| Summary: | This research deals with the impact of aluminum coagulants, used as a tool for the rehabilitation of water bodies, on changes in the phosphorus content in lakes with low alkalinity of water. Mesocosm scale experimental investigations in situ using polyaluminium chloride were carried out, based on five lakes with different levels of water buffering capacity (<1.0−2.5 meq L<sup>−1</sup>). The research was accomplished using three coagulant doses (half a dose, a whole dose, and a double dose calculated for phosphorus inactivation in a lake ecosystem). The results were compared with the ones from the control mesocosms. The study was carried out in two consecutive years. It was shown that the effectiveness of phosphorus removal from a water column was related to the dose of the coagulant, pH value, and the content of organic compounds. The lowest treatment effectiveness was found for lakes, with the most severe symptoms for dystrophy, which were abundant with humic compounds. As the water alkalinity increased, so did the ability of the coagulant to adsorb phosphorus, while the ecosystem’s load of aluminum in toxic forms decreased. Doubling the standard dose did not result in a noticeable improvement in the water quality. The results allow us to evaluate the chances of successful lake restoration by applying aluminum coagulants to natural water bodies which are susceptible to acidification.
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