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Microplastics (MPs) have garnered significant attention due to their widespread presence in the environment and the potential threats they pose to aquatic organisms (Wang et al., 2021). These particles, smaller than 5 millimeters, are classified into primary and secondary types. Primary microplastics are manufactured in micrometer sizes for use in various industries, including aerospace, medicine, and cosmetics (Alomar et al., 2016). In contrast, secondary microplastics are formed from the breakdown of larger plastic debris into smaller particles (Duis and Coors, 2016). Microplastics readily accumulate in aquatic environments and, due to their resistance to degradation, are dispersed globally (Wang et al., 2021). These particles can adversely affect aquatic organisms due to their physical and chemical properties, leading to disturbances in feeding, reproduction, and immune functions (Oliviero et al., 2019). Additionally, microplastics can adsorb pollutants such as heavy metals, exacerbating their harmful effects on aquatic ecosystems. This combination presents a significant threat to marine life health (Prunier et al., 2019). Furthermore, microplastics provide a substrate for microorganisms, facilitating the formation of biofilms. These biofilms can alter the physical and chemical properties of microplastics, influencing their ability to adsorb contaminants (Tu et al., 2020). The objective of this study is to examine the correlation between microplastic pollution and potentially toxic elements in the sediments of the southwestern Caspian Sea coast and assess their impacts on the region marine ecosystem.

The Caspian Sea, the largest enclosed lake in the world, is significantly impacted by human activities such as oil and gas extraction, agriculture, and industrial development. Rivers such as the Volga, Kura, and Ural transport pollutants, including heavy metals, to the sea, posing a threat to the ecosystem, particularly along the southwestern coast (Efendieva, 1994; Simonett, 2006). Sediment sampling was conducted at three polluted stations in Kiashahr, Anzali, and Astara (Kostianoy et al., 2005). Sediment sampling was performed in the spring of 2022 using Van Veen grabs (20×20 cm) with three replicates at each statio (Claessens et al., 2011; Löder and Gerdts, 2015). After being transferred to glass bottles and sent to the laboratory. Microplastic extraction from sediments involves two essential stages. In the first stage, the organic materials in the sediments were digested using hydrogen peroxide (H2O2, 30%). The digestion time varies between 1 and 10 days, depending on the type and amount of organic material (Erkes-Medrano et al., 2015; Zhang et al., 2016). After digestion, the samples were dried at 60°C for 48 hours (Vianello et al., 2013). In the second stage, density separation was used to extract the microplastics. In this step, 100 g of dried sediment was placed in a glass beaker, and 800 mL of saturated NaCl solution (293 g/L) was added (Thompson et al., 2004). After shaking for five minutes, the beaker was left to stand for 45 minutes to allow the high-density particles to settle. The resulting supernatant, containing the floating particles, was filtered through a nitrocellulose filter (Hidalgo-Ruz et al., 2012; Wagner et al., 2014; Duis and Coors, 2016). This process was repeated three times, and the filters were dried at 60°C (Law et al., 2010). Finally, the microplastic particles were examined and counted using a 40x magnification loop, and the number of microplastic particles per gram of dry sediment was reported (Reddy et al., 2006; Morét-Ferguson et al., 2010). The polymer types of the extracted microplastics were identified using FT-IR spectroscopy with ATR, analyzing spectra in the 400-4000 cm⁻¹ range and comparing characteristic peaks with standard polymer databases (Veerasingam et al., 2021). Data analysis was performed using SPSS version 27. The Kolmogorov-Smirnov test was used to check for normality, and to compare pollution levels across stations, ANOVA and Kruskal-Wallis tests were applied. To examine the correlation between microplastic pollution and potentially toxic elements, Pearson and Spearman correlation coefficients were used. All analyses were conducted at a %95 confidence level. Graphs were plotted using Excel 2022.

The average concentration of elements at the three stations revealed that the highest and lowest average concentrations of elements in the sediment were for manganese (Mn) with 760661.80±53.41 µg/kg dry weight at the Anzali station and cadmium (Cd) with 41.44±1.93 µg/kg dry weight at the Astara station. The results of the Kolmogorov-Smirnov test for the distribution of potentially toxic elements in the sediment samples from the stations indicated that some elements did not follow a normal distribution (p˂0.05). To compare the average concentrations of elements across the stations and examine the correlation between elements, parametric tests (one-way analysis of variance and Pearson correlation coefficient) were used for normally distributed data, while non-parametric tests (Kruskal-Wallis and Spearman correlation coefficient) were employed for non-normally distributed data. One-way analysis of variance (ANOVA) results for comparing the average concentration of elements across the stations showed that manganese and zinc had significant differences at all stations (p˂0.05). The cadmium element showed no significant difference between the Kiashahr and Astara stations, but significant differences were observed between Kiashahr and Anzali, as well as Anzali and Astara stations (p˂0.05). The Kruskal-Wallis test results also indicated significant differences for arsenic, cobalt, chromium, copper, iron, mercury, nickel, and lead among the stations (p˂0.05). The Anzali station, with 67±4 pieces per 300 grams of dry sediment, exhibited the highest contamination, while the Kiashahr station, with 45.33±2.30 pieces per 300 grams of dry sediment, showed the lowest contamination. One-way analysis of variance (ANOVA) revealed that the Anzali station had a significant difference when compared to both Kiashahr and Astara stations (p˂0.05). However, no significant difference was observed between the Kiashahr and Astara stations. The microplastics extracted from the sediment samples were categorized into two color groups: blue and red. At the Kiashahr station, red microplastics accounted for 53%, representing the highest abundance, while blue microplastics constituted 47%, representing the lowest abundance. At the Anzali station, blue microplastics were the most abundant, comprising 75%, while red microplastics represented the lowest abundance, constituting 25%. At the Astara station, red microplastics accounted for 53%, representing the highest abundance, while blue microplastics constituted 47%, representing the lowest abundance. A total of 496 microplastic pieces were extracted from the sediment samples at the three stations. All the extracted microplastics were fiber type. The microplastics found in the sediment samples were classified into seven categories: less than 0.5 mm, 0.5-1 mm, 1-2 mm, 2-3 mm, 3-4 mm, 4-5 mm and greater than 5 mm. At the Kiashahr station, the highest abundance of microplastics was found in the 4-5 mm range, followed by the 3-4 mm range. The lowest abundance was observed in the 1-2 mm particles. At the Anzali station, the highest abundance was found in particles larger than 5 mm, followed by the 4-5 mm range, while the lowest abundance was observed in particles smaller than 0.5 mm. At the Astara station, the highest abundance was found in particles larger than 5 mm, followed by the 4-5 mm range, while the lowest abundance was observed in the 1-2 mm particles. The extracted microplastics from the sediments of the southwestern Caspian Sea coast were identified using FTIR-ATR spectroscopy. Five different polymers were identified, including polyethylen (PE), polypropylene (PP), polyester, polystyrene (PS) and nylon. Overall, polyethylene was the dominant polymer in the extracted microplastics from the sediments. The correlation analysis results between the abundance of microplastics and the concentration of potentially toxic elements in the sediments from the Kiashahr, Anzali, and Astara stations indicated no significant correlation between these two variables at the stations under study. Manganese, zinc, and cadmium had a normal distribution in all stations, thus Pearson's correlation coefficient was used to assess the correlation between microplastic pollution and potentially toxic elements. For other elements, whose data did not follow a normal distribution and successful normalization techniques were not applied, the Spearman correlation coefficient was employed. 

Discussion and conclusion: 

In this study, the Anzali station exhibited the highest contamination, with an average of 67 ± 4 microplastic pieces per 300 grams of dry sediment. This finding is consistent with the study by Rasta et al. (2020) which reported a high concentration of microplastic contamination in the sediments of Anzali Wetland. All the microplastics extracted from the sediments in this research were of the fiber type, with blue being the most dominant color, accounting for 58%. Similar results were found in the study by Zhang et al., (2020) in the Shengsi region of China, where fiber-type microplastics were the most abundant, with blue identified as the predominant color. Microplastics at the Kiashahr station were most abundant in the 4-5 mm size range, while the Anzali and Astara stations exhibited the highest abundance in microplastics larger than 5 mm. Similar findings were reported by Kühn et al., (2018) on the coasts of the Netherlands, where microplastics in the size range of 500-2000 micrometers were predominant. In this study, no significant correlation was observed between microplastics and the concentration of elements in the sediments. This lack of correlation may be attributed to differences in the sources of microplastics and elements, the physical and chemical properties of these pollutants, and the varying environmental conditions (Napper and Thompson, 2016). Finally, it can be concluded that the Anzali station has the highest microplastic pollution, primarily composed of fiber type and secondary microplastics, with blue being the predominant color. Anthropogenic sources, such as laundry runoff, fishing gear, and the release of plastic packaging by tourists, contribute to the spread of this pollution in the marine environment. Correlation analysis revealed no significant relationship between microplastics and elements, likely due to differences in sources and the physical and chemical characteristics of these pollutants. A comparison of element concentrations with global standards indicates that the pollution levels are within safe limits; however, continued monitoring and management are crucial to mitigate pollution levels.

The purpose of this study was to determine the frequency of Vibrio parahaemolyticus and to identify pathogenic and non-pathogenic strains of this bacterium using a series of biochemical tests and polymerase chain reactions from the shores of the Caspian Sea (Gilan Province). The water and sediment samples of the Caspian Sea were collected from the depth of 30-50 and 10-20 cm respectively by aseptic method. Sampling date, sampling location, temperature, pH and salinity were measured. Isolation and identification of environmental isolates was done using standard methods in order to determine the genus and species using specific media such as thiosulfate citrate bile sucrose and Vibrio chrome agar. A total of 55 isolates were identified by biochemical, phenotypic and polymerase chain reaction tests. From the total of 55 cases of Vibrio parahemolyticus isolated, the frequency of tdh and trh genes was 1 case (1.8%) and 3 cases (5.4%) respectively. This study showed that Vibrio parahaemolyticus, which have pathogenic potential, should be continuously monitored as agents of contamination of coastal waters. The isolation of these species indicates a health warning for people who use raw seafood or are in direct contact with the waters of the Caspian Sea are in this area.

In this study, a comprehensive assessment of the occurrence and spatial distribution of heavy metals including Nickel, Lead, Zinc, Copper, Cadmium and Chromium, as well as, TPHs in the surface sediments of the coastal areas and far from the coast of the Persian Gulf in Bushehr and Khuzestan provinces were performed to identify the possible sources of sediment contamination and determining the degree of pollution. Eight pollution indices were used to evaluate the quality of sediments in terms of heavy metal and TPH pollution. The investigated areas could be distinguished by a significant degree of heavy metal pollution of Nickel, Cadmium and Zinc. High concentrations of Nickel and Zinc were observed in stations around Nakhiloo, Bushehr. The comparison of average pollution indices between regions showed that both Bushehr and Khuzestan provinces, respectively, with the average of contamination degree (CD) (20.25 and 22.11), modified contamination degree (MCD) (3.38 and 3.69), pollution load index (PLI) (31.17 and 57.57), potential ecological risk index (PERI) (353.54 and 349.39) have a significant degree and risk of Nickel and Cadmium pollution, especially in Nakhiloo area, Bushehr. Investigating the spatial fluctuations of the concentration of TPHs in two provinces showed an increasing trend from Bushehr to Khuzestan. However, areas were located in no pollution or low pollution zonation and were safe from the adverse biological effects of TPHs. Comprehensive and impressive strategies to control and reduce these pollutions, especially in polluted areas, should be considered, so that the sources of these pollutions be managed.

The significance of mangrove species in the world is undeniable and planting artificial mangrove forests is a desirable solution to preserve mangroves. The mangroves provide us with superior ecological services and one of their main services is the bioremediation of heavy metals. Hence, this study aimed to assess the effects of artificial mangroves of Velayat Park of Bandar Abbas on the contamination, adsorption pattern and ecological risk of heavy metals in the sediment samples. Thirty samples were collected from two paralleled sampling sections included those without mangroves and containing mangroves. Each sampling area consisted of upper, middle and lower sections. After drying samples using an oven, the dried sediments were sieved by a 63-micrometer sieve and digested by nitric and hydrochloric acid mixture. The concentration of As, Cu, Fe, Ni, Pb, V, and Zn was determined using ICP. The studied factors suggested a high level of ecological risk for as and Pb and the highest risk was observed in the samples without mangroves and a lower risk was reported for samples containing mangroves. Comparing the results with sediment quality guidelines and other studies indicated a dangerous concentration of As. Pearson correlation coefficient values exhibited that artificial mangroves have significantly affected the adsorption pattern of heavy metals (p-Value < 0.05). The principal component analysis (PCA) showed that As and Pb are dominantly derived from oil products leakages and industrial sewages while Cu, Fe, Ni, V and Zn are predominantly derived from natural and geological sources.

Concerns about the negative effects of microplastics on human health have led to increasing attention to the occurrence of microplastics in the aquatic environments. Recent studies have focus from the marine environment to inland waters, particularly the spatio-temporal distribution of microplastics in rivers. Qarasu basin, leading to Gorgan Bay, is the site of many permanent rivers and the levels of microplastic pollution in those rivers are unknown. This study was conducted along 8 different rivers in 2020. Some 9 fish species were identified and the presence of microplastics was confirmed in 87% of fish. The prevalence of microplastic particles among species were Chelon spp.> Carassius gibelio> Gambusia holbrooki> Rutilus rutilus> Neogobius melanostomus> Cyprinus carpio = Vimba persa> Rutilus caspicus> Barbus cyri. The highest type, color and size of fish microplastics were fiber, black, 0.5-0.1mm, respectively. In conclusion the rivers of Qarasu basin have been polluted by microplastics not only in developed areas with intense human activity but also in upstream areas, which can be considered as a source of pollution in Gorgan bay.

Anzali Wetland is one of the most important wetland in southern Caspian Sea where more than 45 endemic and native fish species were reported. In this study, distribution, abundance and IUCN category of fish species were investigated in Anzali Wetalnd during 2016-2017 according to research fishing and local fishermen data. The result showed that 45 fish species belong to 13 families exist in Anzali Wetland. The highest species diversity belongs to Cyprinidae and Gobiidae families with 25 and 6 species respectively.  There was distinguished 4 groups of fish consisting freshwater resident, Anadromous, estuarine and multi-habitat. Gambusia holbrooki, Carassius gibelio, Hemiculter leucisculus, Blicca bjoerkna and Rhodeus caspius were made 22. 8, 19. 7, 12. 1, 10. 1 and 8. 4% of total caught number, respectively. According to all data, the IUCN groups of Anzali Wetland fishes were categorized as 10 species in least concern, 10 species vulnerable, 5 species endangered and 6 species critically endangered. The protection of fish in Anzali wetland needs more attention and is necessary implementation of rehabilitation programs such as reducing the entry of various pollutants into the wetland, controlling invasive aquatic plants, sediment control, rehabilitating spawning sites and avoiding illegal fishing. Artificial propagation of endangered and critically endangered fish should be done for helping the fish for some years.

Karaj River, as one of main source of drinking and agriculture water for Tehran and Alborz provinces, has biological potentials, and is one of the protected rivers in Iran. The main purpose of this study was to investigate the concentrations of heavy metals: cadmium, chromium and zinc in the water and sediment samples of Karaj River in order to evaluate the quality of water, using biological value index (Z). Sampling of water, benthos and sediments was carried out from January 2018 to July 2018 from 7 point stations along the river. Preparation of water and sediment samples was performed by evaporation and the studied metals concentrations were measured by ICP device. The obtained results demonstrated that both in water and sediment samples, studied metals showed significant differences from upstream to downstream stations. Also, the concentration of all metals in water and sediment was lower in January than in July. The results of the Z biological value index revealed a significant difference in both seasons between the sampling stations, and the highest value of in both seasons belonged to the downstream stations. Contamination by heavy metals, especially in downstream, is due to human interventions such as agricultural, urban and industrial wastewaters. In the cold season, because of declining population density in the river vicinity area, pollution levels have been decreased. Also, in terms of quality, in general, Karaj River can be classified in saprobe class II and III.

The purpose of this study was to evaluate the effects of environmental parameters on the dinoflagellates cyst abundance of Gonyaulacales and Gymnodiales orders in the sediment of Oman Sea coastal regions and the Strait of Hormuz in winter and spring 2015. Dinoflagellate cyst study helps to identify key areas for the feature bloom of harmful species. For this purpose, sediment sampling was carried out by Grab (Ekman) at 15 stations located in the Pasabandar, Ramin, Konarak, Galak and the first Tiab estuary in the Strait of Hormuz. Salinity, temperature, acidity and depth of water column were measured in each station. The sediment grain size and the total organic matter content of the sediment were measured in the laboratory and dinoflagellate cysts were isolated, identified and counted. The relationship between the environmental parameters and the species of dinoflagellate cysts was studied by the ordination method of canonical correspondence analysis(CCA). The ordination results of CCA showed that the abundance of dinoflagellate cysts was significantly affected by the environmental parameters and the sediment grain size (P

Some species of phytoplankton under unfavorable environmental conditions and in their sexual life cycle produce resting cyst that settle to the bottom sediments and become motile in the water column when suitable conditions provided. Phytoplankton species were studied in two stages of motile and cyst for accurate identification of morphology, because some similar cysts may convert to a different phytoplankton and vice versa. The objectives of this study were to culture cysts that isolated from sediments of Lipar zone located in the southeast coast of Iran and to identify excysted samples. Sediment samples from Lipar zone were collected by Ekman grab with 225 cm2 area in 2015. Unknown live and orbicular single cysts were isolated from sediments and cultured in sterile conditions in phycolab using f2 medium under 12 hours light and 12 hours dark cycles at 25±1°C. Assessment of excysted plankton morphology revealed that the cyst is belonged to Dunaliella salina species. In order to confirm this morphological identification, DNA of the plankton was extracted and PCR was performed. The sequence of the sample was compared with the sequences of the similar species from GenBank. Phylogeny and molecular analysis confirmed this identification. The present study revealed that D. salina from the south coast of Iran was able to produce resting cyst.

This study was conducted to determine statement of some metals and oil pollutants in the southern Caspian Sea (Kelarabad-Mazandaran).Seventy five Surface sediments samples (Triplicate) were collected at three stations from autumn of 2011 to winter of 2012. All samples were prepared by digesting and extracting processes and then determined using Atomic Absorption Spectroscopy and High Performance Liquid Chromatography instruments. Results showed that annual mean contents and standard error (±SE) of surface sediments were observed 59 (±3), 25 (±1), 42 (±2), 28 (±1), 14 (±1), 0.16 (±0.06), 0.020 (±0.005) and 2.81 (±0.73) mg/kg.dw for Zn, Cu, Cr, Ni, Pb, Cd, Hg and PAHs, respectively. Also, sediments samples based on Enrichment factor (EF) and Ggeoaccumulation index (Igeo) were showed no polluted conditions for this region, but sediments were classified low polluted based on CF, Cd and PLI. In addition, Hazard Quotation (HQs) of most of PAHs compound were calculated more than unit in surface sediment which according to results of the evaluation’s risks to health of 16PAHs suggest that there is risk for biota. In a conclusion, according to anthropogenic sources of some metals and PAHs of sediment, it is essential that the management and development of the aquaculture industry in the Mazandaran Coast to be made with environmental considerations and cautions.

This study was done for the effects of sediments type on Indomysis nybini survival in Indian White Shrimp brood stock ponds of Kollahi port in Hormozgan province. Sampling place was Tiyab area of Kollahi port of Hormuzgan Province Sediments experiment was done with 4 treatments, without sediment , ponds sediments, peat and sea sediments. Every treatment had three replicates. Stocking density was 20 mysids per liter. This experiment was conducted for 7 dayes. To measure organic matter, amount of ponds sediments, peat and sea sediments were sent to Soil Mechanics Laboratory. Results showed that ponds sediments had 9.31%, peat had 1.55% and sea sediments had 0.78% of organic matter. Experiment results showed that the survival rate was upper than 90% in all of treatments.However there was significant difference in mean of survival. The minimum mean survival (91.04±7.4) was observed in without sediment treatment and the maximum mean survival (99.58±1.8) was observed in ponds sediments treatment. Indomysis nybini able to use of bed sediments, organic matter and soluble compounds in sea water in without food condition

The following research was conducted to study the concentration of Ni and Cd in skeletons and sediments of dominant hard coral family and Poritidae in Zeyton Park, Naz Islands and Shibderaz stations, South of Qeshm Island Persian Gulf. The analysis of these heavy metals in digested samples of skeletons and sediments was done applying Graphite furnace Atomic Absorbtion Spectophotometry SHIMADZU, AA 670 for measuring the levels of Cd and Flame Atomic Absorbtion Spectophotometry SHIMADZU, AA 670 for measuring the levels of Ni.The concentration of Ni in skeleton of coral family Poritidae in Shib-deraz station 29/95, in Naz Islands 27/94 in Zeyton Park 26/16 and the concentration of Ni in sediments of coral in Zeyton Park station 27/91 in Naz Islands station 25/70 and in Shib-deraz station 25/50.The results indicated that there are significant statistical differences toward concentration of Ni in skeleton of coral family Poritidae among Zeyton Park, Naz Islands Shib-deraz stations (P<0.001). But Cd concentration is not significant (P=./992) In addition, considering the concentration of Ni and Cd in sediments of coral familie Poritidae there are significant differences (P<0.05 and P<0.001 respectively). Long term pollution monitoring by heavy metals is of great importance in managing coastal areas. So, concerning the immense importance of coral ecosystem in southern Qeshm Islands and the result of this research, corals can be suggested as a pollution biomonitor

The frequency and distribution of benthos species in the north coast of Gorgan Bay (proposed site for cage and pen culture) were studied. Sampling was carried out in three water depths 1, 2 and 3 meters and 10 stations in each depth using Van Veen sampler with 3 replications. Totally, 12 families were identified: Nereidae, Ampharitidae, Spionidae Amphipodae, Osteracodae, Mytilidae, Dreissenidae, Cardidae, Neritidae, Pyrgulidae, .Lithoglyphidae and Planorbidae of which Osteracodae and Ampharitidae with 26% and 1% had maximum and minimum abundance, respectively in all depths. The maximum and minimum abundance of benthos was 6025 per square meter in waters 3 meters deep and 52 per square meter in waters 1 meter deep. In addition, sediment texture and total organic matter (TOM) were analyzed which showed that 2 and 3 meter deep waters had the maximum percent of T.O.M (7.8%) in spring and autumn while the minimum T.O.M (2.5%) in 1 meter deep waters was observed in winter......

The frequency and distribution of meiobenthoses in northern Creeks of the Bushehr Province during autumn2000- Summer200I were studied for determining the condition of sea sediments using seasonal sampling.
Samplings were carried out in 10 stations from Doube Creek to Bandar Genaveh Creek as well as a control station in the Persian Gulf.
Totally, 4 taxa of meiofauna were identified that Ostracoda and Gastropoda with 29% and 18% had the maximum and minimum percent of abundance, respectively. The maximum percent of abundance in station No.10(control station at sea) with 20735 n/m2 and the minimum abundance in station No.2 (Creek of Farake)with 5320 n/m2 were observed. The maximum abundance of meiobenthose in spring with 167603 n/m2 and the minimum abundance during winter with 61365 nlm2 were determined. In addition, grain size, soil texture and total organic matter (T.O.M) of sediment in 10 stations were analyzed. Results showed that station No.10 with 11.79% had the maximum percent of T.O.M while the minimum T.O.M (3.87%) in station No.4 (Gasir Creek) was observed. The correlation of sampling station based on the abundance of meiobenthose was also analyzed.