Small and Mesoscale Processes of the Mississippi River Plume.
Gary L. Hitchcock
The Mississippi River is the single largest source of freshwater and dissolved inorganic nutrients discharged onto the continent shelf of the United States. During the annual discharge peak in spring the concentrations of inorganic nutrients can exceed 100 µM NO3-N and SiO3-Si in Louisiana shelf waters west of the Mississippi River delta. Although it is clear that the Mississippi River waters originating at the delta are the primary source of `new', or allochthonous, nutrients on the western Louisiana shelf, ship-based surveys have generally failed to discern a clear relationship between surface salinity, surface nutrient concentrations, and phytoplankton pigment concentrations in this region. In spring 1993 we (W. Wiseman, Jr., W. Boicourt, and G. Hitchcock) described the surface properties of the river plume originating at Southwest Pass by following surface drifters deployed in the feature. This Lagrangian view of the plume provided a description of the temporal evolution of water parcels as they transit the Louisiana Shelf.
In May 1993, the path of the plume was to the northwest in response to wind and the Coriolis force. The trajectories of drifters show the plume rapidly expands seaward of the mouth of the delta. Near the mouth of Southwest Pass the plume surface properties are dominated by a salinity minimum that corresponds to the turbidity maximum. As the plume thins and expands, the surface property fields (temperature, salinity, chlorophyll-a fluorescence, and transmittance) become increasingly variable. Surface salinity, chlorophyll-afluorescence, and transmissivity distributions in transects crossing the plume axis show that surface fields become increasingly patchy as the plume waters mix with adjacent shelf waters. The cross-plume gradients in salinity were strong, with minimum salinity at the shoreward edge of the feature. The increasingly patchy nature of the surface property distributions down the axis of the plume reflects mixing processes that facilitate entrainment of shelf waters across the strong pycnocline at the base of the feature. Inorganic nutrient (NO3-N, PO4-P, SiO3-Si) concentrations in plume surface waters decreased linearly with increasing salinity, indicating conservative mixing within the plume. Maximum phytoplankton concentrations in the plume (< 20 µg chl-a 1-1) were much less than those predicted from available nutrient levels. Thus small-scale mixing processes are a major determinant of ambient nutrient concentrations in the plume following the discharge of Mississippi River water from the delta.
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