# _microorganisms_
## 3 Results and Discussion
### Application of qPCR-Based Monitoring in Drinking Water Reservoirs
At all sites, _geo_ copy number and geosmin concentrations increased from June to July and then decreased, with values ranging from 3 to 6.5 \(\times\) 10\({}^{5}\) copies mL\({}^{-1}\) and 5 to 32 ng L\({}^{-1}\), respectively. These results are similar to those of [PERSON] et al. [34], who found that the geosmin concentrations and cyanobacterial cell abundance in drinking water reservoirs over a 14-year period were highest in June and July. The total numbers of cyanobacterial cells, dominant species of cyanobacteria, cell abundances, and odorant concentrations are summarized in Table S1. Cyanobacteria are the most major producers of odorants, which several species have been reported to secrete. Figure 3: Time course of (**A**) _mic_ copy number, 2-MIB concentration, and cyanobacterial cell abundance and (**B**) _geo_ copy number, geosmin concentration, and cyanobacterial cell abundance at the four sampling sites. \(\blacktriangle\), _mic_ or _geo_ copy number as determined with qPCR; \(\bullet\), 2-MIB or geosmin concentration as determined with GC-MS; and \(\blacklozenge\), cyanobacterial cell abundance as determined with microscopy. Multiple morphologically similar species or genera can coexist in water sources, but not all will have the same potential to synthesize odorants [4]. Therefore, 2-MIB/geosmin concentrations and cell abundance counts can be used to determine which species are potential odorant producers within an overall cyanobacterial population. The total number of cyanobacterial cells varied from 10 to 7221 cells mL\({}^{-1}\), and the dominant species were the _Pseudanabaen_ sp. (10-4900 cells mL\({}^{-1}\)), the _Phormidium_ sp. (1600-2100 cells mL\({}^{-1}\)), the _Microcytsi_ sp. (60 cells mL\({}^{-1}\)), the _Anabaena_ sp. (90-120 cells mL\({}^{-1}\)), and the _Merismopedia_ sp. (20-1250 cells mL\({}^{-1}\)). The _Pseudanabaena_ sp. was dominant in most cases at UA, CP, and SB, which are upstream of the North Han River, whereas the _Anabaena_ sp. was dominant at the PD site downstream. When the _Pseudanabaena_ sp. was dominant, the cell number exceeded 3000 cells mL\({}^{-1}\), the 2-MIB concentration was high (16-31 ng L\({}^{-1}\)), and geosmin was detected at a relatively low concentration. The _Phormidium_ sp. was dominant on July 27 at the CP and SB sites, with a 2-MIB concentration of approximately 20 ng L\({}^{-1}\). The _Anabaena_ sp. was dominant at the PD site, and the geosmin concentrations were as high as 29 ng L\({}^{-1}\) and 32 ng L\({}^{-1}\), whereas 2-MIB was hardly detected. The _Pseudanabaena_ sp. is a major producer of 2-MIB in many countries [35; 36] and was confirmed to be the main cause of 2-MIB production in South Korea in 2018 [12]. In this study, the dominant species of cyanobacteria was the _Pseudanabaen_ sp., and the 2-MIB concentration was high. Figure 4 shows the ranges of the _mic_ and _geo_ copy numbers determined at the sampling sites during the study period. The UA site, the most upstream of the study monitoring points, has previously been reported to have high odorant concentrations [20] and was used as a control point to determine how these changed as the odorants moved downstream. For both genes, the copy numbers differed among the four sampling locations and decreased from upstream to downstream. In the t-tests, the copy numbers of the odorous genes were significantly different (_p_ < 0.5 and \(p\) < 0.05) depending on their location in the stream. For _mic_, the copy number was highest (log1.59-log3.95 copies mL\({}^{-1}\)) at the most upstream site (UA) and progressively decreased downstream, such that the lowest copy number (log 1.29-log 2.48 copies mL\({}^{-1}\)) was detected at PD (Figure 4A). A similar pattern was observed for _geo_ (Figure 4B), with the highest gene copy number (up to log 5.82 copies mL\({}^{-1}\)) detected at the upstream sites UA and CP and decreasing downstream. The Han River has two main branches, the South Han and North Han Rivers, which merge at Paldang Dam.