Spatio-temporal dynamics of phytoplankton community and their relationship with water quality in a sub-tropical young reservoir of Northeast India

Abstract This study investigated the spatio-temporal variations in phytoplankton community composition and its correlation with water quality parameters in a newly constructed reservoir in the Northeast Hill region of India. A total of 69 phytoplankton species were identified, marking the highest recorded taxonomic diversity in the region's reservoirs. Bacillariophyceae, represented by 30 species, exhibited the greatest diversity, a common feature of tropical reservoirs. The highest phytoplankton abundance occurred during the monsoon season, while the lowest was recorded in the post-monsoon. Ceratium hirundinella was the dominant species, comprising 59.88% of total phytoplankton abundance, indicating a mesotrophic state and an intermediate stage in the life cycle of reservoir. Spatially, the highest phytoplankton abundance was recorded at near the dam site, followed by mid-stream shallow region and the sites where human intervention was prevalent (tourist area and the cage culture site). The calculated value of diversity indices revealed moderate phytoplankton diversity in the reservoir, with Shannon–Wiener index ( H' ) values ranging from 1.1 to 2.19, Margalef richness ( d' ) from 3.06 to 4.76, and Evenness ( j' ) from 0.303 to 0.593. Pearson's correlation showed positive relationships between total phytoplankton abundance and water quality variables such as temperature, alkalinity, PO 4 -P and NO 2 -N. Further, Canonical Correspondence Analysis (CCA) identified water variables viz ., electrical conductivity, total alkalinity, transparency, NO 3 -N and PO 4 -P as key abiotic factors affecting the phytoplankton community.