182
R.G. Hughes
action. The same may be true of Macoma, and both species may reduce the
potential for these microphytobenthos to be re-deposited and contribute to
sediment stability.
Cerastoderma edule and Arenicola marina occur elsewhere in these estuaries but are not common either in the managed and unmanaged realignment sites, or in the saltmarsh creeks. The reasons are not known, but, were
these species to colonise sediment in realignment areas, they might reduce
further the potential for saltmarsh establishment. Cerastoderma often move
over the sediment, where they disturb other invertebrates (Flach 1996), and
probably also the microphytobenthos and the integrity of the sediment
surface, increasing erosion. Arenicola could deter development of vegetation,
as Philipp art (1994) concluded that the worms restricted colonisation by
Zostera, because of their bioturbatory activities.
8.3 Managing Sediment Accretion and Development
of Saltmarsh Vegetation in Managed Realignment Sites
In managed realignment sites, particularly low-lying ones, further management of the biological and physical processes which affect sediment stability
may be necessary to raise the equilibrium height of the sediment to a level
where saltmarsh vegetation can develop. Even then further management
action may be required to shift the community from an animal-dominated
state to a plant-dominated state.
8.3.1 Physical Factors
French et al. (1999) identified exposure to wave action as a major factor
affecting sediment elevation in unmanaged realignment sites. In managed
realignment sites reducing wave action, or the effects of wave action, would
increase the probability of saltmarsh development, indirectly by promoting
sedimentation to a level where plants could survive, and directly by reducing
disturbance of seeds and seedlings. Wiehe (1935) concluded that the lower
vertical limit of Salicornia was above MHWNTL because a threshold time of 2
or 3 days undisturbed by tides was necessary for the establishment of the
seedlings. This significance of tidal level has been extrapolated to other
species (Davidson et al.199I); however, Gerdol and Hughes (1993) challenged
its ubiquity when they found that transplanted Salicornia seedlings could
survive to maturity below MHWNTL. Although in their sheltered site bioturbation by Corophium prevented establishment of Salicornia, rather than
R.G. Hughes
action. The same may be true of Macoma, and both species may reduce the
potential for these microphytobenthos to be re-deposited and contribute to
sediment stability.
Cerastoderma edule and Arenicola marina occur elsewhere in these estuaries but are not common either in the managed and unmanaged realignment sites, or in the saltmarsh creeks. The reasons are not known, but, were
these species to colonise sediment in realignment areas, they might reduce
further the potential for saltmarsh establishment. Cerastoderma often move
over the sediment, where they disturb other invertebrates (Flach 1996), and
probably also the microphytobenthos and the integrity of the sediment
surface, increasing erosion. Arenicola could deter development of vegetation,
as Philipp art (1994) concluded that the worms restricted colonisation by
Zostera, because of their bioturbatory activities.
8.3 Managing Sediment Accretion and Development
of Saltmarsh Vegetation in Managed Realignment Sites
In managed realignment sites, particularly low-lying ones, further management of the biological and physical processes which affect sediment stability
may be necessary to raise the equilibrium height of the sediment to a level
where saltmarsh vegetation can develop. Even then further management
action may be required to shift the community from an animal-dominated
state to a plant-dominated state.
8.3.1 Physical Factors
French et al. (1999) identified exposure to wave action as a major factor
affecting sediment elevation in unmanaged realignment sites. In managed
realignment sites reducing wave action, or the effects of wave action, would
increase the probability of saltmarsh development, indirectly by promoting
sedimentation to a level where plants could survive, and directly by reducing
disturbance of seeds and seedlings. Wiehe (1935) concluded that the lower
vertical limit of Salicornia was above MHWNTL because a threshold time of 2
or 3 days undisturbed by tides was necessary for the establishment of the
seedlings. This significance of tidal level has been extrapolated to other
species (Davidson et al.199I); however, Gerdol and Hughes (1993) challenged
its ubiquity when they found that transplanted Salicornia seedlings could
survive to maturity below MHWNTL. Although in their sheltered site bioturbation by Corophium prevented establishment of Salicornia, rather than