Most of the early studies on diatom assemblages are simple taxa lists (Eusebio, 1921; Wurtz, 1945; Persoone, et a l , 1968; De vaux, 1969; Omaly, 1969). In the 1970’s, substantial studies were carried out by researchers from Clermont-Ferrand University on the phytoplankton dynamics of some of the largest lakes in the Puy-de-Dôme district. Devaux (Devaux, 1973; Devaux, 1975; Devaux, 1975) applied the ecological succession theory of M argalef (1967) to phytoplanktonic populations of Gour de Tazenat and Lac Pavin (two of the deepest crater lakes of the region). In both lakes diatoms were dominant in the phytoplankton. Specific diversity and primary production (expressed in terms of biomass and chlorophyll concentration) were related to abiotic factors, especially light and temperature. Relationships between zooplankton and phytoplankton in the same two lakes were studied by Lair (Lair, 1975). A synthesis of the work carried out on the phytoplankton, phytobenthos and zooplankton of Lac Pavin has been published by Devaux, et al. (1983). Later on in the 1980’s and 1990’s research at Clermont University focused on phytoplankton metabolism in two lakes. Lac d ’Aydat and Lac Pavin. In these studies, carbohydrate and protein concentrations, fatty acid composition of lipid classes, adenine nucleotides (adenylate energy charge, ATP, ADP, AMP) and photosynthetic assimilation rates were used for the analysis of biomass, metabolism and population structure. The importance of these descriptors for plankton ecological studies was discussed (Alaoui-Mhamdi and Siriki, 1985; Amblard, 1986; Bourdier, 1986; Amblard, 1988; Amblard, et al., 1988). The diatom
Aulacoseira subarctica, being the dominant species in these two lakes, was studied in detail (Amblard and Bourdier, 1990; Amblard, et al., 1992). Models of algal succession were tested on Lac d ’Aydat (Lair and Ayadi, 1989; Lafforgue, et a l , 1995). For the past five years, the focus
Chapter 1.4: The study region 67
has been on Lac Pavin, with studies on phytoplankton metabolism (Maurin, et a l , 1995; Jamet,
et a l , 1997), grazing impact (Quiblier-Lloberas, et a l , 1996), and fatty acid distribution as biomarkers (Desvilettes, et a l , 1997; Léveillé, et a l , 1997), Protozoa and bacteria communities and their relationships with phytoplankton were also intensively investigated (Carrias, et a l,
1996; Maurin, e t a l , 1997; Carrias, et a l , 1998).
In contrast with the substantial amount of research published on three of the largest lakes in the region (le . Gour de Tazenat, Lac Pavin, Lac d ’Aydat), very few studies have been published on the numerous shallow lakes, ponds and peat bogs also present in the French M assif Central. This type o f lake is well represented in the training set of this study (see following chapter). A study on the microbial loop in peatlands, which includes data on diatom assemblages, is one exception (Gibert, et a l , 1998).
In summary, despite detailed knowledge of two or three sites, in general little is known of the modem distribution of diatom algae along the different physical-chemical gradients in the different water bodies of the French M assif Central. Further contemporary research was therefore necessary to allow interpretation, and specifically quantitative reconstruction, of palaeoenvironments from the Ribains core.
Chapter 1.5: Aims and approaches_________________________________________________ ^
1.5. Aims and approaches
The work carried out at the Laboratoire de Botanique Historique et Palynologie, Marseille on the pollen sequences from the French M assif Central crater lakes forms a background to this project. For the past fifteen years researchers from Marseille have been investigating sediments from these craters with a view to reconstructing the vegetation history over the past 400,000 years. In June 1988, a long core was collected from Ribains crater (Haute-Loire, 44°50’09” N and 3°49’16” E) and used for palynological analyses. The location of Ribains crater is particularly interesting because it is intermediate between the sites containing Eemian deposits from Southem Europe (Greece and Italy) and those from Northern Europe, conveniently linking stratigraphy across the Alps (van Andel and Tzedakis, 1996). The results of the pollen analyses include a relative chronology obtained by correlation with other European pollen sequences and a detailed description of the vegetational succession and its interpretation in terms of climate (Beaulieu and Reille, 1992).
The aims of this thesis are twofold. Primarily it is a contribution to ongoing palaeoclimate research on French crater lakes, and especially the site of Ribains. Diatom analysis will offer a unique insight into the former water chemistry and physical limnology of this site. With support from the pollen analyses, the observed changes in the diatom record may be related to regional environmental and climatic fluctuations through time. The diatom record, moreover, should provide a climate history of much greater temporal resolution than the pollen record. The research will focus on the transitions between the penultimate glaciation (or Riss) and the last interglacial period (or Eemian) and between the Eemian and the last glaciation (or Wiirm).
Realising the first objective requires a robust knowledge of modem preferences, tolerances and ecological distribution of individual diatom taxa (autoecological data) and assemblages (synecological information). Although a considerable amount of information can be obtained by an extensive review of the literature on diatom ecology, a valuable complement of information is gained by sampling the living assemblages of diatoms found in the present-day lakes of the study area. Sampling of subfossil diatom assemblages from the surface sediments of these lakes, in association with physical and chemical analyses of the water, is required for quantitative reconstmction of palaeoenvironments. The second aim of this thesis is, therefore, to examine the distribution of contemporary diatom assemblages in lakes of the M assif Central in order to develop transfer functions to reconstmct quantitatively past conditions from sediment records in Ribains crater.
Chapter 1.5: Aims and approaches 69
This thesis does not include a chapter exclusively concerned with methods. Instead, laboratory methods and statistical techniques are described and discussed at the beginning of each relevant chapter.
Chapter 2 is concerned with the study of modem diatom communities found in 25 lakes of the French M assif Central. In this chapter, the geographical and geological situation of these lakes is introduced and their chemical and physical characteristics are analysed. The results of diatom analysis from the living communities sampled in the water of these lakes are also presented in chapter 2. In chapter 3, the relationships between these communities and their physical-chemical environment are analysed using statistical techniques. Diatom analysis of surface sediment samples and the development of transfer functions are the theme of chapter 4. In chapter 5, the transfer functions developed are applied to short cores from contemporary crater lakes of the French M assif Central. The high-resolution diatom record obtained from the analysis of the Eemian sediment from Ribains crater is presented in chapter 6. Interpretation of this record is supported by the application of the transfer functions developed in chapter 4 and by analogue matching of the sediment samples analysed in chapters 4 and 5. In this same chapter, the variations observed in the Ribains diatom record are discussed in terms of climate and comparison with the pollen record is made. Implications for the regional palaeoclimatology are assessed. Finally, a summary of the results is given in chapter 7, along with suggestions for future work.
Chapter 2.1 : Study sites 70