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[MIS19-P05] Numerical model approach on vertical motion of microplastics absorbed into algae aggregations
Keywords:microplastic, aggregate, vertical distribution
Microplastics consisted of polyethylene and polypropylene which have less dense than seawater are mainly floating around sea surface, and their amount leaches more than 20 trillion pieces in the world’s upper ocean. Some observations found these microplastics in oceanic subsurface layers and on ocean floors and, thus, some oceanic process actually occurred to transport them downward from the sea surface to deeper layers. One of expected processes is the absorption into aggregates of phytoplankton (algae). They settle down certainly because of their density higher than seawater (e.g., marine snow) and, therefore, microplastics will be also transported to deeper layer if they are absorbed into the aggregates. The objective of this study is to verify the contribution of this biological process on the vertical distribution of microplastics using a vertically 2D numerical particle tracking model. The modeled particles represented either microplastics or phytoplankton with their interaction. The vertical distributions and size compositions of observed microplastics with sizes larger than 20 micrometers from the sea surface to 1 m above the ocean floor in coastal waters were used for model validation.
First, a model experiment using only buoyant microplastic particles was conducted. The modeled particles had a rise velocity due to their buoyancy and were carried by oceanic turbulence parameterized with wind speed and significant wave height. This model was able to reproduce the size distribution of microplastics 0.2 to 5 mm from the sea surface to 5-m depth, although the observed size distribution smaller than 100 micrometers in the entire water column was not well reproduced. This inconsistency between observations and model experiment only in the physical processes suggested the importance of biological processes.
Second, we carried out a model experiment including both microplastic particles along with phytoplankton particles (or their aggregation) carried by oceanic turbulence and sinking process. The sinking velocity of particles without microplastics was differ from those including microplastics. Phytoplankton particles had chances to include surrounding particles as an aggregation process (multiple particles changing into a single particle), when particles were positioned within a distance predetermined in the model (referred to as ‘aggregation radius’). The microplastic particles were absorbed into aggregate particles when they were positioned closer to the aggregation radius, and when microplastic size is less than or equal to the aggregates. In the model domain, there are 134,600 microplastic and 10,000 phytoplankton particles initially, as actual oceans for microplastic and at spring/autumn bloom and/or red tide for phytoplankton.
This model was able to reproduce microplastics settling to the deep layer with aggregates, and vertical distribution in which the maximum number of microplastic particles was observed in the subsurface layer. Moreover, the modeled aggregates including microplastics had a sinking rate sufficiently larger than or equal to vertical velocities typically observed in the actual ocean, even if the initial concentration of phytoplankton was lower than those typically observed at spring/autumn bloom and/or red tide in the reality. It is also found that settling velocities of the aggregates depended on ambient microplastic abundance, and that downward velocities of aggregates increased as their sizes increased.
First, a model experiment using only buoyant microplastic particles was conducted. The modeled particles had a rise velocity due to their buoyancy and were carried by oceanic turbulence parameterized with wind speed and significant wave height. This model was able to reproduce the size distribution of microplastics 0.2 to 5 mm from the sea surface to 5-m depth, although the observed size distribution smaller than 100 micrometers in the entire water column was not well reproduced. This inconsistency between observations and model experiment only in the physical processes suggested the importance of biological processes.
Second, we carried out a model experiment including both microplastic particles along with phytoplankton particles (or their aggregation) carried by oceanic turbulence and sinking process. The sinking velocity of particles without microplastics was differ from those including microplastics. Phytoplankton particles had chances to include surrounding particles as an aggregation process (multiple particles changing into a single particle), when particles were positioned within a distance predetermined in the model (referred to as ‘aggregation radius’). The microplastic particles were absorbed into aggregate particles when they were positioned closer to the aggregation radius, and when microplastic size is less than or equal to the aggregates. In the model domain, there are 134,600 microplastic and 10,000 phytoplankton particles initially, as actual oceans for microplastic and at spring/autumn bloom and/or red tide for phytoplankton.
This model was able to reproduce microplastics settling to the deep layer with aggregates, and vertical distribution in which the maximum number of microplastic particles was observed in the subsurface layer. Moreover, the modeled aggregates including microplastics had a sinking rate sufficiently larger than or equal to vertical velocities typically observed in the actual ocean, even if the initial concentration of phytoplankton was lower than those typically observed at spring/autumn bloom and/or red tide in the reality. It is also found that settling velocities of the aggregates depended on ambient microplastic abundance, and that downward velocities of aggregates increased as their sizes increased.