The authors present a review of the state-of-the-art in biogeochemical modelling, covering various aspects such as usage, complexity, limitations, cost. A list of common biogeochemical models is also provided.

All in all, the paper covers the topics that one would expect to find when first learning about BGC models, and thus fulfills its role as a chapter in the special issue (if I understood correctly).

I have only a few minor comments:

* line 45, some modelled processes may not conserve mass, if some of the implied variables are not one of the model state variables (e.g. denitrification in the Bamhbi model). Thus, the sentence at it is now may be misleading

* line 78: define DMS, unless it is defined in previous chapters of the special issue ?

* line 102: "most fitted" makes me think of fitting a curve through some points (in a statistical sense). Maybe you meant "most fit" or "fittest" (species) ?

* line 119: define POP

* line 150: remove "several"

* model list: consider adding Darwin to the list (the BGC model associated with MiTgcm) ?

Review of “Numerical Models for Monitoring and Forecasting Ocean Biogeochemistry: a short description of present status“

By Gianpiero Cossarini, Andy Moore, Stefano Ciavatta, and Katja Fennel

The paper provides an overview of the biogeochemical models used for operational oceanography today. It provides basic information about how marine biogeochemical models are coupled with ocean general circulation models and discusses uncertainties related to parameterisations, initial conditions, and the lack of observations. The paper is well written and organized. With the understanding that is is not a classical science paper, but an overview in the context of a report on operational oceanography, I have only a few comments and corrections listed below:

Specific comments

Line 32: “… while detailed descriptions and discussions can be found in the following articles ”.  It sounds like these three papers are the definitive list to read if you want to read all about models in operational oceanography, I suggest to change “while” with “more”.

Line 57: “Rather, equations describing biogeochemical processes rely on empirical relationships based on laboratory experiments (e.g., nutrient limitation experiments, grazing dilution experiments), biological theories, and ecological principles based also on biogeographic relationships.”  Here you should also mention conservation of matter, which is one solid principle that can be applied in these models.

Line 97: I am uncertain about what you mean by this regarding the subdivision of zooplankton “and its role within an end-to-end ecosystem approach (Mitra et al., 2014).“ Is this with respect to who eats them?

Line 119: I suggest to include this paper by Bieser et al., 2023 in the reference list (https://doi.org/10.5194/gmd-16-2649-2023)

Line 122: I think there are quite a few physical models (even if there are more biogeochemical models). I think it would be better to highlight that physical models solve the same equations, but differ mainly in how they are discretized on the horizontal and vertical grid.  Physical models also differ in how they parameterized subgridscale processes.  Biogeochemical models, on the other hand, solve entirely different sets of equations, in addition to being discretized on different grids and having to parameterize processes that are not included explicitly.

Line 140: PICES also exists with variable stoichiometry (PISCES-QUOTA), but the version used operationally uses constant stoichiometry, this should probably be mentioned.

Technical corrections/language

Line  40-41: Use subscript H and V in KH and KV.

Line 49: Suggest: “Different schemes can be used to couple the physical and biogeochemical processes to optimize accuracy and computational cost (Bruggeman and Bolding, 2014; Cossarini et al., 2017).”

Line 78: to define = for

Line 108: The microbial…

Line 117: Just nekton (“organism” is unnecessary)