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Highlights • The biomineral flocs in the Belgian coast were investigated during two contrasting periods. • The observed floc size distributions were decomposed to represent microflocs, macroflocs and megaflocs. • A simple flocculation model that can describe the biofilm growth of flocs was developed. • This model was successfully implemented in the open TELEMAC with five passive tracers. Abstract The Floc Size Distributions (FSDs) of biomineral suspended particles are of great importance to understand the dynamics of bio-mediated Suspended Particulate Matters (SPMs). Field observations were investigated at Station MOW1 in Belgian coastal waters (southern North Sea) during two typical periods with abundant and reduced biomass. In addition, the Shen et al. (2018) [Water Res. Vol 145, pp 473–486] multi-class population balance flocculation model was extended to address the occurrence of suspended microflocs, macroflocs and megaflocs during these contrasting periods. The microflocs are treated as elementary particles that constitute macroflocs or megaflocs. The FSD is represented by the size and mass fraction of each particle group, which corresponds to a temporal and spatial varying mass weighted settling velocity. The representative sizes of macroflocs and megaflocs are unfixed and migrated between classes mainly due to the effects of turbulent shear, differential settling and biofilm growth. The growth of an aggregate because of bio-activities is allotted to each elementary particle. It is further hypothesized that the growth kinetics of biomineral particles due to biofilm coating follows the logistic equation. This simple bio-flocculation model has been successfully coupled in the open source TELEMAC modeling system with five passive tracers in a quasi-1D vertical case. Within an intra-tide scale, the settling velocity (w s) is large during slack tides while it is small during maximum current velocities because of variations in turbulence intensities. Nonetheless, the w s may be largely underestimated when the biological effect is neglected. For a seasonal pattern, the w s is higher in biomass-rich periods in May than in biomass-poor periods in October. While the mean sizes of megaflocs are close during the two periods, the macroflocs during algae bloom periods are more abundant with a larger mean size. This study enhances our knowledge on the dynamics of SPMs, especially the biophysical influences on the fate and transport of estuarine aggregates. [ABSTRACT FROM AUTHOR]

[Display omitted] • Humic-like fluorescence serves as a potential surrogate of TOC regardless of land use. • Protein-like fluorescence shows a strong correlation with nitrogen pollution in urban watershed. • The impact of land use is more significant than that of hydrological conditions. • High TP levels in agricultural watersheds can be detected using protein-like fluorescence. • FI serves as a universal indicator of microbial pollution in watersheds with diverse land use. Fluorescence spectroscopy has demonstrated its potential for assessing organic matter (OM) –related water quality (WQ) and identifying anthropogenically driven pollution sources within watersheds. This study involves a comparison of spatial variations in general WQ parameters and dissolved organic matter (DOM) fluorescence characteristics between two adjacent watersheds characterized by disparate land uses (i.e., urban and agricultural). The investigation encompasses both rain and non-rain events to evaluate the diagnostic utility of DOM fluorescence in monitoring WQ fluctuations. In the urban watershed, variations were predominantly influenced by wastewater effluents, leading to noticeable changes exclusively downstream from sewage treatment facilities, regardless of distinct hydrological conditions. In contrast, the agricultural watershed exhibited more extensive spatial variations, suggesting a prevalence of non-point source pollution stemming from agricultural and livestock activities. The humic-like fluorescent components (C1 and C2) exhibited a robust positive correlation with total organic carbon (TOC), unaffected by varying hydrological conditions and catchment land use. Conversely, the assessment of nutrient levels utilizing the protein-like component (C3) was considerably influenced by catchment land use. A stronger positive correlation between C3 and total nitrogen (TN) was observed in the urban watershed compared to the agricultural watershed. Moreover, the fluorescence index (FI) demonstrated a substantial positive relationship with TN in both watersheds, indicating the dominance of microbial sources in nitrogen pollution for both watersheds. No significant differences were found in the variation trends of WQ and DOM fluorescence between non-rain and rain events. This study emphasizes that catchment land use plays a pivotal role in influencing the associations between fluorescence indices and WQ parameters. Our findings validate the robust potential for replacing conventional WQ parameters with DOM fluorescence indices in watersheds characterized by distinct land uses. [ABSTRACT FROM AUTHOR]