## 2 Materials and Methods
### Data Collection  
The present study was based on 124 sites across the Seomjin River watershed in 2017. The study sites are part of the national water quality monitoring networks run by the National Institute of Environmental Research (NIER) which is operated by the Korean Ministry of Environment (KMOE). All 124 sites are located in the same catchment and are connected to one another by way of the Seomjin River. Due to the accessibility of sediment sampling, most of the study sites are placed on low-order streams (i.e., shallow depth). For data consistency, we conducted the surveys at the same sites twice a year in May and September. The total number of study sites consists of 16 from SDU (catchment area: 763 km\({}^{2}\)), five from SDL (237 km\({}^{2}\)), 12 from Oh-Soo, OS (370 km\({}^{2}\)), 12 from Soon-Chang, SC (431 km\({}^{2}\)), 11 from Yo-Cheon, YC (486 km\({}^{2}\)), seven from Seomjin-Gokseong, SG (183 km\({}^{2}\)), 27 from the lower Seomjin River, SL (1128 km\({}^{2}\)), 24 from JD (1029 km\({}^{2}\)), and 10 from BS (283 km\({}^{2}\)) (Figure 1).  
From the study sites, we investigated geophysicochemical features, such as land-use information and water quality parameters. The land-use data were based on the year 2016, and were obtained from the National Spatial Data Infrastructure Portal ([[http://data.nsdi.go.kr](http://data.nsdi.go.kr)]([http://data.nsdi.go.kr](http://data.nsdi.go.kr))). We specifically extracted the land-use data around the study sites by an arbitrary 1-km circle buffer using ArcGIS software (ESRI, Redlands, CA, USA). We collected water samples on each site (one sample per site). The water quality parameters included biochemical oxygen demand (BOD), total nitrogen (TN), nitrate (NO\({}_{3}\)-N), ammonia (NH\({}_{3}\)-N), total phosphorus (TP), phosphate (PO\({}_{4}\)-P), and chlorophyll \(a\) concentrations (Chl-_a_).  
Figure 1: Map of the study area and land-use information in the Seomjin River catchment. The red circles indicate a total of 124 study sites. The entire watershed was divided into nine subwatersheds.  
Figure 2: Total precipitation of the Seomjin River basin in 2017, based on nine major subwatersheds.  
The water quality parameters, including BOD, TN, NO\({}_{3}\)-N, NH\({}_{3}\)-N, TP PO\({}_{4}\)-P, and Chl-\(a\) concentrations were analyzed in the laboratory using water samples on sites in compliance with the methods proposed by [PERSON] [20].  
For biological data, we sampled three benthic sediments at each site, taking the spatial heterogeneity within the site into account. A Surber sampler (30 cm \(\times\) 30 cm, 500 \(\upmu\)m mesh; [PERSON] et al., 1992) was used to collect benthic macroinvertebrates, at a depth of approximately 10 cm in May and September. Then, we preserved the obtained benthic macroinvertebrates in 7% formalin. In the laboratory, we sorted the invertebrate specimens, identified them up to genus or species level, and counted the number of specimens using a dissecting anatomy microscope. The identification was based on several pieces of literature including [PERSON] [21], [PERSON] [22], [PERSON] et al. [23], [PERSON] [24], and [PERSON] [25].