Fate of Plastic Pollution in the Arabian Seas

Plastic pollution has become of public concern recently and only in the last decades the need of quantifying loads of plastic in the marine environment and identifying their ultimate destination has been urged as a mean to point at where interventions should concentrate. The Arabian seas (Red Sea and Arabian Gulf) have oceanographic features that candidate them as accumulation zones for marine plastics, but, especially the Red Sea, are largely unexplored. The dissertation here presented provides significant advances in the understanding of the marine plastic distribution in the two basins. Despite the initial hypothesis, the Red Sea was found to hold a remarkably low abundance of plastic particles in its surface waters. Similarly, previous assessments have reported the same in the Arabian Gulf. In line with the global estimates, only a small portion of the plastic that is discarded yearly in the marine environment is found in its surface waters, implying the presence of removal processes. However, the unexpectedly low loads of floating plastics in the Arabian seas indicate that sinks are likely more significant here than elsewhere. In the Red Sea, an extensive survey of macroplastic stranded on shores, globally considered a major sink of marine plastic, has indicated that Avicennia marina mangrove forests, through the mesh created by their pneumatophores, contribute significantly more than unvegetated shores in retaining plastics. Loads of plastic in the Arabian Gulf mangrove stands, more impacted by coastal development than stands in the Red Sea, are even larger. The role of mangroves as significant sinks of plastics is further corroborated by the finding that the burial rates of plastic in their sediments follow an exponential increase in line with the global plastic production increase, ultimately demonstrating that plastic is likely sequestered there permanently. Mangrove forests alone are, however, not enough to justify the mismatch between plastic inputs and loads in surface waters. The experimental finding showed here that coral structures can passively trap substantial loads of microplastics and the large extension of reefs, especially in the Red Sea, suggest that reefs might constitute a missing sink of marine plastic in the basin worth exploring.