Seite - 118 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Africa’s freshwaters. Filling this knowledge gap must therefore be the highest priority and an absolute necessity to further understandings of sources and fate. The twostudiesdescribed indetail in thischapterhavebeenconducted in thesame regionandfoundthat thesourcesofplastic (andMP)pollutionwere linkedtourban refuse andfishingactivities.This echoes thefindingsof studies inother freshwater areas,where typeofplastic andMPsreflect theusagesandanthropogenic inputsof thelocalpopulations[10,13,25].ThepopulationofMwanzaisestimatedtobe1.12 millionpeopleby2020 (Table1), andwhilenot an insignificant number, this is by nomeans thelargesturbancenterclose toafreshwaterbody.We, therefore, suggest likely candidates for future research are locationswith highpopulation densities. The River Nile flows through a number of heavily populated cities, most notably, Khartoum in Sudan (almost six million inhabitants estimated by 2020), Alexandria (5.23million), and, of course, Cairo (20.57million) (Fig. 1, Table 1). WhileMPshavenot beendescribed in theNile, other pollutants (i.e., tracemetals Cd,Cr,Cu,Fe,Hg,Mn,Pb,andZn)werefoundintheabioticcompartmentsandthe tissues of resident fish populations [18]. It is worth noting that MPs have been showntoadsorbtracemetals in theenvironment[69,70],andwithin the laboratory, polyethylene MPs were shown to alter the bioavailability and uptake of Ag to freshwater zebra fish [71]. The River Congo similarly flows through densely populated cities, notably Kinshasa (14.12 million inhabitants) and Brazzaville (2.21million), and thesewaters would also be suspected of havingMPs present. Elevated tracemetal concentrations inCongosedimentswere found in thevicinity of urban runoff and domestic and industrial wastewater discharge into the river basin [72]. It would seem obvious to expect MPs to be present alongside other pollutants of urbanorigin in both these rivers. Howtodetermine theprevalenceofMPsrequires thought, and therearevarious sampling techniques to assessMPabundances to consider: (1) shoreline combing, (2) sediment sampling, (3)water trawls, (4) observational surveys, and (5) biolog- ical sampling. In different locations, somemay bemore or less relevant based on practical (the availability of personnel and equipment) and economic factors (i.e., funding). Inour study (CaseStudy II [17]), reporting thepresenceofMPs inLake Victoria,biological samplingwasconsidered tobe themost suitable techniqueas it required little specialist field equipment (i.e.,mantra trawls or trawl nets), and the laboratoryapparatus required todigestgastrointestinal tracts is relativelycommon. Additionally, the study was inexpensive as fish were purchased from the local marketand the researchcouldbeconductedwithinashort spaceof time.However, it isnecessary toselect suitablebiological indictors.Nonselective feedersprovidea better reflectionofMPs in the environment [32, 36]. For instance, the omnivorous fish, Nile tilapia, was used in LakeVictoria, andwater-filteringmussels (Mytilus edulis) and sediment-dwelling lugworms (Arenicolamarina) have been shown to take upMPs from their respective environments [73]. Studies such as the onewe conducted inLakeVictoria only present a “snapshot” ofMPpollution, and longi- tudinal studies are required to describe temporal and spatial differences. Where possible,acombinationof techniquesmaybemoreadvisableparticularlytopresent a complete picture ofMPs in the environment. However,with the current lack of 118 F.R.Khanet al.