PhD conferral Thomas Maes




Lifting the veil on marine litter

Thomas Maes

prof.dr. A.D. Vethaak, copromotor dr. H.A. Leslie

Amsterdam Institute of Molecular and Life Sciences


PhD conferral

The central aim of this thesis was to gain a better understanding of marine litter, including microplastics, in the North-East Atlantic ecosystem. We addressed several knowledge gaps in relation to the standardisation of marine litter and MP monitoring methods, analytical method development, field exposure of MP in both abiotic and biotic matrices, and laboratory exposure and chronic toxicity of MP to marine species.
No significant temporal trend was observed in seafloor litter around the UK for the past 25 years (Chapter 3)
We demonstrated that existing fisheries surveys using trawls can be used to monitor seafloor litter. Macro litter on the seafloor is widespread but patchy within the seas surrounding the UK, ranging from 0 to 1835 pieces km-2 of seafloor and dominated by plastics. Over the entire 25-year period (1992–2017), 63% of the 2461 trawls contained at least one plastic litter item. There was no significant temporal trend in the percentage of trawls containing any or total plastic litter items across the long-term datasets. Statistically significant trends, however, were observed in specific plastic litter categories only. These trends were all positive except for a negative trend in plastic bags in the Greater North Sea - suggesting that behavioural and legislative changes (e.g., plastic bag tax or ban) could reduce the problem of marine litter within decades.
Microplastics accumulate in North Sea sediments with a high organic carbon content (Chapter 4)
Microplastics are present in sediments of the Southern North Sea (0 - 3,146 particles/kg dry weight) and at the sea surface of North West Europe (0 - 1.5 microplastic particles/m3). The highest concentrations of microplastics were found in estuaries and in sediments areas with a high organic carbon content. Sediments act as sinks for microplastics (mainly fibers and spheres), they are less heterogenous and  contain higher concentrations of microplastics compared to surface waters (mainly fragments). Smaller sample sizes and volumes are required for microplastic monitoring in sediments. Sampling for marine sediments is already ongoing, is less prone to error and allows for more precise measurements compared to trawling nets. Standardization of monitoring methods within marine regions is recommended to compare and assess microplastics pollution over time.
North East Atlantic Porbeagle sharks digest microplastics but the health impact is unclear (Chapter 5)
Microplastics are present in high concentrations in top predators living in the North East Atlantic, up to 10.4 particles per g wet weight (w.w.) content and 9.5 particles per g w.w. tissue. This equates to individual microplastics loads as high as 3850 particles per spiral valve. These high concentrations might deliver a first indication of bioaccumulatution. We developed a method for quantifying microplastics in spiral valves of porbeagle sharks. No possible health effects of microplastic contamination were found. There is a potential for microplastic biomonitoring using this species.
Long-term microplastic exposure has adverse health effects on juvenile oysters (Chapter 6)
Juvenile oysters, Crassostrea gigas, exposed for a period of 80 days to 106 particles L-1, represented by 6 µm polystyrene (PS) microbeads, showed an increased death rate compared to a control treatment receiving no microplastics. Weight and shell length remained comparable, but the Condition Index of the oysters in the highest concentration reduced significantly towards the end. The oysters in the highest MP exposure showed the lowest mean Lysosomal Stability score throughout the experiment. Microbeads were detected in the intestines of exposed oysters and in the digestive tubules, but no cellular inflammatory features were observed.
Microplastics in sediments detected using forensic science methods (Chapter 7)
The selective fluorescent staining using Nile Red (NR), followed by density-based extraction and filtration allows for rapid analysis of microplastics in sediments using simple photography through an orange filter at low cost. Image-analysis allows fluorescent particles down to a few micrometres to be identified and counted. The solvatochromic nature of Nile Red also offers the possibility of plastic categorisation based on surface polarity characteristics of identified particles.
Europe underfunds marine litter research on marine litter risks (Chapter 8)
The past decade, the best represented topics within European marine litter projects were ‘Policy, Governance and Management’ and ‘Monitoring’. The underrepresented topics were ‘Risk Assessment’, ‘Fragmentation’ and ‘Assessment Tools’.
From the evidence we gathered, it remains difficult to create a complete understanding of the marine litter issue in the North East Atlantic, much more work is required. What is clear, is that we found marine litter, including microplastic across all investigated sites and samples. In our 25-year study, seafloor litter presence remained constant, although waste types and inputs differed. This suggest that seafloor litter is moving through the marine environment, accumulating where we don’t monitor, or escaping our nets under the form of smaller (micro)plastics. Without strong action to stem plastic production and usage, together with improved waste management, quantities of marine litter and microplastics will increase further, eventually leading to concentrations causing ecosystem impacts and population effects across marine biota. Our oyster and porbeagle shark study indicated that microplastics are taken up and moving through the food chain, affecting marine wildlife and thus potentially also us. Current concentrations of microplastics near point sources are certainly high enough to cause mortality in bivalves as shown in our exposure study. The collected evidence suggests that current properties and quantities of marine litter in the North East Atlantic are expected to cause a significant impact on the ecosystem. If we don’t stop plastic inputs into our oceans, it will only be a matter of time before critical concentrations will be observed more widely. To address this rapidly growing issue, to advice future investments and to steer science needs across Europe, we suggest speeding up the harmonisation of methods, looking into more detail into plastic fragmentation processes and pathways, delivering more risk and life cycle assessments for new and existing products and developing (assessment) tools for each stakeholder. In future, cross collaborative and multi-disciplinary studies and evaluations are needed to improve our understanding and ability to tackle this plastic pollution problem.