| 06 Jul 2026
Freshwater forcing along the Indian Coastal Seas: Impacts on Productivity and Acidification
Abstract. Freshwater fluxes from precipitation and river runoff play a critical role in modulating upper-ocean stratification, nutrient availability, and biogeochemical processes in the coastal waters of the Indian subcontinent. The formation of the barrier layer thickness (BLT) links freshwater input to vertical mixing, influencing both productivity and carbonate chemistry. High-resolution (5 km) MITgcm-BLINGv2 simulations are conducted for the Arabian Sea (AS) and the Bay of Bengal (BoB), and sensitivity experiments are performed to represent reduced and increased freshwater perturbations. We analyzed seasonal variability of buoyancy frequency (N2), mixed layer depth (MLD), net primary productivity (NPP), pH, and phytoplankton biomass across five coastal regions. Reduced freshwater scenarios weakened or eliminated BLT, leading to deeper MLD and N2 maxima, with subsurface nutrient-rich waters entrained upward. This enhanced nutrient availability increased NPP in the coastal regions. However, the upward transport of subsurface carbon also lowered surface pH by 0.03, indicating a trade-off between biological enhancement and increased surface acidification. In the increased freshwater scenario, the BLT strengthened, the MLD shoaled, and NPP decreased, while surface pH increased due to reduced vertical carbon exchange. Interestingly, stratification deepening under reduced freshwater input is more pronounced in the southeastern AS than in the BoB, contrasting conventional understanding. Vertical phytoplankton responses are consistent with these trends, with small and large phytoplankton biomass increasing under weaker BLT and decreasing under enhanced BLT. Freshwater-driven BLT modulation drives a complex interplay between carbon uptake and export along Indian coastal waters. These findings emphasize the importance of accurately representing freshwater fluxes in biogeochemical models to capture regional ecosystem responses.
The manuscript ‘Freshwater forcing along the Indian Coastal Seas: Impacts on Productivity and Acidification’ analyses the role of freshwater fluxes in modulating primary productivity (PP) and ocean acidification (OA) in the coastal regions of the Indian Ocean using numerical modelling experiments. The authors conduct several experiments by removing river fluxes, precipitation, and both together. In another experiment, they increase river fluxes by a certain percentage to represent a possible future scenario based on AR6.
Although the authors perform several numerical experiments, I do not find that the study provides significant new information to our current understanding of the influence of physical dynamics on PP and OA in the Arabian Sea and Bay of Bengal. Several numerical modelling and observational studies published in this region have already established that physical dynamics, particularly vertical mixing associated with regional mixed-layer depth (MLD) and barrier layer thickness (BLT), strongly influence ocean productivity and acidification. The present study largely reiterates this established understanding.
For example, numerical experiments that decrease freshwater input would be expected to reduce stratification, enhance vertical mixing and nutrient entrainment, and consequently increase productivity. Similarly, enhanced freshwater input would strengthen stratification, suppress vertical nutrient supply, and reduce productivity. These mechanisms are already well documented from both observations and numerical modelling studies in the Indian Ocean.
In addition to the limited novelty, I have serious concerns regarding the lack of validation of key variables at the surface and across the water column. Since the manuscript focuses on the effects of freshwater forcing, stratification, and vertical mixing, validation of the relevant essential variables across the water column is particularly important. The absence of such validation considerably weakens the scientific basis of the study.
Overall, I find that the main findings of the manuscript do not provide sufficient novelty. In addition, several major methodological and scientific issues need to be addressed. These concerns, in my view, make the manuscript not suitable for publication in Biogeosciences in its present form. My major concerns are listed below.
Major Concerns 1. Reduced freshwater input and increased productivity
The authors state “Reduced freshwater input may increase productivity, especially in the Bay of Bengal, due to weakened BLT, stronger mixing, and enhanced nutrient entrainment.”
This mechanism hArabian Seaalready been demonstrated by several numerical and observational studies in the region. A specific numerical experiment is not necessarily required to establish that reduced freshwater input can enhance productivity through reduced stratification, stronger mixing, and increased nutrient entrainment.
The well-established seasonal variability of freshwater input, stratification, vertical mixing, productivity, and pH in the Bay of Bengal already demonstrates these relationships. During the pre-monsoon season, river discharge and precipitation are relatively low, and the barrier layer thickness (BLT) becomes weak or nearly absent. This favours enhanced vertical mixing and nutrient entrainment, which can increase productivity and bring carbon-rich subsurface water toward the surface, resulting in lower surface pH. In contrast, during the monsoon and post-monsoon seasons, increased freshwater input and the expansion of the freshwater plume strengthen stratification and BLT, thereby suppressing vertical mixing and influencing both productivity and surface pH.
These processes have already been discussed in several published observational and numerical modelling studies. The authors themselves state “Overall, the freshwater-reduced experiments exhibit a consistent two-layered pH response: lower pH above the MLD and higher pH below, whereArabian Seathe freshwater-enhanced scenario shows the reverse. This behaviour highlights the coupling between freshwater forcing, vertical mixing, and carbon redistribution along the Indian coastal waters, consistent with observed regional contrasts between the Arabian Seaand Bay of Bengal.”
This statement itself indicates that the mechanisms identified in the present study are consistent with previously established understanding. The authors may argue that the specific numerical experiments conducted here have not previously been performed and therefore constitute a new approach. While this may be technically correct, conducting a new numerical experiment does not by itself provide sufficient scientific novelty if the resulting mechanisms simply reconfirm well-established physical and biogeochemical relationships. If these mechanisms had not previously been demonstrated or discussed for the region, the study would have been more useful. However, in its current form, the numerical experiments mainly confirm processes that are already known.
2. Reduced freshwater, lower surface pH, and the productivity-acidification trade-off
The authors state “Reduced freshwater also leads to lower surface pH because carbon-rich subsurface water reaches the surface during the same mixing events that support productivity. This indicates a trade-off between higher productivity and increased surface acidification.”
Again, this is a well-established mechanism. Stronger vertical mixing can simultaneously enhance nutrient supply and bring carbon-rich subsurface water to the surface. Therefore, increased productivity can occur together with enhanced surface acidification and lower pH.
This relationship hArabian Seaalready been discussed in published studies from the region, including Sarma et al. (2021). Therefore, this result does not appear to provide a substantially new scientific understanding.
3. Increased freshwater, stronger BLT, and reduced productivity
The authors state “Increased freshwater strengthens the BLT, reducing mixing and nutrient supply and leading to lower productivity, Arabian Seashown in SENS4 over the Bay of Bengal and parts of the SEAS.
This mechanism is also well established. During the monsoon and post-monsoon periods, enhanced freshwater input and freshwater-plume spreading increase stratification and BLT in the Bay of Bengal. This can suppress vertical mixing and nutrient supply and consequently influence productivity and pH.
Therefore, the result obtained from SENS4 appears to reproduce a mechanism that is already documented through both observations and numerical modelling studies.
4. Changes in stratification associated with reduced freshwater
The authors state “Reduced freshwater also deepens the stratification and slightly weakens it, especially during fall and winter.”
This statement requires further clarification and should be carefully examined in relation to the physical mechanisms discussed elsewhere in the manuscript.
The authors should therefore clearly distinguish what is already known from what is genuinely new from their experiments.
5. Insufficient Literature Review
Taken together, the above results suggest that the manuscript does not provide a sufficiently comprehensive review of the existing literature. This is also evident from the Introduction, where several important numerical and observational studies from the Arabian Sea and Bay of Bengal appear to be missing. A more comprehensive literature review is required, particularly on freshwater forcing and stratification, seasonal variations in BLT and MLD, vertical nutrient transport, productivity response to physical forcing, pH and ocean acidification variability, and the contrasting physical and biogeochemical responses of the Arabian Sea and Bay of Bengal. The authors should clearly establish what hArabian Seaalready been demonstrated by observations and previous modelling studies and then identify the specific knowledge gap addressed by the present work.
Additional Comments 1. Model spin-up and stability
I have serious concerns regarding the model spin-up period. The authors state that the physical model is spun up from a hot start for six years, after which the biological model is introduced and run for another two years, with the eighth year used for analysis.
I am concerned that this period may be insufficient for the model to reach a stable state, particularly for biological tracers and salinity. Tracers can require considerably longer periods to reduce initial-condition effects and reach a quasi-equilibrium state.
Although the study domain is smaller compared to other modelling studies, it is still sufficiently large that one year of biological spin-up may not be adequate to establish a stable biological state. Published modelling studies in the region have used much longer spin-up periods. For example, Malsang et al. (2024) and Liao et al. (2026) describe a 32-year spin-up for a North Indian Ocean model setup. The authors should therefore demonstrate the stability of the model state before using the final year for analysis. At minimum, the authors should show the temporal evolution and drift of the major physical and biological tracers and demonstrate that the model hArabian Seareached a quasi-equilibrium state.
2. Initial and boundary conditions for tracers
The authors should also carefully examine the nitrate fields from the Global Ocean Biogeochemistry Hindcast dataset. The nitrate concentrations, particularly at deeper depths, may not adequately represent observations. I would suggest that the authors consider using the available nutrient fields from the World Ocean AtlArabian Sea(WOA), which may provide a more appropriate mean state for the biological variables.
3. Atmospheric pCO₂ forcing
The authors state “The atmospheric components of pCO₂ are taken from Andrews et al. (2014).”
However, I could not identify the specific atmospheric pCO₂ value or product used from the information provided. The authors should clearly state what value or product hArabian Seabeen prescribed, including the temporal and spatial characteristics of the forcing.
4. Riverine biological tracer concentrations
The authors should provide the values prescribed for the biological tracers in the river input. This is particularly important because riverine inputs can have a substantial influence on productivity and biogeochemistry in coastal regions. The manuscript should clearly specify the concentrations of the relevant nutrients and other biological tracers used for the river forcing and explain the basis for these values.
5. Percentage increase in river flux
The exact percentage increase in river discharge used in the fourth experiment is not clearly provided in the referenced study. The manuscript states that this experiment represents a future scenario, but the basis for the selected percentage increase is not sufficiently clear.
The authors should explicitly state the percentage increase applied to the river flux and explain how this value wArabian Seaderived from the referenced study and its relevance to the AR6-based future scenario.
6. Validation of productivity, pH, BLT, ILD, and MLD
One of my major concerns is the absence of adequate model validation for productivity and pH.
Since the central objective of the study is to investigate the influence of freshwater forcing on stratification, vertical mixing, productivity, and ocean acidification, the model should demonstrate that it can realistically reproduce these variables before the sensitivity experiments are interpreted.
The authors should provide, preferably throughout the water column, validation of surface and depth-resolved productivity, surface and subsurface pH, BLT, ILD, and MLD. The validation of the vertical structure is particularly important because the main conclusions of the manuscript are based on changes in stratification and vertical mixing. The authors should first demonstrate that the model reproduces the observed vertical structure and seasonal evolution of these variables before drawing conclusions from the freshwater sensitivity experiments.
Overall Assessment
The manuscript addresses an important topic and uses numerical experiments to investigate the influence of freshwater forcing on productivity and ocean acidification. However, the principal mechanisms identified by the study are already well established through published observational and numerical modelling studies in the Arabian Sea and Bay of Bengal. The present experiments largely reproduce these known relationships rather than providing a clearly demonstrated new scientific insight.
In addition, the manuscript hArabian Seasignificant concerns related to the adequacy of the literature review, model spin-up and tracer stability, initial and boundary conditions, riverine forcing, atmospheric pCO₂ forcing, and, most importantly, the lack of validation of productivity, pH, BLT, ILD, and MLD.
For these reasons, I do not find sufficient novelty and methodological support in the current manuscript for publication in Biogeosciences. I therefore recommend rejection in its present form.
References
Sarma, V. V. S. S., Prasad, M. H. K., & Dalabehera, H. B. (2021). Influence of phytoplankton pigment composition and primary production on pCO2 levels in the Indian Ocean. Journal of Earth System Science, 130(2), 85.
Malsang, M., Resplandy, L., Bopp, L., Zhao, Y., Ditkovsky, S., Yang, F., ... & Lévy, M. (2024). Contemporary decline in northern Indian Ocean primary production offset by rising atmospheric nitrogen deposition. Frontiers in Marine Science, 11, 1418634.
Liao, E., Resplandy, L., Yang, F., Zhao, Y., Ditkovsky, S., Malsang, M., ... & Stock, C. (2025). A high-resolution physical-biogeochemical model for marine resource applications in the Northern Indian Ocean (MOM6-COBALT-IND12 v1. 0). Geoscientific Model Development, 18(18), 6553-6596.