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.

藻华通过改变水柱的局部结构和物理化学性质的凝胶状外聚合物丰富海水。在某些海洋和沿海地区以及薄层季节性发生的粘液事件中，这些影响尤其强烈。然而，详细的原位检查是有限的，生物改性海水对微观过程的影响仍然知之甚少。在这项研究中，我们通过实验研究了外聚合物凝胶对富含粘液的盐水的流变特性的影响，以及使用添加黄原胶的氯化钠水溶液对快速下沉颗粒的沉降动力学的影响，黄原胶是一种模型外聚合物，形成分散在水。我们研究了两种类型的溶液：胶体分散体和主要含量为颗粒凝胶的分散体。结果表明，由胶体外聚合物组成的分散体比具有主要颗粒外聚合物的分散体具有更强的内部结构，这表明海水的粘弹性随凝胶的结构而变化。两种溶液的沉降速度和施加在固体球体和圆盘上的阻力显着不同，胶体的阻力系数比相应盐度的颗粒溶液高 6 倍，这表明胶体外聚合物可以阻止颗粒落入海洋环境比颗粒凝胶更有效。