The short version
Floating wetlands (rafts planted with marsh plants whose roots hang in the water) are often suggested as a way to pull nutrients out of coastal water, but they have rarely been tested in the field, and even more rarely where winter ice is common. A team from Lithuania and Poland floated small planted islands in two brackish lagoons on the southeastern Baltic Sea for three years, 2019 to 2021, and cut the plants each year to see how much nitrogen and phosphorus left the water with the harvest. The plants grew more each year, and each harvest removed 10.4 to 21.2 grams of nitrogen per square meter of island. Tall reeds removed about twice as much as sedges. The study gives real, measured numbers for an idea CPWB has looked at for Waquoit Bay. This summary is based on the abstract.
What the researchers did
The team ran demonstration floating wetlands at two sites and harvested them for three years in a row.
- They installed small commercial floating islands, 24 to 28 square meters each (about 260 to 300 square feet), in the Curonian Lagoon in Lithuania and the Szczecin Lagoon in Poland.
- The Curonian Lagoon islands were planted mostly with a sedge (Carex); the Szczecin Lagoon island mostly with common reed (Phragmites australis).
- Each year from 2019 to 2021 they harvested the plants and measured the nitrogen, phosphorus and carbon the harvest carried away.
- They compared the balance of nutrients in the plants and in the lagoon water to see whether a shortage of nitrogen was holding the plants back.
What they found
- The plants kept growing. Plant mass rose each year and leveled off at about 2.5 to 3.7 kilograms of fresh plant per square meter of island (Lesutienė et al., 2024).
- 10.4 to 13.1 grams of nitrogen with sedges. Each year’s harvest from the sedge islands in the Curonian Lagoon removed 10.4 to 13.1 grams of nitrogen, 0.6 to 0.8 grams of phosphorus and 318 to 431 grams of carbon per square meter of island.
- 21.2 grams with reeds. The reed island in the Szczecin Lagoon removed about twice as much: 21.2 grams of nitrogen, 1.6 grams of phosphorus and 704 grams of carbon per square meter. By our conversion, the range across both sites works out to roughly 90 to 190 pounds of nitrogen per acre of island each year.
- The plants weren’t starved of nitrogen. The nutrient balance inside the plants did not point to nitrogen holding back their growth.
- The water sometimes was. During the summer blue-green algae bloom in the Curonian Lagoon, dissolved nitrogen in the water ran very low compared with phosphorus, a sign of severe nitrogen shortage.
- Plant choice matters. To remove the most nutrients, the authors suggest tall plants that grow a lot of mass, plants with fine roots that take up the scarcest nutrient efficiently, or both (Lesutienė et al., 2024).
What it means for Waquoit Bay today
Floating wetlands came to Waquoit Bay’s attention when Eco-Machine inventor John Todd spoke at CPWB’s 2021 Annual Meeting and put a price on a floating system for the Bay, and CPWB’s Wetlanders committee later proposed a pilot on the Moonakis River. Field tests in brackish water where winter ice is common are rare, which makes these two lagoons one of the closest field matches to our own coastal waters. There, the plants came back through three seasons and grew more each year.
The study also gives a yardstick for any proposal. The nitrogen removed was counted per square meter of island and left the water only when the plants were cut and carried away, so the total depends on how much island is afloat and on harvesting every year. The reed island removed about twice what the sedge islands did, so plant choice is part of the design. Measured numbers like these are what a floating-wetland plan for the Bay should be checked against, much as nitrogen estimates for a Long Island Sound oyster farm were tuned with a year of local measurements.
Read the paper
Lesutienė, J., Bielecka, M., Petkuvienė, J., & Razinkovas-Baziukas, A. (2024). An assessment of nutrient removal by harvesting constructed floating wetlands in two coastal lagoons in the SE Baltic Sea. Ecohydrology, 17(6), e2682. https://doi.org/10.1002/eco.2682
License: Published by Wiley. The article is not openly licensed and the full text is behind a paywall, so we link to the publisher’s page through its DOI rather than host a copy; this summary is based on the published abstract. The card image is a title card made from the citation.
Drafted with AI assistance from the linked source.
