Effect of exopolymer gels on the viscoelasticity of mucus-rich saltwater and settling dynamics of particles

Algal blooms enrich seawater with gel-like exopolymers modifying the local structure and physio-chemical properties of water column. These effects are particularly intense during mucilage events occurring seasonally in some seas and coastal regions and in thin layers. However, detailed in situ examination is limited, and the effects of biologically modified seawater on microscale processes remain poorly understood. In this study, we experimentally investigated the impact of exopolymer gels on rheological properties of mucus-rich saltwater and consequences on settling dynamics of fast-sinking particles using aqueous solutions of sodium chloride with an addition of xanthan gum, a model exopolymer forming gels dispersed in water. We studied two types of solutions: colloidal dispersion and dispersion with predominant content of particulate gels. The results showed that dispersions consisting of colloidal exopolymers have stronger inner structure than dispersions with predominant particulate exopolymers, suggesting that viscoelastic properties of seawater vary with the structure of gels. Settling velocity and drag exerted on solid spheres and disks were substantially different for the two types of solutions, with the drag coefficient up to six times higher for colloidal than for particulate solutions of corresponding salinity, indicating that colloidal exopolymers may retard particles falling in marine environment more effectively than particulate gels. The findings highlight possible variation in viscoelasticity of mucus-rich saltwater with exopolymer network structure changes in spatial and temporal scales during algal bloom development and implications to the sedimentation of fast-sinking particles such as faecal pellets, carbonate grains, minerals and microplastics.