We performed extensive degradation experiments in the laboratory to characterise the ageing of various polymers and the formations of MPs. This work for the first time demonstrated that the immediate step in polymer degradation involves surface erosion, which generates a considerable amount of nanoscale fragments that we coined nanoplastics. We also observed the formation of MPs, however to a lower extend. Based on these results, we performed degradation experiments in outdoor microcosms to cover a more realistic scenario.
To understand their biological impacts, we performed a range of long-term experiments with the key freshwater invertebrate Daphnia magna. In contrast to previous research, we focused on irregular MPs and population-level effects. Our results on three different polymers and one natural particle as reference demonstrate that the impacts of MPs on daphnid populations are subtle and occur at high concentration only. We further investigated the migration of toxic chemicals from virgin MPs and MPs conditioned with wastewater. Using various in vitro bioassays, we confirmed the leaching of estrogen-like chemicals from all investigated polymers.
In the third part of freshwaterMPs, we conducted a large-scale, long-term mesocosm study to investigate the fate and ecological impacts in semi-controlled model ecosystems. We spiked 1 500 L outdoor ponds with three different MP types (virgin and conditioned) and analysed their behaviour in the water column and sediment, their degradation and their effects on zooplankton and macroinvertebrate communities. We show that all MPs, even buoyant ones that, rapidly sink to the sediment and undergo surface degradation. Similar to our laboratory studies, the impact on biota was limited, if detectable at all.
Finally, data from the literature was then used alongside the project data to begin developing a framework for the environmental risk assessment of MPs. We identified two key issues hampering a risk assessment: First, critical data on the occurrence and toxicity of MPs in a freshwater context are still missing. The project contributed towards filling these gaps. Second, MPs are extremely heterogeneous in terms of their physicochemical properties and, thus, cannot be treated as one stressor. In theoretical exercises, we propose and performed two different risk assessment approaches based on chemical composition and particle properties. Results imply that the risk assessment is very sensitive to data gaps and to how the input parameters are defined. Accordingly, we propose strategies to move forward.
