**Abstract**

Harmful algal "blooms", or HABs, is a hazardous natural phenomenon that often occurs under the influence of anthropogenic factors, for example, during the anthropogenic eutrophication of water bodies. An increase in the frequency and duration of cyanobacterial "blooms" carries a number of serious threats, including local and global degradation of water resources and the impact of cyanotoxins. There are various methods of fighting cyanobacterial "blooms" - physical, chemical, the use of bacterial preparations, etc. However, these methods are not effective enough and, most importantly, do not allow effectively solving the problem of suppressing HABs in water bodies without damage to other components of the aquatic ecosystem. Allelopathy is a natural phenomenon for both stimulatory and inhibitory effects of one plant upon another including microorganisms that resolves this problem. Allelochemicals of macrophytes can be considered as natural algaecides and become the basis of a nature-like convergent technology to suppress the development of plankton cyanobacteria and prevent HABs in water bodies. In our work, we used some allelochemicals of aquatic macrophytes to create a combined algicide of the new generation for suppressing the development of cyanobacteria. The effectiveness of suppressing cyanobacterial "blooms" is demonstrated by the example of field experiments with mesocosms and natural phytoplankton.

**Keywords:** harmful algal "blooms", phytoplankton, cyanobacteria, allelopathy, allelochemicals, field experiments, mesocosms
