Status: this preprint is open for discussion and under review for Biogeosciences (BG).
Coastal Nitrogen Drives Respiration Quotient in the Southern California Bight
Allison R. Moreno,Adam J. Fagan,and Adam C. Martiny
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.
Received: 20 May 2025 – Discussion started: 02 Jun 2025
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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?
Phytoplankton stimulate oxygen production in the surface ocean whereas bacteria will utilize that oxygen. We measure the first coastal r-O2:C - oxygen needed to oxidize carbon- over a 5-yr period in Southern California to determine the controlling factors in this highly dynamic region. We found that seasonality and blooming conditions has a strong impact on this ratio. We also found that a major local oil spill affected r-O2:C, demonstrating that coastal waters are impacted by climate.
Phytoplankton stimulate oxygen production in the surface ocean whereas bacteria will utilize...
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?