the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2024
Diatom diversity and distribution in neotropical karst lakes under anthropogenic stress
Abstract. Lake degradation is an important environmental problem worldwide, particularly in the neotropics where rapid population growth is leading to increasing human impact. However, baseline studies in neotropical lakes are still missing. This study focussed on hydrochemistry, trophic status and in-depth analysis of diatom diversity and ecological distribution in neotropical karst lakes, presenting a high-resolution paleolimnological reconstruction of changing hydrochemical and trophic characteristics in since the late 1950s. We studied sixteen freshwater lakes dominated by bicarbonates, calcium, and magnesium of which four had higher salinity (300–500 mg L-1), sulphate proportions, turbidity and eutrophic conditions. These lakes are considered impacted ecosystem that receive soil-derived sediment, organic matter, urban and agricultural effluents through river inflow. The βw diversity was low (2.6), driven mostly by the hydrochemical and trophic status differences between the four impacted lakes and the rest. Two taxa were characteristic of higher salinity, eutrophic lakes (Aulacoseira granulata var. angustissima and Stephanocyclus meneghinianus) and eight were preferentially present in the low-salinity oligo-mesotrophic lakes. Three of the diatom taxa (Discostella stelligera, A. granulata var. angustissima S. meneghinianus) are cosmopolitan species also present in non-karstic lakes in central Mexico with comparable salinity distributions. Contrastingly, four have restricted neotropical karst distributions (Cyclotella petenensis, Discostella sp, Mastogloia calcarea and Planothidium sp.), in danger of local extirpation as hydrochemical changes and eutrophication increase. C. petenensis described from the Peten Itza record, was present with high abundances in oligo-mesotrophic lakes of low salinity. Paleolimnological analysis allowed to identify that increasing erosion was associated with the first appearance and gradual increase of the diatom taxa characteristic of the impacted lakes since the 1980s, until reaching a critical transition in 2006, demonstrating that currently impacted lakes previously had lower salinity and trophic conditions, comparable with the currently non-impacted lakes.
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RC1: 'Comment on egusphere-2024-914', Anonymous Referee #1, 27 Jun 2024
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I think that this manuscript can make an important contribution to our understanding of neotropical limnology and paleolimnology. The authors have sampled some very interesting and diverse lake sites for this study. The limnological data that were collected are reasonable variables to measure and overall their interpretations about the relationships between diatom community composition in these lakes and their environmental factors are sound and appear justified. There are several issues, though, that I think the authors need to address and information that should be included in the manuscript. Addressing some of these issues, though, might result in a reinterpretation of portions of their data, and might require major revisions.
1. The minimum diatom count sizes for each sample are small, and reflect ~ 100-150 diatom cells/sample. 500 values or ~250 cells are more standard for diatom work, especially if biodiversity assessments are done. The authors clarify that 100 valves were counted for samples that had dissolution, and that is understandable, but a larger count size would have been better and likely revealed higher diversity. Justification for the small count size is needed. The authors might want to consider a greater focus on community composition rather than diversity because of the small count size, especially since some samples might have had a large number of only a fewl taxa. Rare taxa are excluded when count sizes are low.
2. What taxonomic references were used for identifying the diatoms to species level? At what magnification were the diatoms counted? This information should be added to the methods section.
3. More background is needed on how the radiocarbon date was obtained for such recent sediments. Was it a bulk or AMS date? Pb-210 would have been preferable. even though it did not work well at another study site. I was not convinced that the authors could reliably assign dates using interpolation from the basal radiocarbon date. It assumes constant sedimentation, but the color descriptions provided about the sediment stratigraphy suggest that there might be higher sedimentation and more organic-rich sediments in the surface cms of the core. I would recommend the authors explain their assumptions and why they think that a constant sedimentation rate is an appropriate assumption for this lake.
4. The figures and tables are appropriate, but there are many typographical and grammatical errors in some portions of the manuscript. For instance, some of the higher taxonomic classifications are misspelled in the supplemental figures, and there are places, especially in the introduction, where there are missing words in sentences (i.e. and). Upfront the authors state that neotropical records are missing, but I think it would be better to emphasize that they are scarce. There are many paleolimnology and contemporary limnology records from the neotropics, but they are widely dispersed in some areas, and are likely difficult to compare from a regional perspective. Summarizing what is known from Mexico would be important to help emphasize the significance of this study.
Overall, the interpretations for the environmental variables and diatoms make sense and are reasonable, and will provide valuable information for understanding the neolimnology of this region. More information is needed, though, on their methods and assumptions to explain their dating approach for the core sample, and to justify their diatom diversity patterns. Minimally, they need to discuss the effects of small sample sizes on their diversity calculations.
Citation: https://doi.org/10.5194/egusphere-2024-914-RC1 -
RC2: 'Review of egusphere-2024-914', Tom Whitmore, 01 Jul 2024
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Review of "Diatom diversity and distribution in neotropical karst lakes under anthropogenic stress" by M. Caballero et al.
The authors are long experienced in paleolimnological and water-quality research, and are highly qualified to undertake and present this work. The study is important because it examines the diatom flora, its environmental determinants, and historical changes in a rather large set of diverse lakes that are located in an understudied region of Mexico. The Methods are appropriate in most cases and are stated in a clear and comprehensive manner. The diatom plates are of excellent quality.
Comments:
Carlson's trophic state index might not be the best one for lakes that are located in Mexico. In subtropical and tropical lakes, relationships between nutrients, Secchi disk, and chlorophyll a can be different than in the lakes that Carlson used to develop and test his index.
In the core from Lago Penasquito, the use of linear interpolation based on the radiocarbon date of a bottom sample is not a well-justified approach. Even though the 40-cm San Lorenzo core was not datable by 210Pb, the 73-cm core from Penasquito might have been datable with gamma detection. Because this is a karst lake, how was the radiocarbon date corrected for potential hard-water effect, because that can contribute error? Also importantly, sediments are subject to a range of compaction throughout a core, so a linear interpolation would have to assume that compaction is not present and that sediment influx has not changed. If there is sound reason to assume that sedimentation rate has been constant over time, a mass-based interpolation of dates would be preferable to a linear approach because that would correct for compaction differences. In Penasquito, however, the sedimentation rate would not have been constant because the lake was subject to eutrophication. Under such conditions, an assumption of constant linear deposition rate would be violated and dates cannot really be stated to single-digit years (such as 1976-1978, 2006, or 2013). It would be preferable to try gamma detection on remaining sediment if possible, but failing that, it would be better to generalize the dates based on mass-accumulation rates and to report decadal approximations for dates. Also with respect to exploratory analyses, Chla and Zsd should have had high negative correlation.
The diatom floristic information and ecological interpretations are a very valuable part of this study. The biggest concern and criticism I have about the current presentation, however, is the discussion of diatom "diversity" and diversity indices. Diatom sample-count sizes were described as a minimum of 200 valves (what was the maximum or average?), 100 valves in Balamtetic, and 300 valves in the Penasquito core. This is extremely low resolution in diatom counts because diatoms are a diverste taxonomic group, and such small and variable sample sizes would be inappropriate for yielding accurate measures of diversity and for comparing samples. I recall seeing it reported years ago (I don't remember the exact source but believe it was Battarbee) that counts of 300 are sufficient just to approximate the percentages of the 5 most abundant taxa, and that is likely to be true. Smaller samples sizes are likely to be more problematic. More often for defensible statistical analyses, diatom counts are performed on consistent sample sizes of 400-500 valves. Sample sizes that range between 100 and 300 valves would need substantial correction for rarefaction if diversity is being compared, and that would involve random removal of data from larger samples, so it would make diversity calculations even less reliable. A finding of only 50 diatom species is quite small for a regional floristic study, and samples from low-productivity lakes will sometimes yield more species than that in a single sample. Downweighting of rare taxa with CCA would have placed more emphasis on differences among the most abundant taxa across the gradients. The 10 diatom taxa with high regional occupancy are most likely to be accurate and informative. The discussion of various diversity indices in this study is the most questionable and least defensible part of the manuscript. The report of the species that were present, the ecology of taxa with high regional diversity, and the observed changes over time caused by degradation are the strong and most important findings.
The aims stated in lines 62-66 are appropriate because they refer to diatoms with "the highest abundance and frequency of occurrence and those that could be used as indicators of anthropogenic degradation processes". The sample sizes used in this study would be appropriate for identifying the most abundant taxa for such intentions. However, the references to "diatom diversity" in lines 56 and 59 are not really appropriate for samples that have low and variable count sizes.
My strong recommendations are to see if gamma detection of samples could be obtained for the Penasquito core because that would give defensible dates if it were possible. You might have colleagues who could provide that dating as a favor or for inclusion as co-authors. Secondly, i would recommend that the authors focus on the strong points mentioned in the last paragraph above, but remove discussion about diversity because count sizes were too low and variable to yield reliable diversity measures. There is sufficient substance to make this manuscript an important contribution without presenting diversity calculations that could be regarded as questionable.
The manuscript is well worded and is written in an articulate manner. Nevertheless, some sentences could be improved to help with grammatical constructions in English and for clarity. I present some examples of possible corrections for the Abstract, but other corrections could be made throughout the text. If I am asked to review again, I can help redact if I can be provided with a Word version of the manuscript that would enable me to use Track Changes.
Examples of corrections for the Abstract:
change "reconstruction of changing hydrochemical and trophic characteristics in since the late 1950s" to "reconstruction of changing hydrochemical and trophic characteristics since the late 1950s"
change "These lakes are considered impacted ecosystem" to "These lakes are considered impacted ecosystems"
change "sulphate proportions" to "sulphate concentrations"
the phrase "between the four impacted lakes and the rest" is not clear because there is no previous mention about four lakes that were different than the other twelve lakes
Change:
"Paleolimnological analysis allowed to identify that increasing erosion was associated with the first appearance and gradual increase of the diatom taxa characteristic of the impacted lakes since the 1980s, until reaching a critical transition in 2006, demonstrating that currently impacted lakes previously had lower salinity and trophic conditions, comparable with the currently non-impacted lakes."
to:
"Paleolimnological analysis allowed us to determine that the currently impacted lakes previously had lower salinity and trophic conditions that were comparable with the currently non-impacted lakes. (In Lago Penasquito?), increasing erosion since the 1980s was associated with the first appearance and gradual increase in diatom taxa that are characteristic of degradation in impacted lakes, and a critical transition point was observed in diatom assemblages by c. 2006."
Please adopt the recommendations that you feel would be most appropriate.
Sincerely,
Tom Whitmore
Citation: https://doi.org/10.5194/egusphere-2024-914-RC2 -
RC3: 'Comment on egusphere-2024-914', Anonymous Referee #3, 08 Jul 2024
reply
This manuscript will serve as a valuable resource for limnology and water management processes. All information regarding lake degradation is quite relevant for environmental scientists, policymakers, and conservationists aiming to understand and mitigate the impact of human activities on freshwater ecosystems. The authors have conducted a thorough characterization of the lakes' phytoplankton and sampled a comprehensive set of ancillary variables. However, there are certain aspects of the manuscript that require improvement, potentially necessitating major revisions.
Major:
- I suggest that the authors consider including an additional trophic state index to complement the analysis obtained with the Carlson Trophic State Index. This would provide a more comprehensive assessment, especially considering that the Carlson index might not be ideal for lakes in this particular region.
- In my opinion, the resolution of diatom counts is small considering the diversity within this taxonomic group.
Minor:
Line 19: “hydrochemical and trophic characteristics in since the late 1950s”. Please remove the “in”.
Line 113. The authors mention that sampling was conducted in two seasons. However, could you clarify how many samples were collected in total? Also, are the values presented in Table 1 (e.g., water temperature) mean values?
Line 273: “However, it was not clear which lake attributes could be responsible for the higher or lower diversity values (1D or 2D) in the lake groups, with no obvious correlation with trophic status or lake salinity.” Could better monitoring of lake boundary conditions clarify which attributes are responsible for the diversity observed in the lakes? Might this be related to the internal loading of nutrients within the lakes?
Line 280 “The CCA model was significant (p < 0.005), and the variance inflation factors (VIF) of all variables were 280 low (< 12), indicating a low correlation between them.” Please mention Figure 6
Citation: https://doi.org/10.5194/egusphere-2024-914-RC3
Data sets
Hidrogeoquimica de Lagos de Mexico. Margarita Caballero https://datosabiertos.unam.mx
Diatomeas de Lagos de Mexico Margarita Cabllero https://datosabiertos.unam.mx
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