We investigated how physical and chemical factors affect Chlorophyll a concentrations in the Mississippi River, the largest river in North America, by sampling 878 times from February 1997 to December 2018 near its terminus at Baton Rouge, Louisiana. Measured values included temperature, light (secchi disk), suspended sediments, and nutrients (nitrate+nitrite, phosphate, TP, TN, ammonium, silicate) that may affect phytoplankton communities. The data are available for unrestricted use.

Water samples were taken from a wharf overhanging the left descending riverbank of the Mississippi River in Baton Rouge, LA, USA (30^o 26’ 34”N, 91^o11’ 29”W), located 233 km (365 km of river distance) from the debouching end. Sampling started February 1997 and ended December 2018 for a total of 878 collection events occurring at least once monthly. Chl a measured with a Turner Designs fluorometer after extraction in DMSO:Acetone, adjusted for flourometer lamp aging. Sample analysis used a Technicon Autoanalyzer II or LaChat Quick Chem analyzer for inorganic nutrients. Suspended solids were measure gravimetrically. A Secchi disk was used for water clarity.

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