A time series analysis of transparent exopolymer particle distributions and C:N stoichiometry in the subtropical North Pacific: a key process in net community production and preformed nitrate anomalies?

Curran, Kieran; Villareal, Tracy; Letscher, Robert T.

doi:https://doi.org/10.5194/egusphere-2024-1416

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https://doi.org/10.5194/egusphere-2024-1416

Preprints

12 Jun 2024

| 12 Jun 2024

A time series analysis of transparent exopolymer particle distributions and C:N stoichiometry in the subtropical North Pacific: a key process in net community production and preformed nitrate anomalies?

Kieran Curran, Tracy Villareal, and Robert T. Letscher

Abstract. Within the oligotrophic subtropical oceans, summertime DIC drawdown despite nutrient limitation in surface waters and subsurface oxygen consumption in the absence of Redfieldian stoichiometric nitrate release are two phenomena still awaiting a full mechanistic characterization. The distribution, intensity and seasonality of these phenomena are identified with preformed nitrate as a tracer, where negative preformed nitrate (NPN) anomalies below the euphotic zone correspond to oxygen consumption without Redfieldian NO₃^- release, and positive performed nitrate (PPN) anomalies found within the upper 100 m occur where O₂ is produced without stoichiometric NO₃^- drawdown. Many processes that may contribute to these anomalies including N₂ fixation, non-Redfieldian DOM cycling, vertically migrating phytoplankton, heterotrophic NO₃^- uptake and vertical NO₃^- injection events have been measured or modelled, yet generally cannot fully account for the magnitudes of preformed nitrate anomalies and excess DIC drawdown observed in many oligotrophic subtropical waters. One other candidate process that may contribute to both phenomena is the formation of carbon-rich transparent exopolymer particles (TEP) and Coomassie-stainable particles (CSP) from dissolved organic precursors in surface waters and their subsequent remineralization below the subsurface chlorophyll maximum (SCM). However, few data exist to quantify exopolymer production and vertical distributions in oligotrophic oceans over an annual cycle, which is necessary to understand their potential role in the evolution of seasonal preformed nitrate anomalies and DIC drawdown.

To investigate the significance of exopolymer formation and export to North Pacific subtropical gyre biogeochemistry, we undertook a multi-year time-series (Jan 2020 – Sep 2022) analysis of TEP, CSP and total dissolved polysaccharides concentrations at Station ALOHA (22° 45’,158° W), and along a transect from 22° 45’ to 31° N to measure latitudinal variability in June 2021. Exopolymer C:N stoichiometry at Station ALOHA varied between 16.4 – 34.3, with values being more carbon-rich in summer; ratios were higher (32–38) toward the gyre centre at 31° N. TEP concentrations were consistently elevated in surface waters through Spring–Autumn (4–8 µM C after carbon conversion) at Station ALOHA with lower concentrations (~1.5–3 µM C) and more uniform vertical distribution during winter, indicating that TEP accumulated in surface waters may vertically sink and be exported with winter mixing. The accumulation of TEP in surface waters through Spring–Autumn and its subsequent export may account for 6.5–20 % of net community production (NCP), helping reduce the estimated imbalance of N supply and N demand at this site to <10 %. The upper ocean TEP cycle may explain 22–67 % of the observed PPN/NPN anomalies, helping to close the C, N, and O₂ budgets at station ALOHA, while leaving room for significant contributions from other processes such as vertically migrating phytoplankton and heterotrophic nitrate uptake to be further validated. These results suggest that exopolymer production and cycling may be more important to open ocean carbon biogeochemistry than previously expected, with considerable seasonality and spatial variability influenced by physical processes and phytoplankton activity.

Received: 13 May 2024 – Discussion started: 12 Jun 2024

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Kieran Curran, Tracy Villareal, and Robert T. Letscher

Status: final response (author comments only)

RC1: 'Comment on egusphere-2024-1416', Anonymous Referee #1, 03 Oct 2024

The authors present data on TEP and CSP measurements over the course of two years at station ALOHA and a transect that headed north from station ALOHA conducted in June of 2021. The resulting data are then used to estimate the contribution of TEP to carbon cycling in the region. The presentation of the data is challenging at times as the authors skip over the presentation of some data, and assume the readers can recall the climatology of organic nitrogen and carbon at station ALOHA. There is also only limited statistical analysis of these data, which leaves this reader unclear as to significance of some of the differences highlighted by the authors. The presentation of the calculations on the relative impact of TEP on carbon cycling is also not well presented, and it was difficult to follow the assumptions made by the authors.
This manuscript uses an excessive number of abbreviations which makes it challenging to follow the primary points being made by the authors. The manuscript centers on TEP and CSP, adding in abbreviations for NPN, PPN, SCM, NCP, topics that are ancillary to the primary topic makes the abstract hard to follow. This is particularly true since the abbreviations change later in the manuscript when PreNO3 is used.
The abstract should focus more centrally on work presented in this manuscript. The first paragraph is an introduction to the general ideas of the inorganic carbon drawdown, and can be summarized in one sentence before moving on to measurements made in the current project.
Line 199 – here the authors note they will present data in units of umC and umN, but then they go on to present both the xanthan and BA units and carbon/nitrogen units (starting ~ line 233 and in figure 2).
Line 208: are the difference in the conversion factors for nitrogen and carbon between the 5 m and 125m statistically significant? What about the C:N ratios. If they are significantly different, please provide the results if the statistical analysis. If not, please do not highlight differences.
Line 243 ‘Interannual variation in TEP concentrations…’ I am unclear on what data are used for this calculation – bulk concentrations? Surface water? Water-column integrated values? Please be more specific.
Line 247 ‘CSP … more variable than TEP…’ based on what quantifiable metric? This conclusion appears to be based on visual inspection of the data, which is emphasized by the vague mention of differences at ‘specific depths’ and concentrations that ‘appear to be…’ different.
Line 262: Given the interannual variability mentioned on the previous page, I do not think presenting an annual climatology is a valid presentation of the data. Using CSP as an example, the data in figure 2 show March has either the highest water column values or the lowest, so this averages out to a number in the middle. The comparison starting at line 274 to the climatology of PC and PN is also not shown in this manuscript and relies on the user to have a priori knowledge of PC and PN data at station ALOHA. Furthermore, presenting plots of PC vs. PN and TEP-C vs. CSP-N in figure 4 is not an effective means of discussing differences between TEP-C and PC.
line 382 ‘observed ~2 μM latitudinal gradient’ – I am not clear on what is presented here as 2 um, is this an integrated water column value?
Line 492: this is the first time the authors have mentioned a depth integrated carbon value. The conversation comparing existing data with the depth-integrated TEP-C value would be far more compelling if the authors presented the month/year/cruise depth integrated values for their own data (and *not* using the climatology given my previous comment about averaging such disparate years).
Line 501: The section linking the TEP data to net community production is difficult to follow as it jumps from one idea to the next and the reader is never given the citation link to Table 2. Further the various facts from the existing literature are presented, but there is no clear formula provided as to how the calculations are done. This may be an obvious calculation to the authors, but although I am an oceanographer, I am not versed on each step of this calculation and thus the steps used for these calculations are not clear. This is further confused by the mention of one citation (Hannides et al.) in the table legend that does not appear in the paragraph that is apparently describing this calculation.
Line 641: although the manuscript indicates the data are available at BCO-DMO, there are no data available online at this time (October 2024).
Table 1 – since the authors talk about TEP first, it would be easier to interpret this table if the columns start with TEP from the left before proceeding on to CSP and then the C:N ratios.
Figure 2 – the fonts are so small on these figures that it is quite difficult to gather the information needed to understand the figures. Even at 200% magnification, I can just barely read the axes.
Figures 4 & 5 : are these model II regressions statistically significant? Please present the p-values in addition to the r2 values. The colors in figure 5 are difficult to sea, the yellow in particular.
Figure 6: this figure would be easier to interpret if the latitudinal range shown on the x-axis were the same for all three subplots. The shift in axes makes it challenging to line up the three variables being plotted. The data for TEP and CSP particles would also be easier to interpret if the contour lines for fluorescence were imposed upon the color plots for TEP and CSP.
Figure 7 – this figure would be easier to interpret if the same color scheme were used on both subplots.
I am confused as to whether or not the data from station ALOHA from figure 6 are the same data that appear in figure 2. It seems not as the transect data show TEP-C concentrations in the upper 100 meter has having carbon concentrations ~5 µM C, but figure 2 shows values ~3 µM C. Am I missing something here as it seems odd to separate data from a transect that starts at station ALOSH from a regular sampling of station ALOHA, and such a discrepancy in the measurement is an indication of the variability in this measurement.

Citation: https://doi.org/10.5194/egusphere-2024-1416-RC1
RC2: 'Comment on egusphere-2024-1416', Anonymous Referee #2, 09 Oct 2024

Curran and co-authors present an important dataset on carbon-rich exopolymers in the North Pacific Subtropical Gyre, exploring the data to address the contribution of these compounds to net production and the associated surface nitrogen budget. This is an important contribution to the age-old question of why NCP in subtopical gyres is greater than can be accounted for by the particulate sinking flux and DOM flux.
My comments on the manuscript are largely editorial as this is a well executed study.
I found that it took a "long time" to get to the crux of the paper that contextualizes the data in reference to the carbon/nitrogen budgets of the NPSG. My recommendation is to revert to a traditional presentation where results are presented first, followed by a discussion section. As presented, the first sections of the combined Results and Discussion felt repetitive and bordered on tedious. For the short-attention-span readers we have all become, I think this structure would go a long way to better engaging readers.

Citation: https://doi.org/10.5194/egusphere-2024-1416-RC2
RC3:
'Comment on egusphere-2024-1416', Quentin Devresse, 11 Oct 2024
The authors present exopolymer gel particle data from the ALOHA station time series and a detailed oceanographic study. This is a valuable dataset that contributes to the understanding of the estimated imbalance between nitrogen supply and demand in the North Pacific subtropical gyre. Furthermore, the authors give an estimate of the nitrogen content of CSP particles using the colorimetric method, which had not been done before. However, the structure of the manuscript and the quality of the figures make it difficult to read. Major comments:
Overall, the abstract and conclusion are too long, and the introduction is too short, lacking information on TEP and CSP. It should be stated that EPS acts as a bridge between dissolved and particulate fractions of the organic matter which leads to a size continuum of particles in the ocean (see Verdugo et al., 2004; Verdugo, 2012) and that TEP and CSP are independent particle classes with different origins and fates (e.g. Cisternas–Novoa et al., 2015; Zamanillo et al., 2021). It is known that TEP influences the particle/carbon export (i.e. buoyancy, stickiness, aggregation) and therefore influences the distinction between suspended and sinking particles. However, those properties remain unknown for CSP. An overview of the knowledge on TEP and CSP can be found in Mari et al., (2017; https://doi.org/10.1016/j.pocean.2016.11.002), Thornton et al. (2017; https://doi.org/10.3389/fmars.2018.00206) and in the introduction of Marina Zamanillo Campos' thesis (2019).

The results lack the statistical comparisons needed to attest to the significance of the results obtained.

The paragraphs that make up the conclusion should be incorporated into the discussion section. The conclusion should give an exhaustive overview of the study's results.

Many sentences are too long (4-5 lines) and too many abbreviations are used. For consistency with other studies, I suggest replacing maximum subsurface chlorophyll (SCM) with deep chlorophyll maximum (DCM) and deleting the abbreviation SML, which stands for sea surface microlayer. Instead, indicate the mixed layer.

The size of the panels in the figures should be standardized to make them easier to read (figures 2, 3 6, and 7).

Minor comments:
Line 83: Remove sometimes
Line 162: add a space between the number and the unit (6mL to 6 mL) and make the correction through the manuscript.
Line 169: Add the R² of the calibration curves and the calibration curves as a supplementary figure.
Line 251: Correct “0.01 – 0.0.07 μM” to “0.01 – 0.07 μM”
Line 290: change swimmers to “motile microorganisms”
Lines 309-310: You can add that the statement is consistent with Cisternas–Novoa et al., 2015 and Zamanillo et al., 2021 observations.
Line 451: To facilitate the comparison, figure 7 should be removed and incorporated in other figures: Panel A should be included in Figure 2 and Panel B in Figure 6
Lines 449-452: The sentence is too long, divide it into two parts.
Lines 452-457: Confusing you can simplify by stating that here you had dissolved hydrolyzable carbohydrates which were originally different glycans with different degradation properties e.g. sulfated fucoidan (Vidal-Melgosa et al., 2021; https://doi.org/10.1038/s41467-021-21009-6) or laminarin (becker et al., 2020; https://doi.org/10.1073/pnas.1917001117)
Citation: https://doi.org/10.5194/egusphere-2024-1416-RC3

Kieran Curran, Tracy Villareal, and Robert T. Letscher

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Short summary

This work provides a two year record of marine organic gel concentrations from an open ocean site in the subtropical North Pacific Ocean north of Hawaii. These microscopic gels are investigated to understand their importance as an understudied component of organic matter cycling by marine microbes. We find an important role for gel cycling during the summer months, helping explain previously contradictory estimates of nutrient supply and demand for the subtropical ocean.


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