Partitioning whole-lake denitrification using in situ dinitrogen gas accumulation and intact sediment core experiments

Erin M. Grantz, Aki Kogo and J. Thad Scott

Limnol. Oceanogr., 57(4), 2012, 925-935 | DOI: 10.4319/lo.2012.57.4.0925

ABSTRACT: We measured in situ hypolimnion and metalimnion dinitrogen (N2) accumulation rates and N2 production from sediments in contact with the reservoir epilimnia for 3 small (< 1 km2) and shallow reservoirs with inherently low stratification stability. Hypolimnion areal denitrification rates ranged from 98.6 to 168 µmol N2-N m-2 h-1, 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 N2 fixation occurred in these sediments during stratification. The observed shift between net denitrification and net N2 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-2 yr-1. The hypolimnion contribution to annual denitrification was 16-25%. Sediments in contact with epilimnia were the greatest N2 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.

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