Eelgrass seabed vs. seabed without seagrass as a result of unclear water. Copyright: “Havets Planter” by Christensen, P.B. & Høgslund, S. (2011), page 162.
Although eelgrass needs nutrients to grow, it is often nutrient enrichment that restricts the meadows from expanding. Eelgrass is good at economizing on nutrition. The plant accumulates nutrients in both the green leaves and underground biomass for use when the surrounding waters are low in nutrients. The plant takes up nutrients from the sediment as well as the water column, and eelgrass meadows effectively contribute to trapping organic particles from the water column, which are subsequently mineralized and made available to the plant.
Eutrophication and increased nutrient supply to the water cause blooms of plankton and fast-growing macro-algae. They overshadow the eelgrass and sink to the seabed where their decay causes oxygen depletion and makes the seabed fine grained and unstable and therefore less suitable for eelgrass growth.
Nutrients and light are thus closely linked in terms of regulating eelgrass distribution. And eelgrass itself is an active player in this regulation process. Dense eelgrass meadows act as particle filters that attenuate wave energy and settle water column particles, thus ensuring clearer water. This effect is amplified by the eelgrass roots stabilising the sediment and limiting resuspension of seabed material. Throughout spring and summer, dense eelgrass meadows also take up significant quantities of nutrients, making these inaccessible to plankton and fastgrowing algae during that period. Dense eelgrass populations thus have a self-reinforcing positive effect on their growth conditions. However, the opposite, negative self-reinforcing effects occur in areas of seagrass decline. This vicious cycle makes it difficult for eelgrass to recover once it has been displaced from an area.
The challenge is to create the right conditions in terms of light and nutrients that will allow new plants to germinate. The natural recolonisation may still happen slowly in areas where the seabed is unstable, and mother populations are scarce. In these areas, it may be necessary to supplement further with initiatives to limit mechanical disturbances, for example from dragging tools, and support measures such as seed dispersal and stabilising the seabed sediment.