Eelgrass coverage in Odense Fjord. Copyright: REELGRASS
Around year 1900, eelgrass meadows covered approximately one seventh of Danish coastal waters. Most fjords had a thick green carpet of eelgrass meadows, and along the coasts, eelgrass meadows extended down to depths of 11 meters, with scattered patches down to depths of 17 meters. The meadows were dense – with up to almost a kg of dry matter per square meter and an annual output of double that.
In the early 1930s, a fungal disease destroyed eelgrass in the North Atlantic, including Denmark, and wiped out around 90% of eelgrass meadows. A slime mold was the likely causative agent of the disease. Eelgrass occurs predominantly as mono-cultures, making it more susceptible to diseases, and a period of high temperatures possibly contributed to the vulnerability of the meadows.
In the decades following the epidemic, there were only few studies of eelgrass. They show that eelgrass slowly regained some of the lost distribution and abundance through the 1950s and 1960s, but had probably not reached former levels when nutrient leaching from land began in earnest in the 1950s. Nutrient leaching peaked around 1980 and resulted in planktonic algae and other fast-growing algae flourishing in the water column and overshadowing plants on the seabed.
The national monitoring program which started up in 1989 has shown that eelgrass can still be found along the Danish shores but that meadows are not nearly as widely distributed as 100 years ago. In many places, the meadows are but a shadow of themselves and on a national scale, eelgrass depth limits have not improved from the late 1980s until today, despite significant reductions in nutrient leaching. The water remains turbid in many places and leaves no scope for eelgrass expanding to greater depths. Simultaneously, natural disturbances and unsuitable sediment conditions contribute to limiting the distribution. In recent years, however, there has been some signs of a natural reestablishment and expansion of eelgrass, for instance in the Limfjord and in Køge Bay.
It is not only in Denmark that seagrasses are under threat. They are experiencing declines globally. They are disappearing at the same rate as tropical rain forests and are thus among the earths most threatened ecosystems. Some of the main threats to seagrasses are coastal constructions as well as nutrient leaching and sediments that affect the water quality by causing turbidity.