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 22 yr 21 ) 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 22 yr 21 ) 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.

We measured in situ hypolimnion and metalimnion dinitrogen (N 2 ) accumulation rates and N 2 production from sediments in contact with the reservoir epilimnia for 3 small (, 1 km 2 ) and shallow reservoirs with inherently low stratification stability. Hypolimnion areal denitrification rates ranged from 98.6 to 168 mmol N 2 -N m 22 h 21 , while metalimnion rates were up to two times greater. A laboratory experiment simulating the temporary breakdown and reestablishment of the oxycline confirmed that short-term metalimnetic denitrification could be important in reactive nitrogen (N) loss in these reservoirs. Net denitrification occurred in sediments in contact with reservoir epilimnia during seasonal mixing, but net N 2 fixation occurred in these sediments during stratification. The observed shift between net denitrification and net N 2 fixation in epilimnetic sediments corresponded with an annual cycle of nitrate availability in the epilimnia. Annual whole-reservoir denitrification rates ranged from 12 to 23 g N m 22 yr 21 . The hypolimnion contribution to annual denitrification was 16-25%. Sediments in contact with epilimnia were the greatest N 2 source in all of the reservoirs. However, denitrification in the anoxic metalimnion was of similar magnitude to denitrification in epilimnetic sediments at one reservoir, and variability in metalimnetic denitrification rates was related to the strength of stratification stability. Integrating habitat-specific measurements into whole-lake denitrification rates showed that hydrologically dynamic habitats represent important denitrification hotspots, and that periodic disturbances fuel hot moments for denitrification.

Periphyton stoichiometry can vary substantially as a result of differences in stream nutrient availability. A decrease in the periphyton carbon to phosphorus (C:P) ratio should decrease the demand for new P to be immobilized from stream water, but no studies to our knowledge have explored the relationship between periphyton stoichiometry and net P immobilization and release by periphyton. We sought to model biological P immobilization and release (flux) in streams by measuring periphyton stoichiometry and light availability. We measured P flux to and from intact periphyton on stream cobbles (20-100 mm diameter) in 1 L microcosms incubated with streamwater under variable light conditions. Net P immobilization occurred in 75% of microcosms, net P release occurred in only 5% of microcosms, and 20% of microcosms had neither net immobilization nor net release. When normalized to stream conditions, net P immobilization was highest when light availability was high (\60% canopy attenuation) and the periphyton C:P ratio was also high. In contrast, net P release occurred only when light availability was low ([60% canopy attenuation) and the periphyton C:P ratio was also low. A multiple regression model that included both periphyton stoichiometry and light availability from the growing season only, and the interaction term of these two variables, explained 99% of the variation in daily periphyton P flux observed in the study. These results indicate that in order to predict periphyton P immobilization, periphyton stoichiometry and light availability should be considered together. Furthermore, the results indicate that net P immobilization occurs even in very P-rich periphyton, which can act as a P sink when light availability is high.

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