## 4 Discussion  
The HPLC data on marker phytoplankton pigments revealed high chlorophyll \(a\) concentrations (with very high values in the small inland reservoirs Sinyata Reka and Izvornik 2) and demonstrated high proportion of cyanoprokaryotes in the quantitative structure of the summer phytoplankton of the studied waterbodies (Figure 6). The contribution of cyanoprokaryotes to the total biomass exceeded 95% in the Lake Vaya and Sinyata Reka reservoir, and was over 70% in the reservoirs Izvornik 2 and Poroy (Figure 6). All these data are in accordance with the maximum abundance of cyanoprokaryotes in the late summer phytoplankton of the shallow and highly eutrophic waterbodies of the world [57; 58], and of Bulgaria in particular [19; 22; 23]. The results on taxonomic phytoplankton structure obtained during this study by conventional LM (Figures 2-4) demonstrated a high contribution of cyanoprokaryotes to the phytoplankton species composition and abundance. These data coincide with data assessed from the phytoplankton pigment concentrations (Figure 6) and chemically detected cyanotoxins--MCs (Table 3). According to the toxin analyses, MC concentrations varied by sites with a range from undetectable concentrations to 0.46 \(\mu\)g L\({}^{-1}\), with only one recorded type, namely MC-LR (Table 3). This MC is renowned because it has been considered as the most dangerous in the WHO Guidelines for Drinking-water Quality with a provisional value of 1 \(\mu\)g L\({}^{-1}\)[59]. This is the second finding of MCs, and of MC-LR in particular, in the reservoir Mandra and the fourth record of MCs (incl. MC-LR) from the lake Durankulak (for details see [19]). The absence of detectable amounts of MCs in the reservoir Sinyata Reka, in which MC-RR (0.09 \(\mu\)g L\({}^{-1}\)) and MC-LR (0.3 \(\mu\)g L\({}^{-1}\)) were detected in June 2018 [23] is explainable by the different dominants in both studied periods. In June 2018, in Sinyata Reka _Microystis wesenbergei_ dominated, reaching over 90% of the total biomass [9; 23], while in August 2019 heterocytous filamentous _Dolichospermum_ dominated and _M. wesenbergei_ occurred rarely in the samples, in combination with even more sparse _M. aeruginosa_. The difference in dominants and frequent changes of phytoplankton assemblages are typical for shallow waterbodies (e.g., [57]). But in this particular case we have to also mention the strong, atypical for Bulgaria, rains from May until the first decade of August 2019, which preceded the sampling and caused serious dilution of the reservoir waters with lower amounts of TP and TN, the values of which were twice less in comparison with previous year [23]. The results from conducted molecular-genetic analysis (Figures 7 and 8) demonstrated that toxigenic \(mcy\) sequences were found in all three sites, where MC-LR was detected (Mandra West, Mandra East and Durankulak--Table 3). The lack of measurable amounts of MCs (<LOD) in all other wetlands, besides the strong rains mentioned above, can be explained by: (1) the fact that the methods applied by us have limited scope while types of MCs are highly diverse with increasing number of found congeners [13]. Therefore, it is possible to suppose that in shallow Bulgarian waterbodies there are more MC types which have not been detected during the study; (2) quite low _Microystis_ amounts recorded by LM (Table 2), which do not allow the chemical toxin detection; (3) the lack of gene expression during the sampling period due to the specific temporal patterns of \(mcy\) gene expression (e.g., [60]); (4) the influence of other possible factors on toxin production, like the growth phase of the populations [61]. The absence of \(mcyE\) and \(mcyB\) sequences and of detectedMCs in the reservoirs Krapets and Kriva Reka is supported by the fact that no known MC producers were identified in the phytoplankton samples.