Microplastic contamination in the dominant crabs at the intertidal zone of Chongming Island, Yangtze Estuary.

Crabs are one of the most critical and dominant species of the intertidal zone. Their feeding, burrowing, and other bioturbation activities are common and intense. However, baseline data on microplastic contamination in wild intertidal crabs are still lacking. In this study, we investigated the contamination of microplastics in the dominant crabs, Chiromantes dehaani, of the intertidal zone in Chongming Island, Yangtze Estuary, and explored their probable relationship with the microplastic composition in sediments. A total of 592 microplastic particles were observed in the crab tissues, with an abundance of 1.90 ± 0.53 items·g-1 (1.48 ± 0.45 items·ind-1). The microplastic contamination in the tissues of C. dehaani varied significantly among different sampling sites, organs, and size groups, but not among different sexes. Microplastics in C. dehaani were mainly rayon fibers with small sizes (<1000 µm). Their colors were mostly dark, which is consistent with the sediments samples. A linear regression showed significant correlations between the composition of microplastics in the crabs and that in sediments, although they differed in various crab organs and sediment layers. The target group index identified the feeding preference of C. dehaani on the microplastics with specific shapes, colors, sizes, and polymer types. In general, the microplastic contamination in crabs is affected by both objective environmental conditions and subjective feeding habits of crabs. In the future, more potential sources should be considered to completely distinguish the relationship between the microplastic contamination in crabs and adjacent environment.

Distribution characteristics of microplastics and corresponding feeding habits of the dominant shrimps in the rivers of Chongming Island.

There is increasing global concern regarding microplastics. Rivers play a key role in the transportation and storage of microplastics on the Earth's surface. Here, we aimed to investigate the spatial-temporal variations in the distribution of microplastics in water as well as in the dominant species of the macrobenthic fauna Exopalaemon modestus and Macrobrachium nipponense in the river system of Chongming Island by setting up 16 fixed sampling sites. We found that the abundance of microplastics in the water of the rivers on the Chongming Island was 0.48 ± 0.10 n/L. There was no significant difference among different reaches. The abundance of microplastics in the major rivers was significantly higher in summer than in the other seasons. Detection rates of microplastics in Exopalaemon modestus and Macrobrachium nipponense were 50.12 % and 64.58 %, with mean abundances of 1.92 ± 0.52 n/g and 1.49 ± 0.30 n/g, respectively. The composition characteristics of the microplastics in shrimps were affected by the microplastics in the aquatic environment. The microplastic content in the shrimps and water were linearly correlated in terms of the same characteristics (shape, color, and polymer). Shrimps showed a stronger feeding preference {Target Group Index (TGI) > 1} for microplastics with fibrous shapes, transparent and green colors, rayon (RA) and polyethylene (PE) polymers, and relatively small sizes (<400 µm). These results indicate that shrimps prefer to consume microplastics that have similar appearance to their prey. Their benthic dwelling habits may limit their feeding space to the bottom of the water, which in turn leads to an increase in the feeding probability on microplastics of greater densities (e.g., RA). The catabolism of microplastics in shrimps may lead to an overestimation of their feeding preference for smaller sizes. Further controlled experiments should be carried out to obtain deeper insights into the preferences of shrimp for microplastics.

Investigating a probable relationship between the distribution of microplastics and crab burrows in the intertidal zone of Chongming Island, Yangtze Estuary.

Crab burrows are common biogeomorphic structures in the sedimentary environment of the intertidal zone. Existing studies have revealed crab burrows could function as traps and sinks for fine sediment particles and organic detritus transported by the overlying water. However, it was unknown whether the distribution of microplastics was related to crab burrows. To build on past work, we investigated the distribution characteristics of microplastics and crab burrows in the intertidal zone of Chongming Island and explored the probable relationship between them. The detection rate and abundance of microplastics in burrows were higher than in no-burrowing areas, indicating that crab burrow have a trapping function for microplastics. Crab burrows also increased the abundance and detection rate of microplastics in the deeper sediment layer, rather than just the surface layer. Furthermore, microplastics were more abundant in large burrows than in small burrows. The microplastics in the large burrow were significantly smaller in size than those in the small burrows and no-burrowing area. The abundance of microplastics positively correlated with the area occupied by burrows in the majority of study sites. Our findings provide a new perspective for exploring the transport and migration of the microplastics between the sediment and water column in the intertidal zone. Further controlled experiments should be carried to investigate the source-sink effects of crab bioturbation in the intertidal flat on microplastics.

Variation in microplastics composition at small spatial and temporal scales in a tidal flat of the Yangtze Estuary, China.

Microplastics are small, degrade slowly, and easily persist in the water column because they are close to neutrally buoyant. Understanding the distribution of microplastics is fundamental to evaluating the ecological risks that they cause and to identifying ways to control microplastics pollution. Most of the existing research on the distribution of microplastics in the coastal zone has focused on large spatial and temporal scales. To build on past work, we investigated variation in microplastics in a tidal flat of the Yangtze Estuary on small spatial (sediment depth, mudflat vs. vegetation zone) and temporal (fortnightly and semidiurnal) scales. Microplastics were more abundant in surface (0-2 cm) sediments during neap versus spring tide cycles, likely indicating increased deposition during periods with calm waters and increased suspension when water was more turbulent, but did not vary at greater depths in the sediment. Individual microplastics particles were also larger during neap versus spring tide periods. In contrast to the variation between spring and neap tide periods, we found no variation in the abundance of microplastics on the semidiurnal scale. Microplastics were also more abundant in the transect in the vegetation than at slightly lower elevations in the adjacent mudflat. Across all samples, the abundance of microplastics was negatively correlated with the strength of hydrological processes such as submergence time and flow velocity. Our results showed that sampling of microplastics in the intertidal environment needs to consider variation among spring and neap tide cycles, and also among different intertidal habitats that may differ only slightly in elevation. We encourage coupling sampling with direct measures of hydrological processes so that variation in microplastics abundance and size can be rigorously linked to hydrological processes.