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D.M. Paterson and S.E. Hagerthey
physical stress is high while nutrient levels in the sediments tend to be low
leading to low diversity. The mixed assemblage site combines advantages
from both systems (nutrients, attachment sites on particles, light penetration)
and supports high diversity.
5.6 Sediment Stability
From the above discussion it is clear that grain size has a fundamental role to
play in the nature of the microphytobenthos assemblages that colonise
sediments. However, it is now recognised that the activity of organisms also
influences the nature, behaviour and distribution of sediments (Noffke and
Krumbein 1999; Blanchard et al. 2000). The biota stabilising non-cohesive
sediments consists of bacteria (Grant and Gust 1987; Dade et al. 1990), algae
(Neumann et al.1970), diatoms (Holland et al.1974; Vos et al. 1988; Madsen et
al. 1993), and cyanobacteria (Neumann et al. 1970; Grant and Gust 1987;
Madsen et al. 1993; Noffke and Krumbein 1999). The most pervasive effect is
probably through the secretion of organic substances (extracellular polymeric substances, EPS) present in the sediment matrix (Paterson 1997).
Whilst biogenic effects cannot protect sediments against extreme events, the
influence of biogenic mediation on the day-to-day fluxes of material from the
bed can be highly significant (Tolhurst et al. 2000). Most stability studies have
concentrated on the carbohydrate (bulk or colloidal) fraction of the EPS
found in sediments (Underwood et al.1995; De Winder et al.1999; Yallop et al.
2000). Some discrimination between species is now possible. Gyrosigma
limosum and Nitzschia sigma were associated with high total carbohydrate
concentrations in the Colne Point saltmarsh, Essex, UK (Underwood et al.
1998). In contrast, for the diatoms Bacillaria paxillifer, Achnanthes longipes,
and Navicula pelliculosa, mucilage production was low and did not enhance
sediment stability (Holland et al. 1974). Smith and Underwood (1998) found
that the epipelic diatoms Cylindrotheca closterium, Navicula perminuta, and
Nitzschia sigma produced more EPS when migrating. These studies suggest
that the taxonomic composition of epipelic diatom assemblages may determine the stability of cohesive sediments. This possibility is supported by the
recent work of Riethmuller et al. (2000) using remote sensing of algal distribution linked to ground erosion measurements (Riethmuller et al. 1998).
Assemblages containing species capable of high EPS production reduce
sediment erosion more than assemblages producing less EPS. Epipelic forms
seem to be better stabilisers than epipsammic forms. Thus, biogenic stabilisation via organic secretions increases as the proportion of epipelic forms
increases.
D.M. Paterson and S.E. Hagerthey
physical stress is high while nutrient levels in the sediments tend to be low
leading to low diversity. The mixed assemblage site combines advantages
from both systems (nutrients, attachment sites on particles, light penetration)
and supports high diversity.
5.6 Sediment Stability
From the above discussion it is clear that grain size has a fundamental role to
play in the nature of the microphytobenthos assemblages that colonise
sediments. However, it is now recognised that the activity of organisms also
influences the nature, behaviour and distribution of sediments (Noffke and
Krumbein 1999; Blanchard et al. 2000). The biota stabilising non-cohesive
sediments consists of bacteria (Grant and Gust 1987; Dade et al. 1990), algae
(Neumann et al.1970), diatoms (Holland et al.1974; Vos et al. 1988; Madsen et
al. 1993), and cyanobacteria (Neumann et al. 1970; Grant and Gust 1987;
Madsen et al. 1993; Noffke and Krumbein 1999). The most pervasive effect is
probably through the secretion of organic substances (extracellular polymeric substances, EPS) present in the sediment matrix (Paterson 1997).
Whilst biogenic effects cannot protect sediments against extreme events, the
influence of biogenic mediation on the day-to-day fluxes of material from the
bed can be highly significant (Tolhurst et al. 2000). Most stability studies have
concentrated on the carbohydrate (bulk or colloidal) fraction of the EPS
found in sediments (Underwood et al.1995; De Winder et al.1999; Yallop et al.
2000). Some discrimination between species is now possible. Gyrosigma
limosum and Nitzschia sigma were associated with high total carbohydrate
concentrations in the Colne Point saltmarsh, Essex, UK (Underwood et al.
1998). In contrast, for the diatoms Bacillaria paxillifer, Achnanthes longipes,
and Navicula pelliculosa, mucilage production was low and did not enhance
sediment stability (Holland et al. 1974). Smith and Underwood (1998) found
that the epipelic diatoms Cylindrotheca closterium, Navicula perminuta, and
Nitzschia sigma produced more EPS when migrating. These studies suggest
that the taxonomic composition of epipelic diatom assemblages may determine the stability of cohesive sediments. This possibility is supported by the
recent work of Riethmuller et al. (2000) using remote sensing of algal distribution linked to ground erosion measurements (Riethmuller et al. 1998).
Assemblages containing species capable of high EPS production reduce
sediment erosion more than assemblages producing less EPS. Epipelic forms
seem to be better stabilisers than epipsammic forms. Thus, biogenic stabilisation via organic secretions increases as the proportion of epipelic forms
increases.