General comments:

The authors have conducted a study investigating the relationship between groundwater residence time and the bioavailability of organic matter, as well as microbial biomass, metabolism, and production in karstic landforms where groundwater interacts with surface water. This case study provides valuable insights into groundwater microbial communities in the karst Upper Floridan Aquifer. The paper is well-structured and well-written. I recommend accepting the paper after addressing the following minor revisions:

(Xiaogang Chen/Westlake University)

Specific comments:

Results:

In Table 1, some instances of "N.d." were written as "N.d" in the row for "DIC". Additionally, the note below Table 1 for "Sp. Cond" does not match the values in the table.

Table 2 contains two variations of "Not applicable", mentioned as "N.a" or "N.a."

The figures appear blurry, and the color of the graph's axes could be made darker to enhance clarity for readers.

In line 351-352, please provide accurate data or a figure to support the statement regarding "ATP-based biomass concentrations in samples from River Rise at low flow being significantly higher than values at high flow and approximately 3-fold lower than those observed for River Sink at low flow."

Line 357-359 refers to "Fig. 2c," but there is no such figure mentioned in the article.

In line 390-392 and line 407-408, the authors state that the incorporation rate of 3H-thymidine was much higher from Madison Blue Spring. Therefore, it may be more appropriate to use the 3H-thymidine incorporation rate when evaluating doubling times at Madison Blue Spring.

Discussion:

In line 460, please attempt to explain why ATP contents are high at Devil's Eye Spring.

Please address these revisions in the manuscript. Overall, the paper is well-done and provides valuable insights into the topic.