Microplastics in marine waters and their potential risk to marine plankton

The plastic era commenced in 1950s and, since then, production of plastics has been growing exponentially. Part of the plastic waste ends in the oceans and, due to their low degradation, plastic persists and accumulates in the marine ecosystems. Particularly, microplastics (MPs), generally defined as plastic particles between 1 μm and 5 mm, have been found in all marine compartments. The ubiquity and increasing concentration of marine MPs is currently a global environmental concern. Numerous studies have investigated the abundance of MPs > 300μm in surface waters using nets (commonly the Manta net). However, little is still known about the distribution of the smaller size fraction of MPs (< 300 μm), which fall within the size range of phytoplankton and can be ingested by zooplankton and, potentially transferred to higher trophic levels in the marine food web. Additionally, once the plastic enters the aquatic systems, leaching of potentially toxic additives can negatively impacts marine organisms. Plankton play key roles in the functioning of marine ecosystems and therefore knowledge of the interactions between MPs and plankton, and their effects, is essential to evaluate the environmental risk of plastic pollution. While several microcosm studies have demonstrated the adverse effects of small MPs and their leachates on marine organisms, more field data, and environmentally realistic experimental approaches (e.g., mesocosms) are needed to better evaluate the effects of MPs on marine plankton.

In this thesis, the abundance, distribution, and composition of small MPs in surface water (Manuscript I) were investigated at Kattegat-Skagerrak, Denmark. Secondly, the abundance and characteristics of subsurface MPs at different depths were determined and the influence of water stratification on the vertical distribution of MPs in the water column was evaluated (Manuscript II). Also, the concentration of small MPs (>10 μm) in field samples of zooplankton and their fecal pellets (Manuscript III) were assessed. Consequently, the risk of MPs to enter and be transferred into the marine planktonic food webs and the potential influence of fecal pellets on the vertical distribution of MPs (Manuscript III) was assessed. Finally, the effects of long-term exposure to weathered MPs on a planktonic community was analyzed using a mesocosm approach (Manuscript IV). The study hypothesized that (1) MPs are ubiquitous in marine surface waters and the small size fractions are dominant, (2) the vertical distribution of MPs depends on the stratification of the water column, (3) zooplankton ingest few MPs due their selective feeding behavior and consequently few MPs are exported via fecal pellets and (4) MPs affect plankton communities after long term exposure due to leaching of additives.

To address the research questions and test the hypotheses, an extensive field study was conducted in the Kattegat-Skagerrak, Denmark (Manuscript I, II & III) and a land based mesocosm study in the Baltic Sea, Sweden (Manuscript IV). There were different sampling equipment for MPs in the field studies was deployed. A plastic-free pump filter device (Universal filtering object' (‘UFO’)) was used to estimate the concentration of MPs down to 10 μm in surface water at a basin scale (Manuscript I). A pump filter sampler with multi ‘UFO’ units (‘Kraken’) was employed to collect subsurface MPs from different depths (Manuscript II). A multinet and stainless-steel sediment traps were used to collect the zooplankton and fecal pellets respectively (Manuscript III). The collected samples were treated through an enzymatic-oxidative approach and MPs were detected by focal plane array micro-Fourier transform infrared (FPA-μFTIR) spectroscopy. Stringent contamination control and quality assurance measures were implemented throughout the sampling and analysis of MPs. To assess the effects of MPs on the plankton communities, a long-term land based mesocosm experiment was conducted in the Baltic Sea using different concentrations of MPs (<120 μm) from weathered and micronized conventional plastics.

The results from manuscript I revealed that MPs (>10μm) concentrations ranged from 11 to 87 MPs m^-3 in surface Danish marine waters, comparatively lower than in other regions (e.g. Greenland and Canary Islands). Predominant polymers in the samples were polyester, polypropylene, and polyethylene, with approximately 88% of the detected MPs measuring < 300 μm and 56% being fragments. The surface water concentration of MPs tended to be higher in the northern part that opens to the North Sea and the southern parts that connect to the Baltic.

In manuscript II, the MPs concentrations in the samples in different depths ranged from 13 to 156 MPs m^-3, with no significant differences between depths. The dominant polymers across all depths were polyester, polypropylene, polyethylene, and polystyrene, with approximately 94% of MPs being <300 μm. High-density polymers accounted for two-thirds of the total polymers. Interestingly, despite the pronounced water stratification in the study area did not appear to influence MPs distribution throughout the water column. More research in a higher resolution scale is needed for a better assessment of the influence of pycnocline on vertical distribution of plastics.

Manuscript III shows that the concentration of MPs in zooplankton and their fecal pellets was relatively low compared to the concentration in the water. This suggests that the potential risk of MPs transferring to higher trophic levels through zooplankton is low. Although the concentration of MPs in fecal pellets was quantitatively low, it could be ecologically relevant due to the key role of fecal pellets in vertical exportation of particulate carbon in marine ecosystems.

In the Manuscript IV, the results from the mesocosm study on the weathered MPs of conventional plastics indicated minimal to negligible impact on pico, nano, and micoplankton as well as mesozooplankton after long-term exposure at environmentally relevant concentrations.

In conclusion, the present research findings highlight the prevalence of MP > 10μm in Danish marine waters, encompassing both surface and sub-surface waters (Manuscript I & II). The concentration of MPs in the studied area was lower than in other regions and approximately six orders of magnitude lower than MP levels known to cause adverse effects on marine planktonic organisms. Thus, the risk of negative impacts of conventional MPs on the pelagic food web is expected to be minor (Manuscript I & II). The number of MPs found in field-collected zooplankton and fecal pellets was relatively low (Manuscript III). This can be due to the high abundance of natural prey compared to plastic particles in the water column (range of mass ratio phytoplankton to plastic=ca.500 to 100,000:1) and due to the selective feeding behavior of zooplankton. Weathered conventional MPs did not impact the plankton communities after five weeks of exposure, suggesting a minor risk of MPs on plankton at environmentally relevant concentrations of MPs (Manuscript IV). To gain a more comprehensive understanding of the impacts of plastic pollution on plankton communities, further investigations on the abundance and effects of MPs derived from other plastic materials (tire rubbers and bioplastics) are needed, especially those with high levels of functional additives or harmful leachates.