Page - 185 - in Freshwater Microplastics - Emerging Environmental Contaminants?

facilitate biofilm formation [24, 25]. Plastic-colonizingmicroorganisms have also been found to influence the surface properties and buoyancy of polymers [12, 20, 26]. Sincemicroplastics are likely to be transported intomarine environments via WWTP, rivers, and streams [6, 7], factors contributing to initial colonization (such assurfaceroughnessandattachmentbypioneeringcolonizers)canbehypothesized tobeparticularly importantwithin freshwaters. The impacts of particle age and/or weathering on plastisphere consortia may be comparatively pronounced within marine ecosystemswhere the residence times of plastic often exceed thosewithin rivers and streams [24]. However, microplastics additionally accumulate within environments such as lakes, where theymay persist for decades (similar to time- scales predicted for marine habitats) and can be exposed to high levels of UV radiation [2, 27, 28]. Local-scale differences in the composition of plastisphere assemblagesbetweenpolymertypeshavebeenfound[12,29,30],but it isunknown whether there are any general differences in the dominant types of plastic within freshwater and marine ecosystems. Moreover, although it is possible that the ingestion of plastics by higher organisms could have an impact on plastisphere colonizationprocesses, this topic has not been investigated [11, 20, 30]. Ambientconditionssuchas temperature, salinity,pressure,andtheavailabilities of light and oxygen are likely to influence the development of plastic-associated biofilms (Fig. 1) [29, 31].Manyof these conditions differ between freshwater and marineecosystems,andWWTPandunmanaged freshwaters.Forexample, the low temperatures(<5 C),absenceof light,andelevatedpressurewithindeepwatersare likely to impose selective forcesonplastisphereassemblages thatdiffer fromthose within shallow habitats. In contrast with the frequently nutrient-poor conditions present within the open ocean, inland and coastal waters receive high fluxes of nutrients from the surroundingenvironment [14]. In addition tocontributions from organicmatter input andupwelling, highconcentrationsofnutrients (e.g., nitrogen and phosphorus) are released by agriculture and other human activities. Many plastispheremembers have been affiliatedwith pollutant degradation [12, 13, 20, 21], and it is probable that several contaminants play a role in shaping biofilm formationandactivities onpolymers (Fig. 1). Indeed,multiple typesof pollutants, aswell as heavymetals, are known to become adsorbed ontomicroplastics [2, 8, 10]. Further to these factors, physical processes contributing to the movement of suspendedparticles differ between freshwater andmarinehabitats [2].Continuous downstreammovementofwater isakeydistinctionbetweenfreshwaterandmarine ecosystems. In rivers, sediment movement is characterized using the concept of “spiraling” [32, 33]. The components of one spiral include downstream transport, deposition, bed load transport, and resuspension.This concept is awell-developed approach formodelingparticlemovementand isquantifiedusingmeasurementsof deposition length and velocity, turnover time, and the retention-export ratio [34]. To date, direct measurements of spiralingmetrics have not been applied to microplastic (but seeKowalskietal. [26],Longetal. [35],andNizzettoetal. [36]). Microplastic-AssociatedBiofilms:AComparisonofFreshwater andMarine. . . 185