The process for suppressing the growth of green algae in aqueous systems, such as lakes or rivers, comprises introducing carbon dioxide, preferably industrially produced carbon dioxide, into the water to be treated, thereby acidifying the water and suppressing the growth of green algae and promoting the growth of diatoms. The carbon dioxide can be produced by industry. Thus disposal of such industrially produced gases for acidification of waters serves for cleaning water and leads at the same time to clean air.