Abstract
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 72 h. 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.
Highlights
The pool of dissolved organic matter (DOM) in aquatic ecosystems is a heterogeneous mixture of organic compounds that originate both from terrestrial sources and from primary production and processes of decomposition within water bodies
DOM exported during baseflow period has a typically low bioavailability compared to DOM exported during high flow events (Ågren et al 2008; Fellman et al 2009b), and low level of degradable DOM can be expected in Lake Geneva because of the low winter primary production
In this study we investigated the potential occurrence of nonconservative patterns in DOM degradation during mixing events by simulating both lateral river inputs into the lake and vertical turnover occurring during the winter period
Summary
The pool of dissolved organic matter (DOM) in aquatic ecosystems is a heterogeneous mixture of organic compounds that originate both from terrestrial sources (allochthonous DOM) and from primary production and processes of decomposition within water bodies (autochthonous DOM). Perga is typically investigated within a single aquatic environment, i.e. in groundwater (Shen et al 2015), streams and rivers (Balcarczyk et al 2009; Lapierre et al 2013; Catalán et al 2017), wetlands (Bano et al 1998) or lakes (Kothawala et al 2012). These ecosystems are tightly connected within fluvial networks and DOM degradation at the transition zones of these aquatic ecosystems where DOM and bacterial communities from different origins mix has generated growing interest. DOM degradation at mixing interfaces could be characterized by a non-conservative behavior or, in other words, a specific pattern that cannot be predicted from the degradation dynamics observed in the adjacent ecosystems
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