Kienel: Late Weichselian to Holocene Diatom Succession in a Sediment Core
391
in DAZ-4 (45 %), in both cases followed by a strong decrease. From DAZ-6 upwards, they
amount on average to 30 % of the assemblages. Taxa preferring circumneutral conditions
generally prevailed in the diatom assemblages in Lama Lake. Comparatively low percentages
(20 to 30 %) were distinctive in the first recorded assemblages only, coinciding with the
maximum in alkaliphilous taxa. Their proportion tended to increase until DAZ-6 where they
occasionally formed 60 % of the assemblage. Throughout the zones DAZ-7 to 8 the percentage
was relatively stable around 50 %. From the upper DAZ-8 on, the proportion of circumneutral
taxa tended to decrease to 35 %. Taxa classified as acidophilous contributed to the assemblages
only in DAZ 8 and 9, comprising about 3 %.
Discussion
Diatom occurrence
No diatoms have been found in samples below a sediment depth of 6.68 m. Judging from the
basic ecological requirements of diatoms, there are some factors that may inhibit diatom
establishment, but there are counter-indications for some.
l) Light limitation due to a high particle content in the water column caused by melt-water
inflows and turbulence during thawing periods (Patrick, 1977; Sabater and Haworth, 1995).
2) Light and temperature limitation: due to snow-covered (or very thick) ice cover (Doubleday
et aI., 1994), contradicted by the presence of pollen in the whole section (Hahne and Melles,
this volume; Hahne and Melles, 1997).
3) pH-limitation: A pH above 8.5 is considered to be critical for diatom establishment
(Patrick, 1977). Terrestrial input of weathering products from the basaltic bedrock of Lama
Lake and its catchment area can increase lake water alkalinity especially during times of active
runoff.
4) Nutrient limitation due to a strong restriction of runoff processes and weathering and
consequently a restriction of external nutrient supply during cold an dry conditions.
On the other hand, diatom preservation is strongly affected by dissolution processes.
Dissolution of the frustules starts immediately after the death of the organism in the water
column, since lake water is undersaturated in dissolved Si02. The degree of dissolution is
found to increase with raising pH and temperatures (Lewin, 1961), their dwelling time in the
water column, a lower silicification of the frustules and an increasing surface to volume ratio
(Hurd and Birdwhistell, 1983). Dissolution affects diatom assemblages in such a way that
small-sized forms with a low SAIV ratio and strongly silicified parts of the larger forms are left
(Barker, 1992). Post-depositional dissolution has been related to for example turbulenceinduced intense mixing of the water body accompanied by oxygenation of the bottom waters
(McMinn, 1995), dissolved Si-diffusion rates (Rippey, 1983) and the silica content of the pore
water (Flower, 1993).
Corroboration for dissolution's primary role in causing the lack of diatoms at least in the
lower part of the diatom-bearing sequence is provided by the poor preservational stage of the
earliest recorded diatoms. The predominant diatom species in the first assemblage (e.g.
Cyclotella rossii, Cymbella silesiaca, Fragilaria construens, Nitzschia angustata, see Figures.
3 A and B) show alkaliphilous preferences, have small robust forms, and thus may be
considered a dissolution-reduced assemblage (Barker, 1992).
After compiling and evaluating the above data, it appears that a combination of two processes
which are difficult to disentangle may explain the absence of diatoms: I) Non-establishment due
to light deficiency coupled with low temperature, high pH and a lack of nutrients, and 2) nonpreservation as a result of dissolution originating from pore water chemistry (high alkalinity,
undersaturation in silica) andlor turbulence-induced oxygenation processes.
391
in DAZ-4 (45 %), in both cases followed by a strong decrease. From DAZ-6 upwards, they
amount on average to 30 % of the assemblages. Taxa preferring circumneutral conditions
generally prevailed in the diatom assemblages in Lama Lake. Comparatively low percentages
(20 to 30 %) were distinctive in the first recorded assemblages only, coinciding with the
maximum in alkaliphilous taxa. Their proportion tended to increase until DAZ-6 where they
occasionally formed 60 % of the assemblage. Throughout the zones DAZ-7 to 8 the percentage
was relatively stable around 50 %. From the upper DAZ-8 on, the proportion of circumneutral
taxa tended to decrease to 35 %. Taxa classified as acidophilous contributed to the assemblages
only in DAZ 8 and 9, comprising about 3 %.
Discussion
Diatom occurrence
No diatoms have been found in samples below a sediment depth of 6.68 m. Judging from the
basic ecological requirements of diatoms, there are some factors that may inhibit diatom
establishment, but there are counter-indications for some.
l) Light limitation due to a high particle content in the water column caused by melt-water
inflows and turbulence during thawing periods (Patrick, 1977; Sabater and Haworth, 1995).
2) Light and temperature limitation: due to snow-covered (or very thick) ice cover (Doubleday
et aI., 1994), contradicted by the presence of pollen in the whole section (Hahne and Melles,
this volume; Hahne and Melles, 1997).
3) pH-limitation: A pH above 8.5 is considered to be critical for diatom establishment
(Patrick, 1977). Terrestrial input of weathering products from the basaltic bedrock of Lama
Lake and its catchment area can increase lake water alkalinity especially during times of active
runoff.
4) Nutrient limitation due to a strong restriction of runoff processes and weathering and
consequently a restriction of external nutrient supply during cold an dry conditions.
On the other hand, diatom preservation is strongly affected by dissolution processes.
Dissolution of the frustules starts immediately after the death of the organism in the water
column, since lake water is undersaturated in dissolved Si02. The degree of dissolution is
found to increase with raising pH and temperatures (Lewin, 1961), their dwelling time in the
water column, a lower silicification of the frustules and an increasing surface to volume ratio
(Hurd and Birdwhistell, 1983). Dissolution affects diatom assemblages in such a way that
small-sized forms with a low SAIV ratio and strongly silicified parts of the larger forms are left
(Barker, 1992). Post-depositional dissolution has been related to for example turbulenceinduced intense mixing of the water body accompanied by oxygenation of the bottom waters
(McMinn, 1995), dissolved Si-diffusion rates (Rippey, 1983) and the silica content of the pore
water (Flower, 1993).
Corroboration for dissolution's primary role in causing the lack of diatoms at least in the
lower part of the diatom-bearing sequence is provided by the poor preservational stage of the
earliest recorded diatoms. The predominant diatom species in the first assemblage (e.g.
Cyclotella rossii, Cymbella silesiaca, Fragilaria construens, Nitzschia angustata, see Figures.
3 A and B) show alkaliphilous preferences, have small robust forms, and thus may be
considered a dissolution-reduced assemblage (Barker, 1992).
After compiling and evaluating the above data, it appears that a combination of two processes
which are difficult to disentangle may explain the absence of diatoms: I) Non-establishment due
to light deficiency coupled with low temperature, high pH and a lack of nutrients, and 2) nonpreservation as a result of dissolution originating from pore water chemistry (high alkalinity,
undersaturation in silica) andlor turbulence-induced oxygenation processes.