The quarterly collection of phytoplankton was as described earlier for water samples (chapter 2.2). Each sample, preserved in Lugol’s iodine, was concentrated to approximately 10 ml by leaving the algae to settle several days and removing the supernatant. Concentrated samples were digested in a beaker with hot H2O2 (30%) for several hours. After removing the beaker from the hotplate a few drops of HCl (50%) were added to remove any remaining peroxide. Following digestion, the samples were washed 3 times with distilled water. Each wash was followed by centrifugation (1200 rpm for 4 minutes) and removal of supernatant. A subsample of approximately 1 ml was dried onto a coverslip and mounted onto glass slides with Naphrax®. AU materials and slides have been placed in the archive o f the Environmental Change Research Centre (UCL). The prepared slides were examined with a Leitz microscope at 1000 x magnification under oil immersion using phase contrast or bright field illumination. All diatoms
Chapter 2.3: Monitoring o f diatom assemblages 154
were identified to species or varietal levels. Diatoms were enumerated by transect until a total of approximately 300 valves had been counted. Diatom identification and taxonomy, were based on Krammer and Lange-Bertalot (1986, 1988, 1991a, 1991b) and numerous taxonomic publications to aid principally with centric taxa. The identities of some taxa were studied in more detail using scanning electron microscopy (SEM). Because the counts were made on cleaned material and not limited to protoplasm-containing cell, it was not possible to enumerate the percentages of planktonic organisms that were alive in the water samples and those that were merely dead cells in suspension. However, subsamples of fresh material preserved in Lugol’s iodine have been taken from each initial 500 ml water sample and their analysis will be the subject of subsequent research. Taxonomic authorities are presented in the species list in Appendix 2.
2.3.2. Seasonal tre n d s in the diatom assem blages succession, lake by lake
The low periodicity of the sampling does not allow for a detailed discussion of the diatom succession of each lake. Moreover, because the absence of data on cell densities, one should be cautious when discussing the dynamics of the species in terms of relative proportions. However, the seasonal occurrence and dominance of the main species or the stability of the assemblages can be assessed.
• Lac d'A ydat
In the nine samples analysed from Lac d ’Aydat, a total of 83 taxa were identified. Persoone et al.
(1968) found only 23 taxa in four samples taken in 1967-68, but their results are not comparable as they used a plankton net to sample the lake-water. In comparison, Aloui-Mhamdi and Siriki (1985) identified 122 diatom species from the analyses of samples taken weekly from April to December 1984. The species list given by these authors closely resembles the one obtained from the present study and although the names are often different, many are considered as synonyms. Fig. 2.3.1 shows the succession of dominant species found in the water of this lake (expressed as percentages of the total). The spring communities (samples taken in M ay) were analysed on three occasions, each samples were dominated by different species (M ay'96: Cyclotella pseudostelligera, Fragilaria crotonensis, Cyclotella radiosa; M ay’97: Cyclotella radiosa, Fragilaria crotonensis] M ay’98: Asterionella form osa and to a lesser extent Cyclostephanos dubius). Summer samples were characterised by an association of C. radiosa and F. crotonensis
and the large proportions of periphytic Fragilaria spp. (e.g. F. brevistriata, F. robusta and F. elliptica). In autumn and winter the water samples were dominated by the planktonic species
Aulacoseira subarctica. Stephanodiscus parvus was most abundant in Feb. 1998 and absent from the summer samples. A very similar succession of dominant species was observed by
Chapter 2.3: M onitoring o f diatom assemblages 155
Amblard et al. (1988), from the same set of samples taken by Aloui-Mhamdi and Sriki (1985).
Fig 2.3.1: Lac d ’Aydat, diatom succession in water samples.
Fragilaria elliptica " 2 Fragilaria brevistriata X i Fragilaria robusta (1^ Fragilaria capucina v a r. gracilis U n k n o w n p c n n a tc f o n n ( S Y 9 9 8 9 )
Asterionella form osa
g Stephanodiscus parvus {O Cyclostephanos dubius
I
Aulacoseira subarctica ■ _ J ' ■ 1 . I ■ ■ 1 ■ ’ Cyclotella radiosa Fragilaria crotonensis Cyclotella pseudostelligeraJLJL
% 4.//VVVVVVV'
• Lac de BordesFrom this lake seven samples were analysed for diatoms. A total o f 141 taxa were identified. On Fig. 2.3.2., one can see that the diatom assemblages found in this shallow lake were almost exclusively composed of periphytic taxa. Fragilaria elliptica in particular represented ca. 50 % of the total in the first four samples analysed (Aug. 1996 to Aug. 1997) suggesting very little seasonal changes in the diatom community. In the following samples, lower percentages of F. elliptica were found. Fragilaria capucina var. capitellata dominated the sample of Feb. 1998, and Fragilaria nanoides, possibly a planktonic form, was most abundant in May 1998.
Chapter 2.3: M onitoring o f diatom assemblages 156
Fig. 2.3.2: Lac des Bordes, diatom succession in water samples.
I
Ol,
4=1
.9,
§
Fragilaria construens var. venter
Synedra rumpens Tahellariaflocculosa agg. Fragilaria capucina var. gracilis Gomphonema parvulum
Achnanthes minutissima agg.
Fragilaria capucina var. capitellata Nitzschia gracilis Navicula seminulum Fragilaria brevistriata Fragilaria elliptica Fragilaria nanoides Aulacoseira ambigua Fragilaria crotonensis Synedra nana Synedra delicatissima Aulacoseira subarctica Cyclotella pseudostelligera A' A A • Lac du B ouchet
In Lac du Bouchet, eight samples were analysed for diatoms. The water sample taken in August 1997 did not contain enough diatom cells for a reliable count. A total of 153 taxa were identified in the whole set o f samples. The only previous study on the phytoplankton o f Lac Bouchet was published by Eckartz-Nolden and Nolden (1991) who identified only 13 diatom taxa from the analyses of two samples taken in August 1986 and May 1987. It is likely that such contrasting results are due to the different sampling techniques used. These authors used a plankton net with a 20 pm mesh to sample the lake, which is inadequate for the collection of the smallest diatom forms (for example Cyclotella pseudostelligera in this lake is around 5 pm in diameter). Fig. 2.3.3 shows that the planktonic species C. pseudostelligera dominated the assemblages throughout winter and spring. During the summer periods (Aug. 1996 and 1997) and Nov. 1997, the planktonic species were poorly represented. The assemblages were thus dominated by periphytic taxa, Eunotia implicata in Aug. 96, Fragilaria capucina var. capitellata and F. elliptica in November 97. Achnanthes minutissima and Tahellaria flocculosa were constantly present in relatively low percentages throughout the sampling campaign. Fragilaria nanoides
Chapter 2.3: Monitoring o f diatom assemblages 157
Fig. 2.3.3: Lac du Bouchet, diatom succession in water samples.
Fragilaria construens var. venter L Tahellaria flocculosa agg. L Fragilaria capucina var. capitellata
I
CL,
Fragilaria capucina var. gracilis Fragilaria elliptica
Navicula pseudoscutiformis
Achnanthes minutissima agg. Achnanthes straubiana Fragilaria pinnata Eunotia rhomhoidea Eunotia implicata Fragilaria nanoides
I
Cyclotella pseudostelligeraf
I
% • Lac de BourdouzeSeven samples were analysed from this lake and 122 species were identified. By comparison, a list o f 101 taxa was given by Persoone et al. (1968) from the analysis of one sample and from the compilation of results from previous researchers. Surprisingly, except for Fragilaria elliptica
and Fragilaria construens var. venter, most of the dominant species found in the water samples during this study are absent from the list given by these authors. During this survey, the diatom flora was largely dominated by periphytic taxa especially Fragilaria spp. (Fig. 2.3.4). As in Lac des Bordes, the composition of the assemblages appeared to be remarkably stable from Aug. 1996 to Aug. 1997, suggesting little seasonal variations. From Nov. 1997 and in the subsequent samples, proportions of Fragilaria capucina var. gracilis increased steadily to reach ca. 50% in M ay 1998. In this last sample, the proportions o f planktonic taxa such as Cyclotella pseudostelligera, Cyclotella radiosa, Synedra delicatissima and Synedra nana also increased
and reached ca. 30%.
• Lac de la Cassière
Chapter 2.3: Monitoring o f diatom assemblages 158
were identified by Devaux (1969) from the analyses of water and surface sediment samples. The list given by Devaux compares poorly with the one found during the present study and more notably Aulacoseira ambigua, the dominant species during this study, was not reported.
Fig. 2.3.4: Lac de Bourdouze, diatom succession in water samples.
cu <2 0
1
OL,
Fragilaria capucina var. gracilis
Fragilaria vaucheriae
Tahellaria flocculosa agg.
Fragilaria brevistriata
Navicula pseudoscutiformis
Achnanthes minutissima agg.
Nitzschia gracilliformis
Fragilaria capucina var. capitellata ^
Fragilaria construens var. venter
Fragilaria pseudoconstruens Fragilaria pinnata Fragilaria elliptica Cyclotella radiosa Synedra nana Cyclotella pseudostelligera Synedra delicatissima
1
L 1 A ■ ■I
1 \ ■ ■ A1
A 1J
I
I
I
A .5^The succession of species found in this lake is illustrated in Fig. 2.3.5. The assemblages were largely dominated by planktonic species except in Nov. 1997 when the periphytic species
Achnanthes catenata was the most abundant (LM and SEM pictures of this form can be found in Appendix 3.1). Aulacoseira ambigua was the dominant species during the whole period of sampling, representing up to 75% of the total assemblage. Cyclotella radiosa and Fragilaria crotonensis, fairly abundant in May and August 1996, showed only negligible percentages in the same period of 1997-98. Asterionella form osa was dominant in Feb. 1997. Synedra nana
Chapter 2.3: Monitoring o f diatom assemblages 159
Fig. 2.3.5: Lac de la Cassière, diatom succession in water samples.
2 Fragilaria elliptica a
I
Fragilaria capucina var. gracilis Fragilaria brevistriata Achnanthes minutissima agg.Achnanthes catenata
Nitzschia draveillensis Aulacoseira italica var. tenuissima Nitzschia acicularis
Stephanodiscus parvus |
Stephanodiscus hantzschii
Unknown pennate form (SY9989) [
Synedra delicatissima Synedra nana Asterionella formosa Aulacoseira ambigua Rhizosolenia longiseta Fragilaria crotonensis Cyclotella pseudostelligera Cyclotella radiosa J
Ü
c«fc> (A j %• Lac C ham bon
Nine water samples were analysed from Lac Chambon and 119 taxa were identified. From the analyses of their own samples and the compilation of previous studies, Persoone et at. (1968) listed 117 taxa for this lake. Here again the list given by these authors is difficult to compare with our results. For instance no Cyclotella spp. were found and Stephanodiscus astraea var.
minutula is the sole species o f the Stephanodiscus/Cyclostephanos genera mentioned in this list. Despite the shallowness o f this lake (maximum depth: 4m) the diatom assemblages were largely dominated by planktonic species except during May and August 1997 when Achnanthes catenata
was largely dominant (>70%). By comparison with Lac de la Cassière, the absence of a species that constantly dominated the assemblages is the main characteristic of the succession in Lac Chambon (Fig. 2.3.6).
Chapter 2.3: Monitoring o f diatom assemblages 160
Fig 2.3.6: Lac Chambon, diatom succession in water samples.