Transport and settling of particles in the ocean and estuaries

在释放到河流和海洋中的城市塑料废物中，最多有3%被观察到漂浮在海洋表面。为了帮助预测丢失的废物的位置，拟议的研究旨在研究漂浮塑料被运输并最终沉降到河底或海底的三种机制。最近，我们发现高岭土与盐水接触时，会被浮力塑料--由于粘土的密度比海水大，这表明，当泥泞的河流中的塑料进入海洋时，塑料上粘土的积累将导致它们主要沉积在河口、三角洲和海岸附近。这项拟议的研究旨在量化塑料的强化沉降，因为它取决于塑料的类型和大小，粘土的类型和相对浓度，以及这些颗粒悬浮的水的盐度。大多数进入海洋的浑浊河流的密度小于海水，因此它们漂浮在表面上，成为所谓的“低密度流”。尽管如此，粘土最终还是通过水柱下降。虽然单个粘土颗粒沉降非常缓慢，但当与盐水接触时，粘土与微塑料一起聚集成絮凝物，其沉降速度比单个颗粒快数千倍。随着这些絮凝物在低密度流的底部积累，研究表明流体-颗粒混合物变得对流不稳定，进一步提高了沉降速度。这项拟议的研究流将使用颗粒解析数值模拟来研究粘土-粘土和粘土-塑料絮凝，旨在预测沉降速度的提高。对于公海中的漂浮塑料，它们主要通过海流、环流和表面波进行运输。第三个拟议的研究流将研究一种不同的机制，即粒子由内部潮汐运输，内部潮汐与表面潮汐相似，只是它们由于相对温暖的近地表沃茨和较冷的深渊之间的浮力而移动。最近我们发现，当内潮在海岸附近产生并传播到海上时，表面附近的粒子被驱回海岸。然而，特别是在热带地区，波浪会变陡，并将粒子推向波浪的方向--也就是最终破碎的海岸。这一拟议的研究流将通过陡峭的内部潮汐来预测塑料的表面运输。每一项拟议的研究都令人担忧地表明，微塑料主要存在于沿海沃茨。 因此，这项研究将对包括环保主义者，海洋生物学家和加拿大渔业和海洋部在内的广泛社区有用，他们可以更好地评估在这些生物生产区域发现，监测和清理塑料污染的地方。

Of the municipal plastic waste which is released into rivers and the ocean and which is buoyant, at most 3% is observed to be floating on the ocean surface. To help predict where the missing waste is located, the proposed research aims to examine three mechanisms through which buoyant plastics are transported and ultimately settle to the river bottom or ocean floor. Recently we discovered that kaolin clay is electrically attracted to the buoyant plastic, pliolite, when in contact with salt water. Because clay is more dense than sea water, this suggests that, when plastics in a muddy river enter the ocean, the accumulation of clay on the plastics will cause them to settle predominantly in estuaries, deltas and near the coast. This proposed research stream aims to quantify the enhanced settling of plastics as it depends on the type and size of plastic, the type and relative concentration of clay, and the salinity of the water in which these particles are suspended. Most muddy rivers entering the ocean are less dense than sea water, and so they float over the surface as a so-called "hypopycnal current". Nonetheless the clay eventually descends through the water column. Although an individual particle of clay settles very slowly, when in contact with salt water, the clay together with microplastics gather into flocs that, can settle thousands of times faster than an individual particle. As these flocs accumulate at the base of the hypopycnal current, research suggests that the fluid-particle mixture becomes convectively unstable further enhancing the settling speed. This proposed research stream will use particle-resolving numerical simulations to examine clay-clay and clay-plastic flocculation with an aim to develop predictions for the enhanced settling speed. For floating plastics in the open ocean, they are dominantly transported by currents, gyre circulations and surface waves. The third proposed research stream will examine a different mechanism, whereby particles are transported by internal tides, which are similar to the surface tide except that they move due to the buoyancy between the relatively warm near-surface waters and the colder abyss. Recently we discovered that when the internal tide is generated near a coast and propagates out to sea, particles near the surface are driven back toward the coast. However, particularly in the tropics, the waves steepen and drive particles in the wave-ward direction - again toward a coast where they eventually break. This proposed research stream will develop predictions for the surface transport of plastics by steepening internal tides. Each proposed study concerningly suggests that microplastics settle predominantly in coastal waters.  This research will therefore be useful to a broad community including environmentalists, marine biologists and Fisheries and Oceans Canada, who may better assess where to find, monitor and clean up plastic pollution in such biologically productive regions.