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
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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
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