Introduction

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1.1  Phytoplankton pigments as markers for community structure and environmental changes

Recent phytoplankton pigments as markers for community structure: Phytoplankton pigments capture solar energy, mediate its conversion to chemical energy, and ultimately control primary production by regulating the supply of organic carbon to the pelagic food web (cf. Fig. 1). In contrast to higher plants, which all contain a rather similar pigmentation, algae and cyanobacteria have group-specific pigment compositions (Tab. 1). All algae and cyanobacteria contain chlorophyll a (Chla), however other chlorophylls and most of the carotenoids are found in only some taxonomic groups (Tab. 1), thus making them ideal marker pigments. Such marker pigments can be used to quantify the potential contribution of different chemotaxonomic groups making up the phytoplankton community (Weber and Wettern 1981, Gieskes et al. 1988, Everitt et al. 1990, Wilhelm et al. 1991, Mackey et al. 1996, Wright et al. 1996, and others). For instance, lutein is often used as marker pigment for Chlorophyta (´green algae´), alloxanthin for Cryptophyta, fucoxanthin for Bacillariophyceae (´diatoms´) and Chrysophyceae, and zeaxanthin for cyanobacterial picoplankton (cf. Tab. 1).

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Fig. 1. Phyto-plankton pigment production in re-lation to energy capture, grazing, sedimentation, and burial.

Tab. 1. Overview of phytoplankton classes and higher plants along with selected photosynthetic pigments indicating the much greater pigment composition variability in phytoplankton compared to higher plants (after Kohl and Nicklisch 1988, Leavitt et al. 1993, Mackey et al. 1996).

chlorophylls

carotenes

xanthophylls

a

b

c

α

β

alloxanthin

diadinoxanth..

fucoxanthin

lutein

peridinin

violaxanthin

zeaxanthin

Higher plants

x

x

-

-

x

-

-

-

x

-

x

(x)

Cyanobacteria

x

-

-

-

x

-

-

-

-

-

-

x

Chlorophyceae

x

x

-

-

x

-

-

-

x

-

x

(x)

Bacillariophyceae

x

-

x

-

x

-

x

x

-

-

-

-

Chrysophyceae

x

-

x

-

x

-

x

x

-

-

-

-

Pyrrophyceae

x

-

-

-

x

-

-

-

-

x

-

-

Cryptophyta

x

-

-

x

x

x

-

-

-

-

-

-

Eustigmatophyceae

x

-

-

-

x

-

-

-

-

-

x

-

High Performance Liquid Chromatography (HPLC) is central to the study of phytoplankton pigments, since it allows semi-automated and rapid analysis of lipophilic photosynthetic pigments (Gieskes et al. 1988, Wilhelm et al. 1991, Millie et al. 1993, Jeffrey et al. 1997, -1999). Using HPLC the separation and quantification of all chlorophylls and carotenoids as well as their degradation products can be performed, even at extremely low concentration levels. Phycobilins (phycoerythrin and phycocyanin), however, which occur in cyanobacteria mainly, cannot be detected together with chlorophylls and carotenoids using HPLC due to their water-soluble nature.

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The contribution of specific phytoplankton groups to the total phytoplankton community have been estimated for marine systems (Gieskes et al. 1988, Everitt et al. 1990, Letelier et al. 1993, Andersen et al. 1996, Bidigare and Ondrusek 1996, Wright et al. 1996, Latasa et al. 1997, Jeffrey et al. 1997, Rodriguez et al. 2002) and freshwater bodies (Wilhelm et al. 1991, Lami et al. 1992, Soma et al. 1993, -1995, Quiblier et al. 1994, Descy and Métens 1996, Woitke et al. 1996) based on marker pigment analyses using HPLC. In the case of marine systems, Jeffrey et al. (1999) suggested that a HPLC-aided pigment study should be complemented by underway fluorometry and remote sensing that allow in situ and large-scale monitoring (Fig. 2). However, although standardisations and software program for the use of pigments in regular monitorings have been developed for marine systems (Mackey et al. 1996, Jeffrey et al. 1997, -1999), this is not yet the case for freshwater systems.

The advantages of a pigment-based monitoring compared to a microscopic count-based approach are three-fold: (1) it is much less time consuming, and therefore, a larger sample set may be analysed, which is particularly important in large lakes or marine systems; (2) it can also detect picophytoplankton and fragile cells that may be missed using the microscopic counting approach (Gieskes and Kraay 1983, Everitt et al. 1990, Millie et al. 1993); and (3) Chla is closely related to primary production (Gervais and Behrendt 2003), and in comparison to biovolume, Chla is the better parameter to allow primary production estimation because it indicates the photosynthetically active phytoplankton (Wilhelm et al. 1995).

Fig. 2. Methods used for investigating phytoplankton and their pigments in marine systems; adapted from Jeffrey et al. (1999).

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Recent phytoplankton pigments as markers for environmental changes: Different phytoplankton groups have distinct environmental preferences, e.g. light, nutrient and temperature requirements, which affect their growth. Thus, marker pigments that allow the phytoplankton composition to be estimated thereby help to monitor environmental changes. In an early study, Chla and marker pigments were successfully used to monitor the phytoplankton response to environmental changes by monsoon-induced upwelling or downwelling in the Indonesian Sea (Gieskes et al. 1988). In other studies, Chla and marker pigments were also successfully used to study the development and composition of phytoplankton forming deep chlorophyll maximum layers in the North Pacific gyre (Letelier et al. 1993) or blooms in the North Atlantic (Barlow et al. 1993). Furthermore, marker pigments were also analysed in conjunction with physical, chemical and nutrient measurements, in order to describe how El Niño conditions affect phytoplankton populations in the central equatorial Pacific (Bidigare and Ondrusek 1996, Latasa et al. 1997). Phytoplankton pigment distribution was used to document the trophic conditions from coastal to open basin regions of the Mediterranean (Barlow et al. 1997) and in various German lakes (Wilhelm et al. 1995), as well as to describe climate and catchment-related conditions in Canadian shallow mountain lakes and ponds (Vinebrooke and Leavitt 1999). Thus, phytoplankton pigment composition is taken as a measure of the response of phytoplankton to environmental changes in various marine and freshwater systems, and hence is nowadays regularly monitored.

Fossil phytoplankton pigments as markers for community structure and environmental changes: Reconstructions of long-term climatic and other environmental changes have been based on several different biogenic proxies such as pollen, macrofossils, diatom valves, stable isotopes, biomarkers and lipophilic photosynthetic pigments that are preserved in the sediments (cf. Smol et al. 2001). Thereby, the use of photosynthetic pigments for reconstructions of the phytoplankton standing crop has been attempted for many years (e.g. Watts and Maxwell 1977, Züllig 1981, Sanger 1988). Multiproxy approaches that included fossil phytoplankton pigments have successfully tracked climatic and other environmental changes in lakes (Hall et al. 1997, Bianchi et al. 1999, Leavitt et al. 1999, Verschuren et al. 1999, Pienitz et al. 2000, Bennett et al. 2001, Ariztegui et al. 2001, Francis 2001, Lotter 2001, Rusak et al. 2004) and seas (Chondrogianni et al. 2004). In these studies, fossil phytoplankton pigments were established to be able to further complement the information one can extract from the sediment, allowing estimates of the composition of the sedimented phytoplankton and of the total autochthonous primary productivity.

Several single parameter studies have described pigment deposits in lake sediments, which were, nonetheless, also thought to be proxies of changing climate (Vinebrooke et al. 1998, Kowalewska 2001) or physical properties, such as stratification and, thereby, changing redox conditions (Sanger and Crowl 1979, Hodgson et al. 1998, Squier et al. 2002). Similarly, fossil pigments have also been suggested as proxies of changing UV radiation (Leavitt et al. 1997, -2003), changing trophic state (Gorham et al. 1974, Adams and Prentki 1986, Lami et al. 1994), lake acidification (Guilizzoni et al. 1992), food-web manipulations (Leavitt et al. 1993), or anthropogenic influence such as sewage enrichment, land use or dam building (Griffith et al. 1969, Soma et al. 1995). Moreover fossil pigments were used to model the variability and predictability of phytoplankton assemblages after anthropogenic fertilisation (Cottingham et al. 2000).

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Traces of carotenoids, chlorophylls and their degradation products were shown to persist long after the disappearance of morphologically distinguishable remains of the organisms that produced them (Brown 1969) and are often the sole remnants of non-siliceous algae (Leavitt 1993). However, degradation and diagenesis affect all biogenic proxies during sinking and subsequently deposition, and therefore make direct reconstructions difficult. However, despite this, Guilizzoni et al. (1983) found significant correlations between the primary production and total pigment concentration of surface sediments in 12 Italian lakes. Additionally, Gorham et al. (1974), Swain (1985), and Brenner and Binford (1988) obtained similar results in several European and American lakes.

Several studies have also identified pigment-specific correlations between fossil pigments and historical data of the standing crop (Griffiths et al. 1969, Leavitt et al. 1989, Leavitt and Findlay 1994, Hall et al. 1999, Bianchi et al. 2002). For instance, Griffiths et al. (1969) showed in an early sedimentary pigment study, a strong correlation between the fossil oscillaxanthin content and the historical occurrence of Oscillatoria (cyanobacteria). Later, Leavitt and Findlay (1994) established that the ubiquitous pigments ß-carotene and pheophytin a were correlated to total biomass (r = 0.56-0.65) and that the marker pigments lutein+zeaxanthin and pheophytin b were correlated to the biomass of Chlorophyta (r = 0.53-0.55). However, in contrast, these authors also found that the marker pigments α-carotene and alloxanthin were only weakly correlated to Cryptophyta. Moreover, fucoxanthin and Chlc were uncorrelated to Chrysophyceae or Bacillariophyceae and peridinin to Pyrrophyta. These three pigments were strongly degraded. Generally, the correlations between fossil pigments and historical data of biomass were highest when fossil pigments were calculated as units per organic matter (Leavitt and Findlay 1994). Pigment concentrations relative to total organic carbon (TOC) are suggested to remain similar over time, so that TOC-specific pigment plots reduce the problem of differential degradation and allow an interpretation based on changing primary productivity in a lake (Vallentyne 1960, Daley and Brown 1973, Sanger 1988).

The correlations between the sedimentary pigments and the phytoplankton standing crop vary strongly because of differential degradation during sedimentation before permanent burial. Depending on the type of degradation process, chlorophyll can, for example, degrade to chlorophyllide, pheophorbide, pheophytin or steryl chlorin esters (SCE), or ultimately to colourless compounds (Fig. 3). Carotenoids degrade more slowly than chlorophylls, but faster than some chlorophyll degradation products (Sanger 1988). Therefore, the ratios of carotenoids to Chla (including degradation products) often decrease from the euphotic zone to the sediment (Repeta and Gagosian 1984). Moreover, the different carotenoids degrade with different rates. Generally, within the carotenoids, xanthophylls (such as fucoxanthin) degrade faster than carotenes (such as ß-carotene; Vallentyne 1960). An example of xanthophyll degradation towards non-carotenoid products is given for fucoxanthin in Fig. 4.

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Fig. 3. Simplified scheme of Chla degradation. In most of the chlorophyll degradation products, the tetrapyrrol macrocycle remains intact. Magnesium is removed from chlorophylls in the presence of dilute acids (gut passage) and high-light intensities (photooxidation). One of these degradation products is, for instance, pheophytin, where the central Mg-atom is replaced by two H+. Chlorophyllide arises by dephytylation of Chla and Chlb by the ubiquitous catabolic enzyme, chlorophyllase. Chlc is already chlorophyllide-like and does not undergo this process. Pheophorbide arises by removing the central Mg-atom from a chlorophyllide (Llewellyn et al. 1990, Leavitt 1993, Matile et al. 1999). Steryl chlorin esters (SCE) are formed by esterification from chlorophyll degradation products during zooplankton gut passage, and are relatively stable compared to chlorophylls and other degradation products; however their formation and sedimentation processes are not clear at present (King and Repeta 1991, Prowse and Maxwell 1991, Talbot et al. 1999, Soma et al. 2001b).

Fig. 4. Example of carotenoid (fucoxanthin) degradation by herbivore grazers (metabolism) and after sedimentation: Zooplankton herbivores and other heterotrophs hydrolyse carotenoid esters to free alcohols (fucoxanthin to fucoxanthinol) in the water column. Dehydration and epoxide rearrangement occur (fucoxanthinol to isofucoxanthinol 5´-dehydrate) in the surface sediment after burial (Repeta and Gagosian 1982, -1984).

Because pigment degradation is a very complex issue, the degradation processes within the water column and in the surface sediment have to be investigated specifically for each lake and pigment (Cuddington and Leavitt 1999). Differential pigment degradation and losses during deposition in marine and fresh water depend mainly on (1) selective meso- and microzooplankton grazing as well as the different digestibility of cells, (2) light and oxygen availability during and after sedimentation, and (3) sinking rates that differ between species as well as between living and dead cells and faecal pellets (Leavitt 1993, Cuddington and Leavitt 1999, Leavitt and Hodgson 2001, and references therein; Fig. 5).

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Pigment destruction by zooplankton grazing depends on various factors such as gut passage time, edibility and even on food quality. Large Bacillariophyceae, for instance, are less edible than small Chlorophyta or even picoplankton. On the other hand, the extent of the degradation also depends on the grazer size and type (Carpenter and Bergquist 1985, Carpenter et al. 1988). Small protozoa, for example, degrade more efficiently compared to large protozoa (Strom et al. 1998). Also, filter feeders feed on other phytoplankton cells compared to raptorial feeders. Therefore, transformation products that are produced by herbivorous grazing (e.g. pheophorbide and SCE, cf. Fig. 3) were also used to monitor predation in a lake (Leavitt et al. 1993).

Fig. 5. Pathway of pigment production, transformation and degradation within the water column of a lake; scheme adapted from Leavitt and Hudgson (2001).

Oxygen enhances degradation in dead or moribound cells by direct oxidation of the conjugated chromophores or by stimulation of the microbial activity (Leavitt 1993). The fast oxidative degradation in the photic layer is due to photooxidation, whereas in the deeper water, this is caused by chemical oxidation. Further degradation occurs, however, after burial in the surface sediment, especially when the water to sediment interface is oxic (Leavitt 1993). Therefore, only those pigments that reach the deeper sediment layers below the redox zone are well preserved over hundreds of years.

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Sinking rates determine the exposure time to light and oxygen. Factors that reduce the length of light exposure, e.g. high cell sinking rate or faecal packaging, will increase pigment deposition (Cuddington and Leavitt 1999). The incorporation of phytoplankton into zooplankton faecal pellets, for example, enhances the sinking velocity by up to 1000-fold (Welschmeyer und Lorenzen 1985a, -b).

Whether changes in fossil pigment concentrations arose from changes in phytoplankton standing crop or degradation has been a matter of discussion for a long time (Brown 1969, Swain 1985, Sanger 1988, Leavitt 1993, Cuddington and Leavitt 1999). Based on previous studies, an attempt has recently been made to model the extent and rate of lake-specific degradations (Cuddington and Leavitt 1999); however, data to test this model are scarce, especially for deep lakes. Hence, investigating phytoplankton pigments in the photic zone as well as its sedimentation and preservation is still a prerequisite for the interpretation of fossil phytoplankton pigments.

1.2 Phytoplankton and pigments in Lake Baikal

Recent phytoplankton and its pigments: Regular monitoring of a few specific parameters of phytoplankton in Lake Baikal has been conducted by the local institutions for several decades (e.g. Limnological Institute Irkutsk and University of Irkutsk, Fig. 6). Gradients of temperature, insolation, and nutrients are caused in Lake Baikal by its great length over five degrees of latitude, its rift-generated morphometry as well as its large tributaries and bays (Kozhov 1963, Galazii 1993, Kozhova and Izmest’eva 1998). Temperature and stratification regime, as well as ice cover duration and snow cover thickness, are important parameters for phytoplankton development in Lake Baikal, as concluded from recent monitoring (Shimaraev et al. 1994, Kozhov 1963, Kozhova and Izmest’eva 1998) and models based on sedimentary proxies (Mackay et al. 1998, -2003, Semovski 1998) or published data (Verkhozina et al. 2000). However, the regional variation of nutrient supply is also a critical factor (Goldman et al. 1996, Genkai-Kato et al. 2002).

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The extreme intracontinental location of the lake creates highly contrasting seasonal changes, i.e. very cold and dry winters and very warm, cloudy and wet summers (Kozhov 1963, Kozhova and Izmest’eva 1998). In addition to the temperature and wind-induced mixing, ice and snow cover also strongly influence the seasonal succession of the phytoplankton (Mackay et al. 2003). So-called “Melosira” years, which occur every three or four years (at least in the South basin), are characteristic for Lake Baikal. During such years, blooms of endemic Bacillariophyceae, e.g. Aulacoseira baicalensis (its former name was Melosira), develop in the convective layer under the snow-free ice (Kozhov 1963, Kelley 1997, Kozhova and Izmest’eva 1998, Granin et al. 1991, -1999) and these years have been estimated to have a biovolume 10- to 100-fold more than that of “non-Melosira” years (Popovskaya 2000).

Fig. 6. Regular phytoplankton monitoring in Lake Baikal (scan from Galazii 1993).

Moreover, vertical gradients of temperature and insolation influence the distribution of total phytoplankton abundance and composition in the extremely deep Lake Baikal, where the euphotic zone and the wind-induced overturn can reach up to 50 and 300 m water depth, respectively. For example, during periods of overturn, living phytoplankton was found down to over 200 m (Popovskaya 2000) and even during stratification photosynthetically active cells sinking out from the euphotic zone were found down to more than 500 m water depth (Genkai-Kato et al. 2003).

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These previous phytoplankton studies have therefore highlighted the need to investigate three gradients in Lake Baikal: region, depth and season. Additionally, due to the high interannual variability, subsequent years should also be examined for the three gradients. Finally, for a comprehensive study of this lake, all phytoplankton size classes (from large Bacillariophyceae to small picocyanobacteria) should be studied.

Thus far, single phytoplankton studies in Lake Baikal have usually considered only one parameter, e.g. biomass, Chla or primary production. Moreover, conclusions on environmental impact were generally drawn from a series of different studies conducted over several decades and at different sites and seldom supported by statistical analyses (Kozhov 1963, Kozhova and Izmest’eva 1998, and others). Additionally, most studies have used the traditional method of microscopic measurement and counting, which primarily records nano- and microphytoplankton. Autotrophic picoplankton (APP, < 3µm), believed to contribute significantly to the summer assemblage (Popovskaya 2000, Belykh and Sorokovikova 2004, Popovskaya and Belykh 2004), have not been extensively included in the studies until now, partly due to technical difficulties (Boraas et al. 1991, Nagata et al. 1994).

To date, phytoplankton pigments other than chlorophylls have not yet been investigated in the water column of Lake Baikal. Hence, no information about phytoplankton pigment distribution and its driving forces is currently available; thereby hampering also accurate interpretation of sedimentary pigment composition.

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Previously, in situ fluorescence measurements had already indicated the applicability of pigment-based approaches for phytoplankton and environmental surveys in Lake Baikal; however, up until now only total Chla was determined using single-wavelength fluoroprobes (Granin et al. 1991). As yet, multi-wavelength fluoroprobes that allow the estimation of the relative importance of phytoplankton groups (Beutler et al. 2001, -2002a, -2002b) have not been used in Lake Baikal.

Sedimentation through the water column and preservation within the surface sediment: Lake Baikal’s depth, fully oxic water column and low temperatures distinguish it from most freshwater lakes, and its oxic water to sediment interface also sets it apart from marine systems. Hence, this makes Lake Baikal an interesting water mass to study. Three different zones can be distinguished in the water column of Lake Baikal: (1) the euphotic zone, which extends up to 40-50 m; (2) the aphotic zone down to 250 m, which is mixed by wind during homothermy; and (3) the stable aphotic, but oxic deep water zone from approximately 250 m to the lake bottom, which is in the open pelagic regions more than 1 km deep. Deep-water ventilation explains the permanent high content of oxygen in the near-bottom layer and throughout the water body of Lake Baikal (Weiss et al. 1991, Dobretsov 2000). Even up to 2 cm of sediment surface are oxic (Müller et al. 2005) and up to 20 cm are oxidised (Vologina et al. 2000). Ferromanganese (Fe-Mn) crusts indicate redox boundaries in many regions of Lake Baikal (Vologina et al. 2003, Granina 2004).

Strong degradation was, therefore, assumed for biogenic proxies, such as phytoplankton pigments, diatom valves or pollen. For diatom-valve studies, species-specific correction factors have been recently established that allow the composition of the source populations to be reconstituted (Battarbee et al. 2005). Only approximately 1 % of the diatom valves from the phytoplankton crop are preserved in the sediment and some valves are more affected than others (Battarbee et al. 2005). However, as yet such studies were not conducted for pollen or phytoplankton pigments.

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Surface sediment distribution of photosynthetic pigments has been investigated across Lake Baikal, and an attempt has been made to estimate the relevant factors for the sedimentation; although no correlation to the degradation processes in the water column has been undertaken (Soma et al. 2001a). The concentrations of phytoplankton pigments preserved in the surface sediment have been established to be high in the South basin and minimum in the Central basin, while TOC was distributed rather evenly (Soma et al. 2001a). Soma et al. (2001a) hypothesised that the oxidising conditions in the surface sediments and low sedimentation rates in certain regions of Lake Baikal caused intense decomposition of pigments. However, no analysis of the oxidised layers has been undertaken to study such decomposition. Furthermore, this research group also supposed that the river Selenga significantly disturbs sedimentation in the South basin causing uneven spatial pigment distribution; diverse carotenoids were detected in this basin that indicated the former presence of different algal classes although no correlation to the pigments of the standing crop was established (Soma et al. 2001a). Thus, despite this accurate study on the regional pigment distribution in Lake Baikal’s surface sediment the degradation processes of phytoplankton pigments through the water column and within the surface sediment have not been investigated to date. Hence, no information is available on either the pigment distribution in the euphotic zone or their sedimentation in the hypolimnion or their degradation in the surface sediment layers.

Fossil phytoplankton pigments: Lake Baikal is particularly well suited for studies of the natural environment and climate because this lake is believed to have never been glaciated, although glaciers reached the surrounding mountains during the glacial periods. Consequently, sediments have accumulated continuously for 25 million years and today form an up to 7.5 km thick layer. During the past 130,000 years, the Baikal region was subjected to several major glacial and interglacial periods. An alternating deposition of diatom-rich and compact, diatom-barren clayey material reflects the climatic changes in the sediment.

Several biologic parameters, such as diatom valves, biogenic silica and pollen, have been intensively studied in Lake Baikal sediments and the results demonstrate that they act as indicators of climatic and other environmental change (Bezrukova et al. 1991, Qiu et al. 1993, Mackay et al. 1998, Edlund and Stoermer 2000, Horiuchi et al. 2000, Karabanov et al. 2000a, -b, Minoura 2000, Khursevich et al. 2001, BDP Members 2004, and others). Despite important work has been conducted based on the determination of diatom valves or biogenic silica, this traditional method of fossil phytoplankton determination may be insufficient in Lake Baikal as recent reports suggest that (1) in summer more than 50 % of the Chla in Lake Baikal results from APP (autotrophic picoplankton) (Popovskaya 2000), (2) high contributions of the APP to the total primary productivity occur even under ice cover (Straškrábová et al. 2005) and (3) APP were found to be the main chain of the trophic link of Lake Baikal (Popovskaya and Belykh 2004). Hence, to establish the composition of algal populations other than diatoms, chlorophylls, carotenoids and their degradation products should also be determined.

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Due to the oligotrophic, marine-like phytoplankton abundances in Lake Baikal, the microfossils are preserved in very low amounts. However, modern HPLC systems equipped with fluorescence detectors can identify the remaining lipophilic pigments with high sensitivity. In the first ever pigment study at Lake Baikal, Soma et al. (1996) showed that degradation products of chlorophylls were present even in the deepest layers of short cores (41 cm) taken from the South basin. The major chlorophyll degradation product was pheophytin, with pheophorbide occurring only in low amounts. Because pheophorbide has been proposed as a grazing biomarker, the authors considered the effect of grazing in Lake Baikal to be small. No degradation processes were identified either for the water column or for the surface sediment. Moreover, no xanthophylls with epoxide groups (such as fucoxanthin) were detected in this preliminary study, while ß-carotene, alloxanthin, lutein, diatoxanthin and canthaxanthin were found (Soma et al. 1996). The concentrations of the carotenoids varied at intervals of 6-10 cm, while the content of TOC was rather stable through the whole core length (Soma et al. 1996). The authors suggested that phytoplankton changes were manifested in an amplified way in the depth profiles of the pigments in the sediment.

Tani et al. (2001, -2002) continued these preliminary studies by investigating the phytoplankton pigments in two longer and 14C-dated cores from the South basin. In both cores, the transitions from the last glacial period to the Holocene were successfully shown (Tani et al. 2001, -2002). Additionally, Tani et al. (2002) provided initial data on how the phytoplankton pigments could be used as a climatic indicator, in this case for a major cooling event (the Younger Dryas); however these analyses were performed at low resolution (2-5 cm, which in Lake Baikal correspond to c. 100-400 yrs). Also, one of the cores was taken at a site close to the southern coast with rather a steep slope and it was uncertain whether the core was representative of pelagic conditions in the lake. It therefore remains to be determined whether the observation in these two cores can be generally applied to all the basins of the 600 km long Lake Baikal (Tani et al. 2002).

Nonetheless, these studies along with that of Naylor and Keely (1998) revealed that SCEs (steryl chlorin esters, cf. Fig. 3) were well preserved within Lake Baikal´s sediments. Soma et al. (2001b) could reconstruct global climate changes that occurred during the last 2.8 million years using these SCEs as phytoplankton markers. SCEs accounted for 90 % of the total chlorophyll degradation products in sediment layers deeper than 10 m. Soma et al. (2001b) concluded that although records of phytoplankton preserved in long cores of lake sediments, spanning millions of years, have so far been confined to fossil diatoms, their investigation on fossil pigments suggested that SCEs were useful indicators of phytoplankton communities as a whole, including diatoms. However, investigating a 100 m long core, the resolution was low (every 10 cm corresponding to c. every 2400 yrs), and therefore it remains to be proven whether fossil pigments can also be used to track climate changes with accuracy at higher resolutions.

1.3 International interest in paleoclimate and paleolimnologic research in Lake Baikal

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Approximately 150 years ago, a global warming phase began and reconstructions of climate data for the past 1000 years indicate that this warming is unusual and also unlikely to be entirely the result of natural causes (IPCC 2001). Unfortunately, most models predict further worsening (IPCC 2001). Although confidence in the ability of models to accurately predict general future climatic change has increased, there are still a large number of areas for which uncertainties for global extrapolations remain (IPCC 2001). One reason for this is that most models were based on Atlantic studies and may be valid for regions under oceanic influence, but they struggle to explain several events in continental regions. Hence, there is an important gap in our knowledge of how large-scale climate processes affect continental regions (Oberhänsli and Mackay 2005).

Climate models are usually validated by comparing predicted scenarios with measured changes in the past. Traditionally, climatic changes in areas with ocean influence are assessed with, for instance, analysis of ice cores from Greenland or the Antarctic. In continental areas, climatic changes are known from the lake’s sedimentary archives. Thus, for further improvement of climate models one needs the analysis of the biologic record contained in sediment cores in lakes far remote from marine influence. For example, in the northern hemisphere, only few lakes located intracontinentally fit this criterion due to large parts of the northern hemisphere being periodically glaciated. However, one of the lakes which was unaffected by glaciation, and therefore having the advantage of an uninterrupted archive, is the Siberian Lake Baikal.

Lake Baikal is located far from marine influence at the boundaries of major global weather systems such as the North Atlantic Oscillation and Asian Monsoon. Therefore, knowledge on the changes in Lake Baikal and comparison with well-studied European climatic oscillations can considerably aid understanding of the extent of climatic changes and the ecological consequences in continental regions. Thus, the multiproxy analysis of sediment cores from Lake Baikal within the EU framework 5 project CONTINENT (EVK2-CT-2000-0057; http://continent.gfz-potsdam.de; cf. Oberhänsli and Mackay 2005), which besides phytoplankton pigments, also includes plant microfossils, e.g. pollen and diatom valves, as well as techniques such as sediment geochemistry, site survey, and dating, represents a useful tool for climate scenario validation in continental areas.

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However, the biological record contained in Lake Baikal’s sediment is not only central to better understand the aforementioned climate variability and to improve climate models, but it is also pivotal to understand the evolution of the specific fauna and flora of the lake and to the impacts of recent environmental change and pollution. Yet the effect of global warming and local nutrient enrichment on the ecology of the World´s largest lakes, including Lake Baikal, is poorly known.

In 1996, Lake Baikal was granted World Heritage Status by the UNESCO. Since then, the lake’s protection and conservation for the future generations has shifted from a solely local, Russian affair to an international one. This unique ecosystem has changed very little since regular research began in the early 20th century (Kozhova and Izmest’eva 1998), although an increase in air and water temperature as well as precipitation and a decline of the ice cover duration have been reported recently (Magnuson et al. 2000, Shimaraev et al. 2002, Hulme et al. 2003). Additionally, pollution in the river Selenga and South basin due to both industrial and domestic discharge has also been discussed (Galazii 1982, -1991, Martin 1994, Kozhova and Silow 1998, Mackay et al. 1998, Beeton 2002). Spreading of small cosmopolitan species at the expense of the many large endemic Bacillariophyceae was also suggested to have occurred (Popovskaya 1991, Kozhova and Izmest’eva 1998). However, an alternative view is that some of the variations attributed to human impact were within the range of natural changes; hence, the ‘ecological alarm’ was overestimated (Grachev et al. 1989, Grachev 1994, cf. Zumbrunnen 1974, cf. Flower 1998). This controversy has not yet been resolved. However, there is agreement that only changes over and above the ´norms´ can be used to decouple natural from anthropogenic impacts. Therefore, rate and extent of natural changes must be studied to precise the norms. Regular monitoring can assess the recent variations, and the historical variability can be taken from sedimentary records. The CONTINENT project’s overriding aim, and hence under its auspices the present thesis’, was to contribute to the local and international effort to understand the processes of Lake Baikal, and thereby also to its protection.

1.4 Outline of the thesis

The objective of this thesis was to investigate whether and to which extent recent and fossil phytoplankton pigments in Lake Baikal are potential markers for phytoplankton community structure and environmental changes. Moreover, in conjunction with the paleoclimate project CONTINENT and the long-term monitoring programme of the State University Irkutsk, the information gathered by the phytoplankton pigments should be used to complement our knowledge on the current and historical productivity variations in Lake Baikal. Three main aspects were investigated: (1) the distribution of phytoplankton and phytoplankton pigments in the euphotic zone, (2) its sedimentation through the water column and preservation within the oxidised surface sediment, and (3) variation of fossil phytoplankton pigments.

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Distribution of phytoplankton and its pigments in the euphotic zone: Considering the enormous size of Lake Baikal, monitoring the phytoplankton response to anthropogenic and climatic influences requires a less time-consuming method than traditional microscopic cell counts that, however, includes all phytoplankton size classes. I hypothesised that a pigment-based approach would enable regular monitoring to answer the question previously posed by Reynolds (1984) for ecological research on phytoplankton: ´what lives where – and why?´. To test this hypothesis, the present thesis combined pigment-based methods (HPLC and fluorometry) in conjunction with traditional microscopic spot checks.

A preliminary study established the applicability of an HPLC-based approach to determine accurately the summer phytoplankton assemblage in Lake Baikal (Fietz and Nicklisch 2004, Appendix A). This study also demonstrated the need for microscopic spot checks to aid accurate interpretation of the pigment results. This preliminary study of pigments in the Baikal water samples even induced us to revise the autotrophic eukaryotic picoplankton composition, because, due to unusual pigment ratios, one would expect Eustigmatophyceae to be present in the phytoplankton (Fietz and Nicklisch 2004, Appendix A), although none have as yet been described (Kozhova 1987, Bondarenko 1995, Kozhova and Izmest’eva 1998). Three new strains, isolated from Lake Baikal water samples, were identified as Nannochloropsis limnetica Krienitz, Hepperle, Stich & Weiler and were shown to be common members of the Baikalian picoplankton (Fietz et al., submitted, Appendix B).

The recommended remote sensing (Jeffrey 1997, Fig. 2) was studied by Birgit Heim from GeoForschungsZentrum (GFZ) Potsdam, Germany, based on the pigment data reported in the present thesis (Heim et al. 2005, Heim, in prep.). Furthermore, the monitoring was complemented by size fractionated primary production and nutrient availability determinations by Vera Straškrábová and Jakub Borovec from Hydrobiological Institute, Ceské Budejovice, Czech Republic (Straškrábová et al. 2005).

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This suite of methods allowed the phytoplankton to be studied below the water surface as well as from space. Three gradients, region, depth and season, were studied during three consecutive years (2001 to 2003) within or additionally to the long-term monitoring of the Scientific Research Institute of Biology (SRIB) at the Irkutsk State University (Russia). An attempt was made to predict Lake Baikal’s phytoplankton response to further global warming and possible local nutrient enrichment. The knowledge on the phytoplankton pigment distribution also helped interpretation of the sedimentary pigment sequences, as no information about phytoplankton pigment distribution other than Chla has been available until now.

Sedimentation and preservation through the water column and within the oxidised surface sediment: The second task was to determine transfer fluxes of phytoplankton pigments through the water column and to to determine the type and extent of the degradation occurring after burial in the surface sediment. I hypothesised that the phytoplankton standing crop can be projected backward from recently buried pigments or its degradation products. To test this hypothesis sediment traps were moored for two consecutive years in the southern and northern basins of Lake Baikal, and the sedimentary pigments were also analysed in the oxidised surface sediments from open basin and river inflow sites. It was to be determined how the main phytoplankton groups were represented in the deposited material. The potentials and limits of retrospective studies from sedimentary pigments were also documented.

Variation of fossil phytoplankton pigments: The final task was to reconstruct the phytoplankton development during glacial and interglacial periods. It was hypothesised that climate induced changes of total primary production can be tracked based on fossil Chla and that marker pigments tracked changes of phytoplankton composition. For this purpose, the variation of photosynthetic pigment concentrations and organic carbon contents during the Holocene was analysed at three sites within Lake Baikal (South, Selenga Delta, and North). Furthermore, the first continuous photosynthetic pigment sequence of the Kazantsevo Interglacial (European Eemian, Marine Isotopic Stage MIS 5e) at a resolution of c. 150 yr was established. Coring sites were carefully chosen to obtain uninterrupted sequences of sediments and to avoid impacts by secondary processes such as turbidity currents (cf. Vologina et al. 2003) and tectonic activity (Charlet et al. 2005). Age models were specifically established for each site and for the most recent as well as for the up to 200 kyr old segments (Piotrowska et al. 2004, Demory et al. 2005). Diatom, pollen, and for some core sections also biogenic silica and oxygen isotopes analyses have been performed on these cores within the CONTINENT project (Boës et al. 2005, Demske et al. 2005, Granoszweski et al. 2005, Morley et al. 2005, Rioual and Mackay 2005). Both Holocene and Interglacial pigment sequences were compared to the diatom and pollen sequences and to published climate changes.


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