Nearfield flocculation processes along a negatively buoyant river intrusion in a large lake (Lake Geneva)

In estuaries and marine environments, primary particles are frequently transported as large flocs. 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 (i.e., during summer). Direct observations of flocculated particles in the whole water column with a digital holographic camera (LISST-HOLO 30-2000 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 very fine silts (4-8 μm) are dominant in the Rhône River interflow (flowing at thermocline depth), which exhibited the highest suspended sediment loads in the water column. In the hypolimnion below the interflow, where sediment loads were the lowest, microflocs (20-125 μm) and macroflocs (> 125 μm) were most frequent. The size of the largest macroflocs decreased along the interflow pathway, from 272 μm at 350 m from the mouth to 195 μm at 1700 m. In the epilimnion above the interflow, very fine silts and numerous phytoplanktonic organisms (~100-200 μm) were observed. In the hypolimnion, the average estimated fractal dimension (DF_3D) of the flocs ranged between 2.35 and 2.40, highlighting the complexity in floc shape, whereas phytoplanktonic organisms in the epilimnion had DF_3D values ranging between 2.45 and 2.50, suggesting less complex shapes.

 

The transition zone between the bottom layer of the interflow and the hypolimnion (~25-30 m depth) was marked by a sudden increase in the median particle diameter, corresponding to decreasing proportions of clays and very fine silts and to increasing proportions of micro and macroflocs. High-resolution profiles of turbulence collected with a Signature1000 revealed strong turbulence fluctuations and intense shear in this transition zone, compared to the interflow core. These levels of turbulence result in fine particle collisions, and favor the formation of larger flocs (i.e., flocculation) in the transition zone.

 

Furthermore, the influence of instantaneous turbulent kinetic energy as a factor limiting the maximum floc size below the Rhône River interflow was investigated. The observed turbulence level below the interflow corresponded to an estimated Kolmogorov microscale of less than ~320 μm at 350 m from the mouth to ~200 μm at 1700 m, values that are consistent with measurements. This results in the potential breakup of flocs larger than these estimates into smaller finer particles and microflocs, and so can explain the decrease in the macrofloc size along the interflow pathway.