Dredging near holiday homes is driving biodiversity loss in Finland’s archipelago

Finland’s archipelago system of more than 98 500 islands is one of the most iconic in the world, and harbours great biodiversity in a complex mix of habitats. Its coastal areas also attract considerable human recreational activity, and this appeal also makes them hotspots for the construction of holiday homes. There are currently 500 000 such homes in Finland, and interest is increasing.

Small-scale dredging is often carried out in front of second homes to improve access by boat and make conditions easier for recreational activities such as boating and swimming. Dredging can have direct, long-term ecological impacts, such as loss of species and habitats, as well as spreading invasive alien species, but knock-on effects can include changes in water quality. Once areas are dredged for recreational purposes, it is unlikely they can return to their pre-dredged state.

Researchers from the Finnish Environment Institute used data from numerous sources to assess the likely effect of new second homes on the loss of species in the Baltic Sea due to dredging. They analysed data on marine biodiversity collected from over 170 000 underwater sites¹, and identified high-risk zones for dredging using housing data² and aerial images^3,4. They found that such small-scale dredging has a negative impact on coastal species, as it tends to target shallow bays and lagoons with soft sediment seafloors that receive sunlight and are home to a diverse array of algae and marine plants.

According to the modelling, algal species are more likely to be lost due to dredging than aquatic plants, because of their higher coverage area – and the 'footprint’ of impact is expected to extend beyond the boundaries of the dredging activity itself. Larger dredging sites with a diameter over 100 metres (n = 8834) are likely to result in the loss of 20.2 hectares of algal species and 13.7 hectares of marine plants. For smaller sites of 20m (n = 19 094), these figures are estimated to be 5.1 and 4.6 hectares, respectively.

The researchers found that the impact of small-scale dredging is relatively great, due to more than double the number of small, rather than large, projects. Additionally, they did not find a linear relationship between the size of the site and species loss, further underlining the detrimental effect of this type of dredging at any scale. The considerable ecological impacts suggested in the work are likely to be underestimations, as the study only considered the direct impacts of dredging. The full picture is likely to include impacts that are felt well beyond dredging sites, with sediment disturbed and distributed across large parts of the sea, resulting in reduced water quality, as well as the impact of the depositing of dredged material.

The analysis also suggested that while dredging removed many common and widely distributed algae and plants, it also had a notable effect on threatened species, such as the Baltic water-plantain (Alisma wahlenbergii) and fourleaf mare's tail (Hippuris tetraphylla).

The EU’s Biodiversity Strategy for 2030 recognises the importance of marine ecosystems in providing vitally important services such as carbon sequestration. The soft vegetated seabed ecosystems identified by the study as targets for dredging are important for this ‘blue carbon’ role, the researchers stressed, with seabed disturbance likely resulting in the release of harmful emissions.

The researchers conclude that dredging is a significant and understudied driver of marine biodiversity loss. Their study of holiday homes in Finland supports this conclusion, providing evidence to suggest that significant ecological change is driven by recreational land use.

At least 489 new homes are being built annually on Finland’s coast, and the researchers conclude with a call for more stringent land-use regulation to better protect marine biodiversity — a priority of the EU’s wider Marine Strategy Framework Directive. More specifically, they point to a need for greater control over land use in areas of soft, shallow seabed that receive most sunlight, and stricter regulation of dredging.

Footnotes:

1. Kotilainen, A., Erhnsten, E., & Vahteri, P. (2020). Threatened habitat types in Finland 2018: The Baltic Sea: Red List of habitats. Part 2: Descriptions of habitat types. Finnish Environment Institute and Ministry of the Environment.
2. Statistic Finland (2021). Buildings and free-time residences 2021.
3. Kuismanen, L., & Husa, S. (2020). Coastal human activity on the Åland Islands interpreted using aerial photographs. Marine Research Centre, Finnish Environment Institute, Helsinki, Finland and Department of Social Affairs, Health and Environment, Government of Åland.
4. Sahla, M., Turkia, T., Nieminen, A., Räsänen, T., Haapamäki, J., Hoikkala, J., Suominen, F., & Kantanen, J. (2020). Aerial photo mapping of human pressure in the marine areas of the Finnish coast. Marine nature protection, Nature services, Forestry authority

Source:

Virtanen, E.A., Kallio, N., Nurmi, M., Jernberg, S., Saikkonen, L. and Forsblom, L. (2023) Recreational land use contributes to the loss of marine biodiversity. People and Nature. Early View. Available from: https://doi.org/10.1002/pan3.10444

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.