Nutrient Dynamics and Ecosystem Metabolism of Megacity Rivers: Influence of Elevated Nutrient Concentrations in Beijing’s Waterways
Jitendra Pandey
Round 1
Reviewer 1 Report
This study investigates nutrients (nitrate-N, ammonium-N, phosphate and dissolved organic carbon) from sewage treatment effluents in 3 rivers in China) – they were used to estimate nutrients’ fluxes. The influence sewage treatment effluents or any other source of nutrients/contaminants on receiving waters has been widely studied. Such inputs in turn will influence metabolic processes, dissolved oxygen and the wider aquatic ecology. While the role of sediments (sink or source), nutrients inputs and associated impacts on dissolved is widely known in a broad sense i.e., expected implications, they are rarely investigated. The attempt in this study to examine nutrient dynamics and river metabolism is thus novel and interesting.
This a well conducted and presented work. I’ve no real issues, apart from a few points of clarifications:
1. The distance between the upstream and downstream locations of WWTP is variable across the rivers (2-10 km). It is now clear how above the effluent outfall points the upstream sampling locations were. The distance between these sampling points will invariably influence the nutrient dynamics. 10 km is a particularly long distance, and unless there are other sources of nutrient inputs the observations made may not be entirely due to sewage effluent inputs. It would be useful to have some justification of the choice of sampling locations, particularly distances chosen.
2. “The water samples were collected from different depths”. Is this because of the water depth variability due to seasonality? Or was there another reason? Regardless water samples are generally collected just under the surface – in that case water depth wouldn’t be variable.
3. Ideally river flows, the amounts of effluent inputs and nutrients in effluents should be estimated to better understand the nutrient dynamics. Nonetheless, it would be useful to at least to reflect on the extent of uncertainty in the observed findings.
4. It is not clear if the sediment samples were collected from the same locations as those of the water samples. The sediment sampling needs to be better explained – 10 “patches” per 10cm2. 10 “parches” within 10cm2 – not that much – it should have gone across the river width though not in a straight line. Assuming these were combined to make composite samples, it is not clear how the sediment sampling depth was maintained.
Author Response
Thank you for your questions and suggestions, as well as the recognition of our research. We have replied to each question and revised it in the article. Please see the table below for details.
|
Question |
Response |
Location |
|
1. It would be useful to have some justification of the choice of sampling locations, particularly distances chosen. |
Thank you for your question. Our aim was to investigate the impact of sewage effluent and/or tributary inflow rich in nutrients on river ecological processes. Therefore, we selected two sites on each river for comparison. In addition, sampling accessibility and security were also considered, which is also the reason for the different distances between sampling sites. These rivers are urban rivers with no obvious difference in overall habitat or hydraulic conditions, and we achieved the research aims with the selected sites. In order to make the selection of sampling sites clearer to readers, we added the description that the sites considered sampling accessibility. |
See lines 108-109 in the revised paper for these changes. |
|
2. “The water samples were collected from different depths”. Is this because of the water depth variability due to seasonality? Or was there another reason? Regardless water samples are generally collected just under the surface – in that case water depth wouldn’t be variable. |
This is due to seasonal changes in water depth, but we only took water samples within 15 cm below the water surface, and this depth was unchanged. We now explained the sampling depth in the article more carefully. |
See lines 148-149 in the revised paper for these changes. |
|
3. Ideally river flows, the amounts of effluent inputs and nutrients in effluents should be estimated to better understand the nutrient dynamics. Nonetheless, it would be useful to at least to reflect on the extent of uncertainty in the observed findings. |
Thank you for your suggestion. This is useful information, but unfortunately, we have not gotten access to the discharge amounts at sewage outfall or the concentrations of nutrients discharged. These data are not disclosed, and they may also change every day, which is worthy of future research. However, we provided the average daily information and effluent standard of the wastewater treatment plant, which provided some background information for the study. In addition, we measured the nutrient concentrations and fluxes of the affected water bodies, which is the measurement result after nutrients were discharged. Compared with another sampling site on each river, the increased nutrient concentrations caused changes and significant differences in the river ecological processes. |
N/A |
|
4. It is not clear if the sediment samples were collected from the same locations as those of the water samples. The sediment sampling needs to be better explained – 10 “patches” per 10cm2. 10 “parches” within 10cm2 – not that much – it should have gone across the river width though not in a straight line. Assuming these were combined to make composite samples, it is not clear how the sediment sampling depth was maintained. |
We have improved the wording to describe the sampling details of sediments. The sampling locations of sediments were the same as water samples. 10 patches, about 10 cm2 each, were collected at each site, and a total of about 200mL sediment samples were collected. The sampling depth was 2 cm from the sediment surface. |
See lines 150-151 in the revised paper for these changes. |
Reviewer 2 Report
The MS entitled “Nutrient Dynamics and Ecosystem Metabolism of Megacity Rivers: Influence of Elevated Nutrient Concentrations in Beijing’s Waterways” tends to focus on the complex interactions among hydrology, climate, chemistry, and biology. REM has shown direct implications for water quality and is an excellent functional indicator of ecological response to environmental change. Multiple nutrient sources and increased nutrient concentrations are resulting in significant changes in river ecological processes. The study integrates ecological processes that incorporate nutrient fluxes and their involvement in river metabolism. Overall, this research critically evaluates the urban river ecosystem function by studying the flux dynamics of nutrients and river metabolic patterns.
The MS is recommended for publication after a minor revision.
1. Chemical formulas are not properly written with subscripts in many places
2. I suggest the authors should add a small para at the beginning of the Discussion section defining clearly the ecosystem metabolism, and how it relates to the hypothesis tested here, in the particular context of sediment-water interface integrating the river ecosystem resilience. Please refer to Global Change Biology, 2021, DOI: 10.1111/gcb.15662
.
Author Response
Thank you for these questions and suggestions, as well as the recognition of our research. We replied to each question and revised it in the article. Please see the table below for details.
|
Question |
Response |
Location |
|
1. Chemical formulas are not properly written with subscripts in many places |
We corrected them. In addition, due to the font of the writing template, the superscript of nutrients looks lower. |
Change were made at: Line 275; Line 324; Line 600; Line 706 |
|
2. I suggest the authors should add a small para at the beginning of the Discussion section defining clearly the ecosystem metabolism, and how it relates to the hypothesis tested here, in the particular context of sediment-water interface integrating the river ecosystem resilience. |
Thank you for your suggestion. At the beginning of the discussion on river metabolism (Section 4.2), we described the importance and implications of river metabolism and added the content of river pollution stressors (e.g., elevated nutrients in our study) and water temperature driving river metabolic patterns, and explained this point in combination with our research results. However, we agree that stressing the role of multiple stressors would improve this section and have included ideas from the suggested reference. |
See lines 694-702 |
