U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameBiogeochemical Characteristics of the Lower Mississippi River, USA, During June 2003
Document AuthorDagg, M.J.
Bianchi, T.S.
Breed, G.A.
Cai, W-J.
Duan, S.
Liu, H.
McKee, B.A.
Powell, R.T.
Stewart , C.M.
Short DescriptionEstuaries 28, 664–674
CategorySubgroup 2: Characterization of Nutrient Fate-Transport-Sources
Publication Year2005

Abstract: During June 2003, a period of mid level discharge (17,400 m23 s21), a parcel of water in the lower Mississippi River was sampled every 2 h during its 4-d transit from river km 362 near Baton Rouge to km 0 at Head of Passes, Louisiana, United States. Properties measured at the surface during each of the 48 stations were temperature, salinity, dissolved organic carbon (DOC), total dissolved nitrogen, dissolved macronutrients (NO3 + NO2, PO4, Si(OH)4), chlorophyll a (chl a; three size fractions: , 5 mm, 5–20 mm, and . 20 mm), pigment composition by HPLC, total suspended matter (TSM), particulate organic carbon (POC), and particulate nitrogen (PN). Air-water CO2 flux was calculated from surface water dissolved inorganic carbon and pH. During the 4 d transit, large particles appeared to be settling out of the surface water. Concentrations of chl a containing particles . 20 mm declined 37%, TSM declined 43%, POC declined 42% and PN declined 57%. Concentrations of the smaller chl a containing particles did not change suggesting only large particulate materials were settling. There was no measurable loss of dissolved NO3, PO4, or Si(OH)4, consistent with the observation that chl a did not increase during the 4-d transit. DOC declined slightly (3%). 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.