­­­­­­Microplastic load and polymer type composition in European rocky intertidal snails: Consistency across locations, wave exposure and time

Julius A. Ellrich, Independent Researcher, Koblenz, Germany, Sonja M. Ehlers and Jochen H. E. Koop, Department of Animal Ecology, Federal Institute of Hydrology, Koblenz, Germany, Sonja M. Ehlers and Jochen H. E. Koop, Department of Biology, University of Koblenz-Landau, Koblenz, Germany

Background/Question/Methods
Microplastic pollution (by plastic particles < 5 mm, MP) is ubiqitious in oceans worldwide. Due to their small size, MPs are readily ingested by various benthic and pelagic animals. However, information on MPs in rocky intertidal animals and habitats is relatively scant. Therefore, we examined herbivorous snails (common topshells that occur along coastlines worldwide, n= 130) and water samples (n= 24) that we had collected from wave-sheltered and wave-exposed rocky intertidal habitats in Helgoland island (North Sea), Cap Ferrat (Mediterranean), Giglio island (Mediterranean) and Madeira island (Atlantic Ocean) in 2019/2020 for MPs. Additionally, we examined snails collected from the same habitats in 2007-2009. These snails were the related topshell species Steromphala cineraria (from Helgoland), Phorcus turbinatus (from Cap Ferrat and Giglio) and Phorcus sauciatus (from Madeira). A topshell feeds by scratching and brushing small algae and diatoms off the substrate using its flexible and protrusible feeding organ (i.e., the rhipidoglossate radula) and, thereby, typically ingests small particles such as shell pieces and sediments. For MP identification, we used state-of-the-art micro-Fourier-transform (μFTIR) spectroscopy. Altogether, we performed 250 individual μFTIR measurements for MP identification in the snails and 112 individual μFTIR measurements for MP identification in the water samples.
Results/Conclusions
In total, we detected 50 MPs (composed of nine polymer types) in the snails and 24 MPs (composed of six polymer types) in the water samples. Interestingly, the most common snail and water polymer types were acrylic / alkyd fragments (often with styrene and kaolin components) and polyester fibers which indicates that the majority of the MPs derived from paints and synthetic clothing, respectively. Snail MP load and polymer type composition were consistent across locations, wave exposure and time. Likewise, water MP load and polymer type composition were not affected by location and wave exposure. Average MP loads (± SE) were 0.4 ± 0.1 MPs / snail individual, 1.0 ± 0.2 MPs / g snail soft tissue wet weight and 2.4 ± 0.8 MPs / L water. These findings suggest that MPs are common in rocky intertidal snails and habitats. Finally, we detected significant positive relationships between snail and water microplastic loads which indicates that snails are useful bioindicators for MP concentrations in rocky intertidal habitats. Thus, our findings can serve as a baseline to evaluate future MP dynamics in the rocky intertidal. Overall, this is the first comprehensive study of MPs in rocky intertidal organisms across space and time.