Plastic debris in coastal macroalgae

Macroalgae are the most productive marine macrophytes in the coastal ecosystem. Although plastic debris has been ubiquitously detected in marine animals, little is known about plastic pollution in macroalgae and how they interact with each other. In this study, the occurrence of plastic debris including microplastics was investigated in 5 macroalgae species that are commonly found along the Chinese shorelines. These species consisted of Gracilaria lemaneiformis, Chondrus ocellatus, Ulva lactuca, Ulva prolifera and Saccharina japonica. We categorized the plastic debris into 3 size classes, i.e., macroplastics (>25 mm), mesoplastics (5-25 mm), and microplastics (1 μm-5 mm). It was shown that there were 5 loading patterns of plastic debris interaction with the macroalgae. The 5 patterns included entanglement, adherence, wrapping, embedment, and entrapment by epibionts. According to direct observations through the non-digestion method, all 3 size classes of plastics were found in the macroalgae. The abundances were 0-201.5 (macroplastics), 0-1178.0 (mesoplastics) and 0-355.6 (microplastics) items/kg dry weight, respectively. These plastics were dominated by fibers (52.2%) in shape, 1-5 mm (39.6%) in size, and polystyrene (36.5%) in composition. According to indirect observations through the digestion method, only 2 size classes of plastics were identified in the macroalgae: mesoplastics, 0 to 888.9 items/kg dry weight; microplastics, 148.1 to 5889.0 items/kg dry weight. These plastics were prevailing by fibers (71.5%) in shape, 1-5 mm (52%) in size, and polyester (29.3%) in composition. Furthermore, plastic characteristics in the detected macroalgae were related to their species, sampling regions, and beach types based on the results of similarity and principal component analysis. This study indicated that macroalgae utilized diverse pathways for loading plastics in the coastal environment. Meanwhile, environmental factors significantly influenced the distribution of plastics loaded by macroalgae.