Seasonal Variation of Aquatic Insect Biotic Indices and Their Relationship with Water Quality in the Tropical Mong Sen Stream, Lao Cai Province, Vietnam

Freshwater ecosystems in tropical regions are increasingly threatened by anthropogenic disturbances and climate variability, highlighting the urgent need for effective biomonitoring tools. Aquatic insects are among the most widely used bioindicators for assessing ecological quality, yet their application remains underutilized in Vietnam. In this study, we investigated the seasonal biotic indices of aquatic insects and their relationship with physico-chemical parameters in the Mong Sen Stream, a tropical stream in Lao Cai Province, Northern Vietnam, where upstream sites are subject to stronger anthropogenic impacts compared to downstream areas. A total of 86 species belonging to 72 genera, 42 families, and 9 orders of aquatic insects were identified. Both insect diversity and water quality parameters showed clear seasonal variation. Diversity was higher in the dry season, with Shannon-Weaver indices (H’) ranging from 2.1 at site St4 to 2.5 at site St2, compared to the rainy season (H’ = 1.5 at St4 to 2.1 at St2). Water quality was also better in the dry season, with Average Score Per Taxon (ASPT) values ranging from 6.2 to 6.8, relative to 5.0 to 6.1 in the rainy season. Furthermore, aquatic insect diversity indices (d, H’) and ASPT index were positively correlated with dissolved oxygen, whereas other parameters such as BOD5, COD, turbidity, TOC, and TN exhibited negative relationships. These findings demonstrate that aquatic insects provide reliable and sensitive indicators of seasonal water quality variation in tropical streams. This study not only contributes essential baseline data for the Mong Sen Stream but also underscores the scientific and practical significance of integrating aquatic insect-based biomonitoring into freshwater management strategies in Vietnam and other tropical regions.