Polycyclic aromatic hydrocarbons (PAHs) are priority pollutants that are produced through incomplete combustion of modern biomass and fossil fuels. In aquatic systems PAHs are absorbed by suspended matter and ultimately deposited into sediments. PAH fluxes to sediments have been declining in North American since the mid 1960s.Improving technology and regulations were expected to contribute to declining PAH concentrations; however, in some urban sediment there are recent increases in deposition.

Temperatures and precipitations in the Arctic are expected to increase dramatically over the next century, yet little is known about how microbial communities and their underlying metabolic processes will be affected by these environmental changes. To address this knowledge gap, we adopted a unique space-for-time design to analyse sediments sampled from Lake Hazen, NU Canada. Over the past decade, in this world's largest high Arctic lake by volume, climate change has enhanced glacial melt, resulting in increased annual runoff from the watershed. We exploit the spatial heterogeneity created by varying runoff regimes, conducting metagenomic analyses of lake sediments along these spatial gradients to study how a transition from low to high runoff, used as a proxy for environmental change, affects microbial community structure and functional potential. Here we show that increasing runoff leads to a decrease in taxonomic and functional diversity. While our data suggest that rising Arctic temperatures will negatively impact microbial diversity and nutrient cycling in Arctic lakes, it is still unclear how these losses at the microbial community level will affect biogeochemical cycles, or whether this will lead to feedback loops of uncertain direction and magnitude.