Microplastic contamination of Black Sea fish threatens marine ecosystems

Five commercially important fish species from the Bulgarian Black Sea coast were found to be contaminated with microplastics.

A photo of a Knout Goby.
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Guest blog post by Stephany Toschkova, Sevginar Ibryamova, Darina Ch. Bachvarova, Teodora Koynova, Elitca Stanachkova, Radoslav Ivanov, Nikolay Natchev, Tsveteslava Ignatova-Ivanova

One of the main problems of the world’s oceans, reported by many scientific studies, is microplastic pollution. It is also one of the main sources of pollution in the Black Sea. Our new study in BioRisk details microplastic contamination in five fish species important for commercial fishing (Garfish, Мullet, Knout goby, Pontic shad, and Mediterranean horse mackerel). The fish were collected from the Sozopol area of the Bulgarian Black Sea coast.

  • A photo of a Mediterranean horse mackerel.
  • A photo of a Knout Goby.
  • A photo of a mullet fish.

Our results show a wide variety of micropollutants originating from commonly used items such as plastic cups, stirrers, bags, soft drink bottles, fishing nets, packaging, аnd personal hygiene products. These objects systematically enter the Black Sea and degrade into microplastic particles. Microplastics (MPs) were found in all studied tissues of the fish in the form of pellets, fibers and fragments. Pellets were found most frequently, followed by irregularly shaped fragments, while fibers were the least numerous.

Stereomicroscope picture of morphological types of microplastics (arrowheads) recognized in the studied specied from: A) Garfish; B) Mullet C) Pontic shad and D) Mediterranean horse mackerel.

The bulk of the isolated plastics are made of polyethylene (PE) and polyethylene terephthalate (PET). PE is found in plastic bottles, cups, stirrers, and plastic bags. This polymer is very light and floats on the surface of the sea because its density is lower than that of water. PET, on the other hand, is denser than water and more likely to sink and accumulate in it and in organisms living on the seafloor. These polymers are widely used in fabrics, nets, ropes, and strings used for fishing, one of the main economic activities in the Black Sea. The predominant polymer type, PE, corresponds to the content of manufactured plastics all around Europe, as almost half of the plastics produced in Europe are reported as PE.

The sinking and sedimentation of plastics relate to the fact that the upper layer of the Black Sea is less dense than that of other seas. Furthermore, the weight of these particles increases due to the accumulation of marine plants and nutrients on them, and this can affect the distribution of plastics and their sedimentation on the seabed.

A satellite image of the Black Sea. Photo by NASA/GSFC/MODIS

Judging by the obtained results and the amount and type of polymers found in the study and the literature, the source of contamination, in our opinion, can be mainly attributed to domestic wastewater discharges coming from the washing of synthetic fabrics. In Bulgaria, wastewater is discharged – directly or after purification – into marine and freshwater ecosystems, as is the case in other neighbouring countries along the Black Sea coast. However, detailed studies are needed to prove this hypothesis. 

Considering the wide variety of MP types detected in the digestive tracts of the fish, we assume that they regularly ingested MPs during feeding. Many nutrients are also held on the plastic particles, which deceives the fish into perceiving them as food.

It has been reported that plastics smaller than 1000 μm can reach the digestive tract or the gills of fish, and in turn can cause adverse effects such as a weak immune response or reduced fertility.

MPs can also accumulate in predatory fish species. Unfortunately, very limited research was performed on bioaccumulation and biomagnification in food webs, therefore more studies are needed to reach this conclusion.

MPs enter seawater food chains in different pathways and threaten entire ecosystems through their ability to transport pollutants, pathogenic microorganisms, and alien species. Bearing in mind the intensifying economic activity on the Black Sea coast and the consequent influence on the riverine water quality, river mouths can be considered potential sources of MPs. Particularly concerning is the area near the Kamchia River mouth, which is the biggest intra-territorial river in Bulgaria, entering directly into the Black Sea, with a catchment area of more than 5 300 km2 . This catchment and the entire Black Sea coast, where agriculture is well developed is a potential source of MPs, which have the ability to absorb and release toxic chemicals of organic and inorganic origin such as bisphenol A, PCBs and DDT, creating an additional potential risk to human health.

A satellite image showing the Kamchia River mouth.
A satellite image showing the Kamchia River mouth.

Humans are exposed to BPA in the environment they live in, from the air we breathe to the food and drinks we consume. So, even if BPA intake is below accepted limits, this does not guarantee that the additive will not accumulate and cause more pronounced effects and chronic toxicity in the food chain, given its tendency to accumulate.

It is important that future research determines the toxicological side effects of plastic ingestion for fish communities in both benthic and pelagic habitats. However, even if we stop introducing plastics into the water system, both groups of fish will continue to be impacted, since the number of microplastics can increase due to the breakdown of larger plastics in the environment. 

This study shows the need to carry out further studies of microplastics using different types of microscopic and spectral analysis. Even though microplastics may not pose a risk to humans who consume fish, these contaminants pose a potential risk to marine food webs and endangered species. We found particles of different sizes, types and colours in different fish species, and believe the variability of polymer types in fish can indicate the polymer types in water to some extent. Our results show that fish are important as ecological bioindicators and serve as a basis for future studies on microplastic pollution in tourist sandy beaches.

Research article:

Toschkova S, Ibryamova S, Bachvarova DCh, Koynova T, Stanachkova E, Ivanov R, Natchev N, Ignatova-Ivanova T (2024) The assessment of the bioaccumulation of microplastics in key fish species from the Bulgarian aquatory of the Black Sea. BioRisk 22: 17-31. https://doi.org/10.3897/biorisk.22.117668