This paper presents evidence of a seasonal shift from P to N as the nutrient limiting the accumulation of algal biomass in Chesapeake Bay. Following the winter/spring maximum in freshwater runoff, (1) the ratio of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN/PO,) was greater than the N/P of algal biomass; (2) alkaline phosphatase activity was high; (3) phosphate turnover times were short; (4) ammonium turnover times were long; and (5) growth rates of phytoplankton were stimulated by additions of phosphate but not by additions of ammonium or silicate. During the period of low runoff in summer, all indicators reversed, and N limited algal growth rates. Silicate concentrations also showed evidence of biological depletion in spring, which may have limited diatom abundance. Due to the concordance of all indicators at large and small scales, we argue that phytoplankton growth rates exert primary control over biomass accumulation. We conclude that P and Si limit the accumulation of algal biomass along the major axis of Chesapeake Bay in spring, whereas N limits algal accumulation in summer, similar to the conclusions of D'Elia et al. (1986; Can. J. Fish. Aquat. Sci. 43: 397-406) for the Patuxent subestuary. Controlling eutrophication of the Bay and its subestuaries will require basin-specific management practices for both N and P reductions in influent waters. Such management efforts will provlde ecosystem tests of nutrient hmitation on a scale similar to those successfully conducted in lakes.