In this study, we experimentally investigated the degradation of
dissolved organic matter (DOM) during lateral and vertical mixing of
different water masses in a peri-alpine lake. River intrusions and
vertical winter turnover in Lake Geneva (Switzerland, France) were
simulated through short-term laboratory incubations by mixing riverine
and lacustrine waters or lacustrine waters collected at different depths
in winter. Dissolved organic carbon (DOC) degradation was monitored by
dissolved oxygen (DO) measurements and changes in DOM composition were
tracked by fluorescence spectroscopy in pure and mixed treatments during
72h. Initial DOC content and DOM composition were relatively similar
between end-members. The amount of DOC respired at the end of the
incubation was similar between treatments, but decay constants in mixed
treatments derived from a first-order decay model were significantly
higher than expected values calculated based on a simple mass balance
model. Fluorescent dissolved organic matter (FDOM) in mixed treatments
followed non-conservative patterns that could not be predicted based on
observations made in pure treatments. Hence, one protein-like and one
microbial-like fluorophore that were consumed in lake waters were
continuously produced in mixed treatments although lake waters
represented 90% of the mix. No relationships were observed between the
rate of DOC consumption and the initial DOM composition, suggesting that
other factors such as nutrients and/or interactions in microbial
communities were involved. Moreover, no relationships were found between
DOC and FDOM patterns during incubations, suggesting that these
measurements targeted different facets of microbial metabolism of DOM,
respectively microbial respiration (catabolism) and microbial production
(anabolism). While additional investigations are required in order to
identify the drivers of these changes, this study provides evidence of
non-conservative behavior of DOM degradation in mixing zones in lakes.