## 2 Materials and Methods
### Phytoplankton Species Composition and Abundance Assessment by Conventional Light Microscopy (LM)  
In the lab the taxonomic work and counts were done on non-permanent slides on a Motic B1 microscope under magnification 100\(\times\) and immersion. The microphotographs were taken by a Moticam 2.0 mp camera supplied by Motic Images 2 Plus software program. From each site at least four slides were examined.  
The identification of algae followed the standard European taxonomic literature with updates from AlgaeBase ([PERSON], 2019) and relevant modern taxonomic papers. For cyanoprokaryotes, in particular, determination was based on (Dortmund, Germany, and others, 2019; Dortmund, Germany, and others, 2019; Dortmund, Germany, and others, 2019) and additional checks were made in CyanoDB (Dortmund, Germany, and others, 2019). The following morphological features were used for species distinguishing in the genus _Microcystis_: general shape of the colony and presence/absence of subcolonies, density of cells inside the colony, structure of colonial mucilage, cell size, facultative or obligatory occurrence of gas vesicles aggregated in aerotopes. The potential MC-producers were outlined after (Dortmund, Germany, and others, 2019; Dortmund, Germany, and others, 2019).  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Waterbody** & **Alt** & **Latitude** & **Longitude** & **WT** & **pH** & **SD** & **CN** & **TD** & **DO** & **TP** & **TN** \\ \hline Res. Krapets (IBW2000) & 410 & 43\({}^{\circ}\)04.0316\({}^{\prime}\) & 24\({}^{\circ}\)52.3835\({}^{\prime}\) & 28.7 & 8.26 & 5.0 & 870 & 564 & 7.74 & 0.1 & 1 \\ Res. Kriva & 133 & 43\({}^{\circ}\)22.6573\({}^{\prime}\) & 27\({}^{\circ}\)10.9807\({}^{\prime}\) & 23.7 & 8.38 & 0.3 & 662 & 428 & 6.24 & 1.0 & 9 \\ Res. Izvornik 2 (IBW3082) & 255 & 43\({}^{\circ}\)27.3838\({}^{\prime}\) & 27\({}^{\circ}\)21.111\({}^{\prime}\) & 24.5 & 9.44 & 0.15 & 389 & 253 & 13.26 & 8.96 & 4.8 \\ Lake Durankulak (IBW0216) & 4 & 43\({}^{\circ}\)40.5355\({}^{\prime}\) & 28\({}^{\circ}\)33.0806\({}^{\prime}\) & 26.7 & 8.91 & 0.6 & 1048 & 680 & 6.04 & 0.33 & 0.63 \\ Res. Poroy (IBW3038) & 43 & 42\({}^{\circ}\)43.3403\({}^{\prime}\) & 27\({}^{\circ}\)37.5255\({}^{\prime}\) & 27.5 & 8.05 & 0.4 & 644 & 416 & 7.6 & 0.10 & 0.31 \\ Res. Mandra (IBW1720) & 7 & 42\({}^{\circ}\)24.0295\({}^{\prime}\) & 27\({}^{\circ}\)19.1194\({}^{\prime}\) & 25.88 & 7.9 & 0.45 & 676 & 436 & 7.93 & 0.66 & 0.46 \\ - East & 8 & 42\({}^{\circ}\)25.9303\({}^{\prime}\) & 27\({}^{\circ}\)26.7652\({}^{\prime}\) & 27.2 & 8.46 & 0.45 & 578 & 375 & 7.87 & 1.5 & 1.8 \\ Lake Uzungeren (IBW0710) & \(-\)3 & 42\({}^{\circ}\)26.1551\({}^{\prime}\) & 27\({}^{\circ}\)27.2235\({}^{\prime}\) & 27.6 & 8.45 & 0.45 & 1748 & 1132 & 9.7 & 0.40 & 0.28 \\ Lake Vaya (IBW0191) & \(-\)2 & 42\({}^{\circ}\)30.5940\({}^{\prime}\) & 27\({}^{\circ}\)22.075\({}^{\prime}\) & 27.9 & 9.22 & 0.15 & 490 & 17 & 7.69 & 0.50 & 0.26 \\ Res. Sinyata Reka (IBW1793) & 317 & 42\({}^{\circ}\)28.1518\({}^{\prime}\) & 24\({}^{\circ}\)42.0159\({}^{\prime}\) & 28.2 & 10.39 & 0.4 & 490 & 317 & 14.76 & 1.0 & 0.23 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sampling sites and their main environmental parameters in August 2019. Legend: IBWXXXX–number of the waterbody in the Inventory of Bulgarian wetlands (Dortmund, Germany, and others, 2019); SD–sampling date; Alt–altitude (m a.s.l.); WT—water temperature (\({}^{\circ}\)C), SD—Secchi depth (m); CN—conductivity (\(\upmu\)S); TD—total dissolved solids (\(\upmu\) L\({}^{-1}\)); DO—oxygen concentration (mg L\({}^{-1}\)); TP—total phosphorus (mg L\({}^{-1}\)), TN—total nitrogen (mg L\({}^{-1}\)). For details see the text of the paper.  
The quantitative contribution of each species was estimated using the method of the stereometrical approximations [37] on [PERSON] blood-counting chamber with cell considered as the main counting unit [23; 38].