the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
Coastal Nitrogen Drives Respiration Quotient in the Southern California Bight
Abstract. Southern California Bight coastal waters are dynamic and strongly influenced by a changing climate. An increased respiration quotient has been found during high temperature and low nitrogen conditions. These observations are specific to open ocean conditions, and their applicability to coastal environments is uncertain. To disentangle the controlling factors in a coastal environment, we examined environmental conditions, particulate organic matter, and the respiration quotient over five years in the Southern California Bight. Our study revealed clear seasonal variation in environmental conditions and biological parameters. We detected higher than previously reported respiration quotient ratios in open ocean regions. We found a strong inverse relationship between respiration quotient, nitrate and chlorophyll. Our findings also suggest that changes in community structure, triggered by nutrient shifts and a local oil spill, affected the range in respiration quotient and explains some of the variability measured. As climate continues to impact coastal regions, variable r-O2:C patterns and its controls assists in accessing subsurface oxygen concentrations and in turn the health of our coastline.
- Preprint
(1332 KB) - Metadata XML
-
Supplement
(725 KB) - BibTeX
- EndNote
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-2357', Anonymous Referee #1, 06 Oct 2025
-
RC2: 'Comment on egusphere-2025-2357', Anonymous Referee #2, 14 Oct 2025
Moreno et al. present an analysis of respiration quotient (RQ) dynamics in the southern California Bight. The study reveals substantially higher RQ values in this coastal region compared to those typically observed in the open ocean. The authors also identify a strong inverse relationship between RQ and nitrate concentrations, highlighting the underlying biogeochemical drivers of RQ variability. Additionally, they discuss the potential influence of shifts in community composition and episodic oil spills on RQ patterns.
Overall, the manuscript is well written, logically organized, and the conclusions are convincingly supported by the long-term field observations. I have only a few minor comments for the authors’ consideration:
Line 57: I recommend adding a map of the study area (e.g., in the Supplementary Material) to improve clarity.
Line 148: Table S1 appears to be missing.
Line 184: Consider moving the two lower panels of Figure 1 downward and enlarging the markers and axis labels, as the current figure is difficult to read.
Line 187: Please clarify how “clear” is defined—can it be quantified?
Line 188: Indicate in the caption that the squares in Figure S2 denote statistical significance.
Line 212: Justify the use of a 12-point moving average—does this correspond to a 2-month smoothing interval, and why was this window chosen?
Citation: https://doi.org/10.5194/egusphere-2025-2357-RC2 -
AC1: 'Reply on RC2', Allison Moreno, 24 Oct 2025
Reviewer # 2
Moreno et al. present an analysis of respiration quotient (RQ) dynamics in the southern California Bight. The study reveals substantially higher RQ values in this coastal region compared to those typically observed in the open ocean. The authors also identify a strong inverse relationship between RQ and nitrate concentrations, highlighting the underlying biogeochemical drivers of RQ variability. Additionally, they discuss the potential influence of shifts in community composition and episodic oil spills on RQ patterns.
Overall, the manuscript is well written, logically organized, and the conclusions are convincingly supported by the long-term field observations. I have only a few minor comments for the authors’ consideration:
AUTHOR REPSONSE
>>> We appreciate these supportive comments.
Line 57: I recommend adding a map of the study area (e.g., in the Supplementary Material) to improve clarity.
AUTHOR REPSONSE
>>> Thank you. We agree adding a map of our sampling site is beneficial. This is within the Supplementary Material (Figure S1)- we have adjusted all other figure labels to reflect this change.
Line 148: Table S1 appears to be missing.
AUTHOR REPSONSE
>>> Apologies. This has been fixed.
Line 184: Consider moving the two lower panels of Figure 1 downward and enlarging the markers and axis labels, as the current figure is difficult to read.
AUTHOR REPSONSE
>>> Thank you, we have adjusted the Figure 1 to be a 2 x 2 set up.
Line 187: Please clarify how “clear” is defined—can it be quantified?
AUTHOR REPSONSE
>>> We have applied statistical analysis (1 way ANOVA) to add a p-value value. We added “ … annual differences (ANOVA p-value <0.05; Fig. S2).”
Line 188: Indicate in the caption that the squares in Figure S2 denote statistical significance.
AUTHOR REPSONSE
>>> We have added in the Figure S3 (previously S2) description “White squares represent statistical significance at p-value < 0.05.”
Line 212: Justify the use of a 12-point moving average—does this correspond to a 2-month smoothing interval, and why was this window chosen?
AUTHOR REPSONSE
>>> Thank you for this comment. After looking back at our code, this was an error. Previously the 12-point moving average constrained the variability in the r-O2:Cratio the best, which represents a seasonal smoothly. However, we ultimately went with the rlowess moving average because seasonality in r-O2:Cis present but not a strong statistical significance in our dataset. Additionally, our POC and PCOD red lines represent an 8-point moving averaging similar to the environmental data. We have adjusted our Figure 2 description as follows, “. The red line represents an 8-point moving average for A, B, and MATLAB ‘rlowess’ for C moving average. The respiration quotient is a molar ratio.”
Citation: https://doi.org/10.5194/egusphere-2025-2357-AC1
-
AC1: 'Reply on RC2', Allison Moreno, 24 Oct 2025
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 606 | 59 | 19 | 684 | 39 | 12 | 27 |
- HTML: 606
- PDF: 59
- XML: 19
- Total: 684
- Supplement: 39
- BibTeX: 12
- EndNote: 27
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Dear Authors,
This study collected long-term samples of suspended particulate organic matter, phytoplankton community composition, and nutrients. Temporal variations were observed across different years. Due to an accident, the authors compared the relationships among these parameters and ultimately assessed the influence of temperature on phytoplankton growth. This type of multi-parameter, long-term dataset is valuable and should be supported, as it provides new insights into biogeochemical processes and ecosystem dynamics. However, certain aspects—particularly the interpretation of elemental ratios—warrant further clarification and deeper discussion. My major comments are as follows.
Major Comments
1. Redfield Ratio Clarification (Line 132)
Why is the Redfield ratio cited as “1” in line 132? Please explicitly state the formula being used (e.g., -O₂:C or N:P), and clarify the basis for this value. The study reports elemental ratios exceeding the canonical Redfield values, but the associated biogeochemical implications are not clearly explained. Although the ratio (r-O2:C) is compared across different contexts, the manuscript lacks further interpretation of what these deviations represent. The authors should also consider and discuss alternative explanations, including possible sources of error or variability in the observed ratios.
2. Community Structure vs. POC and O₂:C Ratios
The relationship between changes in phytoplankton community composition and the corresponding shifts in oxygen and carbon ratios (e.g., O₂:C) should be more thoroughly examined. An alternative perspective is worth considering: if community structure changes, would the composition and characteristics of particulate organic carbon (POC) also change? When applying a fixed POC conversion factor, how might shifts in species composition affect the resulting POC concentrations?
There is a need to clarify the direction of causality: is the chemical composition driving community shifts, or vice versa? The current version appears to infer changes in community structure from observed chemical outcomes. However, it is equally plausible that structural shifts in the plankton community alter the stoichiometric ratios. This relationship should be explicitly discussed.
Furthermore, beyond community composition, the authors should also address how food web structure may influence the O₂:C ratio. Is this ecosystem predominantly bottom-up or top-down controlled? Could shifts in trophic structure—not just primary producers—impact O₂:C ratios? For example, changes in grazing pressure or predator-prey dynamics may also play a role and should be acknowledged.
3. Implications and Biogeochemical Meaning in the Conclusion
The concluding section could benefit from a stronger emphasis on the broader biogeochemical implications of the observed elemental ratios. In particular, the manuscript should more clearly articulate how these findings inform our understanding of ecosystem function or carbon cycling under changing environmental conditions. Additionally, potential extensions of this work or new hypotheses that emerge from these patterns would enrich the conclusion.
Minor comments:
Sampling periods are unclear in the Methods. By any vessels or boats. Are there any differences between each period as mentioned Line 172.
Line 95, “300 ml for POC and PCOD” is unclear. Each for 300 ml or a total of 300 ml?