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Phytoplankton structure and functional groups as a bioindicator of water quality in an Eastern Amazonian river

ecology
microbiology
freshwater-biology

Abstract

Background. Phytoplankton are natural sensors of aquatic ecosystem dynamics and structure. This study analyzed spatio-seasonal variations in the taxonomic and functional composition of phytoplankton in relation to environmental factors, aiming to evaluate their bioindicator potential.

Methods. We conducted the sampling in the middle Macacoari River, Eastern Amazon, during the Amazonian wet (April–June) and dry (September–November) seasons of 2024.

Results. We identified a total of 207 taxa, dominated by Zygnematophyceae (111 taxa), Bacillariophyceae (38), and Cyanophyceae (26), which together represented 84.54% of all species. Species richness was higher in the wet season, while abundance peaked in the dry season, with significant seasonal variation (Hc=6.488; p=0.0109). Thirty-four species were abundant but non-dominant, consistent with Shannon and Pielou indices, indicating even distribution, low pollution, and high diversity. Functional groups IV and VI accounted for 79.54% of the total abundance. Group IV, dominated by Leptolyngbya sp., Johanseninema constrictum , Mougeotia sp., Closterium acutum , C. aciculare , and Scenedesmus acunae, was associated with hypoxic and acidic conditions during the wet season. Conversely, Group VI, composed exclusively of pennate diatoms, correlated positively with dissolved oxygen and predominated in the dry season. These results highlight phytoplankton community structure as a sensitive bioindicator of seasonal environmental changes in Amazonian rivers.

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