| Summary: | 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.
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