The ice-covered period of boreal lakes has contrasting environmental conditions respect to the ice-free, with cold temperatures, absence of light, and minor gas exchange between water and atmosphere. Focusing on the N cycle, winter seems a suitable period for N-transforming prokaryotes with a high availability of reactive N due to minor assimilation by photoautotrophs. However, there is limited data about winter N cycling rates and the microbes involved on, and about the role of organic matter quality on N cycling. We studied 2 oligotrophic large boreal lakes in North Karelia, Finland, with clear-water and brown-water sides, and an additional side affected by mining resulting in higher nitrate and sulphate levels in the hypolimnion. During winter of 2021 we sampled at the beginning and at the end of the ice-cover. Using the IPT we incubated sediment cores with ¹⁵NO₃^- and quantified the products of 1) complete denitrification (N₂), 2) truncated denitrification (N₂O), and 3) (DNRA, NH₄⁺) to infer the process rates. Also, to see the role of organic matter, we did anoxic slurry incubations with ¹⁵NO₃^- and 1) lake water, 2) miliQ water, 3) algal dissolved organic matter (DOM) extract, or 4) peatland DOM extract. We characterized the DOM using FT-ICR MS. We also explore the genetic potential (DNA) of the sediment microbiome by using several sequencing techniques: 1) 16S rRNA, 2) targeted (N and CH4 functional genes), and 3) shotgun. Preliminary results identify the N-transforming microbes and point to changing nitrate consuming activities and genetic potentials between sites.

Primary Presenter: Carlos Palacin-Lizarbe, University of Eastern Finland (carlos.palacin@uef.fi)

Authors:

Carlos Palacin-Lizarbe, University of Eastern Finland  (carlos.palacin@uef.fi)

Stefan Bertilsson, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden  ()

Henri Siljanen, University of Eastern Finland  ()

Hannu Nykänen, University of Eastern Finland  ()

Christina Biasi, University of Eastern Finland  ()

Moritz Buck, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden  ()

Tong Lui, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden  ()

Mikko Kiljunen, Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, 40014 Jyväskylä, Finland  ()

Saani Aalto, DTU Aqua, Section for Aquaculture, The North Sea Research Centre, Technical University of Denmark, P.O. Box 101, 9850 Hirtshals, Denmark  ()

Antti Rissanen, Faculty of Engineering and Natural Sciences, Tampere University, Korkeakoulunkatu 6, 33720 Tampere, Finland  ()

Anssi Vainikka, University of Eastern Finland  ()

Jukka Pumpanen, University of Eastern Finland  ()