Abstract

Boreal lakes hold more than half of Earth’s freshwater. Understanding their functioning is therefore highly important for preserving the multiple services they provide us with, from clean drinking water to nutritious fish. Organic matter (OM) derived from terrestrial ecosystems has been shown to subsidize secondary production in lakes by contributing between 30-70% of the organic carbon content of aquatic consumers. It is now clear that external inputs (allochthonous) play a crucial role in aquatic food webs, however, the mechanisms underlying initial energy transfer at the base of lake food webs is still poorly understood. The aim of the PhD is to understand mechanisms by which terrestrial resources are integrated into aquatic food webs by focusing on the biological composition of sediments and the way in which processing of OM by bacterial communities depends on interactions with algae. The first step will consist of characterizing microbial community composition via a metagenomics approach. Specifically, the dominant bacterial, archaeal and fungal functional groups will be identified, in order to see how these relate to mineralization rates and to the characteristics of sediments that are shaped by their surrounding watersheds. Secondly, I will determine how the productivity of these communities is influenced by competition with algae for inorganic nutrients. This will be tested in vivo under varying OM conditions (i.e. quantity and quality), and a zooplanktonic consumer will be introduced as an assay of vertical energy transfer to higher trophic levels. In a third part of this PhD, I will work within a larger whole-lake experiment consisting of artificial delta ecosystems and test how changing OM supplies influence energy transfer in the field. Finally, the influence of land use on phytoplankton abundance and nutrient availability will be tested using field observations on watershed characteristics and lake biochemical data. Understanding how terrestrial primary production is mobilized in lakes will help predict how anthropogenic pressures on land use will affect freshwater ecosystems, and will inform us on ways to improve current management practices.