Sensitivity of euphotic zone properties to CDOM variations in marine ecosystem models

In marine ecosystem models, the underwater light intensity is commonly characterized by the shading of phytoplankton in addition to a background light attenuation coefficient. Colour dissolved organic matter (CDOM) is an important component of the background light attenuation, and we investigate how variation in CDOM attenuation affects euphotic zone properties in a general marine ecosystem model. Our results suggest that euphotic zone properties are highly sensitive to CDOM variations occurring in nature. While the nutrient input to the euphotic zone scales the magnitude of the primary production, the vertical structure of nutrients and phytoplankton is largely determined by the variation in CDOM attenuation in our simulations. This suggests that knowledge of CDOM variation is useful to constrain uncertainties in predictions of water column structure in marine ecosystem modelling, but also in analyses utilizing the oceanic nutricline depth as proxy for primary production. Finally, according to our sensitivity analysis, many coastal areas experiencing high loads of terrestrial CDOM are expected to show eutrophication symptoms induced by altered optics.