Multimodality of a particle size distribution of cohesive suspended particulate matters in a coastal zone

Abstract

[1] Particle size distributions (PSDs) of suspended particulate matters in a coastal zone are lognormal and multimodal in general. The multimodal PSD, which is caused by the mixing of multiple particle and aggregate size groups under flocculation and erosion/resuspension, is a record of the particle and aggregate dynamics in a coastal zone. Curve-fitting software was used to decompose the multimodal PSD into subordinate lognormal PSDs of primary particles, flocculi, microflocs, and macroflocs. The curve-fitting analysis for a time series of multimodal PSDs in the Belgian coastal zone showed the dependency of the multimodality on (1) shear-dependent flocculation in a flood and ebb tide, (2) breakage-resistant flocculation in the spring season, and (3) silt-sized particle erosion and advection in a storm surge. Also, for modeling and simulation purposes, the curve-fitting analysis and the settling flux estimation for the multimodal PSDs showed the possibility of using discrete groups of primary particles, flocculi, microflocs, and macroflocs as an approximation of a continuous multimodal PSD.

Spatial and Seasonal Variation of Biomineral Suspended Particulate Matter Properties in High-Turbid Nearshore and Low-Turbid Offshore Zones

“…PSDs in the TMZ skewed toward a smaller size around peak flow (e.g., t = 3, 4 h at location MOW1 on 10 July 2007) and then to a larger size around slack water (e.g., t = 6, 7 h). Except for the PSDs in 23 October 2007, the other PSDs in the TMZ showed bimodality, comprising microflocs (20-200 µm) and macroflocs (>200 µm), as reported in the earlier studies [5]. The primary peak of microflocs in a PSD was prominent around the peak flow.…”

“…D50 increased to the maximum when approaching slack water, but decreased to a minimum around peak flow. Regarding flocculation kinetics, aggregation kinetics dominated over disaggregation kinetics for the slack water, and vice versa for the peak flow [5,18]. In contrast, SPM and POC concentrations and D50 in the OSZ were rather constant, randomly scattered without apparent ups and downs (Figure 4b), showing that aggregation kinetics dominate over disaggregation kinetics for the entire period.…”

“…Biomass and minerals were likely combined in large, settleable biomineral aggregates, because SPM and POC concentrations had the same up-and-down movement during a tidal cycle. Such biomineral aggregates in the TMZ are vulnerable to aggregation and disaggregation (i.e., breakup), depending on the flow intensity (turbulence), available aggregation time to reach the equilibrium aggregate size, and organic matter content, therefore changing D50 in a flow-varying tidal cycle [5,18]. D50 increased to the maximum when approaching slack water, but decreased to a minimum around peak flow.…”

“…However, while approaching the slack water, the secondary peak of macroflocs became dominant over the primary peak. Low flow/turbulence intensity might promote the aggregation of microflocs (i.e., mineral, biomineral aggregates) to macroflocs (i.e., biological aggregates) [4,5,18]. On the other hand, large hardly-settleable biological aggregates which were suspended in the water column might dominate in the slack water.…”

“…The SPM concentration is an important parameter to understand the marine ecosystem as it controls the water turbidity and mediates many physical and biochemical processes [4][5][6].…”