Macro-plastics in coastal environments: effects on dune and marine plants

Plastic pollution is a global problem affecting every natural environment, including coastal sand dunes and seagrass meadows that deliver important ecological services such as coastal protection, erosion control, and wildlife maintenance. Although coastal dunes and seagrass meadows can act as a sink of plastic litter, the impacts of conventional non-biodegradable plastics on plants inhabiting, forming, and maintaining these habitats are substantially unknown. Very little attention has also been paid to the behaviour of biodegradable/compostable plastics in these environments and their potential effects on wildlife and plants. In addition, coastal dunes and seagrass meadows as well as their associated organisms are already threatened by anthropogenic and global change related stressors, but whether and how these factors may interact with plastics in affecting vegetation remains to be explored. Understanding whether plastic litter alone or in combination with other factors could represent a threat to dune and marine plants could be critically important to establish effective and specifically tailored management actions to protect vegetation from plastic pollution. The main aims of this thesis were to assess whether non-biodegradable and biodegradable/compostable plastics can affect the recruitment and the growth of dune and marine plants, and whether these plastics can interact with other factors such as for example increased atmospheric nitrogen (N) deposition, sedimentation and neighbour presence/identity in affecting these plants. Non-biodegradable bags were selected as they constitute a large fraction of beach litter, while biodegradable/compostable bags were chosen as they have recently been commercialized to reduce plastic pollution and the market of biodegradable plastics is predicted to greatly increase in the future. To answer these questions, manipulative experiments were performed under laboratory, mesocosm and field conditions.