As previously shown, the results attested to the presence of microplastic in all trophic levels of the investigated food chain. The results show that partly there is a correlation in size or weight of the species and amount of fibres found in the investigated species. This
supports the statement made in the hypothesis that the consumption of an individual higher trophic level fish is more harmful than an individual lower level species due to greater microplastic presence. Moreover the presence of a greater number of fibres found in higher trophic levels is arguably evidence for the bioaccumulation of microplastic in the food chain of the yellowmouth barracuda, and thus marine ecosystems. The bioaccumulation of microplastic in this food chain in which ultimately humans are the final consumer indicates that microplastic fibres do indeed pose a very serious threat to human health, particularly in maritime areas of heavily pescetarian diet.
5.1 Sample Period and Location
The samples were all taken around Samos Island and the fish are highly likely to travel along the whole coastline even though some samples were taken in the north and some in the south of the Island. The samples were taken over one and a half months, with the exception of zooplankton samples which were taken latest due to the weather conditions;
strong wind, currents or waves. This might have influenced the amount of fibres found because the weather conditions changed significantly from strong winds and 20°C to a moderate climate. Due to this change in conditions the water temperature increased which might have had an influence on the activity of the fish and with that the possibility of ingesting microplastic fibres was higher.
5.2 Correlation Between the Total Fibres and Length/ Weight
The occurring differences in the standard deviations could be caused by different day of catch, location of catch, sex, low difference between size and weight of the individuals.
The B. boops show a small standard deviation which could be a result of them being caught at the same day (see Appendix 4 “B. boops Information Samples”). These individuals were probably part of a fish school with similar eating and traveling behaviour. All those factors result in similar size/ weight and presence of fibres or other contaminants.
T. mediterraneus and S. viridensis standard deviation are much bigger which could be a result of the specimen being caught at different locations and days. This results in different of lifestyle. Though some fish were caught at the same day and showed similar characteristics (see Appendix 6 and 8).
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The findings show that there were more fibres present in the tested yellowmouth barracuda than in horse mackerel, bogue and surface water. This correlates with other research, which states that heavy metals, mercury, and pesticides accumulate within the food chain and are more highly concentrated in high trophic levels than in lower ones (Cresson, et al., 2014). It means that the highest trophic level of this investigated food chain have the most fibres compared lower trophic levels as the B. boops and T. mediterraneus. This correlates with the behaviour of other pollutants which accumulate in a food chain. Considering the average length, weight and amount of fibres also supports this test result. The smallest and lightest fish, B. boops, has the lowest amount of fibres and the tallest or heaviest fish, S. viridensis, has the highest amount.
On the other hand, the results show that the number of fibre per weight or length is higher in smaller or lighter specimens. This correlates with other studies which argue that herbivore accumulate higher concentrations of metals than carnivore species due to their feeding habit (El-Moselhy, Othman, El-Azem, & El-Metwally, 2014).
5.3 Quality Control During Research
The control sample shows little evidence of micro fibres with an average of 1,1 ± 1,2 which indicates almost no contamination from outside (see Appendix Control Sample). This insures the accuracy of properly conducted work. Contamination control measures were:
filtering the distilled water due to its storage in plastic containers; cleaning the needed equipment first with soap or alcohol and afterwards three times with filtered distilled water.
The occurring contamination could be a result of the atmospheric or the tank pollution. These sources of contamination were checked on weekly basis (see Appendix Quality Control).
Further contamination with fibres can result from the salt used for the salt solution which was packaged in plastic, or stirring due plastic coating of the stirring magnet which could erode (see appendix saltwater contamination). To counteract these interferences, every saltwater solution was filtered twice before for the analysis of microplastic fibres present in the fish stomach. Contamination with fibres might have occurred due to filtering the distilled water with contaminated filter paper though the rate of contamination decreased (see Appendix Quality Control). Due to the contamination of the new filter paper with which the distilled water from plastic containers was filtered, some contamination might have occurred. Though, the contamination of filter paper became less over the period of research see Appendix Quality Control, Filter Paper Contamination. Apart from the Surface water filter papers, but the threat of contamination wasn’t as high as from the others (see Graphs in the Appendix Quality Control, Filter Paper Contamination).
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5.4 Identification Problems of Microplastic Fibres
The report “A comparison of microscopic and spectroscopic identification methods for analysis of microplastic in environmental samples” by Young Kyoung Song et al., pointed out the discrepancies in the identification of microplastic fibres and other fibres (see figure below).
Figure 22 Pictures of natural fibres: (a) non-plastic (organic), (b) non-plastic (cotton) and (c) non plastic (rayon) and synthetic fibres: (d) and (e) impact polypropylene (Song, et al., 2015)
Therefore, it can only be assumed that the discovered fibres are from microplastic origin though it cannot be certain but is highly possible.
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5.5 Health Hazard For Human Food Resources
It is mentioned before, in the theoretical framework chapter, that fibres pose threats to organisms that consume them as they can cause blockage in the digestive tract, become translocated to different tissues within the organism, and undergo accumulation (Mathalon &
Hill, 2014). In reference to this regardless of being of plastic origin, fibres are a threat to the organisms that consume them. There is a health risk in fish from which the organs are not taken out and a possible health risk for other fish from which the organs are taken out due to the probability of fibres transferring into the tissue the surrounding tissue (Mathalon & Hill, 2014).
Assuming that the occurring fibres are from microplastic origin, there is also a threat by pollutants. Toxicants leaching out of the plastic debris might be introduced to organisms which were incorporated as additives while manufacture, to improve the properties of the plastic. But also due to their hydrophobic properties which leave them susceptible to accumulation of hydrophobic organic contaminant which could dissociate after ingestion (Cole, et al., 2013). This higher bioavailability of contaminants in the low trophic levels has an impact on the whole ecosystem. The coastal structure could be interrupted substantially and with that produce a trophic cascade and extinct species (Bacelar et al., 2008).
Furthermore, it is already proven that microplastic ingestion takes place in zooplankton species which act as the primary consumer. They are capable to ingest small plastic particles and also show clumping in the posterior mid-gut. This in turn has impact on their algal
ingestion which decreased significantly (Cole, et al., 2013). One effect could be
bioaccumulation of fibres in the food chain which makes micro fibres available for the higher trophic levels (Bacelar et al., 2008) which could be the reason for fibres occurring in the fish stomachs of S. viridensis, T. mediterraneus and B. boops. Another effect could be change in feeding behaviour and with that higher risk of infection or starvation and death which leads to a decrease in population.
As already mentioned, sudden regime shift and ecosystem collapses is more likely to occur in stressed ecosystems, due to top-down (e.g. overfishing) versus bottom-up (e.g. increase of nutrition input that causes eutrophication (Bacelar et al., 2008). All this might influence the availability for food resources for humans but also points out the risks for human health by eating fish containing fibres and other pollutants. Additionally, it shows that the health risk is higher when human eat high trophic levels which are fish like the yellowmouth barracuda or horse mackerel. Smaller fish on the other hand show an increased risk due to the higher amount of fibres per weight and length.
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