title: Effects of microfiber and light pollution on coastal ecosystem services provided by the blue mussel, Mytilus edulis
Creators: name: Christoforou, Eleni
Publisher: name: University of Glasgow
Year: 2022

description: Filter-feeding bivalves, such as the blue mussel, Mytilus edulis, provide important ecosystem services within coastal ecosystems. Amongst them is the control of phytoplankton abundance and species composition while also contributing to nutrient cycling, which can diminish the impacts of coastal eutrophication and Harmful Algal Blooms (HAB), two major stressors of coastal environments. However, the healthy functioning of mussels and the provisioning of related ecosystem services can in turn be compromised by coastal anthropogenic pressures. This thesis focuses on the effects of two prevailing forms of coastal pollution: microfibers and artificial light at night (ALAN) on coastal ecosystem services provided by mussels. Microplastics (&lt;5mm) are found at coastal ecosystems around the world and their impact on marine organisms has been investigated by multiple studies therefore an initial review of literature on the effects of microplastic pollution on coastal bivalves was conducted. The review aimed to assess whether the experimental settings employed in laboratory studies are relevant to field observations on microplastics characteristics and concentrations. This investigation revealed that previous studies have used a wide range of shapes, materials, sizes, and concentrations of microplastics; and in many cases, these did not coincide with the characteristics of microplastics found in coastal waters. For instance, the concentrations of microplastics used were frequently orders of magnitude higher than environmental levels. Moreover, 48.5% of studies exposed bivalves to spherical microplastics, whereas in the field, fibres were the prevailing shape. Despite the prevalence of microfibers in the coastal marine environment, the review revealed that research on the effects of microfibers on bivalves is scarce. To fill this gap in literature and enhance our understanding on the impacts of microfibers on the phytoplankton consumption by bivalves, a short-term experiment was performed simulating acute microfiber concentrations and microalgae bloom conditions. The results showed that microfiber exposure did not cause any immediate effect on the phytoplankton clearance capacity of mussels. However, a 10.5% decrease was observed in the mussel clearance capacity of microfiber-exposed mussels after five days of microfiber-free conditions, suggesting that even short-term exposure to microfibers can result in long-term interference of the removal of phytoplankton from the water column. Subsequently, it was imperative to investigate the effects of chronic exposure to microfibers. Hence, a long-term (52days) laboratory experiment followed, where mussels exposed to microfibers (&lt;100 μm) showed a significantly less phytoplankton clearance capacity (-21%) in comparison to mussels in the microfiber free treatment, after 39 days of exposure. This could be attributed to the accumulated microfibers in the digestive gland of the experimental mussels, although the exact mechanism remains to be clarified by further research. Furthermore, at the end of the experiment it was evident that the mussels with the highest filtration accumulated the most microfibers, suggesting that prolonged exposure to microfibers could negatively affect the phytoplankton removal capacity of the mussels with the highest clearance capacity. This can consequently result in a decrease in the ecosystem services provided by mussel populations. Artificial light at night (ALAN) is another prevalent form of pollution which affects more than 22% of the world’s coastlines. This anthropogenic stressor can have negative impacts scaling up from individual to community level as most organisms have adapted their biology to the natural daily, tidal, and seasonal light cycles. Currently, research and conservation efforts in terrestrial ecosystems are focusing on identifying ALAN wavelengths that cause the least disturbance; but similar research on coastal ecosystems and specifically bivalves is still scarce. To test the effect of different ALAN wavelengths on mussels, a controlled laboratory experiment was performed exposing different mussels to green, red, and white LED ALAN wavelengths and a control dark treatment. The results reveal that both activity and clearance capacity, as well as the relationship between them, depended on the wavelength of ALAN. Specifically, mussels exposed to green ALAN had the greatest open/close frequency and lower phytoplankton consumption in comparison to mussels under the red light, however there was no significant difference to the control treatment. The phytoplankton clearance capacity of mussels was also dependent on the season the experimental organisms were collected in, which coincides with the reproductive cycle of the mussels. This suggests a seasonal variation in the phytoplankton removed from the water column and therefore the ecosystem services provided by mussels. These studies help us understand the severity of the threat which microfibers and light pollution pose on marine bivalves and the ecosystem services they provide. A major step to alleviate the effects of microfibers is the development of appropriate filtration systems within waste treatments plants, capable of retaining microfibers and preventing their introduction in coastal waters. It is also essential to implement appropriate legislation regarding the production and disposal of plastic materials. Furthermore, these results can act as a basis for future experiment on the effect of ALAN wavelengths on bivalves as they suggest that green ALAN may cause more disturbance to mussel population that red ALAN. These effects should also be considered in a broader ecological perspective, even though red ALAN did not impose a negative effect on mussels, when phytoplankton was exposed to red ALAN there was an increase in its abundance and change in community composition which could also lead to harmful algae events.