|The study of Koch et al. has now been revised and individual sections of the manuscript improved considerably. And while I still think there is substantial novelty in this data set, there is numerous issues that would need to be seriously addressed before publication. In fact, the manuscript contains ‘scientific errors’ that must be removed and draws conclusions that are not supported by the outcome of the study (see below). |
Major issues are:
(1) To my opinion, nitrate values found in urban groundwater are comparably low and to my opinion do not point to a strong contamination. Moreover, there is numerous studies that underline that nitrate at concentrations below 50mg/L does not directly affect groundwater fauna. In consequence, one cannot expect much of an outcome in that respect. Indeed, correlations with nitrate have been shown but through indirect effects in agricultural areas. Since only a few physical-chemical parameters have been measured, and only temperature and land use, that show clear alterations to a ‘natural’ reference situation, I would put my focus on these two ‘impacts’.
(2) I fully agree that groundwater fauna is temperature sensitive and in central Europe stygobionts are almost exclusively (with some exceptions) cold stenothermic. I do not agree with the thresholds mentioned in the manuscript and the sources cited. It is stated (P2-L37) that groundwater fauna ‘cannot withstand’ water temperatures over 16°C (Brielmann et al. 2009) or rather 14°C (Spengler et al. 2017) for an extended period. This is definitely not true. I went back into the cited sources and what is stated there is as follows: Brielmann et al. (2009) says “True groundwater invertebrates (stygobites) are assumed to be cold stenotherm and can hardly persist at water temperatures exceeding 16°C for extended periods of time (T.Weber & S.I. Schmidt, unpublished data).” It says ‘hardly’ and cites work ‘not published’ and the paper is 10 years old. The study itself found that “… faunal abundance showed no relation to impacted groundwater temperatures, but faunal diversity decreased with temperature, possibly emphasizing the sensitivity of individual groundwater invertebrates towards heat discharge.” No relationship between temperature and faunal abundance! In Brielmann et al. (2011) it is stated that „Niphargus inopinatus (groundwater amphipod) when allowed to move freely in a temperature gradient preferred a temperature between 8 and 16°C; in 77% of the observations the speciemen were found there, but in consequence in 23% of the cases the animals were outside this range. For the isopod Proasellus cavaticus, specimen were in 66% of the observations found between 8 and 16 °C. In Glatzel (1990) a species-specific critical threshold temperature of 19°C is mentioned for Parastenocaris phyllura (harpacticoid copepod) beyond which a significantly higher mortality occurred. A study on groundwater microbes and fauna in local aquifers below basins collecting surface runoff during extreme rain events found that groundwater fauna was almost absent at spots that were impacted by significant temperature dynamics, with maximum temperatures of up to 22°C (Foulquier et al. 2011). Spengler et al. (2017) reports about declining fauna biodiversity at temperatures above 14°C. In fact, there is species found that start to disappear from the communities at higher temperatures while others are still found.
If we summarize all this information, then it is clear that there is a variability in temperature tolerance among groundwater faunal groups and species. No clear threshold at 14°C or 16°C appears proven, more likely individual thresholds are somewhere between 14°C and 18-20°C, based on what has been reported so far. It is really essential to carefully interpret findings from other studies and data published.
(3) To my very personal opinion there is two ways to publish scientific results and findings. First, to do the minimum necessary. Second, to explore the data best possible. My feeling is, and this was already said in the first round of review, that the data set has not yet been explored and analyzed in a proper way. Although there was substantial criticism from both reviewers because of a lack of statistical analyses, the only change that was done is applying now a simple Withney-Mann-U-Test to all data. That is sad and boring, and to my opinion does not deserve publication in a high ranked journal. Only from the papers cited, the authors could have derived ideas about the application of additional, more sophisticated multivariate tests like PCA, CCA, … Sorry to be so direct.
(4) I like the idea of testing the ecological assessment schemes of Hahn (2006), Griebler et al. (2014) and Korbel & Hose (2017) in an urban setting. However, such an application needs to be done with some care. In the first tier (step) of the scheme described in Griebler et al. (2014) which is somehow similar to what was published by Korbel & Hose (2011), it is recommended to choose five or more criteria with a minimum of 3 biological ones. If criteria are selected that are partly dependent to each other, e.g. proportion of crustaceans and proportion of oligochaetes, then the resolution of the assessment is very low. Surprisingly, although several assessment indices have been considered by the authors (GHI, GESI, GFI), results of none are presented in the paper. Obviously, as I got from the reply to reviewers’ comments, things have not worked out as clear as expected. I would have liked to read in the discussion about the ‘pitfalls’ of the individual assessment schemes. Again, an assessment scheme cannot compensate the lack in use of multiple sensitive criteria. Finally, although, the prerequisite to sample stations more than once is fulfilled, sites that are compared have been sampled in different years, a fact that should at least be discussed.
(5) I guess, we all agree that this first study of groundwater fauna and assessment of the groundwater ecological status in an urban setting was accompanied by some limitations. There have been only a few physical-chemical parameters measured, the number of wells sampled werde very different for the two land use categories, and regional and local reference conditions for the groundwater fauna were missing, to give just three examples. This is normal, and one can nicely build on this first experience. And yes, the reply of the authors to several of the reviewer recommendations was: “The aim of this study was to provide a first overview of the ecological groundwater conditions of the study area”. What I really disliked is that although the results are of limited validity and transferability, and need to be confirmed in follow-up investigations, at the end of the discussion section it is stated that: “Areas with no or little groundwater fauna could be used for to store thermal energy at higher temperatures.” and “HT-ATES could be established in urban environments.” How can this conclusion be drawn from the findings presented?
I am very sorry to disappoint the authors, after putting efforts in the revision of the original submission, but the manuscript to my opinion is still far from being ready to be published in HESS. I recommend another round of major revision.