the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 Oct 2023
Phytoplankton Retention Mechanisms in Estuaries: A Case Study of the Elbe Estuary
Laurin Steidle
Ross Vennell
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Laurin Steidle and Ross Vennell
Status: open (until 30 Nov 2023)
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RC1: 'Comment on egusphere-2023-2231', Anonymous Referee #1, 02 Nov 2023
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General Comments:
The article predominantly emphasizes numerical aspects, as evident from their consistent use of the term 'particles' throughout the text, as opposed to 'phytoplankton' as stated in the title. I would advise the authors to be cautious about this choice of terminology. If their intention is to underscore the biological aspects of their research, the paper requires further clarification in three main areas:
- Terminology: Instead of referring to 'particles,' using 'phytoplankton cells' would better align with the biological focus of their study.
- Model Validation: Including aspects related to model validation, such as capturing the seasonal cycle, the duration of bloom events, the spatial distribution of blooms in relation to distance from the estuary, and other relevant parameters, would enhance the biological relevance of their research and provide a more comprehensive understanding of the dynamics at play.
- The aspect that phytoplankton cells survive in the dry grid cell (without water) needs to be justified. Otherwise, this should be corrected in the method, post-processing (without redoing all the tests) – these cells can be excluded from the final count, and further conclusions should be corrected.
Another crucial aspect to consider is that the authors apply the physical outputs derived from another model to drive the passive movement of their particles. In my view, it would be valuable to compare their retention areas and the vertical distribution (represented as violins) with the physical aspects governing lateral movement. Specifically, exploring the impact of tides on distance from the estuary, tide amplitude, and current structure could provide deeper insights into why the oldest cells tend to remain close to the coast.
By examining these additional physical factors in relation to the behavior of phytoplankton cells, the study could offer a more holistic perspective on the dynamics of the ecosystem and help elucidate the mechanisms that influence cell retention and distribution. This approach would enhance the overall quality and relevance of the research.
In general, it is clear that the authors are modelers. All biological parts should be reviewed by a specialist in phytoplankton biology. In my opinion, the paper contains some interesting novel approaches, such as considering a variety of different phytoplankton species with their varying reproduction rates and vertical migrations together. However, results need further exploration, focusing on the physics of the current, and justifying questionable model parameterization choices. I believe the authors should put more effort into this work for publication. Thus, I propose a major revision.
Specific Comments:
- Page 2, lines 30-35: It appears that the author may be conflating two distinct diel migration behaviors observed in planktonic species. One type of diel migration is exhibited by phytoplankton, which is primarily driven by the availability of sunlight for photosynthesis. This behavior is solely dependent on the sun's position in the sky, as phytoplankton are primary producers that rely on light for their metabolic processes. On the other hand, carnivorous planktonic species, like certain zooplankton and dinoflagellates, exhibit a different diel migration pattern. Their vertical movements are not directly driven by the sun but are instead motivated by the distribution of their prey, mainly phytoplankton, which, in turn, is influenced by sunlight-driven photosynthesis. These species engage in diel migration as a survival strategy, often to avoid predators or to exploit variations in food availability. In this context, it is essential to emphasize the distinction between these two types of diel migration patterns to provide a more accurate and biologically informed account of the behaviors of planktonic organisms. Recognizing the ecological drivers behind these migrations is crucial for a comprehensive understanding of aquatic ecosystems.
- Page 2, line 44: Please correct the reference "St. Lawrence Estuary while() (Kimmerer et al., 2014)."
- Page 4, lines 83-84: What is the spatial resolution of the three-dimensional unstructured grid used to represent the Elbe estuary in this model, and how does it vary within the dataset?
- Page 5 lines 107-110: The statement, "A particle starts its life with a light budget of 28 days, and each minute below 1m reduces this budget by one minute, while the opposite applies when they are above 1m. Children of light-limited parents inherit the remaining light budget of their parents," should be supported by relevant laboratory studies or evidence. Additionally, the terminology used, such as "children" and "parents" for phytoplankton, might be confusing and should be rephrased for clarity.
- Page 5 lines 118-122: The statement that "particles become stranded when the current grid cell becomes dry, and once this cell is rewetted, all stranded particles resuspend and are able to move again" should be justified based on ecological principles and the behavior of phytoplankton. It's important to explain the reasoning behind this choice, as phytoplankton typically cannot survive when completely dry.
- Page 6, line 150: Please provide an explanation for the choice of population doubling times in idealized conditions ranging from 40 to 404 days. This choice should be based on scientific rationale and may require further clarification.
- Page 7, Section "Results": Before analyzing the retention success, it's advisable to perform some form of model validation. Consider whether your model or specific scenarios with their parameters successfully reproduce the seasonal cycle of phytoplankton, including the duration of bloom events and the number of particles over distance from the North Sea. Model validation is crucial to ensure the reliability of your results.
- Page 7, line 171: Please clarify the intention behind looking at the state of phytoplankton after one year in terms of estimating areas where they "successfully retain."
- Page 7, line 175: In the statement, "to successfully retain," please provide a clear definition or criteria for what constitutes successful retention in the context of your study.
- Page 7, line 177: When stating "approximately 3 months," consider providing supporting evidence or references to confirm the accuracy of this time frame based on relevant observations or studies.
- Page 9, Figure 4: The positive depths shown in Figure 4 may be related to tidal oscillations. It would be valuable to describe the tide variabilities or free surface level variability in the site section to help explain these depth variations. Page 9, line 196: Please clarify which tests or scenarios were chosen to be plotted on Figure 5. Explain whether this is an average over all the tests conducted and provide justification for this choice.
- Page 9, line 195: The statement regarding the parameterization of drifting particles as phytoplankton and their tendency to strand near riverbanks should be approached with caution. Phytoplankton typically cannot survive away from water. To provide a more accurate assessment of phytoplankton behavior, consider excluding particles that become stranded in dry grid cells and correlating their behavior with currents over the coasts and tides, as these factors are usually lower near the coasts, favoring retention.
- Page 10, Figure 5: If possible, mark the important sites labeled as "a," "b," "c," etc., on Figure 1 to provide a clearer reference for readers.
- Page 10, lines 210-220: Please cite relevant observations or studies where phytoplankton survival without water is documented to support the statement made in this section. If it cannot be supported, all conclusions about retention in tidal flats should be rewritten.
- Conclusion section is very poor and need to be revised.
Kindly note that the combined text incorporates both the general and specific comments for improved clarity.
Citation: https://doi.org/10.5194/egusphere-2023-2231-RC1 -
RC2: 'Comment on egusphere-2023-2231', Anonymous Referee #2, 03 Nov 2023
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This paper studies the retention mechanisms of phytoplankton in an estuarine environment. The paper is clearly written and the results are of much interest. However, since this work is strongly based in model simulations, I miss a lot of information about how the numerical simulations have been performed. Also, there is a lack of sensitivity analysis of the results when the parameters are changed.
Thus, before the paper can be accepted the author should provide details (probably including appendixes):
- On the numerical velocity fields: boundary conditions, resolution, vertical components, explicit bathymetry, etc...
- On the Lagrangian transport model: interpolation schemes, use of eddy diffusion?, sticking of particles to land, etc... You may also compute other Lagrangian quantities to show like exit times, retention times, or accumulation zones (vertical and horizontal).
- On the "population dynamics": how particles divide, die (is it a Gillespie simulation)? number of particles, density, etc...
- Provide a sensitivity analysis of the many parameters of the model, in particular those concerning phytoplankton demography like mortality, reproduction rates, etc...
Citation: https://doi.org/10.5194/egusphere-2023-2231-RC2
Laurin Steidle and Ross Vennell
Laurin Steidle and Ross Vennell
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