Studying the effects of marine plastics on coastal ecosystems in the Balearic Sea

[eng] Due to their persistence, pervasiveness, and global abundance, microplastics are emerging as structural elements of global oceans. Like other structural elements, microplastics flow between biotic and abiotic reservoirs through multiple pathways, modifying and being modified by the environment. The potential strength and effects of this alteration on ecosystem functioning and global Earth processes are raising concerns and highlighting the urgency of better characterizing the different reservoirs and pathways connecting them within the plastic cycle. This thesis aims to deepen the understanding of the spatial distribution, composition, and ecological interactions of microplastics in marine coastal ecosystems of the North western Mediterranean Sea, focusing on the Balearic Islands as a representative hotspot of plastic pollution. Integrating field sampling, laboratory analysis, and qualitative and quantitative systematic reviews, this work contributes to quantifying and characterizing microplastic stocks across marine compartments and gradients of anthropogenic pressures, while investigating the potential of copepods to act as local and global temporary microplastic reservoirs, shaping microplastic distribution patterns in the marine environment. Spatial distribution patterns of neustonic microplastics under increasing anthropogenic pressures were assessed in the North Western Mediterranean Sea, and their vertical distribution was subsequently analyzed from the sea surface to the sediments, including the water column and the hyperbenthic layer. Moving along with the vertical gradient microplastics ingestion was evaluated in a wide range of fish and invertebrate’s species inhabiting different depths. The study encompasses the investigation of plastic pollution in critical environments, such as the hyperbenthic layer hosting Posidonia oceanica seagrass meadows, and the sea surface microlayer at the interface between the ocean and the atmosphere. Results confirm the Balearic Islands as a hotspot for plastic pollution, characterized by high abundances of neustonic microplastics regardless of proximity to contamination sources or protection status, and hosting marine litter windrows where less diverse zooplanktonic communities are observed. Microplastic abundances increase along vertical gradients, with sediments hosting two orders of magnitude of microplastic abundances than surface waters. Fibers are the most represented fraction across the water column, in marine sediments, and ingested by marine species. The extensive polymer characterization performed in this thesis highlights the strong dominance of cellulosic polymers in the fiber fraction, marking a clear difference between the microplastic inventories found in biota and below the sea surface, compared to those floating at the sea surface, mainly composed of polyethylene and polypropylene. When primarily focusing on micro- and meso-scale assessments at high spatial resolution, this thesis transitions from a local to a global perspective to evaluate interactions between microplastics and copepods within the plastic cycle. Despite the low microplastic ingestion values observed in the hyponeustonic copepod P. mediterranea, with a mean of 0.11 ± 0.05 MPs/ind., this species retains a mean of 45 ± 65 MPs/m3, emerging as an extended reservoir of microplastics within the sea surface microlayer. Systematic reviews and meta-analyses confirm copepods as extensive temporary reservoirs of microplastics in global oceans, owing to their abundance, and key candidates for shaping the distribution and fate of microplastics. This thesis contributes to establishing a methodological framework for studying the interactions between microplastics and copepods, emphasizing the importance of considering the abundance of studied taxa and evaluating microplastic retention as a key parameter to assess the ecological implications of these interactions. Overall, this thesis highlights the importance of implementing comprehensive approaches to assess ecosystem health in relation to plastic pollution, including the fiber fraction in major assessments and regulatory frameworks. It also emphasizes the potential key role of copepods in shaping the distribution of plastics and the need to further explore the significance of this interaction in the main pathways through which microplastics could influence global-scale processes.