Nutrient and phytoplankton data indicate poor environmental health in four oxbow lakes in central Louisiana suggesting that long-term agriculture practices and increases in shoreline development have accelerated eutrophication. Surface-water quality and phytoplankton indicators of eutrophication were examined at Lake Bruin, Lake St. John, Lake St. Joseph, and False River Lake along an eutrophication gradient. These oxbow lakes are cut-off meanders of the Mississippi River that do not receive overbank flow from the river due to the levee system built in the early twentieth century. Oxbows have formed at various times in the last few hundred years as the Mississippi River carves a more efficient hydrologic route to the Gulf of Mexico and exhibit a succession of stages in lake evolution, from deep and oligotrophic to shallow and eutrophic. Water-quality samples were collected three times per year: once in late spring, once in late summer-early fall, and once in winter. Water samples were analyzed for major ions, nutrients, suspended sediments, pesticides, dissolved organic carbon, and chlorophyll. At each site, physiochemical properties (water temperature, specific conductance, dissolved oxygen, and pH) were recorded at multiple depths within a single vertical profile. Phytoplankton community samples and cyanotoxin samples were collected from the photic zone at the time of water-quality sample collection at one site per lake. This data release provides water quality profile and phytoplankton data for these lakes.