International Journal of Marine Science, 2025, Vol.15, No.5, 245-254 http://www.aquapublisher.com/index.php/ijms 247 fragments in environmental samples. Its potential application in microplastic pollution detection is mainly reflected in two aspects: indirect biological indicators, and changes in gene fragment abundance of specific marine organisms can reflect the existence and degree of microplastic pollution. It is conceived that by detecting changes in mussel DNA content in water samples, the effect of microplastics on mussel communities is indicated. Microbial community analysis shows that the surface of microplastics often attaches special microbial communities (so-called "plastic circles"). Through eDNA sequencing, the composition of bacteria and algae species adhered to the microplastics can be identified, thereby indirectly confirming the existence of microplastics in the environment (Ali et al., 2024). Although microplastics themselves do not contain DNA, they can act as a carrier to carry marine microorganisms, and analyzing these microbial eDNAs helps track the microplastic's environmental migration pathways and residence environments. For example, some attachment-resistant bacteria only reproduce on plastic surfaces. By detecting the DNA of such indicator bacteria, it is possible to quickly screen for plastic debris in seawater or sediments. Environmental DNA technology has the advantages of non-invasiveness and high sensitivity, and can directly detect DNA signals in the laboratory without the need for a large number of filtering samples. 2.3 European sea microplastic monitoring projects Developed countries and international organizations have already carried out a number of large-scale programs and projects in marine microplastic monitoring. In Europe, the "Beihai Microplastic Monitoring Program" jointly developed by scientific research institutions from multiple countries is committed to establishing a systematic microplastic long-term monitoring network in the Beihai region. The plan unifies sampling and analysis methods, and regularly collects samples from seawater, beaches and organisms in the North Sea and adjacent Atlantic Ocean and Baltic Sea to monitor the changes in the abundance and distribution of microplastics over time. Europe has also adopted the Marine Strategic Framework Directive (MSFD) to require member countries to monitor offshore microplastic pollution and has developed detailed technical guidelines. For example, the German coast has set up an environmental sample library project with the support of the government, collecting unified specifications of biological samples such as mussels every year to analyze the changing trends of microplastic load in the body (Halbach et al., 2022). These measures provide a scientific basis for evaluating the effectiveness of governance measures. In the "North Sea" program, researchers found that there are on average about 4 plastic fragments per square meter of sand on the German coast of the North Sea, but there are large differences between locations (Walther et al., 2024). The project report also recommends that methods such as random sampling and vertical profile sampling be used to improve monitoring representativeness. In addition to government-led monitoring, Europe also encourages the public to participate in the "plastic detection" citizen science project and expand the monitoring scope through volunteer sampling. 3 Distribution and Migration Mechanism of Marine Microplastics 3.1 The influence of ocean current, wind and settlement on microplastic migration The spatial distribution of marine microplastics is highly uneven, mainly driven by the joint force of marine dynamic processes and atmospheric processes. Ocean circulation is the main way to transport plastic debris from a long distance: ocean currents can carry microplastics from nearshore sources to the open ocean and gather in the circulation center to form a "garbage belt" (Wichmann and Delandmeter, 2019). Waves and tides can also affect the distribution of microplastics in vertical directions and near shorelines. Storm surges can push some of the floating plastic to the shore or bury them into nearshore deposits. The role of atmospheric wind power cannot be ignored. Some small-sized and low-density microplastics can be entrained into the atmosphere by aerosols generated by sea surface wind and waves, and then settle to the sea surface or land after long-distance transmission. Research points out that microplastics have been transported to remote alpine and polar regions through the atmosphere for a long distance. Sedimentation is the main mechanism of vertical migration of microplastics. Higher density of plastic particles or biologically attached weight-enhancing debris will gradually sink to the underlying seawater or even the seabed. Observations found that the number of microplastics on the surface of the ocean is much lower than the total input amount. It is speculated that a large number of microplastics settled to the seabed through agglomeration, biofooding and excretion, and become plastic reserves
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