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R.G. Hughes
algae (particularly Enteromorpha sp.) or pioneer zone vascular plants (e.g.
Salicornia spp. and Spartina spp.). The secretion of extracellular polymeric
substances (EPS) by the microphytobenthos leads to the development of
biofilms that increase sediment cohesion and stability by increasing To erit (see
reviews by Paterson 1997; Paterson and Black 1999). Sediment that accretes in
this way may induce a positive feedback, for the biomass of microphytobenthos generally increases with increased elevation (Austen et al.1999) so, as
the biomass increases further, so will the erosion threshold and accretion is
further enhanced. While it is generally understood that biofilms help stabilise
sediments, the erosional characteristics of cohesive sediments are less well
understood than those of non-cohesive sediments (e.g. sands), principally
because the unit of erosion is a floc, a variable heterogeneous mix of inorganic
particles in an organic matrix. Consequently, a large number of physical,
chemical and biological variables can affect the stability of cohesive sediments (Paterson and Black 1999), to the extent that sedimentological techniques alone cannot be used to predict their behaviour.
In nutrient -enriched estuaries, like those of SE England, colonisation of
mud by mats of cyanobacteria (e.g. Oscillatoria spp.) and algae, such as
Enteromorpha spp., is typical. These filamentous forms grow into the sediment and stabilise it. They also protect the surface from erosive water movement by slowing the currents immediately above the surface, and may
enhance deposition by trapping suspended sediment. Vascular plants may
further aid accretion by increasing deposition, by slowing water movement at
the sediment surface, and reducing erosion by binding sediment with their
roots (van Eerdt 1985). The effect of the plants may be dependent on their
morphology and on their density. For example, Spartina plants, which are
relatively large and complex, may have a greater effect than the smaller,
relatively simple, smoother Salicornia. Individual plants may have little effect
as the currents may simply flow around them increasing velocity and causing
local scouring of the sediment. A thicker cover of vegetation will increase the
depth of the boundary layer of slow moving water above the sediment surface.
8.2.2.2 Effects of Invertebrates
As the sediment accumulates it may become colonised by invertebrates that
may have a major effect on sediment stability, and ultimately the development
of saltmarsh vegetation. Infaunal invertebrates may help sediment accretion,
by compacting the sediment as they burrow, and by binding sediment grains
together with the mucous secretions used in the construction of their burrows
(e.g. Meadows and Tait 1989). However, the weight of evidence is that the
overall effect of infaunal invertebrates is to reduce sediment stability. In the
unmanaged and managed realignment sites in SE England four species are
R.G. Hughes
algae (particularly Enteromorpha sp.) or pioneer zone vascular plants (e.g.
Salicornia spp. and Spartina spp.). The secretion of extracellular polymeric
substances (EPS) by the microphytobenthos leads to the development of
biofilms that increase sediment cohesion and stability by increasing To erit (see
reviews by Paterson 1997; Paterson and Black 1999). Sediment that accretes in
this way may induce a positive feedback, for the biomass of microphytobenthos generally increases with increased elevation (Austen et al.1999) so, as
the biomass increases further, so will the erosion threshold and accretion is
further enhanced. While it is generally understood that biofilms help stabilise
sediments, the erosional characteristics of cohesive sediments are less well
understood than those of non-cohesive sediments (e.g. sands), principally
because the unit of erosion is a floc, a variable heterogeneous mix of inorganic
particles in an organic matrix. Consequently, a large number of physical,
chemical and biological variables can affect the stability of cohesive sediments (Paterson and Black 1999), to the extent that sedimentological techniques alone cannot be used to predict their behaviour.
In nutrient -enriched estuaries, like those of SE England, colonisation of
mud by mats of cyanobacteria (e.g. Oscillatoria spp.) and algae, such as
Enteromorpha spp., is typical. These filamentous forms grow into the sediment and stabilise it. They also protect the surface from erosive water movement by slowing the currents immediately above the surface, and may
enhance deposition by trapping suspended sediment. Vascular plants may
further aid accretion by increasing deposition, by slowing water movement at
the sediment surface, and reducing erosion by binding sediment with their
roots (van Eerdt 1985). The effect of the plants may be dependent on their
morphology and on their density. For example, Spartina plants, which are
relatively large and complex, may have a greater effect than the smaller,
relatively simple, smoother Salicornia. Individual plants may have little effect
as the currents may simply flow around them increasing velocity and causing
local scouring of the sediment. A thicker cover of vegetation will increase the
depth of the boundary layer of slow moving water above the sediment surface.
8.2.2.2 Effects of Invertebrates
As the sediment accumulates it may become colonised by invertebrates that
may have a major effect on sediment stability, and ultimately the development
of saltmarsh vegetation. Infaunal invertebrates may help sediment accretion,
by compacting the sediment as they burrow, and by binding sediment grains
together with the mucous secretions used in the construction of their burrows
(e.g. Meadows and Tait 1989). However, the weight of evidence is that the
overall effect of infaunal invertebrates is to reduce sediment stability. In the
unmanaged and managed realignment sites in SE England four species are