TRAINING SET SITES AND ENVIRONMENTAL VARIABLES
5.4 Numerical analysis of the diatom data 1 Summary statistics
A total of 310 taxa was recorded in the 31 surface sediment samples. The full taxon names and authorities are listed in Appendix 5. A large number of infrequent taxa usually occur in species data-sets and here a total of 102 taxa were present with a relative frequency of > 1% in a minimum of 2 sangles. These frequent taxa are maiiced "*" in Appendix 5. Table 5.1 lists the most common taxa present (with a relative frequency > 5%) in the surface sediment of each site, illustrating the importance of the small centric planktonic taxa (eg. Stephanodiscus and
Cyclostephanos) and the Fragilaria species in the data-set.
Table 5.2 presents the total number of taxa recorded in each sample, the total valve count and a simple measure of floristic diversity, that is, the ratio of diatom taxa to valves counted. Total valve counts ranged from 499 in site No. 50, which was a difficult slide to count with a high clay content and badly broken diatom frustules, to 656 valves in site No. 113, which was dominated by one species {Stephanodiscus parvus). The total valve count was increased at this site to produce accurate relative percentages of the other taxa.
The total number of taxa recorded in a single surface sediment sample ranged from only 19 in site No. 112, which was dominated by Fragilaria spp, to 81 taxa in site No. 79, in which no single taxon exceeded a relative frequency of 15%. Hence, floristic diversity across the data-set was quite variable, ranging from 0.03 in site No. 112 to 0.16 in site No. 79. A total of 22 sites had a diversity score of 0.10 or less, reflecting the predominance of one or two species in shallow, nutrient-rich waters of this kind. This contrasts with a total of only 9 sites out of 34 with a floristic diversity of 0.10 or less in a study of surface sediment diatom assemblages from 34 relatively acid, nutrient-poor lakes in Galloway, south-west Scotland (Flower et al., 1986).
5.4.2. Detrended Correspondence Analysis (DCA)
DCA was employed to investigate the principal patterns of floristic variation in the data. In the DCA presented here, detrending was by segments with non-linear rescaling of the axes and rare species were downweighted (Hill & Gauch, 1980; ter Braak, 1987b, 1987c). Analysis was performed on the most common 102 taxa listed and marked in Appendix 5. Table 5.3 and Figures 5.1 and 5.2 present the results of DCA of all 31 surface sediment samples.
Table 5.3 Eigenvalues and cumulative variance accounted for in a DCA of the 102 common taxa in the surface sediment diatom assemblages of 31 sites
Eigenvalue Cumulative variance
represented
Axis 1 0.617 13.3%
Axis 2 0.382 21.5%
Axis 3 0.249 26.9%
Axis 4 0.168 30.5%
It is not surprising that with a large number of taxa and "noisy” data with many zero values, that the DCA axes do not capture a large proportion of the cumulative variance of the species data (Stevenson et al., 1991). Despite the fact that the first two DCA axes only explain 21.5% (Table 5.3) of the variance in the species data, the ordination plot is still informative.
A major outcome of the analysis was that Mundon Hall Pond (No. 88) was a clear outlier in terms of its diatom assemblage (Figure 5.1) and it is evident from Figure 5.2 that the eight uppermost diatom taxa on the plot were so located because of their high relative abundance at this site (ie. AMOllA, GO013A, NA004A, NA007B, NA022A, NI008A, NI014A, and SU073A). These taxa were not recorded in similarly high percentages at any other site in the data-set. Owing to the high scores on axis 2 for this sample, the scaling concentrates the remaining samples into a small area of the plot, making it difficult to interpret, therefore the DCA was re run treating site No. 88 as passive and thus downweighting its significance. The results are presented in Table 5.4 and Figures 5.3 and 5.4.
Table 5.4 Eigenvalues and cumulative variance accounted for in a DCA of the
10 2 common taxa in the surface sediment diatom assemblages of 30
sites
Eigenvalue Cumulative variance
represented
Axis 1 0.617 14.5%
Axis 2 0.277 21.0%
Axis 3 0.197 25.6%
Axis 4 0.104 28.0%
Table 5.4 shows that the results are very similar to those in Table 5.3. However because site No. 88 was treated passively and its impact on axis 2 was reduced, the eigenvalue for the second axis was not as high and a larger proportion of the cumulative variance of the species data was captured by axis 1 in the second analysis. The less condensed distribution of the samples and species on the plots enabled a clearer interpretation of the DCA results.
Axis 1 appears to be partly a nutrient axis as diatom taxa known to be indicators of very eutrophic waters are positioned on the right-hand side of Figure 5.4, eg. Cyclostephanos dubius
{CCO^lA),Stephanodiscus hantzschii A),Stephanodiscus hantzschiiio. tenuis (ST002A),
Cyclostephanos [cf. tholiformis] (CC9997), Stephanodiscus parvus (STOlOA) and Cyclotella
meneghiniana (CY003A), and the sites with high annual mean TP concentrations are positioned
on the right-hand side of Figure 5.3, eg. Nos 34, 65, 98, 100 and 120.
Conversely, diatom taxa known to prefer less-rich, mesotrophic waters are positioned on the left- hand side of Figure 5.4, eg. Achnanthes clevei (AC006A), Fragilaria construens (FR002A), and
Fragilaria construens var. venter (FR002C), and the sites with the lower annual mean TP concentrations are positioned on the left-hand side of Figure 5.3, eg. Nos 4,42, 83,101 and 108.
Although axis 1 appears to be mainly related to nutrients, there are clearly a number of other important factors controlling the species distributions, as there are exceptions to the general pattern described above. A number of sites with extremely high annual mean TP concentrations are also positioned towards the left-hand side of Figure 5.3, eg. Nos 31 and 112, and a number of diatom taxa known to favour eutrophic waters are positioned on the left-side of Figure 5.4,
eg. Fragilaria pinnata (FROOIA), Fragilaria brevistriata (FR006A) dnà Achnanthes lanceolata
var. rostrata (ACOOIB).
If the diatom taxa are grouped into two broad categories: ''planktonic” and ”non-planktonic”, based on published ecological descriptions (although the exact habitat of many taxa remains uncertain), it becomes evident that axis 1 is also partly a "habitat” or "life-form” axis. The planktonic taxa occur largely on the right-hand side of Figure 5.4, eg. the small centric taxa in the genera Cyclotella, Cyclostephanos and Stephanodiscus (CY002A, CY003A, CYOllA, CCOOIA, CC002A, CC9997, STOOIA, ST002A, STOlOA) and other planktonic forms such as
Asterionella formosa (ASOOIA), and Aulacoseira granulata var. angustissima (AU003B). The
non-planktonic taxa are located largely on the left-hand side of Figure 5.4, eg. the small
Fragilaria species (FROOIA, FR002A, FR006A), and some of the small Navicula species
(NA005A, NA114A), and in the centre of the diagram, eg. taxa in the genera Achnanthes
(ACOOIA, AC013A, AC023A) and Cocconeis (COOOIA, COOOIB, C0005A).
If the sites are classified on a similar basis, according to whether they are dominated by planktonic or non-planktonic taxa, a similar pattern emerges with the samples dominated by planktonic taxa clearly located on the right-hand side of Figure 5.3 (eg. Nos 34, 65, 69, 76, 98, 100,107,113,120), samples dominated by non-planktonic Fragilaria taxa on the left-hand side of the plot (eg. Nos 4, 31, 42, 83, 108,112), and samples dominated by non-planktonic taxa of the genus Achnanthes, positioned towards the centre of the plot (eg. Nos 57, 79, 85, 105).
Therefore, based solely on the known ecological preferences of the major diatom taxa, axis 1 appears to be a combination of a nutrient and a life-form axis. This suggests that in future, it may be advisable to separate planktonic and non-planktonic forms to form distinct data-sets, in order to single out nutrients as the major explanatory variable. The statistical relationship between the ccxnmon diatom taxa and the environmental variables, including nutrients, is explored in the following chapter.
Figure 5.1 DCA plot of the 31 surface
Table 4.1 for site codes)
sediment samples on axes 1 and 2. (See
088 4.0 o A
2.0
# 1 0 7 # 1 2 0 # 0 3 4 # 0 3 7 # 0 8 2 112 . , 0 8 # 0 8 6 # 0 7 9 # 1 0 5 1.0- 004 # 0 8 3 # 0 0 7 # 0 4 : t o o # # 0 9 8 # 1 1 3 # 0 8 5 # 0 5 3 # 0 7 3 # 0 6 5 # 0 7 6 ,# 0 6 0 0 74' 4.0 3.02.0
Axis 1= 0.617 198Figure 5.2 DCA plot of the 102 common diatom taxa in the 31 surface sediment samples on axes 1 and 2. (See Appendix 5 for taxon codes)
*
5.0 4.0- 3.02.0
1.0
0.0
■1.0 -2.0
■3.0 FH006A Z i , Afri i c006A A ^ * « » ’P n a o o t bA NK)14AA Aa m o ii a G 0013aA NA022A A A NA0O4A 8 U 073B A NK»SA A NI028A A NA0O4B A ASUOIBA AP001A A c c o o i aA fr o o/aA Agyozqa A A A STOOIA A , ACC9997 ST9999 A ^ A A ^ ^ ^ STOlOA A ACC002A A A ^ AG0003|AST002A A A ^ ^ A ^ ... •A CTOOOA--- ASYOOZAA ANW 72A
AU003B A A A C 032A I I I -1.0 0.0 1.0 2.0 Axis 1= 0.617 3.0 4.0 5.0 199