Assessment of primary production in optically complex waters: Towards a generalized bio-optical modelling approach

Primary production is the most fundamental ecological process in surface waters. Bio-optical models allow for the estimation of aquatic primary production from optical remote sensing data, providing a cost-effective alternative to incubation measurements. Existing bio-optical models use different parameters for common model components, namely: (1) the vertical resolution of chlorophyll- a , (2) the planar-to-scalar irradiance conversion factor, and (3) the photosynthesis-irradiance relationship. Using a unified modelling framework, we evaluate the impact of these three model components for optically complex inland and transitional waters by testing 16 model configurations against a comprehensive in situ dataset spanning oligotrophic to eutrophic conditions in Lake Geneva, three Estonian lakes, and three stations in the Baltic Sea. Our results demonstrate that the choice of photosynthesis-irradiance function is the most critical component, causing a 23.2 percentage range in median magnitude error and a 0.26 range in median vertical shape correlation. In contrast, using depth-resolved chlorophyll- a profiles shows a modest impact on primary production estimates, while the choice of irradiance conversion factor has a negligible impact. Time-series analysis further reveals that while the models successfully reproduce seasonal dynamics, their ability to resolve short-term variability is limited. Our findings underscore that model accuracy is constrained by the parameterisation of model components, uncertainties in empirical constants, and the in situ validation data itself. This highlights the need to prioritise the refinement of photosynthesis-irradiance relationships alongside community efforts to develop standardised validation datasets that include concurrent measurements of carbon isotope incubations and inherent optical properties.