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Feasibility of using the bivalve Anodonta anatina and sediment layers as biochronometers for retrospective assessment of environmental changes in Anzali Wetland
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N. Pourang   |
| Iranian Fisheries Science Research Institute (IFSRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran |
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Abstract: (11 Views) |
Introduction
Wetlands are among the most sensitive ecosystems on Earth, increasingly threatened by anthropogenic activities and climate variability (Mitsch and Gosselink, 2015). Anzali International Wetland, a Ramsar Convention site in northern Iran connected to the Caspian Sea, faces escalating pressures from urbanization, agriculture, and industrial effluents (Jamshidi-Zanjani and Saeedi, 2013; Abbasi, 2019). Effective environmental monitoring requires reliable natural proxies capable of recording changes with adequate resolution (Schöne and Gillikin, 2013). Among these, bivalve shells and sediment cores have proven valuable for preserving elemental and isotopic signals (Poulain et al., 2015; Schöne et al., 2023). Bivalve shells grow incrementally, incorporating elements under both environmental and physiological controls, a process known as sclerochronology (Marali et al., 2017; Fröhlich et al., 2022). Sediment cores provide complementary long-term records of deposition reflecting pollution and diagenetic processes (Schneider et al., 2021; Zhang et al., 2023). Integrating both archives enhances understanding of source-to-sink dynamics and bioavailability, yet combined studies remain rare (Luoma and Rainbow, 2008). This study evaluates, for the first time, the combined use of Anodonta anatina shell growth layers and sediment cores as high-resolution biochronometers for reconstructing temporal variations of 14 elements (Ba, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Ni, Pb, Sr, Zn) in Anzali Wetland.
Methodology
Sampling was conducted in February 2025 at three sites based on A. anatina distribution. Five live bivalves were collected per site. Sediment cores were collected using PVC tubes (1 m length, 5 cm ID) and sectioned at 2 cm intervals. For one core per site, TOM, grain size (GS1>500, GS2 250–500, GS3 125–250, GS4 63–125, GS5<63 µm), pH, EC, and Eh were measured following MOOPAM (2010) and Nishimuta et al. (2021). The second core was digested using HNO₃ and H₂O₂ in high-pressure vessels and analyzed for elements by ICP-MS (Agilent 7700x), with quality control using GBW07318 and SRM 2711 (Gao et al., 2017; Armiento et al., 2022). Recoveries were 86–104% and RSD <5.7%. For bivalve analysis, the oldest shell per site was selected. After soft tissue removal and cleaning, the left valve was embedded, sectioned along the maximum growth axis, ground, polished, and etched with 0.01 M HCl. Acetate peel replicas were examined under a digital microscope to count annual growth lines (Richardson, 2001; Ridgway et al., 2011; Pourang et al., 2014). Element: Ca ratios in prismatic and nacreous layers were measured by LA-ICP-MS (193 nm ArF excimer, 80 µm beam, 2.5 J/cm², 6 Hz) coupled to an Agilent 7900 ICP-MS, with calibration using NIST SRM 612/614 and LD-5 calcite, and data processed with Iolite software (Schöne et al., 2010; Vihtakari et al., 2017). Statistical analyses included ANOVA followed by Duncan's test, Pearson correlation, HCA with average linkage and Euclidean distances (Sokal and Rohlf, 2012; Gelfand et al., 2020), using SPSS 29 and Excel 2021, after log or square-root transformation when needed (Zar, 2010).
Results
Significant inter-site variability was observed in sediment element concentrations for most elements except Fe and Zn (ANOVA, p ≤ 0.0001). Site 3 exhibited the highest Ba concentration (mean 1066.26 ppm), while Site 2 showed the highest Cd (1.16 ppm) and Pb (59.05 ppm). Grain size distribution differed significantly among sites: Site 3 was dominated by fine particles (GS5, mean 55.19%), Site 1 by coarser fractions (GS3, mean 47.45%), and Site 2 had intermediate characteristics. TOM was highest at Site 2 (mean 18.65%), and EC was also highest at Site 2 (mean 1812 µS/cm), while Eh was lowest at Site 3 (mean 166 mV). HCA of sediment elements showed distinct clustering patterns: Fe consistently formed a separate cluster across all sites. At Site 1, four clusters were identified; at Site 2, four clusters including separation of K with Mn; and at Site 3, two major clusters (all elements except Fe in one group, and Fe alone). In bivalve shells, element: Ca ratios exhibited layer- and site-specific patterns (Tables 5, 6; Figure 7). The nacreous layer showed greater inter-site variability than the prismatic layer, particularly for Ba/Ca (F = 25.185, p ≤ 0.0001), Sr/Ca (F = 53.150, p ≤ 0.0001), and Cr/Ca (F = 36.421, p ≤ 0.0001). Ba/Ca ratios in the nacreous layer were highest at Site 3 (4.23 mmol/mol), intermediate at Site 1 (2.49 mmol/mol), and lowest at Site 2 (1.69 mmol/mol). Sr/Ca ratios in the nacreous layer were highest at Site 2 (65.24 mmol/mol), followed by Site 3 (50.82 mmol/mol) and Site 1 (35.94 mmol/mol). Several elements (Cd, Fe, K, Mg, Mn, Zn) showed no significant inter-site differences in shell layers despite varying sediment concentrations, indicating strong physiological control. Mn/Ca ratios were consistently higher in the prismatic layer than in the nacreous layer across all sites (e.g., Site 1 prismatic mean 170.62 vs. nacreous 74.04 mmol/mol). Pearson correlation revealed significant relationships: at Site 2, Cd and Pb showed strong positive correlations with TOM (r = 0.626 and 0.822, respectively, p ≤ 0.01); at Site 1, Fe correlated strongly with fine particles (GS5: r = 0.842, p ≤ 0.01); and Eh showed a negative correlation with Mn (r = -0.589, p ≤ 0.05). HCA of shell element: Ca ratios (Figure 8) consistently separated Mn and Sr into distinct clusters across most site-layer combinations, while other elements (Cd, Pb, Co, Ni, Zn, Cr, Li, Cu, K, Ba, Mg, Fe) often grouped together.
Discussion and conclusion
The integrated analysis reveals complex controls on element distribution, including spatial heterogeneity, biological regulation, and temporal synchrony (Dick et al., 2007; Lazareth et al., 2013). High Ba at Site 3 may indicate terrestrial runoff or phytoplankton-driven inputs (Schneider et al., 2021; Fröhlich et al., 2022), while elevated Cd and Pb at Site 2 suggest agricultural/urban pollution (Zare Khosheghbal et al., 2013). The higher TOM and coarser sediments at Site 1 enhance metal sorption through organic complexation, whereas the reducing conditions at Site 2 likely promote dissolution of Fe/Mn oxides and associated trace metal mobility (Luoma and Rainbow, 2008; Mehdizadeh et al., 2023). The greater inter-site variability in the nacreous layer compared to the prismatic layer supports the hypothesis that the nacreous layer, due to its faster growth rate and different microstructure, is more sensitive to environmental fluctuations (Pourang et al., 2014). Ba/Ca ratios in shells, particularly in the prismatic layer, serve as a reliable proxy for phytoplankton bloom dynamics, with the highest values at Site 3 consistent with known patterns of algal biomass distribution and bloom cycles in Anzali Wetland (Fallahi et al., 2016; Bagheri et al., 2025). Higher Sr/Ca at Site 2 may reflect unique hydrological conditions or physiological vital effects. The lack of correlation between environmental availability and shell composition for elements such as Cd, Fe, K, Mg, Mn, and Zn highlights the importance of biological regulation in element incorporation. The constant Mg/Ca ratios across sites support this interpretation, as Mg is known to be strongly physiologically controlled in freshwater bivalves (Lazareth et al., 2013). The consistent enrichment of Mn in the prismatic layer across all sites aligns with previous findings and may result from its association with organic-rich growth bands formed during periods of reduced shell growth (Siegele et al., 2001; Shirai et al., 2014). The strong positive correlation of Cd and Pb with TOM at Site 2 indicates that organic matter plays a significant role in accumulating these metals, likely reflecting historical anthropogenic inputs (Schneider et al., 2021). The negative correlation between Eh and Mn confirms the redox-sensitive behavior of manganese (Poulain et al., 2015). HCA results further support these interpretations: the consistent separation of Fe in sediments reflects its dominance as a lithogenic element and its association with organic matter and redox-sensitive processes (Luoma and Rainbow, 2008); the separation of Mn and Sr in shells highlights their strong environmental and physiological controls (Schöne et al., 2023); and the grouping of Ba with other trace metals at Site 3 suggests common terrestrial or phytoplankton-driven inputs. The prismatic layer generally records environmental signals (e.g., Ba and Cu) more reliably than the nacreous layer due to its higher organic matter content (Becker et al., 2005). Conversely, Pb, Zn, and Mn are strongly regulated by physiological processes (vital effects), limiting their direct use as environmental proxies (Schöne et al., 2011). The observed mismatches between sediment and shell signals for certain elements underscore the necessity of multi-proxy approaches that combine complementary archives. The findings emphasize that while Ba and Cu in the prismatic layer are promising proxies for primary productivity and contamination, respectively, elements like Pb, Zn, and Mn require cautious interpretation due to biological modulation. This study demonstrates that A. anatina and sediment cores provide complementary, high-resolution archives for environmental reconstruction in Anzali Wetland. The integrated approach allows distinguishing geogenic from anthropogenic inputs and provides a robust framework for assessing source-to-sink dynamics, bioavailability, and ecological responses. Future research should incorporate isotopic dating methods (²¹⁰Pb and ¹³⁷Cs) for sediment cores to establish accurate age-depth models and improve temporal correlations between shell growth increments and sedimentary events (Abbasi, 2019; Coral-Carrillo et al., 2023). Additionally, stable isotope analyses (δ¹³C, δ¹⁸O, and metal isotopes) in both bivalve and sediment samples would help distinguish anthropogenic inputs from natural backgrounds and elucidate biogeochemical cycling under varying redox conditions. The use of longer-lived bivalve species could further extend chronological records and improve temporal resolution.
Conflict of interest
The author declares that they have no conflict of interest.
Acknowledgment
I would like to express my gratitude and appreciation to all esteemed colleagues at the Inland Waters Aquaculture Research Center who participated and collaborated in various stages of this research, especially Alireza Mirzajani, Ali Abedini, Mohammad Sayad Bourani, Siamak Bagheri Jounaghani, and Reza Mohammdidoost Noveiri. |
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| Keywords: Growth layers, Sediment layers, Major and trace elements, Environmental conditions, Anzali Wetland, Anodonta anatina |
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Full-Text [PDF 2421 kb]
(5 Downloads)
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Type of Study: Research |
Subject:
اكولوژي محيطهاي آبي
Received: 2026/04/13 | Accepted: 2026/06/29
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