The Missing Plastic Paradox of Ocean

The amount of plastic entering our oceans is tied to the production of plastic. However, it's puzzling that the observed amount of plastic floating on the sea surface is significantly lower than expected based on the plastic production and release since the 1950s. This mystery is known as the "missing plastic paradox." Several explanations have been proposed, falling into two main categories:

Overestimated Plastic Transport to the Sea

1. Riverine transport: Rivers are often considered a major source of plastic marine debris (PMD), but it is suggested that this source might be overestimated.

2. Atmospheric deposition: Surprisingly, atmospheric deposition of plastic remains unaccounted for in current estimates, even though it could be as significant as other land-sea transport processes combined.

Underestimated Removal Processes

1. Sinking plastics: While initially floating PMD can sink, the majority of submerged and sedimented plastic types differ from the buoyant plastic found at the ocean's surface.

2. Shore re-deposition: A substantial portion of floating plastic eventually beaches in coastal areas, further reducing the plastic load on the ocean's surface.

3. Accumulation in subtropical gyres: Floating PMD that avoids beaching accumulates in subtropical gyres and enclosed basins, where additional degradation processes come into play.

The Role of Solar UV Radiation Solar ultraviolet (UV) radiation is particularly influential in subtropical and tropical regions, where floating PMD tends to accumulate. Photodegradation is a crucial process initiated by UV radiation, leading to the degradation of plastic. Here are the key aspects of photodegradation:

1. Radical Reactions: UV radiation triggers radical reactions in plastics, causing chain breakage of the polymer. This, in turn, releases various organic and inorganic byproducts.

2. Byproducts: These byproducts can remain dissolved in the water, contributing to dissolved organic carbon (DOC) or dissolved inorganic carbon (DIC), or escape into the gas phase, including volatile molecules like short-chain hydrocarbons.

The Study's Findings The study investigated the kinetics of photodegradation of common plastic types found floating on the ocean's surface. Under conditions representing the sea surface in subtropical latitudes, the research found that UV light degrades larger microplastic particles by over 1.7% per year, equivalent to half-lives of less than 44.4 years. In a simplified model, this suggests that up to 22% of all floating plastic ever released into the ocean could have been degraded by solar UV radiation.

Implications The full impact of photodegradation on the marine environment remains a subject of ongoing research. One significant implication is the potential increase in nanoplastics concentrations in subtropical and tropical oceans where plastic accumulates. The consequences of this rise in nanoplastics for marine life in these ecosystems are yet to be fully understood.

Conclusion Solar UV radiation is a key player in the degradation of plastic at the ocean's surface, shedding light on the missing plastic paradox. Understanding the effects of photodegradation and its byproducts on marine ecosystems is vital for crafting effective policies to combat ocean plastic pollution. Further research will be essential to comprehensively grasp the implications of photodegradation on our oceans and marine life.

Ref: https://doi.org/10.1016/j.marpolbul.2022.114544