Freshwater ecosystems under natural and anthropogenic stressors : The impact of ultraviolet radiation and nanoplastics on plankton and the ecosystem

Since life emerged on Earth, organisms have had to handle threats and stressors of different kinds. In freshwater ecosystems, one of these ancient stressors is the solar ultraviolet (UV) radiation. Life has been continuously exposed to it, and various behavioural and morphological adaptations have evolved to cope with the stressor. On the other hand, human activities have introduced novel stressors to the Earth system, for example, different pollutants. Plastic pollution is a novel stressor in the history of Earth, and a complex, global, socio-environmental issue. Nanoplastics, the smallest size fraction (< 1 μm) formed when plastic material breaks down, negatively affect a variety of freshwater organisms. However, a comprehensive understanding of the effects of nanoplastics on natural freshwater ecosystems is still needed.The aim of this thesis is to address how these two different stressors affect freshwater plankton, including effects at other levels of biological organization. To do this, I performed both laboratory and mesocosm scale experiments to assess the effect of UV radiation and nanoplastics, exploring, for example, organism growth, morphology, behaviour, and fitness, but also population abundances, community composition, and ecosystem processes.Solar UV radiation is a temporally variable abiotic factor. It fluctuates during the year, but also daily, and over short time scales with the position of the sun and rapidly occurring variations in cloudiness. Despite its variable nature, most studies on organisms’ responses to UV radiation have assessed the effects of a constant exposure. In Paper I, I experimentally investigated individual survival, reproduction, and behaviour of the zooplankter Daphnia magna when exposed to constant or fluctuating UV radiation. I found that D. magna has the potential to adopt alternative behavioural strategies to deal with either constant exposure or repeatedly fluctuating UV radiation, and that the response to the fluctuating environment implies a fitness cost.Nanoplastics, compared to larger-sized plastics, have different transport pathways, interact differently with organisms, and require the use of specific analytical techniques for their characterization and quantification. Currently, there is a need to assess the transport, fate, uptake, and effects of nanoplastics in natural ecosystems. I investigated all these aspects in freshwater wetland mesocosms and laboratory experiments, using model plastic nanoparticles. The results of these studies showed that nanoplastics can be retained by freshwater wetland mesocosms, being mostly accumulated in sediments of the aquatic compartment, but also in organisms as D. magna, Asellus aquaticus and macrophytes (Paper II). Moreover, nanoplastics negatively affect the abundance of freshwater key organisms, as D. magna, and change the community composition of phytoplankton, favouring cyanobacteria over diatoms (Paper III). Further, these plastic nanoparticles interact differently with two different phytoplankton species (Paper IV) causing differential effects on growth and group formation.Collectively, this thesis analysed different organism responses to both natural and anthropogenic stressors. Despite long-standing adaptations, organisms still face costs associated with responding to natural stressors as UV radiation, and these differential costs might fuel population differentiation and local adaptation. The impacts that humans are causing on natural ecosystems through plastic pollution also press natural populations and communities. Further, these pollutants can cause shifts that might be irreversible and harmful to the whole freshwater ecosystem, and to us humans. New regulations but also major social changes are needed to really change the current trend of increasing plastic pollution.