How fast, how far: Diversification and adoption of novel methods in aquatic microplastic monitoring

Since 2004, there has been a marked diversification in the methods used to determine aquatic microplastic (MP) concentrations. Despite calls for a unified approach to MP sampling, the proliferation of new methods has accelerated in recent years. Both minor method adaptations and entirely novel approaches have been introduced to overcome barriers to reliable MP sampling, extraction and quantification, resulting in a variety of complimentary but also competing approaches. However, there is little clarity regarding the extent to which new methods are acknowledged and adopted, or of the apparent drivers of, as well as barriers to, said adoption. To explore these issues, the rate of method diversification was examined in a systematic review. The rate and degree of diversification were determined by scoring each method by its "degree of novelty": highly novel methods, secondary adaptations of existing methods and smaller, tertiary adaptations of existing methods. This analysis revealed that the rate of method diversification has been greatest since 2011. Our results indicate limited use of these novel methods and adaptations in the subsequent literature, with many researchers falling back on methods that are well established in the existing literature. Importantly, there is little consistency in the units used when reporting MP concentrations. However, these differences are seldom driven by method selection and are rather the result of discrepancies between researchers. Thus, in understanding the requirements of comparability and consistent reporting for monitoring purposes, we can apply a diverse approach to sampling whilst maintaining the applicability and usefulness of the resulting data.

Long-term microplastic retention causes reduced body condition in the langoustine, Nephrops norvegicus

Microplastic represents a rising proportion of marine litter and is widely distributed throughout a range of marine habitats. Correspondingly, the number of reports of species containing microplastics increases annually. Nephrops norvegicus in the Firth of Clyde have previously been shown to retain large aggregations of microplastic fibres. The potential for N. norvegicus to retain plastic over an extended time period increases the likelihood of any associated negative impacts to the individual. This study represents the longest observation of the impacts of microplastic retention in invertebrates. We exposed N. norvegicus to plastic over eight months to determine the impacts of extended exposure. Over this period we compared the feeding rate, body mass, and nutritional state of plastic-fed N. norvegicus to that of fed and starved control groups. Following the experimental period, the plastic-fed langoustine contained microplastic aggregations comparable to those of small individuals from the Clyde Sea Area. Comparisons between fed, unfed and plastic-fed individuals indicated a reduction in feeding rate, body mass, and metabolic rate as well as catabolism of stored lipids in plastic contaminated animals. We conclude that N. norvegicus exposed to high levels of environmental microplastic pollution may experience reduced nutrient availability. This can result in reduced population stability and may affect the viability of local fisheries.

Environment and gut morphology influence microplastic retention in langoustine, Nephrops norvegicus

Over the past twenty years microplastic pollution has been recorded in all major marine habitats, and is now considered to be of high environmental concern. Correspondingly, the number of reports of microplastic ingestion by marine species is increasing. Despite this, there are still relatively few studies which address the uptake and retention of microplastic in wild populations. Langoustine, Nephrops norvegicus, sampled from the Clyde Sea Area, have previously been seen to contain large aggregations of microplastic fibres. The large proportion of contaminated individuals and size of the microplastic aggregations observed suggests that Nephrops are at high risk of microplastic ingestion. In this study the levels of ingested microplastic in populations of N. norvegicus from the Clyde Sea Area, North Minch and North Sea are examined. Animals in the near-shore, Clyde Sea population showed both a higher percentage of microplastic containing individuals and much greater weights of microplastic retained in the gut. N. norvegicus revealed that only a small percentage of individuals from the North Sea and Minch contained microplastic, predominantly single strands. An expanded sample from the Clyde Sea Area was examined to identify the factors influencing microplastic retention. This revealed that males, larger individuals, and animals that had recently moulted contained lower levels of microplastic. The presence of identified food items in the gut was not seen to correlate with microplastic loads. Observations of microplastic in the shed stomach lining of recently moulted individuals and the lack of aggregations in wild-caught individuals suggests that ecdysis is the primary route of microplastic loss by N. norvegicus. Therefore the large aggregations observed in wild-caught animals are believed to build up over extended periods as a result of the complex gut structure of N. norvegicus.