Greulich, Maria (2020) Effects of plastic and natural microparticles on the performance of the marine filter feeder Mytilus galloprovincialis. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 71 pp.

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Abstract

Mussels perform several key ecological functions in the oceans, such as water filtration, reef-building and the enhancement of the benthic-pelagic coupling. They are exposed to a variety of abiotic microparticles in their natural environment and have developed mechanisms to cope with these indigestible particles. A newly introduced type of microparticle in the oceans are microplastics (plastics < 5 mm) that mainly derive from the fragmentation of larger plastic debris. The production of plastic has increased exponentially over the last decades, and so has the pollution of the environment by plastics. It was shown that microplastics are ubiquitous in the marine environment, with largely unknown effects on marine life. For this reason, laboratory effect studies are required to find out whether microplastics pose a novel risk to marine biota or whether they can be treated as just another component of abiotic seston. In this study, I exposed juvenile farmed mussels (Mytilus galloprovincialis) in a controlled laboratory experiment to both microplastics (recycled PMMA, mean size: 120 μm) and natural inorganic particles (diatom shells, mean size: 86 μm). Both particles were irregularly shaped fragments that were constantly resuspended in the water column. The microparticles were applied at three different concentration levels (1.5 mg/l, 15 mg/l, 150 mg/l), to mimic seston concentrations naturally occuring in the oceans. After 42 days of exposure, the respiration rates (by up to 67 %), the dry weight (by up to 23 %) and the body condition index (by up to 20 %) were lower in mussels exposed to microplastics than in mussels treated with natural particles. The significant differences occured independently of particle concentrations. No effects were found for the clearance rates, the byssus thread formation and the mortality of the mussels. The negative effects of microplastics on the physiology of the mussels are presumably caused by oxidative stress and an altered feeding behaviour due to gill damages, as the mussels were always fed in the presence of microparticles. Nevertheless, M. galloprovincialis showed robustness against high particle loads, which verifies the fact that mussels originate from turbid environments and are well adapted to these conditions. The negative effects of microplastic exposure on the respiration rates of the mussels were only detectable after the long-term exposure of 42 days, which underlines the importance of extended exposure times in laboratory effect studies. Since the effects have always been negative for mussels exposed to microplastics compared to mussels treated with natural particles, I assume that microplastics have specific physical and chemical properties, e. g. surface tension, charge and hydrophobicity, that make them a novel stressor in the marine environment.

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