Drivers of the spatial phytoplankton gradient in estuarine–coastal systems: generic implications of a case study in a Dutch tidal bay

Jiang, Long; Gerkema, Theo; Kromkamp, Jacco C.; van der Wal, Daphne; Carrasco De La Cruz, Pedro Manuel; Soetaert, Karline

doi:https://doi.org/10.5194/bg-17-4135-2020

Articles | Volume 17, issue 16

https://doi.org/10.5194/bg-17-4135-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-17-4135-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 16

Research article

|

17 Aug 2020

Research article |

| 17 Aug 2020

Drivers of the spatial phytoplankton gradient in estuarine–coastal systems: generic implications of a case study in a Dutch tidal bay

Long Jiang, Theo Gerkema, Jacco C. Kromkamp, Daphne van der Wal, Pedro Manuel Carrasco De La Cruz, and Karline Soetaert

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Final revised paper (published on 17 Aug 2020)
Preprint (discussion started on 03 Mar 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Jiang et al.', Tom Fisher, 06 Mar 2020
- AC1: 'Response to Referee 1', Long Jiang, 11 May 2020
RC3: 'BG-2020-40_review', Nicole Millette, 11 Mar 2020
- AC2: 'Response to Referee 2', Long Jiang, 11 May 2020
RC4: 'Review for Long Jiang et al., BG-2020-', J. Blake Clark, 07 Apr 2020
- AC3: 'Response to Referee 3', Long Jiang, 11 May 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (15 May 2020) by Carol Robinson

AR by Long Jiang on behalf of the Authors (16 May 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 May 2020) by Carol Robinson

RR by Nicole Millette (05 Jun 2020)

Suggestions for revision or reasons for rejection

Overall, the authors put thought into my previous comments and either incorporated it into their paper or gave a detailed response. I do have some minor comments still I would like them to consider.

Comments
• Page 4, line 13-15: At least provide a reference for where people can get more detailed information about the NIOZ sample collection and analysis.
• Page 4, line 21-22: As with the NIOZ sampling data, at least provide a reference for the method that was used to estimate primary production with 14C. However, since these experiments were done for this project, or at least this is the first time this data is being published, I would suggest that more information is needed than one sentence. What containers were used, what volume of water, how much water was used in analysis, how was analysis done, etc. This was information I was left wondering about.
• Figure 13: The authors have made it clear that they still want to include this figure in the paper, and none of the information that they present is wrong so I cannot say that it should not published. However, I would like to explain my concerns. I primarily have an issue with the figure and the naming of different types of [chla] spatial gradients, not the text of the synthesis section. I do think this figure is really interesting and useful, but to include it at the end of a paper about a single estuary is a disservice to the figure. What the authors have is the basis for a potentially interesting review paper on [chla] spatial patterns in estuaries-coastal systems and causes for these patterns. I would argue that it is not appropriate to introduce a new scientific categorization at the end of an original research paper. It is fine to compare spatial patterns in Oosterschelde and what is driving these patterns to other systems, as the authors do in the text, as a way to demonstrate the wider context on these findings. However, the authors decide to go a step further and classify the different types of spatial gradients (as type I-V). My opinion is that if you are presenting a new categorization for scientists to use, then more effort is needed to explain and demonstrate each different “type”. Most of the “types” described here are in relation to Oosterschelde, and not as their own type. Again though, I acknowledge that this is just my opinion and I cannot challenge any of the information they present in the figure. I just think this figure and the categorization is a step too far for this paper.

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RR by Koenraad Muylaert (16 Jun 2020)

Suggestions for revision or reasons for rejection

I agree with the other reviewers that the paper is well-written and the results are convincing. I have no major comments. I only have a few remarks that the authors might take into consideration.

The main conclusion of this study is that the gradient in decreasing chlorophyll a concentration along the longitudinal axis of the embayment is caused by input of phytoplankton from the sea and grazing pressure exerted by bivalves in the inner bay. A similar conclusion was previously reached in another paper by Jiang et al that explored the fate of seston in the Oosterschelde bay. The difference between this and the previous paper is that this paper includes a primary production function (backed up by in situ primary production measurements). This begs the question: ‘what is the relative importance of in situ primary production in the bay versus import from productive coastal waters’?

Fig 10 seems to suggest that import of phytoplankton from coastal waters is of little importance for chlorophyll a concentrations in the Oosterschelde bay, except for the station close to the mouth. Fig 12, however, gives a different impression and suggests that chlorophyll a concentration is almost halved along most of the bay when import is turned off. Or is import from coastal waters only important during the spring bloom? Some quantification of the relative importance of import versus local production in the bay during the course of the year would therefore be valuable.

Presenting only chlorophyll data along a longitudinal gradient of an estuarine system can be misleading because the volume of the system generally decreases exponentially from the seaward to the landward end. Therefore, what is happening near the mouth of the bay is quantitatively much more important than what is happening near the head of the bay.

It is hard to get an idea of the amount of water that is exchanged between the bay and coastal waters. The tidal prism is given, but this value is of little use if the total volume of the bay is not known. Would it be possible to add this information?

The bivalves seem to exert a very strong grazing pressure on phytoplankton. At least part of the bivalve population in the Oosterschelde system is cultivated. It would be interesting to know whether this strong top-down control is the result of commercial bivalve farming of of natural populations of bivalves. Which bivalves contribute most to top-down control of phytoplankton?

I agree with the other reviewers that great care should be taken when a fixed C:N ratio and chlorophyll:C ratio are used, especially in a N-limited system. These ratios can vary strongly between spring and summer. In spring, low light intensity and high N availability will result in a high chlorophyll:C ratio, while in summer high light intensity and N limitation will result in a low chlorophyll:C ratio.

The model is only very summarily described in the methods section. It would be interesting to add some general information on how N, light and temperature effects on primary production were modeled, and how grazing and nutrient cycling by bivalves were modeled.

I agree with the previous reviewers that the remote sensing data add very little to the conclusion of the study. I think the statement in the abstract ‘satellite data substantiate the roles of benthic grazing and tidal import’ is not supported by the spatial distribution of chlorophyll a concentration shown in the satellite image.

How important is nutrient remineralisation by bivalves for supporting primary production in the bay? That nutrient recycling by bivalves is important is briefly mentioned on p. 9 l. 27, but it is not quantified, nor supported by data.

The “synthesis” that gives a kind of classification of estuaries and bays is very interesting but it too short to be of much value. This could be very interesting if it were elaborated in a review paper that includes an in-depth discussion on the relative importance of different processes that control phytoplankton in estuaries. The “synthesis” also includes much information that is not relevant for this study. It is a classification of estuaries while the Oosterschelde is not really an estuary but a coastal bay. Because so much of the discussion deals with other systems, the comparison with similar systems where phytoplankton distribution is governed by bivalve filtration is too brief. This phenomenon has been reported in other estuaries and many of the other systems where bivalves control spatial distribution of phytoplankton are not mentioned (e.g. Willapa Bay and San Francisco Bay). It would be valuable to have more in-depth discussion of the conditions that lead to such a situation of strong top-down control by bivalves.

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RR by Tom Fisher (16 Jun 2020)

ED: Publish subject to minor revisions (review by editor) (18 Jun 2020) by Carol Robinson

AR by Long Jiang on behalf of the Authors (30 Jun 2020) Author's response Manuscript

ED: Publish as is (04 Jul 2020) by Carol Robinson

AR by Long Jiang on behalf of the Authors (08 Jul 2020)

Short summary

A seaward increasing chlorophyll-a gradient is observed during the spring bloom in a Dutch tidal bay. Biophysical model runs indicate the roles of bivalve grazing and tidal import in shaping the gradient. Five common spatial phytoplankton patterns are summarized in global estuarine–coastal ecosystems: seaward increasing, seaward decreasing, concave with a chlorophyll maximum, weak spatial gradients, and irregular patterns.