| 24 Jun 2026
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.
The authors present a manuscript investigating the effects of browning and increased temperature on C:N:P stoichiometry, phytoplankton nutrient limitation, and ecosystem metabolism in experimental pond mesocosms. The study fits well within the scope of Biogeosciences by linking C, N and P cycles with ecosystem responses. The results demonstrate that browning (as increased DOC concentrations) affects macronutrient ratios in pond ecosystems. Furthermore, the authors show that phytoplankton was N and P co-limited, rather than DOC-induced light limitation. The presented metabolism data indicate that increasing DOC concentrations lead to a stimulation of bacterial production, whereas primary production decreases with increasing DOC concentrations and higher temperature. This could be connected to higher temperature stress and competition with heterotrophs for nutrients. The authors conclude that warming and browning affects primary and bacterial production in different ways, favoring bacterial production over primary production.
Overall, I found this manuscript interestingly and well written, without unnecessary redundancy. The study addresses a relevant scientific question using a novel and reproducible experimental approach that suits to answer the stated research questions. The manuscript is concise, clearly structured, and the conclusions are supported by the presented data. However, there are several aspects that would benefit from clarification. Particularly, the manuscript would benefit from an improved explanation why each C:N:P (total and dissolved, with DIC or DOC) is used. Additionally, the manuscript would benefit from a careful revision to improve the consistency of abbreviations and chemical formulas. There is no clear patterns for elements and chemical compounds which are sometimes written as abbreviations and sometimes spelled out. I believe that this manuscript has the potential to make a valuable contribution to the field after the authors have addressed the comments raised below.
General comments:
The manuscript would benefit from a clearer explanation of the different stiochiometric ratios used throughout the analyses and the reason for their selection. The DOC:TN:TP ratio is described as the total (unfiltered) ratio, whereas the DIC:DIN:PO4 ratio represents the filtered fraction. However, DOC and DIC represent different C pools, affecting different ecological processes. When ecosystem metabolism is considered, DIC is particularly relevant for primary production, whereas DOC is more closely linked to bacterial production. I therefore encourage the authors to clearly define each ratio and explain why it is the most appropriate metric for addressing the respective aspect.
Figure 1 could be improved to communicate the main message more effectively. I recommend adapting the color scheme to match that used in the mixed model (=changing from green to blue, also having in mind to use colorblind friendly palettes) to improve the consistency throughout the manuscript. Furthermore, the legend currently explains the ambient and warmed treatments, but the meaning of the color intensity is not described. Adding this information would make the figure easier to interpret.
Figure 2 would be presented more clearly by modifying the axis labels following the style of Andersen et al. (2025). In addition, I recommend including at least the boundaries of the depletion zones to panel B and C. This would help readers to identify more easily in which nutrient depletion zone the individual data points are located.
Specific comments:
Line 168: It is not clear why DOC is included as a macronutrient for algal nutrient requirements, please clarify.
Lines 185-187: Please explain more explicitly that increased DOM inputs (with DOC concentration as proxy) may also introduce DON and DOP as nutrients. Otherwise, this statement that higher DOC concentrations increase C:N and C:P ratios appears somewhat self-evident.
Lines 195-198: This sentence is very long and difficult to follow. Splitting it into two shorter sentences would improve readability.
Lines 201-203: I do not think this is the appropriate interpretation of the ternary diagram. An optimal stoichiometric ratio would be located near the 1/3 or each axis (Redfield ratio in the middle), rather than indicating, e.g. 0-15% of the algal P requirements. In my opinion, the manuscript already communicated the main finding clearly without this sentence.
Line 207: Please specify the saturation aspect in the context of C:N:P ratios. As currently written, "saturated" could be interpreted as referring to gas exchange equilibrium, which is not illustrated in the referenced figure).
Line 208: Please specify what the reported optimum refers to.
Line 217: Please add the unit to the value of 2.5
Technical corrections:
Line 290 (Table 1): For the row "log(TN:TP) - sampling time", the interpretation refers to "Strong seasonal control of TOC:TP", which I assume is a typo.
Line 290 (Table 1): In row log(DIC:PPO₄³⁻), there appears to be a typo (double P). In addition, the interpretation for sampling time states "no evidence", although the reported p-value is 0.04