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D.M. Paterson and S.E. Hagerthey
5.5.1 Non-cohesive Sediments
In regions of high hydrodynamic stress, benthic assemblages consist of
organisms capable of firmly attaching to sediment particles and resistant to
abrasion. Extreme environmental conditions can physically remove and
damage epipsammic algae (Delgado et al. 1991). Under very extreme conditions few sediment grains may be colonised at all. Typical assemblages
consist of epipsammic monoraphid diatoms, such as Cocconeis and Achnanthes (Round 1979; Yallop and Paterson 1994; Yallop et al. 1994),
euglenoids (Kingston 1999), and coccoid cyanobacteria, such as Merismopedia (Wachendorfer et al. 1994; Noffke and Krumbein 1999). Attachment to
sediment particles allows these micro algae to survive the hazards of physical
transport, mainly particle collisions and burial. The density and composition of epipsammic algal assemblages are influenced by physical exposure
and the duration, frequency, and timing of disturbance events that directly
mediate key variables, such as mean grain size (Oh and Koh 1995) and
organic content (Peletier 1996). The diversity of the microbial assemblages
tends to increase as the magnitude of disturbance decreases. This is a
statement of "The Intermediate Disturbance Hypothesis" (IDH, Connell
1978). Thus, the composition and structure of epipsammic assemblages
varies depending on the frequency and extent of disturbance. This can be
considered as a "disturbance continuum" which influences system diversity
through the capabilities of the individual taxa in relation to their ability to
adhere to particles (Wetherbee et al. 1998), their motility and rapidity of
colonisation (Noffke and Krumbein 1999). At lower magnitudes and frequencies of disturbance, benthic diversity and biomass increase. Monoraphid diatom cell density can increase and araphid diatoms (e.g. Fragilaria,
Raphoneis) appear. In regions of moderate hydrodynamic stress, Noffke and
Krumbein (1999) found that a filamentous cyanobacterium, Oscillatoria
limosa, could stabilise sandy surfaces because of its high mobility, rapid
colonisation and binding efficiency.
As the effects of disturbance become even less, diversity may be affected in
three ways. Firstly, for epipelic assemblages, diversity decreases as interspecific competition increases. Secondly, epipelic diatoms (e. g., Navicula and
Gyrosigma), more familiar from muddy assemblages, colonise the non-cohesive sediments. This combination of epipsammic and epipelic forms further
increases diversity. Examples of such mixed assemblages are reported in the
literature. For example, epipsammic taxa dominated sediments in the
Westerschelde Estuary and accounted for 90 % of the cells counted from 74
species (Sabbe 1993). Yet, in terms of diversity, only 32 of the 74 taxa were true
epipsammic forms belonging to the genera Achnanthes, Amphora, Catenula,
Cocconeis, Fragilaria, and Opephora. Thus, although epipelic diatoms contributed very little to the overall biomass (10 %) in the Westerschelde Estuary,
D.M. Paterson and S.E. Hagerthey
5.5.1 Non-cohesive Sediments
In regions of high hydrodynamic stress, benthic assemblages consist of
organisms capable of firmly attaching to sediment particles and resistant to
abrasion. Extreme environmental conditions can physically remove and
damage epipsammic algae (Delgado et al. 1991). Under very extreme conditions few sediment grains may be colonised at all. Typical assemblages
consist of epipsammic monoraphid diatoms, such as Cocconeis and Achnanthes (Round 1979; Yallop and Paterson 1994; Yallop et al. 1994),
euglenoids (Kingston 1999), and coccoid cyanobacteria, such as Merismopedia (Wachendorfer et al. 1994; Noffke and Krumbein 1999). Attachment to
sediment particles allows these micro algae to survive the hazards of physical
transport, mainly particle collisions and burial. The density and composition of epipsammic algal assemblages are influenced by physical exposure
and the duration, frequency, and timing of disturbance events that directly
mediate key variables, such as mean grain size (Oh and Koh 1995) and
organic content (Peletier 1996). The diversity of the microbial assemblages
tends to increase as the magnitude of disturbance decreases. This is a
statement of "The Intermediate Disturbance Hypothesis" (IDH, Connell
1978). Thus, the composition and structure of epipsammic assemblages
varies depending on the frequency and extent of disturbance. This can be
considered as a "disturbance continuum" which influences system diversity
through the capabilities of the individual taxa in relation to their ability to
adhere to particles (Wetherbee et al. 1998), their motility and rapidity of
colonisation (Noffke and Krumbein 1999). At lower magnitudes and frequencies of disturbance, benthic diversity and biomass increase. Monoraphid diatom cell density can increase and araphid diatoms (e.g. Fragilaria,
Raphoneis) appear. In regions of moderate hydrodynamic stress, Noffke and
Krumbein (1999) found that a filamentous cyanobacterium, Oscillatoria
limosa, could stabilise sandy surfaces because of its high mobility, rapid
colonisation and binding efficiency.
As the effects of disturbance become even less, diversity may be affected in
three ways. Firstly, for epipelic assemblages, diversity decreases as interspecific competition increases. Secondly, epipelic diatoms (e. g., Navicula and
Gyrosigma), more familiar from muddy assemblages, colonise the non-cohesive sediments. This combination of epipsammic and epipelic forms further
increases diversity. Examples of such mixed assemblages are reported in the
literature. For example, epipsammic taxa dominated sediments in the
Westerschelde Estuary and accounted for 90 % of the cells counted from 74
species (Sabbe 1993). Yet, in terms of diversity, only 32 of the 74 taxa were true
epipsammic forms belonging to the genera Achnanthes, Amphora, Catenula,
Cocconeis, Fragilaria, and Opephora. Thus, although epipelic diatoms contributed very little to the overall biomass (10 %) in the Westerschelde Estuary,