Microphytobenthos in Contrasting Coastal Ecosystems: Biology and Dynamics
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epipelic species (primarily Navicula) accounted for the remaining 42 species
and greatly increased the diversity of the assemblage. Thirdly, dense mats of
cyanobacteria often cover non-cohesive sediments (Wachendorfer et al.1994;
Yallop and Paterson 1994; Yallop et al. 1994; Noffke and Krumbein 1999).
Complex mixtures of cyanobacteria and diatoms have been described for
both freshly colonised sediments and mature mat systems (Riege and
Villbrandt 1994; Yallop and Paterson 1994; Yallop et al. 1994). Typically, these
mats are dominated by filamentous cyanobacteria such as Microcoleus
chthonoplastes, Oscillatoria spp., Spirulina sp., Phormidium sp. and the coccoids Gloecapsa sp., and Merismopedia sp.
The microscale distribution of particles of varying sizes can also directly
influence diversity. Saburova et al. (1995) suggested that the spatial distribution of micro algae on the mesoscale depends mostly on the composition of
sediments (and on factors influencing particle size and density). They found
that the maximum numbers of micro algae coincided with the crests of sandy
mounds presumably due to sediment texture, as the depressions had a higher
percentage of silt. The distribution and abundance of crevices in particles
may also influence microphytobenthic diversity (Bergey 1999). Small crevices
contain young stages or small taxa. Moderate crevices are effective refugia
and sustain greater diversity. Very large crevices do not enhance diversity
because there is less protection from disturbances (e.g., predators and particle collisions). Other environmental variables can also influence diversity on
non-cohesive sediments. Growth rates for cyanobacteria were higher on
coarse sand (>63 flm) at 15°C, whereas Nitzschia sp. was the competitive
dominant for all sediment types at 10°C. At 25 DC, the filamentous cyanobacterium Microcoleus chthonoplastes dominated (Watermann et al. 1999).
5.5.2 Cohesive Sediments
Hydrodynamic stress is less influential in determining diversity on cohesive
sediments. However, while surveys of microphytobenthic assemblages are
common (e.g., Oppenheim 1991; Underwood et al. 1998), few studies have
directly compared diversity between non-cohesive and cohesive sediments or
reported data on sediment characteristics (cf. Underwood 1994; Yallop et al.
1994). Whereas epipsammic algae dominate non-cohesive sediments, epipelic
algae dominate cohesive sediments. Epipelic diatoms have adaptations to
cope with the heterogeneity in physicochemical conditions (AdmiraaI1984)
and appear to be more competitive on finer grained sediments. For example,
diatoms Phaeodactylum tricornutum and Nitzschia sp. have higher growth
rates at 10 °C and 15°C on mixed sediment (50:50 fine sand [<63 flmJ/mud)
than on fine sand alone and under these conditions outcompete cyanobacteria (Watermann et al. 1999).
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epipelic species (primarily Navicula) accounted for the remaining 42 species
and greatly increased the diversity of the assemblage. Thirdly, dense mats of
cyanobacteria often cover non-cohesive sediments (Wachendorfer et al.1994;
Yallop and Paterson 1994; Yallop et al. 1994; Noffke and Krumbein 1999).
Complex mixtures of cyanobacteria and diatoms have been described for
both freshly colonised sediments and mature mat systems (Riege and
Villbrandt 1994; Yallop and Paterson 1994; Yallop et al. 1994). Typically, these
mats are dominated by filamentous cyanobacteria such as Microcoleus
chthonoplastes, Oscillatoria spp., Spirulina sp., Phormidium sp. and the coccoids Gloecapsa sp., and Merismopedia sp.
The microscale distribution of particles of varying sizes can also directly
influence diversity. Saburova et al. (1995) suggested that the spatial distribution of micro algae on the mesoscale depends mostly on the composition of
sediments (and on factors influencing particle size and density). They found
that the maximum numbers of micro algae coincided with the crests of sandy
mounds presumably due to sediment texture, as the depressions had a higher
percentage of silt. The distribution and abundance of crevices in particles
may also influence microphytobenthic diversity (Bergey 1999). Small crevices
contain young stages or small taxa. Moderate crevices are effective refugia
and sustain greater diversity. Very large crevices do not enhance diversity
because there is less protection from disturbances (e.g., predators and particle collisions). Other environmental variables can also influence diversity on
non-cohesive sediments. Growth rates for cyanobacteria were higher on
coarse sand (>63 flm) at 15°C, whereas Nitzschia sp. was the competitive
dominant for all sediment types at 10°C. At 25 DC, the filamentous cyanobacterium Microcoleus chthonoplastes dominated (Watermann et al. 1999).
5.5.2 Cohesive Sediments
Hydrodynamic stress is less influential in determining diversity on cohesive
sediments. However, while surveys of microphytobenthic assemblages are
common (e.g., Oppenheim 1991; Underwood et al. 1998), few studies have
directly compared diversity between non-cohesive and cohesive sediments or
reported data on sediment characteristics (cf. Underwood 1994; Yallop et al.
1994). Whereas epipsammic algae dominate non-cohesive sediments, epipelic
algae dominate cohesive sediments. Epipelic diatoms have adaptations to
cope with the heterogeneity in physicochemical conditions (AdmiraaI1984)
and appear to be more competitive on finer grained sediments. For example,
diatoms Phaeodactylum tricornutum and Nitzschia sp. have higher growth
rates at 10 °C and 15°C on mixed sediment (50:50 fine sand [<63 flmJ/mud)
than on fine sand alone and under these conditions outcompete cyanobacteria (Watermann et al. 1999).