Mineralization pathways of organic matter deposited in a river–lake transition of the Rhone River Delta, Lake Geneva

Abstract

During the éLEMO endeavour (a research project in which the Russian MIR submersibles were used for studying Lake Geneva) four sediment cores were retrieved on a transect from the delta of the Rhone River towards the profundal part of the lake. The degradation pathways of organic material (OM) were investigated considering different electron acceptors. Essentially, OM at the delta sites had a higher fraction of terrestrial material than the lake sites indicated by higher C/N ratios, and higher long-chain n-alkane and alcohol concentrations. The concentrations of chlorins were higher at the distant sites indicating more easily degradable OM in the sediments. However, the chlorin index that was used to determine the degradation state of the OM material indicated that pigment derived OM of deltaic sediments was less degraded than that of the profundal sediments. The fluxes of reduced species from the sediments decreased from the delta to the profundal for CH4 (from 2.3 to 0.5 mmol m(-2) d(-1)) and NH4(+) (from 0.31 to 0.13 mmol m(-2) d(-1)). Fluxes of Fe(ii) and Mn(ii), however, increased although they were generally very low (between 9 × 10(-5) and 7.6 × 10(-3) mmol m(-2) d(-1)). Oxygen concentration profiles in the pore waters revealed lower fluxes close to the river inflow with 4.3 and 4.1 mmol m(-2) d(-1) compared to two times higher fluxes at the profundal sites (8.8 and 8.2 mmol m(-2) d(-1)). The rates for totally mineralized OM (Rtotal) at the shallower sites (4.7 mmol C m(-2) d(-1)) were only half of those of the deeper sites (9.7 mmol C m(-2) d(-1)). Accordingly, not only the rates but also the mineralization pathways differed between the shallow and profundal sites. Whereas only 0-6% of the OM was mineralized aerobically at the shallow sites (since almost all O2 was used to oxidize the large flux of CH4 from below) the situation was reversed at the deeper sites and the fraction of aerobically degraded OM was 72-78%. We found a better efficiency in CH4 production per carbon equivalent deposited at the deeper sites as a result of the higher degradability of the mainly autochthonous OM in spite of the lower deposition rate and the higher degradation state of the OM compared to the delta sites.

Hypolimnetic oxygen depletion rates in deep lakes: Effects of trophic state and organic matter accumulation

“…Deeper sedimentary horizons (> 10 cm) however exhibited elevated CH 4 and NH 4 + concentration of up to 7.0 and 0.6 mmol L −1 , respectively. These resembled values measured from river deltas or from shallower, more productive lakes (Sobek et al 2009; Randlett et al 2015; Steinsberger et al 2017). In contrast to the Great Lakes, the deep peri‐alpine lakes investigated by this study accumulated inorganic matter from weathering of rocks and OM from their watersheds.…”

Lateral variations and vertical structure of the microbial methane cycle in the sediment of Lake Onego (Russia)

“…Methane concentrations were also generally lower than those observed in Lake Geneva. As an example, the Shuya River-related site P1 showed similarities in concentrations to deeper (50-60 m) and inactive areas of the Rhone delta in Lake Geneva (Sollberger et al 2014, Randlett et al 2015 or to the trenched sediment of the central basin (Brandl et al 1993), which are all much deeper than P1 sediment cores (90-250 m). Lake Onego and the atypical Petrozavodsk Bay are thus relatively small CH 4 producers, in particular during winter.…”

Terrestrial Vegetation Drives Methane Production in the Sediments of two German Reservoirs

“…Unlike the majority of the terrestrial POC, the autochthonous OC may be readily available 8,39,45–47 and preferentially decomposed under aerobic conditions or by other energetically more efficient pathways than methanogenesis such as denitrification, iron und sulphate reduction. This can take place already in the water column and at the sediment-water interface 48 . Mass balance calculations using 13 C revealed that the degradation of autochthonous OC contributed to the TIC pool in the hypolimnion of both reservoirs 49 .…”

“…In reservoirs larger than in this study, internal photosynthesis may become a more significant source. It has been shown that sedimentation areas at river inflows into lakes are rich in terrestrial material 48 and exhibit high rates of methanogenesis 57 . Our results suggest that terrestrial carbon is a major driver of such methanogenesis hotspots.…”