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

Abstract

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.

Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir

“…This is consistent with Deemer et al (2011) and Grantz et al (2012), who also report N 2 accumulation in excess of NO { 3 depletion. There are several mechanisms that may explain the imbalance.…”

“…Hypolimnion thickness may also influence denitrification rates by controlling the mass of NO { 3 trapped in the hypolimnion during stratification. Nitrate was nearly exhausted (i.e., , 5 mmol N L 21 ) from the compressed hypolimnia (i.e., , 3 m) of the systems studied by Grantz et al (2012) after as little as 1 month of stratification, thereby limiting in situ denitrification rates. By contrast, 4 months of stratification were required to deplete NO { 3 in the larger hypolimnion in Harsha Lake.…”

“…One factor that may explain differences in denitrification rates among these studies is the thickness of the hypolimnion. While it is frequently assumed that most denitrification in reservoirs is sedimentary (Wall et al 2005;David et al 2006), several studies have documented denitrification in hypoxic water columns (Terai and Yoh 2001;Bratkic et al 2012;Grantz et al 2012) and the importance of water column denitrification will increase with the ratio of water volume to benthic surface area. The hypolimnion in Harsha Lake was hypoxic, bordering on anoxic near the sedimentwater interface (dissolved oxygen , 0.02 mg L 21 ), and the mid-water column N 2 O spike in June suggests the hypolimnion supported denitrification while NO { 3 was available.…”

“…Alternatively, the concentration of N 2 O and other N species, excluding N 2 , have been measured in situ and denitrification rates assumed based on their distribution (Mengis et al 1997). Membrane inlet mass spectrometry (MIMS), however, allows for sufficiently precise and accurate dissolved N 2 concentration measurements to quantify biological N 2 production rates in situ (Kana et al 2006;Deemer et al 2011;Grantz et al 2012). When combined with measurements of N 2 O and other solutes, MIMS can provide substantial insight into the relationship between denitrification and N 2 O production.…”

Nitrification contributes to winter oxygen depletion in seasonally frozen forested lakes

“…Similar approaches have been employed relatively recently to calculate whole-system fluxes (Beaulieu et al 2014;Deemer et al 2011;Grantz et al 2012). Each approach incorporates different assumptions about the weighting of surface, middle, and deep samples in ecosystem-level quantification.…”