The biofiltration capacity of bivalve populations is known to alleviate the effects of coastal eutrophication. However, this important ecosystem service could potentially be impaired by the increasing microplastic abundance in near shore environments. It is known that relatively large microplastics (~500µm) impair the filtration capacity of bivalves, however, the effect of smaller microplastics, and specifically microfibers, is not known even though they are more common in many natural systems and similar in size to phytoplankton, the main food source of mussels. Here, we investigated the effects of long-term exposure to microfibers (MFs), which are smaller than 100µm, on the biofiltration capacity of the blue mussel, Mytilus edulis. Our findings show that longterm exposure (here 39 days) to microfibers significantly reduced (21%) the clearance of phytoplankton (Tetraselmis sp). While previous studies have shown that larger microplastics can decrease the filtration capacity of mussels after short-term exposure, our findings suggest that, for smaller MFs, mussel's clearance capacity is significantly affected after long-term exposure (39 days in this study). This may be due to the accumulation of MFs in the digestive system. In addition, the most efficient phytoplankton consumers were more susceptible to MF accumulation in the digestive system. This suggests that prolonged exposure to MF of coastal mussels could 2 negatively impact the biofiltration of more potent individuals, thus decreasing the ecosystem service potential of the population as a whole. CapsuleWe found that long-term exposure to small microfibers can impair the phytoplankton clearance by coastal mussels.

Artificial light at night (ALAN) is increasingly recognised as a disruptive form of environmental pollution, impacting many physiological and behavioural processes that may scale up to population and community-level effects. Mounting evidence from animal studies show that the severity and type of the impact depends on the wavelength and intensity of ALAN. This knowledge has been instrumental for informing policy-making and planning for wildlife-friendly illumination. However, most of this evidence comes from terrestrial habitats, while research testing alternative wavelength illumination in marine environments is lagging behind. In this study we investigated the effect of such alternative ALAN colours on marine primary producers. Specifically, we tested the effect of green, red, and natural white LED illumination at night, compared to a dark control, on the growth of a green microalgae as well as the biomass, diversity and composition of a phytoplankton assemblage. Our findings show that green ALAN boosted chlorophyll production at the exponential growth stage, resulting in higher biomass production in the green algae Tetraselmis suesica. All ALAN wavelengths affected the biomass and diversity of the assemblage with the red and green ALAN having the stronger effects, leading to higher overall abundance and selective dominance of specific diatom species compared to white ALAN and the dark control. Synthesis. Our work indicates that the wavelength of artificial light sources in marine areas should be carefully considered in management and conservation plans. In particular, green and red light should be used with caution in coastal areas, where there might be a need to strike a balance between the strong effects of green and red light on marine primary producers with the benefit they bring to other organisms.

Artificial light at night (ALAN) is a disruptive form of pollution, impacting physiological and behavioural processes that may scale up to population and community levels. Evidence from terrestrial habitats show that the severity and type of impact depend on the wavelength and intensity of ALAN; however, research on marine organisms is still limited. Here, we experimentally investigated the effect of different ALAN colours on marine primary producers. We tested the effect of green (525 nm), red (624 nm) and broad-spectrum white LED ALAN, compared to a dark control, on the green microalgae Tetraselmis suesica and a diatom assemblage. We show that green ALAN boosted chlorophyll production and abundance in T. suesica . All ALAN wavelengths affected assemblage biomass and diversity, with red and green ALAN having the strongest effects, leading to higher overall abundance and selective dominance of specific diatom species, some known to cause harmful algal blooms. Our findings show that green and red ALAN should be used with caution as alternative LED colours in coastal areas, where there might be a need to strike a balance between the effects of green and red light on marine primary producers with the benefit they appear to bring to other organisms.

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