3.1. Rainfall Patterns
The rainfall amount during the monsoon period was the largest in 2013 and the lowest in 2015 in all rivers (
Table 2). The rainfall pattern sampled for a month was different for each river. The respective sums of rainfall in each river a week before the second sampling in 2013, 2104, and 2015 were 10.0 mm, 77.0 mm, and 17.6 mm for the Geumgang River; 14.6 mm, 33.7 mm, and 11.7 mm for the Mangyeonggang River; and 0.3 mm, 97.9 mm, and 18.7 mm for the Dongjingang River, which showed the highest rainfall in 2014. However, the Sapgyocheon stream had 70.7 mm, 55.5 mm, and 6.5 mm of rainfall in 2013, 2014, and 2015, respectively. This was the same pattern as that of the monsoon rainfall.
The rainfall frequency over a certain intensity during the monsoon period was counted. The frequency of 10 mm or more of precipitation was the highest in 2013 and the lowest in 2015 in the Geumgang River, but the frequency of rainfall of at least 30 mm was the highest in 2014. For the Mangyeonggang River and Dongjingang River, the frequency of rainfall ≥10 mm was the highest in 2014. The frequency of rainfall ≥30 mm was highest in 2013 and the lowest in 2015, similar to the rainfall pattern for the monsoon period. For the Sapgyocheon stream, the frequencies of rainfall of all intensities were the highest in 2013 and the lowest in 2015. Both the rainfall amount and frequency in the Sapgyocheon stream were the highest in 2013 and the lowest in 2015, as was the case with the rainfall patterns during the monsoon period.
3.2. Land Use and Soil Characteristics
The land use map showed a distinctive difference from river to river (
Figure 4). The proportion of land devoted to agriculture was high around the Geumgang River at 52% and around the Dongjingang River at 60% (ANOVA,
p < 0.01), while the proportion devoted to urban development was relatively high in the Sapgyocheon stream at 54.6% (ANOVA,
p < 0.01). In the Mangyeonggang River, the proportions of urban and forested areas were high at 44.1% and 23.5% (ANOVA,
p < 0.05 for all), respectively. Land use around the rivers had a significant impact on water quality [
49]. Nutrients and pollution were lower in forested areas as forests can naturally purify water quality [
50]. However, the agricultural and urban areas had high levels of pollution compared with the forested areas due to the effects of non–point-source pollution [
51]. In particular, pollution was reportedly high due to an impermeability layer and commercial and industrial zones in urban areas [
19].
Soil characteristics by the rivers were also surveyed (
Figure 5). There was no significant difference in the proportion of class A, B, and C soils in the rivers, but the proportions of class D soil were lower in the Geumgang River (10.0%) and the Dongjingang River (9.1%) and higher in the Mangyeonggang River (53.3%) and Sapgyocheon stream (49.1%) (ANOVA,
p < 0.01). Soil characteristic is an important factor in analyzing runoff due to rainfall. For example, Class D soil causes relatively higher runoff [
13].
3.3. Environmental Characteristics
The environmental factors in the four studied rivers showed distinctive differences before and after monsoons (
Table 3). The water temperature increased significantly from 20.8 °C to 24.4 °C in the Geumgang River, from 21.2 °C to 25.1 °C in the Mangyeonggang River, from 19.9 °C to 25.1 °C in the Dongjingang River, and from 21.2 °C to 24.7 °C in Sapgyocheon stream before and after monsoons (ANOVA,
p < 0.01). Before the monsoon, the Sapgyocheon stream had clearly higher DO levels (8.2 mg/L to 6.9 mg/L) compared with that of the Geumgang (7.3 mg/L to 7.8 mg/L), Mangyeonggang (7.3 mg/L to 7.9 mg/L), and Dongjingang (7.7 mg/L to 8.3 mg/L) rivers (ANOVA,
p < 0.05). After the monsoons, DO was significantly reduced in the Sapgyocheon stream, reaching its lowest value (
t-test,
p < 0.01, ANOVA,
p < 0.01).
Electrical conductivity increased in all rivers before and after monsoons, but it varied from river to river. Electrical conductivity was relatively low in the Geumgang River (287.9 μS/Cm; 397.0 μS/Cm) and the Dongjingang River (191.3 μS/Cm; 288.2 μS/Cm), whereas it was relatively high in the Mangyeonggang River (342.2 μS/Cm; 530.0 μS/Cm) and Sapgyocheon stream (420.1 μS/Cm; 498.7 μS/Cm) (ANOVA, p < 0.05). It increased in all rivers after monsoons.
Turbidity levels decreased in all rivers and were the highest in the Geumgang River (84.2 nephelometric turbidity units [NTU]; 67.5 NTU) before and after the monsoon. TN levels were the highest in the Sapgyocheon stream (4.37 mg/L; 2.95 mg/L) before and after the monsoon (ANOVA, p < 0.01), but no significant difference was seen in the Geumgang River (2.72 mg/L; 2.51 mg/L), Mangyeonggang River (2.63 mg/L; 2.35 mg/L), and Dongjingang River (2.12 mg/L; 1.97 mg/L). TN levels decreased in all rivers after the monsoon, particularly in the Sapgyocheon stream (t-test, p < 0.05). NH3-N was also the highest in the Sapgyocheon stream (1.79; 0.80) before and after the monsoons, whereas no significant difference was found in the Geumgang River (0.32; 0.39), Mangyeonggang River (0.75; 0.31), and Dongjingang River (0.06; 0.07). TP levels before the monsoon were the highest in the Sapgyocheon stream, and no significant change was found in any rivers before and after the monsoon.
The environmental factors in the rivers differed significantly depending on the sampling period (
Figure 6). Water temperatures increased after monsoons, particularly in 2015, when the rainfall amount and frequency were small. Electrical conductivity increased distinctively in 2013 when the rainfall amount and frequency were large and decreased in 2014 and 2015 after the monsoons when the rainfall amount and frequency were small. Rainfall in the monsoon periods causes rain runoff, introducing pollution sources into the rivers [
51]. Urban and agricultural areas, in particular, where the soil permeability is lower than that of the wetlands and forests, had a large amount of soil and particulate-matter runoff, intensifying point and non-point pollution sources and increasing nutrient runoff [
52,
53]. The sampling sites were located in the mid-to-downstream reaches of the rivers and included many agricultural and urban areas with multiple pollution sources.
Variation in electrical conductivity in 2013 was high, with rich nutrients, compared with that of other years. TN, NH
3-N, and TP were higher in the Sapgyocheon stream but low in the Geumgang River, Mangyeonggang River, and Dongjingang River, with fewer nutrients. The variation range in the Sapgyocheon stream was clear before and after monsoons. In particular, TN and NH
3-N decreased more dramatically in 2015 when the rainfall amount and frequency were larger than in 2013. The influx of pollution sources was closely related to runoff from rainfall and surrounding land [
54,
55]. Soil with low permeability caused more runoff [
13]. Sapgyocheon stream had a higher concentration of nutrients and a higher proportion of soil with low permeability compared with the three rivers.
The annual influx of pollution was introduced primarily by rainfall [
56], which can dilute pollutants in the rivers but also increase surface runoff and transport soil nutrients to the rivers, where sediment and pollutants are concentrated downstream due to the water flow [
21]. The high rainfall amount and frequency in 2013 supplied nutrients to the rivers, making the variation before and after the monsoons relatively small. However, the small rainfall amount and low rainfall frequency in 2015 supplied fewer nutrients to nutrient-rich rivers, creating a larger variation before and after the monsoon.
3.5. Epilithic Diatom Communities
The epilithic diatoms that appeared for three years in the rivers in the mid-western region of the Korean Peninsula represented 222 taxa consisting of 2 orders, 3 suborders, 9 families, 37 genera, 200 species, 19 varieties, 2 forma, and 1 subspecies. The variation of major species in each water system before and after the monsoon is as follows (
Figure 7).
Geumgang River: Five major species, including
Melosira varians C. Agardh,
Navicula minima Grunow, and
Aulacoseira granulata (Ehrenberg) Simonsen, were found. The highest-standing crops were
Melosira varians and
Nitzschia palea (Kützing) W. Smith before monsoons and
Aulacoseira granulata and
Melosira varians after monsoons. The standing crop of
Melosira varians, which was high throughout the survey period, occurred primarily in eutrophic waters [
59]. The standing crop increased more in 2015 than in 2013 and 2014, and
Navicula minima,
Aulacoseira granulate, and
Cyclotella meneghiniana Kützing increased after the monsoons.
Mangyeonggang River: Three major species,
Nitzchia palea,
Melosira varians, and
Fragilaria elliptica Schumann, were found. The highest-standing crops were
Melosira varians and
Nitzchia palea before the monsoons and
Nitzchia palea and
Fragilaria elliptica after the monsoons.
Nitzchia palea, which was a high-standing crop throughout the survey period, was an indifferent species. It was resistant to pollution and appeared as a dominant species in polluted waters [
60]. The standing crop of
Nitzchia palea and
Melosira varians increased in 2013, and the standing crop of
Melosira varians increased in 2014. In 2015, the populations of
Melosira varians and
Fragilaria elliptica, which were floating species, increased significantly.
Dongjingang River: Five major species, including
Nitzchia palea,
Melosira varians, and
Nitzchia amphibia Grunow, were found. The highest-standing crops were
Nitzchia palea and
Melosira varians before the monsoons and
Nitzschia palea and
Nitzschia amphibia after the monsoons. The standing crop of
Nitzschia palea increased in 2013, and the most standing crops decreased in 2014. The proportion of
Nitzschia amphibia significantly increased in 2015. It is known as an indifferent species and often appears in nutrient-rich waters [
59].
Sapgyocheon stream: Six major species, including
Navicula minima,
Nitzchia palea, and
Gomphonema lagenula Kützing appeared. The highest-standing crops were
Nitzchia palea and
Navicula minima before and after the monsoons.
Nitzchia palea is known to appear in nutrient-rich waters as a dominant species [
60], and
Navicula minima are known to grow in a wide range from freshwater to weak brackish water. The standing crops of major species decreased in 2013 and 2014, and most standing crops distinctively increased in 2015. Most standing crops of the major species in the Sapgyocheon stream significantly decreased after the monsoons of 2013 and 2014 and increased in 2015. In 2013 and 2014, when the rainfall amount and frequency were relatively large, the standing crop of epilithic diatoms decreased after the monsoons. This was the result of the ease of detachment of epilithic diatoms caused by the flow that occurred during rainfall and erosion by suspended sediment [
61]. However, in 2015, when the rainfall amount and frequency were small, the standing crop increased because detachment of epilithic diatoms occurred less frequently, and growth was dominant due to nutrient uptake. In particular, the Sapgyocheon stream, where the pollution level and concentration of nutrients were high, saw a significant increase in the standing crop of epilithic diatoms.
3.6. Changes in Indicator Species
To identify epilithic diatoms that can represent variable environments along with rainfall on the rivers in the mid-western region of the Korean Peninsula, ISA was performed on each group’s biological community (
Table 5 and
Table 6):
Geumgang River: Five indicator species were identified before the monsoon, including
Navicula capitatoradiata H. Germain ex Gasse and
Achnanthes hungarica (Grunow) Grunow in 2013,
Navicula veneta Kützing in 2014, and four species including
Cyclotella stelligera (Cleve & Grunow) Van Heurck and
Navicula cryptotenella Lange-Bertalot in 2015 (
Table 5).
Navicula capitatoradiata, whose indicator value was high in 2013, is an indifferent species and inhabits brackish to freshwater [
62].
Navicula veneta, which was an indicator species in 2014, was an indifferent species and often found in brackish waters with high electrical conductivity and heavily eutrophic waters [
62].
Navicula crytotenella, which was an indicator species in 2015, was an indifferent species. It is known to inhabit oligotrophic to eutrophic waters but was rarely distributed in heavily polluted areas [
62]. The indicator species in the Geumgang River after the monsoons were
Gomphonema parvulum (Kützing) Kützing and
Aulacoseira ambigua (Grunow) Simonsen in 2013, three species, including
Navicula veneta and
Navicula amphiceropsis Lange-Bertalot & U. Rumrich in 2014, and
Aulacoseira granulata var.
angustissima (O. Müller) Simonsen and
Navicula schroeteri F. Meister in 2015.
Gomphonema parvulum, which was dominant in 2013, appeared in eutrophic water bodies where there was considerable sewage inflow [
63].
Navicula veneta, which was the same indicator species before the monsoon, appeared with a high indicator value after the monsoon in 2014.
Aulacoseira granulata var.
angustissima, which was an indifferent species, appeared in 2015. The species that appeared in the Geumgang River before the monsoons showed a high indicator value in indifferent species. Saproxenous species appeared after the monsoon in 2013, when the rainfall amount and frequency were large, and the same species appeared in 2014 before and after the monsoon. Indifferent species appeared after the monsoon in 2015.
Mangyeonggang River: Three indicator species were identified before the monsoon in 2013, including
Fragilaria bidens Heiberg and
Gomphonema pseudoaugur Lange-Bertalot and
Navicula viridula var.
rostellata (Kützing), Cleve and
Navicula trivialis Lange-Bertalot were identified in 2014, followed by
Fragilaria capucina Desmazières,
Cyclotella stelligera, and
Surirella angusta Kützing in 2015 (
Table 5).
Fragilaria bidens,
Navicula viridula var.
rostellata, and
Fragilaria capucina, which had the highest indicator value each year, are known to inhabit largely mesotrophic waters and prefer weak alkalinity [
46,
64]. Three indicator species were identified after the monsoon in the Mangyeonggang River, including
Cymbella affinis Kützing and
Nitzschia inconspicua Grunow, five species, including
Cocconeis placentula var.
lineata (Ehrenberg) Van Heurck and
Navicula gregaria Donkin were identified in 2014, followed by
Cymbella leptoceros (Ehrenberg) Kützing and
Navicula decussis Østrup in 2015.
Cymbella affinis, which appeared in 2013, is known to prefer waters with average levels of electrolytes [
65].
Cocconeis placentula var.
lineata, which appeared in 2014, is often found in mesotrophic and eutrophic waters [
59].
Cymbella leptoceros, which appeared in 2015, is known to prefer mesotrophic waters [
65]. Species that inhabit mesotrophic waters appeared in the Mangyeonggang River as indicator species before the monsoons. However, species that preferred mesotrophic waters appeared after the monsoons.
Dongjingang River: Eight indicator species appeared before the monsoon, including
Cocconeis placentula var.
lineata and
Aulacoseira ambigua in 2013. Six species, including
Gomphonema parvulum and
Fragilaria bidens, appeared in 2014, and
Cyclotella atomus Hustedt was found in 2015 (
Table 5).
Cocconeis placentula var.
lineata,
Gomphonema parvulum, and
Cyclotella atomus, whose indicator values were high, were indifferent species that frequently appeared in eutrophic rivers [
63]. Six species, including
Navicula recens (Lange-Bertalot) Lange-Bertalot and
Navicula notha J.H. Wallace, appeared in 2013 as indicator species after the monsoons. An indicator species in 2014,
Fragilaria delicatissima Proshkina-Lavrenko, did not appear in 2015.
Navicula recens, which appeared in 2013, is an indifferent species that is widely distributed in eutrophic rivers with high electrical conductivity [
62].
Fragilaria delicatissima, which appeared in 2014, is known to prefer a neutral pH and is indifferent to organic pollutants [
33]. Species that often appeared in eutrophic rivers were identified as indicator species before the monsoons in the Dongjingang River. After the monsoons, species that were often distributed in eutrophic streams with high electrical conductivity appeared in 2013, when the rainfall amount and frequency were large. In 2014, indifferent species appeared, and, no indicator species appeared in 2015.
Sapgyocheon stream: Indicator species before the monsoons were
Navicula cari Ehrenberg and
Fragilaria bidens in 2013,
Navicula cryptocephala Kützing in 2014, and
Fragilaria elliptica and
Navicula veneta in 2015 (
Table 5).
Navicula cari and
Fragilaria elliptica, whose indicator values were high in 2013 and 2015, preferred fresh or eutrophic waters with high electrical conductivity [
62,
66].
Navicula cryptocephala, whose indicator value was high in 2014, was known to appear frequently in waters with high electrical conductivity [
62]. As an indicator species after the monsoons,
Navicula seminulum Grunow and
Nitzschia subacicularis Hustedt appeared in 2013, preferring eutrophic waters. In 2015,
Nitzschia paleacea (Grunow) Grunow and
Cyclotella stelligera appeared. In particular,
Cyclotella stelligera is known to prefer oligotrophic or mesotrophic waters [
67]. In the Sapgyocheon stream before the monsoons, species that preferred eutrophic waters with high conductivity emerged. In 2013, when the rainfall amount and frequency were high after the monsoons, species that preferred eutrophic waters with high pollution were found, whereas species that preferred mesotrophic waters appeared in 2015 when the rainfall amount and frequency were small.
In summary, the ecological characteristics of the indicator species that appeared in the Geumgang River, Mangyeonggang River, and Dongjingang River before and after the monsoon were not distinctively different. In the Sapgyocheon stream, saproxenous species that preferred eutrophic waters appeared before the monsoons each year, and after the monsoons, saproxenous species appeared in 2013. In 2015, when the rainfall amount and frequency were low, species that preferred mesotrophic waters appeared, indicating that the ecological characteristics of the indicator species were distinctively differentiated according to the rainfall pattern.
3.7. Community Variation
To identify the distribution of epilithic diatoms before and after the monsoon, PCA was conducted using the number of species that appeared for three years and the standing crop of each species. The analysis revealed that the epilithic diatom community could be divided by survey period. Before the monsoon in 2013, diatoms were distributed in the positive direction along axis 1, with a similar trend evident after the monsoon. The diatoms were distributed in both positive and negative directions before and after the monsoons in 2014. In 2015, they were distributed in the negative region along axis 2 (
Figure 8a). To quantitatively identify the effect of rainfall on the epilithic diatom community, a CDI was calculated and then comparatively analyzed by river and year (
Figure 8b).
The results of the analysis of the CDI in each river showed that the highest CDI (ca. 5.50) occurred in 2015 when the rainfall amount and frequency were small in all rivers (
Figure 8c). The Sapgyocheon stream exhibited the lowest value is 2013 and the highest value in 2015 (ANOVA,
p < 0.01). Rainfalls cause an influx of nutrients from urban and agricultural areas to the rivers [
68,
69], and more runoff occurs on impervious surfaces in urban areas or soils with low permeability [
13,
70]. The high rainfall amount and frequency in 2013 caused an influx of nutrients and pollutants into nutrient-rich rivers, making no significant difference in the environment before and after the monsoon, reducing community variation. However, the low rainfall amount and frequency improved water quality, resulting in an environmental change and large community variation before and after the monsoons. This is consistent with the changes in environmental factors.
In a study by Cho et al. (2020) [
37], the variation in the epilithic diatom community was large when the rainfall amount was high, which was the opposite result of the present study. The survey sites of Cho et al. (2020) were the main tributaries of the Imjingang River, which is not a large river compared with those in this study. In contrast, the present study selected survey sites from mid- to downstream reaches of relatively large rivers with multiple tributaries. Matter in upstream reaches is transported downstream, and pollutants that are introduced from tributaries [
71], non-point pollution sources, point pollution sources, and surrounding lands are concentrated in downstream areas [
72]. Intensive rainfall may dilute nutrients but can also introduce a large number of nutrients to rivers by increasing surface runoff [
11,
21]. A large amount of rainfall did not create an environmental difference from that before the monsoon due to the influx of various components, whereas a small amount of rainfall introduced a low level of nutrients and pollutants to the rivers, making an environmental difference. When epilithic diatoms move from eutrophic waters to an unpolluted environment with fewer nutrients, their community composition changes [
73].
In conclusion, the largest change in the epilithic diatom community occurred in 2015, when the habitat environment changed distinctively due to a small amount of rainfall, and the Sapgyocheon stream experienced significant yearly changes because of the large number of urban impermeable surfaces and low-permeability soils.
3.8. Relationship between Rainfall Patterns and Epilithic Diatom Communities
The relationship between community variation and rainfall showed a mainly negative correlation, with a significant correlation seen in the Mangyeonggang River and Sapgyocheon stream (
Table 6). Sapgyocheon stream showed a weak correlation between the sum of rainfall one month before sampling and the CDI but a significant negative correlation with entire rainfall during the monsoon periods (
p < 0.05). The sum of rainfalls for one and two weeks in the second survey and the rainfall amount during the monsoon periods showed a significant correlation with the CDI in the Sapgyocheon stream, and the rainfall amount within two weeks had the highest correlation (
p < 0.01).
The CDI and rainfall frequency also showed a primarily negative correlation, with a significant correlation seen in the Mangyeonggang River and Sapgyocheon stream. The frequency of rainfall events ≥30 mm and ≥50 mm influenced the CDI in the Mangyeonggang River. The highest correlation was with the intensity and frequency of rainfall ≥30 mm. In the Sapgyocheon stream, the frequency of rainfall ≥10 mm, ≥30 mm, ≥50 mm, and ≥70 mm exhibited a correlation, and the frequency of rainfall ≥10 mm had the highest correlation with the CDI (p < 0.01).
The rainfall pattern and the CDI were significantly correlated in the Mangyeonggang River and Sapgyocheon stream, which had a high percentage of soil with low permeability. In particular, the frequency of rainfall ≥10 mm had a high correlation with the biotic change in the Sapgyocheon stream, which had a high level of pollution.