the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Dec 2024
Understanding ecohydrology and biodiversity in aquatic nature-based solutions in urban streams and ponds through an integrative multi-tracer approach
Abstract. Rapid urbanization and climate change affect ecohydrology, biodiversity and water quality in urban freshwaters. Aquatic nature-based solutions (aquaNBS) are being widely implemented to address some of the ecological and hydrological challenges that threaten urban biodiversity and water security. However, there is still a lack of process-based evidence of ecohydrological interactions in urban aquaNBS, and their relationship to water quality and quantity issues at the ecosystem level. Through a novel, integrative multi-tracer approach using stable water isotopes, hydrochemistry and environmental DNA we sought to disentangle the effects of urbanization and hydroclimate on ecohydrological dynamics in urban aquaNBS and understand ecohydrological functioning and future resilience of urban freshwaters. Stable isotopes and microbial data reflected a strong influence of urban water sources (i.e. treated effluent, urban surface runoff) across stream NBS. The results show potential limitations of aquaNBS impacts on water quality and biodiversity in effluent-impacted streams, as microbial signatures are biased towards potentially pathogenic bacteria. Urban ponds appear more sensitive to hydroclimate perturbations, resulting in increased microbial turnover and lower microbial diversity than expected. Furthermore, assessment of macrophytes revealed low diversity and richness of aquatic plants in both urban streams and ponds, challenging the effectiveness of NBS in contributing to aquatic diversity. This also demonstrates the need to adequately consider aquatic organisms in planned restoration projects, particularly those implemented in urban ecosystems in terms of habitat requirements. Our findings emphasize the utility of integrating tracer approaches to explore the interface between ecology and hydrology, and provide insights into the ecohydrologic functioning of aquaNBS and their potential limitations. We illustrate the benefit of coupling ecological and hydrological perspectives to support future NBS design and applications, that consider the interactions between water and the ecosystem more effectively.
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RC1: 'Comment on egusphere-2024-3537', Anonymous Referee #1, 06 Jan 2025
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Title: Understanding ecohydrology and biodiversity in aquatic nature-based solutions in urban streams and ponds through an integrative multi-tracer approach
Author(s): Maria Magdalena Warter et al.
MS No.: egusphere-2024-3537
MS type: Research article
Summary:
The authors collected water samples monthly from urban streams and ponds around the City of Berlin over a one-year period. Some streams received treated effluent from a wastewater treatment plant, and some did not. The samples were analyzed for stable isotopes of hydrogen and oxygen in water and a suite of physicochemical parameters (e.g., major ions). Samples were also collected for environmental DNA analysis, but only seasonally (n=4). In September of the study period, macrophyte (aquatic vegetation) surveys were carried out at two sites per each stream and pond site. Spatial and seasonal patterns in physiochemical parameters were assessed and spatial patterns in microbial diversity and community structures were assessed. Ultimately, the authors were interested in identifying the spatial and seasonal drivers of microbial variability and evaluating the significance of their results for assessing the benefits (and efficiency) of aquatic nature-based solutions (aquaNBS). The authors found that both urban streams and ponds have low macrophyte diversity, suggesting that current aquaNBS may not effectively support aquatic biodiversity. Indeed, the authors showed that microbial diversity is still strongly influenced by urban water sources (e.g., treated effluent, impervious surface runoff) despite implementation of aquaNBS. Effluent-impacted streams had a high abundance of bacterial communities specific to the human digestive system. Urban ponds had the lowest microbial diversity and were sensitive to hydroclimatic shifts. Ultimately, the authors advocate for implementing this multiple-tracer approach to assess the performance of aquaNBS across scales.
General Comments:
This is a high-quality study. The number of sites and number of tracers (that holistically characterize the ecosystem) makes up for the shorter time frame of the study. The integration of isotope, hydrochemistry, macrophyte, and microbial results is well done and relatively easy to follow. There are a few places where the methods could use some clarity (see suggestions below), but they do not detract at all from the overall understanding of what was done.
I think the scientific importance of this paper is high. While some of the results may not be surprising, I think the integration of multiple tracers is quite novel, especially in the context of aquatic ecosystem restoration. I think the paper would benefit from the authors leaning a bit more into the discussion of restoration goals. See my suggestions on this below. This could also include some discussion of the logistical constraints (e.g., time, cost, expertise) involved in implementing a multi-tracer approach more broadly. Ecological restoration is often implemented by municipalities, often with support from consultants. What kind of partnerships could be developed to make this approach accessible to all involved?
The paper is well-written with relatively few grammatical and linguistic errors. The figures are high quality, and all seem well placed in the main manuscript. I have provided some suggestions below on how to improve the clarity of some of the more data heavy figures.
Specific Comments (and some Technical Corrections):
Lines 97 to 100 are repetitive.
The research objectives do not mention macrophytes, despite them being discussed in the methods.
Lines 72 to 73 are unclear. How do macrophytes drive competition?
Lines 80 to 82 seem important. Can you expand on how these previous studies used macrophytes and microbial communities as indicators of ecosystem health and functioning?
The wording on lines 83 to 85 does not make sense to me. Can this long sentence be broken up?
On lines 97 to 100 there are two aim statements that seem repetitive. I recommend combining them into one statement.
The research objectives do not mention macrophytes which are clearly an important part of this study and presented in the methods, results and discussion. I recommend integrating them into objectives one and/or two.
On line 103 aquatic nature-based solution efficiency is mentioned. This term should be defined earlier in the introduction.
Double check the whole paper for missing or problematic punctuation. For example, on line 108 there is a missing period.
On line 132, the authors say that the word ‘restored’ will not be used for the remainder of the paper because all of the sites are restored. But then the word is included in the subsequent subtitles.
What is the significance of the gray area in figure 1a. Why are the upstream portions of the Panke, Wuhle, and Erpe catchments not included in this gray area? And in figure 1c and 1d, why are those upstream portions not included.
On lines 204 and 205, can the authors explain how the natural and disturbed sites for the macrophyte surveys were chosen? Was it a random selection? How did the authors avoid bias in the choice of sites?
On lines 216 to 218, can the authors provide a reference and/or an equation for Shannon's diversity index.
In section 2.6, there are a number of statistical tests and methods mentioned, but it is not clear if these are traditional or novel (or somewhere in between) methods for this particular application. I think this could be addressed through a few references indicating previous studies that have applied these methods in a similar context. Also, what software was used for the analyses? This should be stated and referenced.
I think the word ‘spatial’ can be removed from line 228. If it is left in, it gives the impression that within group spatial variability is shown as well.
On line 229, ‘strong evaporative’ does not make sense.
On line 232 how do the authors know that there are multiple sources of water contributing to this stream. I think a reference to prior work would be helpful here.
On lines 234 to 240 can the authors provide an estimate of the uncertainty in the young water fraction. Does the frequency of sampling impact the estimates.
On line 274 DO is defined a second time. See line 186.
On lines 282 to 283 if groundwater contributions vary, does that mean that calcium concentrations would vary? The authors describe Ca concentrations as ‘elevated’. This sentence needs clarity.
On line 290 percentages would be easier to interpret.
On line 334 what is meant by stability of diversity?
On lines 336-338 what is meant by ‘more modified’? Is this referring to the artificial sites?
In figure 6 can the headings for each panel indicate bacteria or diatoms and streams or ponds. This would be instead of having bacteria and diatoms only on panels a and c and streams and ponds written in the lower left corner.
It is not clear in figure 6 what the different symbols mean. For example, in the streams panels could a legend be added for effluent and non-effluent impacted? And what are the different subgroups for the pond sites?
On line 369 and line 392, instead of ‘green space’ can the authors say ‘percentage of green space in the catchment’ (assuming this is correct)? The same applies to ‘sealed surfaces’ (and is applied on line 394).
On line 396 it is not clear what the authors mean by ‘pond and stream macrophytes were assessed together and not separated’.
Why is there a greater precipitation influence in ponds than streams (lines 400 and 401)?
The sentence that runs from line 400 to 404 is too long and doesn’t completely make sense to me. At the beginning, ponds and stream are being compared, but then the sentence transitions to talking about macrophyte diversity across ponds. Can this sentence be broken up for clarity?
On lines 420 to 422, is the bacterial community observed in the effluent impacted stream influenced by the harmful substances and nutrients? I am trying to understand if this sentence is directly or indirectly related to the information right before it.
On line 426, what is meant by low water quality?
On line 429 the authors refer to a mismatch between the initial restoration goals and restoration measures undertaken. Do the authors know with certainty that the restoration goals included a bacterial community that was reflective of pre disturbance conditions?
There appears to be an extra parenthesis on line 439.
Line 455 and 456: are urban ponds not designed with stormwater management as the main goal?
Line 463: missing e in i.e.
I am having a hard time extracting the key point from the last paragraph in section 4.1 (lines 462 to 467). In particular the first couple of lines which refer to ‘successfully implemented’ and ‘contributions to biodiversity and water quality ensured’ make me think twice. Are the authors advocating for this type of coupled isotope/hydrochemistry/microbial assessment to be implemented more broadly across NBS projects? Is it a feasible approach to see if the cumulative effects of multiple NBS have a positive ecohydrological effect at the watershed scale? This goes back to my earlier comments about restoration goals. Should we expect a stream or pond that is integrated into the urban mosaic (and especially one that receives WWTP effluent) to regain any semblance of its pre-disturbance ecohydrological characteristics? If yes, then maybe this multi-method assessment is a good idea. If not, then why go to the trouble? The example on lines 512 to 514 gets at this type of decision-making, but I think it could be explored more.
Could the authors expand on what is written on lines 471 to 473? Are there studies that review the most common stream and/or pond restoration objectives? Are ‘increased aquatic diversity’ and ‘increased habitat quality’ common goals? If so, how are they usually assessed? In the introduction (e.g., lines 45-47), goals associated with water quantity and heat are mentioned. Vegetation, habitat, and societal benefits are mentioned in the next paragraph. Perhaps some of these references could be revisited in this part of the discussion.
The paragraph starting on line 479 is very interesting. So ultimately hydrology will dictate the long-term benefits of NBS.
Citation: https://doi.org/10.5194/egusphere-2024-3537-RC1
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