Terrestrial Carbon Additions to Zooplankton Prey Influence Juvenile Estuarine Fish Growth

Round 1

Reviewer 1 Report

environments-2208056: “Terrestrial carbon additions to zooplankton prey influence juvenile estuarine fish growth” by Johnson et al.

In this paper, authors examine whether the increased terrestrial resource availability (terrestrial dissolved organic matter-tDOM), such as after inflow events, could positively influence somatic growth of juvenile Australian bass Macquaria novemculeata. With as hypothesis that increased carbon availability from the leachate would result in increased somatic growth of juvenile fish in the higher tDOM treatments. By mimicking under experimental conditions the heterotrophic conditions after periods of influx where algal production is suppressed and terrestrial carbon becomes the dominant energy resource, this study provides potential support to the notion that terrestrial carbon uptake by zooplankton after inflows can positively influence the somatic growth of juvenile fish, providing increased energetic resources. I found the study very interesting with information that would be valuable to the scientific community. However, the study still needs some answers to the questions below and improvements before any consideration.

Many scientific names presented in the text should be italized see L69, L72, L79, L93-94, L373, L376, L398

L93-98 These need to be referenced. Please provide some references.

L105 "following methods of Fielder and Heasman (2011)". Please improve this presentation according to the MDPI guidelines for author citation.

L118 What is meant by"... studies on the species 37."?

L120-126 Authors should briefly explain and motivate why the use of such experimental conditions?

L147 “Australian Bass” = “Australian bass”. See also L304.

L180 What is meant by “Samples were refrigerated (3oC) before”? Please check.

L171-182  Please provide at least one reference.

L185 “Thirty (30) fish” = “Thirty fish”

L186 “… measured with callipers for standard, fork and total length.” Authors should justify the use of these 3 fish length descriptors and define them precisely. In my opinion and with regard to the morphology of the Australian bass, the fork length would be the most precise among these descriptors and seems more robust for studying linear growth.

L197-198 “Survival rates (as percentage) of each treatment and timepoint are detailed in Appendix 1 and were all above 75%.” I suggest that survival be integral to the outcome and take it out of the Appendix 1 (L424-426). An appropriate statistical test can also be applied. Similarly, growth performance such as length gain (%) and weight gain or loss (%) as well as specific growth rate (%/day) are calculated and included in the study results. In this case, the standard length and the total length do not provide more information than the fork length. SL and TL can be therefore deleted.

L226-231"Post hoc testing indicated that bacteria concentrations were significantly higher in the 10 mg/L treatment at 0 hours after the DOC additions compared to the other two treatments (Figure 1, Tukey’s HSD, p < 0.05). After 40 hours, bacterial cell concentrations in the control treatment (0 mg/L) on-growing tanks were less 229 than in the 5 and 10 mg/L treatment on-growing tanks (Figure 1) though non-significantly (Tukey’s HSD, p >0.05)." I can't read these in Figure 1 without reading the main text.

L257-274 See comments already mentioned above.

L296-297 “Our results indicate that terrestrial carbon positively influenced growth of juvenile Australian bass (Figure 2) when additions of DOC to Artemia were at higher concentrations (10 mg/L).” This can be supported by calculating Pearson’s correlation coefficients in results of FL (best length descriptor) for experimental carbon treatments. However, this statement is really confusing because growth involves length and weight performance in fish. Please rephrase. See also L379, L397 and L400.

L384-385 “Numerous studies have demonstrated links between inflows, terrestrial carbon assimilation and growth of juvenile fish in estuaries, attributing this relationship to increased prey resources [8,9,26,67].” This is unclear. Please rephrase.

L397-401 “Our results indicate that terrestrial carbon can positively influence somatic growth of juvenile Australian bass (Macquaria novemculeata) after additions of tDOM to the food web. Additions of tDOM in concentrations of 10 mg/L of DOC to Artemia nauplii ongrowing tanks, led to increased growth of food limited juvenile Australian bass fed these zooplankton.” It is not clear because there has been a loss of weight compared to the initial weight. Please rephrase.

L427-596 Please check the reference list to improve it according to the MDPI guide for citing authors in the list.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

General appraisal:

Very interesting well conducted study almost in conditions to be published. I only advise to add cautionary notes along the document (abstract, introduction, discussion and conclusions) recognizing that freshwater flows may have other type of impacts on estuarine communites and fish organisms and, therefore, the global impact of freshwater flows may be much different from when we are only considering terrestrial organic matter isolated.

 

Specific comments:

Keywords: Revise to avoid repetitions with the title.

Lines 36-37: I think diadromous term is here used incorrectly as most of the fishes that colonize estuaries are really not diadromous (catadromous ou anadromous) but species that use them as nursery areas or occasionally.

Line 118: Correct the typo.

Author Response

We thank the reviewer for their comments.

Keywords have been revised to reduce repetition from title.

Line 36-37 has been revised to reflect the influence of inflows on longitudinal movement of fish, rather than the influence on diadromous species.

Typo on line 118 has been corrected.

These changes have not been tracked within the document due to their brevity. 

Reviewer 3 Report

The manuscript covers a highly interesting topic, the supply of terrestrial carbon inputs to the growth of fish in an estuary. The manuscript is well written and the results novel and interesting. The discussion puts the results well into the context of the state of the art literature. I have only minor comments in the attached pdf and merely suggest that the authors check the manuscript for minor spelling and word use errors before publication.

Comments for author File: Comments.pdf

Author Response

The minor comments in the attached pdf have been addressed in the manuscript. An additional review of the manuscript for minor spelling and errors has been performed as well.