388
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
used to clearly subdivide the Lama Lake sequence into three major units (Figure 7).
The first unit with non-planktonic taxa comprising over 50% of the total assemblage
coincided in range roughly with DAZ-1. The Ep/Np ratio reached its lowest value (0.46) in the
sequence.
The second unit, including the DAZ-2 to 7, was characterized by Ep/Np ratio values above 1.
Tychoplanktonic taxa occurred with a relative maximum (33.4%) in DAZ-4 concomitant with
the relative maximum of euplanktonic taxa (72.1 %). "True" planktonic taxa, however, reached
a maximum in relative abundance in DAZ-3, terminating a strong increase following the
proportional maximum of non-planktonic taxa (68.4%) in DAZ-1. After a retreat in DAZ-5
(with values approximating 1), the Ep/Np-ratio increased again in DAZ-6 and had values
around 2. A steady decrease to finally equal proportions of euplanktonic and non-planktonic
taxa marked DAZ-7. In the third unit, comprising the DAZ-8 and 9, this general trend continued
towards assemblages dominated by non-planktonic taxa. This trend was interrupted shortly in
DAZ-9 and in the uppermost part the Ep/Np ratio increased, mostly as a result of higher
proportions of tychoplanktonic taxa.
Trophic indications
Nitrogen and phosphorus are the most important nutrients governing algal growth (e.g. Hecky
and Kilham, 1988). Nutrient enrichment leads to well-defined functional responses of the
plankton species present and many of these changes have an autecological basis, as indicated by
numerous phytoplankton monitoring and experimental studies (e.g. Reynolds, 1984; Sommer,
1990; van Donk and Kilham, 1990).
The nutrient ratio and limitations (Tilman et a!., 1982) and the availability of dissolved silica
regulate the composition of the phytoplankton community (species composition). Several
studies showed a ,resource-based competition' under changing nutrient ratios (e.g. Tilman et
a!. 1982; Kilham and Kilham, 1984). For instance, the availability of dissolved silica (e.g.
Bradbury, 1975; Bradbury, 1988; Conley and Schelske 1993; Reavie et a!., 1995) and
phosphorus enrichment (e.g. Carney, 1982; Anderson, 1989; Stoermer, 1993; Sabater and
Haworth, 1995), processes coupled with lowering Si:P ratios during lake eutrophication, are
responsible for a characteristic replacement of plankton species groups. Pappas and Stoermer
(1995) found that inorganic nitrogen enrichment in Lake Huron affected the phytoplankton
species composition as well. Most remarkable was an increase in relative abundance of
Cyclotella comensis.
A number of methods have been developed to infer past trophic conditions in lakes from
diatoms. Some examples include the use of indicator species (Battarbee, 1978; Brugam, 1988),
ratios such as Araphidineae : Centricaceae (Stockner, 1971) or Centrales : Pennales (Nygaard,
1956), and indices (e.g. diatom inferred trophic index by Agbeti and Dickman, 1989). In recent
years, statistical models including ordination methods and weighted-averaging regression and
calibration have been developed. Finally, diatom-based transfer functions enabling the
reconstruction of lake trophic status have been generated (e.g. Agbeti, 1992; Anderson et a!.,
1993; Bennion, 1995; Hall and Smol, 1992; Reavie et a!., 1995).
In order to assess relative changes in past trophic conditions in Lama Lake, the diatom taxa
have been grouped according to the concept of trophic tolerance groups (Hofmann, 1994),
based on the Vollenweider model (Vollenweider, 1979), which describes the freshwater trophic
status as a function of total phosphorus (TP) concentration. Additional data from the literature
(e.g. Stoermer, 1993; Stoermer et a!., 1985; Stoermer et a!., 1996), an extensive compilation in
Voigt (1997) and TP tolerances from Reavie et a!. (1995) and Bennion (1995) were included.
The relative abundance data of the observed diatom taxa are placed in five categories (Figure 8).
The proportion of tolerant taxa is higher in DAZ-l. Their highest proportion in DAZ-8
Land-Ocean Systems in the Siberian Arctic: Dynamics and History
used to clearly subdivide the Lama Lake sequence into three major units (Figure 7).
The first unit with non-planktonic taxa comprising over 50% of the total assemblage
coincided in range roughly with DAZ-1. The Ep/Np ratio reached its lowest value (0.46) in the
sequence.
The second unit, including the DAZ-2 to 7, was characterized by Ep/Np ratio values above 1.
Tychoplanktonic taxa occurred with a relative maximum (33.4%) in DAZ-4 concomitant with
the relative maximum of euplanktonic taxa (72.1 %). "True" planktonic taxa, however, reached
a maximum in relative abundance in DAZ-3, terminating a strong increase following the
proportional maximum of non-planktonic taxa (68.4%) in DAZ-1. After a retreat in DAZ-5
(with values approximating 1), the Ep/Np-ratio increased again in DAZ-6 and had values
around 2. A steady decrease to finally equal proportions of euplanktonic and non-planktonic
taxa marked DAZ-7. In the third unit, comprising the DAZ-8 and 9, this general trend continued
towards assemblages dominated by non-planktonic taxa. This trend was interrupted shortly in
DAZ-9 and in the uppermost part the Ep/Np ratio increased, mostly as a result of higher
proportions of tychoplanktonic taxa.
Trophic indications
Nitrogen and phosphorus are the most important nutrients governing algal growth (e.g. Hecky
and Kilham, 1988). Nutrient enrichment leads to well-defined functional responses of the
plankton species present and many of these changes have an autecological basis, as indicated by
numerous phytoplankton monitoring and experimental studies (e.g. Reynolds, 1984; Sommer,
1990; van Donk and Kilham, 1990).
The nutrient ratio and limitations (Tilman et a!., 1982) and the availability of dissolved silica
regulate the composition of the phytoplankton community (species composition). Several
studies showed a ,resource-based competition' under changing nutrient ratios (e.g. Tilman et
a!. 1982; Kilham and Kilham, 1984). For instance, the availability of dissolved silica (e.g.
Bradbury, 1975; Bradbury, 1988; Conley and Schelske 1993; Reavie et a!., 1995) and
phosphorus enrichment (e.g. Carney, 1982; Anderson, 1989; Stoermer, 1993; Sabater and
Haworth, 1995), processes coupled with lowering Si:P ratios during lake eutrophication, are
responsible for a characteristic replacement of plankton species groups. Pappas and Stoermer
(1995) found that inorganic nitrogen enrichment in Lake Huron affected the phytoplankton
species composition as well. Most remarkable was an increase in relative abundance of
Cyclotella comensis.
A number of methods have been developed to infer past trophic conditions in lakes from
diatoms. Some examples include the use of indicator species (Battarbee, 1978; Brugam, 1988),
ratios such as Araphidineae : Centricaceae (Stockner, 1971) or Centrales : Pennales (Nygaard,
1956), and indices (e.g. diatom inferred trophic index by Agbeti and Dickman, 1989). In recent
years, statistical models including ordination methods and weighted-averaging regression and
calibration have been developed. Finally, diatom-based transfer functions enabling the
reconstruction of lake trophic status have been generated (e.g. Agbeti, 1992; Anderson et a!.,
1993; Bennion, 1995; Hall and Smol, 1992; Reavie et a!., 1995).
In order to assess relative changes in past trophic conditions in Lama Lake, the diatom taxa
have been grouped according to the concept of trophic tolerance groups (Hofmann, 1994),
based on the Vollenweider model (Vollenweider, 1979), which describes the freshwater trophic
status as a function of total phosphorus (TP) concentration. Additional data from the literature
(e.g. Stoermer, 1993; Stoermer et a!., 1985; Stoermer et a!., 1996), an extensive compilation in
Voigt (1997) and TP tolerances from Reavie et a!. (1995) and Bennion (1995) were included.
The relative abundance data of the observed diatom taxa are placed in five categories (Figure 8).
The proportion of tolerant taxa is higher in DAZ-l. Their highest proportion in DAZ-8