Diversity of caddisflies (Trichoptera), water quality factors, and microplastics in a northern Thai stream

The larvae of various caddisfly species are used to assess water quality at various levels of pollution. The purpose of this study was to determine the species composition and seasonal distribution of Trichoptera larvae in a northern Thai mountain stream, as well as their relationship to physicochemical parameters. Seasonally collected benthic macroinvertebrates were sampled between October 2024 and April 2025. A total of 2748 individual caddisfly larvae, distributed across 13 families and 24 genera were found. The most abundant larvae were Hydropsychidae, followed by Odontoceridae and Philopotamidae. The most prevalent species in this study were Marilia sumatrana, Chimarra sp., Cheumatopsyche dubitants, and Potamyia phaidra. The results from applying the indices of caddisfly biodiversity in the seven streams indicated that the Shannon index (H') ranged from 2.393 to 2.901, the Simpson index (D) ranged from 0.8763 to 0.9374, and the Evenness index (E) ranged from 0.835 to 0.943, respectively. Pearson correlation analysis showed that environmental factors such as water pH, temperature, dissolved oxygen, ammonia-nitrogen, nitrate-nitrogen, and total dissolved solids were significantly associated with caddisfly larvae (p < 0.05, p < 0.01). Polyplectropus sp. was significantly related to dissolved oxygen. Amphipsyche meridiana, Hydropsyche askalaphos, and Oecetis sp. were significantly related to ammonia-nitrogen content. Ganonema sp. and Leptocerus sp. were significantly related to nitrate-nitrogen content. Marilia sumatrana was significantly related to water pH. Triaenodes sp. was significantly related to water temperature. The larvae of Chimarra akkaorum, Cheumatopsyche dubitants, Macrostemum sp., and Leptocerus sp. were all significantly associated with total dissolved solids. All individuals tested contained microplastics. Small MPs (less than 100 to 250 μm) were found in the largest concentrations, in the shape of fibers, and primarily in the color blue. A total of seven types of polymers have been identified, of which cellulose acetate butyrate (CAB) and polyethylene terephthalate (PET) are the most common.