Chapter 6 - Behavior of Microplastics in Coastal Zones

Abstract

Physical and dynamic properties of marine microplastic (MP) particles are summarized, with relevance to their behavior in the coastal zone. The density range, size classes, and typical particle shapes are presented, and their variation with time due to weathering, biofouling, and mechanical degradation in the swash zone is considered. The dependence of the time rate of increase of integral particle density on the particle shape due to its biofouling shows that floating fibers and threads (“one-dimensional” (1-D) particles) are the first to begin sinking, followed by 2-D films and flakes, and then 3-D fragments. Mechanical fragmentation in the swash zone of plastic samples made of low-density polyethylene (LDPE), polystyrene (PS), polypropylene (PP), and foamed polystyrene (foamed PS) shows qualitative features of the generated MP particles and indicates that increase in mass of MPs with time is exponential. Dynamic behavior of MPs is discussed on the base of knowledge from classical hydrodynamics, marine sedimentology, and physical oceanography. Shape-dependent manner of the particle sinking and the value of the terminal settling velocity are characterized in terms of the particle size, density, and angularity. Critical velocity of resuspension by a unidirectional flow is addressed in terms of the Shields diagram, summarizing available published data on laboratory experiments on the particles with properties similar to those of MPs. Consideration of motion of particles in an oscillatory flow under surface waves and in roll structures illuminates mechanisms of mixing of MPs and natural marine debris. Analysis of the observed sorting of sediment grains under the influence of surface waves raises the question of probable sorting of MPs in the water column of the coastal zone, leading to different transport for different kinds of MP particles, including the offshore export of smaller MPs, onshore transport of larger particles, and possible effect of arresting of finer MPs under coarser sediment grains. Typical velocity scales of natural convective flows and upward/downward motions within the Langmuir circulation cells are shown to be of the same order of magnitude as the settling velocity of common MPs—units of centimeters per second—which suggests the importance of these mixing mechanisms for the motion of MP particles. As a natural case study, the events of washouts of heavily intermixed patches of seaweed, amber stones, and MPs onto the beaches of the Baltic Sea after severe storms are examined. It is concluded that stormy episodes seem to bring repeatedly larger plastics to the surf and swash zone and export smaller pieces offshore.