4.1. Distribution of diatom communities from surface peat sediments
The dominant diatoms belong to Pennatae and most of them are benthic or epiphytic diatoms in peatlands. The most diverse genera in both shrubby-herbaceous and herbaceous peatlands are Navicula, Pinnularia and Eunotia, followed by Cymbella, Gomphonema, Nitzschia, Fragilaria and Aulacoseira (Fig. 2). Kingston (1982) found similar diatom assemblage (Eunotia, Navicula and Pinnularia) from peatlands in northern Minnesota. Hargan et al. (2014; 2015) and Chen et al. (2014; 2020) also implied the genera with the most species were Eunotia and Pinnularia in peatlands of Canada, in montane mires of central China and in Sphagnum peatlands of northeastern China, respectively. Navicula is abundant in freshwater and can grow in water with various characteristics (Van Dam et al., 1994; Ma et al., 2018). Pinnularia is also the freshwater algae and develop well especially in low electrolyte or mineral water environments, while only a few species could live in marine and brackish waters (Rühland et al., 2000). Eunotia widely distributed in oligotrophic, acidic environments, and some species attach to the peat mosses in swamps (Chen et al., 2014; Ma et al., 2018). It is freshwater genus which mostly occurring in soft water with low content of calcium and chloride, such as lakes, rivers and ponds. It is common in oligosaprobic and clean water conditions (Cubizolle et al., 2005). Cymbella is freshwater genus with wide range of temperature adaptation, mostly epiphytic and can also shed from the substrate and live free-floating. Generally grow in circum-neutral or alkaline environments, as well as in the humid subaerial habitats. Except for a few pollution-tolerant species, it generally likes to grow in meso- to oligotrophic, clean or cooler water bodies, which can be used to indicate the water environment with low phosphorus concentration (Van Dam et al., 1994; Dong et al., 2015). Gomphonema often lives in relatively clean running or still water, however, a few species have pollution tolerance and can grow in a polluted environment. Some species are circum-neutral, and most of the species grow in water of pH ≥ 7 (Proske et al., 2017). Nitzschia can live in almost all kinds of water bodies, and some species can live in high concentration organic wastewater, polysaprobic conditions (Van Dam et al., 1994; Cubizolle et al., 2005; Mangadze et al., 2017). Fragilaria is generally predominant in freshwater (Cremer et al., 2001). Aulacoseira is Centricae diatom which mainly planktonic, widely distributed in freshwater and often appear in inland water and terrestrial sediments (Van Dam et al., 1994).
The dominant species of diatoms in shrubby-herbaceous peatlands are Eunotia paludosa (49.6%), Eunotia bilunaris var. bilunaris (16.6%), Achnanthes minutissima (13.0%), Nitzschia palea (4.1%) and Eunotia mucophila (3.4%) (Fig. 2). Eunotia paludosa has absolute superiority in most of the samples, with the highest species content of 91.05%. Eunotia paludosa, Eunotia bilunaris var. bilunaris and Eunotia mucophila have the highest occurrences, the dominant acidophilous species indicated that the water environments of shrubby-herbaceous peatlands in the study area are acidic. Hargan et al. (2014) also implied the acidobiontic diatoms Eunotia paludosa and Eunotia mucophila dominated when water pH lower than 5.5 in peatlands of the Boreal Shield and Hudson Plains, Canada. As we could see in Fig. 5, the pH optima and tolerance ranges of Eunotia mucophila and Eunotia paludosa are 5–6 in our study area. The main dominant diatoms species in herbaceous peatlands are Nitzschia palea, Navicula minima, Fragilaria capucina, Hantzschia amphioxys, Gomphonema parvulum, Navicula radiosa, Nitzschia terrestris, Pinnularia pulchra, Caloneis silicula and Achnanthes minutissima which are suitable for neutral-alkaline habitats (Hargan et al., 2014; Proske et al., 2017; Mangadze et al., 2017). The dominance of these diatom species indicates that the water pH of herbaceous peatlands is higher than that of shrubby-herbaceous peatlands in the northern Greater Khingan Mountains.
4.2. Relationships between modern diatom communities and water environments
The maximum CODMn is 58.7 mg/L at sample 15 which belonging to shrubby-herbaceous peatland (Table S2), Eunotia paludosa (64.18%) and Achnanthes minutissima (33.8%) are the dominant species at sample 15 (Fig. 2). As we can see in Fig. 4, there are significant positive correlations between CODMn and Eunotia paludosa and Achnanthes minutissima. Also Eunotia paludosa and Achnanthes minutissima have the highest optima and tolerance ranges of CODMn among the main diatom taxa (Fig. 5). Sample 15 also has lower diatom species diversity, Shannon-Weiner index is 0.8, Simpson index is 0.5 and Pielou index is 0.4 (Table S2). There exist significant negative correlations between CODMn and diatom species diversity indices in Table 2. The minimum value of CODMn is 1.03 mg/L, appears at sample 16 which is herbaceous peatland (Table S2). The dominated diatoms at sample 16 are Hantzschia amphioxys (17.2%) and Fragilaria capucina (12.9%) which have the lower optima and tolerance ranges of CODMn (Fig. 5). CODMn suggests significant negative correlations with Hantzschia amphioxys and Fragilaria capucina (Fig. 4). The species diversity at sample 16 is relatively high, the Shannon-Weiner, Simpson and Pielou indices are 2.7, 0.9 and 0.8, respectively (Table S2). The negative correlation between diatom species diversity and CODMn may indicate that freshwater with high cleanness in the study area is more suitable for diatom species flourishing.
The maximum pH value 7.7 appears at sample 16 (herbaceous peatland) (Table S2), which are dominated by Hantzschia amphioxys (17.2%), Fragilaria capucina (12.9%), Nitzschia terrestris (11.2%), Nitzschia palea (7.9%), Navicula mutica (7.3%), Meridion circulare (5.6%) and Caloneis silicula (4.3%) (Fig. 2). Most of these above diatom species are alkaliphilous or circum-neutral types (Hargan et al., 2014; Proske et al., 2017; Mangadze et al., 2017), which positively correlated with water pH (Fig. 4) and have higher optima and tolerance ranges of water pH (Fig. 5). The minimum pH 4.8 occurs at sample 11 (shrubby-herbaceous peatland) (Table S2), Eunotia paludosa accounts for 68.4%, which is a typical species in acidic peatlands (Poulíèková et al., 2004; Hargan et al., 2014). Besides, the diatoms species diversity in shrubby-herbaceous peatlands is lower than that in herbaceous peatlands (Fig. 3g, h, i). Diatom species diversity increases with water pH and suggests significant positive correlation with water pH (Table 2). Shurtliff et al. (2017) noted that alkaline, peat-rich and nutrient-rich wetland waters were consistent with high diatom species diversity. Yang et al. (2005) suggested that diatom species diversity increased from mesotrophic to meso-eutrophic waters, but decreased in eutrophic lakes, the number of diatom species in hypereutrophic lakes was lowest. Kingston (1982) and Hargan et al. (2014) also suggested that diatoms diversity and richness on hummocks were lower than in hollows, in acidic bogs were lower than in rich fens due to the increase of minerotrophy. Water pH has a significant impact on the growth, reproduction and metabolism of diatoms. Many diatoms species have specific optima and tolerance ranges on the pH of water habitats (Chen et al., 2014). If the pH of the habitat exceeds this specific range, these species will quickly change or disappear. The maximum pH in the study area is 7.7, we infer that the increase of pH in this range is conducive to the increase of diatom species diversity.
4.3. Diatoms as biomonitors of hydrological environments
Caloneis silicula content in the samples with pH > 7 is much higher than that in the samples with pH < 7, the highest content (over 10%) of this species is at sample 17 herbaceous peatland (Fig. 2). Caloneis silicula is positively correlated with pH (Fig. 4), which has the highest pH optima and tolerance range in our study area (Fig. 5), indicating that Caloneis silicula is not suitable for acidic water and can be used as an indicator of high pH water environment in the northern Greater Khingan Mountains.
Eunotia bilunaris var. bilunaris is an epiphytic species and distributed worldwide, prefers to living in acidic, oligo-mesotrophic, low conductivity swamps, lakes and shallow waters (Ma et al., 2018; Chen et al., 2020). The relative content of Eunotia bilunaris var. bilunaris is the highest in samples 5 and 12, which exceeding 40% (Fig. 2). The EC is 43 and 39 µs/cm, pH is 5.5 and 5.2 in samples 5 and 12, respectively, both EC and pH in samples 5 and 12 are less than the average EC and pH in shrubby-herbaceous peatlands (Table S2). The lower optima and tolerance ranges of pH and EC (Fig. 5), indicating that Eunotia bilunaris var. bilunaris can be used as an indicator of low conductivity and acidic water environments.
Eunotia mucophila is epiphytic and generally found in oligotrophic and acidic lakes and bogs (Chen et al., 2014; Hargan et al., 2014, 2015; Ma et al., 2018). The relative content of Eunotia mucophila are the highest at samples 10 and 13, exceeding 15% (Fig. 2). The pH of these two samples are less than 5.5 (Table S2), the lowest pH optima and tolerance range of Eunotia mucophila (Fig. 5) among the main diatom taxa in our study area indicated that this species is dominant in acidic environments, little or no presence in alkaline conditions. Eunotia mucophila prefers to acidic water conditions and negatively correlated with water pH (Fig. 4), it could be used as indicator species in acidic water environments.
Eunotia paludosa prefers to living in low pH and oligotrophic pools and streams, and can be found in water with moderate EC (Poulíèková et al., 2004; Hargan et al., 2014, 2015). Eunotia paludosa has the highest relative content of more than 70% in samples 1, 4 and 6 (Fig. 2). The pH values of these three samples are all less than 6 and the average EC is 53 µs/cm (Table S2). Little or no content of Eunotia paludosa is found in samples when EC over 110 µs/cm. The relative content of Eunotia paludosa in the samples with pH > 6 is much lower than that in the samples with pH < 6. The lower optima and tolerance ranges of pH and EC in Fig. 5 indicated that Eunotia paludosa prefers acidic water and can be used as an indicator of low conductivity and acidic water environments.
Fragilaria capucina which epiphytic or planktonic is a universal species in oligotrophic freshwater (Chen et al., 2021). The species can not exist in the samples with pH less than 6.5, which positively correlated with pH (Fig. 4) and has relatively high pH optima and tolerance range (Fig. 5). In samples 23 and 29, the concentrations of NH4-N, NO3-N and PO43− are the lowest, TN and TP are also close to the lowest value in the study area (Table S2), while the relative content of Fragilaria capucina is the highest (over 25%) (Fig. 2). The relative content of Fragilaria capucina in the samples with high TN and TP in the study area is much lower than that in the samples with low TN and TP. Fragilaria capucina has low TN optima and tolerance range (Fig. 5) and prefers nutrient-poor habitats, which can be used as an indicator of nutrient-poor environments.
Hantzschia amphioxys is widespread in circum-neutral pH conditions (Hargan et al., 2014; Proske et al., 2017). The relative content of Hantzschia amphioxys is the highest at samples 16,18, 19 and 30, exceeding 10% (Fig. 2). This species tends to grow in herbaceous peatlands with higher pH than in shrubby-herbaceous peatlands in the northern Greater Khingan Mountains which exhibits positive correlation with pH (Fig. 4) and has relatively high pH optima and tolerance range (Fig. 5). It can be used as an indicator of high pH water environments in the study area.
Navicula pupula prefers medium-high conductivity, eutrophic, weakly alkaline water (Dong et al., 2006). At samples 20 and 29, the relative contents of Navicula pupula are the highest (more than 3%), and the EC of sample 20 is up to 208 µs/cm (Table S2). The relative contents of Navicula pupula in the samples with low water conductivity in the study area are lower than that in the samples with high conductivity, indicating that Navicula pupula prefers water conditions with medium to high conductivity and can be used as an indicator of medium-high conductivity.
Navicula radiosa is a benthic, alkaliphilous species and is widely distributed (Hargan et al., 2014). In sample 23, the relative content of Navicula radiosa is the highest, over 17% (Fig. 2). The concentrations of TP and PO43− at this point are the lowest, TN and CODMn are also relatively low (Table S2), lower optima and tolerance ranges of TN and CODMn (Fig. 5) indicated that Navicula radiosa may be suitable for living in clean and nutrient-poor water. The content of Navicula radiosa is little or absent in the samples with pH less than 6, higher pH optima and tolerance range (Fig. 5) indicate that Navicula radiosa is alkaliphilous and prefers water with higher pH, which can be used as an indicator of clean water environment with high pH in the northern Greater Khingan Mountains.