Data Analysis

The varied methods by which microplastic uptake by biota is measured understandably result in differing levels of recording. At the highest level, researchers record the number of items, often in relation to organism size. This may be recorded simply, as the percentage of individuals seen to ingest microplastic [36,66,71,76,77,79,84,89], the number of microplastics per individual [16,53,54,57,58,65,74,78,80,85,88-91,102] or as the number of microplastic items by length or weight [38,54,60,61,73,77,89]. Many types of plastic, for example, microfiber boluses, do not lend themselves to the enumeration of individual plastic items. In addition, mastication and peristaltic action may break down plastic items within the gut; as a result, the number of items in the gut may exceed that originally ingested. In such cases, researchers have reported the weight of plastic aggregations [41], descriptions of the aggregation of microplastic observed [34] or a combination of the two. Such issues in enumeration are more often observed in studies of wild-caught organisms, where the initial level of microplastic exposure is not known and the type of microplastic recovered is susceptible to tangling. A similar issue may be observed in the study of microplastic uptake in laboratory experiments; here, concentrations of introduced microplastic may be recorded solely by number or mass per individual [19,161] or as a value in relation to mass of food [33,34,92,99,161-163] or volume of water [10,26,164].

The use of multiple methods to quantify the level of microplastic uptake by fish and invertebrates is also an issue in the reporting of environmental plastic levels. Inconsistency in the use of units can mask or inflate the apparent impact of microplastics on a species or location. This increases the likelihood of errors arising when comparing multiple studies carried out by unrelated researchers. The manner in which plastic abundance and concentration is recorded influences the range of statistical analyses available; for example, grouping aggregations into specific classifications reduces the power of the available tests. In field experiments, a range of techniques have been used to determine the relationship between microplastic uptake and both biological and environmental factors. Many of these methods combine continuous and categorical variables in linear models of varying complexity and require careful structuring in statistical software such as R statistical software [165].

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