| 25 Sep 2025
Heavy precipitation-induced Yangtze River runoff greatly regulates heterotrophic prokaryotes production and induces P-limited growth in the northern East China Sea
Abstract. Although heterotrophic prokaryotes (HP) play a crucial role in biogeochemical carbon cycles, microbial oceanographic studies associated with heavy precipitation-induced large-scale freshwater runoff are understudied in the East China Sea (ECS), the largest continental shelf in the northwest Pacific. To elucidate the impact of Yangtze River Diluted Water (YRDW) on HP production (HPP) and growth-limiting resources, we conducted comprehensive microbial oceanographic measurements in combination with analysis of satellite images and optical property analyses of dissolved organic carbon (DOC) over three consecutive years in the northern ECS. Our results revealed that the HPP and chlorophyll-a were consistently highest in summer due to the supply of excess DOC and nutrients via YRDW, which is intriguing considering the enhanced HPP coupled with spring phytoplankton bloom in middle latitudes in general. However, the exceptionally great YRDW runoff induced by heavy rainfall resulted in excessive supply of terrestrial-origin recalcitrant DOC and nutrients imbalance with high N:P ratio (34), which was responsible for the limited DOC bioavailability and phosphorus-limitation for the HPP. Accordingly, the enhanced HPP to primary production ratio (> 0.5) in summer suggested enhanced carbon flow via microbial loop, which ultimately affects fishery structure and production by reducing energy efficiency in food web process. Our results demonstrating the contrasting impact of YRDW on regulating (i.e., either stimulating or suppressing) the HPP provide new insights into the microbial responses to climate change-induced large-scale freshwater discharge, which is applicable to other ocean basins receiving great freshwater inputs (e.g., Amazon River and Arctic Ocean) accompanied by increasing precipitation.
General comments
This manuscript presents a valuable three-year dataset (2020-2022) exploring how heavy precipitation and the resulting YRDW influence microbial carbon dynamics in the nECS. The authors integrate satellite-based salinity, PP, HPP, and FDOM measurements with nutrient-addition bioassays. The topic is timely and relevant to Biogeosciences, addressing the interaction between hydrological forcing and microbial processes in marginal seas.
The manuscript is well organized and clearly written, and the field observations are impressive in scope. However, several aspects of the analysis and interpretation require further clarification before the conclusions can be fully supported. In particular, the paper tends to extend beyond the range of the presented evidence, and the causal links between precipitation, DOM quality, and microbial carbon partitioning are not fully demonstrated.
I outline below the main points that, in my view, require attention:
1.The paper draws a direct causal chain from precipitation and YRDW variability to microbial carbon partitioning and trophic balance. While the correlation between hydrological forcing and microbial parameters is evident, the discussion extrapolates local observational patterns to ecosystem-scale mechanisms (e.g., "enhanced microbial loop and reduced food-web efficiency") without direct process measurements of carbon transfer (e.g., respiration or bacterial growth efficiency).
The authors are encouraged to clarify which interpretations are empirically supported and which remain conceptual or inferential. This distinction will help strengthen the credibility of the conclusions.
2. The use of the HPP:PP ratio as an indicator of microbial loop intensity is informative but limited. Without concurrent measurements of respiration or BGE, this ratio reflects only relative production rates rather than the efficiency of carbon transfer or sequestration. The authors briefly acknowledge this point in the manuscript, but a more explicit discussion in the main text would strengthen the interpretation and clarify the limitations of using HPP:PP as a mechanistic indicator.
3. The manuscript would benefit from a better balance between the Results and Discussion sections. The results are presented rather succinctly, whereas the discussion is extensive and mechanism-oriented. Presenting slightly more quantitative detail in the Results—especially regarding interannual trends and variability—would make the argumentation in the Discussion easier to follow and more persuasive.
4. The study attributes seasonal variability of HPP mainly to precipitation and phosphorus limitation. However, temperature, grazing pressure, or water-column stability may also influence microbial activity. A brief discussion acknowledging these potential co-drivers would provide a more balanced interpretation.
5. The nutrient-limitation bioassay (Fig. 7) is compelling; however, the differences between 12-h and 26.5-h incubations could affect comparability. Please discuss potential bias due to unequal incubation times.
Minor comments
1. It would be helpful to clarify whether HPP and PP were measured concurrently at the same sampling stations. If not, a short note on how this may affect the interpretation of HPP:PP ratios would be valuable.
2. The authors might consider adding a simple schematic summarizing how YRDW affects nutrient stoichiometry, DOM composition, and microbial carbon flow.
3. In the Conclusion, tone down generalizations such as “greatly regulates” or “strongly enhances”.
4. The use of 0.2 μm filtration followed by a 9:1 mixture of filtered and unfiltered seawater seems appropriate for reducing grazing while preserving the original bacterial assemblage. Still, the rationale could be clarified briefly in the Methods to help readers understand the intention behind this experimental setup.
Technical corrections
The panel labels (A–L) in Figure 1 seem unnecessary, as they are not mentioned in the main text. Consider removing them or clarifying their meaning in the caption.