Wetlands have played an important role for humankind in all continents. They are characterized by a large number of ecological niches and a significant percentage of biological diversity. Wetlands are among the most productive ecosystems in the world comparable to rainforests and coral reefs. Zooplankton community is cosmopolitan in nature and they inhabit all freshwater habitats of the world and their diversity and density refers to variety within the community. There are often important links in the transformation of energy from producers to consumers due to their large density, drifting nature, high group or species diversity and different tolerance to the stress  . Zooplankton diversity is one the most important ecological parameters as there is the intermediate link between phytoplankton and fish and plays a key role in cycling of organic materials in an aquatic ecosystem.
Due to their short life span, the zooplankton community often exhibits quick and dramatic changes in response to the change in the physicochemical properties of the aquatic environment. They do not only form an integral part of the lentic community but also contribute significantly to the biological productivity of the fresh water ecosystem. In the investigation, the data of zooplankton density and diversity in a moderate ecosystem of Anzali International wetland was studied monthly for one year, January 2012-December 2013 at 9 selected sites with 3 replicates.
2. Materials and Methods
2.1. Study Area
Present study has been carried out in Anzali wetland (Figure 1). This wetland
Figure 1. Sampling stations in Anzali wetland.
lies between 49˚55'E and 37˚20'N of Gilan province, North of Iran. Total area of the wetland is approximately 15000 ha with maximum depth of about 1.2 meter. The water temperature rises up to 23˚C during the month of May and falls below 10˚C in January. The wetland is a principal source of food and fishing for local dependent communities of the rural areas, whilst household sewage and agricultural run-off from surrounding rice fields are discharged into it.
2.2. Zooplankton Collection, Preservation, Identification and Density Analysis
The samples of zooplankton were collected from each selected study site of this wetland for a period of one year (January 2012 to December 2013). The nylon Plankton net of conical shape and reducing cone of (30 μ mesh size) were used for collection of zooplankton. For a precise collection of zooplankton, the plankton net was towed in open water area of each site three times. After transferring the sample in air tight plastic bottles, it was carefully labeled and preserved immediately on site using 5% formaldehyde. Later the collected samples were brought to the laboratory for identification of each genus, after that the density of zooplankton was calculated as per the lackey drop method  .
3. Result and Discussion
The zooplankton population of Anzali wetland consisted of 61 various species of zooplankton (Table 1). The recorded species were categorized into 4 different groups as Protozoa, Rotatoria, Copepoda and Cladocera.
Diversity analysis revealed that Rotatoria dominated the zooplankton assemblage of Anzali wetland with 29 species. Protozoa also is dominated with 22 species and Copepoda were the third dominant group followed by 5 species and Cladocera were the forth group with 4 species.
Distinct peaks of Protozoa, Rotatoria, Copepoda and Cladocera were observed during spring and summer  . However, the minimum population of the groups was registered during winter (Figures 2-5).
The net zooplankton abundance increased during summers probably corresponding to the water quality, decaying vegetation increased levels of organic matter in the sediment and higher abundance of bacteria in the wetlands during this time (Table 2 and Table 3)   . Sudden reduction in the zooplankton population during the winter as noticed in the present finding could be due to sudden fall of temperature and dilution in concentration of minerals and salts in wetland water (Figure 6 and Figure 7)  .
Qualitative dominance of Rotatoria over other zooplankton assemblages has been observed in Anzali wetland. Similar observations have been obtained   . Summer peak obtained for Rotatoria members in the Anzali wetland was observed may be due to optimal nutrient and temperature conditions and lower DO contents in this season. Low Rotatoria density during the cold season can be attributed to turbulence generated by the excess water flow during the season
Table 1. Identified zooplankton species in Anzali wetland.
 . Dominance of Copepoda among zooplankton peak was found during summer might be due to optimal thermal and nutritional conditions and lower concentration of oxygen  . Effect of rains may explain low records of Cladocera from November to March. Copepods developed better in warm periods as noticed in the present study. Lesser abundance of copepods as recorded in the present study had also been observed   . The low abundances of copepods in Anzali wetland appear to be due to mainly predation pressure from fishes  .
Table 2. Significant difference of zooplankton densities in different seasons.
Table 3. Significant difference of zooplankton densities in different stations.
Figure 2. Seasonal average of all groups in spring.
In general, it can be concluded that in Anzali wetland like other similar ecosystems in the world, zooplankton society has mainly contained Rotatoria, Protozoa, Copepoda and Cladocera, among them, Rotatoria and Protozoa have been observed more than other groups   . The dominant group of zooplankton communities of Anzali wetland is Rotatoria. The result of the survey by
Figure 3. Seasonal average of all groups in summer.
Figure 4. Seasonal average of all groups in autumn.
Figure 5. Seasonal average of all seasons in winter.
Figure 6. Time variation of species density.
Figure 7. Spatial variation of species density.
Consultant Engineering group showed that Rotatoria, Protozoa and Copepoda were the dominant groups of Anzali wetland which has been approved by other researches  . Finally, it was confirmed that although the rate of changes in abundance and diversity of zooplankton groups are nearly close in different stations and season but east and central part of the wetland have less population and diversity due to increasing amount of pollution in these areas.