Warming vs. browning: a dual mechanism behind net ecosystem production of shallow brown aquatic systems

Abstract. Northern lakes are warming and browning due to increased cDOM inputs, altering water column C:N:P ratios and temperatures that regulate photoautotrophic and heterotrophic production. Yet the relative importance of warming versus browning, and their combined effects on nutrient stoichiometry and microbial dynamics remain unclear. We experimentally manipulated boreal ponds along a cDOM gradient under ambient and +2 °C warming to quantify impacts on nutrient conditions and microbial production. Browning shifted C:N:P stoichiometry in both total and dissolved pools, with increased C:N and C:P ratios in the total nutrient pools, while reducing relative inorganic nutrient availability. Despite declining N:P ratios in both total and dissolved pools, both pools were strongly carbon-saturated and depleted in both N and P. Nutrient‑addition assays confirmed NP co‑limitation. Browning increased bacterial production, while browning and warming combined reduced primary production, lowering PP:BP ratios, although net heterotrophy didn’t occur, likely because the ponds were so shallow. We show that browning -not warming- drives boreal freshwaters toward carbon‑rich, N‑ and P‑ colimited conditions, while warming and browning together modulate microbial production by amplifying DOC‑driven heterotrophy and suppressing photoautotrophic production. Our study reveals a dual mechanism behind the shifting of brown systems toward net heterotrophy. As browning and warming are pervasive trends across Northern Hemisphere lakes, these findings have wide applicability and advance general understanding of how freshwater ecosystems respond to ongoing environmental change.