Introduction
Wetlands occupy only about 6% of the land on Earth, yet they provide habitat for 40% of all species and offer countless benefits (Mitsch et al., 2015; Kingsford et al., 2016; Khatun et al., 2021). In Iran, large wetlands cover approximately 1.7% of the country’s surface area; however, their destruction is occurring rapidly, with a loss of about 14% of this area between 2011 and 2021 (Mahdian et al., 2024). Anzali Wetland, located southwest of the Caspian Sea, is notable for its unique characteristics and was one of the first wetlands registered on the Ramsar Convention list in June 1975 (RIS, 2023). Like other aquatic ecosystems in Iran, Anzali Wetland has undergone rapid destruction. Despite being geologically young (Leroy et al., 2011), it has aged quickly and is approaching its final stage of succession. Long-term monitoring shows that the wetland's water surface area decreased by approximately 80%, from about 258 km² in 1930 to about 52 km² in 1989. This trend continued from 1989 to 2020, with a decrease of approximately 0.75 km² per year, resulting in a shrinkage of the wetland to 25.9 km². This pattern of degradation raises concerns about the wetland's potential complete disappearance in the near future (Aghsaei et al., 2020; Mahdian et al., 2023). Currently, the Caspian Sea level is decreasing by 8 cm per year (Chen et al., 2017; Prange et al., 2020; Lahijani et al., 2023), contributing to the wetland's shrinkage, as it is hydraulically connected to the sea. Finally, the Anzali Wetland will be desiccated between 2058 and 2062 in non-conservative approach (Mahdian et al., 2024). Due to these factors and other anthropogenic influences, Anzali Wetland was included in the Montreux Record, highlighting the urgent need for conservation and restoration efforts (JICA et al., 2005; RIS, 2023). The connection between the Caspian Sea and Anzali Wetland enhances its role as a habitat for spawning and as a nursery ground for anadromous fish. Historically, more than 80% of the total fishing along the Iranian coast of the Caspian Sea occurred in Anzali Wetland (Holčík and Oláh, 1992). Historical data indicate that total fish catches in the wetland between 1932 and 1940 ranged from 4,000 to 7,500 tons (~218 kg/ha), primarily consisting of Caspian Sea fish such as kutum, bream, and pike-perch. The fishery value of Anzali Wetland has declined due to reductions in water depth and surface area, reaching only 17 kg/ha by 1990 (Hydropriject, 1965; Kimbal and Kimbal, 1974; Nezami, 1994). The severe decline in fish stocks prompted a limnological investigation of Anzali Wetland in collaboration with Iranian and World Fisheries Organization experts in 1980 (Holčík and Oláh, 1992). This investigation yielded a report on the status of fishing and proposed solutions for stock rehabilitation. Subsequent studies have periodically explored various topics, including the determination of fisheries potential, hydrology, hydrobiology, and the ecology of Anzali Wetland (Nezami, 1994; Khodaparast, 2003; Mirzajani, 2009; Fallahi, 2018). Aquatic macroinvertebrates are crucial organisms in all aquatic ecosystems. Their abundance and distribution respond significantly to nutrient levels and pollution, making them valuable indicators for ecological assessments ( Hilsenhoff, 1988; Barbour et al., 1996; Lenat, 1998; Overton, 2001; Bode et al., 2002). They also serve as food resources and contribute to energy transfer for fish and other aquatic organisms (López–López et al., 2015). Additionally, macroinvertebrates act as a link between primary producers, decomposers, and higher trophic levels, playing a major role in the detritus food chain (Neogi et al., 2016) and enhancing the productivity of aquatic environments (Sarkar et al., 2020; Karmakar et al., 2022). Their biomass positively affects fish density and growth rates (Richardson, 1993). Macroinvertebrates are among the most important organisms in Anzali Wetland, and they play a crucial role in fisheries production, yet only a few studies have been conducted on them. Notable research from the last two decades includes assessments of the inlet rivers to Anzali Wetland based on benthic communities (Mirzajani et al., 2008), surveys of macro-benthic organisms in the southwestern region of the wetland (Jalili et al., 2011), identification of Oligochaetes (Annelida, Clitellata) in Anzali Wetland (Nazarhaghighi et al., 2014), identification of Limnodrilus species in the wetland (Naeemi et al., 2015), studies on benthic macroinvertebrates during 2014 (Ghane et al., 2017), and health assessments of the Shanbeh-Bazar River using macroinvertebrates and water quality parameters (Foomani et al., 2020). None of the previous studies examined the relationship between fish production and macroinvertebrates. This study aims to investigate the abundance and biomass of benthic and epiphytic macroinvertebrates to assess the potential for benthophage fish production in Anzali Wetland.
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 was comprised 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., 2020). Today, the water bodies in most areas, including Shijan, Sorkhankul, and much of Siah Keshim, have dried up or become very shallow or limited in extent.
Benthic and epiphytic macroinvertebrates were sampled from various locations within the wetland, and their biomass was measured. For benthic sampling, a Van Veen grab with a surface area of 225 cm² was used at 11 stations, with three replicates taken at each location. The sampling periods occurred approximately every 45 days in March, April, June, August, October, November 2023, and December 2024. Epiphytic macroinvertebrates were seasonally sampled from submerged plants at 10 stations, specifically in the western part of the wetland in April, July and October 2023 and February 2024. Aquatic plants were collected with three replicates using a rake with a 30 cm diameter, rotated in a circular motion. The plants were washed several times, and the organisms were carefully separated. In the laboratory, the collected specimens were classified into taxonomic groups at the family or genus level, using established references (Macan, 1968; Merritt et al., 2008; Thorp and Covich, 2009). The biomass of the organisms was measured with a balance accurate to 0.0001 g. The prediction of benthophage fish productivity was based on macroinvertebrate biomass, calculated using the following equation (Li and Mathias, 1994):
Fish productivity=
where B is the biomass of macroinvertebrates, P/B is the ratio of production to standing biomass of food organisms, which was considered to be 4 according to Li and Mathias (1994). The food utilization coefficient (Uf) and the feed conversion ratio (FCR), were set at 25% and 5, respectively (Li and Mathias, 1994).
Results
Benthic macroinvertebrates were classified into 12 families across 8 orders, including insects, worms, mollusks, and crustaceans. Diversity and abundance were higher at stations located in the open water bodies, particularly in the Siah Keshim and western parts, as well as at stations 4 and 8. The families Naididae and Chironomidae were the most abundant among the benthic organisms, with densities of 128 and 61 individuals/m², respectively. The highest occurrence percentages were also observed in these families, while Coenagrionidae and Simuliidae had the lowest percentages. The epiphytic macroinvertebrates were identified across 18 families belonging to 11 orders. The highest occurrence percentages were found in the orders Amphipoda and Gastropoda. Chironomidae were the most abundant epiphytic macroinvertebrates, with 82 individuals/m², followed by Planorbidae, with 58 individuals/m². The total abundance of benthic organisms was higher at stations 9, 10, and 11 compared to other stations, ranging from 400 to 625 individuals/m², primarily due to the dominance of the Naididae, Chironomidae, and Lumbriculidae families. Benthic biomass was highest at station 4, followed by stations 11, 10, and 8, varying between 6.4 and 11.1 g/m², linked to the dominance of the Sphaeriidae and Lumbriculidae families. Total biomass of epiphytic macroinvertebrates varied from 1.9 to 6.4 g/m², dominated by the Gammaridae and Lymnaeidae families. The mean (±SE) total biomass of benthic and epiphytic macroinvertebrates was 4.4 ± 0.9 g/m² and 3.1 ± 0.6 g/m², respectively. Temporal changes in macroinvertebrate biomass showed that the biomass of organisms was greater in winter-spring compared to summer-autumn, decreasing to less than 1 g/m² in summer and early autumn. Overall, benthophage fish production was estimated at 15.1 kg/ha.


Discussion and conclusion
In this study, the greatest diversity of macroinvertebrates was observed at the Bahmbar station, which has been reported to have good water quality, with total nitrogen and phosphorus levels lower than in many other sites (Abedini et al., 2018). The least diversity and abundance of organisms were found in most regions such as Siahdarvishan, the Sorkhankul outlet, and the eastern stations. This decline is attributed to the degradation of these areas to a riverine state, minimal water availability, and high exposure to nutrients or pollutants for most of the year. Nutrient levels were also greater in the eastern and Sorkhankul areas compared to Siah Keshim and the western areas (Abedini et al., 2018). In this study, the diversity of epiphytic macroinvertebrates was greater than that of benthic macroinvertebrates (Table 2). Annual investigations on benthic organisms from 1992 to 2002 (Mirzajani, 2009) and in 2014 (Ghane et al., 2017) observed a similar number of macroinvertebrate groups, with Chironomidae and Tubificidae having the highest percentages in terms of occurrence, abundance, and biomass among benthic organisms. The total biomass of benthic organisms was reported to range from 1.15 to 7.76 g/m² during different years from 1992 to 2002 (Mirzajani, 2009) and with a maximum biomass of 5.4 g/m² in 2014 (Ghane et al., 2017). Temporal changes in organism abundance can be interpreted through their biological cycles; for instance, the low abundance of Chironomidae in summer has been attributed to their departure from substrates and water for the final metamorphosis stage (Valipour, 1997). The decline of Anzali Wetland, coupled with the drying of large areas, excessive growth of invasive species like water hyacinth, and increasing pollutant concentrations, has severely impacted the diversity and abundance of macroinvertebrates. Even non-native species, such as Macrobrachium nipponense, which previously exhibited high abundance and biomass (Ghane et al., 2021), have seen a sharp decline in density and were not observed in the current survey's sampling units. On the other hand, high concentrations of pollutants in some regions have led to a notable decline in benthic communities. The Naididae family was the most abundant benthic group in this study (Table 2). This family can tolerate low oxygen levels (Aston, 1973; Nijboer et al., 2004); however, a mass mortality of Naididae was observed during field surveys in May 2024 in the eastern region, where there is significant discharge from the highly polluted city of Rasht. Field observations also indicated a lack of benthic organisms beneath the cover of water hyacinth, which has become extensively distributed in Anzali Wetland recently (Mirzajani, 2024). The recent negative degradation in Anzali Wetland has decreased its fisheries potential, estimated at 15.1 kg/ha for benthophage fish in this study. The highest average fish catch recorded was 218 kg/ha in 1941, followed by sharp decreases to 105 kg/ha in 1951 and 19 kg/ha in 1962 (Hydropriject, 1965). There was an increase to 75 kg/ha in 1993 due to rising Caspian Sea levels (Holčík and Oláh, 1992). Currently, fish production potential is lower than in many Iranian inland waters, such as Shovir, Taham, and Todebin, which average around 30 kg/ha (Mirzajani et al., 2020), and in various wetlands worldwide, including those in Italy (30–300 kg/ha) and countries in West Africa (~80 kg/ha) (Nezami, 1994). Considering the current water body area of 3,700 ha (Mirzajani, 2024), the total benthophage fish production is predicted to be 55.9 tons. In 2023, fish harvests in Anzali Wetland reached 93.1 tons, with cyprinid species comprising 56.6% of the total catch; common carp accounted for 24.8%, and Prussian carp for 22.1% (Daghigh Roohi et al., 2025). Anzali Wetland has undergone significant environmental changes and cannot revert to its previous status. Furthermore, predictions indicate a continued decline in Caspian Sea levels at a rate of 8 cm per year (Chen et al., 2017; Lahijani et al., 2023; Mahdian et al., 2024). Habitat rehabilitation could effectively enhance the growth and survival of many aquatic organisms, particularly macroinvertebrates and fish. Expanding water body areas, creating depth in various regions, controlling sediment in the watershed area, managing water hyacinth, and optimally harvesting aquatic plants could help restore some of the wetland’s lost capacity.
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 14-73-12-037-01051-011055. We also appreciate the help of our colleagues in Inland Waters Aquaculture Research Center.