Abstract. Nitrate pollution of freshwaters and methane emissions into the
atmosphere are crucial factors in deteriorating the quality of
drinking water and in contributing to global climate change. The n-damo (nitrite-dependent anaerobic methane oxidation), nitrate-dependent anaerobic methane
oxidation and the anaerobic oxidation of ammonium (anammox) represent
two microbially mediated processes that can reduce nitrogen loading of
aquatic ecosystems and associated methane emissions to the
atmosphere. Here, we report vertical concentration and stable-isotope
profiles of CH4, NO3-, NO2-, and
NH4+ in the water column of Fohnsee (lake in southern
Bavaria, Germany) that may indicate linkages between denitrification, anaerobic oxidation of methane (AOM), and anammox. At
a water depth from 12 to 20 m, a methane–nitrate transition
zone (NMTZ) was observed, where δ13C values of methane
and δ15N and δ18O of dissolved nitrate
markedly increased in concert with decreasing concentrations of
methane and nitrate. These data patterns, together with the results of
a simple 1-D diffusion model linked with a degradation term, show that
the nonlinear methane concentration profile cannot be explained by
diffusion and that microbial oxidation of methane coupled with
denitrification under anaerobic conditions is the most parsimonious
explanation for these data trends. In the methane zone at the bottom
of the NMTZ (20 to 22 m) δ15N of ammonium increased
by 4 ‰, while ammonium concentrations decreased. In
addition, a strong 15N enrichment of dissolved nitrate was
observed at a water depth of 20 m, suggesting that anammox
is occurring together with denitrification. The conversion of
nitrite to N2 and nitrate during anammox is associated with
an inverse N isotope fractionation and may explain the observed
increasing offset (Δδ15N) of 26 ‰
between δ15N values of dissolved nitrate and nitrite at a
water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in
the NMTZ at a water depth between 16 and 18 m. The associated
methane concentration and stable-isotope profiles indicate that some
of the denitrification may be coupled to AOM, an observation supported
by an increased concentration of bacteria known to be involved in
n-damo/denitrification with AOM (NC10 and
Crenothrix) and anammox (“Candidatus
Anammoximicrobium”) whose concentrations were highest in the methane
and ammonium oxidation zones, respectively. This study shows the
potential for a coupling of microbially mediated nitrate-dependent
methane oxidation with anammox in stratified freshwater ecosystems,
which may be important for affecting both methane emissions and
nitrogen concentrations in lakes.
High rates of anaerobic methane oxidation in freshwater wetlands reduce potential atmospheric methane emissions
