‘Floating Island’ plant rafts create wildlife hotspots, nurture beneficial microbes and pond ecosystems.

These floating islands typically comprise mat rafts made of plastic, foam or cork, sown with locally available native plants that would otherwise be unable to grow there due to the water depth. 

These features are sometimes used on stormwater ponds as an affordable, low-maintenance, nature-based solution. They absorb and remove pollutants and excess nutrients from wastewater, eutrophic water or urban stormwater runoff and offer a promising method of bioremediation (the use of organisms to remove contamination). In this way, the rafts deliver benefits for the environment, society and the economy. 

So far, research in this area has primarily focused on their ability to clean contaminated or polluted water, but the new study confirms how they also boost biodiversity thanks to the role of plants and the beneficial microbes which colonise the plant rafts.

As an experiment, researchers set up floating islands in a 48-metre-perimeter, 1.2-metre deep artificial stormwater pond in rural Portugal. The cork-based rafts contained four locally available perennial plant species: bearded iris Iris germanica (4 plants), marsh marigold Caltha palustris (2 plants), sweet flag grass Acorus gramineus (4 plants), and bulrush Typha latifolia (2 plants). The islands were established in 2018 and sampled again in 2022. The pond was fed by water from a second artificial pond, mixed with stormwater from surrounding woods and agricultural fields. 

The researchers assessed the culturable bacterial communities that colonised the floating platform and root system, and the invertebrate biodiversity associated with the rafts and the pond margin. The study identified 30 freshwater ecosystem bacterial strains living among the plant roots and on the platform surfaces. They found that bacteria capable of producing plant-growth-boosting molecules were thriving on the rafts. Among the plants, A. gramineus dominated the platforms, with a well-established root system.

The islands were found to be functioning as biodiversity hotspots and promoting ecosystem regeneration, providing shelter, food and breeding sites for insects. Of particular note, researchers found the rafts supported the complete lifecycle of at least 10 different species of dragonflies and damselflies (Odonata). These predatory insects play crucial roles in ecosystem food webs, and they are used as biological indicator species of aquatic ecosystem health. Other studies have found these platforms can also support a wide range of invertebrates, amphibians, fish, reptiles and birds.

The rafts mostly retained their structural integrity, however they were releasing cork oak particles after six years. The type of cork used in this study might be less suitable for longer-term use than, for example, a cork oak bark. 

Overall, the study showed that these floating islands can promote and support diverse beneficial pond bacteria communities, without compromising bioremediation, aesthetics or water management. The study broadens existing research on their use to address ecosystem degradation, widening the focus from bioremediation to biodiversity enhancement. The findings show the potential of artificial floating islands for establishing biodiverse communities in stormwater or agricultural runoff pools.

The European Commission is funding research and innovation on nature-based solutions, as part of projects such as Horizon Europe, to help achieve Sustainable Development Goals and protect people and planet.  Under the Nature Restoration Regulation, a key element of the EU Biodiversity Strategy for 2030 and the European Green Deal, member states are required to restore ecosystems through national restoration plans. These plans can include a wide variety of restoration measures, ranging from passive recovery to active interventions.

Nature based solutions also feature prominently in the Kunming-Montreal Global Biodiversity Framework adopted by the UN Convention on Biological Diversity (CBD) in 2022 and are being targeted by a global partnership, ENACT, launched the same year at COP 27 to address climate change, biodiversity loss and ecosystem degradation, with a focus on protecting, conserving and restoring aquatic ecosystems.

Reference:

Calheiros, C. S. C., Ilarri, M., Godinho, M., Castro, P. M. L., and Pereira, S. I. A. (2025) Biodiversity assessment in a floating treatment wetland established in a stormwater pond. Ecological Engineering. 215 (2025) 107598.

https://doi.org/10.1016/j.ecoleng.2025.107598