Fates of dissolved and particulate materials from the Mississippi River immediately after discharge into the northern Gulf of Mexico, USA, during a period of low wind stress

M.J. Dagg1,*, T. Bianchi2,a, B. McKee2,b, and R. Powell1

 

1Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, Louisiana 70344, USA

2Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA

 

 

*Corresponding author: mdagg@lumcon.edu

 

Abstract: In June 2003, we conducted a two-part field exercise to examine biogeochemical characteristics of water in the lower Mississippi River during the 4 days prior to discharge and in the Mississippi River plume over 2 days after discharge. Here we describe the fates of materials immediately after their discharge through Southwest Pass of the Mississippi Delta into the northern Gulf of Mexico. Changes in surface water properties immediately after discharge were much larger and more rapid than changes prior to discharge. Total suspended matter (TSM) declined, probably due to sinking, dissolved macronutrients were rapidly diminished by mixing and biological uptake, and phytoplankton populations increased dramatically, then declined. This decline appeared to begin at salinities of approximately 10 and was nearly complete by 15. A large increase in dissolved organic carbon (DOC) occurred over approximately the same salinity range. Weak winds (< 2 m s-1) during and preceding this cruise apparently led to the formation of an extensive but thin freshwater lens from the river. This lens spread widely without much mixing, and the bloom of phytoplankton that occurred between discharge and a salinity of 10 was probably a freshwater community seeded from the lower river. Phytoplankton bloomed for a period of about 1-2 days, then declined dramatically, apparently releasing large amounts of DOC. Macronutrients from the river were utilized by the river phytoplankton community in the extensive freshwater lens. This contrasted with the more typical situation in which river nutrients stimulate a marine phytoplankton bloom at salinities in the mid-20s.  We concluded that the direct effects of dissolved and particulate bio-reactive materials discharged by the Mississippi River were spatially restricted at this time to low salinity water, at least as surface phenomena. After being transported through the lower river essentially unaltered, these materials were biogeochemically processed within days and 10s of km. More generally, the mixing rate of plume water with receiving oceanic water has profound effects on the food web structure and biogeochemical cycling in the plume.

 

REFERENCE: Dagg, M.J., T. Bianchi, B. McKee, and R. Powell. 2006. Fates of dissolved and particulate materials from the Mississippi River immediately after discharge into the northern Gulf of Mexico, USA, during a period of low wind stress. Cont. Shelf Res. (in press November 2006).

 

a Current address: Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA

b Current address: Department of Marine Sciences, University of North Carolina at Chapel Hill, CB 3300, 12-7 Venable Hall, Chapel Hill, North Carolina 27599, USA