An integrated image of the effect of multiple stressors (temperature rise & plastic chemical pollution) on the harpacticoid copepod Nitokra spinipes

Since the Industrial Revolution, human activity has reinforced climate change to alarming rates and has introduced new stressors such as plastic pollution to the environment. Currently, 4.8 - 12.7 million tons of plastic litters enter the oceans every year. Globally alternatives to replace synthetic-based plastics are being developed, to find more sustainable solutions for materials and applications. Bio-based plastics, for example, form a major group of the newly introduced alternatives. However, the effect of bio-based plastics relative to synthetic-based plastics is poorly understood. Additionally, the potential impact of bio and synthetic-based plastics and their combined effect with other environmental stressors on aquatic life is rarely investigated. This leaves a knowledge gap in understanding the realistic effects of plastics in the environment. The goal of this study was to assess the combined effects of the leaching additives extracted from two polymers, bio and synthetic based plastics, combined with a temperature increase (+ 2 °C) on aquatic invertebrates. To do so, we used the brackish water copepod Nitokra spinipes, an important and relevant species in toxicity testing. In this study the toxicity of the bio-based plastic Poly Lactic Acid (PLA) and the synthetic-based plastic Polypropylene (PP) were examined. Additionally, a next generation trial test was included in this study to present a baseline for further testing of the possible effect of multiple stressors on the next generation of N. spinipes. In the test with PLA leachates a significant effect was measured of decreasing larval development rates (LDR) at increasing leachate concentrations. In the PP leachates test no significant effect of the applied stressor (PP) on the measured LDR was observed. In the multiple stressor scenario performed with PLA, a significantly lower LDR was observed compared to the treatments in which N. spinipes was exposed to only one stressor. This may indicate that N. spinipes was affected by the thermal stress indicating increasing vulnerability to additional stressors, such as plastic leachates of PLA. The multiple stressor scenario with PP leachate did not result in any observed delay in LDR’s after 7-days exposure. Further testing in the next generation trial experiment was therefore performed with PLA leachates. First results of this experiment indicate that PLA possesses chemicals that impact the spawning success of N. spinipes within the tested period of 2 weeks. No effects from the assessed stressors on the spawning trajectory, as well as the number of ovigerous females was found. Further testing to examine the exact extent of the effect of estrogenic activity (EA) exhibiting chemicals on the life cycle of N. spinipes, as well as research to other factors that may influence reproduction rates is still required. All together this study concludes that PLA, unlike PP, induces toxicity, affecting the survival of N. spinipes and that this effect is enhanced under a multiple stressor scenario.