré Spillway (BCS) in southeastern Louisiana to protect the City of New Orleans. The BCS diverted approximately 21.9km3 of river water into the oligohaline Lake Pontchartrain Estuary over the course of 43days. We characterized estuarine dissolved organic matter (DOM) dynamics before, during, and after the diversion in order to better understand the biogeochemical dynamics associated with these immense freshwater inflows. Dissolved organic carbon (DOC) exhibited a large degree of variability during and after the period of elevated primary productivity that occurred following the diversion. Furthermore, DOC analysis provides limited insight into carbon cycling during these dynamic periods. In order to overcome the limitations of DOC, spectroscopic methods were used to gain insights into chemical composition dynamics. Both ultraviolet visible (A254, A350, SUVA254, spectral slope, and normalized UV/Vis) and fluorescence spectroscopy (excitation emission matrices and fluorescence and biological indices) were used to study the compositional changes of DOM over time. Collectively, our results document a perturbation in DOM chemistry in Lake Pontchartrain due to the diversion and a subsequent return toward pre-diversion conditions. Immediate increases in A350 indicate that BCS freshwater contained elevated concentrations of lignin of terrestrial origin. Ensuing declines in A350, along with changes in the fluorescence and biological indices, indicate that DOM rapidly became more microbial in composition. Our results provide insights into estuarine DOM dynamics relevant to systems receiving flood pulses of freshwater due to either hydrologic manipulation or precipitation events.