Drivers and effects of plastic contamination in aquatic ecosystems

Marine litter indicates any solid material manufactured or processed by man, which is discarded or disposed in the marine environment after its use. The most prevalent sort of marine litter is plastic, which is found in all aquatic ecosystems, even in remote areas far from human activities. Plastic already dispersed in the marine environment undergoes through environmental degradation and is further fragmented into smaller particles which called MicroPlastics (MPs), with a dimension comprised between 1μm and 5mm. The thesis aims to analyse the spatial and temporal variability of seafloor macro-litter in the Food and Agriculture Organization (FAO) Geographical Sub-Area 11 (Sardinian seas, western Mediterranean) and to test whether different litter categories may result in geographically segregated seafloor litter hotspots. Analyses are based on data on different litter categories (plastic, rubber, metal, glass, cloth, wood, paper, other) collected over a 7 years period, in the framework of the MEDiterranean International Trawl Survey. Results show the absence of any temporal trend in seafloor macro-litter density and weight, but rather indicate a spatial and bathymetric segregation of different litter categories. The data shows how different sources and physical features of macro-litter items (i.e., plastic and fishing gear, rubber, glass, metals, and cloth) led to spatially segregated accumulation hotspots. (Chapter 2). Accumulation hotspots, moreover, can be source of MPs that can eventually contaminate benthic fauna. In Chapter 3, the exposure to MPs was evaluated using economically and ecologically key crustaceans Palinurus elephas (Fabricius, 1787) and Nephrops norvegicus (Linnaeus, 1758) collected from sites located on the western coast of Sardinia, at depths comprised between 25 and 660 m was investigated. The results suggest that both P. elephas and N. norvegicus could be valuable bioindicators and charismatic species that could represent excellent flagship species for raising awareness toward the global issue of plastic in the marine environment (Chapter 3). Finally, moving to freshwater environments, the last chapter is a study looking at the effects of MPs pollution on fish behaviour. The target species were the native crucian carp (Carassius carassius) and invasive gibel carp (Carassius gibelio), that were exposed to MPs via contaminated food pellets (using polyethylene or tire wear particles) and compared to a control group for 60 days. Then, differences in behavioural changes and swimming capacity were tested. The results show that exposure to MPs affected fish behaviour, and in particular the native species changed its behaviour toward bolder levels, potentially exposing them to higher predation rates in the wild. (Chapter 4). The thesis further emphasizes that plastic pollution, an ever-arising challenging issue to all environmentalists needs to be taken care of. The study highlights the smaller yet dense hotspots of the accumulated plastic litter along the Sardinian coasts due to various factors such as oceanic currents and geomorphology. Contributing to the diverse scientific literature for the ingestion of the MPs by several species, here in our study the decapod crustaceans such as P. elephas and N.norvegicus share a similar feeding habitat that is scavenging activity and digestion. This common feeding habitat of the species suggests that they could be potential bioindicators for the MP contamination. Lastly, the MPs in their sub-lethal doses of exposure to the fish exhibited behavioural changes after two-month treatment eventually impact the species interactions. All these results contribute to the present literature and set an example for the toxicity experiments wherein the higher does could be tested for interactions with other organisms especially with predators.