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 (R_total) 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.