Scale characteristics of the bloom event: A case study in the Iranian coastal waters of the Southern Caspian Sea

Nutrient enrichment in water and sediments due to excessive anthropogenic activities in recent years has caused excessive algal growth in the Caspian Sea. The current study was conducted to determine the abundance of phytoplankton community, the dominant species and chlorophyll-a [Chl-a] concentration during algal blooms in the Iranian coastal waters of Caspian Sea through four seasons from 2013 to 2014. The minimum and maximum phytoplankton abundance recorded were 73±31 and 505±55 million cells m-3 in summer and winter, respectively. The median concentration of Chl-a increased to 5.81 mg m-3 in autumn, as compared to the annual median value (2.43 mg m-3). The results indicated that the bloom started in autumn and it continued falling with a low concentration during winter (Chl-a: 2.59 mg m-3). The three species Stephanodiscus socialis, Binuclearia lauterbornii and Thalassionema nitzschioides were classified in medium bloom class (100-1000 million cells m-3) in spring, summer and autumn, respectively. While in winter Pseudonitzschia seriata (harmful species) and Dactyliosolen fragilissima were classified in medium bloom class with high relative frequency. The scaling of bloom abundance revealed that bloom initiation coincided with 10 million cells m-3 of the dominant phytoplankton species. The bloom at the regions with more than 100 million cells m-3 of total phytoplankton abundance and dominant species was overlapped with the bloom regions based on Chl-a concentration.