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Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera
Mikkelsen, O.; Hill, P.S.; Milligan, T.G. (2006). Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera. J. Sea Res. 55(2): 87-102. dx.doi.org/10.1016/j.seares.2005.09.003
In: Journal of Sea Research. Elsevier/Netherlands Institute for Sea Research: Amsterdam; Den Burg. ISSN 1385-1101, more
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  • Mikkelsen, O.
  • Hill, P.S.
  • Milligan, T.G.

Abstract
    Fine-grained (< 63 μm) particles in the aquatic environment flocculate into larger, porous entities (flocs). Flocculation models have been proposed wherein the particles in the water exist either as single grains or as part of flocs. This approach, however, contradicts the idea put forward by Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183-199.] that a flocculated suspension would consist of large porous, fragile flocs, macroflocs (> ~125 μm), made up of smaller, more sturdy flocs (microflocs, < ~125 μm) and single grains. This paper combines results from a LISST-100 laser diffraction particle sizer and a digital floc camera in order to produce full-size spectra covering in situ particle sizes > 2.5 μm, the smallest size resolvable by the LISST-100. For the first time, this allows a detailed investigation of changes in floc volume during break-up and flocculation. In accordance with Eisma [Eisma, D., 1986. Flocculation and de-flocculation of suspended matter in estuaries. Neth. J. Sea Res. 20, 183-199.], flocs are divided into macroflocs (here > 133 μm) and microflocs (36-133 μm). In addition, a single-grain fraction (<36 μm) is introduced as particles of this size are of importance for the optical properties of the water column. The variation of these three fractions over a range of forcing conditions is examined using a particle volume size range from 2.5 to 9900 μm. When stress increases, the volume occupied by macroflocs decreases while the volume occupied by microflocs increases. It is also demonstrated that when stress decreases, the volume occupied by macroflocs first increases and then decreases as flocs settle out during quiescent conditions. However, in general no overall relationship between stress and floc size was found because parameters other than stress influence floc size, most notably resuspension, settling, advection, and biological activity. On average, macroflocs make up 40-65% of the total suspended volume, whereas single grains and microflocs each make up 15-34% of the total suspended volume. Because an inverse relationship between floc size and density generally exists, this means that occasionally the majority of the suspended mass can be found in single grains and microflocs. This has implications for the optical properties of the water column as scattering is dominated by the smallest particles in suspension. The findings also have implications for existing flocculation models that would probably benefit from inclusion of the microfloc fraction.

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