Floating islands on Lake Ceresio against algae and cyanobacteria

Three AlternativA councillors in Mendrisio, Jacopo Scacchi, Monika Fischer Kiskanc and Giampaolo Baragiola, have tabled a question proposing floating islands on Lake Ceresio. They ask the municipal executive whether it is aware of the project and whether it would consider a trial, particularly in the Capolago area.

The problem behind the request is a recurring one across the whole basin. During warmer periods the lake experiences blooms of algae and cyanobacteria, which affect water quality, bathing safety and the region's appeal to visitors. Cyanobacteria in particular can produce toxins and lead to the temporary closure of swimming areas.

How they work

The proposed structures are floating modules on which aquatic plants grow. The roots reach directly into the water and take up excess nitrogen and phosphorus, the nutrients that drive algal growth. Removing nutrients reduces the conditions that favour a bloom. It is a form of phytodepuration, a technique already used in artificial basins and river stretches, applied here to a natural lake. The project was developed with the involvement of Supsi.

The councillors present the idea as an attempt to bring together three aims usually treated separately: environmental protection, landscape quality and the enhancement of the territory. A planted floating module, they argue, is not a technical installation to be hidden but an element that can sit within the lakeside landscape.

The question the request leaves open is effectiveness at scale. The amount of nitrogen and phosphorus a few floating islands can absorb has to be set against the loads reaching the basin from treatment plants, agricultural run-off and diffuse discharges. Lake Ceresio also has a slow water turnover, one of the reasons nutrients accumulate in it. A trial would serve precisely to measure that ratio, which on paper remains uncertain.