Introduction
Anzali Wetland, located in the southwest of the Caspian Sea, is notable for its unique characteristics. It was one of the first wetlands registered on the Ramsar Convention's list. However, it is rapidly deteriorating and has been included in the Montreux Record, highlighting the need for conservation and restoration efforts (Ramsar Convention Bureau, 2014). Aquatic plants are a key component of the wetland ecosystem, playing a crucial role in nutrient cycling and providing habitat for many animals (Kurniawan et al., 2021; Dissanayaka et al., 2023). In recent years, the increase in nutrient influx has led to significant growth of aquatic plants in various parts of Anzali Wetland. This growth, including both floating and submerged species, has negatively impacted the oxygen levels, temperature, and pH of the wetland's water (Filizadeh and Khodaparast, 2005). Numerous studies have focused on the identification of aquatic plant species in different areas of Anzali Wetland. The most important studies include the investigation of plant species biodiversity in the 1960s, which identified 194 genera and 291 species (Ghahraman and Atar, 2003), as well as a more recent document that provides a checklist detailing 69 families and 362 species in Anzali Wetland (Zehzad, 2017). However, research on the growth and density of plant communities remains limited. Notably, only one study has addressed the distribution and abundance of aquatic plants in the western part of Anzali Wetland (Hosseinjani et al., 2017; Mirzajani et al., 2020a). Monitoring the distribution and biomass of dominant plant species from different ecological types is essential for effective wetland management. Therefore, this study investigates the biomass of dominant floating and submerged plants in the water body of Anzali Wetland and examines the distribution of prominent marginal plants along the wetland's boundaries.

Methodology
Anzali Wetland is located at a latitude of 37°28' North and a longitude of 49°25' East, with an average elevation of -23 meters below sea level. In the recent past, Anzali Wetland consisted of four main sections: the eastern part (Shijan), the central part (Sorkhankul), the western part (Abkenar), and the southern part (Siah Keshim) (Mirzajani et al., 2020b). Today, the water bodies in most areas, including Shijan, Sorkhankul, and much of Siah Keshim, have dried up or become very limited and shallow.
Sampling of submerged plants was carried out using a rake at 10 stations during four seasons in 2023. The aquatic plants were extracted from the water by rotating the rake with a circular motion, harvesting diameter of 30 cm. The samples were then washed with water to remove mud before being identified using reference materials (Abbasi, 1377; Riazi, 1996). Afterward, the samples were air-dried for 8 to 24 hours, and their wet weight was measured using a scale with an accuracy of 0.1 grams. The samples were then oven-dried at 70°C for 24 hours and weighed again. To measure the organic matter and ash content of each species, several subsamples were weighed on a balance with an accuracy of 0.0001 grams, burned in a furnace at 450 degrees Celsius for 4 hours, and weighed again (ASTM, 2000). The extent of marginal plants was determined using satellite images and the Normalized Difference Vegetation Index (NDVI), while the distribution of floating plants within the water body was assessed through visual interpretation of Google Earth images.
Results
In the western part of the Anzali Wetland, the submerged species Ceratophyllum demersum, Myriophyllum spicatum, Stuckenia pectinata, and Hydrilla verticillata exhibited high densities, while Najas marina and Potamogeton crispus were abundant in some areas. The average biomass of submerged aquatic plants was 547.7±313.9 g/m², varying from 400 to 983 g/m² across different stations. C. demersum showed the highest biomass at 53.8%, followed by H. verticillata at 18.9% and M. spicatum at 10.6%. The biomass of C. demersum ranged from 146 to 595 g/m² at various stations. The highest average plant biomass was measured in July (730 g/m²), while the lowest was recorded in May (388 g/m²) (Fig 1). The organic matter content of submerged plants was found to be 1.72% of their dry weight, and the contribution of organic matter to the sediment of Anzali Wetland due to winter die-off was estimated at 246-560 grams per square meter. Among the floating plants, Nelumbo nucifera and Pontederia crassipes were the dominant species, covering areas of 350 and 970 hectares, respectively. The final weight of water hyacinth was estimated at 150 tons per hectare. Other floating plants, including Hydrocotyle spp., Nymphoides cristata, Lemna spp., Spirodela spp., Salvinia natans, and Trapa natans, were observed in limited and patchy distributions across some stations. The leaf dry weight of N. nucifera was measured at 219.5 g/m², while the total dry biomass of P. crassipes was measured at 1620 g/m². Their total organic matter values were approximately 88.5% and 78.2% of their dry weight, respectively. P. crassipes occupied marginal areas and shallow zones of the western wetland, along all canals, drains, and entrances that have slow water flow, with no area completely devoid of this plant. When this plant dies back in winter, it contributes about 1267 grams per square meter of organic matter to the wetland substrate. The common reed, Phragmites australis, is also a dominant species in the marginal areas of the wetland. Its distribution was estimated to cover 10,290 hectares, as determined through visual interpretation of Google Earth images.


Figure 1: Average dry weight of submerged plants in different areas of the western part of Anzali Wetland (a) during the months of 1402
Discussion and conclusion 
In recent years, the shrinking of the Anzali Wetland and the conversion of water bodies to other land uses have intensified. Comparing the extent of land uses within the wetland bed in 2022 to that in 2001 showed that the water body had decreased by about 40%, while urban areas increased by 114%, paddy fields by 4.7%, forests by 54%, and common reeds by 3.6%. Comparing the results of this research with wetland plant surveys from previous years (Hosseinjani et al., 2017; Mirzajani et al., 2020a) indicates rapid changes in the aquatic plant communities of the wetland. The highest distribution and density of Azolla filiculoides was recorded in the summer of 2017, while it was not observed in the main areas of the wetland in subsequent years. The biomass of submerged plants in this survey was almost similar to that of the 2017 survey, but the composition of dominant species changed slightly. In 2017, the submerged species C. demersum and M. spicatum had high distribution and density (Mirzajani et al., 2020a), while in this study, the dominant species were C. demersum and H. verticillata. In 2017, two floating species, Nelumbo nucifera and Hydrocotyle spp., were dominant, especially in marginal areas (Mirzajani et al., 2020a). However, after five years, N. nucifera significantly occupied more areas of the wetland, and the invasive water hyacinth P. crassipes has grown explosively, occupying a large area of Anzali Wetland at high densities. This indicates that the high density of aquatic plants in the water body and marginal areas has become a serious problem. The excessive growth of both floating and submerged species impacts the growth and survival of many aquatic organisms, especially fish and epiphytic macroinvertebrates, sometimes leading to their mortality due to decreased water oxygen levels and changes in water temperature and pH. Therefore, optimal harvesting of aquatic plants is essential for the management of Anzali Wetland and should not be overlooked.
Conflict of Interest
The authors have no conflicts of interest to declare that are relevant to the content of this article.
Acknowledgment
The authors would like to thank the Department of Environment of Guilan Province for financially supporting this project, registered in AREEO under the code number 14-73-12-036-01051-011054. We also appreciate the help of our colleagues at Inland Waters Aquaculture Research Center.