Authors studied allochthonous and autochthonous importance in steppe stream food webs using the novel method with carbon isotopes of essential amino acids. You also show that d13C values of essential amino acids can be used to distinguish allochthonous and autochthonous carbon signals. Your results conclude the importance of autochthonous support of fishes. I keep this ecologically important study, and the authors have done a great job sampling and analyzing amino acids from hundreds of fishes. I have only two concerns about your study. First, stable isotopes studies often refer to energetic pathways even if bulk or amino acid carbon values do not tell us energetic pathways but carbon cycle or here origin of essential amino acids. For example, in microbial studies, carbon dioxide production is the end product of utilized energy from organic substrates, and carbon in microbial biomass (e.g. PLFA or proteins) is new biomass. Therefore, even if fish does not use terrestrial sources for building amino acids or new tissues, this does not reveal if they could utilize terrestrial carbohydrates for energy. Actually, Taipale et al. (2016; Scientific reports 6.1 (2016): 1-15) noted that zooplankton can utilize terrestrial carbohydrates under phytoplankton deficiency. However, since fish does not directly usually feed on terrestrial particles, it is possible that the highest consumption of terrestrial sources (e.g., carbohydrates) occurs by benthic invertebrates in stream food webs. Altogether, I would like the authors to be careful in wording throughout the manuscript. My second point is a related correction of your amino acid carbon isotopes values, which you do not seem to do. For example, if you prepared propyl chloroformates of amino acids, your carbon isotope value should be corrected by the propyl group generated during the derivatization process as done regularly with fatty acids (e.g. Bec et al. 2011; doi:10.1111/j.2041-210X.2011.00111.x). Based on your method description, it seems that you prepared methyl chloroformates, and this should take into account in correcting d13C values of amino acids.

Minor points
Add temperature to line 207. Which temperature did you hydrolyze amino acids?
Line 366 please do not abbreviate RCC and SBGC

In your revised version of your manuscript, you have improved your manuscript according to all referees. Since readers can be inspired by your study, I keep it important that your protocol is repeatable. Even though you have provided more information about carbon isotope measurement from the amino acid, it is still not repeatable. See my comment below.
Line 228. Did you run each amino acid separately with EA-IRMS? Based on this protocol, it means that you should have a correction factor for each amino acid. Please clarify that reader could easily repeat this. I recommend you add the names of amino acids which you analyzed and the company name where you purchased reference materials. You could publish correction factors as a supplement.