Purpose. The purpose of the study consists in identifying the characteristic features of the distribution of hydrophysical and biogeochemical parameters of marine environment depending on the tide phases in the Onezhskiy Bay (the White Sea) in September.

Methods and Results. In the cruise of the R/V «Ekolog» (September 6–11, 2019), synchronous integrated hydrological and biogeochemical studies were for the first time performed in the Onezhskiy Bay depending on the phase of a tidal cycle in September. The standard methods applied for this purpose included two sections in the White Sea, namely along the Western Solovetskaya Salma Strait and through the Onezhskiy Bay from north to south. This permitted to determine the chlorophyll a and nutrients contents, the taxonomic composition, abundance and biomass of phytoplankton including its vertical distribution within the photic zone, as well as the qualitative and quantitative composition of zooplankton. Organic forms of nitrogen (0.62–0.83 mg/l) prevailed among the nutrients in the Onezhskiy Bay, the contents of Pmin and Porg were close (on average 9 μg/l), the concentration of phosphorus mineral forms was predominant in the water bottom layer at the deep-sea stations. In the Western Solovetskaya Salma, the phytoplankton biomass average values during high and low water were 6.75 ± 1.18 mg C/m³ and 10.25 ± 11.34 mg C/m³, and in the Onezhskiy Bay – 8.07 ± 2.43 mg C/m³ and 16.61 ± 13.54 mg C/m³, respectively. Phytoplankton was represented by diatoms, dinophytes, cryptophytes and dictyochas. In the southern part of the Onezhskiy Bay, a significant increase in the abundance of all common zooplankton species was found at night.

Conclusions. In the area under study, the impact of the tidal cycle phases on spatial and temporal variability of the marine environment characteristics was manifested in a change in the thickness of the layer of temperature and salinity surface anomalies; position of the Onezhskiy frontal section shifted by 8–9 km; the changes in the nitrites, ammonium ions and chlorophyll a concentrations, and also in the phytoplankton biomass were statistically insignificant; the composition of dominant phytoplankton species did not change; the horizontal distribution of zooplankton, primarily its warm-forms, corresponded to the water temperature horizontal gradient: in the southern part of the Onezhskiy Bay, the abundance of boreal species is by orders of magnitude higher than that near the boundary with the basin.