This is a very clear and nicely written manuscript. I have limited comments to make, mainly because the presentation is clear and the interpretations appear sound and based on a well treated dataset. A couple of very minor comments below:

L138. Should this be C2?

L164. Is there an advantage to using a flat distribution for the source end member (DIC) and a normal distribution (PIC) for the samples in the Monte Carlo simulation? If not, a sentence on the justification of this approach here would be useful.

Section 4.3. Are the values calculated here just scaled based on the surface area of the lake? I appreciate that this is delineated as a first order estimate, but is a simple areal scaling of these values appropriate? Some clarification/justification here would be appreciated to add a bit more veracity to this section, which is important to clarify the importance of the study’s findings.

In this manuscript, Mittelbach et al. present a novel approach employing sediment traps at two locations in Lake Geneva (France/Switzerland), a large peri-alpine lake. By analyzing particulate organic carbon (POC) and particulate inorganic carbon (PIC) from traps situated near and distant from the main river inflow, the authors convincingly demonstrate that calcite precipitation can significantly contribute to carbon sequestration in hardwater lakes.

The manuscript is clearly written, well structured, and methodologically robust. The results are effectively presented and offer valuable insights into carbon biogeochemical cycling in lacustrine systems. I particularly appreciate how a relatively simple sediment-trap setup can inform broader lake-scale carbon budgets and align well with longer-term perspectives derived from sediment cores. The methodology appears applicable to other systems to explore the role of pelagic calcite precipitation and dissolution in lake carbon cycling.

I commend the authors for this excellent contribution. I have only a few comments and questions, as the manuscript is nearly ready for publication in its current form. Please find my detailed feedback below.

Major Comments

Line 121: Were dissolution rates of settling calcite crystals considered? If estimable, could this affect the study's conclusions?

Line 154: The assumption that "PICAuto was assumed to derive from the lake’s DIC pool" excludes any calcite formation in the Rhône or other tributaries. Please clarify why this possibility was ruled out.

Lines 168–169: It appears the potential lateral transport of allochthonous material from the Rhône to the distal trap was not accounted for. How can we be confident that a significant portion of allochthonous POC is not laterally transported? Given the overlap between riverine particle input and peak primary productivity, please justify this methodological assumption and discuss any potential implications for the results and their interpretation.

Figure 4: Could the authors explain the abrupt increase in ΔPI¹⁴C observed in December 2022?

Minor Comments

Line 300: Please specify “calcite precipitation events” to avoid confusion with meteorological precipitation.

Line 311: Regarding "such as May 2023," is there supporting data or a reference confirming elevated primary productivity during this period?