the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Contrasting seasonal patterns in particle aggregation and DOM transformation in a sub-Arctic fjord
Abstract. Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.
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Status: open (until 29 Jul 2024)
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RC1: 'Comment on egusphere-2024-1314', Anonymous Referee #1, 23 Jul 2024
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The authors collected samples in a sub-Arctic fjord in different seasons and conducted incubation experiments to investigate changes in DOM and POM concentrations, DOM compositions and other metrics before and after incubations. While the study is interesting, there are two major concerns that need to be addressed before it can be considered for publication.
- The authors attempted to interpret their experimental results in the context of POM and DOM transition, arguing that DOM aggregation and POM dissolution determine the fate of these organic matters. However, the results of the incubation experiments are not strong enough to support this interpretation because: 1) the trends of changes in POC and DOC concentrations during the incubation seem to be complex and they are not always having opposite trend. The authors also mentioned this in the section 5.3.2, and 2) the magnitudes of changes in POC and DOC concentrations are significantly different (for example,~ 2 μM changes in POC vs >200 μM changes in DOC in Sep incubation), making it difficult to compare these changes to draw conclusions about DOM aggregation and POM dissolution.
- The authors also tried to compare the changes in DOM compositions before and after incubation. However, the analysis is questionable because the changes in metrics such as H/C, O/C, MW, and AI in Table 2 are small compared to the standard deviations. Most of the discussions are based on these changes, which are smaller than the errors. While the authors argue that these small changes are significant, they do not provide any statistical test reports to support this argument. The authors should conduct statistical analysis to support those small changes are still significant.
Major comments:
Lines 32-35: The sentence seems to imply that DOC cannot contribute to carbon sequestration, which is misleading. The authors should acknowledge that DOC can also contribute to carbon sequestration.
Line 44: The sources listed for DOM are incomplete. For example, zooplankton grazing can also release DOM from phytoplankton. I recommend referencing “Carlson, C. A., & Hansell, D. A. (2015). DOM sources, sinks, reactivity, and budgets. Biogeochemistry of marine dissolved organic matter, 65-126.”
Lines 50-55: The authors mention labile and semi-labile DOM but omit recalcitrant DOM, which has a turnover time of millennia. To be thorough and consistent with previous categorization of DOM based on lability, recalcitrant DOM should be included .
Line 145: I do not fully understand why using headlight with red light can reduce the possibility of biological production. Chlorophyll a absorbs red light.
Lines 253-254: Why pre-soaking SPE column sorbent with Methanol is needed ?
Line 259: The DOC recovery is highly variable. Could you explain why?
Line 280: Did the authors try other criteria for formula assignment, such as allowing nitrogen between 0 and 4 ?
Line 281: Why remove formulas which contained both nitrogen and sulfur, 13C and nitrogen or sulfur and 13C? Could the authors justify it?
Lines 395-400 and Figure 5: The changes in DOM parameters like H/C, ALmod, MW, and O/C are much smaller than the standard deviation. Although the authors state these small differences are statistically significant, statistical test results should be provided to justify this claim .
Minor comments:
Line 108: should be “characteristics” instead of “character”?
Line 131: (TPM and PIM)
Line 141: should be “every two months”?
Citation: https://doi.org/10.5194/egusphere-2024-1314-RC1
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