Marine bio-optical properties applied to biogeochemical modelling

The seminal idea of optical oceanography is that by inspecting the colour of the ocean we can get a grasp on the biogeochemical composition of the water body. The field is used in many applications, ranging from ecology and biogeochemistry, to understanding the possible hazards in our oceans and the emerging trends of climate change. The term \u2018ocean colour\u2019 stems from the fact that the visible part of the spectrum is used by the ocean ecosystem for photosynthesis, which accounts for almost half of the global photosynthesis on Earth. The final goal of the thesis project is to improve the quality of Copernicus Marine Environment Monitoring Service (CMEMS) biogeochemical products for the Mediterranean Sea through the development of a new optical module for the MedBFM forecasting model system. CMEMS products quality assessment requires the comparison of model outputs with observations and the use of specific metrics. A quality-controlled bio-optical in-situ data set from the Biogeochemical-Argo Mediterranean floats network (BGC-Argo, with 4 radiometric, 2 physical and 1 biogeochemical variable) and remote sensing products from the Copernicus Marine Data Stream (inter-annually variable weekly data of diffuse attenuation coefficients of downward planar irradiance, Kd(var), at 490 nm) were used for such purpose. In both cases, the optical data (PAR profiles and Kd(490) maps respectively) served as model input for the MedBFM system (in 1- and 3-dimensional settings), whilst the biogeochemical data from BGC-Argo floats (fluorescence derived chlorophyll concentration profiles) and HPLC-obtained chlorophyll data from an openly accessible database were used for validation purposes. The work included two different MedBFM model configurations: firstly, in the form of a non-assimilative 1-dimensional model with various bio-optical and mixing parametrizations, where the former might serve both as a first step towards more complex optical representations and could on the other hand have a diagnostic utility by inspecting the product quality through the use of BGC-Argo floats. The combined use of a biogeochemical model of medium complexity) with a rich data set enabled also an in-depth study on the optics-related biogeochemical properties of the examined basin. The second configuration focused on the impact of using weekly variable Kd(var) versus climatological Kd(clim) values as a full 3-dimensional model optical forcing, thus estimating the effect of an updated data set in terms of spatio-temporal variability of the chlorophyll field and output quality. Such an integrated approach is useful as a first step towards the improvement of the new optical component of the 3-dimensional biogeochemical Mediterranean Sea model, striving towards the implementation of a hyperspectral radiative transfer model, which would present a fundamental upgrade to obtain a more accurate description of the underwater light field, impacting both biogeochemistry and hydrodynamics

Merging bio-optical data from Biogeochemical-Argo floats and models in marine biogeochemistry

In numerical models for marine biogeochemistry, bio-optical data, such as measurements of the light field, may be important descriptors of the dynamics of primary producers and ultimately of oceanic carbon fluxes. However, the paucity of field observations has limited the integration of bio-optical data in such models so far. New autonomous robotic platforms for observing the ocean, i.e., Biogeochemical-Argo floats, have drastically increased the number of vertical profiles of irradiance, photosynthetically available radiation (PAR) and algal chlorophyll concentrations around the globe independently of the season. Such data may be therefore a fruitful resource to improve performances of numerical models for marine biogeochemistry. Here we present a work that integrates into a 1-dimensional model 1314 vertical profiles of PAR acquired by 31 BGC-Argo floats operated in the Mediterranean Sea between 2012 and 2016 to simulate the vertical and temporal variability of algal chlorophyll concentrations. In addition to PAR as input, alternative light and vertical mixing models were considered. We evaluated the models\u2019 skill to reproduce the spatial and temporal variability of deep chlorophyll maxima as observed by BGC-Argo floats. The assumptions used to set up the 1-D model are validated by the high number of co-located in-situ measurements. Our results illustrate the key role of PAR and vertical mixing in shaping the vertical dynamics of primary produces in the Mediterranean Sea. Moreover, we demonstrate the importance of modeling the diel cycle to simulate chlorophyll concentrations in stratified waters at the surface