the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2024
Sea ice-associated algae and zooplankton fecal pellets fuel organic particle export in the seasonally ice-covered northwest Labrador Sea
Abstract. Ocean warming and Arctic sea ice decline are expected to affect biological pump efficiency by altering the timing, quantity, quality, and composition of export production. However, the origins and composition of sinking organic matter are still understudied for the oceans generally, and in ice-covered areas especially. Here we use compound-specific isotope analysis (CSIA) of amino acids (AAs) to investigate the sources and composition of exported organic matter from a sediment trap-derived time-series of sinking particles collected at depths of 469 m and 915 m at the edge of Saglek Bank in the northwest Labrador Sea from October 2017 to July 2019. The outer edge of Saglek Bank is located at the confluence of cold and fresh Arctic outflow and relatively warmer Atlantic waters. The area is subject to seasonal sea ice cover and is a biological hotspot for benthic organisms including deep-sea corals and sponges. Sea ice was present for ~50–60 % of the deployment days in both cycles. Phytoplankton blooms at our study site cooccurred with the onset of sea ice melt. Microalgal taxonomy indicated the presence of ice-associated diatoms in the sinking particles during the spring bloom in 2018, confirming that sea ice algae contributed to the organic particle export at our study site. Abundant copepods and copepod nauplii caught in the sediment traps was consistent with a high abundance of copepods in overlying epipelagic waters. Stable carbon isotopes (δ13C) of essential amino acids (EAAs) of the sinking particles revealed a potentially important contribution of sea ice algae as a carbon source at the base of the food web to sinking particles, with only minor modification by microbial resynthesis. Stable nitrogen isotopes (δ15N) of AAs of sinking particles provided independent evidence of the minor bacterial degradation and Bayesian mixing models based on normalized δ15N-AA values revealed dominant contribution of fecal pellets (76–96 %) to the sinking particles. Our study demonstrates the importance of sea ice algae and fecal pellets to the biological pump in the seasonally ice-covered northwest Labrador Sea, with sea ice algae exported either directly via passive sinking or indirectly via zooplankton grazing, with fecal pellets dominating the organic particle fluxes.
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RC1: 'Comment on egusphere-2024-3265', Yuchen Sun, 15 Nov 2024
I have read the manuscript (egusphere-2024-3265) entitled “Sea ice-associated algae and zooplankton fecal pellets fuel organic particle export in the seasonally ice-covered northwest Labrador Sea”. This manuscript reports the results of the δ13C and δ15N analyses of amino acids in sinking particles collected by sediment traps in the seasonally ice-covered arctic sea. Applying several statistical tools, the authors found that 1. sinking particles is mainly originated from fecal pellets; 2. sea ice algae is the main ultimate carbon source of sinking particles; 3. sinking particles experienced only minor microbial reworking. In general, I think this research is well-conducted. Although some similar conclusions have been demonstrated in previous studies in some other polar regions, this study reveals the mechanism and dynamics of organic carbon sink in this specific area, and provides a good reference for future researches. Thus, I recommend the publication of this manuscript in BG after revision. Here are some comments to the authors:
Line 176: Please clearly indicate which method you applied, and add a few descriptions of it. At least list the name of derivatization method here.
Line 205, 208: I think the terms “TPmet” and “TPpro” may be misleading, because Met and Pro are the abbreviations of two amino acids, and someone may think that the TP is calculated from the isotope ratio of these two amino acids. I recommend you to use some other abbreviations, for example, “TPmeta” and “TPproto”.
Table 2: AA-related indices for many samples are not determined. What is the difficulty in obtaining these data? Not enough amount? Some chromatographic problems? Or just didn’t have enough time to analyze all of them?
Figure 5: I think Lys should be EAA, not NEAA. In my understanding, Lys cannot be synthesized by marine consumers.
Figure 6. About the PCA analysis, I recommended you to try adding Lys in the PCA model (if you agree that it is an EAA). It may help the classification of different end members, because it is known that Lys has different synthetic pathways in plants and bacteria. Also, I feel that you can discuss a bit more about the PCA results in the text. It seems that Thr and Leu are two informative amino acids in terms of distinguishing sea ice algae and pelagic algae. Do you think we can propose a new indicator using these two AAs to distinguish the contribution from sea ice algae and pelagic algae?
About the LDA results, your sea ice algae and pelagic algae data look like in the middle of microalgae and heterotrophic bacteria, instead of showing “pure algae-like” signal. It makes your sinking particles look even “more like” algae than your algae samples. I think it will be interesting if you use your sea ice algae and pelagic algae data as the training data to construct a new LDA model, and put your sediment trap data into it. It may provide us a better semi-quantitative estimation of the relative contribution from sea ice algae and pelagic algae.
Figure 7, 8: I recommend you to add the word “Microbially” before “Degraded OM”.
Line 465: Most lipids and carbohydrates don’t contain N, so it sounds strange to me to say that they are responsible for the bulk δ15N values. I prefer to list some other N-containing compounds here, such as heterocyclic molecules (including nucleotides and pigments), and amino sugars.
Line 472: I don’t think “Phe does not undergo deamination reactions during heterotrophic metabolism”. A more accurate expression should be like “deamination/transamination reaction is not the first and ‘rate-limiting’ step in the ‘dominant’ metabolic pathway of Phe in animals”.
Line 489-491: Could you explain a little about the discrepancy between ΣV values and the Bayesian mixing model using Phe-normalized δ15N of Ala and Thr? Because there are several high ΣV values for sediment trap samples which are comparable to degraded OM, but we don’t see the same results in the output of the Bayesian mixing model.
Line 523: While I understand it is necessary to exclude the zooplankton end-member in the mixing model because zooplanktons were removed from the samples before analysis, I wonder what the relative contribution from the zooplankton biomass in the N fraction of sinking particles will be.
Line 531-532: Because copepods are the only dominant type of zooplankton in the area, do you think that using the end-member containing a much larger variety of species will cause a larger uncertainty/error in the estimation of relative N contributions?
Citation: https://doi.org/10.5194/egusphere-2024-3265-RC1 -
RC2: 'Comment on egusphere-2024-3265', Anonymous Referee #2, 25 Nov 2024
In this paper, the effects of sea ice algae and fecal pellets on carbon and nitrogen cycling processes in the Labrador Sea are investigated using an integrated isotope approach, including amino acid stable isotope methods. Overall this paper did a good job of analyzing the bulk and amino acid stable isotope data and obtaining very meaningful results of the analysis. I still have some questions to discuss with the authors at the level of analytical methods.
1. The authors used different TDF and beta values cited in the literature as the reference for trophic level calculations for different animal groups, and the system of calculations included not only Glx-Phe but also Ala-Phe. In the same ecosystem, the use of different criteria to calculate trophic levels may lead to biased conclusions, and would it not be better to use a unified method for calculating trophic levels in the same region and ecosystem? In addition, the food chain structure in this study is not complex.
2. Why estimate the proportional contributions of three end-members by using Phe-normalized δ15N-Ala and δ15N-Thr? I don't particularly understand the actual meaning of Phe-normalized δ15N-Ala and δ15N-Thr. Generally Thr is not a trophic amino acid nor a source amino acid, what is the significance of using Thr here?
3. I don't really agree with the estimation of nitrogen sources in Figure 8, which is not in line with the general understanding. And unlike carbon, nitrogen usually undergoes a very complex mineralization and denitrification process that leads to severe isotopic fractionation. Also, using End-menber data from the literature instead of actually measuring it yourself may lead to calculation errors.
4. Overall, some of the data in this paper does not support the conclusions very strongly, and some of the less definitive conclusions considered can be left out of the discussion. For example, the analysis of organic composition in 4.2 is merely inferential and does not require a great deal of space for extensive discussion of a less than robust conclusion.
Others:
Line 143-146: What is the depth of collecting the zooplankton with a multi-net plankton sampler? And '200-0 m layer' means from 200 to 0 m or at the depth of 200m?
Line 257: It will be better to add ' 2017 ' to the left side of the abscissa in Figure 2, which is consistent with the following figure. The same applies to Figure 8.
Line 309-311: I don’t fully understand the sentence. The δ13C-EAA patterns of sinking particles were more similar to sea ice algae than to pelagic algae? Maybe you can express it in detail.
Line 627: Please check your reference format again. For example, the name of the journal (e.g., Annual review of marine science, Limnology and oceanography, Global change biology) should be capitalized, consistent with other references in this article. And the format of cited reference in line 672-673 is wrong. You need to modify and check again.Citation: https://doi.org/10.5194/egusphere-2024-3265-RC2
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