Dear Julia,

two reviewers have provided comments on your manuscript, and in your reply you indicated how you are going to modify the paper in response to their suggestions and critics. I encourage you to submit the revised version of the manuscript and the point-by-point response letter, where you show what exactly was changed in manuscript in response to the reviewer comments. Please note that one of reviewers is going to review the revised paper again.

From my side, I have just one minor point. In response to the reviewer #1, you've wrote "Hence, relatively deep lakes might play the role of ‘hot spots’ in the tundra landscape, where decomposition of organic matter in the deep water column and in the upper sediment is accelerated by the isolation from the atmospheric oxygen." My understanding is that aerobic decomposition is much faster than anaerobic one. Is it different for the thermokarst lakes?

Best regards,

Victor

Dear Julia,

the revised manuscript seems to be appropriate for another review round, but I have missed a file with your point-by-point replies to the reviewer comments. Your response in the interactive discussion is not exactly the same as modified in the manuscript. For example, in response to the referee #1 comment you wrote:

Discussions on the stratification importance for the biogeochemistry and its dependence on the lake morphometry have been added to the revised manuscript in the discussion section 5, pp.6659 L22:
"Our observations revealed, and the model was able to simulate, appearance of short stratification periods in summer in the studied lakes (Figs. 4& 5). While this has importance of these stratification events for the lake biogeochemistry is not quantified yet, though the seasonal mixing regime is apparently the major physical factor affecting all biogeochemical processes in lakes. In particular, duration of the thermal stratification in summer affects immediately the amount and the vertical distribution of dissolved oxygen: longer summer stratification provokes deep anoxia and favors by this methanogenesis in the deep water column and in the upper sediment (Golosov et al. 2012). Under equal climatic forcing, lake depth is the primary factor determining duration of summer stratification (the second one being the water transparency, Kirillin 2010). Sensitivity model runs with the lake depth varying in the range 2-12 m using the same meteorological input data from Samoylov demonstrate that lakes in this climatic zone with mean depths >5 m should have dimictic stratification regime, i.e. develop continuous stratification in summer with duration of 1 month or longer (Fig. R1). This also supports the observation of summer stratification in deeper (> 6 m) Alaskan thermokarst lakes (Sepulveda-Jáuregui et al., 2015). In deeper lakes of ~8 m depth or more, the summer stratification duration significantly increases on account of high thermal inertia resisting to vertical mixing during the autumn cooling in August-September (Fig. R2)."

Revised manuscript:
"We observed and simulated short stratification periods in summer in the studied lakes (Figures 4 & 5). The effect of these stratification events on lake biogeochemistry is probably the major physical factor affecting biogeochemical processes in lakes. In particular, the duration of the thermal stratification in summer affects the concentration and vertical distribution of dissolved oxygen. Longer summer stratification provokes deep anoxia and favors methanogenesis in the deep water column and upper sediment (Golosov et al., 2012). Under equal climatic forcing, lake depth is the primary factor determining the duration of summer stratification (the second one being the water transparency, Kirillin, 2010). Sensitivity model runs with the lake depth varying in the range 2-12 m using the same meteorological input data from Samoylov demonstrated that lakes in this climatic zone with mean depths >5 m should have dimictic stratification regimes, i.e. develop continuous stratification in summer with duration of 1 month or longer (Figure 10). This also supports the observation of summer stratification in deeper (> 6 m) Alaskan thermokarst lakes (Sepulveda- Jáuregui et al., 2015). In lakes of about 8 m depth or more, the summer stratification duration significantly increases since high thermal inertia prevents vertical mixing during the autumn cooling in August-September (Figure 11)."

I fully understand that you modified your response to the reviewer during manuscript rewriting. This is why I ask you to provide point-by-point answer to reviewer comments EXACTLY AS IN THE REVISED MANUSCRIPT. As soon as I get this point-by-point response, I will continue with the review process.

Best regards,

Victor

Dear Julia,

I am very pleased with the revised version. Before I finally accept it, please clarify one point:

Last para in the section 5 was not changed:
"This heat loss may occur through transmission into
both the sediment and the atmosphere, the latter being of minor importance due to isolation of the water column by the ice cover"

The reviewer #2 was complained that this is not correct:
"P 6661, L 6-8: Is it really the case that heat transfer to the atmosphere is of minor importance during winter? Ice has a large thermal conductivity, and the temperature gradient between ice (/water below) and atmosphere is large. Are there any references for that?"

You acknowledged that reviewer is correct (see below) but have not changed the text. Why?

"The reviewer is absolutely right. The heat flux from lakes to the atmosphere is much higher than the heat flux from snow covered soils (for example, shown by Langer et al. 2011b for ponds and Jeffries et al. 1999 for Alaskan lakes; both cited in this paper). In particular during winter the subsurface heat flux becomes an essential component in the surface energy balance due to the lag of incoming short wave radiation. The sub surface heat flux balances up to 90% of the radiative losses. Thus, lakes could play an important role in the atmosphere heat budget during winter."


Best regards,

Victor

Dear Julia,

thank you for clarifying the point on the heat loss.

I am pleased to accept your revised paper for publication in Biogeosciences.

Best regards,

Victor