Biological N₂ fixation hypothetically could balance the N∶P supply ratio in lakes and reservoirs, causing P to be the ultimate limiting element. However, the efficiency and time scale of this mechanism has been poorly studied and remains largely unknown. We compared the relative rates of N supply to the epilimnion of 3 reservoirs of similar age and morphology but with differing external nutrient inputs. Our objective was to determine if N₂-fixation rates in these reservoirs exceeded other internal N supply pathways during the growing season, a situation that would suggest that these systems are still evolving in terms of nutrient-limitation status. Phytoplankton N₂-fixation rates ranged from 3.4 to 16 g N m⁻² y⁻¹ in the reservoirs. These rates are among the highest N₂-fixation rates reported in the scientific literature. Measureable N₂ fixation also occurred in sediments in contact with reservoir epilimnia (0.85–1.5 g N m⁻² y⁻¹). Sediment N₂-fixation rates exceeded rates of N regeneration from epilimnetic sediments (0.10–0.24 g N m⁻² y⁻¹), a result suggesting that much of this fixed N either accumulates seasonally in sediments or is lost via coupled nitrification–denitrification. NH₄ flux from hypolimnia (0.13–1.8 g N m⁻² y⁻¹) and lower metalimnia (∼0.00–1.8 g N m⁻² y⁻¹) was a substantial internal N supply to epilimnia during summer stratification. However, phytoplankton N₂-fixation rates always exceeded rates of N input from these internal sources. These findings suggest that, even though the annual cycle of ecosystem N inputs via N₂ fixation has occurred over many years or decades in these reservoirs, reactive N accumulation has not been sufficient to alleviate seasonal N deficiency and lead to perpetual P limitation of phytoplankton.