Stoichiometric imbalance in rates of nitrogen and phosphorus retention, storage, and recycling can perpetuate nitrogen deficiency in highly‐productive reservoirs

We measured the nutrient stoichiometry of inputs, outputs, retention, storage, and recycling in three seasonally nitrogen (N)‐deficient reservoirs by incorporating watershed mass balances with measurements of internal N and phosphorus (P) transformations. Our objective was to determine if the reservoirs were accumulating N and thereby likely to develop strict P deficiency over time. For the eutrophic reservoirs, the N : P (by atoms) of annual outputs was two to five times greater than that of inputs, reflecting higher retention efficiency for P than N (∼ 90% vs. ∼ 50%, respectively) and resulting in retention stoichiometry indicative of N deficiency (N : P < 20). The N : P of these fluxes differed less for the mesotrophic reservoir because of similar N and P retention efficiencies, and the N : P of retained nutrients indicated strict P deficiency (N : P > 50). Denitrification (12–23 g N m −2 yr −1 ) removed ∼ 50–100% of N retained by the reservoirs annually, increasing N deficiency in storage relative to retention for all the reservoirs (N : P < 1–30). The combined effects of more efficient P than N retention and efficient denitrification were also evident in the low N : P (< 10) of internal recycling. N 2 fixation (7–11 g N m −2 yr −1 ) was inefficient in balancing system N deficits and did not increase the low N : P of annual watershed inputs or seasonal epilimnion nutrient concentrations into the range of strict P deficiency. Low N : P storage and internal recycling strongly suggested that these reservoirs are not accumulating N relative to P and are thereby unlikely to become strictly P deficient over time.