Monitoring the Extraordinary Ephemeral Emergence of Myriophyllum spicatum L. in the Coastal Lagoon Albufera of Valencia (Spain) and Assessing the Impact of Environmental Variables Using a Remote Sensing Approach

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.Many land-based sensitive bands or inversion algorithms can serve water bodies, but the differences between land and water need to be considered, especially for aquatic plants. Water bodies attenuate greatly in the near-infrared band, so the spectral reflectance information carried by the near-infrared band is basically unusable. Therefore, these bands need to be excluded when considering the construction of algorithms.

 

 

2.Section 2.2, lines 152-159. Was the spectral data not measured in the field? Even if the sky was clear, the satellite data still had large deviations after atmospheric correction, so the collection of field data, besides suspended sediments, phytoplankton, chlorophyll, etc., still required spectral data measurement.

 

3.Line 208-211, it should be noted that the plant-sensitive spectra of land may not be suitable for aquatic plants.

 

 

4. Line 215-217, are you sure red light is okay? Can you add a figure to show the spectral differences between other plants?

 

5. Line 236-243 I am still not sure about the author’s description. First, how was Myriophyllum spicatum measured? Was it exposed to air or submerged in water? If it was in water, we should not be able to detect its reflectance spectrum. This could cause misjudgment, as even healthy Myriophyllum spicatum could be considered unhealthy due to the lack of reflectance spectrum caused by water absorption.

 

6. Change the labels of the horizontal axis in Figure 3 to be slanted, not vertical.

 

7. Add more remote sensing images of different times in Figure 5, which can increase the differentiation.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Please see my review report in the attached file. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Minor editing of English should be considered. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript describes a way to utilize remotely sensed imagery to identify/quantify submerged aquatic vegetation and then links this to water quality. I believe this is an interesting topic that could have important implications for management, but I feel like this manuscript does not fully discuss why the SAV returned. Although I agree that water quality is an important factor in determining SAV, I do not feel that only 1 year of data provides enough evidence to state that this is the cause of SAV disappearance. Myriophyllum spicatum typically dies back each year, regardless of water quality – I think there needs to be a mention of seasonality. Are there any long-term data sets that could describe why the SAV returned to the lagoon (see study by Gurbisz and Kemp 2014: https://doi.org/10.4319/lo.2014.59.2.0482)? Also, I think this imagery analysis could be useful for environmental/coastal managers for understanding SAV coverage/extent and be used as a proxy for water quality. I would like to see this idea developed more fully in the discussion.

Some specific comments: 

Line 38 – Does increased DO directly lead to greater water transparency?

Line 48 – “more cover” of what?

Lines 60-62 – I think proper management implies intervention. Were there updated environmental laws that were enacted between 1972 and 2018?

Figure 2 – It is hard to see the box on the left. I suggest either showing a smaller regional area or adding another box of only Spain. Also, the scale bars/north arrow are a little fuzzy.

Line 136 – What other species of aquatic macrophytes are found?

Line 142 – Can you include the Jucar River on the site map? Also, was the clean quality water anomalous, relative to previous years?

Sampling Methods – How many field sites did you sample? It would be good to also include this in the site map. How was SAV sampled? What metrics did you use?

Vegetation indices – Was regression analysis used to estimate biomass? Were all the mentioned indices used? But only NDVI provided useful results? How was vegetation area calculated?

Figure 5 – The scalebar/north arrow is hard to see.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Please see my comment for 2nd in the attached file. 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Minor English editing should be considered. 

Author Response

We agree with your review, please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Lines 99-112: Instead of focusing on all the remote sensing challenges of aquatic macrophytes by Malthus in the intro – I would emphasize that there is a need for remote sensing studies in this field and talk about the one(s) being addressed by this work. The additional challenges can be brought up again in the discussion as additional needs.

Line 176: How many water samples were collected – where were they collected (multiple depths? Multiple sites? Outside/within the SAV bed?)

 

Line 192: This is a very general statement that can probably be rephrased to state why remote sensing is used in this study (e.g., takes frequent measurements, covers a large area, different band combinations reflect vegetation health).

I would also recommend condensing some of the details of this section. For example, you could say that you obtained Sentinel and Landsat imagery from sources and these were atmospherically corrected using algorithms/software.

Lines 236-253: I do not feel like all of these indices should be included in the main text since they were not chosen for analysis. I suggest omitting the ones not used, or referencing a supplemental table/reference that includes these other indices that can be used.

Lines 284-285: Describe how vegetation area was calculated. Were all cells with a value between 0.25-0.7 assumed to be fully vegetated (i.e., this would be shown as vegetation present [1] and anything not in this range would be vegetation absent [0]). Then, was area calculated by summing up all cells where vegetation was present? Or was partial coverage included?

Line 345: While there may be a correlation between water clarity and plant onset, temperature is an important environmental cue for plant emergence. Can you show plant area versus temperature?

Overall, I think there is an opportunity to frame the manuscript as combining water quality/physical parameter measurements with remote sensing coverage. Rather than only coming at it from a water clarity angle, you can also compare to the other measurements (e.g., temperature) to show the seasonal changes in SAV. I think a longer-term monitoring program (multiyear effort) would be necessary to understand the interannual changes.

Comments on the Quality of English Language

There are some instances where I think the grammar/word choice can be improved. For example, there are many sentences that start with "in this sense/context". Or line 157, I'm not sure "notorious" is the appropriate word choice. 

Author Response

We agrre with your review, please see the attachment

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors covered all my concern and questions. The manuscript can be now considered for publication. 

Comments on the Quality of English Language

Minor English editing if needed. 

Author Response

Thank you very much for your review.

Reviewer 3 Report

Comments and Suggestions for Authors

Abstract says “compared to previous years” – was this done?

Lines 102-116 – I feel like this needs to be framed in the context of what is being addressed in this study. I think it is reasonable to talk about the gaps that are not addressed in this study in the discussion. However, I feel like the overview of all of the key challenges identified by Malthus fits in the introduction (i.e., “the relative position of the sun with respect to the sensor”, or “standardized methods for assessing morpho-ecological gradients, structural complexity, and functional status…”)

I don’t think the results here (transparency vs. cover) necessarily contradict what is reported in the literature. Plants often have a temperature cue that leads to emergence. However, this often happens in the spring, which is usually a period of higher river discharge (and as a result higher turbidity) due to more rainfall, snow melt. Also, because the plants were absent over 40 years – and there was no comparison to data in previous years, there is not enough information to determine if the seasonal measurements reflect the ideal conditions for SAV growth. Often, when vegetation has been absent for so long, there is a “hysteresis effect” where improvements beyond the historic conditions are needed for vegetation to return.   

Author Response

Thank you very much for your review, please see the attachment.

Author Response File: Author Response.pdf