116
D.M. Paterson and S.E. Hagerthey
In a study of the epipelic diatoms of the Severn Estuary, UK, Underwood
(1994) found that diversity on the low shore was higher than mid and high
shore but did not differ between the three sampling sites (Severn Estuary:
Sand Bay; Portishead; and Aust). On average, Shannon index values for each
site and number of taxa were less than 1.78 and 16, respectively. However, the
taxonomic composition for the mid-shore site was different from the low and
high shore. Sediment shear strength, bulk carbohydrate, and colloidal carbohydrate were all greater for the mid-shore site than for the low- and highshore sites. Navicula phyllepta, Cylindrotheca signata, Gyrosigma limosum
(formerly spencerii), Entomoneis paludosa, and Nitzschia epithemioides were
positively correlated with sediment bulk carbohydrate and colloidal
carbohydrate and had higher relative abundances for the mid-shore site,
suggesting that EPS production was higher for these taxa and directly
increased sediment stability. The mid-shore site also had high relative
abundance of euglenoids in mid summer. Taxa associated with both the highand low-shore sites, Navicula pargemina, Navicula flanatica, Raphoneis
minutissima, Coscinodiscus sp. 1, and Cymatosira belgica, were negatively
correlated with colloidal carbohydrate indicating that these species do not
affect sediment stability as much.
5.5.3 Niche Diversity
The diversity of microphytobenthos therefore varies between sandy and
muddy sites. Maximum diversity appears to occur at the interface between
clean sands and pure mud where epipsammic and epipelic forms can coexist.
This may reflect the availability of sites and resources for the cells and the
variety of niches available. The interfaces between the sediment and overlying
media are areas of intense vertical gradients (StaI1995; Taylor and Paterson
1998; Paterson and Black 2000). These gradients are created and maintained
by the activity of organisms at the sediment surface (Jickells and Rae 1997).
The severity of the gradients is related to the nature of the sediment (porosity,
permeability) and the intensity of the biological activity. In general terms,
gradients are more severe where biomass is high and where sediment
particles are small. Thus, cohesive sediments of reasonable nutrient status
maintain the sharpest gradients of change. This implies very restricted spatial
niche separation along the gradients in cohesive sediments. Thus, within the
aerobic region, spatial zonation cannot be established easily but there is some
evidence indicating that temporal displacement occurs. Early observations of
diatom migration noted that cells came to the surface and were replaced by
different forms at various stages of the tidal cycle (Round 1981). This suggestion has been given support by electron microscopy of intertidal sediments showing euglenids replacing diatoms at the surface (Paterson et al.
D.M. Paterson and S.E. Hagerthey
In a study of the epipelic diatoms of the Severn Estuary, UK, Underwood
(1994) found that diversity on the low shore was higher than mid and high
shore but did not differ between the three sampling sites (Severn Estuary:
Sand Bay; Portishead; and Aust). On average, Shannon index values for each
site and number of taxa were less than 1.78 and 16, respectively. However, the
taxonomic composition for the mid-shore site was different from the low and
high shore. Sediment shear strength, bulk carbohydrate, and colloidal carbohydrate were all greater for the mid-shore site than for the low- and highshore sites. Navicula phyllepta, Cylindrotheca signata, Gyrosigma limosum
(formerly spencerii), Entomoneis paludosa, and Nitzschia epithemioides were
positively correlated with sediment bulk carbohydrate and colloidal
carbohydrate and had higher relative abundances for the mid-shore site,
suggesting that EPS production was higher for these taxa and directly
increased sediment stability. The mid-shore site also had high relative
abundance of euglenoids in mid summer. Taxa associated with both the highand low-shore sites, Navicula pargemina, Navicula flanatica, Raphoneis
minutissima, Coscinodiscus sp. 1, and Cymatosira belgica, were negatively
correlated with colloidal carbohydrate indicating that these species do not
affect sediment stability as much.
5.5.3 Niche Diversity
The diversity of microphytobenthos therefore varies between sandy and
muddy sites. Maximum diversity appears to occur at the interface between
clean sands and pure mud where epipsammic and epipelic forms can coexist.
This may reflect the availability of sites and resources for the cells and the
variety of niches available. The interfaces between the sediment and overlying
media are areas of intense vertical gradients (StaI1995; Taylor and Paterson
1998; Paterson and Black 2000). These gradients are created and maintained
by the activity of organisms at the sediment surface (Jickells and Rae 1997).
The severity of the gradients is related to the nature of the sediment (porosity,
permeability) and the intensity of the biological activity. In general terms,
gradients are more severe where biomass is high and where sediment
particles are small. Thus, cohesive sediments of reasonable nutrient status
maintain the sharpest gradients of change. This implies very restricted spatial
niche separation along the gradients in cohesive sediments. Thus, within the
aerobic region, spatial zonation cannot be established easily but there is some
evidence indicating that temporal displacement occurs. Early observations of
diatom migration noted that cells came to the surface and were replaced by
different forms at various stages of the tidal cycle (Round 1981). This suggestion has been given support by electron microscopy of intertidal sediments showing euglenids replacing diatoms at the surface (Paterson et al.