Suspended sediments in Lake Geneva: Observations, settling behaviour and mechanisms in the Rhône River plume

In estuaries and marine environments, primary particles are frequently transported as large flocculated particles. This study provides, for the first time, evidence of in situ flocculation in Lake Geneva, a glacier-fed freshwater lake on the Swiss/French border. Measurements were focused in the nearfield of the Rhône River plume as it flows as an interflow into the stratified lake. Direct observations of flocculated particles in the whole water column with a digital holographic camera (LISST-HOLO 20-2000 μm), permitted estimation of the variability of sediment floc properties (size, nature and shape) with depth. Combined with full depth in situ laser particle sizing (LISST-100X), the measurements revealed that primary particles (< 4 μm) and microflocs (4-100 μm) are dominant in the Rhône River interflow, which exhibited the highest suspended sediment loads in the water column. In the hypolimnion below the interflow, where the sediment loads were the lowest, macroflocs (100-450 μm) were most frequent. The estimated fractal dimension (DF3D) of the flocs in the hypolimnion range between 2.0 and 2.6, highlighting a large variability in the floc shape. Estimates of the settling velocities (Ws) showed an increase from 0.05 to 3 mm s-1 when floc sizes increased from 30 to 500 μm. This emphasizes the important influence of flocculation of fine sediments on the increase of Ws. These estimates are similar to previous estimates of Ws for fluvial flocs (Thonon et al., 2005). However, they are slightly faster (up to 2 times) than Ws estimated for flocs in the vicinity of the Rhône River plume as it flows into the Mediterranean Sea (Many et al., 2019). This difference in Ws between the two Rhône River plume environments is explained by the shapes of the flocs, which overall are less complex and therefore flocs settle faster in Lake Geneva compared to the Mediterranean Sea. Furthermore, the influence of instantaneous turbulent kinetic energy as a factor limiting the maximum floc size within the Rhône River interflow was investigated. The observed turbulence level in the interflow corresponded to an estimated Kolmogorov microscale of less than ~200 μm. This results in the potential breakup of flocs larger than 200 μm into smaller primary particles and microflocs and thus can explain the smaller floc size in the interflow compared to that in the hypolimnion.