the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
Implementing Riverine Biogeochemical Inputs in ECCO-Darwin: a Critical Step Forward for a Pioneering Data-Assimilative Global-Ocean Biogeochemistry Model
Abstract. Resolving riverine biogeochemical inputs in ocean biogeochemistry models is pivotal for capturing the spatiotemporal variability of nutrients and carbon in coastal regions and in the global ocean. ECCO-Darwin is a pioneering data-assimilative global-ocean biogeochemistry model, which, to date, has focused on the pelagic zone. As a key step towards improving the representation of coastal regions in ECCO-Darwin, we add lateral inputs of carbon, nitrogen, and silica and evaluate the model response with regard to primary production and ocean carbon cycling. We generate riverine inputs by combining point-source freshwater discharge from JRA55-do with the Global NEWS 2 watershed model, accounting for lateral inputs from 5171 watersheds worldwide. While adding carbon and nutrients along with freshwater improves biogeochemical skill in river plume regions and coastal waters, the open-ocean response may be overestimated due to an excess of carbon and nutrients advected offshore. This highlights the need for a more nuanced representation of land-to-ocean and nearshore processes for quantifying how global-ocean primary production and carbon cycling respond to land-to-ocean inputs.
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RC1: 'Comment on egusphere-2025-1707', Anonymous Referee #1, 04 Sep 2025
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In their manuscript, Savelli et al. describe their implementation of river fluxes in the ECCO-Darwin model, and evaluate the performance of the model after this implementation. The paper is well written, the methods are generally sound, and the analysis of the effects of river fluxes in the model is well reflected and seems robust. The manuscript is also well suited for GMD. While I think the paper is close to publication, it might lack a bit of really novel aspects, since the implementation of river fluxes has been described in a few GOBMs in recent years, as cited in the study. I believe it would be relatively straight forward to add a few more interesting aspects originating from the implemention into the ECCO-Darwin model specifically, and would strongly recommend to do this (phytoplankton species shifts? More detailed process-based explanations of divergent FCO2 responses in the different regions?). I also have a few major points that should be clarified and I hope they will also improve the manuscript.
- I would not refer to the implementation as a critical one, as in the title, since the effects of the implementation shown for the global ocean are shown to be quite limited here. While it would be a different case if the model was focussing on coastal fluxes, there does not seem to be any other advancements in this direction presented in this paper.
- I think the introduction could be improved. While the important points are there in my opinion, it reads a bit disconnected. I would try to streamline and especially underline the following points: 1. Rivers transport carbon, nutrients and alkalinity to the ocean, 2. These transports are central for biogeochemistry and biological life in coastal regions. They also can affect open ocean biogeochistry due to offshore transport. 4. Models of the current generation do not well represent these transports and their implications.
- The methodologies section is missing the component of describing general nutrient an carbon sources and losses in the model, which I think is highly relevant to understanding the role of river inputs and their fate in the ocean. Where was biogeochemical matter originating from previous to the river implementation. How does the loss through burial work in the model? Maybe this is less relevant for ECCO-Darwin due to corrections made through assimilation, but it should still be described.
- The river implementation accounts for present-day river fluxes as inputs, and the authors use these fluxes to spinup the model, if I understand correctly. These fluxes have however been subject to strongly dynamical changes over the past century. Due to the longtime scales in the ocean, most of the ocean is thus likely more closely equilibrated with preindustrial fluxes. Thus spinning up the model with present-day fluxes will likely overestimate their contributions in the open ocean (which the authors explain due to offshore transport processes solely). I think this should definitely be discussed and taken account for future work.
- The CO2 source simulated by the model is strongly underestimated compared to what is currently estimated and used in the Global Carbon Budget (Regnier et al., 2022; Friedlingstein et al., 2024). The magnitudes of C, N and alkalinity river inputs, seem however consistent with literature values, which are thought to favor CO2 outgassing taking account input-to-export/burial ratios. Could the authors potentially better explain the discrepencies of the flux simulated in the model with presently used values (Is it a question of timescale of the spinup? Assuming present-day atmospheric CO2 versus preindustrial concentrations? Stoichiometries?).
Regnier, P., Resplandy, L., Najjar, R.G. et al. The land-to-ocean loops of the global carbon cycle. Nature 603, 401–410 (2022). https://doi.org/10.1038/s41586-021-04339-9
- I would suggest additionally using regional coastal water residence times estimated in literature and their differences between regions to explain differences for the different coastal responses in different regions of focus (e.g. regional values in Liu et al., 2019; Lacroix et al., 2021). For instance, high residence times in Arctic region could favor more outgassing of terrestrial organic material, versus less uptake in these light-limited regions through the nutrient inputs.
Liu, X., Dunne, J. P., Stock, C. A., Harrison, M. J., Adcroft, A., & Resplandy, L. (2019). Simulating Water Residence Time in the Coastal Ocean: A Global Perspective. Geophysical Research Letters, 46, 13910–13919. https://doi.org/10.1029/2019GL085097
Lacroix, F., Ilyina, T., Laruelle, G. G., & Regnier, P. (2021). Reconstructing the preindustrial coastal carbon cycle through a global ocean circulation model: was the global continental shelf already both autotrophic and a CO2 sink?. Global Biogeochemical Cycles, 35, e2020GB006603. https://doi.org/10.1029/2020GB006603
Specific Comments
L17 “At the same time, most of the refractory part of riverine organic carbon is transported offshore from river mouth regions as it is remineralized at slower turnover rates.” This is a bit of a jump from the previous sentence. Did you mean to add that river transports play a central role for biogeochemical processes in the coastal ocean first?
L25 I would also add that terrestrial OC is thought to cause a source of CO2 to the atmosphere (after degradation).
L27 “Globally, this lateral input increases ocean primary productivity and contributes to an estimated coastal-ocean sink of∼ 0.25 Pg C yr−1, which is roughly 17% of the global-ocean sink (Cai, 2011; Lacroix et al., 2021; Gao et al., 2023).”
I would use Dai et al. (2022) and Resplandy et al. (2024) here as more recent estimates:
Dai, M., Su, J., Zhao, Y., Hofmann, E. E., Cao, Z., Cai, W. J., ... & Wang, Z. (2022). Carbon Fluxes in the Coastal Ocean: Synthesis, Boundary Processes, and Future Trends. Annual Review of Earth and Planetary Sciences, 50, 593-626.
Resplandy, L., Hogikyan, A., Müller, J. D., Najjar, R. G., Bange, H. W., Bianchi, D., ... & Regnier, P. (2024). A Synthesis of Global Coastal Ocean Greenhouse Gas Fluxes. Global Biogeochemical Cycles, 38(1), e2023GB007803.
L157 “Additionally, the data-based products exhibited lower surface-ocean pCO2 compared
to ECCO-Darwin Baseline (Figure 1i) in the Arctic Ocean and near the periphery of Antarctica; regions where observations are highly limited in space and time.”
Could this potentially also be a sea ice representation problem?
L193 “the increase of NPP” -> The areal increase in NPP?
L197 “In Baseline, ARCT results in a CO2 uptake of roughly 0.21 Pg C yr−1.” This reads as if ARCT was a simulation, would slightly revise the wording.
Table 4. In terms of global FCO2, I would add the estimates of Aumont et al., 2002 and Lacroix et al., 2020. The table also only shows model derived estimates, whereas some budget-derived estimates also exist and are, as of now, preferably used in assessments (e.g. Regnier et al., 2022).
Aumont, O., J. C. Orr, P. Monfray, W. Ludwig, P. Amiotte-Suchet, and J.-L. Probst (2001), Riverine-driven interhemispheric transport of carbon, Global Biogeochem. Cycles, 15(2), 393–405, doi:10.1029/1999GB001238.
Citation: https://doi.org/10.5194/egusphere-2025-1707-RC1
Data sets
Compiled outputs and code R. Savelli https://doi.org/10.5281/zenodo.15512392
Model code and software
ECCO-Darwin biogeochemical runoff GitHub R. Savelli https://github.com/MITgcm-contrib/ecco_darwin/blob/master/v05/1deg_runoff
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