Biogeochemical characteristics of the lower Mississippi River, USA, during June 2003

M.J. Dagg1,*, T.S. Bianchi2,a, G.A. Breed1,b, W.-J. Cai3, S. Duan2, H. Liu1,c, B.A. McKee2,d, R.T. Powell1 and C.M. Stewart2


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

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

3Department of Marine Sciences, University of Georgia, Athens, Georgia 30602-3636



*Corresponding author:


Abstract: During June 2003, a period of mid-level discharge (17,400 m-3 s-1), a parcel of water in the lower Mississippi River was sampled every 2 h during its 4 d transit from 362 km upriver near Baton Rouge, Louisiana, USA to km 0 at Head of Passes, Louisiana, USA.  Properties measured at the surface during each of the 48 stations were temperature, salinity, dissolved organic carbon (DOC) and total dissolved nitrogen (TDN), dissolved macro-nutrients (NO3 + NO2, PO4, Si(OH)4), chlorophyll a (three size fractions: < 5 mm, 5-20 mm, and > 20 mm), pigment composition by HPLC, total suspended matter (TSM), and particulate carbon and nitrogen (POC and PN). In addition, air-water CO2 flux was calculated from surface water dissolved inorganic carbon (DIC) and pH. During the 4 d transit, large particles appeared to be settling out of the surface water. Concentrations of chlorophyll a containing particles > 20 mm declined 37%, TSM declined 43 %, POC declined 42 % and PN declined 57%.  Concentrations of the smaller chlorophyll a containing particles did not change suggesting only large particulate materials were settling. There was no measurable drawdown of dissolved NO3, PO4 or Si(OH)4, consistent with the observation that chlorophyll a did not increase during the 4 d transit. Dissolved organic carbon (DOC) declined slightly (3 %). Overall, these data indicate there was little autotrophic or heterotrophic activity in the lower Mississippi River at this time, but the system was slightly net heterotrophic.


REFERENCE: Dagg, M.J., T.S. Bianchi, G.A. Breed, W.-J. Cai, S. Duan, H. Liu, B.A. McKee, R.T. Powell and C.M. Stewart. 2005. Biogeochemical characteristics of the lower Mississippi River, USA, during June 2003. Estuaries 28: 664-674.


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

b Current address: Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

c Current address: Department of Biology; Atmospheric, Marine, and Coastal Environment (ACME) Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

d 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