Thesis title: Understanding pathways of microplastics through marine food webs: Developing a system perspective to achieve a reliable monitoring strategy
The diffusion of synthetic compounds and materials in natural environments is an irreversible change posing emerging threats to the structure and functioning of ecosystems. Over the last two decades many studies have highlighted the presence of small plastic particles in every kind of environmental sample. Most studies about microplastics (i.e., synthetic particles ≤ 5 mm in size) emphasize contamination of marine environment, which represents the main sink for this class of contaminants. However, the comprehension of the pathways that microplastics follow from sources to sinks and the dynamic of their entry and transfer through the food webs are still important knowledge gaps to be filled. One of the main research priorities on microplastic pollution is to develop monitoring strategies to assess the impact of microplastic ingestion on wild organisms. Although fish species are regarded as good bioindicators, the complex mechanisms linking microplastic ingestion to the biology and ecology of different species complicate the collection of comparable data over time and across different marine regions. This thesis has the ultimate aim of providing basic recommendations for the development of monitoring programs based on fish species. To achieve this objective, the analysis of the distribution of microplastics in seawater and on the seabed has been combined with studies investigating the relationship between microplastic ingestion and the ecology and biology of candidate target species. Results highlighted that the environmental fate of microplastics with different physicochemical characteristics is determined by the arrangement of several factors, including the circulation of seawater on different spatial scales, the effects of deep currents, the topography of the seabed, as well as the presence and location of contamination sources (e.g., river flows, urban settlements, and seaports). The relationship between the contamination of environmental matrices and the bioavailability of microplastics has turned out to be even more complex. In particular, the exploitation of different habitat underlies the ingestion of different microplastic types. At the same time, species-specific ecological traits affect microplastic ingestion rates, which could be further influenced by the individual features of the specimens examined. Overall, the present work indicates that effective monitoring programs should be based on the definition of specific monitoring objectives for assessing the impact of determined microplastic types, which should be defined according to the actions implemented by the policy. From this perspective, future studies on microplastic pollution should give more attention to the meaning of categories used to classify microplastics.