A new study led by the Institut de Ciències del Mar (ICM-CSIC) has found that plastic pollution contributes to ocean acidification. Plastic pollution and acidification are two of the biggest issues threatening the ocean today.
The increase in ocean acidity is making it difficult for organisms that calcify, such as corals, to maintain their skeletons. Every year, up to 13 million tons of plastic reach the sea, and it is estimated that there are around 250,000 more tons floating throughout the ocean.
The study reveals that as sunlight degrades plastic, dissolved organic carbon compounds are released. These organic chemical compounds lead to a drop in pH levels.
“Thanks to this study we have been able to prove that in highly plastic-polluted ocean surface areas, plastic degradation will lead to a drop of up to 0.5 pH units, which is comparable to the pH drop estimated in the worst anthropogenic emissions scenarios for the end of the 21st century,” said study first author Cristina Romera-Castillo.
The sun’s ultraviolet light leads to plastic degradation, causing the plastic to break into smaller pieces. Some of these particles, known as microplastics, are less than 5 millimeters in length.
When exposed to sunlight and erosion, the plastic is “aged.” This exposure results in an increased release of chemical compounds into the water and causes a lowering of the water’s pH.
The chemical compounds released by plastic can come from the plastic itself or additives that give plastic color or resistance. Some of these compounds are organic acids, which is why they contribute to the pH drop.
For this study, the researchers exposed new and aged plastic to temperature and solar radiation. The water pH and the amount of dissolved organic carbon released by microplastics were also analyzed.
With only six days of exposure to sunlight, aged plastics released a large amount of dissolved organic carbon compounds – which led to a significant decrease in water pH. Experiments using new polystyrene, low-density polyethylene (LDPE), and biodegradable plastic fragments did not show a substantial drop in pH.
“These results show that aged plastic affects acidification much more than new plastic, which is very worrying, since most of the plastic found in the sea, whatever type it is, is degraded,” concluded Romera-Castillo.
The study is published this week in the journal Science of the Total Environment.