Seasonal dynamics of phytoplankton biomass in the coastal waters of Okhotsk Sea at southeastern Sakhalin

Seasonal dynamics of phytoplankton biomass is analyzed on the base of phytoplankton samples collected within monitored 4 polygons located near the mouths of spawning rivers at southeastern Sakhalin in May-October of 2019–2022. During the surveys, the water temperature ranged from –1.55 to 22.50 оC and salinity – from 11.8 to 32.8 psu that reflected strong freshwater influence. The phytoplankton biomass at certain stations ranged from 4.7 to 26058.1 mg/m³ indicating high spatial heterogeneity driven by local hydrodynamics. Three distinct biomass pulses were observed in spring, summer, and autumn. In 2019 when the Dolinka mouth was surveyed, the peaks of biomass dynamics occurred in May-June (with domination of Alexandrium cf. tamarense and Kryptoperidinium triquetrum) and October. In 2020 at the Dudinka mouth, the peaks were recorded in May (Sundstroemia setigera), June (A. cf. tamarense  with the biomass up to 3.2 g/m³), and September. In 2021 at the Manuy mouth, two peaks were observed in May and September; the former bloom was formed under water temperature at the bottom of –0.9 оC by Thalassiosira nordenskioeldii with the biomass up to 25.1 g/m³ – the first record of such high biomass in shallow coastal waters of the Okhotsk Sea. In 2022 at the Gornaya mouth, the succeeding peaks appeared in June (K. triquetrum and Rhizosolenia hebetata), July (Leptocylindrus danicus and Noctiluca scintillans), August (Coscinodiscus asteromphalus), and October (Guinardia striata and Thalassionema frauenfeldii). Dominant species in spring changed with temperature: T. nordenskioeldii and R. hebetata dominated in the years with cold spring (below 4 оC) but S. setigera dominated in warmer years (above 5 оC). The autumn peaks of biomass were observed in all years. This phenomenon contradicts with typical succession of phytoplankton on the shelf of Okhotsk Sea where an autumn peak is not usual, that reflects local specificity of pre-estuarine areas where the river runoff, wind and tidal mixing, and upwelling provide additional abilities for diatoms development. Among dinoflagellates, A. cf. tamarense reached high biomass under stable thermal conditions (~8 оC), whereas K. triquetrum dominated under windy and colder conditions. However, both species were rare at the Manuy mouth (in 2021), possibly because of rocky substrates unfavorable for the dinoflagellate cysts accumulation.

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