the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2025
Impact of Internal Tides on Chlorophyll-a Distribution and Primary Production off the Amazon Shelf from Glider Measurements and Satellite Observations
Abstract. The ocean region off the Amazon shelf including shelf-break presents a hotspot for Internal Tides (ITs) generation, yet its impact on phytoplankton distribution remains poorly understood. These baroclinic waves, generated by tidal interactions with topography, could modulate nutrient availability and primary production both by mixing and advection. While previous studies have extensively examined the physical characteristics and dynamics of ITs, their biological implications, particularly in nutrient-limited environments,remain underexplored. To address this question, we analysed a 26-day glider mission deployed in September–October 2021 sampling hydrographic and optical properties (chlorophyll-a) at high resolution along an IT pathway, satellite chlorophyll-a and altimetry data to assess mesoscale interactions. Chlorophyll-a dynamics were analysed under varying IT intensities, comparing strong (HT) and weak (LT) internal tide conditions. Results reveal that ITs drive vertical displacements of the Deep chlorophyll Maximum (DCM) from 15 to 45 meters, accompanied by a remarkable 50% expansion in its thickness during HT events. This expansion is observed with a dilution of the chlorophyll-a maximum concentration within the DCM depth. Turbulent cross-isopycnal exchanges driven by tides redistribute chlorophyll-a into adjacent layers above and below the DCM. At the surface, turbulent fluxes contribute to 38% of the chlorophyll-a supply, which directly influences primary production. Notably, the total chlorophyll-a content in the water column increases by 14-29% during high internal tide phases, reflecting a net enhancement of primary productivity. This increase results from the combined effect of vertical mixing and stimulated biological activity in the surface layer. These findings highlight the role of ITs as a key driver of chlorophyll-a distribution and short-term biological variability, reshaping the vertical chlorophyll-a profile and regulating primary productivity and potentially carbon cycling in oligotrophic oceanic systems.
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Status: open (until 15 Jul 2025)
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RC1: 'Comment on egusphere-2025-2141', Anonymous Referee #1, 07 Jun 2025
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Summary:
The authors utilize measurements from a 26-day glider deployment, complemented with satellite products, to investigate how internal tides modulate mixing, nutrient availability, and primary production in the vicinity of the shelf break offshore of the Amazon River mouth. The results suggest that internal wave-driven vertical mixing drives variability in the ocean state and vertical distribution of chlorophyll over the deployment.
Merit:
This paper utilizes glider data to explore how physical processes modulate the vertical distribution of chlorophyll. I think that this work on the interdisciplinary applications of the physics would be of interest to many readers. The approach is generally sound, although additional detail and discussion is needed in places as commented below. The presentation quality and structure is mostly clear, and any points of confusion are noted.
General comments:
- The results focus on a small area where it is known that internal tides have a significant influence on dynamics/mixing, termed a “hotspot” by the authors. I think something more needs to be said regarding the spatial distribution of such features; that is, whether the observations represent an extreme case or are typical. That might help clarify some of the implications of the work, e.g., whether internal tides are a key driver of variability as mentioned on line 39-41 in the abstract.
- I think there needs to some more discussion regarding the separation of spatial and temporal variations in the glider data. That is, inherently a glider that moves in space will capture variations in both space and time but without additional context it will be unclear which is more important. There are related elements already in the text; e.g. the discussion of eddy evolution and the location of the segments relative to the position of the eddy. But, I don’t see any explicit mention of it. I think that is needed, even if the aliasing turns out to be minimal. Some discussion of how diurnal-weekly-monthly temporal variations, and the impacts on the observed spatial variability, would improve the manuscript.
- I think some of the information provided in the methods section is not sufficient for the results to be reproducible. I have mentioned a few places where I think specific details are needed in the comments below.
- No direct turbulence or mixing estimates are used in this paper. While I do not think this is a problem, I think it should be clear earlier in the manuscript. Much of the language in the abstract/intro/early sections attributes changes to vertical mixing, which may be (likely is) the case but is not shown directly in the paper. I would recommend a careful edit of these sections so this is clear to the reader. Related to this (see my comment for L518), it is implied that vertical mixing is entirely a result of internal tides. While possible alternate contributing factors are clearly mentioned in the discussion, I think it would be helpful if it were mentioned earlier before presenting the results.
Line-by-line comments and suggestions:
L26-27 – I think this sentence is unnecessary. It is already stated later that they do modulate nutrient availability/productivity.
L33 – “remarkable” compared to what? Please clarify
L36 – clarify what contributes to the other 62%
L55 – This sentence feels a bit disconnected; discuss how it influences climate variability, i.e. through air-sea interaction.
L96-97 – I think it would be helpful to add a sentence/references regarding the seasonality of internal tides and mesoscale features.
L111 – clarify that this is in optimal conditions with no currents
L116 – change “thanks to” to “from” or “using”
L117 – strange wording. Reword “enabling to estimate”
L139 – change “imagery” to “images”?
L153 – extra space after 05
L157 – “merges”
L177 – the URL could be moved to a data statement at the end
L194 – What specific hydrographic properties were used to classify the data into these periods? Was this done objectively?
Fig 1 – On a related note, there seem to be breaks between periods A&B and B&C. Are these transitional periods? Why were they not classified into any of the primary subregions?
L204 – How was the aggregating done? Is it assuming that every measurement within that depth range is treated the same? Or, was there some type of vertical averaging?
L210 – I’m curious how large of a contribution is expected from the SMS term? Is this a source of uncertainty?
L226 – How are high and low tidal forcing defined?
L233 – “integrated in DCM at the DCM” – I don’t understand what this wording means. I think you mean integrated in depth within the layer.
Fig 2 – Nice schematic that shows how Chl changes vertically due to internal tides. I think the colors are somewhat ambiguous. It is unclear whether green refers to a) the sum of CHL and SMS or b) just Chl-a from SMS.
L254 – Would a Spearman correlation analysis potentially be more appropriate, considering that I think we would not expect a linear relationship?
L261 – missing space after the period
L268 – I disagree with this… it looks to me like euphotic depth Zeu decreases in the eddy core and increases on the eddy periphery, in a similar pattern to chlorophyll as described in the later text.
L277/279 – 11th / 12th
L277-280 – I’m not convinced there was “expansion” of the eddy. That seems, from the figure, to be an artifact of the cutoff ADT used to define the edge. Please reword to say that (or clarify if I am mistaken).
L288-300 – Following my previous comment, it looks like Zeu and chlorophyll are correlated within the eddy, but that this correlation seems to break down when outside the eddy. I think an explanation of this would be helpful.
L336 – Fig 6 appears to show that salinity was always above 35.5
L333-352 – Nice summary. Much of the discussion on stratification is descriptive, however, and some of the trends mentioned in the text are not clearly apparent on the figure. For example, I do not clearly see more salinity stratification in region A than B, as is mentioned at L342. I think including quantitative information in a few places (i.e., dT/dz and dS/dz) would strengthen this section.
L365-378 – The answers to some of my earlier comments are here. I think, perhaps, this should be moved earlier. I.e., discuss the four periods, then discuss their differences?
L368 – I think better to use 3 significant figures to be consistent here and in other places for the isopycnals
L371 – “a distance was recorded” – odd wording; please rephrase
Fig 8 – Add units on the y-axis (& for Fig 13). Also, it seems strange to me that the spectra are so smooth (or, perhaps I am mistaken). Was any smoothing done to the lines on this plot? If so, I would suggest to just plot the raw spectra.
L398-408 – It seems from the earlier plots that there is strong variability in surface chlorophyll. But this is not clear from Fig 9. Please explain this apparent discrepancy.
L419 – Odd wording. Maybe say “the peak is more pronounced”?
L422 – Was there significant temporal variability in the chl-a profiles? If so, perhaps a proportional criterion for thickness might work better?
L425 - While the relationship between thickness and high/low internal tide activity is very clear, I’m less convinced about the relationships between thickness and chlorophyll within the two IT regimes. It seems from Fig 11 that the high R values are because of peak thickness varies by much more than delta CHL, rather than a large change in chlorophyll. As suggested earlier, I think calculating a Spearman ranked correlation coefficient might be more appropriate, and would tell whether high values of Chla are associated with low values of thickness.
L428-430 – I think this is probably a stronger conclusion than the correlation coefficients (and is more clear in Fig 11). Maybe move earlier in the paragraph?
L447 – I’m a bit confused on how “chlorophyll-a loss” is calculated. From Fig 12, it does not look like the decrease in Chl at the peak is as high as 64%. Please clarify.
L458 – Is there any SMS contribution to the DCM layer? I assume not based on the calculations in this paragraph.
L470-481 – nice summary of the differences between A and B.
L493 – extra space between “ability”
L518-524 –I think it would be useful to try to contextualize this more with existing literature – i.e. are there papers quantifying the impact of NIWs and fronts on chlorophyll. If not, are there any that have quantified physical turbulence parameters relating to these issues? I think having some additional background is important here, considering that the paper is based off of an implied assumption that the entirety of vertical mixing results from internal tides (which may be mostly true, but it would be helpful it this was put in context).
L575 – I’m not sure about the specific journal policy for this special issue regarding whether having data available upon request is acceptable.
Citation: https://doi.org/10.5194/egusphere-2025-2141-RC1
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