Optimal Assimilation Number of Phytoplankton in the Siberian Seas: Spatiotemporal Variability, Environmental Control and Estimation Using a Region-Specific Model

The maximal value of the chlorophyll-specific carbon fixation rate in the water column or the optimal assimilation number (P-opt(b)) is an important parameter used to estimate water column integrated primary production (IPP) using models and satellite-derived data. The spatiotemporal variability in the P-opt(b) of the total and size-fractionated phytoplankton in the Siberian Seas (SSs) and its links with environmental factors were studied based on long-term (1993-2020) field and satellite-derived (MODIS-Aqua) observations. The average value of P-opt(b) in the SSs was equal to 1.38 +/- 0.76 mgC (mg Chl a)(-1) h(-1). The monthly average values of P-opt(b) decreased during the growing season from 1.95 mgC (mg Chl a)(-1) h(-1) in July to 0.64 mgC (mg Chl a)(-1) h(-1) in October. The average value of P-opt(b) for small (<3 mu m) phytoplankton 1.6-fold exceeded that for large (>3 mu m) phytoplankton. The values of P-opt(b) depend mainly on incident photosynthetically available radiation (PAR). Based on the relationship between P-opt(b) and PAR, the empirical region-specific algorithm (E-0reg) was developed. The E-0reg algorithm performed better than commonly used temperature-based models. The application of E-0reg for the calculation of P-opt(b) will make it possible to more precisely estimate IPP in the SSs.