CHARACTERISTICS OF MICROPLASTICS IN DIFFERENT CULTURE STAGES OF TYPICAL CRAB PONDS

Microplastics in freshwater has received wide concern recently. Microplastics in natural waters have been the subject of extensive research. However, research on microplastics in the aquaculture water is still limited. China has the largest aquaculture industry in the world. Large area of aquaculture ponds not only provide abundant aquatic products but are also considered to be an important source of pollutants. Ropes, nets, fences, boats, and packaging, among other plastic goods, are widely used in the fishery and aquaculture industries, and they are a potential source of microplastics in the aquaculture environment. Microplastics tend to be mistaken by organisms and possibly cause damage. Research has shown that the abundances of microplastics of several aquaculture areas in China are relatively high. From the previous research, crab ponds tended to have relatively high abundance of microplastics. The goal of this study is to learn about the features of microplastics in crab ponds during various culture periods in order to gain a better understanding of how they vary throughout the culture process. In this study, the abundance and characteristics of microplastics in the water of crab ponds at different culture stages were studied. The samples were collected from 5 crab ponds around Honghu Lake in May, July, and October 2020. Surface water was collected using a stainless-steel bucket and filtered with a 500-mesh stainless steel mesh. The microplastics were filtered on a membrane for analysis. The suspected plastic particles on the membrane were first observed in a stereo microscope to record their number, shape, size and color. Several particles were randomly selected to be tested by a Raman spectrometer to identify their polymer types. The results showed that the abundance of microplastics in crab ponds is at the level of 200—1640 items/m³, which is at a medium level compared with other water bodies. The abundance of microplastics of May, July and October were ranged from 200—880 items/m³ (average 532 items/m³), 280—880 items/m³ (average 528 items/m³), and 280—1640 items/m³ (average 976 items/m³), respectively. No significant difference in the abundance of microplastics in the three samples from different month was shown by the results of the one-way analysis of variance (P=0.175), but according to the average and maximum values, the abundance of microplastics was relatively low during the early and middle stage of cultivation but high at the end of cultivation. The water diversion and drainage processes, the use and aging of plastic products, and the changes in the water environment during the culture process of crab might all have impacts on the change in the abundance of microplastics in the crab ponds. The culture process not only affected the abundance of microplastics, but also other features of microplastics in crab ponds. The shape, size, color and polymer type of microplastics all changed in different cultivation periods. The particle size composition of the microplastics with 0.5 mm as the boundary was similar and evenly distributed in different month. The shape of microplastics were classified as fiber, sheep and fragment with fibrous microplastics being most abundant. Fibrous microplastics accounted for a relatively low proportion in the early stage of cultivation, and a relatively high proportion in the middle and end stages. The proportion of transparent microplastics continued to increase while the proportion of colored microplastics dropped significantly during the process of aquaculture. Seven different types of polymers including polypropylene (PP), polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polyamide (nylon, PA) and polymethyl methacrylate (acrylic, PMMA) were detected in the samples with PP, PET, and PE being most abundant. The polymer types were more various in the end of cultivation compared with the early and middle stage. The higher abundance of microplastics at the end of aquaculture suggested that attention might need to be paid to the impact of aquaculture tail water discharge on the occurrence of microplastics in natural water bodies. At the same time, the influence of the characteristics of microplastics in different culture periods on the ingestion of microplastics and ecological risks of cultured organisms also needs to be further explored in follow-up studies.