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.

1. Global change, such as exotic invasions, dramatically affects ecosystem functioning. However, the mechanisms behind the impacts are often unclear and despite extensive experimental work, we know little about the importance of biodiversity loss as a component of global change effects in real-world ecosystems. 2. We disentangled several mechanisms by which an exotic invader affected ecosystem functioning in East African drylands in Kenya and Ethiopia. We used structural equation modelling to separate direct effects of a woody invader Prosopis juliflora on a range of ecosystem functions from indirect effects mediated through changes in biodiversity (plant species richness) and ecosystem functioning (herbaceous biomass). We analysed effects on ecosystem functions linked to soil biogeochemical cycling and transfer of energy between trophic levels. 3. We found that the mean size of individual indirect effects mediated by biodiversity and herbaceous biomass was about twice as large as the mean size of individual direct effects of Prosopis on ecosystem functions, showing that indirect effects are an important component of the invader's overall environmental impacts. Changes in both herbaceous biomass and biodiversity were approximately equally important as drivers of indirect effects, indicating that we need to expand our view of indirect effects to consider a wider range of mechanisms. Simple univariate models failed to capture some Prosopis invader effects because positive direct effects on soil stability and predatory invertebrates were counteracted by negative indirect effects on biodiversity or biomass loss. The majority of effects were similar in both study systems. Suggestions that woody invaders are able to | 2661 Journal of Ecology LINDERS Et aL.

Declining pollinator populations have caused concern about consequences for food production, and have initiated an increasing number of initiatives that aim to mitigate pollinator loss through enhancement of floral resources. Studies evaluating effects of mitigation measures generally demonstrate positive responses of pollinators to floral resource enhancement. However, it remains unclear whether this represents landscape‐level population effects or results from a spatial redistribution of individuals from otherwise unaffected populations. Here, we present a method for estimating landscape‐level population effects using data from commonly used standardized pollinator transect surveys. The approach links local density responses of pollinators in both mitigation sites and surrounding landscape elements to the area these habitats occupy in mitigation landscapes as well as control landscapes to obtain landscape‐level population estimates. We demonstrate the method using data from a 2‐year study examining the effects of experimental wildflower enhancements on wild bumblebees and solitary bees in Dutch agricultural landscapes. The results show that conclusions based on local responses may differ significantly from those based on landscape‐level responses. Wildflower enhancements significantly enhanced landscape‐level abundance of both bumblebees and solitary bees. Bumblebees showed a pronounced positive local density response in mitigation sites and the surrounding landscape that was in line with significant landscape‐level increases in abundance. However, solitary bees showed no local response to mitigation sites, and the landscape‐level increases in abundance only became apparent when the area of bee habitat was taken into account. Incorporating the area of both newly created and pre‐existing pollinator habitats into effect estimates accounts for density‐dependent processes such as dilution, spillover and local concentration of individuals. It, therefore, results in more reliable estimates of the response to mitigation measures of pollinators, as well as other mobile arthropod groups that are often being surveyed using transect surveys.