Common Structures and Properties of Seagrass Beds Fringing the Coasts of the World 205
gales, and grazing by birds, the sediment is levelled each year. As a result, the
geomorphological character of the area does not change from year to year
(Nacken and Reise 2000). Sedimentation and erosion are more or less in
equilibrium. In areas where frost is less frequent in winter, the seagrass may
also maintain a coverage of 20 %-30 %, in spite of grazing by geese and ducks
(Tubbs and Tubbs 1983). In the most western reaches of the Channel where
frost is an exception, and which are off the main migration routes of the birds,
the seagrass beds remain green in winter and are not or only partly levelled by
gales. As a consequence, the bottom becomes raised, and, as more coarse
material settles along the margins of the beds than in the center, the beds
become heterogeneous and show a considerable relief. In the highest situated
beds, in particular, this leads to a decline of the seagrasses, because of increasing drainage during low tide and the impact of erosion.
Almost all available data on the dynamics of seagrass beds relate to their
expansion and decline. The existence of a bed is taken for granted. In spite of
that, there are still no answers to the question why in a certain area a seagrass
bed occurs, and why the direct surroundings are without seagrass, even as
there does not seem to be any obvious physical cause for the difference. In
some areas, the beds seem to have more or less stable positions, but, in other
areas, the beds show continuous changes in size and topographical position,
and 'move' through the area. Establishment of new beds has not yet been
studied. The opportunity to study this process exists along the North American Pacific coast, where the introduced Zostera japonica is still expanding in
area, or in the Mediterranean where Halophila stipulacea is still expanding
(Lipkin 1975; Den Hartog and Van der Velde 1993; Di Martino 1999). Between
the actual first settlement and the formation of a well-established bed, many
thresholds have to be overcome. Successful settlement is probably only
possible in years or series of years with very special climatic conditions.
Another factor of importance is whether a species is completely absent in an
area or whether there are some small relict populations present.
9.6 Worldwide Decline of Seagrass Beds
Seagrass beds are declining all over the world. In many places this is gradual as a
consequence of progressive pollution of the seawater. At first the epiphytic
growth on the leaves increases to the point that the plants begin to suffer, and
after that a gradual decrease of the bed sets in. This is characterized by a decrease
in the density of the shoots in the bed, later by fragmentation of the bed. Often
this process is not recognized in the first stages because it is so gradual.
A 'wasting disease' reduced the beds of Zostera marina in the North
Atlantic region in the 1930s (Milne and Milne 1951; Martin 1954; Rasmussen
gales, and grazing by birds, the sediment is levelled each year. As a result, the
geomorphological character of the area does not change from year to year
(Nacken and Reise 2000). Sedimentation and erosion are more or less in
equilibrium. In areas where frost is less frequent in winter, the seagrass may
also maintain a coverage of 20 %-30 %, in spite of grazing by geese and ducks
(Tubbs and Tubbs 1983). In the most western reaches of the Channel where
frost is an exception, and which are off the main migration routes of the birds,
the seagrass beds remain green in winter and are not or only partly levelled by
gales. As a consequence, the bottom becomes raised, and, as more coarse
material settles along the margins of the beds than in the center, the beds
become heterogeneous and show a considerable relief. In the highest situated
beds, in particular, this leads to a decline of the seagrasses, because of increasing drainage during low tide and the impact of erosion.
Almost all available data on the dynamics of seagrass beds relate to their
expansion and decline. The existence of a bed is taken for granted. In spite of
that, there are still no answers to the question why in a certain area a seagrass
bed occurs, and why the direct surroundings are without seagrass, even as
there does not seem to be any obvious physical cause for the difference. In
some areas, the beds seem to have more or less stable positions, but, in other
areas, the beds show continuous changes in size and topographical position,
and 'move' through the area. Establishment of new beds has not yet been
studied. The opportunity to study this process exists along the North American Pacific coast, where the introduced Zostera japonica is still expanding in
area, or in the Mediterranean where Halophila stipulacea is still expanding
(Lipkin 1975; Den Hartog and Van der Velde 1993; Di Martino 1999). Between
the actual first settlement and the formation of a well-established bed, many
thresholds have to be overcome. Successful settlement is probably only
possible in years or series of years with very special climatic conditions.
Another factor of importance is whether a species is completely absent in an
area or whether there are some small relict populations present.
9.6 Worldwide Decline of Seagrass Beds
Seagrass beds are declining all over the world. In many places this is gradual as a
consequence of progressive pollution of the seawater. At first the epiphytic
growth on the leaves increases to the point that the plants begin to suffer, and
after that a gradual decrease of the bed sets in. This is characterized by a decrease
in the density of the shoots in the bed, later by fragmentation of the bed. Often
this process is not recognized in the first stages because it is so gradual.
A 'wasting disease' reduced the beds of Zostera marina in the North
Atlantic region in the 1930s (Milne and Milne 1951; Martin 1954; Rasmussen