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The seasonal variation in concentration of transparent exopolymer particles (TEPs), particulate organic carbon (POC) and particulate organic nitrogen (PON) were investigated together with floc size and the concentration of suspended particulate matter (SPM) along the cross‐shore gradient, from the high turbid nearshore toward the low‐turbid offshore waters in the Southern Bight of the North Sea. Our data demonstrate that biophysical flocculation cannot be explained by these heterogeneous parameters, but requires a distinction between a more reactive labile ("fresh") and a less reactive refractory ("mineral‐associated") fraction. Based on all data, we separated the labile and mineral‐associated POC, PON, and TEP using a semi‐empirical model approach. The model's estimates of fresh and mineral‐associated organic matter (OM) show that great parts of the POC, PON, and TEP are associated with suspended minerals, which are present in the water column throughout the year, whereas the occurrence of fresh TEP, POC, and PON is restricted to spring and summer months. In spite of a constantly high abundance of total TEP throughout the entire year, it is its fresh fraction that promotes the formation of larger and faster sinking biomineral flocs, thereby contributing to reducing the SPM concentration in the water column over spring and summer. Our results show that the different components of the SPM, such as minerals, extracellular OM and living organisms, form an integrated dynamic system with direct interactions and feedback controls. Plain Language Summary: Particles suspended in coastal waters occur as loose aggregates of tiny mineral and organic particles, also known as flocs. Their mass concentration is higher in winter than in summer, but their sizes are smaller in winter. The seasonal cycle of phytoplankton activity drives this phenomenon. In spring, phytoplankton blooms and starts to produce fresh and sticky organic matter. This glue binds particles together after collisions and promotes increasingly larger and yet stable flocs. Analytical lab methods cannot distinguish between the freshly produced sticky and the older inactive organic material that is stored in the sediments and entrained into the water by erosion and resuspension. Therefore, previous studies were not able to detect the relation between the occurrence of the sticky material and the floc‐sizes in coastal waters. However, our innovative model approach allows a separation into both fractions. We observe a clear increase in floc sizes during spring and summer when fresh sticky organic material is available. As larger particles sink faster, they are removed from the water column, which allows a higher light penetration. In this way, phytoplankton is exposed to changing conditions, an illustration of how direct interactions and feedback loops occur in an integrated dynamic system. [ABSTRACT FROM AUTHOR]

• The effects of anionic, cationic and neutral organic matters on the aggregation of microplastics are investigated. • All of the organic matters show positive effects on the growth of biofilm-coated microplastic aggregates. • The two-class population balance model is used to track the small and large microplastic subgroups under turbulence. Biofilm-coated microplastics are omnipresent in aquatic environments, carrying different organic matter (OM) that in turn influences the flocculation and settling of microplastic aggregates. In this study, the effects of chitosan, guar gum, humic acid, and xanthan gum on the flocculation of anthropogenic microplastics are examined under controlled shear through the mixing chamber experiments. The results show that all of the selected OMs have positive effects on biofilm culturing and thus enhance the growth of microplastic flocs, with more evident promoting effects for cationic and neutral OMs (i.e., chitosan and guar gum) than anionic OMs (i.e., humic acid and xanthan). No critical shear rate is observed in the size vs. shear relationship based on our measurements. In addition, the quadrature-based two-class population balance model is employed to track the development of bimodal floc size distributions (FSDs) composed of small and large microplastic flocs. The model predictions show reasonable agreement with the observed FSDs. The largest error of settling flux from the two-class model is 7.8% in contrast with the reference value measured by the camera-based FSDs with 30 bins. This study highlights the role of different OMs on microplastic flocculation and indicates that a two-class model may be sufficient to describe microplastic transport processes in estuaries. [Display omitted] [ABSTRACT FROM AUTHOR]

• The current findings of the heterogeneous composition of flocs are summarized. • The effect of the composition of flocs on their structure and dynamics is reviewed. • The interaction among heterogeneous components within flocs is explained. • Human activities affecting the heterogeneous composition of flocs are also summarized. Flocculation is a key process for controlling the fate and transport of suspended particulate matter (SPM) in water environments and has received considerable attention in the field of water science (e.g., oceanography, limnology, and hydrology), remaining an active area of research. The research on flocculation has been conducted to elucidate the SPM dynamics and to diagnose various environmental issues. The flocculation, sedimentation, and transportation of SPM are closely linked to the compositional and structural properties of flocs. In fact, flocs are highly heterogeneous in terms of composition. However, the lack of comprehensive research on floc composition and structure has led to misconceptions regarding the temporal and spatial dynamics of SPM. This review summarizes the current understanding of the heterogeneous composition of flocs (e.g., minerals, organic matter, metals, microplastic, engineered nanoparticles) and its effect on their structure and on their fate and transport within aquatic environments. Furthermore, the effects of human activities (e.g., pollutant discharge, construction) on floc composition are discussed. Graphical abstract [Display omitted]. [ABSTRACT FROM AUTHOR]