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Large and turbid rivers have varying temperatures, light conditions, nutrient availability, and nutrient ratios that may affect phytoplankton communities and occur within a changing world of point and nonpoint source nutrient loadings. We investigated how these physical and chemical factors affect Chlorophyll a(Chl 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. We hypothesized that nutrient concentrations and ratios were significant factors limiting phytoplankton biomass accumulations in this turbid river. The Chl aconcentrations were in the “poor” water quality category when above 20 μg Chl aL−112% of the time. Two percent of the samples were > 40 μg Chl aL−1and occurred on declining discharges. Results from graphical analysis and a principal component regression analysis showed that the highest Chl avalues were constrained to when dissolved silicate: dissolved inorganic nitrogen (DIN) molar ratios < 1 : 1 and dissolved DIN : phosphate molar ratios < 16 : 1, which is when the phytoplankton community likely consists of non‐siliceous phytoplankton. Increasing light conditions and reducing turbulence, which happens when river water is diverted into calmer and shallower waters, will create a phytoplankton bloom—perhaps becoming a harmful algal bloom—as has happened previously.