Eelgrass and other seagrasses are the meadows of the sea. They are widely distributed along the coastlines of the world and occur in continuous belts or patches under the right growing conditions. Seagrasses need light and only grow as deep as the penetrating light permits. In clear waters such as the Mediterranean Sea, seagrasses can grow to about 50 meters depth, while in Denmark, eelgrass typically grows to about 3-5 meters depth in the fjords and a few meters deeper along the more open coastline. The seabed should be soft/sandy and relatively protected and, in Denmark, eelgrass is virtually present along all coastlines except on the west coast, where the waves are too strong.

Eelgrass and other seagrasses are flowering plants (not seaweeds) – and have flowers, leaves and roots just like plants on land. But while there is a great variety of land plants, there are only about 60 seagrass species in the world, all very similar. In Denmark, there are 2 genuine seagrass species: Eelgrass (Zostera marina) and Dwarf eelgrass (Zostera noltii) – the former is by far the most common.

Eelgrass shoots consist of 3-7 grassy leaves attached to a rhizome and roots that form a dense matrix in the seabed. Eelgrass is propagated by seeds or offshoots. With offshoot propagation, eelgrass can grow a few decimeters per year from the border of an existing meadow, whereas seeds allow for dispersal over far greater distances.

Eelgrass meadows play a key role in the coastal ecosystems. They are very productive and provide habitats and nursery grounds for a wide range of plant and animal species, thus stimulating coastal biodiversity. They also act as natural coastal protection because the leaves attenuate wave ebergy, and the roots and rhizomes stabilise the sediment. The plants also function as particle filters, thus contributing to keeping the water clear; and they store carbon and retain nutrients. All these features place eelgrass meadows among the most valuable ecosystems in the world.

The meadows are vulnerable to disturbances, though, and this vulnerability – in combination with their important ecosystem services – is the reason why seagrasses are used as indicators of good ecological status in coastal waters. Despite their distribution and importance, seagrasses are poorly known by most people because they are hidden beneath the sea.