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 ‘ocean colour’ 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.