Aggregation of Suspended Particles Limited by Preferential Settling

In coastal waters, the settling velocity of suspended particulate matter (SPM) is primarily governed by particle size, which varies during tidal cycles due to aggregation and disaggregation. Although high turbulence is typically viewed as the primary factor limiting particle size through disaggregation, the large variation in turbulence level does not fully explain the limited variation in mean particle size observed during tidal cycles. This study aims to identify the key processes driving SPM size dynamics and assess their relative influence. We introduce preferential settling into a distribution-based SPM-aggregation model, where larger particles settle faster, effectively removing them from the water column. Our model, applied to both lab and field cases, demonstrates that preferential settling significantly limits particle size, particularly in shallow, SPM-rich waters, as it is independent of turbulence. Changes in particle size range only become significant on timescales beyond a few tidal cycles, and are driven by factors such as biological ones. To account for these changes, we introduced a time-dependent reduction in coagulation efficiency to represent the degradation of organic matter over time. Biological processes such as this degradation should be considered in future studies focused on longer timescales, whereas turbulence-independent processes such as preferential settling need to be included in SPM models for tidal-scale dynamics.