1. Climate change is altering both dissolved organic matter (DOM) properties and phytoplankton dynamics in lakes. High-altitude lakes are good sentinels of global change. However, their value as sentinels depends on how well we understand their functioning. This study analyzed physical, chemical, and biological data during three pre-defined periods running from complete ice-cover right to the end of the ice-free season, in a high-altitude oligotrophic lake. A functional approach was used to assess how relationships between DOM and the planktonic community vary over time. 2. Phytoplankton functional change was found to occur with shifts in dissolved organic matter. During the ice-influenced period, from February to the end of the thaw, the phytoplankton community was dominated by small autotrophs and mixotrophic flagellates and DOM from sediment and terrestrial origin dominated the DOM pool of the lake. Phytoplankton diversity and richness increased during the post-snowmelt overturn period, when terrestrial DOM dominated the DOM pool. Finally, large siliceous autotrophs, competitive under low nitrogen concentrations and high temperature, dominated almost exclusively during the late summer period. Increased phytoplankton biomass meant that phytoplankton-derived DOM was dominant during the late summer period. 3. These phenological changes in the phytoplankton community resulted in functional shifts at the base of the food web. Based on the relationships between the variables in the study, it can be deduced that the nature of the relationship between phytoplankton and bacteria progressively shifted from strong top-down control exerted by phytoplankton over bacteria toward predominantly bottom-up control at the end of the ice-free season. 4. Synthesis. Using a field survey starting from complete ice-cover and lasting right to the end of the ice-free season in a sentinel lake, we show seasonal shifts in the link between dissolved organic matter properties and plankton community traits. Differences in environmental conditions and DOM origin explained variation in phytoplankton community structure and function, pointing to seasonal shifts in microbial food web interactions. Our research suggests that, under climate change, the balance between mixotrophy and autotrophy and the control exerted by phytoplankton over bacterioplankton will change in oligotrophic high-altitude lakes with large, vegetated catchments.