| 03 Sep 2026
Biogeochemical responses to ocean alkalinity enhancement in the North China Sea
Abstract. Ocean Alkalinity Enhancement (OAE) has been proposed as a promising marine carbon dioxide removal strategy. Yet its ecological consequences remain poorly understood, particularly in dynamic coastal systems characterised by high environmental variability. Here, we investigated the effects of OAE on plankton community structure and biogeochemical processes in the eutrophic North China Sea (NCS) using a microcosm experiment with natural seawater collected offshore. Two OAE scenarios were evaluated: CO2-equilibrated and CO2-non-equilibrated treatments, both targeting an increase in total alkalinity (ΔTA) of approximately 500 µmol kg−1. Neither OAE treatment significantly affected the abundance of phytoplankton and microzooplankton. Similarly, plankton communities exposed to OAE maintained their natural levels of chlorophyll a, taxonomic composition, biodiversity, and particulate organic carbon throughout the experiment. In contrast, substantial TA loss occurred in the CO2-non-equilibrated OAE treatment, indicating reduced alkalinity retention under non-equilibrated conditions. This study was conducted within the framework of the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project (OAEPIIP), which was established to standardise OAE microcosm experiments and systematically assess the environmental impacts of OAE across diverse marine ecosystems. Our findings indicate that NCS phytoplankton and microzooplankton exhibited high tolerance to both CO2-equilibrated and non-equilibrated OAE scenarios under the experimental conditions tested, while highlighting the need to monitor and mitigate TA loss alkalinity loss during OAE implementation in warm coastal environments.
This manuscript addresses a timely and important topic focusing on the ecological impacts of OAE in coastal systems, and contributes valuable data to the growing OAEPIIP intercomparison effort. The manuscript is generally well-written and the experimental design follows standardized protocols. The experimental design is appropriate for the research question, and the inclusion of both equilibrated and non-equilibrated treatments is a strength. However, methodological details are insufficiently described, and the conclusions need to be more carefully qualified. I recommend minor revision before the manuscript can be considered for publication in Biogeosciences.
Comments
L15-20: The "substantial TA loss" in the non-equilibrated treatment is arguably the most important result of the study. However, the abstract provides no mechanistic explanation. Please propose potential mechanisms.
L24: "TA loss alkalinity loss" — duplicated phrase; correct to "TA loss" or "alkalinity loss."
L14: "Neither OAE treatment significantly affected...", please specify the statistical test in the abstract, or at least indicate that this is based on GAMMs.
L50-60: The paragraph on OAE and plankton cites several studies but does not discuss the mechanistic pathways by which OAE might affect plankton (e.g., pH stress, CO₂ limitation, trace metal availability, nutrient speciation). A brief mechanistic overview would strengthen the introduction.
L140-147: Statistical analysis: GAMMs are mentioned but not described in detail. What was the model structure? What were the fixed and random effects? How was temporal autocorrelation handled? Provide more detail.
L165 Fig1: The claim of a TA:DIC loss ratio of ~2:1 (Section 4.3, "Implications for OAE") is the primary evidence for CaCO3 precipitation. This ratio must be shown explicitly.
L172-174: "Initial BSi levels were high, likely due to resuspended particles caused by windy conditions at the time of sample collection." — This is speculative. Move to Discussion or remove.
L185: "Phytoplankton assemblages were dominated by small-sized cells (< 5 µm) until day 4, with no larger plankton observed by microscopy." — This is important for interpreting the lack of response. Discuss whether the community was dominated by pico- and nanoplankton throughout, and whether this affected sensitivity to OAE.
L235: "The lack of detectable responses is likely be attributed..." — grammatical error; correct to "is likely attributable to".
L255-260: It is suggested to discuss seasonal and interannual variability in the NCS and how this might affect generalizability.
L262: Section 4.2 concludes that "there was little evidence that moderate alkalinity enhancement influenced the measured pools of particulate organic carbon and nitrogen." However, the claim that "OAE can alter inorganic carbon cycling without necessarily producing short-term changes in plankton-derived particulate organic matter" is strong. Please rephrase.
L300: "Consequentially" — use "Consequently."
L302: The limitations section does not discuss the closed-system artifact (no gas exchange, no dilution, no vertical mixing) or the bottle effect.
L328: "moderate OAE can be implemented without detectable short-term effects" — this is too strong. Replace with "did not detect short-term effects under the experimental conditions tested."
References: Check for consistency in citation style. Some references are cited as "(Bach, 2024)" while others are "(Bach et al., 2019)."