## 3 Results and Discussion
### Spatial Distribution of Benthic Macroinvertebrate Communities Before and After Summer Rainfall  
The distribution of benthic macroinvertebrate communities was spatially distinct (Figure 4). At all nine of the subwatersheds, the abundance of benthic macroinvertebrates decreased after the summer rainfall in September. A decreasing level of benthic communities differed from the location of subwatersheds. It was a particularly remarkable pattern that while the abundance of _Gammarus_ spp. (Amphipoda) slightly increased, the abundance of _Asellus_ spp. (Isopoda) dramatically decreased after the heavy rainfall (Figure 4). Moreover, the relative abundance decreased from 41% to 3.3% (Table 1). However, in this respect, _Gammarus_ spp. also dramatically increased in terms of relative abundance (26.6% in May to 69.6% in September, Table 1). This pattern was clearer at the SDU site, which was a forest-dominated area. We could not find evidence to support their inverse abundance pattern associated with rainfall, especially in a forest area. To understand their relationships and ecological  
Figure 3: Comparison of water quality and biological indices between May and September. Error bars indicate standard errors. Asterisks represent statistical significances at \(P<0.05\). (**a**) TN; (**b**) NH\({}_{3}\)-N; (**c**) NO\({}_{3}\)-N; (**d**) TP; (**e**) PO\({}_{4}\)-P; (**f**) Chl-\(a\); (**g**) BOD; (**h**) DI; (**i**) EI; (**j**) H\({}^{\prime}\); (**k**) RI; (**l**) BMI. interactions, a long-term monitoring is highly required to depict the inter-annual variation in specific land-use coverage. In relation to unfavorable benthic macroinvertebrates in polluted environments, it was found that Diptera (Chironomidae and _Simulium_ spp.) prevailed across the Seomjin River watershed (Figure 3). However, it was also observed that their abundance was significantly low in September after summer rainfall. Chironomidae abundance consistently decreased across all of the study sites except for SDL. A slight increase (21.9% to 24.1%) of the abundance might be longitudinal flush effects, since the SDL site was located downstream of the Seomjin Dam. _Simulium_ spp. also decreased after summer rainfall, but their low abundance appeals to further surveys over a long term. On the other hand, the key benthic macroinvertebrates such as Coleoptera and Ephemeroptera were scrutinized. Interestingly, the abundance patterns of _Elmidae_ spp. and _Eubrianax_ spp. were opposite between May and September. The former was commonly higher in May, while the latter was higher in September (Table 1). Ephemeroptera were slightly more abundant in September. Particularly, _Ecdyonurus_ spp. abundance was distinctly high in September relative to the other Ephemeroptera (Table 1). The most dominant Ephemeroptera, _Baetis_ spp., showed irregular spatial pattern in their abundance. Interestingly, the longitudinal pattern of abundance looked like a zigzag, which implies that _Baetis_ spp. (e.g., larva) could be influenced serially from upstream to downstream by hydrological factors. Another key group Trichoptera showed higher abundance, particularly at the SDL and OS sites (Table 1). _Hydroysyche_ spp. appeared to be sensitive to summer rainfall, while _Cheumatopsyche_ spp. seem to be more tolerant. Figure 4: Average abundance of benthic macroinvertebrate communities based on (**a**) order and (**b**) subcategory (family, genus, and species) in Seomjin River.