Seasonal Occurrence of Caddisfly Larval Communities on a Coarse Substrate in the Upper Nan River, Northern Thailand

Abstract Caddisfly larvae are commonly found in the Nan River. Larvae were collected monthly from a cobble habitat for a period of two years to study the influence of seasonal changes on larval communities. A Surber sampler was placed in the cobble habitat in the riffle zone at four study sites along the upper Nan River. Field-collected larvae were identified at the family level in the laboratory. Physicochemical factors were measured in terms of water temperature, pH, turbidity, dissolved oxygen (DO) and biochemical oxygen demand (BOD5), as well as for concentrations of ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N) and orthophosphate (PO43-). Fifteen families of the caddisflies were found. The cool season was associated with the highest degree of density followed by the summer and rainy seasons. Hydropsychidae was the most common caddisfly present throughout the year. Hydroptilidae was common in the summer and cool seasons. Values of water temperature, pH, turbidity, DO, BOD5 and NO3-N among the three seasons were observed to be significantly different. According to the CCA analysis, water temperature, turbidity, BOD5, NO3-N and NH3-N factors had a positive relation to the density of Ecnomidae, Goeridae, Odontoceridae and Polycentropodidae. Remarkably, caddisfies in the families Hydropsychidae, Rhyacophilidae and Stenopsychidae were intolerant to variations in DO and pH values. The results of our study indicate that the physicochemical factors varied in conjunction with the different seasons, while communities and the degree of density of some caddisfly larvae were associated with variations of the physicochemical factors. Keywords: Trichoptera, Cobble, Environmental factors, Season