Changes in near-shore phytoplankton community and distribution, southwestern Caspian Sea

Sampling was done over a year (December 2020–November 2021) to study the distribution and species composition of phytoplankton in the southwestern Caspian Sea. Sea water from various depths was collected along four transects: Lisar, Anzali, Sefidrood and Chaboksar, with a total of 12 stations. A comparison was then made with the data collected in 1996 to look at the long-term changes. The increase in sea water temperature and salinity, especially in summer, was observed in the present study, which was possibly due to global warming and climate change. Amongst 48 identified phytoplankton species, Bacillariophyta (80.0%) and Cyanophyta (11.0%) were dominant. Phytoplankton abundance was measured as 99,775 ± 30,200 cell L⁻¹ on average. The highest abundance was a non-native species Pseudo-nitzschia seriata (H. Peragallo 1900) from Bacillariophyta with more than 17,000 cell L⁻¹, followed by another toxic species Nodularia spumigena (Mertens ex Bornet and Flahault 1888) from Cyanophyta. The N. spumigena species had the highest abundance as 5,000,000 cell L⁻¹ in July 2021 at the Anzali region. Nutrient increase of more than threefold in 2020–2021 has caused a sevenfold increase of chlorophyll-a as compared to that in 1996. The increases in nutrients levels, sea surface temperature (> 30 °C) and salinity are generally regarded as important factors to cause variations in phytoplankton abundance and composition. The sevenfold increase of phytoplankton abundance between 1996 and 2020–2021 and change of trophic state from oligotroph to mesotroph, indicated the increase of stress and pressure on the native phytoplankton community.