Amorphous polymers have larger surface area that allows a higher sorption of POPs

Recent studies point out that the distribution coefficient of organic pollutants, such as PCBs, on MPs increases with hydrophobicity . In our study, MPs had high levels of dioxins and PCBs while barely detectable levels of pesticides. Dioxins and PCBs have logKow values ranging from 7 to 8 , while pesticides tend to be less hydrophobic. For example, aldrin and dieldrin have logKow values ranging from 5.68 to 7.4, and 4.32 to 6.2, respectively . Thus, hydrophobicity could partly explain the sorption patterns of POPs on MPs observed in this study. PET and PVC MPs placed for 3 months close to salmon farms showed significantly higher levels of POPs than HDPE MPs. Sorption of POPs to MPs depends, in addition to the properties of the POPs mentioned previously, on the properties of the MPs and the water or other matrix surrounding the MPs. Such important properties ar size, crystallinity degree, polarity, colour, occurrence of specific functional groups and surface area of the MPs as well as pH, salinity, temperature of the water and biofilm formation around the MPs . MPs used in this study were non-coloured. PE, PP and PET MPs were similar in size and shape, while uPVC MPs were smaller . Nevertheless, the levels of POPs sorbed to uPVC MPs were similar to the levels found on PET MPs, suggesting that the MP size was not the main factor influencing the amount of POPs sorbed to the different polymers. In addition, PET and PVC in presence of water tend to acquire positive and negative charges, respectively , suggesting that polarity might not play a significant role on the sorption of POPs in this study. The crystallinity of MPs, by contrast, varied among polymers. Crystallinity of polymers is an important factor that affects the sorption of POPs. Crystalline polymers have a well-ordered and firm structure that does not favor sorption of chemicals.

HDPE is characterized by a relatively high crystallinity , while PET and PP are considered semi-crystalline polymers,ebb and flow tray and PVC has amorphous structure. Thus, differences in the sorption of POPs to the four polymers studied may be explained by the degree of crystallinity. However, the level of crystallinity of a polymer can vary considerably as a result of the production process , which could explain differences observed between our study and earlier reports. A study carried out in California, USA, found that HDPE, LDPE and PP MPs deployed for several months in San Diego Bay had significantly higher levels of PCBs and PAHs than PET and PVC MPs . In another study, PE MPs collected in Japanese coastal areas had higher amount of PCBs adsorbed than PP MPs, although the concentrations of PCBs in single pellets from same locations had a high variability . Differences in water temperature, salinity and biofilm formation could also explain the discrepancies with those studies, which were carried out at lower latitudes. Our study was carried out north of the Arctic Polar Circle during the winter, with low seawater temperatures and when the lack of light reduces biofilm growth . To our knowledge there are no previous studies that report sorption patterns of POPs in MPs in waters above the Arctic Polar Circles or in Arctic waters. Thus, comparison with previous studies is not straightforward. Furthermore, studies focused on the mechanisms of sorption of pollutants on MPs have primarily focused on laboratoryscale experiments, which can only heed limited known factors that might influence sorption behaviour. Such cannot sufficiently explain all the mechanisms by which MPs sorb organic pollutants under complex environmental conditions. Several types of pollutants that can exert synergistic or antagonistic effects on one another exist in the natural environment, and the interactions between MPs and pollutants become very complex with the constant changes in environmental conditions.

The above highlights the need to further investigate interactions of chemicals and MPs in diverse regions of the planet to better understand the impact of MPs in the environment. This study focused on the sorption of dioxins and PCBs to MPs and this is, to our knowledge, the first report to show that MPs can bind relatively high levels of dioxins close to salmon farms. The group of POPs evaluated in this work might therefore be a relevant factor for the differences observed between this study and other reports. Previous studies have mainly analysed the sorption of PCBs, brominated flame retardants, pesticides and PAHs on MPs . Very few reports are available on the levels of dioxins bound to MP polymers in the sea. To our knowledge, only one study has reported levels of PCDD/ PCDFs on MPs and such pollutants were only detected in charred MPs collected at the coast of the Maldives . Different types of pollutants can have different affinities to polymers, as shown, inter alia, in our study. For instance, PAHs and chlorinated benzenes were reported to sorb stronger to PE than PP, while PP had higher sorption capacity than PE for hexachlorocyclohexanes . Thus, pollutants with higher affinities to polymers may out compete other pollutants. For example, in a mixture of DDT and phenanthrene, it was observed that the first chemical out competed the latter in terms of MP adsorption . This process could potentially explain the non-correlation observed between the composition of POPs found in the mussels and the MPs. Bio accumulated levels of dioxins in mussels placed next to the MPs for three months in the sea were different to those sorbed to the MPs. Mussels are sentinel species often used to bio-monitor aquatic pollution since they are regarded to generally accumulate pollutants present in the water and have low bio-transformation capacity . Thus, pollutants found in their tissues tend to reflect those found in the surrounding environment.

One possible explanation for the different levels of POPs in MPs and mussels in waters with a cocktail of pollutants could therefore be the competitive binding of pollutants to plastic. In terms of fish farming, this study suggests that PET and PVC MPs could have a higher environmental impact than HDPE MPs. PET and PVC are high-density polymers . Since their densities are higher than seawater , these MP polymers tend to sink and accumulate in benthic sediments , unless they are very small, in the nanoplastic scale, where floatability might vary . Benthic areas beneath fish farms are usually enriched with organic waste, resulting from fish faeces and non-eaten feed pellets, and associated pollutants, which are likely fed on by wild benthic feeding biota. Sediment beneath fish farms could, therefore, be potential sources of polluted MPs to wild organisms. However, the impact of MPs derived pollutants on resident organisms is probably insignificant compared to the exposure to the same pollutants through other pathways , although polluted MPs could represent a environmental threat if carried to non-polluted areas by, for instance, ocean currents. PP MPs also sorbed a significant amount of POPs associated to fish farming. PP has a lower density than seawater and PP MPs will therefore remain for longer periods in the pelagic zone . Thus, PP MPs could have higher capacity to transport POPs from aquaculture facilities to the surrounding areas than other MP polymer types . Furthermore, PP is used in fish farming materials such as mooring ropes, which eventually release MPs as a result of wear and tear . Thus, the impact of this polymer in the environment and in relation to aquaculture could be more important than previously expected. Based on this study, HDPE MPs might play a less important role as vector of POPs from aquaculture facilities to the environment. However, because feeding pipes in salmon farms are a known source of HDPE MPs to the environment , and because the vast majority of materials used in fish farming are made of HDPE, this MP type might still play a significant role in spreading pollutants from fish farms.

Moreover, weathering of plastic and changes in the degree of crystallinity of polymers in the environment could modify the sorption patterns observed in this study . Considering all the above, the role of MPs as potential vectors of pollutants from aquaculture facilities should be studied more in depth and considered in future assessments of the environmental impact of fish farms with open-nets. The role of MPs as vector of such pollutants to organisms or other environments is still a controversial matter . Some studies have shown that pollutants sorbed to MPs can be transferred to organisms under specific conditions. For instance, Murray River rainbow fish exposed to MPs spiked with PBDEs bio-accumulated greater amount of such pollutants compared to individuals exposed to virgin MPs . Other studies have reported that exposure of organisms to pollutants sorbed to MPs is insignificant compared to exposure of pollutants through other pathways, 4×8 flood tray such as diet or environmental exposure . Furthermore, it has been suggested that MPs might reduce bio-availability of such compounds in polluted environments by sorbing pollutants in the water . The aim of this study was to evaluate the potential of MPs to sorb POPs associated with fish farming and, consequently, to act as a possible vector of such pollutants. Our results show that the composition of POPs sorbed to MPs placed for three months next to two fish farms were similar to that of MPs incubated with fish feed for three days, but were significantly different to MPs placed for three months in a harbour and the reference station. This suggests that MPs found in the surroundings of salmon farms can sorb POPs present in the fish feed. However, the ability of MPs to transfer POPs from fish farms to organisms will depend on several factors that were not addressed in this study. Pollution levels in the surrounding environments, ocean current dynamics in the area, species affected or even size and shape of MPs are some important factors to assess when studying the transfer of pollutants from MPs to organisms, which requires further investigation. Therefore, the role of MPs in transfering POPs from salmon farming remains uncertain based solely on our results.

Modern day agriculture in Europe has evolved towards a highly industrial sector by intensification and farm scale enlargements in order to contribute to global food production . The produced commodities compete on world markets resulting in low consumer prices, but also forcing farmers to continuously decrease costs and increase yields through technological innovations and management intensification to maintain their competitiveness . Although food production has considerably increased, it has also led to many adverse impacts on the environment and biodiversity . As a response and triggered by societal pressure, a wide spectrum of sustainable forms of agriculture has been developed over time . These sustainable production systems depend less on external and synthetic inputs and may result in reduced environmental degradation and biodiversity conservation. In many instances, forms of sustainable agriculture start as grassroot movements initiated by social interests . Today, many types exist but are relatively immature to study a long-term sustainability transition . Organic farming emerged in Europe in the early 20th century largely independently by private activities . From 1991 it has been ‘institutionalized’ by the establishment of a European wide organic regulation, the EC Regulation 2092/91 . This replaced most national policies which were established in the 1980s . The regulation of 1991 was repealed, and the current organic legislation falls under council regulation EC NO 834/2007 . For the period from 2014 to 2020 the CAP provided funding for organic farming through the European Agricultural Fund for Rural Development . Each EU country implements their own Rural Development Programme specifically tailored to their own challenges and capabilities . Currently, the European Commission has set out an ambitious action plan for the further development of organic production by member states towards 25% of organic agricultural area by 2030 . Due to the relatively long history, the long term sustainability transition of organic farming can be well studied. Interestingly, despite the more than 30 years of EU legislation and a common internal market, organic farming in EU member states has developed at different rates .