Journal of Paleolimnology 8: 15-26, 1993. 9 1993 Kluwer Academic Publishers, Printed in Belgium. 15. Stratigraphy of fossil pigments and Cladophora and its ...
Journal of Paleolimnology 8: 15-26, 1993. 9 1993 Kluwer Academic Publishers, Printed in Belgium.
15
Stratigraphy of fossil pigments and Cladophora and its relationship with deposition of tephra in Lake M vatn, Iceland Arni Einarsson ~,2, Hlynur 0skarsson ~ & Haflidi Haflidason 3 l Institute of Biology, University of Iceland, Grens6svegur 12, 108 Reykjavlk, Iceland; 2Mf;vatn Research Station, c/o Nature Conservation Council, P.O. Box 5324, 125 Reykjavik, Iceland," 3Department of Geology, sec. B, University of Bergen, AllOgt. 41, 5O07 Bergen, Norway Received 24 February 1992; accepted 18 September 1992
Key words: Fossil pigments, Cyanobacteria, myxoxanthophyll, palaeolimnology, Lake M~catn, Iceland, tephra, Cladophora
Abstract
Fossil plant pigments and Cladophora fragments were analyzed in a 6.53 m long sediment profile from Lake M~vatn, Iceland, covering most of its history of about 2300 years. A decrease in myxoxanthophyll (produced by Cyanobacteria) with time and an increase in the benthic Cladophora reflects a gradual shift from planktonic to benthic primary production as water depth is reduced (to 3.15 m at the core site) because of sediment accumulation. Two periods of relatively high concentrations of myxoxanthophyll coincided with relatively frequent deposition of tephra (volcanic ash) but did neither conform with tectonic activity which might have changed the water level nor the available climatic record. Sediment depth, tephra content and percent undegraded chlorophyll (an indicator of pigment preservation) together could explain 56.7~o of the variation in myxoxanthophyll. It is hypothesized that temporary increases in myxoxanthophyll resulted from periodic nutrient enrichment by flesh tephra deposited in the watershed or because tephra increased the erosion of organic soil in the water catchment area. Fluctuations in Cladophora show an inverse relationship with myxoxanthophyll in the uppermost 4 m of the core, and may result from a shading effect of planktonic Cyanobacteria on the phytobenthos or competition for nutrients released by the bottom sediments.
Introduction
Fossil plant pigments have been used increasingly to reconstruct the trophic history of lakes (Vallentyne, 1956; Ztillig, 1981, 1982; Guilizzoni et aL, 1983; Engstrom etal., 1985; Rybak & Rybak, 1985; Leavitt et al., 1989; see also Douglas et al., 1978). Most often fossil pigments have been used to trace the relatively recent history of eutrophi-
cation. This has been done successfully by analyzing pigments derived from Cyanobacteria (blue-green algae) (see ZOllig, 1961; Swain, 1985), which tend to form blooms in lakes with high loading of nutrients, especially phosphorus (see Schindler, 1977; Lean etal., 1978; Lundgren, 1978; Pick & Lean, 1987; Cullen & Forsberg, 1988). Lake Mbvatn, North Iceland is a large, natu-
16 rally eutrophic lake, in which cyanobacterial blooms occur regularly. Situated in an active volcanic area in high latitudes (65 o 35' N), the lake is exposed to relatively large environmental variation, both climatic and geological (tectonic movements and tephra (volcanic ash) fallout). At the same time human impact has been minimal and the hydrological environment stable. The lake is shallow, but its sediment is relatively thick. Hence, sedimentation has significantly reduced the lake volume (Einarsson & Haflidason, 1988). Recent monitoring programs and historical data show that plant and animal populations of Lake M2~vatn exhibit large fluctuations on time scales varying from a few years to decades or even centuries (Gardarsson et al., 1988), but it has not been possible to link the long term biological record to variation in chemical or physical characteristics. Palaeolimnological studies have revealed considerable changes in the lake's biota during the past 2000 years (Einarsson, 1982; Einarsson & Haflidason, 1988). The aim of the present study is to reconstruct the long term plant succession in the main basin of Lake M2~vatn, primarily by using fossil plant pigments and Cladophora fragments. The samples analysed were taken from a 653 cm long sediment core covering most of the 2300 years history of the lake. Analyses of crustacean and chironomid remains in the core have been published elsewhere (Einarsson & Haflidason, 1988). We examine the fossil record of plant pigments and fragments of Cladophora with respect to changes in water depth, climate and nutrient loading.
ment from the former lake have been found in lava formations on the shores of the present lake, and also underlying the sediment formed in the present lake (Einarsson, 1982). The geology of the area is described by Tborarinsson (1979) and S~emundsson (1991). The lake is divided into two main basins, the North basin (8.5km 2) and the South basin (28.2 km 2) (Fig. 1). Extensive areas in the South basin are between 3 and 4 m deep, maximum water depth is about 4.0 m. In the North basin a large bottom area has been dredged for diatomite, and the depth has increased from about 1 m to 2-5.5 m. Water enters the lake almost entirely from springs on its east shore. Most of the springs are cold (about 6 ~ but springs in the North basin are warmer (up to 30 oC) (61afsson, 1979a). The watershed (about 1500 km 2) is a highly permeable, sparsely vegetated lava terrain, partly covered with aeolian sand. Average duration of ice cover is about 190 days per annum (Rist,
Lake M~vatn Lake M~rvatn (37 km 2) was formed about 2300 years ago following a major volcanic eruption taking place in the vicinity of the lake area (Thorarinsson, 1951; Einarsson, 1982; Saemundsson, 1991). The lake rests in a shallow depression in an extensive lava field produced during the eruption. Another lake existed at the same site before the eruption, but it appears to have been wiped out by the lava. Traces of much disturbed sedi-
Fig. l. Position of Lake M~rvatn and its watershed (shaded) in Iceland and the location of the sediment core.
17 1979). The water column is thoroughly mixed during the summer months, but thermal stratification develops locally in mid winter (Olafsson, 1979a). External loading of phosphorus and nitrogen has been estimated to be 1.5 g/m 2 year and 1.4 g/m 2 year, respectively (Olafsson, 1979b), but nitrogen fixation by Cyanobacteria and internal loading from sediments are important in the total nutrient budget. Primary production has been estimated to be 3800kcal/m 2 year (J6nasson,1979), of which 600kcal/m 2 year come from phytoplankton. Most of the primary production therefore takes place on the bottom of the lake, mainly by diatoms, but also by Cladophora aegagropila which covers a large part of the South basin floor. Vascular plants (mainly Myriophyllum spp. and Potamogeton spp.) cover considerable areas in other parts of the lake (Gardarsson et al., 1987). Cyanobacterial blooms, formed by Anabaena flosaquae, occur regularly, but may fail in some years. During the last 22 years Anabaena blooms failed in 7 years. Oscillatoriaceae appear to be only represented by Oscillatoria limnetica in the plankton of Lake M~vatn (J6nasson & Adalsteinsson, 1979). The average sediment accumulation rate has been estimated at 1.5-2.8mm/year or 500600gdw/m 2 year (Olafsson, 1979b; Einarsson etal., 1988). Average sediment thickness in the South basin is about 4.3 m (Lindal, 1959). The sediment is diatomaceous gyttja with numerous thin inorganic layers, visible only in X-ray photographs, but thicker (1-2 cm) layers occur. Most of the inorganic layers are made up of tephra (Einarsson etal., 1988). Diatom frustules comprise about 55 ~o and minerogenic material (mostly tephra) about 30?o of the dry weight of the sediment in the North basin (Lindal, 1959). Wind induced resuspension of sediment occurs frequently, and bioturbation is induced mainly by chironomid larvae and diving ducks. Mixing of sediments has not been measured directly, but based on seston concentration in the water column during storms and penetration depth of the animals in the sediment, it would be unrealistic to expect time resolution to be finer than about 40
years, corresponding to about 8-10 cm of sediment (Einarsson et al., 1988). Tephra is mainly deposited in the lake during volcanic eruptions as air fallout. Several factors operate to mix the tephra particles with the sediment between tephra layers. These include wind induced wave action, bioturbation, sinking in the sediment due to density difference and blowing of tephra from the surroundings (see also Ruddiman & Glover, 1972 and Thompson et al., 1986). The local human population has until recently been composed of farmers (10-15 farms) subsisting on sheep farming and fishing for trout (Salmo trutta) and arctic charr (Salvelinus alpinus). In the last two decades the human population has grown as a result of industrialization (diatomite production) and increased tourism. Methods
Sampling The core was retrieved from the sediment of the South basin in January 1985 (Fig. 1). The core was taken through the ice with a 'Russian corer' (Jowsey, 1966) at a water depth of 3.15 m. Each core segment was 50 cm long and 5 cm in diameter. The core segments were frozen at the coring site and stored in the deep freeze. The core stratigraphy is described by Einarsson etal. (1988). The following samples were taken from the core: (1)Minerogenic layers (as seen on X-ray films); (2) At 10 cm intervals: 2 cm 3 for measuring water content, loss on ignition and manganese; 2 cm 3 for counting microfossils other than diatoms (see Einarsson & Haflidason, 1988); 6 cm 3 for pigment analyses and about 1 cm 3 for diatom analyses.
Pigments Four types of pigments were analysed in the present study: Chlorophyll derivatives (mainly phaeophytin and phaeophorbide) (665 nm); total carotenoids (448 nm) and the xanthophylls myxoxanthophyll and oscillaxanthin.
18 Myxoxanthophyll is only produced by Cyanobacteria (bluegreen algae) (Z~illig 1981; Swain 1985) and oscillaxanthin is primarily found in the Oscillatoriaceae (Hertzberg et al., 1971; Ztillig, 1981). The methods used for extracting and measuring the pigments were described by Swain (1985). Chlorophyll derivatives were measured at 665 nm and expressed as absorbance per g organic matter where 1 equals an absorbance of 1.0 in a 10 cm cuvette in 100 ml solution. Native (undegraded) chlorophyll was estimated after acidification of the sample. Total carotenoids were determined at 448 nm and expressed in the same way as the chlorophyll derivatives. As pointed out by Swain (1985) 'total carotenoids' is a misnomer since it does not include myxoxanthophyll or oscillaxanthin. Oscillaxanthin and myxoxanthophyll were determined at 412, 504 and 529 nm, which are the absorption peaks for phorbin, myxoxanthophyll and oscillaxanthin, respectively. Concentrations of myxoxanthophyll and oscillaxanthin were calculated from equations given by Swain (1985).
Cladophora Cladophora remains (Fig. 2) were counted on microscope slides prepared from 2 cm 3 of fresh sediment. The sample was deflocculated in 10~o KOH at 60 ~ for 24 hours' sieved with a 63 #m brass sieve (Endecotts) and prepared in gelatin glycerin stained with gentian violet. Each cell was counted as a unit, or, if cell boundaries were not preserved, the cell length was estimated as 5 times the cell width. This estimate was based on observations of living material.
the Mn content of the sediment. Before the core was opened the core segments were X-ray photographed and the films were analyzed with a densitometer at 0.5 cm intervals. The densitometer readings were corrected for differences in exposure between films and attenuation of the X-rays towards the ends of the core segments. According to Einarsson etaL(1988), tephra particles comprise the bulk of the minerogenic material in the sediment of Lake M)vatn. Manganese is about 0.2~ by weight of the tephra deposited in the area (Einarsson etal., 1988). Other significant local Mn-sources are not known. We assume that most of the Mn in the sediment is associated with tephra particles and use manganese as an indicator of the tephra content (see also Olafsson, 1979a). Samples for Mn analysis were taken from the core after distinct minerogenic layers had been removed. Hence the samples represent the fraction of tephra which was well mixed with the organic sediment. Mn was measured colorimetrically (525 nm) as potassium permanganate (KMnO4) after bulk decomposition of the ignited sample with sulphuric acid (H2SO4) and hydrofluoric acid (HF) and oxidation in potassium periodate (KIO4). A tephrochronological record was established by comparing the geochemical and the petrographical signature of the tephra layers in the core with the signature of dated tephra layers traced into the lake area (Einarsson et al., 1988).
Results
Myxoxanthophyll and oscillaxanthin Organic matter and tephra content Loss on ignition was defined as the mass reduction of dried sediment after one hour of combustion at 550 ~ (H/~kanson & Jansson, 1983). Minerogenic matter was estimated by two methods: (1) by a densitometric analysis of X-ray photographs of the core and (2)by measuring
The average concentration of myxoxanthophyll in the sediment core was 432.9/~g g - 1. Two zones had relatively high concentrations: at 4.0-5.5 m depth (relative to water surface) and 6.7-8.0 m (Fig. 3). Other depth intervals had relatively low concentrations. By far the highest concentration (1208 #g g l) occurred at the lower end of the core.
19
Fig. 2. Remains of Cladophora from the M~,vatn sediment. No. 2 also has a carapace of the chydorid cladoceran Chydorus sphaericus. Illumination is phase contrast except in no. 5 where normal illumination was used. Scale bars are 50 #m long.
20 OSCILLAXANTHIN
MYXOXANTHOPHYLL
CHLOROPHYLL DERIVATIVES
TOTAL CAROTENOIDS
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