AME prepress abstract  -  DOI: https://doi.org/10.3354/ame01911

Calculation of hypolimnic denitrification in a dimictic freshwater lake during summer stratification

Marion Martienssen*, Jörg Böllmann, Brigitte Nixdorf, Kristina Rathsack

*Email: marion.martienssen@b-tu.de

ABSTRACT: The present study compares different methods for calculating hypolimnic denitrification in the stratified water column in Lake Scharmützelsee (Germany) during summer stratification. Denitrification rates were estimated either from the nitrate loss under anoxic conditions or with a Michaelis–Menten based approach. The latter included correction terms to cope for temperature variations and oxygen inhibition. For this kinetic approach, 2 critical parameters (cell count and maximum turnover rate) are needed. As nirS gene copy numbers obtained by qPCR do not distinguish between viable and non-viable cells, MPN-based cell counts were also tested for their suitability in the Michaelis-Menten approach. At the beginning of the stratification period, about 1% of the denitrifiers calculated from nirS gene copy numbers were cultivable. Nitrate loss and calculated pPCR-based denitrification rates differed by only one order of magnitude (mean rates 40.74 and 1.27 µmol l–1 d–1, respectively). During the stratification period, the difference between qPCR-based calculated denitrification rates and measured nitrate loss increased by up to 4 orders of magnitude, resulting in an enormous overestimation of the calculated denitrification rates. Calculations based only on the numbers of cultivable cells from the MPN counts were significantly lower than the nitrate loss in the lake. Neither nirS gene copy numbers nor MPN counts were suitable for determining the real number of metabolically active denitrifiers. Therefore, additional methods (e.g. PMA-PCR, viability staining or model strains) are needed to identify the real portion of metabolically active cells.