Aquatic plant chokes waters across Southwest Finland

Thick growths of tähkä-ärviä, an aquatic plant, are spreading across Southwest Finland’s archipelago, in places covering several hectares and preventing people from swimming or boating, Yle reports.

The plant is not an invasive species. Its rapid expansion is linked to environmental changes, particularly eutrophication caused by excess nutrients and warmer waters. The exact mechanism behind the growth is not yet known.

“This summer, growth has been weaker than last year. That may be because there has been less sunshine,” said Pekka Alho, a project manager at Turku University of Applied Sciences.

The university is mapping the growths with drones, testing the effects of cutting them down and looking for uses for the plant material. Trials were carried out in Rymättylä, Naantali, last year, while this year’s work is focused on Ykskoivu in Uusikaupunki.

In shallow bays, the growths can form mats covering up to several hectares, said project planner Nanni Piiparinen. Drone images can identify the plant in some conditions, while other growths must be inspected on site.

The project is also collecting reports from the public. Nearly 40 observations have already been submitted from Uusikaupunki. In the worst cases, people have reported being unable to use motorboats.

Cutting the plant carelessly could make the problem worse because it can spread from small fragments. The material must therefore be taken onto dry land after removal.

Because the plant contains large amounts of nutrients, researchers are investigating whether it could be used as a soil improver or fertiliser. Laboratory tests will also examine its nutrient content and any heavy metals it may have accumulated. The large volume of wet material means that disposal sites should be located nearby to keep transport costs down.

Removal has been tested this summer in Uusikaupunki with a specialised vessel that cuts the growths and lifts them directly aboard. Contractor Tero Almi, who develops removal methods, said he had brought more than 100,000 kilograms of submerged plants ashore over the past two years.

There is still little research on the long-term effects of removing submerged plants, including how much cutting reduces growth the following summer. Alho said the work may become an annual operation because large quantities of nutrients have accumulated in the sea. The project’s results are due to be published next winter.