the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 May 2022
Effects of seasonal and diel variations in thermal stratification on phytoplankton in a regulated river
Abstract. Thermal stratification in lotic systems has occurred globally and more frequently in recent decades due to global warming and artificial river modification, often with negative ecological effects. However, studies on river stratification have been restricted to rivers below dams or reservoirs affected by their water release and lacked examination of diel stratification and its impact on phytoplankton, in particular. In the present study, we assessed the degree of thermal stratification, its environmental drivers, and the response of water quality and phytoplankton assemblage against stratification in the mid-lower reach of the Nakdong River, whose morphology has been highly modified, including the construction of eight weirs. We implemented vertical temperature profiling at three study sites, both seasonally and diurnally. Then, we calculated three stratification indices: relative water column stability, Schmidt stability, and maximum temperature gradient. Three indices for assessing the degree of stratification showed that most sites experienced diel stratification during summer. Principal component analysis showed that stratification significantly led to seasonal and diel variations in the water environment. Solar radiation and air temperature were positive controllers, while a negative controller (in this case, the river flow rate) existed only for diel variation in the stratification. Higher abundance and surface cell accumulation of cyanobacteria Microcystis were observed at the stratified sites, and the diel variations in its biomass (chlorophyll a) in the surface water were primarily associated with the stratification indices instead of with other temperature and nutrient variables. Overall, the results suggest that the river has summer stratification, which is involved in amplifying cyanobacterial bloom intensity. Without a suppressing factor, summer stratification is expected to be recurrent in the river, and thus mitigating the developed stratification is needed by promptly regulating the river flow.
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Interactive discussion
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RC1: 'Comment on bg-2022-42', Anonymous Referee #1, 10 Jun 2022
General comments
The authors present an interesting manuscript about the different effects of seasonal and diel variations in a river’s thermal stratification on phytoplankton community. This work is timely given the recent intensification of interest in global temperature increase and helps predicting its short/long term consequences in freshwater ecosystems. I am pleased that the authors made a valuable contribution to the field with the high-frequency data of phytoplankton community rarely seen in other studies. The authors hypothesized that river stratification would have different environmental drivers and effects on phytoplankton in the two different time scales then analyzed them separately. Interestingly, the authors found that the seasonal shifts in phytoplankton community structure were either insensitive and showed a limited response to the stratification indices. Summer cyanobacterial bloom intensity, here cell abundance and accumulation into the surface water, was positively affected by the diel variations in the stratification indices and thermocline. Based on the environmental drivers of stratification, the authors discuss the generalization of stratification events for the river system and the implementation strategy for flow management to mitigate cyanobacterial blooms. Overall, the manuscript has interesting research questions and the data collection/analysis/interpretation seem sound. However, the manuscript needs to be revised before publication. I hope the comments below can help the authors improve their manuscript.
Specific comments
Section 2.2: the used thermal stability indices are all based on the vertical temperature difference (potential energy), but their calculations lack the concept of vertical mixing (mixing energy) that against the formed stratification. Prandtl number, Richardson number, or Lake number could be additionally considered when appropriate (Kirillin and Shatwell, 2016).
Section 2.3: It would improve the readability of the materials and methods section, if the different data analyses were more clearly linked with specific hypotheses which already stated in the results section.
Section 3.1: temporal variations in the stratification indices are investigated, but why are the authors interested in the scales of variation? What do they expect? This is one of many examples, where the formulation of a hypothesis would improve the storyline. Are the authors expecting that short-term stratification will have a different ecological mechanism or consequence from lake stratification which persists longer?
Fig 4: why are the authors presenting additional information on the thermoclines and their vertical variations? It would be easier to read if the authors formulated a hypothesis about how the diel variation of the thermoclines affect the vertical distribution of phytoplankton cell in Fig 8 and then investigate these.
Section 3.2: the first paragraph summarizes the changes over seasons and sites in the multiple parameters which were later analyzed against the stratification indices. From reading, it is not clear why all this information (and with the standard error of detail) is presented. Parts of the paragraph are trivial and the text could easily be reduced substantially (e.g. the two first sentences could be removed).
Section 3.3, relationships between phytoplankton assemblage and multiple environmental factors including the stratification indices are investigated. It is described that the diel CCA showed a positive relationship between air temperature and cyanobacterial density. The authors must draw a conclusion by combining the PCA results, which showed a strong relationship between chlorophyll a and the stratification indices.
Fig 8. I suggest the authors to present cyanobacterial cell density, which was used in the CCA analyses in Fig 7. This may give a reason for the different stratification-phytoplankton relationships between the PCA and CCA.
Section 4.2: ‘The PCA ordinations revealed that thermal stratification is one of the most important drivers of water environments in the Nakdong River, largely accounting for their seasonal and diel variations’. What do the authors mean by this?
Reference
Kirillin, G., & Shatwell, T. (2016). Generalized scaling of seasonal thermal stratification in lakes. Earth-Science Reviews, 161, 179-190.
Citation: https://doi.org/10.5194/bg-2022-42-RC1 - AC1: 'Reply on RC1', Eunsong Jung, 13 Sep 2022
-
RC2: 'Comment on bg-2022-42', Anonymous Referee #2, 14 Aug 2022
I recommend the authors consider the following comments and develop the manuscript in a global context and perspective.
General comments
This manuscript focuses on river stratification, which was not common in the past but has become frequent due to human river modification and climate changes. The authors traced the causes of stratification and its impact on phytoplankton in the middle of the Nakdong River in South Korea after a river modification. They conducted vertical profiling seasonally and diurnally between 2017 and 2018, and diagnosed the stratification degree using three stratification indices. They also implemented various analyses to reveal the effect of stratification on the river microbial ecology, including associations between different water quality parameters and phytoplankton assemblages. The authors concluded that: 1) The summer stratification in 2018 exceeded the stratification thresholds generally accepted in many aquatic environments, and 2) Diel variations in the stratification intensity and thermocline depth were distinctive from the typical stratification of lakes or dams.
As a reviewer, I concur with the authors in their conclusions. I see several interesting points of the study, which are unfortunately not stressed at the current state of the manuscript. River utilization patterns become complex worldwide, and stratification often occurs in lotic systems. Most importantly, the structure and function of the aquatic ecosystem of the Nakdong River must be shifted to a new phase due to a large-scale river transformation project (as the authors illustrated). I ask the authors to relate the stratification occurrence and the river project; hence the readers may find helpful information comparable to their respective study sites..
The authors provided a lot of analytical results, but they are not well related, nor successfully emphasize their findings. Consequently, the manuscript must be restructured largely. In my reading, I also found several unclear sentences. I have listed some of these under my minor comments. Please ask a native English speaker in the related scientific field to revise the English writing.
I hope that the authors’ findings will be successfully conveyed to the readers of the journal Biogeosciences. I believe that an appropriate revision is necessary for this manuscript to meet the goals pursued by the Journal. Please revise the manuscript by carefully reflecting on this general and the following specific comments.Specific comments
- Title
Title of the manuscript may be reconsidered as the authors revise the manuscript. Please contact your Editor of the journal the possibility.- Abstract
Line 11. In the discussion part, the authors referenced the researches on diel river stratification and response of phytoplankton. However, in this line they suggested the studies are “lacking”. Please revise it.
Lines 23-25. If the authors accept the general comment, the abstract must be rewritten. Especially, please stress your new findings or suggestion in this part.- Introduction
Lines 40-43. Differentiation of stratification characteristics in rivers from the typical lake stratification would be one of the main conclusions of this study. Moreover, I think these lines must be restructured, in relation to the river project the Nakdong River has experienced.
Lines 44-50. This is more appropriate for the study site description part so please move the sentences. Instead, explain why the Nakdong River stratification problem is import in global context.- Methods
Line 60. It is strongly recommended the study site establishment strategy: i.e. I hope to understand why the authors set their sites there. In the results section, The different river morphologies or distances from the weirs brought different spatial variations in the stratification indices. This must be also related to the objectives of the study.
Lines 113-139. It is strongly recommended to reformulate this part. I realized that they first examine descriptive characteristics of the study sites, then stratification events were investigated with the application of three indices. Then they applied several multi-variate statistical analyses such as PCA and CCA. Please detail the purpose and differences these analyses to facilitate the readers’ understanding. I suggest the authors to reinspect the uses of similar analyses.
Throughout the method part: Please keep methods the authors used in order of the analysis sequence, hence should be linked to the sequence of results exhibition.- Results
Please revise the Results part as the authors are asked to rewrite the methods part.
Lines 153-160. The authors diagnosed stratification in the river using three stratification indices. They presented much information of seasonal and spatial variation in the stratification for the respective stratification indices into the Figure 2. From reading, it is not clear which information they think the most important and relevant for phytoplankton.
Lines 166-174. Same comment. In the Discussion, it is explained that frequency of destratification and timing of the maximum stratification are important for cyanobacterial prolieferation. Timing of the maximum stratification and depth of the thermocline were investigated. However how would they affect phytoplankton was not rigorously discussed in the Discussion.
Section 3.3. I concur with many comments already made by Referee #1. In addition to and supporting suggestions by Referee #1, I would recommend that the authors to identify the roles of diel variation in stratification and thermocline depth using the figure 7and 8 for the novelty of the study.
Lines 225-236. Phytoplankton distribution seems well responded to the stratification event (only by Figure 6); however, its corresponding text does not clearly explain. Please rewrite this part.
Lines 242-249. Relationships between phytoplankton assemblage and environmental variables including several stratification indices were analyzed. In interpreting the results, do they assume the response of phytoplankton in an “assemblage-scale” or a linear relationship in an individual phytoplankton phylum? Also, please highlight and distinguish these finding from the similar researches in the Discussion section.- Discussion
Subsection 4.1. They entitled this part as “the characteristics of the Nakdong River,” however the Nakdong River circumstance was not well discussed. The spatial difference of the stratification indices were found between the study sites. They should explain how the different river morphologies or distances from the weirs brought these consequences. For example, the authors may relate future mitigation strategy for cyanobacterial proliferation to stratification patterns.
Moreover, they only stressed that the river had different stratification characteristics than the typical lake system by comparing the stratification indices and behaviors of stratification to referenced lakes. Finally, they concluded that these characteristics are important in shaping the aquatic ecosystem. Please relate the results in the manuscript to the storyline. Please suggest how unique or general the stratification in the Nakdong river is by comparing to other river stratifications.
Section 4.2. They expect that summer stratification would become more intensive in the Nakdong river based on the dependence of the stratification indices on the meteorological variables. It would be more persuasive if evidences were provided properly. For example, they can show the highest record of air temperature of the study site in 2018 or extended years then compare it with the air temperature at the date of surveys. This will tell how much the stratification would further intensify as it reaches the middle of summer and how the stratification would be recurrent in every summer.
Section 4.3. The authors are asked to discuss their results with the references shown in the texts. If the authors accept my suggestion shown above, the texts in this section must be largely restructured.- Conclusion
Is there really a need for a concluding section? This section is basically a summary and not a terse concluding paragraph. It needs to be shorter and highlight the novelties of the study. If this cannot be done, then it is not needed.Technical comments
Lines 57-62. As aforementioned, the sentences of Nakdong River explanation may be merged to this part. Please do not forget to clarify the Nakdong River characteristics, comparable to other rivers in the world.
Line 70. Please explain your diel monitoring strategy in detail.
Line 76. The authors examined phytoplankton species data from their diel monitoring, but here they did not explain how they monitored, identified, and enumerated.
Figure 1 (the map). Please add a small map showing the East Asian region to understand where the river is located.
Lines 97, 102. I don’t think the in-line equations are necessary, because the authors already showed them in Table 1. Please consult the journal style.
Line 103. Acceleration due to gravity ï gravitational acceleration
Table 1. Please check whether the authors used those threshold values as the meaning of a magnitude or intensity that must be exceeded for stratification. Why they provided two values for the respective indices? In my reading, the terms “low- and high-threshold” could be misunderstood to mean the lowest and the highest values of each of the indices reported previously.
Also, the authors did not provide equation of “Maximum temperature gradient” in the table, and what is “many” right after the name of this index?
Line 148. Please show the version numbers for respective R packages the authors used.
Lines 155-157. Different results between the stratification indices were explained. Please move it to the Discussion section.
Lines 181-190. I think this descriptive part may appear in the first part of the results.
Table 3. This would be appropriate for supplementary materials.
Lines 259-267. Please move this paragraph to the previous diel pattern explanation part, and concentrate on Microcystis distribution. Also, please explain what data were used to calculate each average value.
6. Figure 8. Please check the parentheses.Citation: https://doi.org/10.5194/bg-2022-42-RC2 - AC2: 'Reply on RC2', Eunsong Jung, 14 Sep 2022
Interactive discussion
Status: closed
-
RC1: 'Comment on bg-2022-42', Anonymous Referee #1, 10 Jun 2022
General comments
The authors present an interesting manuscript about the different effects of seasonal and diel variations in a river’s thermal stratification on phytoplankton community. This work is timely given the recent intensification of interest in global temperature increase and helps predicting its short/long term consequences in freshwater ecosystems. I am pleased that the authors made a valuable contribution to the field with the high-frequency data of phytoplankton community rarely seen in other studies. The authors hypothesized that river stratification would have different environmental drivers and effects on phytoplankton in the two different time scales then analyzed them separately. Interestingly, the authors found that the seasonal shifts in phytoplankton community structure were either insensitive and showed a limited response to the stratification indices. Summer cyanobacterial bloom intensity, here cell abundance and accumulation into the surface water, was positively affected by the diel variations in the stratification indices and thermocline. Based on the environmental drivers of stratification, the authors discuss the generalization of stratification events for the river system and the implementation strategy for flow management to mitigate cyanobacterial blooms. Overall, the manuscript has interesting research questions and the data collection/analysis/interpretation seem sound. However, the manuscript needs to be revised before publication. I hope the comments below can help the authors improve their manuscript.
Specific comments
Section 2.2: the used thermal stability indices are all based on the vertical temperature difference (potential energy), but their calculations lack the concept of vertical mixing (mixing energy) that against the formed stratification. Prandtl number, Richardson number, or Lake number could be additionally considered when appropriate (Kirillin and Shatwell, 2016).
Section 2.3: It would improve the readability of the materials and methods section, if the different data analyses were more clearly linked with specific hypotheses which already stated in the results section.
Section 3.1: temporal variations in the stratification indices are investigated, but why are the authors interested in the scales of variation? What do they expect? This is one of many examples, where the formulation of a hypothesis would improve the storyline. Are the authors expecting that short-term stratification will have a different ecological mechanism or consequence from lake stratification which persists longer?
Fig 4: why are the authors presenting additional information on the thermoclines and their vertical variations? It would be easier to read if the authors formulated a hypothesis about how the diel variation of the thermoclines affect the vertical distribution of phytoplankton cell in Fig 8 and then investigate these.
Section 3.2: the first paragraph summarizes the changes over seasons and sites in the multiple parameters which were later analyzed against the stratification indices. From reading, it is not clear why all this information (and with the standard error of detail) is presented. Parts of the paragraph are trivial and the text could easily be reduced substantially (e.g. the two first sentences could be removed).
Section 3.3, relationships between phytoplankton assemblage and multiple environmental factors including the stratification indices are investigated. It is described that the diel CCA showed a positive relationship between air temperature and cyanobacterial density. The authors must draw a conclusion by combining the PCA results, which showed a strong relationship between chlorophyll a and the stratification indices.
Fig 8. I suggest the authors to present cyanobacterial cell density, which was used in the CCA analyses in Fig 7. This may give a reason for the different stratification-phytoplankton relationships between the PCA and CCA.
Section 4.2: ‘The PCA ordinations revealed that thermal stratification is one of the most important drivers of water environments in the Nakdong River, largely accounting for their seasonal and diel variations’. What do the authors mean by this?
Reference
Kirillin, G., & Shatwell, T. (2016). Generalized scaling of seasonal thermal stratification in lakes. Earth-Science Reviews, 161, 179-190.
Citation: https://doi.org/10.5194/bg-2022-42-RC1 - AC1: 'Reply on RC1', Eunsong Jung, 13 Sep 2022
-
RC2: 'Comment on bg-2022-42', Anonymous Referee #2, 14 Aug 2022
I recommend the authors consider the following comments and develop the manuscript in a global context and perspective.
General comments
This manuscript focuses on river stratification, which was not common in the past but has become frequent due to human river modification and climate changes. The authors traced the causes of stratification and its impact on phytoplankton in the middle of the Nakdong River in South Korea after a river modification. They conducted vertical profiling seasonally and diurnally between 2017 and 2018, and diagnosed the stratification degree using three stratification indices. They also implemented various analyses to reveal the effect of stratification on the river microbial ecology, including associations between different water quality parameters and phytoplankton assemblages. The authors concluded that: 1) The summer stratification in 2018 exceeded the stratification thresholds generally accepted in many aquatic environments, and 2) Diel variations in the stratification intensity and thermocline depth were distinctive from the typical stratification of lakes or dams.
As a reviewer, I concur with the authors in their conclusions. I see several interesting points of the study, which are unfortunately not stressed at the current state of the manuscript. River utilization patterns become complex worldwide, and stratification often occurs in lotic systems. Most importantly, the structure and function of the aquatic ecosystem of the Nakdong River must be shifted to a new phase due to a large-scale river transformation project (as the authors illustrated). I ask the authors to relate the stratification occurrence and the river project; hence the readers may find helpful information comparable to their respective study sites..
The authors provided a lot of analytical results, but they are not well related, nor successfully emphasize their findings. Consequently, the manuscript must be restructured largely. In my reading, I also found several unclear sentences. I have listed some of these under my minor comments. Please ask a native English speaker in the related scientific field to revise the English writing.
I hope that the authors’ findings will be successfully conveyed to the readers of the journal Biogeosciences. I believe that an appropriate revision is necessary for this manuscript to meet the goals pursued by the Journal. Please revise the manuscript by carefully reflecting on this general and the following specific comments.Specific comments
- Title
Title of the manuscript may be reconsidered as the authors revise the manuscript. Please contact your Editor of the journal the possibility.- Abstract
Line 11. In the discussion part, the authors referenced the researches on diel river stratification and response of phytoplankton. However, in this line they suggested the studies are “lacking”. Please revise it.
Lines 23-25. If the authors accept the general comment, the abstract must be rewritten. Especially, please stress your new findings or suggestion in this part.- Introduction
Lines 40-43. Differentiation of stratification characteristics in rivers from the typical lake stratification would be one of the main conclusions of this study. Moreover, I think these lines must be restructured, in relation to the river project the Nakdong River has experienced.
Lines 44-50. This is more appropriate for the study site description part so please move the sentences. Instead, explain why the Nakdong River stratification problem is import in global context.- Methods
Line 60. It is strongly recommended the study site establishment strategy: i.e. I hope to understand why the authors set their sites there. In the results section, The different river morphologies or distances from the weirs brought different spatial variations in the stratification indices. This must be also related to the objectives of the study.
Lines 113-139. It is strongly recommended to reformulate this part. I realized that they first examine descriptive characteristics of the study sites, then stratification events were investigated with the application of three indices. Then they applied several multi-variate statistical analyses such as PCA and CCA. Please detail the purpose and differences these analyses to facilitate the readers’ understanding. I suggest the authors to reinspect the uses of similar analyses.
Throughout the method part: Please keep methods the authors used in order of the analysis sequence, hence should be linked to the sequence of results exhibition.- Results
Please revise the Results part as the authors are asked to rewrite the methods part.
Lines 153-160. The authors diagnosed stratification in the river using three stratification indices. They presented much information of seasonal and spatial variation in the stratification for the respective stratification indices into the Figure 2. From reading, it is not clear which information they think the most important and relevant for phytoplankton.
Lines 166-174. Same comment. In the Discussion, it is explained that frequency of destratification and timing of the maximum stratification are important for cyanobacterial prolieferation. Timing of the maximum stratification and depth of the thermocline were investigated. However how would they affect phytoplankton was not rigorously discussed in the Discussion.
Section 3.3. I concur with many comments already made by Referee #1. In addition to and supporting suggestions by Referee #1, I would recommend that the authors to identify the roles of diel variation in stratification and thermocline depth using the figure 7and 8 for the novelty of the study.
Lines 225-236. Phytoplankton distribution seems well responded to the stratification event (only by Figure 6); however, its corresponding text does not clearly explain. Please rewrite this part.
Lines 242-249. Relationships between phytoplankton assemblage and environmental variables including several stratification indices were analyzed. In interpreting the results, do they assume the response of phytoplankton in an “assemblage-scale” or a linear relationship in an individual phytoplankton phylum? Also, please highlight and distinguish these finding from the similar researches in the Discussion section.- Discussion
Subsection 4.1. They entitled this part as “the characteristics of the Nakdong River,” however the Nakdong River circumstance was not well discussed. The spatial difference of the stratification indices were found between the study sites. They should explain how the different river morphologies or distances from the weirs brought these consequences. For example, the authors may relate future mitigation strategy for cyanobacterial proliferation to stratification patterns.
Moreover, they only stressed that the river had different stratification characteristics than the typical lake system by comparing the stratification indices and behaviors of stratification to referenced lakes. Finally, they concluded that these characteristics are important in shaping the aquatic ecosystem. Please relate the results in the manuscript to the storyline. Please suggest how unique or general the stratification in the Nakdong river is by comparing to other river stratifications.
Section 4.2. They expect that summer stratification would become more intensive in the Nakdong river based on the dependence of the stratification indices on the meteorological variables. It would be more persuasive if evidences were provided properly. For example, they can show the highest record of air temperature of the study site in 2018 or extended years then compare it with the air temperature at the date of surveys. This will tell how much the stratification would further intensify as it reaches the middle of summer and how the stratification would be recurrent in every summer.
Section 4.3. The authors are asked to discuss their results with the references shown in the texts. If the authors accept my suggestion shown above, the texts in this section must be largely restructured.- Conclusion
Is there really a need for a concluding section? This section is basically a summary and not a terse concluding paragraph. It needs to be shorter and highlight the novelties of the study. If this cannot be done, then it is not needed.Technical comments
Lines 57-62. As aforementioned, the sentences of Nakdong River explanation may be merged to this part. Please do not forget to clarify the Nakdong River characteristics, comparable to other rivers in the world.
Line 70. Please explain your diel monitoring strategy in detail.
Line 76. The authors examined phytoplankton species data from their diel monitoring, but here they did not explain how they monitored, identified, and enumerated.
Figure 1 (the map). Please add a small map showing the East Asian region to understand where the river is located.
Lines 97, 102. I don’t think the in-line equations are necessary, because the authors already showed them in Table 1. Please consult the journal style.
Line 103. Acceleration due to gravity ï gravitational acceleration
Table 1. Please check whether the authors used those threshold values as the meaning of a magnitude or intensity that must be exceeded for stratification. Why they provided two values for the respective indices? In my reading, the terms “low- and high-threshold” could be misunderstood to mean the lowest and the highest values of each of the indices reported previously.
Also, the authors did not provide equation of “Maximum temperature gradient” in the table, and what is “many” right after the name of this index?
Line 148. Please show the version numbers for respective R packages the authors used.
Lines 155-157. Different results between the stratification indices were explained. Please move it to the Discussion section.
Lines 181-190. I think this descriptive part may appear in the first part of the results.
Table 3. This would be appropriate for supplementary materials.
Lines 259-267. Please move this paragraph to the previous diel pattern explanation part, and concentrate on Microcystis distribution. Also, please explain what data were used to calculate each average value.
6. Figure 8. Please check the parentheses.Citation: https://doi.org/10.5194/bg-2022-42-RC2 - AC2: 'Reply on RC2', Eunsong Jung, 14 Sep 2022
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Cited
2 citations as recorded by crossref.
Eunsong Jung
Gea-Jae Joo
Hyo Gyeom Kim
Dong-Kyun Kim
Hyun-Woo Kim
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