Occurrence in Freshwater Systems

As compared to marine environments, the occurrence of microplastics in freshwater environments is less understood [41,42]. The early investigations of freshwater systems suggest microplastic presence and interactions to be equally as far reaching as observed in marine systems. In the marine environment, microplastics cover a wide range of habitats: at the sea surface, on shorelines, in the deep sea, in sea ice and in biota. They are reported in geographic locations around the globe and widely reported in different depositional environments. They affect the ecological functioning of biogenic habitat like pelagic open ocean habitats to deep-sea benthic habitats and also the coastlines of all continents. They are found even within organisms far from the human presence, such as in deep-sea corals [9].

Identification and quantification of plastic particles are very sensitive and are key steps to address the plastic contamination. The detection and analytical confirmation of purified samples of MPs require access to sophisticated equipment like micro-FTIR [43], Raman spectroscopy [33] scanning electron microscopy, and energy-dispersive X-ray spectrometry (SEM-EDS) (Rocha- Santos and Duarte 2015) to analyze the chemical constituents of microplastics. After pyrolysis, gas chromatography and mass spectrometry (GC-MS) can also be carried out, but the information about shape and size of particle is lost [44].

Recently, substantial data have been collected on microplastics in freshwater habitats by Faure et al. [54]. The presence of primary and secondary microplastics in Lake Geneva, between Switzerland and France, have been found. Moore et al. emphasized that microplastics entered the environment upstream of oceans [50]. The MP concentrations in surface water samples of the Rhine river (Germany) were reported at an average of 892,777 particles/km2, with a peak concentration of 3.9 million particles/km2 [32]. In river shore sediments, the number of particles ranged from 228 to 3763 and 786 to 1368 particles/kg along the rivers Rhine and Main (Germany), respectively [45]. Su et al. showed microplastic pollution levels during 2015 in Lake Taihu in China and found that microplastics reached 0.01 x 106-6.8x 106 items/km2 in plankton net samples. The microplastics distribution in plankton net samples was different from that in sediments, and microplastics were dominated by fibers 100-1000 pm in size [46]. The Three Gorges Reservoir in China revealed high surface-water contamination with fibers being the most abundant plastics. High-density microplastics were likely to deposit in the sediments [47]. Research on microplastics in freshwater systems is gaining impetus, and microplastics are now recorded in freshwater systems of several continents around the globe.

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