Mangrove Propagule Dispersal in a Shallow and Narrow Coastal Lagoon: A Simulation-Based Assessment of the Setiu Wetlands, Malaysia

Round 1

Reviewer 1 Report

“Mangrove Propagule Dispersal in a Shallow and Narrow Coastal Lagoon: A Simulation Based Assessment for Setiu Wetlands, Malaysia (By Z. Zainol et al.)”

   This study deals the movements of Rhyzophora mucronata propagules after detachments from mother trees in the lagoon of Setiu Wetland, Malaysia. Authors use a simulation to pursue the movements after release. A mark-recapture experiment is also used in the field to validate the simulation results. The manuscript is well-written and the obtained results give an important ecological information for mangrove studies. However, I have some comments and queries;

1.       I believe that the results of mark-recapture experiment actually conducted in fields is the most powerful method. Please describe concisely on this experiment: for examples, on the release timing (at high-tide or low-tide? at spring tide or neap tide? It decides the initial movement), on the size (length/weight) effects to trajectory if you have any data, and on some adequate observation in field.

2.       For the results of mark-recapture experiment, a similar graph to Figure 4 will be necessary.

3.       Each propagule has different tracking from others in the simulation. Please show the given factors to the simulation in “2.3 Numerical modelling”. I could not catch it from the manuscript, sorry.

4.       In Figure 1, the scale of distance is necessary to write in the graph.

5.       How large is the annual freshwater discharge of the Ular River? Please write if you have any information.

6.       What is the concrete meaning of “a settling velocity (LINE 168)”? Need explanation.

7.       In Figure 2 (a), the position of blue letters of “First quarter”, “Full moon”, “Third quarter”, “New moon” … above the graph are seemingly a bit strange. If the position written is right, the tidal gap (water current speed also) might become small at spring tide and large at neap tide seen from the present Figure 2. Please check again.

8.       In Figures 3 and 5, the unit of velocity and a small caption will be necessary for the colored bars shown beside the figure.

9.       In Figure 6, many points are overlapped between (a) and (b). As the graph (b) is a portion of (a), only the graph (a) might be enough to show to readers. Also, what does the box caption “Time flooded” means? Author must explain it relating to the simulation methods.

10.   I agree with you: VSDD and SDD are majority and LDD is rare seeing from the results of simulation. If this simulation result is confirmed, the phrase “LDD are rather uncertain” will be a bit too weak in the expression (LINE 32).

11.   With the Discussion (around LINE 287), if you select other places for the point of propagule release, what will happen for the trajectory of propagules? Now, you select the secluded point S2 in the inlet of northern sector for the point. The result of simulation may be much different when you release propagules, for an example, at S1. Please mention why you select S2 for the point (relating to the Materials and Method around LINE 130 too).

Author Response

To begin with, we would like to thank you for your constructive comments and suggestions as well as your willingness to review this manuscript. We consider each of your comments and suggestion in the revision process. We appreciate all the comments and changes have been made accordingly, otherwise strong justifications are provided to retain certain arguments. Enclosed is a point-by-point response to your comments and suggestion.

Author Response File: Author Response.pdf

Reviewer 2 Report

Review Mangrove Propagule Dispersal.....Forest 1849563 

By Zuraini Zainol et al. 

The present manuscript describes the results of a tracking simulation model and a mark recapture experiment to identify the dispersal behavior of Rhizophora mucronata propagules in an enclosed coastal lagoon influenced by fresh inputs and tidal forcing within a timespan 30-40 days. 

This work is valuable to understand the dispersal mechanisms of mangrove propagules within an area of extensive mangrove stands and subject to pressure from human activities that are modifying the natural hydrodynamic, decreasing the freshwater input and inducing sediment inputs from tidal forcing. 

The authors proposed to investigate three hypotheses: 

1. The landscape configuration of the lagoon limits the propagule dispersal to very short to short distances 

2. Most of the propagules are entrapped by the vegetation due to calm water movement 

3. Increased upstream discharge could induce long distance dispersal. 

The first two hypotheses were clearly tested based on the data from both, the tracking simulation model, and the mark recapture experiment. 

Hypothesis 3 is a mere conjecture since no data was provided to test its validity. Therefore, I would not consider this statement as a testable hypothesis based on the data used. Instead, I recommend approaching this statement within the discussion section or providing data that allows to validate it. 

Specific comments and recommendations 

Introduction 

Line 57 …..interaction between propagule traits, surface water conditions.  

Also considered  “ forest type and landscape configuration” 

Materials and Methods 

Study area 

Line 101-103 

Here I found key information on forest type of mangroves within the lagoon: The fact that there is a clear patterns of forest type, where norther section is dominated by basin mangroves, the central section by fringing mangroves and the southern section by riverine mangroves has implications for the type of processes and mechanisms of propagule dispersal. Hence, the fact that the study was carried out in the norther section where basin forest dominates, the scope of the experiment is limited to those specific conditions where VSDD to SDD is a naturally occurring mechanism. 

I recommend considering this aspect in the discussion section 

Line 131-134 

Justify why one week of water current data describes the 40-day period of tracking simulation. 

Results 

Hydrodynamic characteristics of the lagoon 

Figure 3 a and c shows an opposed current direction to b and d, which is not explicitly described in the results section. This opposite current direction, does not have implications for the distance dispersal data? 

Under low tide conditions, either neap or spring, the dispersion of propagules will be limited or completely prevented from the north to the central and south section. However, I do not see this aspect either clearly described in the results sections or in the discussion section. 

Line 187-188. 

Comment: Except for spring high tide. 

Figure 5 is a static representation of simulation results, although what would be the interaction of the bidirectional current pattern and the dispersion of propagules form the north considering different time periods and lagoon sections? 

Discussion and conclusion section 

There are several conjectures and assumptions that can not be supported by the data the authors have gathered. 

1. The effect of fish cages was not considered in the tracking simulation model, nor in the mark recapture experiment. For example, where are the cages located? 

2. Vertical mixing in the shallow and narrow water channels. Vertical mixing is forced by wind and tide, and it is a normal condition in shallow water. How, the gathered data indicate that longer residence time is due to vertical mixing when the main stated barrier was prop roots and pneumatophores of the various species present in the area? Especially when the experiment was conducted in the northern section of the lagoon where basin forest type dominates the landscape? 

3. Is there any data on salinity changes in Setiu Lagoon that adds support to the hydrodynamic changes? Evidence of increased salinity in the norther section that can be linked to the decrease in freshwater input? 

4. 40 day period for the tracking simulation model and the mark recapture experiment under the described conditions of hydrodynamics favor VSDD and SDD, what about extreme conditions of high freshwater inputs due to unusual rainy conditions? Could it be this type of mechanism that favors more LDD? 

Final remarks 

I acknowledge that my expertise is not in modeling or physical oceanography, so I hope the authors find useful my comments around the modeling and the physics of coastal waters. Although, I considered that there is room for improving the ecological interpretation of the data.

Author Response

First of all, we would like to thank you for your sincere comments and suggestions of this manuscript. We appreciate all the comments from reviewer. Changes have been made accordingly, otherwise strong justifications are provided to retain certain arguments. Enclosed is a point-by-point response for your comments and suggestion.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Final review 

Typos 

Line 274. Characteristics 

Line 295, considering using other word than “better”,   (further / wider) 

Line 313 chage “are” for “were” 

Line 337 …..eddies that ”are” known to occur 

Line 346-348 In addition, obstacles like fish cages are practically affecting....... 

The above conclusion is not based on the actual mark-recapture experiment data or the tracking simulation model. Although reasonable it is just a conjecture.  

Line 348-349 The longer residence time of propagules is attributable to the vertical mixing........ 

There is a straightforward explanation of longer residence time of propagules, as was mentioned in the results section. Line 245-246 “the majority of recapture propagules landed in dense Rhizophora prop roots or Avicennia pneumatophores, favor by a shallow water column and weak currents. Water current is modulated by external forcing such as tides and river inputs, as well as depth of the water column and any structures that attenuate water movement. All these processes are in a continuous feedback and influence each other. How is this link to vertical mixing? I do not find it clear. 

Author Response

Dear Reviewer 2,

Thank you for your comments to ensure that we produce a high quality manuscript. We appreciate your time to review this manuscript for the second time. Here, we enclosed the response to your comments for reference.

Thank you.

Author Response File: Author Response.pdf