the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jul 2025
Tracing Water Masses and Particle Dispersion on the Northern South China Sea Shelf Using POM Signatures under Contrasting Wind Conditions: A New Perspective
Abstract. River-dominated shelf systems are shaped by complex hydrodynamic and biogeochemical interactions, making source-to-sink (S2S) tracking a challenge. To explore the influence of physical processes on particle distribution in the northern South China Sea, two research cruises were conducted along the Guangdong coast in the summers of 2018 and 2020. Hydrographic profiles and particle volume concentrations were measured using a CTD and a laser in-situ scattering transmissometer (LISST). Seawater samples from the surface, pycnocline, and bottom layers were analyzed for chlorophyll-a (Chl-a), particulate organic matter (POM, including POC, PN, and δ¹³CPOC), and nutrients. Empirical orthogonal function (EOF) analysis revealed distinct patterns of hydrodynamic control in both years. In 2018, stratification was intensified by local rainfall from a cyclonic system and the subsurface seawater, leading to the accumulation of larger particles at the pycnocline. In 2020, however, the Zhujiang River plume (ZRP) played a dominant role in stabilizing the water column under the southwesterly monsoon, entraining bio-particles enriched in δ¹³CPOC. This process facilitated the northeastward transport of biogenic material into the Taiwan Strait, possibly influencing sediment composition along the plume pathway. Although the ZRP and subsurface chlorophyll maximum shared similar biogeochemical signatures, the ZRP exhibited higher POC-to-Chl-a ratios and greater particle bulk densities, indicating more advanced POM degradation. Notably, the significantly negative correlation between δ¹³CPOC and salinity along the ZRP highlights δ¹³CPOC serves as a robust tracer of riverine water, more effective than the traditional N/P ratio in this river-dominated shelf system.
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RC1: 'Comment on egusphere-2025-2502', Anonymous Referee #1, 13 Aug 2025
The authors conducted two cruises along the Guangdong Coast in summer to reveal how water mass mixing, physical and biogeochemical features of suspended particles affect the horizontal and vertical distributions of particles in the Northern South China Sea (NSCS) shelf. Multiple physical and biochemical parameters were used to define the possible source of particles and the underlying processes. The manuscript is well written and presents an interesting story. However, I don’t think some of the data are properly interpretated in this manuscript. More details need to be clarified. I would recommend a major revision.
Major comments:
- It is more prevalent that the d13C of POM increases as salinity increases in the coastal sea because the terrestrial OM (e.g., -27‰) has a more depleted d13C than marine OM (e.g., -20‰). However, it has a opposite trend in the Appendix Fig. A7 of this manuscript. Can the authors compile more data (d13C-POC vs. Salinity) from the published paper and see if it’s a general feature for the particles in the NSCS?
- With a decreasing trend of d13C-POC vs C/N similar to the Fig. 10 in KK Liu et al. (2007), 11 in the main text should reflect the physical mixing between terrestrial and marine OM, while their corresponding salinity seems unreasonable, i.e., higher salinity for terrestrial OM and lower salinity for marine OM. Moreover, in the main text the author neglected the contribution from terrestrial source, and only interpretated the more depleted d13C-POC value as a result of phytoplankton composition and isotopic fractionation during growth without proper explanation for the simultaneous high C/N values. I suggest the authors add the coverages of different end-members in panel (a) of Fig. 11, similar to the author's previous work, i.e., Fig. 6F in Lee et al. (2023).
- The sources (terrestrial vs. marine) and bioavailability (fresh/labile vs. degraded) of particles and their correlation with the particle size and water stability should be clarified in the main text. For instance, the contribution fractions of different sources can be calculated using an isotope mixing model (Yu et al., 2010 ECSC).
Minor comments:
P2, L29: Specify what biogeochemical signatures.
P2, L31: Only one parameter like d13-POC may lead to misinterpretation. It is better to use multiple parameters to constrain the source and transport of the suspended particles.
P3, L58: Delete the period after NSCS.
P4, L84: Delete the period after systems.
P6, L135: The two 500 mL samples are used for what parameters?
P6, L136: “Onboard”
P8, L160-162: Check out the over-citation problem.
P10, L232-233: Is there any validation plot from the author?
P11, L255: “fresher”?
P15, L301: How about the C/N ratio in the SCM water? Why the terrestrial source can be excluded?
P15, L303-305: Is there any other paper presenting such depleted d13C-POC in the shelf sea?
P20, L406: Please check “finer” or “coarser”.
P22, L440: to shoal “with” depths…
P25, L497: resulting in
P28, L546-537: It’s wired to see such kind of seawater end-member values. Can you provide any other studies using similar definition?
P28, L542-544: The interpretation of d13C-POC mainly composed of phytoplankton in the ZRP (stable fractionation) is contradict to that in the SCM (distinct fractionation). Any explanation for such difference?
Table 1&2: Some elements of the table (e.g. the footnote) are repeated several times. Please refine the tables.
Fig. 11: It’s better to remove panel (b), and add the coverages of the different end-members.
Fig. 12: Add the C/N ratio, and try to reveal the fractions between marine vs terrestrial.
Appendix Fig. A6 and A7: Add the plots of C/N ratio.
Citation: https://doi.org/10.5194/egusphere-2025-2502-RC1 -
RC2: 'Comment on egusphere-2025-2502', Anonymous Referee #2, 21 Aug 2025
Review Commons:
The study investigates the impact of physical processes on partical distribution in the northern South China Sea based on two cruises conducted along the Guangdong coast in the summer of 2018 and 2020. The authors identified that in 2018, heavy rainfall induced by a tropical cyclone resulted in strong stratification. In 2020, the Zhujiang River plume played a dominant role in stablizing the water column under southwesterly winds, leading to the accumulation of large particles due to the formation of pycnocline.
Overall, the paper is well-written in clear and understandable English. The figures are clear and easy to read. However, I found the scientfic questions to be unclear and lacking significance. The analysis and interpreation of the results were shallow and lacked logical conherence. Some explainations regarding the implications of the results seemed disconnected from the the results themselves. I believe the paper requires substaination revision to reach the standards expected for publication in Biogeosciences. Below are my specific suggestions on how to improve part-by-part
Abstract
At the end of the abstract, the broader significance of the research should be addressed. Consider highlighting what new results and findings contribute to our understanding of particulate organic matter (POM) transport in coastal regions, not just limited to the northern South China Sea (NSCS) itself.
1. Introduction
The first paragraph (L38-L46). You should focus to introduce the object you want to study instead of general hydrological and biogeochemical environment of NSCS. What you want to study here is the POM. You should give some introduction about why study POM is important. This was partially mentioned in L70-L75, which should be placed in front
L88-L90: I found the statement to be unclear and vague. What exactly do you mean by “complexity”? There are several statements in the text that are similarly ambigous and require more specific
L90: from -> induced by
L96-L98: It is repetive of first paragraph
L102-L111: Regarding the three questions. I felt they need to be further clarify
For example, in question 2, which asks, “which particles are transported to downstream regions, such as the Taiwan Strait?” It appears that there is no data provided to support claims of POM transport downstream.
Question 3: “Can particulate matter … processes?” The questions seems somewhat lacking in significance. It is a well-known fact that POM is impacted by water mass transport and biogeochemical processes. Therefore, please clarify what you specifically intend to ask.
2. Materias and methods
Figure 1. Observation period - > Observational period. Figure 1c illustrates the 2018 typhoon case; however, the shape of the plume typically changes significantly before, during and after a typhoon. It appears that Figure 1c represents the conditions prior to the typhoon. To clarify this distinction, it would be beneficial to include wind vectors in the figure.
2.2 Water stratification
E represents the brunt vasala frequency, a commonly known factor used to measure the strength of stratification. It would be more appropriate to refer to it as BVF for clarity
L177: There should be a space between the notation ± and the number that follows it
L237: The interval of the ECMWF wind data should be specific. Is it provided hourly or every 3hr?
3. Results
3.1 River runoff and hydrographic changes on the NSCS shelf
L250-L253: I do not understand why mention “the additional mechiansms” here. It was quite obvious the distribution of the river plume was the determined by the wind conditions, especially for your observational period. It was determined by the episodic wind. The mentioning of dam and climate oscillation make the satements very diverge and lack of logic conherence.
L256: I am curious about the precipitation mechanism. While a typhoon can indeed bring heavy rainfall, it moves through the region quickly. Therefore episodic rainfall may not have a significant impact on the upper layer sea surface salinity.
L282: Potential regions for the high N/P ratio at stations ZHJ4 and J1 should be provided.
3.2 Contrasting POM feathers in the shelf system.
L303-L305: Once again, I feel that the stament regarding the ECS is distracting in the context of the NSCS. Again, I felt the stament about the ECSF was distractive with the facts of NSCS. Although it was important to compare what you found with previous studies, but are they sharing the same mechanisms or not, why they are similar. You should be more clearly to describe the linkage.
4. Discussion
I do not think the results were organized appropriately. Some of the materials should belong in the methods section, while others are more suited for the results section. The discussion section should have deeper analysis of the mechainsms and a comparison with previous work, highlighting what is new in your findings. However, based on the descirptions provided, it feels like all the findings have been previously reported, which raises concerns about what is new.
4.1 Co-variability among properties of water masses and particles:
L355-L372: I think this should belong to the method part.
L373-L430: Should belong to the result part
L370: “below 0.1 were considered insignificant”. Statistical rational for excluding values below 0.1 threshold should be given.
4.2 Partical distribution assoicate with water stratification
L424-L445. I do not understand the logic behind mentioning your finding that “the highest concentrations of coarse particles were observed within strongly stratified layers” in conjuction with the previous work of Tian et al. (2022). Are these findings the same, only documented in a different coastal region? If so, it would be helpful to clearly articulate the connections or similarities between the studies and explains how they contributed to the broader understanding of this phenomenon. Clarifying the relevance and implications of these findings would greatly enhance their significance.
Section 4.2 and 4.3: I don’t think the mechanisms were fully explored in these two sections. For example, the title of the paper is “Tracing Water Mass and Particle Dispersion on the Northern South China Sea Shelf Using POM Signatures under Contrasting Wind Conditions: A New Perspective” While the 2018 case focused on typhoon-induced rainfall changing stratification, the mechanisms mentioned primarly was about typhoon-induced rainfall. However, there is little discussion regarding the impact of winds? Typhoons can induce on-offshore winds in the region you studied, as well as stong vertical mixing. It is important to explore how these wind conditions might affect the distribution of particles and POM. A more comprehensive discussion of these factors would provide a clearer understanding of the overall dynamics.
L456: vertical transport of particulate matter, is it associated with typhoon induced strong vertical mixing?
L459-L463: The discussion may have diverged from the central theme of your study, particularly regarding the “Contrasting Wind conditions.” If your research focuses on how different wind conditons impact mechanisms such as particulate dispersion and water mass dynamics, then bringing in topics like dam impacts and climate variability may seem off-topic or overly broad.
L482-L487: Again, the discussions may have strayed from a direct connection to your study’s results, specifically regarding the comparisons made with Xu et al.(2022). If their study focused on offshore dynamics while your research is centered on inshore conditions, it is important to clearify the implications of this distinction. For example, provide specific mechianms or observations that differ between the two studies. Discuss any varing hydrodynamic conditions, stratification patterns, or particle types that might lead to different outcomes.
5. Concusions:
L579-L581: It seems your results do not specifically address or demonstrate transport to the Taiwan Strait
Citation: https://doi.org/10.5194/egusphere-2025-2502-RC2
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