Eva Álvarez

Spectral ocean-colour reflectance is increasingly used to constrain marine ecosystem models, yet its potential to provide direct observational constraints on process-based biogeochemical models remains underexplored. Here we calibrate multi-wavelength remote sensing reflectance (RRS(λ)) using a genetic algorithm within a one-dimensional configuration of the Biogeochemical Flux Model. The approach is applied to two contrasting Mediterranean environments: an open-ocean ecosystem in the Ligurian Sea (BOUSSOLE observatory) and an optically complex coastal site in the Northern Adriatic Sea (Acqua Alta oceanographic tower – AAOT). At both sites, the calibration successfully reproduces observed RRS(λ) at six visible wavelengths, demonstrating that the model captures the main features of the RRS(λ) spectra across contrasting trophic regimes. However, the mechanisms used by the model to match RRS(λ) differ markedly and reveal distinct controls on particle scattering. At BOUSSOLE, RRS(λ) calibration primarily adjusts phytoplankton and small-particle optical properties, improving phytoplankton biomass estimates and increasing particulate backscattering. This highlights the dominant role of small and algal particles in shaping backscattering in oligotrophic waters. In contrast, at AAOT, RRS(λ) calibration mainly modifies suspended particulate matter (SPM) optical parameters, with limited changes in the biogeochemical state. Overall, RRS(λ) calibration reproduces observed optical signals while revealing site-dependent controls on particulate backscattering. Sensitivity and calibration exercises improved predictive capability within the calibrated parameter space, but residual error patterns revealed potential structural limitations or simplifications in the model's optical representation. This work demonstrates the potential of RRS(λ) observables to guide improvements in the representation of particle optical properties and biogeochemical processes in marine ecosystem models.

The origins of ecosystem stability have long intrigued ecologists. Recent theoretical advancements highlight three primary mechanisms driving stability: dominance, asynchrony and averaging. While plant community stability has been extensively studied, our understanding of marine community stability remains limited. Plankton, as key components of marine ecosystems, play vital roles in carbon and nutrient cycling as well as climate regulation. Identifying the drivers of plankton community stability is therefore essential for effective ecosystem management and conservation. Using a biogeochemical model that realistically represents plankton biodiversity, we identified dominance and averaging as the main mechanisms driving plankton community stability. By studying plankton communities across different temporal, spatial and trophic scales, we were able to show that changes in these dimensions can alter the dominant stabilizing mechanism. Stronger stabilization arising from dominance was linked to reduced biomass evenness, while stronger stabilization through averaging was linked to increased biomass evenness. Understanding the mechanisms that underpin the stability of plankton communities is essential to anticipate how marine ecosystems will respond to environmental change and which types of disturbances most threaten plankton communities. If stability is primarily driven by dominance, the system may appear stable but remain highly vulnerable, as its balance relies on a few functional groups. In contrast, when stability arises from temporal asynchrony or the averaging effect among many groups, the community can better buffer environmental fluctuations but becomes more sensitive to biodiversity loss.

Diatoms are key contributors to marine biogeochemical cycles, yet their distinctive reproductive strategies are rarely represented explicitly in ecosystem and biogeochemical models. In particular, the alternation between asexual size reduction and sexual size restitution introduces intrinsic size-structured dynamics that may influence plankton community structure and biodiversity. Here we incorporate a size-structured formulation of diatom reproductive dynamics into a coupled optical–biogeochemical plankton model to assess their impact on plankton biomass, diversity, and trophic interactions. The model represents size-dependent asexual reproduction as a shrinking flux across diatom size classes and sexual reproduction as a restitution process activated below a critical size threshold. Using long-term simulations and a Monte Carlo ensemble exploring uncertainty in reproductive parameters, we evaluate the sensitivity of plankton communities to variations in reproduction rates. Results show that asexual reproduction exerts a dominant control on diatom biomass and size structure, producing nonlinear, threshold-like responses capable of reorganizing phytoplankton functional composition and propagate upward through the food web. Sexual reproduction acts mainly as a secondary modulator, while trophic interactions buffer variability at higher trophic levels. These findings highlight reproductive dynamics as important regulators of plankton biodiversity and biogeochemical functioning.

Diatoms are key contributors to marine biogeochemical cycles, yet their distinctive reproductive strategies are rarely represented explicitly in ecosystem and biogeochemical models. In particular, the alternation between asexual size reduction and sexual size restitution introduces intrinsic size-structured dynamics that may influence plankton community structure and biodiversity. Here we incorporate a size-structured formulation of diatom reproductive dynamics into a coupled optical–biogeochemical plankton model to assess their impact on plankton biomass, diversity, and trophic interactions. The model represents size-dependent asexual reproduction as a shrinking flux across diatom size classes and sexual reproduction as a restitution process activated below a critical size threshold. Using long-term simulations and a Monte Carlo ensemble exploring uncertainty in reproductive parameters, we evaluate the sensitivity of plankton communities to variations in reproduction rates. Results show that asexual reproduction exerts a dominant control on diatom biomass and size structure, producing nonlinear, threshold-like responses capable of reorganizing phytoplankton functional composition and propagate upward through the food web. Sexual reproduction acts mainly as a secondary modulator, while trophic interactions buffer variability at higher trophic levels. These findings highlight reproductive dynamics as important regulators of plankton biodiversity and biogeochemical functioning.

Diatoms are key contributors to marine biogeochemical cycles, yet their distinctive reproductive strategies are rarely represented explicitly in ecosystem and biogeochemical models. In particular, the alternation between asexual size reduction and sexual size restitution introduces intrinsic size-structured dynamics that may influence plankton community structure and biodiversity. Here we incorporate a size-structured formulation of diatom reproductive dynamics into a coupled optical–biogeochemical plankton model to assess their impact on plankton biomass, diversity, and trophic interactions. The model represents size-dependent asexual reproduction as a shrinking flux across diatom size classes and sexual reproduction as a restitution process activated below a critical size threshold. Using long-term simulations and a Monte Carlo ensemble exploring uncertainty in reproductive parameters, we evaluate the sensitivity of plankton communities to variations in reproduction rates. Results show that asexual reproduction exerts a dominant control on diatom biomass and size structure, producing nonlinear, threshold-like responses capable of reorganizing phytoplankton functional composition and propagate upward through the food web. Sexual reproduction acts mainly as a secondary modulator, while trophic interactions buffer variability at higher trophic levels. These findings highlight reproductive dynamics as important regulators of plankton biodiversity and biogeochemical functioning.

Chromophoric dissolved organic matter (CDOM) significantly contributes to the non-water absorption budget in the Mediterranean Sea. The absorption coefficient of CDOM, a_CDOM(λ), is measurable in situ and can be retrieved remotely, although ocean-colour algorithms do not distinguish it from the absorption of detritus. These observations can be used as indicators for the concentration of other relevant biogeochemical variables in the ocean, e.g. dissolved organic carbon. However, our ability to model the biogeochemical processes that determine CDOM concentrations is still limited. Here we propose a novel parameterization of the CDOM cycle that accounts for the interplay between the light- and nutrient-dependent dynamics of local CDOM production and degradation, as well as its vertical transport. The parameterization is included in a one-dimensional (1D) configuration of the Biogeochemical Flux Model (BFM), which is here coupled to the General Ocean Turbulence Model (GOTM) through the Framework for Aquatic Biogeochemical Models (FABM). Here the BFM is augmented with a bio-optical component that resolves spectrally the underwater light transmission. We run this new GOTM-(FABM)-BFM configuration to simulate the seasonal a_CDOM(λ) cycle at the deep-water site of the Bouée pour l'acquisition de Séries Optiques à Long Terme (BOUSSOLE) project in the northwestern Mediterranean Sea. Our results show that accounting for both nutrient and light dependence of CDOM production improves the simulation of the seasonal and vertical dynamics of a_CDOM(λ), including a subsurface maximum that forms in spring and progressively intensifies in summer. Furthermore, the model consistently reproduces the higher-than-average concentrations of CDOM per unit chlorophyll concentration observed at BOUSSOLE. The configuration, outputs, and sensitivity analyses from this 1D model application will be instrumental for future applications of BFM to the entire Mediterranean Sea in a three-dimensional configuration.

Chromophoric dissolved organic matter (CDOM) significantly contributes to the non-water absorption budget in the Mediterranean Sea. The absorption coefficient of CDOM, α_CDOM(λ), is measurable in situ and remotely from different platforms and can be used as an indicator of the concentration of other relevant biogeochemical variables, e.g., dissolved organic carbon. However, our ability to model the biogeochemical processes that determine CDOM concentrations is still limited. Here we propose a novel parametrisation of the CDOM cycle that accounts for the interplay between the light- and nutrient-dependent dynamics of local CDOM production and degradation, as well as its vertical transport. The parameterization is included in a one-dimensional (1D) configuration of the Biogeochemical Flux Model (BFM), which is here coupled to the General Ocean Turbulence Model (GOTM) through the Framework for Aquatic Biogeochemical Models (FABM). Here BFM is augmented with a bio-optical component that revolves spectrally the underwater light transmission. We did run this new GOTM-FABM-BFM configuration to simulate the seasonal α_CDOM(λ) cycle at the deep-water site of the BOUSSOLE project in the North-Western Mediterranean Sea. Our results show that accounting for both nutrient and light dependence of CDOM production improves the simulation of the seasonal and vertical dynamics of α_CDOM(λ), including a subsurface maximum that forms in spring and progressively intensifies in summer. Furthermore, the model consistently reproduces the higher-than-average concentrations of CDOM per unit chlorophyll concentration observed at BOUSSOLE. The configuration, outputs and sensitivity analyses from this 1D model application will be instrumental for future applications of BFM to the entire Mediterranean Sea in a 3D configuration.

The Mediterranean Forecasting Systems produces operational analyses, reanalyses and 10-day forecasts for many Essential Ocean Variables (EOVs), from currents, temperature to wind waves and pelagic biogeochemistry. The products are available at a horizontal resolution of 1/24 degrees (approximately 4 km) and 141 unevenly spaced vertical levels. The core of the Mediterranean Forecasting System is constituted by the physical (PHY), the biogeochemical (BIO) and the wave (WAV) components coupled offline, consisting of both numerical models and data assimilation modules. The 3 components together constitute the so-called Mediterranean Monitoring and Forecasting Center (Med-MFC) of the Copernicus Marine Service. Daily 10-day forecasts are produced by the PHY, BIO and WAV components as well as analyses, while reanalyses are produced for the past 30 years about every ~3 years and extended (yearly). The modelling systems, their coupling strategy and evolution is illustrated in detail. For the first time, the quality of the products is documented in terms of skill metrics evaluated on a common three-year period (2018–2020), giving the first complete assessment of uncertainties for all the Mediterranean environmental variable analyses.

The Mediterranean Forecasting Systems produces operational analyses, reanalyses and 10-day forecasts for many Essential Ocean Variables (EOVs), from currents, temperature to wind waves and pelagic biogeochemistry. The products are available at a horizontal resolution of 1/24 degrees (approximately 4 km) and 141 unevenly spaced vertical levels. The core of the Mediterranean Forecasting System is constituted by the physical (PHY), the biogeochemical (BIO) and the wave (WAV) components coupled offline, consisting of both numerical models and data assimilation modules. The 3 components together constitute the so-called Mediterranean Monitoring and Forecasting Center (Med-MFC) of the Copernicus Marine Service. Daily 10-day forecasts are produced by the PHY, BIO and WAV components as well as analyses, while reanalyses are produced for the past 30 years about every ~3 years and extended (yearly). The modelling systems, their coupling strategy and evolution is illustrated in detail. For the first time, the quality of the products is documented in terms of skill metrics evaluated on a common three-year period (2018–2020), giving the first complete assessment of uncertainties for all the Mediterranean environmental variable analyses.