Daily dynamics of phytoplankton photosynthetic parameters in the surface layer

Study of the daily dynamics (DD) of photosynthesis quantum yield (φ) is relevant for estimation of primary production. Dependence of the DD of the maximum quantum yield of the photochemistry of photosystem II (Fv/Fm) and the light saturation coefficient (Ik) on the photosynthetically available radiation (PAR) in the surface layer was analysed in the Black Sea. The Fv/Fm decreased (10% and 50%) at midday, whereas the Ik increased (two- and threefold) at midday on 11 and 20 June 2024, correspondingly. We proposed to retrieve the Ik DD based on the Ik value at the reference time (t0) and the sun DD. The most accurate retrieval of the Ik DD was at t0 close to midday, when the Ik reached maximum values. The proposed approaches for calculation of the φ daily average (φ¯) are based on: (1) Fv/Fm and PAR measured at t0 and retrieved Ik DD; (2) PAR measured at t0, measured Fv/Fm DD and retrieved Ik DD, and (3) measured Fv/Fm and Ik at t0 and measured daily PAR. All proposed approaches provided correct assessment of the φ¯ for all times of day except for early morning and late evening.