Phytoplankton enhances the flocculation of suspended particulate matter in a turbid estuary

Over the past decades, improvements in water quality—particularly increased oxygen and reduced nitrogen concentrations—have led to changes in phytoplankton biomass and community composition in the Schelde estuary, a macrotidal estuary in Belgium/SW Netherlands. We argue that these changes have affected SPM dynamics by modifying floc stability and size. Seasonal and spatial variability in SPM flocculation dynamics were examined using water samples collected monthly from December 2021 to December 2022 at five stations in the freshwater and brackish tidal reaches of the estuary. In a custom-built flocculation chamber, these samples were first subjected to a high turbulent shear rate (45 s⁻¹) to break the flocs, followed by a lower shear rate (20 s⁻¹) that promoted aggregation for 120 min. Floc size distribution changes were analyzed to determine key flocculation parameters, in particular, equilibrium floc size (D_e), and flocculation speed (V_f). Using generalized additive models, we assessed the influence of seasonal, spatial, environmental, and biotic factors on D_e and V_f. Unlike in the brackish stations, the freshwater stations displayed clear seasonal patterns in flocculation dynamics, with D_e and V_f increasing in spring and summer. The strong positive correlations between D_e, V_f, and Chl a, as well as between V_f and phytoplankton-derived particulate organic carbon, suggest that phytoplankton plays a key role in the flocculation process. Transparent exopolymer particles (TEP), which because of their sticky properties can promote particle coagulation, were only weakly correlated with enhanced flocculation, suggesting that TEP properties may be affected by factors such as its specific composition, age, and remineralization.