It has recently been discovered that plastic debris can form crusts on intertidal rocks. This phenomenon has been termed- plasticrust, but the formation process and potential plastic sources are unknown. In this study, we show for the first time that discarded maritime ropes are plasticrust sources and that summer rock surface temperature can contribute to plasticrust formation.

Discarded fishing gear, including maritime ropes, are a source for marine and coastal plastic pollution worldwide. Animals can get entangled in such fishing gear or ingest the plastic material. However, the role of maritime ropes in the formation of plasticrusts was unknown. Plasticrusts are crusts that are left behind by larger plastic items which are abraded on intertidal rocks. They have been reported for the first time from Madeira Island in the Atlantic Ocean. During a field trip to Madeira Island in January 2020, we discovered a green plasticrust next to a green maritime rope in the rocky intertidal. Since information on plasticrust sources and formation processes were lacking, we collected material from the green crust and the green rope. At the lab, we then identified the polymer types using Fourier-transform infrared spectroscopy (FTIR). We found that the plasticrust and the rope consisted of the common polymer type polypropylene and that the FTIR spectra of the rope and the crust showed a 96.5 % similarity. Therefore, we were able to prove that the green plasticrust derived from the green maritime rope. It is also the first finding of a plasticrust that does not consist of polyethylene and that is not of blue or white colour (Figure 1).

What could be the source of blue and white plasticrusts?

Additionally, we then collected a blue maritime rope in the Madeiran rocky intertidal and compared its FTIR spectrum with the FTIR spectra of blue and white polyethylene plasticrusts that we found on Madeira Island. Again, the resemblance of the spectra was very high with a similarity of 93.7 % between the blue plasticrust and the blue rope and 92.3 % between the white plasticrust and the blue rope. The white plasticrusts may once have been of blue colour and may have lost their colour due to environmental influences in the rocky intertidal. To sum up, our results show that maritime ropes can be the sources of green, blue and white plasticrusts. 

Effects of summer rock surface temperature on plasticrust formation

Since all plasticrusts showed signs of melting, we used a heating plate to expose fibres of the collected green and blue ropes to simulated summer rock surface temperatures for the duration of one low tide. As a control, we placed blue and green fibres on the lab bench at room temperature. Afterwards we examined all fibres for signs of melting and analysed the fibre thickness. Our analyses showed that the heated green polypropylene and the heated blue polyethylene fibres from the maritime ropes were glued together at the end of the experiment, thereby increasing their thickness. This was not observed for the control treatments which shows that summer rock surface temperatures can contribute to plasticrust formation in the field.

Implications of the findings for marine organisms

The fact that maritime ropes leave behind plasticrusts in rocky intertidal habitats is concerning. Since these crusts cover rock surfaces which are inhabited by invertebrates such as marine snails and crabs, these animals could ingest plasticrust material while foraging. Furthermore, it is currently unknown for how long plasticrusts last in the rocky intertidal and if plasticrust material detaches from rocks. Depending on its size, such detached plasticrust material could then contribute to microplastic pollution. Microplastics are plastic particles below 5 mm in size which can easily be ingested by various organisms.

The paper was authored by Sonja M. Ehlers, Julius A. Ellrich and Ignacio Gestoso and published in Marine Pollution Bulletin (DOI 10.1016/j.marpolbul.2021.112841).