# Centennial Lake Environmental Evolution Reflected by Diatoms in Yilong Lake, Yunnan Province, China
## 3 Results
### PCA Results and Ecological Affinity of Ditatoms  
A PCA method was performed on diatom data of Yilong Lake using C2 [70], excluding four samples (7-9 cm; 31 cm) in which diatom could not be found (Figure 5). Figure 5: PCA ordination of diatom species in Yilong Lake. The eigenvalues of axis 1 and 2 were 0.21 and 0.10, respectively, which explained 31% of the total. The relationships between each diatom species and each axis are as follows: (1) Each diatom species can be represented by the corresponding vector with the arrow direction pointing to its extreme. (2) The angles between vectors represent the correlation of diatom species. The smaller the angle is, the more positive the correlation is. On the contrary, the greater the angle is, the more negative the correlation is. In addition, there is no correlation between two diatom species if the angle is 90\({}^{\circ}\). (3) Meanwhile, the correlations between the vectors and axis are the same as above. According to the ordination results of PCA (Figure 5), the diatoms in the positive direction of axis 1 mainly include _E. soex_ and _G. gracilis_. Except for the Antarctic region, _E. soex_ is distributed in all continents and the Arctic region and is a typical worldwide species [53] and is also reported all over China [61]. It mainly lives in a freshwater environment with extremely weak running water and can survive in pH values ranging from 4.7 to 9 [53] and occurs in an acidic, neutral environment with a pH value equal to 6 in China [71]. _G. gracilis_ is also a widely distributed species in the world, which is common in streams, waterfalls, and rivers, except in the South and North Poles [53]. It is also found in China within the pH value range of 4.78-8.04 [72] and is regarded as a typical neutral pH species [73]. Therefore, these two diatoms found in Yilong Lake can be classified as acidic to neutral species. The diatoms in the negative direction of axis 1 are mainly _F. construens_ and _F. construens_ var. _venter_, which occur in rivers, lakes, wetlands, and oceans and prefer to live in a neutral-alkaline freshwater environment [53]. Simultaneously, _A. exigau_, _N. menisculus_, _N. viridula_, _F. inflata_, and _N. denticula_ occur around them as well, and they all live in an environment of a pH value greater than 7 [53]. _A. exigau_ is a worldwide diatom that prefers alkaline waters [74]. It is reported in lakes, rivers, springs, and hot springs all over the world [75]. _F. inflata_ can survive in water with a pH value of 7.4-9.0 [53] and is distributed widely in China [61]. _N. menisculus_ is a freshwater species that prefers alkaline environments [76; 77]. _N. viridula_ is a cosmopolitan diatom and occurs in rivers, lakes, wetlands, estuaries, and oceans; they are especially abundant in low-gradient streams and rivers with tolerance to high pH value waters [78]. _N. denticula_ is widely distributed in China. It grows in flowing water environments and is common in swamps, lakes, rivers, and other environments [79]. Its habitat is closely related to alkaline freshwater [79]. Therefore, it can be considered that the environmental variable represented by axis 1 should be the pH value of the water, with the positive direction representing the acidic environment and the negative direction representing the alkaline environment. Similarly, the diatoms in the positive direction of axis 2 mainly include _A. islandica_, _E. soex_, and _C. cynphiformis_. _A. islandica_ is commonly found in oligotrophic to mesotrophic waters with high latitudes or altitudes [80; 81], including boreal lakes, such as Winnipeg Lake and Forest Lake in North America [82]. _E. soex_ is a benthic species and likes to live in clean waters with low nitrogen [83]. _C. cynphiformis_ is widely distributed in the arctic and temperate zones, mainly in oligotrophic lakes and streams [84]. _F. construens_ and _F. construens_ var. _venter_, occurring in mesotrophic and eutrophic lakes [61], are distributed near the original point of axis 2, which may often indicate clean waters [85; 86; 87]. _A.