Comparative analysis of nutrient data in the lower Mississippi River

As the second phase of a large-scale Tulane University Mississippi River water-quality database project, nutrient data on the lower Mississippi River have been examined for potential differences among sampling agencies, geographic locations and chemically similar nitrogen parameters. These data represent the most comprehensive source of information on nutrients in the Mississippi River available as a single database. Monthly means, grouped by parameter, sampling location and agency, were calculated and compared as paired sets, excluding months where data were not available for both sets. Evaluations using ANOVA indicated few differences among agencies providing nitrogen-compound data, although differences were observed in phosphorus-compound data from different sources. Unfiltered and filtered fractions of nitrate–nitrite were found to be comparable throughout the study area. Spatial examination of the nutrient data indicated few differences in inorganic nitrogen concentrations throughout the study area, but significant variability in total Kjeldahl nitrogen (TKN). Orthophosphate and total phosphorus concentrations increased through the study area from Arkansas [river miles (RM) 500–810] downstream to St. Francisville (RM 231–315), but with little or no change from Baton Rouge (RM 111–230) to the Gulf of Mexico. Results provide a combined data set from which preliminary calculations of daily nutrient loads in the Mississippi River between 1960 and 1997 were conducted.