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 (similar to 90% vs. similar to 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 similar to 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.