40
J.A. Commito and N.M.J.A. Dankers
2.2 The Dynamic Nature of Mussel-Bed Structure
In 1877, Karl Mobius used the term "biocoenosis" to describe the beds of
oysters around the Wadden Sea island of Sylt. He described the positive and
negative interactions among the biotic and abiotic components of these
important suspension-feeding bivalve populations. In doing so, he formalized
the new field of community ecology, a discipline that has been strongly influenced by benthic ecologists ever since. Like oysters, mussels are commercially valuable suspension-feeding bivalves found in shallow subtidal and
intertidal sites, often in dense, well-defined populations. Reise and Schubert
(l987) discussed the long-term changes in benthic community structure at
subtidal Wadden Sea sites. They concluded that mussel beds in the 1920s had
been restricted to the shallows, with oyster beds and tubicolous polychaete
reefs in deeper water, whereas by the 1980s mussels occupied both shallow
and deep depths. The reasons for these changes are uncertain, but Reise and
Schubert argue that they result from a combination of coastal eutrophication,
disruption of the bottom by dredging and trawling for oysters and mussels,
and the intentional transplanting of small, shallow water mussels to deeper
sites where they reach commercial size more quickly.
As is the case with subtidal mussel beds, there are intertidal sites in Europe
with long-term data sets that simply do not exist for coastal locations in other
parts of the world. These data demonstrate that some locations have persistent mussel beds, generally where the bottom is sheltered from the effects
of storms and winter ice scouring. Examples include the protected embayment of Konigshafen on the island of Sylt in the Wadden Sea (Nehls et al. 1997)
and the River Exe estuary in Devon, England (McGrorty et al. 1990). True
persistence stability occurs when a system stays the same in the face of external forces of change. If these sites are sheltered from storms and ice, then
we cannot say for certain if they are exhibiting persistence stability. On the
other hand, exposed sites in the Wadden Sea subjected to the removal of
mussels by storms and ice and burial of mussels by sediment often show
dramatic changes in the spatial distribution and abundance of mussel beds
(LandahI1988; Dankers and Koelemaij 1989; Dankers 1993; Nehls and Thiel
1993; Beukema and Cadee 1996). Fisheries activities have similar effects
(Dankers and Koelemaij 1989; Dankers 1993). Mussel beds can become reestablished in the same location after the occurrence of a disturbance, especially where layers of old shell fragments remain (Dankers and Koelemaij
1989). Thus, beds may not be persistent, but they do demonstrate adjustment
stability, or the ability to return to a former state after being perturbed.
Figure 2.1 summarizes the major processes that occur on mussel beds, determine their abundance and distribution, and affect the soft-bottom species
living in association with them.
J.A. Commito and N.M.J.A. Dankers
2.2 The Dynamic Nature of Mussel-Bed Structure
In 1877, Karl Mobius used the term "biocoenosis" to describe the beds of
oysters around the Wadden Sea island of Sylt. He described the positive and
negative interactions among the biotic and abiotic components of these
important suspension-feeding bivalve populations. In doing so, he formalized
the new field of community ecology, a discipline that has been strongly influenced by benthic ecologists ever since. Like oysters, mussels are commercially valuable suspension-feeding bivalves found in shallow subtidal and
intertidal sites, often in dense, well-defined populations. Reise and Schubert
(l987) discussed the long-term changes in benthic community structure at
subtidal Wadden Sea sites. They concluded that mussel beds in the 1920s had
been restricted to the shallows, with oyster beds and tubicolous polychaete
reefs in deeper water, whereas by the 1980s mussels occupied both shallow
and deep depths. The reasons for these changes are uncertain, but Reise and
Schubert argue that they result from a combination of coastal eutrophication,
disruption of the bottom by dredging and trawling for oysters and mussels,
and the intentional transplanting of small, shallow water mussels to deeper
sites where they reach commercial size more quickly.
As is the case with subtidal mussel beds, there are intertidal sites in Europe
with long-term data sets that simply do not exist for coastal locations in other
parts of the world. These data demonstrate that some locations have persistent mussel beds, generally where the bottom is sheltered from the effects
of storms and winter ice scouring. Examples include the protected embayment of Konigshafen on the island of Sylt in the Wadden Sea (Nehls et al. 1997)
and the River Exe estuary in Devon, England (McGrorty et al. 1990). True
persistence stability occurs when a system stays the same in the face of external forces of change. If these sites are sheltered from storms and ice, then
we cannot say for certain if they are exhibiting persistence stability. On the
other hand, exposed sites in the Wadden Sea subjected to the removal of
mussels by storms and ice and burial of mussels by sediment often show
dramatic changes in the spatial distribution and abundance of mussel beds
(LandahI1988; Dankers and Koelemaij 1989; Dankers 1993; Nehls and Thiel
1993; Beukema and Cadee 1996). Fisheries activities have similar effects
(Dankers and Koelemaij 1989; Dankers 1993). Mussel beds can become reestablished in the same location after the occurrence of a disturbance, especially where layers of old shell fragments remain (Dankers and Koelemaij
1989). Thus, beds may not be persistent, but they do demonstrate adjustment
stability, or the ability to return to a former state after being perturbed.
Figure 2.1 summarizes the major processes that occur on mussel beds, determine their abundance and distribution, and affect the soft-bottom species
living in association with them.