Diversity and Physiological Tolerance of Native and Invasive Jellyfish/Ctenophores along the Extreme Salinity Gradient of the Baltic Sea

Round 1

Reviewer 1 Report

The paper, "Diversity and physiological tolerance of native and invasive jellyfish/ctenophores along the extreme salinity gradient of the Baltic Sea", is an excellent and thorough overview of gelatinous zooplankton populations in the Baltic Sea. Due to the potentially increasing impact of jellyfish in ecosystems worldwide, this review is a timely and important contribution to the field of marine ecology. The paper is well-written, and I have no suggestions or comments. I thoroughly enjoyed reading it!

Author Response

Response: Thank you very much for the evaluation and positive comments.

Reviewer 2 Report

Diversity #1068685: Diversity and physiological tolerance of native and invasive jellyfish/ctenophores along the extreme salinity gradient of the Baltic Sea

General comments

The review by Jaspers, Bezio and Hinrichsen summarizes the current state of knowledge on the diversity and distributions of the jellyfish and ctenophore species in the Baltic Sea, and how they are shaped by the unique environmental factors characterizing this marginal sea. This review is timely and important, as with the intensification of climate change and other anthropogenic-driven environmental impacts, invasive gelatinous species that are evidently increasing in some regions such as the Baltic Sea can lead to drastic changes in ecosystem functioning and services. Here, Jaspers et al. provide comprehensive information on species status, biological background, and predictions, along with identification of current knowledge gaps. They conclude that jellyfish and ctenophores should be included into ongoing Baltic Sea monitoring activities to elucidate their impact on fish communities. This is a strong and important message to scientists, policy-makers and other stock holders.

The information included in this review is comprehensive, balanced, and the amount of citations is sufficient. The review is well written, and the flow makes it easy to read from start to end. Nevertheless, I had few comments, listed here below.

1) I think that such a review paper that focuses much on the species distributions throughout the Baltic Sea, would benefit much from distribution maps instead of text only. If possible, density/abundance data can be visualized on top of such maps as well.

2) In some cases, density estimates are missing or limited to one area or one year (eg, A. aurita). If possible, please provide density data as well as biomass. If no data is available, please add this to the list of knowledge gaps.

3) Implications discussed are mainly foodweb implications (to fisheries) and others are briefly mentioned (stinging bathers, fishing gear loss, clogging intake pipes). However, other ecosystem effects should be mentioned as well. This include oxygen depletion (eg, Møller, L.F. and Riisgård, H.U., 2007. Population dynamics, growth and predation impact of the common jellyfish Aurelia aurita and two hydromedusae, Sarsia tubulosa and Aequorea vitrina, in Limfjorden (Denmark). Marine Ecology Progress Series, 346, pp.153-165) or change in nutrient dynamics (West, E.J., Welsh, D.T. and Pitt, K.A., 2008. Influence of decomposing jellyfish on the sediment oxygen demand and nutrient dynamics. In Jellyfish Blooms: Causes, Consequences, and Recent Advances (pp. 151-160)).

Minor comments

• In the abstract and throughout the text – both jellyfish and ctenophores are referred to as “grazers”. The biological definition of grazing involves herbivory, thus is not always applicable for jellyfish, and mostly not for ctenophores. I would suggest to replace “grazers” with “consumers” or “predators” (when applicable).
• Throughout the manuscript, species names are often not presented in italics.
• In paragraph 1.2 (Physical environment of the Baltic Sea), when referring to upwelling in Eckernförde and Kiel Bay during autumn, the following publications can be used instead of personal observation (Saderne, V., Fietzek, P. and Herman, P.M.J., 2013. Extreme variations of pCO 2 and pH in a macrophyte meadow of the Baltic Sea in summer: evidence of the effect of photosynthesis and local upwelling. PloS one, 8(4), p.e62689. Lehmann, A. and Myrberg, K., 2008. Upwelling in the Baltic Sea—a review. Journal of Marine Systems, 74, pp.S3-S12).
• In paragraph 1.2 (p. 4), should be “Baltic” instead of “Belt Sea”.
• Same paragraph, “synergistically” instead of “synergistic”.

Author Response

General comments: The review by Jaspers, Bezio and Hinrichsen summarizes the current state of knowledge on the diversity and distributions of the jellyfish and ctenophore species in the Baltic Sea, and how they are shaped by the unique environmental factors characterizing this marginal sea. This review is timely and important, as with the intensification of climate change and other anthropogenic-driven environmental impacts, invasive gelatinous species that are evidently increasing in some regions such as the Baltic Sea can lead to drastic changes in ecosystem functioning and services. Here, Jaspers et al. provide comprehensive information on species status, biological background, and predictions, along with identification of current knowledge gaps. They conclude that jellyfish and ctenophores should be included into ongoing Baltic Sea monitoring activities to elucidate their impact on fish communities. This is a strong and important message to scientists, policy-makers and other stock holders.

The information included in this review is comprehensive, balanced, and the amount of citations is sufficient. The review is well written, and the flow makes it easy to read from start to end. Nevertheless, I had few comments, listed here below.

1) I think that such a review paper that focuses much on the species distributions throughout the Baltic Sea, would benefit much from distribution maps instead of text only. If possible, density/abundance data can be visualized on top of such maps as well.

Reply: Thank you very much for the positive evaluation and pointing out that distribution maps would be a useful addition. We fully agree, however, abundance data presentations are currently hampered due to lack of investigations which cover the entire Baltic Sea, especially during recent decades and covering the different seasons. We include this as knowledge gap in the revised version.

2) In some cases, density estimates are missing or limited to one area or one year (eg, A. aurita). If possible, please provide density data as well as biomass. If no data is available, please add this to the list of knowledge gaps.

Reply: Thank you for pointing out that abundance and biomass data would be very useful. We fully agree, however, this needs sophisticated investigations where the current data knowledge is too weak to comprehensively and conclusively include this. We have highlight this knowledge gap in the conclusion section page 18/19, line 581-585 and page 5, line 138/139.

3) Implications discussed are mainly foodweb implications (to fisheries) and others are briefly mentioned (stinging bathers, fishing gear loss, clogging intake pipes). However, other ecosystem effects should be mentioned as well. This include oxygen depletion (eg, Møller, L.F. and Riisgård, H.U., 2007. Population dynamics, growth and predation impact of the common jellyfish Aurelia aurita and two hydromedusae, Sarsia tubulosa and Aequorea vitrina, in Limfjorden (Denmark). Marine Ecology Progress Series, 346, pp.153-165) or change in nutrient dynamics (West, E.J., Welsh, D.T. and Pitt, K.A., 2008. Influence of decomposing jellyfish on the sediment oxygen demand and nutrient dynamics. In Jellyfish Blooms: Causes, Consequences, and Recent Advances (pp. 151-160)).

Reply: Thank you very much for these suggestions which are now included on page 5, line 141/142.

Minor comments

• In the abstract and throughout the text – both jellyfish and ctenophores are referred to as “grazers”. The biological definition of grazing involves herbivory, thus is not always applicable for jellyfish, and mostly not for ctenophores. I would suggest to replace “grazers” with “consumers” or “predators” (when applicable).

Reply: Thank you for pointing this out, we have replaced grazer (only once mentioned in line 164) with predator: “novel predators imposing a significant grazing impact on zooplankton in …”.

• Throughout the manuscript, species names are often not presented in italics.

Reply: Thanks, the ms is now spell checked for it, the abstract was from the online submission system, where italics were not copied in. sorry for this!

• In paragraph 1.2 (Physical environment of the Baltic Sea), when referring to upwelling in Eckernförde and Kiel Bay during autumn, the following publications can be used instead of personal observation (Saderne, V., Fietzek, P. and Herman, P.M.J., 2013. Extreme variations of pCO 2 and pH in a macrophyte meadow of the Baltic Sea in summer: evidence of the effect of photosynthesis and local upwelling. PloS one, 8(4), p.e62689. Lehmann, A. and Myrberg, K., 2008. Upwelling in the Baltic Sea—a review. Journal of Marine Systems, 74, pp.S3-S12).

Reply: Thank you for pointing this out, we have removed pers. com. and included the two references on page 4 line 111.

• In paragraph 1.2 (p. 4), should be “Baltic” instead of “Belt Sea”.

Reply: Changed to Baltic Sea – line 125.

• Same paragraph, “synergistically” instead of “synergistic”.

Reply: Changed to synergistically.

Reviewer 3 Report

Dear Authors,

In my opinion the work represent an important contribution to understand gelatinous carnivorous role in an important marine ecosystem as Baltic Sea. The work is well written and organised. Even we felt that is more regional, it describe an important community in one of the largest brackish-water systems of the world, which represent  a matter of concern due to the higher abundance of non-indigenous gelatinous zooplankton species. Particularly worrying are their high grazing impact on ecosystem level. I strongly recommend the publication of this work in Diversity. 

Some minor corrections: pay attention to some species names that are not in italic.

Author Response

Response: Thank you very much for the positive evaluation, we have checked that all species names are in italic, the abstract was actually taken from the online submission system, the word document is now 100% checked and ok. Thanks for pointing this out! The MS has now also been proof read by a native speaker.