The effects of wind-driven resuspension events on the flux of metals across the sediment-water interface in a shallow estuarine environment, Lake Pontchartrain, LA

Lake Pontchartrain is a shallow estuary (~ 3.7 m average depth), which undergoes frequent storm and wind events. These events provide the energy necessary to resuspend the near-surface sediment, allowing metals in the dissolved and particulate phases to enter the water column. In this study a suite of redox sensitive elements (Fe, Mn, Mo, U, V, Cr) were measured in porewaters, on a monthly basis, to determine the effects of mixing on the porewater chemistry of the surface sediment (down to 20 cm depth). Porewater concentrations of Mn, Mo, U, V, and Cr were affected by resuspension events, however Fe did not show any major changes in its concentration over time. This is most likely due to Lake Pontchartrain being a Mn-dominated system where Fe reduction does not occur at the depths affected by resuspension. Porewater Mn concentrations were greatest in May and June possibly due to a short period of quiescence prior to sampling allowing a build-up of reduced Mn in the porewaters. Soluble U concentrations were also greatest in May reflecting an association with Mn oxides. Mo showed a similar trend. Porewater Cr and V concentrations were highest in June and August suggesting that processes other than Mn cycling contribute to the control of V and Cr geochemistry. Previous studies have suggested that the seasonality of wind and storm events promote a similar seasonality in the resuspension and mixing of sediment at depth. The frequency and intensity of storm events increases in the fall and winter months. However, the porewater metals data did not suggest that there were any seasonal trends in the flux of dissolved metals across the sediment-water interface. The data suggested that there is sufficient energy in the smaller events to alter the surface-sediment chemistry that large winter storms are not needed. A fully instrumented weather station was constructed at our field site in the western portion of Lake Pontchartrain (30 18.800 N, 90 16.831 W). Using the measurements from the platform combined with short-lived radioisotope data, a resuspension model was created that describes the influences of wind speed and direction on depth of resuspension. This model also suggests that the resuspension events affecting the porewater chemistry are less seasonal and more stochastic.