Diatom Assemblages

The overall diatom species assemblages differed in their composition and relative abundance of taxa between the two substrata. This was expected from the results obtained in Chapter 3 and reflects the findings from other comparative studies between different artificial substrata (Siver 1977, Tuchman & Stevenson 1979, Stevenson & Lowe 1986, and Cattaneo & Amireault 1992). A. minutissima was the most common species in the tile samples and achieved the highest relative abundance in both training sets. The most common taxon in the rope samples was A. lanceolata, followed by G.

parvulum and C. placentula v. euglypta, the latter taxa being more typical of the natural epiphytic communities. Small species, normally associated with the natural epilithon, were more common on the tile samples (e.g. N. atomus and A. pediculus).

Species Diversity

The H ill’s N2 diversity of the samples from both training sets was very variable between the sites but, in the majority of cases, was similar between the two different substrata at any one site. There were notable exceptions to this: at sites PA N G l and TILL2 the tile samples were very diverse but the rope samples were dominated by A. minutissima and C. placentula v. euglypta, respectively. Conversely, at M O LEl and SLEA l (Nov. sample) the rope samples were very diverse and the tile samples dominated by only one taxon: N. gregaria and N. lanceolata, respectively. Overall the mean and range of species diversity for the rope and tile samples was very similar: rope samples had a very slightly higher mean H ill’s N2 value but a tile sample achieved the highest diversity at any one site (Fig. 4.26).

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Figure 4.26 Com parative H ill’s N2 diversity between the two training sets

Rope Training Set

The detrended correspondence analysis of the rope species data revealed the major gradient in the diatom assemblages to be associated with alkalinity. Sites with high

alkalinity tended to be dominated by A. minutissima. The exception to this was high

alkalinity sites which also had high phosphorus where A. minutissima was normally

replaced by C. placentula v. euglypta. The dominance of A. minutissima at sites of high

alkalinity was instrumental in the distribution of samples along DCA axis 1. Samples with low alkalinity had high DCA axis 1 scores and were characterised by taxa

including: G. pannilum, S. minima, M. varians, Eunotia spp., N. cryptocephala and N.

rhynchocephala. The H ill’s N2 diversity of the lower alkalinity sites was generally

higher than the A. minutissima dominated sites with high alkalinity.

The remaining variation within the species data was less clearly related to the measured environmental variables. The strongest correlation on the second DCA axis, using the environmental data passively, was with silica. Silica was high at all sites and thus it is unlikely that it would have a direct controlling influence on the diatom assemblages due to silica limitation. On the contrary, it would appear that this relationship is most likely

due to some Fragilaria species showing a strong preference for high silica

concentrations (Fig. 4.27). The reason for these Fragilaria species performing better at

very high silica concentrations is not clear, and does not seem to have been recorded in other diatom studies. Silica was correlated with phosphorus but the response of the Fragilaria species to TP and FRP was less clearly defined.

10.0 % 0.0 20.0 % 0.0

Fragilaria pinnata o 7.5 Fragilaria elliptica 1.5 Ctenophora pulchella

% ° % ° =0 O o 0 O _0.0 0.0

Fragilaria brevistriata 50.0 Fragilaria construens °

V. venter 7 .5 Fragilaria oldenburgiana % % o°dS> 8n B ° ° 0.0 " 0O®ntl?° ° 0.0 0 2.8 22.4 2.8 22.4 2.8 22.8 Silica m gL ' (log)

F igu re 4.27 Distribution o f C. pulchella and F ragilaria spp. along the silica gradient (rope)

Phosphorus was important in the observed distribution of diatom samples in the DCA, with TP being significantly correlated to species axes 1, 3 and 4. The fact that TP was correlated with three DCA axes suggests that its importance in determining species assemblages was dependant on the other environmental conditions at any one site. Under such conditions, where species may be reacting to numerous environmental gradients, detrended correspondence analysis becomes very difficult to interpret and hence the use of constrained ordination techniques (CCA), see below.

The correlations of the remaining environmental variables with the DCA axes, were generally low. Interestingly, only current velocity appeared to have no significant relationship with the diatom assemblages despite the wide range of flow conditions between the sample sites (3.6-98.6 cm sec'^). This is contrary to the findings of other studies, where current velocity was seen to have a marked effect on the diatom communities (Douglas 1958, Stevenson 1983, Homer et al. 1990 and Biggs 1996). This may be due to the structure of the rope substratum causing the effects of flow rates to be limited. Reynolds (1996) reported on the reduction of velocity within the dense fronds of river macrophytes, due to the creation of local micro-environments, thus allowing unattached and less robust forms of algae to develop at relatively high flow conditions.

The structure of the rope can be considered analogous to submerged macrophytes and will, therefore, be likely to create similar micro-environmental conditions.

Tile Training Set

The distribution of tile samples, within the DCA, was somewhat different from that observed for the rope data-set. The gradient length on the first axis was greater in the tile data, and the total variation in the species data was also larger. Unlike the rope samples, however, the sites were more evenly distributed along the first two axes, without the dispersion of samples at the high end of axis 1 seen in the rope DCA (cf. Figs. 4.12 & 4.17). The main pattern of species variation was correlated with alkalinity but not as strongly as the rope samples. This was mainly because of the response of Achnanthes minutissima to the alkalinity gradient, but unlike the rope samples there was also a stronger correlation (negative) between DCA species axis 1 and phosphoms. These results suggest that the diatom taxa growing on the tile substratum reflect the trophic conditions slightly better than the rope samples.

The second DCA axis was similarly correlated with silica, but negatively in the tile samples. Again this was due to the greater occurrence of some Fragilaria species at higher silica concentrations. Unlike the rope data, however, DCA axis 2 was also noticeably correlated to pH and flow. The relative importance of flow in determining species distributions in the tile training set is doubtless due to the increased physical stresses caused by current velocity across the flat surface of the tile. Under such conditions some motile taxa and delicate chain forming diatoms are unable to maintain stable conditions for growth (Reynolds 1996). The distribution of some taxa in the tile training set was found to be restricted to sites of low flow rates; e.g. M. varians, F. pinnata and Navicula veneta. Other, non-attached taxa, appeared to be able to exploit the microphytic boundary layer, where laminar flow occurs, and maintain relatively high abundances; e.g. N. dissipata, N. lanceolata, N. gregaria, N. tripunctata and N. cryptotenella. Similar relationships, between flow and species distributions, were not observed in the rope training set.

On the remaining DCA axes, there were no strong correlations (P < 0.005) between the species assemblages and the environmental variables. The even distribution of sites

across the first two DCA axes, coupled with the strong passive relationships with the environmental variables, suggests that the tile samples reflected their overall environment more closely than the assemblages growing on the rope substratum. The response to phosphorus alone, however, is more difficult to assess from these data. In the tile training set TP was more strongly correlated to DCA axis 1, and to a lesser extent to axis 2. In the rope data TP was strongly correlated with three axes, and thus the use of constrained ordination techniques (CCA) were needed to identify the direct response of the species assemblages to phosphorus.