| February 28th, 2025 | Publications

To mark the third annual World Seagrass Day (1 March), we dive into this important habitat. Found in shallow coastal areas across the world, seagrass meadows may seem like unassuming habitats, but they are critical for biodiversity and human well-being globally.

Seagrass meadows support many ecosystem services, including providing habitat for fish. Photo: Dmitris Poursanidis, GRID-Arendal.

Seagrasses are flowering plants that have migrated back to the sea and adapted to the pressures of living in a marine environment. Unlike algae, they have a root system that anchors them to the sediment, and they produce flowers, pollen, and seeds like many familiar land plants. Seagrasses are foundation species – the meadows formed by seagrass plants transform coastal areas, providing a habitat rich in food and shelter for all types of species, including many valuable fisheries species. Their leaves dampen waves and their roots and rhizomes bind the sediment which trap harmful substances, including pollutants, nutrients, and microplastics, from the water column into the sediment. This also helps to protect coastal areas from damage caused by storms. Seagrass meadows, along with other blue forest ecosystems such as mangroves and seaweed forests, are also important in mitigating the effects of climate change, as they remove carbon from the atmosphere and water column and sequester it into the sediment – a form of long-term carbon storage. The functions provided by seagrass, termed ecosystem services, mean seagrass meadows are critically important coastal ecosystems for both marine biodiversity and human wellbeing.

Seagrass meadows are important in Norway and beyond

In Norway, the main seagrass species is common eelgrass (Zostera marina), which is found along the entire coastline, up to 10 meters deep in some areas. Dwarf eelgrass (Zostera noltii) and widgeongrass (Ruppia spp.) are also found in some shallow areas. Though ecological research on seagrass meadows in Norway has historically been limited, in recent years we have learned much about their importance. Norwegian seagrass meadows provide juvenile nursery grounds for important fish such as cod, thus supporting one of the main pillars of the Norwegian economy.  The meadows also support high biodiversity of invertebrates, such as crustaceans, bivalves, and polychaetes.  Though small, these play an essential role in keeping the eelgrass healthy by grazing filamentous algae, and in forming the base of food webs by providing food for fish and larger invertebrates such as crabs. Researchers in Norway are currently researching how seagrass communities respond to stressors including climate change and eutrophication in Norway and across the Nordic countries in the NORSE project. The project also explores how biodiversity of associated species can provide resilience to Nordic seagrass meadows, allowing them to recover and thrive in the future.

Research on the pollution and climate change mitigation role of Norwegian seagrass is also underway. Recently, the first large-scale assessment of carbon stocks in Norwegian seagrass meadows showed that seagrass, particularly meadows in shallow sheltered coastal areas, stored extremely high amounts of carbon. Future research will focus on how climate change and other stressors affect the pollution and climate change mitigation capacity of seagrass meadows in Norway and beyond.

Ecosystem services of seagrass meadows. Infographic courtesy of GRID-Arendal.

Seagrass meadows under pressure…

Seagrass meadows are dependent upon light, temperature, and sediment. They are therefore very sensitive to environmental changes, and an estimated 20-30% of seagrass area has been lost globally since the 19^th century. High nutrient and sediment inputs into coastal waters from human activities such as agriculture, industry, and coastal development can lead to blooms of filamentous algae and phytoplankton, reducing the amount of light available for seagrass plants. When overfishing pressure cascades down the food chain to affect the invertebrate grazers that can otherwise control algal blooms, seagrass ecosystems suffer and can oftentimes die off completely. More recently, the effects of climate change and marine heat waves have become an increasing threat to seagrass meadows around the world. Projects such as NORSE are studying the effects in conjunction with other stressors in the hopes of understanding how to improve the resilience of seagrass meadows and protect their valuable ecosystem services.  The loss of seagrass has cascading effects across the whole marine ecosystem and on human communities that depend on these habitats.

In Norway, seagrass loss has been observed in the Oslofjord and in the neighbouring Swedish Skagerrak. Current research is ongoing to improve the mapping and monitoring of seagrass in other areas of Norway, in order to better plan management efforts.

…but signs of hope for the future

Several countries in Europe and around the world have long-standing seagrass restoration projects, and though the overall trend for seagrasses globally is negative, local efforts to reduce nutrient inputs and replant seagrass has turned the trend around in several areas, including many parts of Europe. In Norway, seagrass restoration is still in the early stages, but a pilot project in the Oslofjord, where populations have been greatly reduced, shows some promise.

Eelgrass restoration in the Skagerrak. Photo: Eduardo Infantes, University of Gothenburg

There is also increasing national and international awareness of the importance of seagrass meadows. Seagrass has been acknowledged in regional and international frameworks and agreements on preserving and restoring biodiversity (e.g. the Kunming-Montreal Global Biodiversity Framework and the EU Restoration Law) and in climate change mitigation and adaptation (e.g. the implementation of the Paris Agreement on climate change). In 2022, the UN officially proclaimed 1 March as World Seagrass Day, to emphasize the need for increased awareness of the importance of this habitat.

By both conserving and properly managing existing meadows along with restoring lost meadows, the critical ecosystem services of these coastal habitats can be ensured into the future.