## 3 Results and Discussion  
### Comparison of Water Quality and Biotic Indices  
Water quality parameters and biotic indices were compared in nine subwatersheds between May and September. In the temporal scale, the nitrogen level was slightly higher in May than in September (Figure 3). It appeared that a significant amount of nutrients entered the stream by summer rainfall (see Figures 2 and 3). We speculate that incoming nutrients are more dissolved forms of nitrogen and phosphorus than particulate, because we observed frequent increases of NH\({}_{3}\)-N, NO\({}_{3}\)-N, and PO\({}_{4}\)-P (Figure 3). The previous studies have reported that the high flux of NO\({}_{3}\)-N could be driven by specific agricultural activities, such as manure applications and conservational tillage [37; 38]. Nonetheless, TN concentrations exhibited a gradual decrease from upstream to downstream (Figure 3), and specifically exhibited approximately from 2 mg L\({}^{-1}\) at sites SDU, SDL, and OS, to 1 mg L\({}^{-1}\) at sites SG and SL. A study by [PERSON] [39] reported that unexpectedly high nutrient concentrations in headstreams are often observed because of the relatively undeveloped wastewater treatment and septic tanks in the rural area of South Korea, thereby inducing an increase of BOD. In contrast to TN and NO\({}_{3}\)-N, NH\({}_{3}\)-N concentrations were very high at the SC site, regardless of time (Figure 3). Given the largest fraction (49%) of cropland among nine subwatersheds, the high ammonia concentrations could be associated with agricultural activities. In China, which is geographically close to Korea, it was reported that agricultural nitrogen accounted for more than 40% of the variability of TN, and subsequently it drives ammonia increases [40; 41].  
The phosphorus pattern was spatially similar to the nitrogen pattern in May. The level of TP concentrations gradually decreased from upstream to downstream, except at the headstream SDU site in May (Figure 3). However, it was comparatively notable that phosphorus concentrations rebounded in the lower part of the river, such as at the BS and SL sites. This longitudinal trend could be consistent with the fact that the lower part of the Seomjin River was dominated by agricultural lands (38.8% at the IS, SC, YC, BS, and SL sites) and urban areas (6.6% at the YC and BS sites) relative to the upper part (27% at the SDU, SDL, SG, and JD sites, and 3.3% at the SDU, SDL, and JD sites) (see Figure 1). In relation to agricultural management practices (e.g., manure applications) and urbanization, a large amount of dissolved phosphorus could be generated [42; 43].  
Chl-_a_ concentrations were also slightly higher at the upper part than at the lower part of the river. However, based on the current level of Chl-_a_ concentrations, the Seomjin River changed from being oligotrophic to mesotrophic (Chl-_a_\(<\) 10 \(\mu\)g L\({}^{-1}\)). The BOD values mostly ranged from 1 to 3 mg L\({}^{-1}\), and appear to differ from the spatial pattern of nutrient concentrations (Figure 3). Additionally, we also compared five biotic indices, and most of the values of biotic indices were not statistically significant across all the study sites (Figure 3). Thus, it was difficult to distinguish the spatial pattern of benthic communities based merely on the biotic indices.