In this blogpost, Sonja Ehlers reports how microplastic loads in rocky intertidal snails from the North Sea, Mediterranean and Atlantic Ocean are consistent across space and time. The majority of the contained microplastics consisted of paint chips that likely derived from ships or maritime equipment. Such paint chips can contain toxic antifouling substances and heavy metals.

Regarding marine plastic pollution, people usually think of animals like whales, turtles and fish that consume plastic or get entangled in discarded fishing gear. However, while those pelagic organisms have been intensively studied for plastics over the last decades, some marine organisms and habitats remain largely unexplored.

Microplastics are plastic particles below 5 mm in size which can be created by fragmentation and weathering of larger plastic items such as plastic bottles, plastic food packaging and synthetic clothing. Once in the ocean, plastic items can be abraded on rocky coasts by wind and wave action. The resulting microplastics can then be consumed by various organisms. Interestingly, rocky intertidal organisms have rarely been analyzed for microplastics.

Therefore, we collected rocky intertidal snails (topshells, Fig. 1) from four European coasts in 2019 and 2020: Helgoland Island (North Sea, Germany), Cap Ferrat (Mediterranean Sea, France) and Giglio Island (Mediterranean Sea, Italy; Fig. 2) and Madeira Island (Atlantic Ocean, Portugal). At the lab, we then gently removed the snails from their shells, weighed them and chemically digested their soft tissue for subsequent microplastic analysis. We verified and identified microplastics by conducting micro-Fourier-transform infrared (µFTIR) measurements for polymer type identification. Since we had already collected topshells in Helgoland, Cap Ferrat and Giglio between 2007 and 2009, we were able to compare microplastic loads and compositions across locations and time (2007-2009 vs 2019-2020).

Furthermore, we wanted to find out whether the microplastic loads in the snails were correlated with the microplastic concentrations in the surrounding seawater. Therefore, we also collected and analyzed water samples from the water surrounding the snails for microplastics. Topshells are submerged in seawater during high tide and start feeding once they are covered by water. Hence, it is likely that microplastics in the snails have once been suspended in the water column.

Snails are useful bioindicators for microplastic concentrations in rocky intertidal waters

Our analyses revealed that the 130 snails that we analyzed contained 50 microplastics (consisting of nine polymer types) and that the water samples contained 24 microplastics (incorporating six polymer types). In the snails, we mostly found acrylic/alkyd paint chips but also common microplastic types such as polyester fibers, polyethylene and polypropylene. In the water samples, we mainly found polyester fibers and acrylic/alkyd paint chips. Snail microplastic loads did not differ across locations and years suggesting that microplastics have been fairly common pollutants in European rocky intertidal snails during the past 13 years. Furthermore, snail and water microplastic loads were significantly correlated indicating that snails are fine-tuned bioindicators for microplastic concentrations in rocky intertidal waters.

How can paint chips be accurately identified and what might be their source?

Interestingly, our µFTIR spectra revealed traces of kaolin and styrene in several microplastics. Such substances are added to paints in order to increase water resistance and to reduce moisture uptake. Thereby, our µFTIR analysis showed that these microplastics were actually paint chips. The paint chip colors were blue, red and green and resembled boat and ship colors in nearby harbors. Such paints can be toxic for animals since they commonly include antifouling substances and heavy metals.

In total, we found that 50 % of all microplastics in the snails consisted of paint chips. This constitutes the highest paint chip proportion which has ever been detected in marine animals. Previous studies have found 30 % paint chips in Pacific fish, 27 % in turtles, 25 % in jellyfish and 17 % in Atlantic fish. This suggests that coastal organisms like snails might be more prone to paint chip ingestion than such pelagic organisms. Thereby, our study underlines the importance of including paint chips in microplastic studies.

The paper was authored by Sonja M. Ehlers, Julius A. Ellrich and Jochen H. E. Koop and published in Environmental Pollution (DOI 10.1016/j.envpol.2021.118280)