## 2 Materials and Methods  
### Sites and Sampling  
The sampling was carried out between the 14 th and 21 st August 2019 in nine shallow waterbodies situated in Central and Eastern Bulgaria (Figure 1, Table 1). In Table 1 we provide the unique number (IBWXXXX) from the Inventory of Bulgarian wetlands [18] where detailed descriptions of the waterbodies are available. However, it is worth mentioning that: (1) the reservoirs Krapets, Kriva Reka, Izvornik 2 and Sinyata Reka are inland, while the lakes Durankulak, Vaya and Uzunegeren, and the reservoirs Poroy and Mandra are coastal; (2) three waterbodies are included in the Red List of Bulgarian Wetlands [18] as critically endangered (lakes Vaya and Durankulak) and endangered (reservoir Mandra); (3) reservoirs Kriva Reka and Izvornik 2 were sampled for first time.  
The sampling was preceded by the use of a drone DJI Mavic 2 Enterprise Dual Pro (DJI Technology Co., Ltd. Shenzhen, China), supplied by a photo camera and with the possibility to measure the surface water temperature. In this way we observed in real time the whole waterbody and spots with visible differences in the color were chosen for sampling of cyanoblooms from inflatable boats in accordance with our former study [23]. In cases of visible water homogeneity, the sites from our previous studies were repeated (for details see [23]). Therefore, the Mandra reservoir was sampled in two sites--in its eastern part, and its western part, presented hereafter as Mandra East and Mandra West, respectively. The coordinates, altitude above sea level, water temperature, pH, total dissolved solids, oxygen concentration, and conductivity were measured in situ by Aquameter AM-200 and Aquaprobe AP-2000 from Aquared water monitoring instruments (Aquaread  
Figure 1: Map of R. Bulgaria with indication of the studied waterbodies (modified after ([[http://www.ginkgomaps.com](http://www.ginkgomaps.com)]([http://www.ginkgomaps.com](http://www.ginkgomaps.com)) and Google Maps). Explanations are provided in the text.  
Ltd., Broadstairs, UK). The water transparency was estimated using a Secchi disk, while total nitrogen (TN) and total phosphorus (TP) were measured ex situ with the Aqualytic AL410 Photometer from AQUALYTIC(r) (Dortmund, Germany) (Dortmund, Germany) (Dortmund, Germany) (Dortmund, Germany). Data from all these measurements are shown in Table 1.  
Phytoplankton samples for taxonomic identification and counts, for pigment analysis and for molecular-genetic studies (each in a volume of 0.5 L) and for toxin identification (in a volume of 1 L) were collected from the water surface (0-20 cm) due to the holo-polymitic character of the studied wetlands. The samples for phytoplankton identification and counts were fixed immediately with 2% formalin and transported to the lab, where they were further concentrated by sedimentation (Dortmund, Germany, and others, 2019). Additionally, from each site a living sample in a volume of 0.05 L was inoculated in plastic tubes containing Bristol media and Allen-Arnon solution (50:50) (Dortmund, Germany, and others, 2019) for stimulation of cyanoprokaryote growth and further cultivation in the Algal Collection of Sofia University ACUS (Dortmund, Germany, and others, 2019). The samples for toxin analyses were immediately placed in a box with dry ice and were provided frozen to the relevant lab. The water samples for pigment analyses and PCR-studies were filtered under a mild vacuum, using Macherey-Nagel GF5 filters porosity 0.4 \(\upmu\)m (MACHEREY-NAGEI GmbH & Co. KG, Duren, Germany), and Whatman(r) 0.45 \(\upmu\)m cellulose filters (Merck KGaA, Darmstadt, Germany), respectively, within a few hours after collection. The filters were placed in 15 mL sterile plastic tubes (Falcon) and preserved in dry ice until further treatment (see below).