Lake Okeechobee experiences blooms of cyanobacteria, perhaps due in part to the low nitrogen:phosphorus (N:P) ratios found in the water column, which may favor cyanobacteria over other phytoplankton taxa. The current strategy for managing the lake is to reduce external P loads. Some have recommended that pumping of N-rich water from the Everglades Agricultural Area (EAA), located adjacent to the south end of the lake, into Lake Okeechobee could increase N:P ratios and reduce cyanobacterial blooms until P loads are reduced. We conducted experiments to assess potential impacts of that strategy on phytoplankton in the lake's south pelagic region. Threein situexperiments were conducted during July 1994, with triplicated treatments (P, N, N+P, or EAA canal water additions) applied to 20-L clear plastic carboys, which were incubated in the lake for 3 days. In the first experiment, additions of P (50 μg · L−1) did not stimulate chlorophyllaor net primary productivity. Additions of N (500 μg · L−1) led to significant increases in both parameters. The greatest biomass and productivity increases (5-fold) occurred when the N and P were added together. In the second experiment, additions of N at 1X, 2X, and 4X the concentrations utilized in experiment 1 caused 3-fold increases in chlorophyllaand primary productivity. Nearly the same results were obtained in the third experiment, where EAA canal water was added to lake water in a 1:10 or 1:5 ratio, increasing soluble reactive P (SRP) by 17 and 34 μ · L−1, and dissolved inorganic nitrogen (DIN) by 175 and 350 μg · L−1, respectively. Although agricultural canal water often has been shown to contain herbicides which can suppress algal growth, no such effect was observed here. The yields of chlorophyllaper unit SRP or DIN in experiment 3, where canal water was added, were nearly identical to those in experiments 1 and 2, where N or N+P were added. In none of the experiments was there a shift from cyanobacteria to other algal taxa, despite the presence of small rapidly-growing chlorophytes in the lake water inoculum. The phytoplankton was dominated by cyanobacteria (70 to 81% of total phytoplankton densities), especiallyLyngbya, Oscillatoria, Chroococcus, andAphanocapsa, in all treatments, which encompassed DIN:SRP ratios from below 1:1 to over 140:1. From a management standpoint, these results indicate a potential for increased biomass of cyanobacteria in this N-limited lake if water pumping from the EAA were to be substantially increased.