Comparison of two light attenuation parameterization focusing on timing of spring bloom and primary production in the Baltic Sea

The physical-biogeochemical coupled model HMB-ERGOM is used to investigate the effects of light attenuation on the timing of spring bloom (TSB) in the Baltic Sea. When light attenuation was not included, the predicted TSB was earlier than observed values in shallow areas (<50 m) and the predicted primary production tended to be lower, especially in the open-sea areas. Tuning the value of related parameters could not resolve these two discrepancies simultaneously. In the present study, a new light attenuation parameter was introduced to incorporate the effects of inorganic suspended particulate matter (SPM) using bathymetry depth and vertical turbulent diffusivity. A variable optimal photosynthesis irradiance in ERGOM was replaced with a constant value. The new parameterization led to improvement in three aspects of modeled results: nutrients and chlorophyll concentrations, TSB, and primary production. However, insufficient light utilization and under-estimation of primary production in some coastal regions remain problematic. The present study demonstrates the possibility of examining the potential impacts of inorganic SPM without explicitly coupling a complicated SPM model and highlights the importance of inorganic SPM modulating TSB in shallow areas. The new parameterization could be used to examine spatial variation of TSB in the Baltic Sea.