Determination of algal bloom potential and water quality based on chlorophyll-a, abundance and biomass of phytoplankton in the coastal waters of southern Caspian Sea (2018-2019)

In the last two decades, the Caspian Sea has faced various ecological events, such as harmful algal blooms and increasing in the trophic level. It is necessary to conduct monitoring studies in this water body, because of applying the Caspian Sea for fisheries activities. The objectives of the present paper are the study of algal bloom potential and trophic level in the Iranian coastal of southern Caspian Sea up to 30m depth, during the year of 2018-2019 as well as the comparison of the obtained results to the condition in previous years. Boundary lines for the beginning of blooms (annual median+5%) based on the chlorophyll-a concentration, abundance, and biomass of phytoplankton were obtained 2.02 (mg/m3), 176 (million cells/m3), and 312 (mg/m3), respectively. The lowest percent frequency of bloom (13%) was recorded in spring and summer based on the chlorophyll-a, abundance, and biomass of phytoplankton, respectively. According to calculations based on the accumulation of the aforementioned parameters, water has excellent quality (oligotrophic state) in summer and moderate quality (mesotrophic state) in other seasons. An interannual study of trophic states showed oligotrophic, mesotrophic, oligo-mesotrophic, and mesotrophic level in 1996-97, 2009-10, 2013-2014, and 2018-2019, respectively. The highest seasonal abundance of species (187 cells/ml) was recorded in Pseudonitzschia seriata. This species is considering a serious ecologic problem and decreasing water quality in the winter season. The bloom potential of Pseudonitzschia seriata was obtained in the “medium density” group. It should be noted that the increasing trend of inter-annual trophic levels is the alarm of environmental health and show the ability of the ecosystem to thrive the harmful algal blooms. The environmental alarm is more emphasized in the central region (with a history of macroscopic toxic algal blooms) because the water quality in the central area (medium quality) was lower than in the eastern and western areas.