## 3 Results and Discussion
### Assessment of the PCB Concentration in the Water of Lake Baikal
#### 3.3.1 Statistical Analysis of the Results of 'Conventional' Monitoring  
The results of the PERMANOVA analysis (Table S6) indicated that among all studied factors only year and month of sampling significantly influenced the concentrations of the investigated PCB congeners, including \(\Sigma_{7}\)PCBs, in Baikal water (year factor--R\({}^{2}\) = 0.163; month factor--R\({}^{2}\) = 0.074; \(p\)-value \(<\) 0.05). At the same time, based on the coefficients of the R\({}^{2}\) covariation, the sampling year factor had the maximum effect on the concentrations of PCBs. Further data analysis by PCA based on the PERMANOVA results was carried out for the values averaged by the factors of the pelagic zone of the lake's basin and sampling depth.  
The analysis of the results of the determination of PCB indicator congeners by PCA revealed that all data were divided into two groups (Figure 4a). The first group in the second quarter of the coordinate plane included the average data obtained in September 2016, May 2018-2019, and June and September 2020-2021. All points of this group were characterised by minimum concentrations of all indicator congeners and \(\Sigma_{7}\)PCBs concentration ranging from \(\leq\)0.11 to 0.47 ng/L. The second group (the first quarter of the coordinate plane) includes the data obtained in May 2016 and September 2019. These data referred to extreme concentrations of all indicators congeners and \(\Sigma_{7}\)PCBs (1.3-2.0 ng/L) relative to the minimum level determined during the monitoring between 2015 and 2021.  
On the PCA biplot (Figure 4a), the PCB concentration gradient vectors form a codirectional group that includes all investigated indicator congeners of PCBs and \(\Sigma_{7}\)PCBs, except for congener 28 (a congener group with a positive correlation). The vector of congener 28 is directed almost perpendicular to the main group, and the concentration of this pollutant depends (correlates) to a lesser extent on the concentrations of other indicator PCBs. Perhaps, the sources of congener 28, its further fate in the aquatic ecosystem, and (or) congener properties (the minimum hydrophobicity and the maximum water solubility among indicator congeners) affect its concentration in Baikal water.  
The PCA data on the concentrations of PCBs in Baikal water allowed us to draw the following conclusions: (i) an increase or a decrease in all indicator congeners, except for congener 28 and the concentration of \(\Sigma_{6}\)PCBs, occurred synchronously; (ii) within the framework of conventional monitoring, the assessment of the PCB concentrations in Baikal water based on the total concentration of six indicator congeners (\(\Sigma_{6}\)PCBs, excluding congener 28) would meet more stringent criteria.  
Figure 4: Results of the concentration distribution analysis of the investigated PCB types and the total PCB concentration based on PCA. **(a)** Analysis of water samples: point on the plane characterise the average PCB concentrations in Baikal water (entire water area of the lake) by the year and month of sampling (a) point is the average PCB concentration in water samples taken in a certain month of one year; **(b)** analysis of the distribution of PCB concentrations in only specimens: points on the plane characterise the average PCB concentrations in only the year, month, and age of the individuals (a point is the average PCB concentration in individuals of the same age collected in the same year and the same month). Vectors on the planes show the directions of the concentration gradients of the investigated PCB types (blue vectors and green vectors are dioxin-like PCBs), the total PCB concentration (orange vectors), and the age of o