Vertical Distribution and Community Structure of Meiofauna in Wakod Dam, Maharashtra, India

Abstract

Freshwater reservoir sediments support diverse meiofaunal communities that respond to sedimentary, hydrological and physicochemical conditions. The present study assessed the taxonomic composition, abundance, vertical distribution and community structure of meiofauna in Wakod Dam, Maharashtra, India. Sampling was conducted at four sites representing inlet, central, dam-wall and anthropogenically influenced littoral conditions during pre-monsoon, monsoon and post-monsoon periods. Sediment cores were divided into four depth intervals: 0–2, 2–5, 5–10 and 10–20 cm. Meiofauna were extracted, identified morphologically and expressed as individuals per 10 cm³ of sediment. Nematoda and Harpacticoida were the principal groups recorded. The inlet site showed the highest reported seasonal abundance, with abundance generally greater during the monsoon than during pre- and post-monsoon periods. Most meiofauna occurred within the upper 5 cm of sediment, with 52.6% recorded at 0–2 cm and 34.8% at 2–5 cm. Nematoda showed the broadest vertical distribution, whereas Gastrotricha, Rotifera and Harpacticoida were strongly concentrated near the sediment surface. Site I exhibited the highest reported taxon richness and Shannon diversity, whereas Site III showed lower diversity and stronger nematode dominance. Spearman correlations indicated positive associations of total abundance and Harpacticoida with dissolved oxygen and redox potential, while suspended solids and phosphate showed negative associations with total abundance. The findings demonstrate clear spatial, seasonal and vertical structuring of reservoir meiofaunal communities and indicate their potential value as complementary biological indicators of sedimentary environmental conditions.

Introduction

This study investigates the composition, abundance, seasonal variation, vertical distribution, diversity, and environmental relationships of freshwater meiofauna in Wakod Dam, Maharashtra. Meiofauna are microscopic sediment-dwelling organisms such as nematodes, copepods, rotifers, ostracods, gastrotrichs and other groups that play important roles in nutrient cycling, energy transfer and aquatic food webs. Because they respond to environmental conditions, they can also serve as useful complementary indicators of sediment and ecosystem quality.

The study was conducted at four sites—the inlet, central reservoir, dam-wall and anthropogenically influenced littoral region—during pre-monsoon, monsoon and post-monsoon seasons. Sediment cores were divided into four depth intervals: 0–2, 2–5, 5–10 and 10–20 cm. Meiofauna were separated, microscopically examined and identified into major taxonomic groups. Environmental variables including dissolved oxygen, redox potential, chlorophyll-a, suspended solids, phosphate and organic carbon were also considered. Diversity and community structure were evaluated using taxon richness, Shannon diversity, Pielou's evenness, maturity index, nematode:copepod ratio and trophic diversity, with Spearman correlations used to examine environmental relationships.

Nine major meiofaunal groups were recorded. Nematoda and Harpacticoida occurred at all four sites and were the dominant and most consistently distributed groups. Ostracoda and juvenile/small Oligochaeta were also widespread, whereas groups such as Turbellaria and Tardigrada had more restricted distributions.

Strong spatial and seasonal differences were observed. Site I (inlet) had the highest abundance, particularly during the monsoon (52,864 ± 7,920 individuals/10 cm³), while Site III (dam-wall) had the lowest (12,064 ± 1,680 individuals/10 cm³ during monsoon). Abundance was generally highest during the monsoon season, potentially reflecting changes in water inflow, sediment deposition, organic matter and habitat conditions.

A major finding was the strong surface concentration of meiofauna. Approximately 87.4% of all recorded organisms occurred within the upper 5 cm of sediment, with 52.6% in the 0–2 cm layer. Harpacticoida, Rotifera and Gastrotricha were especially concentrated near the surface, while Nematoda showed a comparatively deeper distribution.

Site I had the greatest taxon richness (28), Shannon diversity (2.68), and evenness (0.78), whereas Site III had the lowest values. The nematode:copepod ratio was highest at Site III (14.2), suggesting greater relative nematode dominance there, although this ratio should be interpreted as a community metric rather than a definitive pollution indicator.

Environmental relationships showed that dissolved oxygen and redox potential were positively associated with meiofaunal abundance and diversity, while suspended solids and phosphate generally showed negative relationships with abundance and diversity. Organic carbon was strongly positively associated with nematode abundance (r = 0.82). However, these correlations demonstrate associations rather than direct cause-and-effect relationships.

Conclusion

The study demonstrated clear spatial, seasonal and vertical structuring of meiofaunal communities in Wakod Dam. Nematoda and Harpacticoida were the principal meiofaunal groups and were recorded at all sampling sites. The inlet region exhibited the highest reported seasonal abundance, while the dam-wall site showed lower abundance and diversity. Approximately 87.4% of the recorded meiofaunal abundance occurred within the upper 5 cm of sediment, demonstrating strong vertical concentration near the sediment surface. Site I showed the highest reported taxon richness, Shannon diversity and evenness, whereas Site III exhibited lower diversity and stronger nematode dominance. Positive associations with dissolved oxygen and redox potential and negative associations with suspended solids and phosphate indicate relationships between meiofaunal community characteristics and environmental conditions. Overall, meiofauna show potential as a complementary biological component for assessing spatial and seasonal ecological variation in reservoir sediments. However, confirmation using replicate-level data, standardized sampling, detailed taxonomic identification and appropriate inferential statistics is required before the numerical results are used for definitive environmental assessment.

References

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Copyright

Copyright © 2026 Madhuri R. Kharat, Premchand B. Sirsat. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.