At the moment the ecosystem suffers under an extremely complex matrix of stressors which are the driving role in qualitatively and quantitatively changing the freshwater ecosystems and biota. Their capacity to offer sustainable products and services to human society is significant, one of the main categories of these identified stressors being water pollution in general 1,2.
Among the high diversity of pollutants, microplastics are one of the 21st century’s greatest environmental problems since they are so ubiquitous. In addition, once they are dispersed into the environment, they are very difficult to remove, making their way into drinking water and aquatic food sources, harming the environment as well as human health. It is of great significance to identify organisms that have a high level of biological capacity to ingest, bio-accumulate, and transfer microplastics through trophic networks in detrimental ways. Plastic production has reached record levels, with more than 350 million tons produced between 2010-2020 3. The exponential growth of the production of plastics began in the 1960s 4 and their use in all sectors of the economy and improper disposal has led to their spread, making their presence ubiquitous 5. This situation coincides with consumer demand among an ever-growing world population, which is exacerbated by a lack of suitable waste management 6. As a result, plastic pollution is now a common feature of aquatic ecosystems even in Earth’s remote areas 7.
Every year, about 4% of the plastic waste generated worldwide ends up in the ocean 8,9 What exactly happens with the substantial missing fraction of plastics, especially microplastics, on their way from continental hydrographic contexts to the ocean is relatively poorly understood.
Microplastics are defined as plastic fragments with dimensions of less than 5 mm 10 and generally result from the degradation of plastic waste. Microplastics can be categorised according to their source of origin: primary microplastics used, for example, in personal care products, and secondary microplastics resulting from the degradation of larger pieces 11. The negative effects of microplastics on humans and other organisms are generated by their structure and size, producing wounds to the digestive and respiratory systems by ingestion or inhalation due to their small size. They can also be transported to other organs through the circulatory system. Also added to such physical damage is the chemical impact of substances that are adsorbed on the surface of microplastics and that are released due to their increased lipophilicity in the body 11-14.
The significance of alluvial systems as transport vectors for terrestrial plastic debris has been neglected for a long time. It is now known, however, that flowing water often tends to have high concentrations of a variety of plastics with differing chemical compositions, physical properties and size. Above all, microplastic fragments smaller than 5 mm in diameter pose a complex risk to lotic ecosystems all over the world. Amongst other effects, these particles are ingested by various aquatic organisms, leak endocrine-disruptive compounds, and act as vectors for waterborne contaminants, pathogens, and alien species 15. The impacts of microplastics originating within terrestrial ecosystems and washing into aquatic systems is now a major environmental concern 16,17. This fact underscores important questions concerning the roles of a wide variety of abiotic and biotic components of lotic systems in terms of the transport, retention, sharing, and deviation of microplastics. Exactly how important biologically mediated plastic transport might be is in general unknown and it is only a presumption in the literature that biological processes are responsible for the uptake and exporting of a significant portion 18.
In comparison with all over the world marine fish species 17,19,20, freshwater fish species have been much less studied in terms of microplastic contamination. For example in Romania, which detains over 29% of the Danube Basin surface 2, only a single study has been done examining the concentrations of microplastics and this only in sediments from the Danube River to the Black Sea 22.
Elsewhere in Europe, numerous studies have detected high concentrations of microplastics within freshwater aquatic sediment systems: 100-629 particles/kg from the Antuã River, Portugal 23, 250-300 particles/kg at Urban Lake in the United Kingdom 24, 4000 particles/kg in the Rhine-Main River, Germany 25, and 24-620 particles/kg in the Danube River, Romania 22.There are also several key studies of microplastics in freshwater fish: in France 26, Switzerland 27,28,the UK 29,30, Belgium 31,Germany 32, and Poland 33.
The Danube River basin is shared by 19 countries including 79 million people of different cultures and socio-economic systems, which makes it the world’s most international river 34,as well as a very complex area to be properly managed sustainably. In the Lower Danube Basin, traces of hominin activities date to at least the Middle Pleistocene and, over the millennia, the negative effects of human activities have become increasingly acute and complex in the region, especially in terms of the ecological status of fisheries 35-37.
The Danube Basin is well-known historically as an important area for the richness and diversity of its fisheries 38,39. This includes its all Danubian countries sub-basins’ habitats and ecosystems, which are characterised by a high level of overall biodiversity including fish 40-58.
The Transylvanian Depression, to which the Mureş River basin mainly belongs, is a well individualized geographical and ecological area bordered by the Carpathian Mountains. Over seven million people occupy this huge geographical depression, significantly transforming it through their activities. This region includes numerous Danube tributaries of different order, fish habitats, trophic webs, species, and communities all of which are negatively affected under these circumstances 59-69.
Fish are an ideal taxon for environmental assessment and monitoring, including for the presence of plastics 26,30,70-76. Fish are widely consumed by humans as a prevalent source of protein worldwide 77. In addition, fish are a well-known intermediate trophic link between other ecosystem components and are one of our food sources 78-81. Different fish organ tissues were selected in this study as logical places to search for the presence of microplastics.
The fish species selected for this research was the common nase, a protected freshwater, benthopelagic, potamodromous species 39 listed in Appendix III of the Bern Convention. Its flesh is tasty 82 and the nase is commonly consumed by humans in the study area. Its sharp, low, slit-like mouth is a perfect tool for scraping away the algal growth on substrata, also ingesting detritus, debris, and diatoms—as well as other small particles such as those coming from anthropogenic sources 39,83. This scraping “vacuum cleaner” mouth morphology makes this fish species for the study of microplastic particles present in lotic environments.
The aim of the study is to reveal the natural characteristics of the nase that make it a biological uptake vector for microplastics in lotic ecosystems.