Effects of biofouling on the sinking behavior of microplastics

Abstract

Although plastic is ubiquitous in marine systems, our current knowledge of transport mechanisms is limited. Much of the plastic entering the ocean sinks; this is intuitively obvious for polymers such as polystyrene (PS), which have a greater density than seawater, but lower density polymers like polyethylene (PE) also occur in sediments. Biofouling can cause large plastic objects to sink, but this phenomenon has not been described for microplastics <5 mm. We incubated PS and PE microplastic particles in estuarine and coastal waters to determine how biofouling changes their sinking behavior. Sinking velocities of PS increased by 16% in estuarine water (salinity 9.8) and 81% in marine water (salinity 36) after 6 weeks of incubation. Thereafter sinking velocities decreased due to lower water temperatures and reduced light availability. Biofouling did not cause PE to sink during the 14 weeks of incubation in estuarine water, but PE started to sink after six weeks in coastal water when sufficiently colonized by blue mussels Mytilus edulis, and its velocity continued to increase until the end of the incubation period. Sinking velocities of these PE pellets were similar irrespective of salinity (10 vs. 36). Biofilm composition differed between estuarine and coastal stations, presumably accounting for differences in sinking behavior. We demonstrate that biofouling enhances microplastic deposition to marine sediments, and our findings should improve microplastic transport models.

Microplastics increase the marine production of particulate forms of organic matter

“…This resulted in negative correlations of both TEP (particles L −1 , Spearman rho=−0.69, p=0.001, n=18) and CSP (particles L −1 , Spearman rho=−0.49, p=0.04, n=18), as well as in the number of Synechococcus cells (Spearman rho = −0.860.65, p<0.0001, n=33) to microplastics concentration ( figure S2). This suggests that biological production and rapid aggregation of organic gels are a possible removal mechanism of microplastics from the water column, supporting recent observations (Kaiser et al 2017, Michels et al 2018. The presence of both heterotrophic bacteria and suspended particles may increase the stickiness of Synechococcus-derived TEP or its precursors, enhancing cell aggregation and export (Deng et al 2015, Cruz andNeuer 2019).…”

The physical oceanography of the transport of floating marine debris

“…(5) Plastic buoyancy may be affected by biofouling (Kooi et al 2017). Epibionts growing on plastic particles add mass and, depending on the organism's specific density, cause changes in overall buoyancy of the fouled particle (Zettler et al 2013), increasing settling velocity (Kaiser et al 2017). These effects are more important on particles with large surface area to volume ratios that have lower initial buoyancy (Ryan 2015) and can rapidly lead to sinking of fouled particles (Fazey and Ryan 2016).…”

“…The dynamics of fibres or very small plastics, on the other hand, remains poorly documented, and overall the interaction between particles and turbulence is a major knowledge gap in a realistic description of the vertical distribution of plastics in the ocean. More experiments are also needed to parameterise the influence of plastic degradation and biofilm formation on the rise/settling velocity (Kaiser et al 2017). The vertical distribution is also influenced by particle concentration, through affecting the turbulence (Bennett et al 2013), and size (Bennett et al 2014).…”

Transport of marine microplastic particles: why is it so difficult to predict?

“…Numerical modelling is a promising tool to elaborate the required holistic view of MPs (as well as marine plastic litter in general) in the world ocean. In its turn, relevant modeling depends on thorough conception of MPs dynamics in the real environment; however, only initial steps have been made in this regard (Ballent et al 2012;Reisser et al 2015;Chubarenko et al 2016;Kaiser et al 2017;Khatmullina and Isachenko 2017;Kooi et al 2017). In contrast, behavior of other particles in the water column (e.g., natural sediment grains, fecal pellets, phytoplankton) have been quite extensively studied in sedimentology and hydrodynamics (e.g., Soulsby 1997;Turner 2002).…”