Salt Marshes and Seagrass Meadows
Salt marshes are coastal habitats that form a transitional zone between marine and terrestrial environments. They are distributed worldwide and cover a total area of approx. 45,000 km2, of which 395 km2 can be found in the Wadden Sea. Salt marshes can develop in coastal areas that are less exposed to wind and waves and additionally show high sedimentation rates. As salt marshes are regularly flooded with seawater, their vegetation (mainly consisting of herbs, grasses and shrubs) is well adapted to waterlogging and high salinity values. Moreover, the physical structure of the salt marsh vegetation reduces flow velocities and turbulences in the overlying water layer during each flood event. The reduced flow conditions enable suspended sediments in the water layer to deposit onto the marsh platform which facilitates vertical accretion. This process is important for the development of a salt marsh, but also for its ability keep up with the rising sea levels, which ensures its persistence. Through this salt marshes make also an outstanding contribution to coastal protection as they dampen the sea’s hydrodynamic impact on the coast. However, besides trapping sediment particles, salt marshes also filter pollutants out of the water, which means they play a significant role in the nutrient cycle. Furthermore, they are of immense importance as breeding, feeding and resting areas for many coastal and migratory birds. They are also home to highly specialized animal and plant species, meaning that salt marshes are habitats which make a great contribution to the global biodiversity. Another ecosystem service that recently get more and more in the focus of science is the ability of salt marshes to take up CO2 from the atmosphere and sequester it permanently in the sediment. Carbon sequestration in salt marshes usually exceeds the sequestration rates of terrestrial habitats (e.g. tropical rainforests) by far. This is mainly due to the high productivity of salt marsh vegetation, the reduced decomposition processes in the sediment and the additional carbon input from the adjacent sea.
Despite their great importance, the salt marsh area is annually declining on a global scale by 1-2%. This decline is largely caused by human activities such as land use (e.g. land reclamation for agriculture), eutrophication and the effects of climate change. Human activities have been also taking place in salt marshes in the Wadden Sea, particularly in form of grazing, creation of artificial drainages or sedimentation fields. Even though these measures have very different effects on the salt marsh development, they all influence its natural genesis. However, due to the growing awareness of the importance of salt marshes and the designation of the Wadden Sea as a national park, salt marshes have been put under protection and human interventions were significantly reduced. This has led to a positive development and an increase in salt marsh area of around 15% in the last 30 years.
Seagrass meadows are often found in the sea adjacent to or in close proximity to salt marshes. They occur worldwide, except for the Arctic and Antarctic, and are the dominant vegetation of shallow, sandy coasts. Globally, they cover an estimated area of about160,400 - 266,600 km2. Most seagrass meadows grow in shallow water and are therefore constantly covered with sea water but they also occur on tidal flats, where they are regularly exposed at low tide. This is also the case in the Wadden Sea.
Seagrass plants occur in communities and can therefore compose large meadows. These meadows are characterized by a high productivity and their ability to fulfil a variety of ecologically important functions. They provide habitat for a variety of organisms and protect them from predation. Therefore, they are hotspots for biodiversity. Seagrass meadows also serve as spawning areas and nursery grounds for fish and are an important source of food, e.g. for sea turtles, dugongs and migratory birds. Similar to salt marsh plants, seagrasses increase sedimentation by increasing the surface roughness with their leaves and therefore reducing current velocities. With their dense root system and high vegetation cover, they also reduce erosion of the seafloor and thus contribute to coastal protection. Further parallels are that seagrasses also filter nutrients from the water and that they can store significant amounts of carbon. Carbon is taken from seawater or the atmosphere in form of CO2 and stored permanently and efficiently in the sediment. However, the efficiency to store carbon depends on many internal and external factors, so that the storage capacity varies greatly between regions. Compared to warmer regions (such as the Mediterranean or the tropics), seagrass in the Wadden Sea can only store small amounts of carbon.
Seagrass meadows are declining worldwide. This was also observed in the Wadden Sea, especially in the 1970s to 1990s. However, long-term observation from the air and on the ground has shown that seagrass meadows in the northern Wadden Sea have recovered significantly over the past 25 years. Since the late 1990s, seagrass area has increased more than six-fold. This is primarily attributed to a reduction of nutrient inputs into the Wadden Sea. Since nutrients are usually discharged into the Wadden Sea by rivers, the recovery of seagrass meadows is limited to the northern Wadden Sea as it is far away from the major river mouths. The central and southern Wadden Sea are located near the big river estuaries such as the Elbe, Weser, Ems and Rhine. Here, seagrasses are in close proximity to the source of pollution and suffer from excessive nutrient loads. Further reduction of nutrient discharges would be needed in order to initiate a seagrass recovery here.
