Seasonality of riverine macroplastic transport

Abstract

Marine plastic pollution is an increasing environmental threat. Although it is assumed that most marine plastics are transported from land to the ocean through rivers, only limited data on riverine plastic transport exists. Recently, new methods have been introduced to characterize riverine plastics consistently through time and space. For example, combining visual counting observations and plastic debris sampling can provide order of magnitude estimations of plastic transport through a river. In this paper, we present findings from multi-season measurement campaign in the Saigon River, Vietnam. For the first time, we demonstrate that macroplastic transport exhibits strong temporal variation. The monthly averaged plastic transport changes up to a factor five within the measurement period. As it is unclear what drives the variation in plastic transport, relations between rainfall, river discharge, presence of organic material and plastic transport have been explored. Furthermore, we present new findings on the cross-sectional and vertical distribution of riverine plastic transport. With this paper we present new insights in the origin and fate of riverine plastic transport, emphasizing the severity of the emerging thread of plastic pollution on riverine ecosystems.

The Way of Macroplastic through the Environment

“…Based on their studies, they also assume that films and foils are more affected by turbulence due to their larger surface-to-volume ratio and are therefore more easily distributed in the water column. Similar to Schwarz et al (2019) [85], they conclude that the vertical distribution of MaP is somewhat independent of the item's density, as they found many plastic items with densities less than water below 0.5 depth, which was possibly due to density changes after biofilm development or, as already mentioned, turbulence [87].…”

“…However, in a comparison of the polymer types found in rivers and in the oceans, Schwarz et al (2019) [85] were able to demonstrate that the stronger turbulences in rivers hinder MaP from settling, which leads to increased horizontal transport. In this context, van Emmerik et al (2019) [87] investigated the horizontal distribution of MaP in the Saigon River and found strong variations in where the plastic is transported throughout the year. They concluded that the horizontal plastic distribution is mainly influenced by the river's flow velocity, wind speed, and direction as well as navigation.…”

Plastic debris in rivers

“…Mangrove forests tend to retain plastic debris for long periods (months–years) (Ivar do Sul et al, ). Results from the first long‐term riverine plastics debris monitoring effort demonstrated that the magnitude of plastic transport was mainly related to the presence of aquatic vegetation, mainly consisting of water hyacinths (van Emmerik, Strady et al, ). Water hyacinths often accumulate and form large vegetation patches, which also retain plastic debris.…”

“…However, most estimates range between 16 and 119 tonnes/year (Seine, Rhine, Po) (Gasperi et al, 2014; van der Wal et al, ). In Southeast Asia, estimates range between 2.1·10 3 tonnes/year for the Jakarta waterways (van Emmerik, Loozen et al, ) to 1.3·10 3 tonnes/year for the Saigon river (van Emmerik et al, ; van Emmerik, Strady et al, ). This shows that river plastic emission may be a more distributed problem than previously assumed (Schmidt et al, ) as also urban drainage systems and smaller rivers emit several orders of magnitude more plastic than large rivers.…”

An evaluation of the River-OSPAR method for quantifying macrolitter on Dutch riverbanks

“…All monitoring long-term efforts of floating (plastic) litter to date have been done using the visual counting method (e.g. Crosti et al, 2018;Castro-Jiménez et al, 2019;van Emmerik et al, 2019b). Floating litter observations can also be facilitated through citizen science.…”