Purpose. The purpose of the study is to analyse the spatial variability of chlorophyll a concentration (TChl-a) and spectral coefficients of light absorption by phytoplankton (a_ph(λ)) as well as to determine the relationship between a_ph(λ) and TChl-a in the wavelength range from 400 to 700 nm at 2 nm intervals for different layers of the euphotic zone.

Methods and Results. The research was carried out at 79 stations in August–September 2023. Water samples were collected using Niskin bottles, chlorophyll a concentration was determined spectrophotometrically, and spectral absorption coefficients were measured on moistened glass fiber filters using an integrating sphere. The vertical structure was analysed based on the CTD data, chlorophyll a fluorescence, and photosynthetically available radiation (PAR). The upper mixed layer (UML) and the underlying layers down to the depths with 1 and 0.1% of PAR were identified. The parameterisation was performed using a power function a_ph(λ) = A(λ) · TChl-a^B(λ) for two layers of the euphotic zone and the underlying layer. TChl-a in the surface layer varied within 0.7–14 mg·m⁻³ in the Pacific Ocean and within 0.87–8.9 mg·m⁻³ in the Sea of Okhotsk. Four types were identified. Their formation resulted from water stratification, its location within the euphotic zone and nutrient supply. The coefficients A(438) and A(675) were 0.058 and 0.023 m²·mg⁻¹ for the UML, and 0.043 and 0.020 m²·mg⁻¹ for the layer between UML and the bottom of the euphotic zone. It was shown that at the same light intensity, the chlorophyll a-specific absorption coefficient a_ph*(λ) decreased with increasing TChl-a, which was related to the stronger pigment package effect as the proportion of large-celled species in phytoplankton increased.

Conclusions. For the first time, parameterisation of light absorption by phytoplankton for the study area was performed within the 400–700 nm range (at 2 nm intervals) for two layers in the euphotic zone and for the underlying one. Statistically significant differences between the parameterisation coefficients of these layers were revealed. The obtained relationships can be used to retrieve the a_ph(λ) spectra from TChl-a, which is necessary for assessing the photosynthetic characteristics of phytoplankton and determining primary production using the spectral method, as well as for developing regional satellite algorithms and operational monitoring of water productivity off the Kamchatka Peninsula.