1. High-altitude lakes are sentinels of change. Anticipated increases in inputs of dissolved organic carbon (DOC) from catchments in high-altitude lakes could affect planktonic interactions, especially if inputs occur during the winter ablation phase.However, the responses of plankton communities to added DOC interacting with nutrients during this period are still not well understood.2. We sampled under-ice water and used a multifactorial experimental design to investigate the DOC effect on planktonic organisms (phytoplankton and heterotrophic prokaryotic plankton [HPP]) and their interactions during the winter ablation phase. The plankton community was subjected to DOC additions (0, 2, 4, and 6 mg C/L) under two nutrient-availability treatments (natural concentrations or enriched in N and P), two light conditions (dark or light), and two temperature conditions (10 or 18℃).3. We found HPP to be co-limited by N, P, and C in our microcosms. Added glucose as available C-source for bacterioplankton was highly consumed at the end of the experiments. This consumption was not always related to an increase of the HPP biomass, due to a rising predatory control by ciliates and mixotrophic phytoplankton over bacteria. In dark conditions, the biomasses of both autotrophic and mixotrophic phytoplankton were substantially reduced, and the HPP biomass under DOC and nutrient additions principally depended on the predatory control exerted by ciliates. In light conditions, a diversification of top-down control over bacteria was observed, with the HPP response to DOC and nutrient additions depending on both mixotrophs and ciliates. Moreover, when heterotrophic ciliates and mixotrophs were present together in the experiments, the ciliates replaced the mixotrophs in phagotrophy over bacteria. 4. Our experimental results indicate that DOC inputs can rapidly alter the trophic interactions of the planktonic food web, depending on nutrient limitation in highaltitude lakes. We found decreased commensalistic interactions between bacteria and phytoplankton, but increased competition and predation, after DOC additions. | 1649 DORY et al.

The predicted increase in allochthonous dissolved organic carbon (DOC) in high‐altitude lakes is expected to alter the phytoplankton–bacterioplankton relationship. However, few studies address the influence of summer phytoplankton dynamics on microbial responses to DOC additions. We sampled natural plankton assemblages during two contrasting periods of summer in a high‐altitude lake in the French Alps and subjected them to glucose and nutrient enrichments under two light conditions (dark or light) and two temperature conditions (10°C or 18°C). Our results indicate that glucose use by bacteria differs over the summer, depending on the availability of autochthonous DOC and the nutrient limitation. Glucose was consumed by bacteria more in early summer; however, biomass increased with glucose addition more in late summer than in early summer. This pattern arose from the greater availability of phytoplankton‐derived DOC in late summer, reducing the need for alternative carbon sources in late summer, when phytoplankton biomass was high. Mixotrophic taxa were stimulated after glucose additions both in early summer and in late summer. We found greater competition between bacteria and phytoplankton in late summer after glucose addition, linked to the summer nutrient limitation pattern. Our study thus highlights a differential response depending on the timing of summer DOC inputs. The global changes forecast for the French Alps should increase heterotrophic and mixotrophic processes in planktonic communities of shallow high‐altitude lakes with vegetated catchments. This experimental study provides insights that will be useful in predicting ecological trajectories and in refining predictions of sentinel lakes' responses to global changes.

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 analysed 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 DOM. 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 (LSP).Increased phytoplankton biomass meant that phytoplankton-derived DOM was dominant during the LSP.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 towards predominantly bottomup 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 DOM 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 K E Y W O R D S functional traits, global change, microbial interactions, mountain lakes, trophic interactions | 47

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