Towards continuous mass and size distributions for beach plastic litter: Spatiotemporal analyses of abundance and composition.

Beaches are known as hotspots for the accumulation of plastic debris and are widely used for monitoring marine litter on a global scale. However, there is a significant knowledge gap regarding temporal trends in marine plastic pollution. Moreover, existing studies on beach plastics and popular monitoring protocols only provide count data. Consequently, it is not possible to monitor marine litter based on weights, which hampers the further application of beach plastic data. To address these gaps, we conducted an analysis of spatial and temporal trends in plastic abundance and composition using OSPAR beach litter monitoring data from 2001 to 2020. We established size and weight ranges for 75 (macro-)plastic categories to estimate the total plastic weight, enabling us to investigate plastic compositions. While the amount of plastic litter exhibits significant spatial variation, most individual beaches displayed notable temporal trends. The spatial variation in composition is primarily attributed to differences in total plastic abundance. We describe the compositions of beach plastics using generic probability density functions (PDFs) for item size and weight. Our trend analysis, method for estimating plastic weight from count data, and PDFs for beached plastic debris represent novel contributions to the field of plastic pollution science.

Restoration of degraded grasslands, but not invasion by Prosopis juliflora, avoids trade-offs between climate change mitigation and other ecosystem services.

Grassland degradation and the concomitant loss of soil organic carbon is widespread in tropical arid and semi-arid regions of the world. Afforestation of degraded grassland, sometimes by using invasive alien trees, has been put forward as a legitimate climate change mitigation strategy. However, even in cases where tree encroachment of degraded grasslands leads to increased soil organic carbon, it may come at a high cost since the restoration of grassland-characteristic biodiversity and ecosystem services will be blocked. We assessed how invasion by Prosopis juliflora and restoration of degraded grasslands in a semi-arid region in Baringo, Kenya affected soil organic carbon, biodiversity and fodder availability. Thirty years of grassland restoration replenished soil organic carbon to 1 m depth at a rate of 1.4% per year and restored herbaceous biomass to levels of pristine grasslands, while plant biodiversity remained low. Invasion of degraded grasslands by P. juliflora increased soil organic carbon primarily in the upper 30 cm and suppressed herbaceous vegetation. We argue that, in contrast to encroachment by invasive alien trees, restoration of grasslands in tropical semi-arid regions can both serve as a measure for climate change mitigation and help restore key ecosystem services important for pastoralists and agro-pastoralist communities.

Establishment of wildflower fields in poor quality landscapes enhances micro-parasite prevalence in wild bumble bees.

The current worldwide pollinator decline is caused by the interplay of different drivers. Several strategies have been undertaken to counteract or halt this decline, one of which is the implementation of wildflower fields. These supplementary flowers provide extra food resources and have proven their success in increasing pollinator biodiversity and abundance. Yet such landscape alterations could also alter the host-pathogen dynamics of pollinators, which could affect the populations. In this study, we investigated the influence of sown wildflower fields on the prevalence of micro-parasites and viruses in the wild bumble bee Bombus pascuorum, one of the most abundant bumble bee species in Europe and the Netherlands. We found that the effect of sown wildflower fields on micro-parasite prevalence is affected by the composition of the surrounding landscape and the size of the flower field. The prevalence of micro-parasites increases with increasing size of sown wildflower fields in landscapes with few semi-natural landscape elements. This effect was not observed in landscapes with a high amount of semi-natural landscape elements. We elaborate on two mechanisms which can support these findings: (1) "transmission hot spots" within the altered flower-networks, which could negatively impact hosts experiencing an increased exposure; (2) improved tolerance of the hosts, withstanding higher parasite populations.

The effect of particle properties on the depth profile of buoyant plastics in the ocean.

Most studies on buoyant microplastics in the marine environment rely on sea surface sampling. Consequently, microplastic amounts can be underestimated, as turbulence leads to vertical mixing. Models that correct for vertical mixing are based on limited data. In this study we report measurements of the depth profile of buoyant microplastics in the North Atlantic subtropical gyre, from 0 to 5 m depth. Microplastics were separated into size classes (0.5-1.5 and 1.5-5.0 mm) and types ('fragments' and 'lines'), and associated with a sea state. Microplastic concentrations decreased exponentially with depth, with both sea state and particle properties affecting the steepness of the decrease. Concentrations approached zero within 5 m depth, indicating that most buoyant microplastics are present on or near the surface. Plastic rise velocities were also measured, and were found to differ significantly for different sizes and shapes. Our results suggest that (1) surface samplers such as manta trawls underestimate total buoyant microplastic amounts by a factor of 1.04-30.0 and (2) estimations of depth-integrated buoyant plastic concentrations should be done across different particle sizes and types. Our findings can assist with improving buoyant ocean plastic vertical mixing models, mass balance exercises, impact assessments and mitigation strategies.