Evelyne Franquet

Allochthonous and autochthonous dissolved organic matter (DOM) and nutrient availability (nitrogen [N], phosphorus [P]) are expected to increase in lakes as a consequence of climate change. These increases can alter the planktonic food web, especially in oligotrophic systems. This study addresses the consequences of DOM and nutrient increase on heterotrophic bacterioplankton (Hbact) and phytoplankton in high‐altitude lakes. A planktonic community from a high‐altitude lake was incubated for 10 days and laboratory microcosms were subjected to a full factorial design 4 × 2 × 2 of three factors: carbon (C‐control, glucose, algal lysate, soil extract), inorganic nutrients (no added nutrients, addition of N and P) and light (light:dark cycle, dark). Hbact was co‐limited by organic C and inorganic nutrients, resulting in lower biomass and production with glucose or inorganic nutrients added alone than with the two sources added in combination. Algal‐derived DOM was more labile, more consumed, and resulted in higher bacterial production, while soil‐derived DOM was less labile and less consumed. Phytoplankton was strongly limited by inorganic nutrients. DOM additions, via the interactions with Hbact, increased phytoplankton biomass and modified the community composition. Mixotrophs increased with C under light or nutrient limitation, pointing them as major factors in controlling mixotrophy in high‐altitude lakes. Soil‐derived DOM led to higher algal biomass, higher proportion of mixotrophs, and more taxa adapted to nutrient‐enriched conditions. This study reveals a shift in planktonic interactions with changes in DOM quality and quantity. Under the projected climate change, we expect an increase in heterotrophy and a modification of trophic interactions with important repercussions on the C cycle in high‐altitude lakes.