Processes That Affect Saltmarsh Erosion and Saltmarsh Restoration
177
drained unvegetated mud. These authors also concluded that the natural
development of saltmarsh in realignment sites can not be taken for granted.
In both Clementsgreen Creek and Wallasea Island, where pioneer zone plants
could colonise the mud and begin the facilitated successional development of
saltmarsh vegetation, the dominant processes were bioturbation and herbivory by the invertebrate infauna. The lower limits of the plant distributions
were not determined by sea level (or water movement) directly, but by the
activity of the invertebrates. The lower limits of their distributions were not
those of their potential niches but had been raised to those of their realised
niches.
Hughes (1999) proposed a hypothesis relating the losses of saltmarshes
to the activities of the infauna. The hypothesis is that the pioneer zone,
including that within saltmarsh creeks formed initially by drainage of water,
will be colonised by invertebrates. In SE England these will be particularly
the polychaete Nereis diversicolor (hereafter Nereis), which some evidence
indicates has increased in range and abundance over the past few decades,
and to a lesser extent the amphipod Corophium volutator (hereafter Corophium), and the gastropod Hydrobia ulvae (hereafter Hydrobia). Herbivory
and bioturbation by these animals prevent colonisation by pioneer zone
vegetation directly and render the sediment more erodable, by waves at the
face of the marshes and by tidal currents in the creeks. Thus, if SLR is affecting the loss of vegetation it may be doing so indirectly, by allowing an upward displacement of the distributions of the infaunal species, and by
increasing the duration and intensity of their activities in any given location.
In addition to SLR, Burd (1992) reported an increase in the tidal range on
this coast of 45 cm since 1870. A higher tidal range could increase erosion
because of higher tidal currents that flow through the creeks.
Further evidence of the effects of Nereis on plant colonisation was provided
by Hughes et al. (2000) who demonstrated that Zostera noltii transplants protected from the polychaetes had a higher survivorship, lower index of root
damage, and greater biomass than unprotected ones. Emmerson (2000)
reported that Nereis could also affect negatively transplanted Spartina
anglica. Z. noltii and Nereis have almost non-overlapping distributions which
led Hughes et al. (2000) to the hypothesis that there are two broadly alternative stable states on the upper mudflats. In one state plants are dominant
and prevent colonisation by burrowing infauna. In this case the plants were
Z. noltii, but the roots of a variety of plants, or filamentous algae, could also
deter burrowing. In the other state the infauna dominate and prevent colonisation by plants, through herbivory and bioturbation. Managing these two
states could be the key to establishing pioneer zone plants and the subsequent
successional development of saltmarsh.
In this chapter, the interactions of the important biological and physical
processes relevant to sediment stability and saltmarsh development are re-
177
drained unvegetated mud. These authors also concluded that the natural
development of saltmarsh in realignment sites can not be taken for granted.
In both Clementsgreen Creek and Wallasea Island, where pioneer zone plants
could colonise the mud and begin the facilitated successional development of
saltmarsh vegetation, the dominant processes were bioturbation and herbivory by the invertebrate infauna. The lower limits of the plant distributions
were not determined by sea level (or water movement) directly, but by the
activity of the invertebrates. The lower limits of their distributions were not
those of their potential niches but had been raised to those of their realised
niches.
Hughes (1999) proposed a hypothesis relating the losses of saltmarshes
to the activities of the infauna. The hypothesis is that the pioneer zone,
including that within saltmarsh creeks formed initially by drainage of water,
will be colonised by invertebrates. In SE England these will be particularly
the polychaete Nereis diversicolor (hereafter Nereis), which some evidence
indicates has increased in range and abundance over the past few decades,
and to a lesser extent the amphipod Corophium volutator (hereafter Corophium), and the gastropod Hydrobia ulvae (hereafter Hydrobia). Herbivory
and bioturbation by these animals prevent colonisation by pioneer zone
vegetation directly and render the sediment more erodable, by waves at the
face of the marshes and by tidal currents in the creeks. Thus, if SLR is affecting the loss of vegetation it may be doing so indirectly, by allowing an upward displacement of the distributions of the infaunal species, and by
increasing the duration and intensity of their activities in any given location.
In addition to SLR, Burd (1992) reported an increase in the tidal range on
this coast of 45 cm since 1870. A higher tidal range could increase erosion
because of higher tidal currents that flow through the creeks.
Further evidence of the effects of Nereis on plant colonisation was provided
by Hughes et al. (2000) who demonstrated that Zostera noltii transplants protected from the polychaetes had a higher survivorship, lower index of root
damage, and greater biomass than unprotected ones. Emmerson (2000)
reported that Nereis could also affect negatively transplanted Spartina
anglica. Z. noltii and Nereis have almost non-overlapping distributions which
led Hughes et al. (2000) to the hypothesis that there are two broadly alternative stable states on the upper mudflats. In one state plants are dominant
and prevent colonisation by burrowing infauna. In this case the plants were
Z. noltii, but the roots of a variety of plants, or filamentous algae, could also
deter burrowing. In the other state the infauna dominate and prevent colonisation by plants, through herbivory and bioturbation. Managing these two
states could be the key to establishing pioneer zone plants and the subsequent
successional development of saltmarsh.
In this chapter, the interactions of the important biological and physical
processes relevant to sediment stability and saltmarsh development are re-