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In this research, an attempt has been made to investigate the state of biodiversity, distribution and abundance of phytoplankton and zooplankton in the Anzali wetland, and to compare the results with previous studies and determine the changes in the plankton structure. The present study was conducted during 2023 - 2024 in the Anzali wetland. The findings showed that the number of phytoplankton phylum decreased from 8 and its genera from 117 genera in 1994 to 6 phylum and 68 genera, respectively in 2023-2024.
The findings showed that the average abundance of phytoplankton was 7.6 million cell.L-1, which decreased by 9- folds compared to 2001-2002. The highest abundance of Cyanobacteria was 1.2 million cell.L-1 and Bacillariophyta with the number of 5 million cell.L-1 in 2023-2024 had the highest abundance among phytoplankton phyla. Comparison of the abundance of cyanobacteria with 2001-2002 shows a 48-fold decrease in the abundance of cyanobacteria in 2023-2024. Also, a three-fold decrease in the abundance of Bacillariophyta was observed compared to 2013-2014. Among Bacillariophyta, the dominant genera were Nitzschia and Cyclotella, and of Chlorophyta, Ankistrodesmus and Scenedesmus, and of Cyanobacteria, the genus Oscillatoria were the most abundant. Also, the abundance of Euglenozoa increased from 1994 to 2014, so that the abundance of Euglenozoa reached 3-folds in 2023-2024.
The results showed that 58 species of zooplankton were identified in the Anzali wetland, the most genus belonged to the Rotifera phylum with 28 genera. Also, 12 genera of Protozoa and 9 genera of Arthropoda were identified. Among the zooplankton phylum, Rotifera was the most abundant with 148 ± 38 ind.l-1, followed by Protozoa with 81 ± 11 ind.l-1 and, Arthropoda with 72 ± 12 ind.l-1. The average abundance of the zooplankton was 305 ± 48 ind.l-1, which decreased by 7-folds compared to 2001-2002 ( abundance; 2200-2300 ind.l-1) and 2014-2015 (4000 ind.l-1) by 13-folds. The abundance of Rotifera and Protozoa decreased by 13 and 19-folds in 2023-2024, respectively compared to 2014-2015. The dominant genera were Brachionus and Rotatia from Rotifera; genera Centopyxis and Arcella from Protozoa; and genus Cyclocypris and group Copepoda from Arthropods. Probably, factors such as the decrease in the height of the Caspian Sea, the decrease in rainfall due to climate change, the increase in bed sediment and the bloom of the alien species Eichhornia crassipes, and the lack of proper management of the wetland have caused a sharp decrease in the biodiversity and abundance of the phytoplankton community of the Anzali wetland compared to the previous decades.
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