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The nitrogen stable isotopic signature (δN) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δN of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN), %N in the sediments and lake morphometric variables were the best predictors of sedimentary δN, explaining 66 % of its variation. Significant relationships were also found between sediment δN and human-derived N load ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.23, p < 0.001), the latter being a strong predictor of TN ( $$ R_{{{\text{adj}} .}}^{2} $$ = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δN. Alberta lake sedimentary δN signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δN was best explained by %N in the sediments and lake volume. Overall, our findings support the use of δN in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δN are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context. [ABSTRACT FROM AUTHOR]

Here we harnessed the power of metaproteomics to assess the metabolic diversity and function of stratified aquatic microbial communities in the deep and expansive Lower St. Lawrence Estuary, located in eastern Canada. Vertical profiling of the microbial communities through the stratified water column revealed differences in metabolic lifestyles and in carbon and nitrogen processing pathways. In productive surface waters, we identified heterotrophic populations involved in the processing of high and low molecular weight organic matter from both terrestrial (e.g. cellulose and xylose) and marine (e.g. organic compatible osmolytes) sources. In the less productive deep waters, chemosynthetic production coupled to nitrification by MG-I Thaumarchaeota and Nitrospina appeared to be a dominant metabolic strategy. Similar to other studies of the coastal ocean, we identified methanol oxidation proteins originating from the common OM43 marine clade. However, we also identified a novel lineage of methanol-oxidizers specifically in the particle-rich bottom (i.e. nepheloid) layer. Membrane transport proteins assigned to the uncultivated MG-II Euryarchaeota were also specifically detected in the nepheloid layer. In total, these results revealed strong vertical structure of microbial taxa and metabolic activities, as well as the presence of specific "nepheloid" taxa that may contribute significantly to coastal ocean nutrient cycling., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)