The Late Quaternary palaeoenvironmental history of a presently deep freshwater lake in east-central Alberta, Canada and palaeoclimate implications

PAI O ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178

The Late Quaternary palaeoenvironmental history of a presently deep freshwater lake in east-central Alberta, Canada and palaeoclimate implications Michael Hickman a, Charles E. Schweger b "Devonian Botanic Garden and Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada b Department of Anthropology, University of Alberta, Edmonton, Alberta, T6G2H4, Canada Received 8 November 1994; revised and accepted 4 September 1995

Abstract

The palaeoenvironmental history of Moore Lake, Alberta, Canada (54°30'N; l10°30'W) has been investigated through analysis of pollen, diatoms, total chrysophyte stomatocysts, and sedimentary pigments, including the bluegreen algal xanthophylls, oscillaxanthin and myxoxanthophyll. The initial lake was shallow. Vegetation of the region was treeless and no doubt reflected the unique late-glacial pioneering environment. Planktonic diatoms characteristic of a deep, eutrophic lake were dominant by ca. 11400 yr B.P., when a birch-spruce-dominated Boreal Forest developed. However, in response to early Holocene warmth and aridity the lake had become saline by ca. 10,000 yr B.P. when Chaetoceros spp. and Cyclotella caspia became dominant, while an open parkland/grassland vegetation had developed, subject to increased fire frequency. Freshwater taxa reappeared between ca. 9100 and ca. 8400 yr B.P. providing evidence of a wetter period. However, after ca. 8400 yr B.P. planktonic saline taxa returned as dominants. By ca. 6200 yr B.P. birch-spruce forests were once again developed in the area in response to more effective precipitation. Between ca. 5800 and 4000 yr B.P. oscillations between fresh and saline diatom taxa occurred suggesting some aridity reversals. During the last ca. 4000 years planktonic eutrophic diatom taxa dominated. Diatom numbers were initially low but increased ca. 11,400 yr B.P. and remained high until ca. 10,000 yr B.P. They were minimal during the saline interval. Afterwards as the lake freshened numbers of diatoms increased and several large peaks occurred, perhaps in response to internal nutrient loading coupled with increased runoff and erosion. Oscillaxanthin concentrations were initially high, but during the saline interval they became undetectable, except during the freshwater interlude, and then, as the lake became fresh large maxima occurred. In contrast, myxoxanthophyll concentrations were high during this saline interval. Large myxoxanthophyll peaks occurred simultaneously with those of oscillaxanthin as the lake freshened. Overall lake palaeoproduction levels during the Holocene appear to have altered little. The Moore Lake record demonstrates short-lived palaeoclimatic changes of unknown cause superimposed upon the broad trends, and the value of the palaeolimnological record which can be much more sensitive than fossil pollen vegetation reconstruction in demonstrating climate history. Results are compared with others for western Canada and the Great Plains.

0031-0182/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0031-0182(95)00089-5

162

M. Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

1. Introduction

Holocene palaeoenvironmental history of the northern Great Plains has focused upon midHolocene altithermal drought (Antevs, 1955) or hypsithermal warming (Deevey and Flint, 1957). For Alberta, both have been invoked to explain a wide range of phenomena including vegetation patterns (North, 1976), and archaeological sequences (Reeves, 1973). However, warmer than present conditions appear to have affected northwestern Canada as early as the late glacial (Ritchie et al., 1983), and dated lake core studies in Alberta indicate significant aridity during the early to midHolocene (Schweger and Hickman, 1989). The cause has been attributed to the summer insolation maximum ca. 9000 yr B.P. Moore Lake, Alberta, now provides a record, largely based upon changing salinity, of a much more complex Holocene palaeoenvironmental history. Closed lakes or those having only minor inflowing and outflowing streams in arid or semi-arid regions, respond sensitively to changes in effective precipitation with concentration or dilution of dissolved salts occurring (e.g., Schweger and Hickman, 1989; Fritz et al., 1991). Small catchments only serve to make such lakes even more sensitive. Some ultimately become saline, while water levels just fall in others but not enough to cause major chemical changes (Hickman and Schweger, 1991b). Such changes, from fresh to saline water, are reflected in the composition of the lacustrine microfossils, particularly diatoms. Thus, these systems become sensitive recorders of palaeoclimatic and palaeohydrological changes (Street-Perrot and Roberts, 1983; Schweger and Hickman, 1989). Moore Lake, situated in east-central Alberta where the present-day climate is drier and more arid compared to the Foothills and central regions of the province (Williams and Masterton, 1983), was saline between ca. 10,000 and ca. 6000 yr B.P. as indicated by the presence of both Ruppia pollen and saline diatom taxa (Hickman and Schweger, 1993). The lake became saline because of drought caused by high summer insolation induced by orbital fluctuations (Hickman and Schweger, 1989).

As well as documenting variations in Holocene palaeoclimate using both Ruppia pollen and diatoms (Hickman and Schweger, 1993), we investigated the palaeoenvironmental record in detail to determine (1) vegetational and lake histories, (2) how oscillations between fresh to saline conditions affected (a) the succession of the diatom species, (b) palaeoproduction through analysis of sedimentary chlorophyll a and total carotenoid derivatives, and (c) changes in the blue-green algal populations through analysis of oscillaxanthin and myxoxanthophyll concentrations, and how significant fluctuations between saline and freshwater could impact upon this component of the biota, and (3) palaeoclimate implications are discussed.

2. Description of the region and the lake

Moore Lake (54°30'N; l10°30'W; 550 m a.s.1.) is a headwater lake in east-central Alberta (Fig. 1) possessing a catchment only four times its surface area. A morphometric map of the lake and major catchment features are presented in Fig. 2a,b. A description of lake characteristics is given in Hickman and Schweger (1993). The lake itself has a surface area of 9.28 km 2 and a maximum depth of 26.0 m. The dominant diatom taxa include Fragilaria crotonensis Kitton, Stephanodiscus niagarae Ehr., S. hantzschii Grun., Cyclotella radiosa (Grun.) Lemm and Synedra acus v. radians (Kutz.) Hust. Of the Cyanobacteria, species of Oscillatoria are most prominent (e.g., Oscillatoria agardhii Gomont, O. limosa Agardh, O. limnetica Lemm., and O. tenuis Agardh). Today Cyanobacteria comprise up to 20% of the phytoplankton standing crop of Moore Lake (Mitchell and Prepas, 1990), with Oscillatoria spp. the dominant taxa.

3. Methods

The lake sediment was cored using a 5 cm diameter modified Livingstone piston corer (Cushing and Wright, 1965) from a winter ice surface (Fig. 2a) (Hickman and Schweger, 1993). Two cores were retrieved from the deep eastern

M. Hickman, C. E. Schweger/ Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178

163

i

.!

Alberta !

!

L_ t


BOREALFOREST ForestZone " ~ Lokes Ogre) of

Ma

.

greshwoter

(200-500

%.

i

~'~

'/k~MONTANE~Fe

[

Lakes

Zone "~ ( 500 - 120 CO0

SOhn¢

ppm)

ASPEN PARKLAND

BOREAL

"O ESTt'' \

pro*tieLoke~

k,

GRASSLAND

0

zuu

Ill

Fig. 1. Locations of Moore Lake, and the other Alberta lakes to which reference is made in the text (F= Fairfax Lake; W= Lake Wabamun; 1= Lake Isle; B = Buffalo Lake; Ma = Mariana Lake). (a) Vegetation zones (solid lines; Government of Alberta and the University of Alberta, 1969) and lake zones (dashed lines); Northcote and Larkin, 1963), and (b) mean annual precipitation (contours are in ram). (Government of Alberta and the University of Alberta).

basin (M-1 and M - 2 ) (Fig. 2a). A third core ( M - 3 ) was taken from the shallower western basin. M-1 and M-3 provided material for pollen analysis and

radiocarbon dates, while M-2 provided subsamples for siliceous microfossils and sedimentary pigments.

M. Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

164

Coring Site M - 1 : M - 2 M-3

0

,.~

3 km

I

I

(b)

Forest Wetlands 0 I

3kin

I~ ~

Agricultural/open Resident ial/rec teat ional

i

Fig. 2. (a) Morphometric map of Moore Lake (depth contours are in m) and (b) features characterizing the Moore Lake catchment. Coring sites are shown in both (a) and (b).

diagram was divided into five assemblage zones by visual inspection for convenience in description. Total chrysophyte stomatocysts were determined simultaneously with the diatoms. TAP and TC pigments were determined as described in Hickman et al. (1990). Results are expressed as sedimentary pigment units g 1 organic matter (the latter determined as loss-on-ignition where the 1 cm 3 subsample is ignited at 550°C for 1 hour). To provide an independent measure of pigment preservation the proportion of chlorophyll not degraded to breakdown products was determined by acidifying the extract as described by Swain (1985). Blue-green algal carotenoids were determined as described in Hickman and Schweger (1991a). Results are expressed g-1 organic matter. Pollen samples were prepared using standard methods (Faegri and Iversen, 1975). A relative percentage pollen diagram was constructed from a sum of total pollen, but plotting only the ten most important taxa, using the lower portion of M-3 and all of core M-1 (Fig. 3). Charcoal fragments and pyrite frambroids were calculated using the total pollen as sum. The pollen record was subdivided into local pollen assemblage zones by visual inspection for convenience in description Radiocarbon analysis of the organic lacustrine sediments provided chronological control. All dates cited are uncorrected for atmospheric 14C flux, and therefore, should read as "radiocarbon years B.P.". The ages of diatom and pollen zones were extrapolated from the 14C results.

3.1. Core and chronology In the laboratory each core section was remeasured and stratigraphic features were described and used to match cores M-1 and M-2. Afterwards, l cm 3 subsamples were removed at 5 - 1 0 c m intervals for pollen, loss-on-ignition, diatom and sedimentary pigment analyses (chlorophyll--TAP and total carotenoids--TC). Approximately 10 g wet sediment subsamples were taken at 10cm intervals for analysis of the blue-green algal carotenoids. Subsamples for diatom analyses followed procedures outlined in Hickman et al. (1984) and Hickman and Klarer (1981 ). A relative percentage diatom diagram was constructed of the important taxa from the total diatom sum (Fig. 4). The

Core M-2 measured 985 cm, some 75 cm longer than M-1. The difference in lengths of M-I and M-2 was the result of loss of sediment from the core barrel as it was brought to the ice surface. Both were laminated throughout except for depth intervals between 85 and 100, 350-370, 400 and 445,475 and 500, 590 and 610, and 910 and 985 cm (Hickman and Schweger, 1993). The upper sediments were predominantly laminated clay-gyttja, while the lower unlaminated section comprised clays. Seven sediment samples taken from M-l, and three from M-3, were dated by radiocarbon analysis (Table 1). Only the lowermost date from

M. Hickman, CE. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology123 (1996) 161-178

^o%°

b,_ L

:--'_

_

-L

165

/ 3 ~'o

~__~___ M~:_6.• 2 1 4 0 ± 130 • 2380 + 140

,"

'rD

~

Core M-1

M-5

• 4470 ± 110

P

p,

'

.

.

•

I

!

t

.

.

.

.

rob.

~ - !

,

-'-'~LI

• 5800 + 80

I

I I~ r_ L

M-4 9250 + 80

l--- 1

~Mi-~;-2

Core

10200 + 160 ' 11300+ 170 • 11830 ± 330

M-3 0

20

40

60

80

0

20

40

0

20

400

0

0

0

20

40

600

0

0

0

5000

1000

2000

Percentage S u m = total Pollen

Fig. 3. A summary percentage pollen diagram for the basal region of core M-3 retrieved from the western basin of the lake, and M-1 retrieved from the deeper eastern basin.

Table 1 Radiocarbon dates for the lacustrine sediments samples taken from the cores (GSC: Geological Survey of Canada; AECV: Alberta Environmental Centre, Vegreville) Core

Depth (cm)

Date (yr B.P.)

Laboratory number

25-30 100-105 400-405 600-605 765-770 850-855 895-900

2140 ___130 2380 _+ 120 4470 _+ 110 5850 _+80 9250 _+80 10,200 _+160 11,300 _+ 170

GSC-2900 GSC-2907 GSC-2910 GSC-2870 GSC-2858 GSC-2921 GSC-2856

357.5-362.5 417.5 422.5 517.5-522.5

6420+ 160 7650+ 130 11,830+330

AECV-409C AECV-410C AECV-411C

M-3

core M-3 will be discussed here. The dates from the lowermost level sampled in the core M-2 (895-900cm) and core M-3 (517.5-522.5cm) demonstrate ages in excess of 11,300 and 11,830 yr B.P., respectively, hence the entire Holocene period is included in these lacustrine records.

3.2. Pollen and vegetation history The summary pollen diagram (Fig. 3) accounts for about 90% of the total frequency for any sample horizon.

Zone

1:

Artemisia-Cyperaceae-Gramineae,

Core M-3, 558-522 cm; > 11,830 yr B.P. Pollen is rare over the lowest 20 cm, and some, certainly Pinus, and perhaps Picea, may have been rebedded. Reliable assemblages above 538 cm are dominated by Artemis&, Cyperaceae and Gramineae. Salix is the only shrub/arboreal taxon recorded. Charcoal fragments are most abundant over this portion of the core. N o pyrites were recorded. The sharp increases in Picea pollen, 890 cm in core M-I, and 516 cm in core M-3, are used to correlate Moore Lake pollen records M-1 and M-3. Treeless vegetation was initially established on the stagnant ice moraine of this region following deglaciation (Fenton and Andriashek, 1983). Whether this vegetation should be considered grassland or tundra, or as more likely the case, a reflection of a unique late-glacial pioneering envi-

166

M. Hiekman, C. E. Schweger/ Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996~ 161 178

ronment, is still a matter of discussion (Hutton et al., 1994). Zone 2." Picea Betula, Core M-l, 910-825 cm; ca. 11,300-ca. 9900 yr B.P. This zone begins with the abrupt rise in Betula pollen followed by Picea which dominates the rest of the zone. Artemisia;Cype'raceae, and Gramineae decline in abundance, while Salix and Populus reach their maximum values. AInus appears in small amounts. Charcoal is present, while pyrites are abundant through this zone. Therefore, catchment vegetation underwent a rapid transformation between ca. 11,600 and 11,400 yr B.P. with the arrival of Betula and Picea, and the development of a spruce-birch-dominated Boreal Forest. Since Pinus and Alnus were not components of this late glacial forest, it probably has no analogues in the modern Boreal Forest communities of western Canada (MacDonald and Ritchie, 1986). Further the presence of Populus, Salix and relatively high NAP suggest that this was an open forest environment. Zone 3: Betula-Picea, 825-745 cm; ca. 9900-8800 yr B.P. Betula pollen rises sharply at the beginning of the zone, while Picea declines in importance. Pinus pollen gradually increases and Populus frequencies are suppressed over this interval. Charcoal persists and pyrites show two brief peaks. This was a period of rapid vegetation change in the Moore Lake area. Betula became an important element along with Picea in forming open mixed forest communities. The increasing frequencies of Pinus pollen suggest that long distant Pinus populations were advancing toward the Moore Lake area. Zone 4: NAP-Populus, 745-610 cm; ca. 8800-ca. 6200 yr B.P. Populus increases and remains abundant through the zone. Cyperaceae, Gramineae and Artemisia pollen all reach maximum values, while Ruppia pollen is only found in this zone. Pinus and Picea exchange dominance. Picea reaches minimum values in the middle of this zone, where a Pinus peak occurs. Betula rises to a peak at the top of the zone. Charcoal persists with a peak occurring at the top of the zone, while pyrite values drop to trace amounts.

The declines in Betula, then Picea pollen abundance, and the rise of Populus and NAP taxa to modern levels (MacDonald and Ritchie, 1986) suggest that parkland and grassland vegetation replaced forests in the Moore lake region. Zone 5." Pinus Picea, 610 50 cm; ca. 6200 ca. 2200 yr B.P. At the beginning of zone 5 Pinus pollen rises to modern values and remains dominant. Picea displays no clear trends, while Populus persists at low values over the zone and NAP taxa decline in abundance and remain relatively unchanged through the zone. Alnus reaches its greatest abundance, while Betula exhibits a slow trend to higher values near the top of the zone. Salix remains relatively unchanged throughout. Charcoal exhibits three brief peaks, while pyrites once again become abundant above 494 cm. Betula Picea forests once again developed in the area and were quickly changed by the local presence of Pinus, and subsequently, Alnus. The greater abundance of Alnus points, perhaps, to the development of moist peatlands in the region. Over the following 4000 5000 yr, regionally stable southern Boreal Forest existed around Moore Lake. Zone 6." Betula-Pinus, 50-0 cm; ca. 2200 yr B.P. to the present Pollen frequencies change quickly. Betula first rises, then drops as Picea and then Pinus pollen frequencies rise sharply. Alnus, Populus, Salix, Cyperaceae and Artemisia all exhibit increased pollen frequencies. The pollen assemblages over the upper half metre of the core suggest development of a more open or parkland vegetation. However, an obvious explanation for these changes is not clear. Low charcoal percentages preclude increasing fire frequency, and it has been suggested that European settlement was marked by increased forest cover brought about by the demise of the bison herds (Campbell and Campbell, 1994). The influence of a drier climate on the dry mixed wood vegetation around Moore Lake would probably result in increased grassland and jack pine vegetation (Strong and Leggat, 1981). Unfortunately, the 14C chronology for zone 6 is inadequate, and problematic by being too old. Either the rate of deposition was extremely slow, the top of the sediment core

M. Hickman, C.E. Schweger/Palaeogeography,Palaeoclimatology,Palaeoecology123 (1996) 161-178

was lost during coring or there may be old carbon contamination. The pollen stratigraphy of Moore Lake is very similar to that of Mariana Lake, 180 km to the northwest (Hutton et al., 1994). However, there are two important differences. First, the vegetation development at Moore Lake appears to have taken place earlier than at Mariana Lake. The Moore Lake boundaries and appearance of Betula and Picea are about 1000 yr earlier than equivalent levels at the more northerly and slightly more elevated Mariana Lake. There remains the possibility of 14C contamination from older carbon sources, possibly dissolved carbonate. Unfortunately, no terrestrial macrofosiils suitable for AMS dating were found, and there are no volcanic tephra horizons for comparison. That the 14C chronology of Moore Lake may be too old by as much as 1000 yr is a caveat attached to all subsequent discussion. Second, peak aridity at Mariana lake was marked by development of Populus parkland vegetation. During the same interval, ca. 9100-6200 yr B.P., grassland and parkland vegetation expanded at the more southerly Moore Lake indicating greater aridity. The vegetation regions of eastern Alberta (Strong and Leggat, 1981) expanded at least 200 km northward during this interval so that aspen parkland reached Mariana Lake and aspen groveland surrounded Moore Lake. 3.3. Lake history Diatoms and chrysophyte stomatocysts Zone 1: D-l; 985-950cm; ca. >11,830-ca. 11,600 yr B.P. The initial diatom assemblage comprises primarily pioneering benthic taxa (Fig. 4). Small species of Fragilaria (F construens and varieties, particularly F. construens v. venter) are particularly important along with Amphora ovalis and its varieties, especially A. ovalis v. pediculus. Such taxa are primarily epipelic, living upon the submerged sediments, while A. ovalis v. pediculus is primarily an epipsammic species living attached to sand grains (Round, 1965; Hickman, 1974; Hickman and Round, 1970). It can also be epiphytic upon other larger diatoms. All these taxa are widely adapted

167

forms common to shallow lakes, ponds and the shallow littoral regions of deeper lakes. They are commonly the initial "pioneering species" in nearly all lakes of glaciated regions. In Alberta lakes investigated, they comprise the dominant pioneers in the early Holocene as well as the mid-Holocene in response to reduced water levels (Hickman et al., 1984; Hickman and White, 1989; Hickman and Schweger, 1991b). They are also dominant in montane lakes, whether alpine or sub-alpine (Hickman and Reasoner, 1994). These taxa are cosmopolitan, indicative of alkaline lakes and ponds, as well as well-oxygenated water (Archibald, 1971). Their initial dominance suggests the early Moore Lake was shallow, and certainly much shallower than at any time afterwards, and one that formed when the catchment had a sparse, treeless vegetation. The peak of Tabellaria fenestrata at the end of this zone along with Fragilaria erotonensis and Stephanodiscus hantzschii indicates higher water levels. Also, the presence of Tabellaria fenestrata and the low D:C ratios of this zone (Fig. 5) suggest lower lake nutrient levels toward the end of this interval (Stool, 1985, 1988), since this species develops best in more oligotrophic to mesotrophic lakes (Round, 1959; Hickman, 1975); however, it is tolerant of a wide range of conditions and often occurs during the summer months when eutrophic lakes are stratified, and nutrient concentrations within the epilimnion are low. As in other studies, diatom numbers and accumulation rates are initially very low which probably results from either high turbidity or perhaps a high base status resulting from catchment leaching, and not low water temperatures. Low temperatures are not limiting to the growth of diatoms in present-day lakes. Diatoms can be very abundant in the early cold months of the year. In fact in Lake Wabamun, Alberta, where today portions of the lake remain ice-free every winter, very large diatom populations develop in these areas attached to rocks and emergent vegetation during both winter and early spring months when water temperatures are around 4.0°C (Hickman, 1974, 1982). In Moore Lake, as in other lakes at this stage of their development, high turbidity caused by suspended materials in the water, as well as

I t

.o

20 010010 0 400 Percentage ( ~< 1~o)

i

H

~ ~.,

SO

+°

0

I

3

0 10

P

¢°

+

! .*

0 100

J

O

/2

i: d

40

6. d

40

I =o

.

0

+o

200100

¢4

200

~,

30

.O I=W 2 + . e .e ..e +=.

o,o /

0

S

20

o,°

.+

0

40

/

0

~.

20

dd

.o

()

'

~ ' ss

-)'2

)3

--B

I-4

b-S

--

"11300+-170

"10200±160

°9250+-80

-58oo*-=o

=4470±110

2140-'130 .2380-~140

1 unlaminated [ ~ laminated

1

I l

I

I

g

Fig. 4. Diatom stratigraphic changes presented as a relative percentage diagram. Only the dominant taxa are included• Also shown are the changes in diatoms ~umbers and accumulation rates.

0-

g,

8

7.

6"

3

1.

o

W

°

L

I I

Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178 D:C

Diatom

80

0 0,6 2-

ratio

'

yrs BP

'

zones

• 2140 +- 130

IF

• 2 3 8 0 +_ 140 D-5

m m ~

E

4-

0)

6-

w4470±110

D-4 , , 5 8 0 0 '+ 8 0

.

.

.

.

.

.

C)

r-

D-3 • 9250 -+ 80 - "k 1 0 2 0 0 + 160 _ _m11300+ 170 .

.

.

.

.

.

.

.

......

D-2 ---.--_-I):I-

Fig. 5. The diatom: chrysophyte stomatocyst ratio changes (D:C ratio). Diatom zones are also shown.

from resuspension of the sediments, was probably most important in limiting diatom productivity. During this zone the sediments are not laminated which again provides evidence, perhaps, for a much shallower lake and/or sediment mixing. Zone 2." D-2: 950-815 cm; ca. ll,600-ca. 9600 yr B.P. Planktonic diatom species characteristic of eutrophic lakes (e.g., Fragilaria crotonensis and Stephanodiseus hantzschii) dominate until the latter part of zone D-2. Stephanodiseus hantzschlii is described as being found in strongly eutrophic water (Hustedt, 1949; Bradbury, 1975), and responds vigorously to increasing nutrient inputs (Brugam, 1979). The lake during this interval was very much deeper than in D-1 with laminated sediments and beautifully preserved long, and delicate frustules of Fragilaria crotonensis, as well as those of Synedra ulna v. danica. This suggests little or minimal sediment disturbance and resuspension. Diatom production and accumulation rates increase rapidly, and remain high until the latter portion of the zone. Highly productive and more nutrient rich conditions are also suggested by the large D:C ratio (Smol, 1985, 1988). One can envisage Moore Lake stratifying each summer, as well as inversely under winter-ice cover, with nutrients accumulating in the anoxic hypolimnion. As a result these nutrients would have been released into the photic epilimonion in the autumn and the

169

spring when complete water circulation was restored. Further with basin filling nutrients could enter the lake via the inflowing streams and possibly through erosion, since the catchment vegetation was still sparse, and the spruce-birchdominated Boreal Forest did not completely form until ca. 11,400 yr B.P. Toward the end of this zone dissolved salts were becoming more and more concentrated, until ultimately the lake became saline ca. 10,000 yr B.P. (Hickman and Schweger, 1993). The small size of the Moore Lake catchment, and insignificant inflowing streams probably contributed to the lake's sensitivity to a warmer, dryer climate. As a result of increased salinity freshwater species were replaced as the dominants by planktonic, saline species, namely Chaetoceros and Cyclotella caspia toward the end of this zone. This change in water chemistry also resulted in a reduced diatom production. Zone 3." D-3: 815-575 cm," ca. 9600-ca. 5800 yr B.P. Parkland and grassland vegetation replaced forests around Moore lake. Fire frequency was high, while within the lake the submersed hydrophyte Ruppia appeared (Fig. 3), which indicates saline, and possibly shallower water (Husband and Hickman, 1985; Hickman et al., 1984; Schweger and Hickman, 1989; Vance et al., 1992; Vance et al., 1993). Ruppia occidentalis S. Wats has a broad ecological range (0.2-20% TDS) (Rawson and Moore, 1944); however, growth and biomass allocation are correlated with water chemistry, and the number of flowering plants increases with increasing lake salinity (Husband and Hickman, 1985). It is also found in the fossil records of several other lakes across central Alberta. In Lake Isle (53°38'N; 114°41'W) (Fig. 1) it occurs ca. 8000 yr B.P., while in Lake Wabamun, Moonlight Bay (53°35'N; 114°30'W) (Fig. 1) it was present between ca. 8500 and 4000 yr B.P. (Schweger and Hickman, 1989). This plant also grows today in Buffalo Lake (53°15'N; 112°51'W) (Fig. 1) but higher fossil pollen frequencies between 7000 and 3000 yr B.P. indicate larger populations during the mid-Holocene, and more saline water. Its presence in lakes across central Alberta reflects the effects

170

M. Hickman, C E. Schweger/ Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178

of the early to mid-Holocene arid and warm climate. The dominant planktonic diatom taxa Chaetoceros and Cyclotella caspia are characteristic of saline lakes (Fritz and Battarbee, 1986; Fritz et al., 1991). Two species of Chaetoceros were encountered. At the beginning of this zone cysts of Chaetoceros amanita occurred. These were quickly replaced by those of C. muellerii and C. muellerii v. subsalina. C. amanita has been found living in Blue Lake Warm Spring, Utah, USA (Kaczmarska et al., 1985), and is described as being mesohaline (TDS = 4831 mg 1-1) and mesothermal (25-29°C). Similarly, the other species is also described as a mesohalobe (Hustedt, 1930; Johansen and Rushforth, 1985). Vegetative cells were only found during the months of June and August in the study of Johansen and Rushforth (1985). In an experimental study using C. muelleri optimal growth occurred between salinities of 2-12.5% and at temperatures between 20 and 30°C, while calcium and potassium salts tended to reduce growth in comparison to sodium and magnesium (Blinn, 1984). Thus, Moore Lake was saline with perhaps summer water temperatures higher than at present. Also, such a succession of species perhaps reflects increasing salinity of the water over the zone. Many of the frustules of the taxa encountered within this zone were badly eroded, particularly the benthic species. In contrast, the heavily silicified Chaetoceros cysts and Cyclotella caspia frustules appeared unaffected by taphonomic processes. Furthermore, compared to diatom assemblages of today's Alberta saline lakes the sedimentary assemblage in this zone appears to have no modern analogue, nor does it compare with other published material. This may be due to the shallowness of the present-day saline lakes in Alberta, as well as frustule dissolution. This saline water period was interrupted between ca. 9100 and ca. 8400 yr B.P. by a freshwater interval because the freshwater planktonic species Fragilaria crotonensis and Stephanodiscus hantzschii reappeared. This suggests increased effective moisture and dilution of the dissolved salts. No evidence of a period of effective moisture is indicated by the pollen record. N o similar event

has been observed in the diatom or pollen records of other central Alberta lakes that we have investigated (Hickman and Klarer, 1981; Hickman et al., 1984; Hickman, 1987; Hickman et al., 1990; Hickman and Schweger, 1991a,b), perhaps because these other lakes were not as sensitive as Moore Lake. A wetter period dated ca. 9-;00 8400 yr B.P. has been recorded by the pollen record of Toboggan Lake, southwestern Alberta (MacDonald, 1989), although alternative nonclimate explanations may be possible (MacDonald, 1993). The size of the freshwater response in Moore Lake suggests that this is possibly a real event, and not the result of sediment redistribution, since the sediments remain laminated throughout this period, and again preservation of the long, delicate frustules of Fragilaria crotonensis was excellent. Only examination of cores from other lakes in the Moore Lake region or elsewhere in western Canada will provide a definitive answer. Epipelic taxa (Navicula oblonga, Anomoeoneis costata, and Surirella ovalis) become more prominent during the latter half of this zone after the freshwater interval, perhaps indicative of lowered water levels. By the end of zone D-3 both Chaetoceros and Cyclotella caspia had declined to minimal amounts dur to increased effective precipitation and dilution of the dissolved salts during the latter stages of D-Y Catchment vegetation changes were also occurring with the appearance of a Betula-Pinus forest, which again suggests more effective precipitation. Diatom productivity was very low throughout this predominantly saline zone. Perhaps overall lake productivity decreased, since absence of pyrite spherules during this zone would suggest welloxygenated water and no prolonged periods of hypolimnetic anoxia, and subsequent accumulation of nutrients in the hypolimnion. Further, phytoplankton production, in general, is inhibited in lakes whose conductance exceeds 1000 gS cm 2 (Bierhuizen and Prepas, 1985), and the conductance of Moore Lake during D-3 must have attained an average of at least 6500 gS c m - 2 which is optimal for the growth of Ruppia (Husband and Hickman, 1985). Today in such saline lakes benthic algal production is far more important than that

M. Hickman, CE. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178

of the phytoplankton (Hickman, 1978; Mitchell and Prepas, 1990). Events in Moore Lake are similar in some respects to those found by Fritz et al. (1991) for Devils Lake, North Dakota, USA with an initially freshwater diatom assemblage being replaced by one dominated by saline taxa ca. 8000 yr B.P. However, the saline period only lasted until ca. 7000 yr B.P., after which a series of oscillations between fresh and saline conditions occurred. In Medicine Lake, South Dakota, an abrupt change from fresh to saline conditions also occurs ca. 9000 yr B.P., and is associated with the transition from forest/parkland to prairie vegetation (Radle et al., 1989). Zone 4." D-4." 575 430 cm; ca. 5800 ca. 4000 yr B.P. Changes in the diatom assemblages suggest some aridity reversals because of the oscillations among saline and freshwater taxa. For example, peaks of the saline taxa Chaetoceros and Cyclotella caspia occur at 550 and 440, and 520 and 475 cm, while taxa such as Synedra ulna v. danica, a species indicative of alkaline water with a fairly high conductance (Patrick and Reimer, 1966) formed peaks at 505, 460, and 450cm. Other freshwater species, namely Fragilaria erotonensis, Stephanodiscus hantzschii, and Synedra acus v. radians formed peaks of varying size at 530, 510 and 470cm, respectively. Similar oscillations between fresh and saline conditions have been described by Teller and Last (1982, 1990) for shallow Lake Manitoba based upon declines in sediment pore water content, and development of soil horizons that formed during periods of low water and dry stages. Fritz et al. (1991) also describe oscillations between fresh and saline conditions after ca. 7000 yr BP. Also, sections of unlaminated sediments occurred within this zone suggesting sediment disturbance possibly due to storm-induced turbulence. Both diatom numbers and accumulation rates increased during this zone, with peaks becoming progressively larger, perhaps in response to freshening conditions, and nutrient inputs from the catchment through rain-caused erosion. The D:C ratio also suggests increased lake nutrient levels at 520, 510 and 420 cm during this zone. These peaks

171

occur when the sediments were unlaminated (Hickman and Schweger, 1993) which suggests complete lake mixing, possible storm events, sediment resuspension, and erosional input of nutrients into the lake. The end of this zone is marked by a rapid decrease in both diatom numbers and accumulation rates, possibly because of a brief return to more saline water, since a small peak of saline taxa re-occurs at the end of this zone. Zone 5." D-5: 430-5 cm," ca. 4000 yr B.P. to the present By zone D-5 Boreal Forest was well established in the region, and essentially a modern climate existed, which was cooler and wetter than the early to mid-Holocene period (Schweger and Hickman, 1989). Thus, the catchment was more stabile; nutrients became sequestered in the forest vegetation, and were made unavailable to the lake and its biota (Hobbi and Likens, 1973; Vitousek and Reiners, 1975; Bormann et al., 1974; Bormann and Likens, 1979; Hornbecker, 1975; Gorham et al., 1979). However, some dynamic changes evidently occurred within the lake during this zone. Freshwater planktonic diatoms characteristic of alkaline, eutrophic conditions dominated. The decline in importance of Synedra ulna v. danica in this zone perhaps indicates a further freshening of the lake and reduced conductance levels. Fluctuations in prominence of taxa such as Fragilaria crotonensis and Stephanodiscus hantzschii perhaps indicate shifts in the trophic status of the lake from periods of mesotrophy to more eutrophic intervals. The lake was possibly more mesotrophic during mid-zone when Asterionella formsa, Stephanodiscus niagarae and Synedra acus v. radians were more prominent, because major peaks in diatom numbers and accumulation rates occurred prior to their increased prominence. Increases in the prominence of Stephanodiscus hantzschii in the upper two metres of the core possibly mark a return to more eutrophic conditions. These dynamic shifts in dominant diatom taxa, numbers, accumulation rates and the D:C ratio (Fig. 4) are probably indirectly related to climate but directly related to lake nutrient level changes, and hypolimnetic nutrient accumulation under anoxic conditions. Perhaps for periods, as today,

M. Hickman, C.E. Schweger/Palaeogeography, Palaeoclirnatology, Palaeoecology 123 (1996) 161 178

172

deposition, fossil and algal abundance remain correlated through time so long as there is no change in basin morphometry, light penetration, stratification or deep oxygen content. The percentage undegraded chlorophyll levels would suggest that preservation conditions throughout the Moore Lake record have been good, and have altered little even during the saline period. However, water levels in the lake appear to have fluctuated, while basin morphometry has changed during the Holocene with nearly 10 m of sediment accumulating upon the lake bottom. Such events could possibly affect pigment levels and may mask any Holocene trends. Low TAP and TC concentrations characterize the very early part of the Holocene record of Moore Lake in zone D-l, (Fig. 6) which is consistent with records from other Alberta lakes (Hickman and White, 1989; Hickman and Schweger, 1991b). These low values could result from an unstable catchment, and sediment resuspension. Consequently, light penetration could

complete water circulation did not occur every year, or for periods of years. Nutrients then accumulated within the hypolimnion, and when released provided the stimulus for production of large populations of diatoms characteristic of eutrophic lakes.

3.4. Sedimentary chlorophyll and carotenoids In any investigation using lacustrine sedimentary plant pigments to estimate fluctuations in palaeoproduction certain caveats must be placed upon the data. Though sedimentary pigments contain detailed information concerning the history of an individual lake, interpretation of stratigraphic changes is often difficult (Binford et al., 1983; Swain, 1985) due to the various pigment degradation pathways and conditions affecting preservation. However, according to Leavitt (1993) over the widest range of production, pigment deposition and fossil concentration are proportional to the algal standing crop, and despite losses during

TAP

TC

TAP TC °/oUN

Osc

Myx

Diatom

Osc/Myx

Yrs B P

zones

0 100 0

2000

0.2

60 0 i

0 ' ' ' ! .....

I

'

i

;~

i

i

400 0 l

i

i

,

I~~ :=~ 0 0 ~

400 r

i

i

0

4

i

,

1

I

=2140 *-130 =2380-+140

~

~-,

~_

I

~

_l_e----=-=~--__ _"_

I

, ~ 5 0 ~=~-' 0 1000 >

2

4

~

D 5

• 4470-+ 110

~,~2Zzzz:====~

.---~---_-~. . . .

,/". . . . D-4

E I

• 5800 + 80 D-3 =9250+80 10100+_160 =11300+_170

•

10

-

222222222

___-

i

Fig. 6. Sedimentary pigment changes. Total chlorophyll a derivatives (TAP) and total carotenoids (TC), both expressed as sedimentary chlorophyll pigment units g ] organic matter, the TAP:TC ratio, concentrations of oscillaxanthin (Osc) and myxoxanthophyll (Myx), both expressed as/~g g ] organic matter, the osciilaxanthin:myxoxanthophyll ratio (Osc:Myx ratio) and the percent undegraded chlorophyll (%UN). The diatom zones are also shown.

M. Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

have been reduced which, in turn, reduced algal production. Sediment resuspension would also lead to increasing degradation of pigments through photodegradation (Carpenter et al. 1986). However, percent undegraded chlorophyll within D-1 is high compared to other Alberta lakes investigated, therefore, preservation conditions do not appear to be a major factor in the early stages of the Moore Lake record. TAP concentrations, in particular, display no precise trends unlike records from other Alberta lakes in which maximum levels occur between ca. 9000 and 4000 yr B.P. (Hickman et al., 1984; Schweger and Hickman, 1989; Hickman and Schweger, 1991b). The largest TAP peak occurs within diatom zone D-2 corresponding to when freshwater planktonic diatoms are dominant, while the lowest concentration was toward the end of the saline period. A small TAP peak occurs during the freshwater interval (ca. 9100-8400 yr B.P.), perhaps due to increased lake production. Consistent with other Alberta records is the trend of little change during the last 4000 yr. Thus, overall palaeoproduction levels have changed little throughout the Holocene, and the record is characterized by an irregular pattern of highs and lows. In contrast, TC concentrations are highest within the saline period with the largest peak coinciding with that of TAP during freshwater interval. The general trend of high TC concentrations during the saline period could also reflect the dominance of blue-green algae because, they produce more carotenoids then do other algae (Harris, 1978). Otherwise TC concentrations alter little.

3.5. Oscillaxanthin and myxoxanthophyll Very evident cyanobacterial carotenoid changes occur during the Holocene in Moore Lake (Fig. 6). Like the Alberta Foothills lake, Fairfax Lake (Hickman and Schweger, 1991b) (Fig. 1), members of the Oscillatoriaceae are important during the early part of the lacustrine record (diatom zones D-1 and D-2). However, unlike Fairfax Lake where highest oscillaxanthin values occurred very early in the record (prior to ca. 11,300 yr B.P.) when lake nutrient levels are interpreted as being high, the catchment unstable and sparsely

173

vegetated, in Moore Lake largest oscillaxanthin values occur at the end of the saline period when diatom numbers and accumulation rates were also very high. Oscillatoria species are often among the first genera of Cyanobacteria to dominate lakes as they become eutrophic (Eberly, 1964; Edmondson, 1968; Skullberg, 1978), and these large peaks emphasize the eutrophic and productive nature of the lake during this period. These massive oscillaxanthin and myxoxanthophyll peaks occurring at the end of diatom zone D-4 and during the early part of D-5 correspond to the freshening of the lake water. Such peaks provide further evidence for increased nutrient inputs into the lake, and possible internal nutrient loading characteristic of stratifying lakes with hypolimnetic anoxia occurring. They also indicate, as the diatom record does, a more productive, eutrophic lake. Also, TAP:TC ratio values are generally low which is also characteristic of eutrophic lakes (Swain, 1985). Afterwards, during the last 4000 yr B.P. a series of small peaks of both oscillaxanthin and myxoxanthophyll occur, which again are probably related to nutrient release from an anoxic hypolimnion. The last large peaks of both oscillaxanthin and myxoxanthophyll coincided with the last unlaminated section of the core, hence complete circulation and possible sediment disturbance, and consequently, nutrient release. In Lake Wabamun, Alberta (Fig. 1) in which there is also an early to mid-Holocene saline interval, myxoxanthophyll concentrations were high in the early Holocene (ca. 9000 yr B.P.) prior to this saline period (Hickman and Schweger, 1991 a). Oscillaxanthin concentrations, on the other hand, were high when the lake is interpreted as being saline. Perhaps the differences in the bluegreen algal carotenoid records reflect differences in salinity of the lakes, with Moore Lake becoming so saline that species of Oscillatoria were unable to grow. They became replaced by other species of Cyanobacteria. This was probably related to climate, since today Moore Lake is in an area presently more arid than Lake Wabamun, The species found dominating in Moore Lake today are not found in saline lakes which provide optimal growing conditions for Ruppia. Species of Oscillatoria reappeared briefly during the fresh-

174

M. Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

water period ca. 9100-ca. 8400 yr B.P. which provides further evidence of their sensitivity to increasing salinity levels. 3.6. Palaeoclimate

The late glacial was a period of rapid ice retreat and stagnation in western Canada (Bobrowsky and Rutter, 1992; Klassen, 1989; Teller, 1987). In the Moore Lake region a unique late-glacial pioneering vegetation developed and was quickly succeeded by a spruce-birch forest. These geological and ecological events were in response to the warmer climate which saw summer temperatures exceed those of today (Ritchie et al., 1983; Schweger and Hickman, 1989; Hickman and Schweger, 1993). Vegetation changes that occurred between ca. 11,600 and 6200 yr B.P. suggest continued warmth and aridity. Diatom changes indicate general aridity ca. 9600 5800 yr B.P., punctuated by a moist period ca. 9100-8400 yr B.P., and short-lived aridmoist oscillations between ca. 5800 and ca. 4500 yr B.P. Re-establishment of the southern Boreal Forest and permanent freshwater conditions at Moore Lake no doubt reflect more effective precipitation and less evaporation stress over the last half of the Holocene with an essentially modern climate established by ca. 4000 yr B.P. The western Canadian palaeoclimatic record, including that of Moore Lake, differs from the moist/cool-dry/hot-moist/cool early-mid-late Holocene sequence most often seen in records elsewhere in North America. For western Canada, warmth and aridity began in the late glacial and extended through the early Holocene, so that by 6000 yr B.P. the climate was already in transition and peak aridity had passed (Vance et al., 1995). Deglaciation began with the Cordilleran ice and the western edge of the laurentide ice, proceeding west to east across western Canada even before significant ice retreat had occurred in midcontinent or eastern North America (Dyke and Prest, 1987a, 1987b). Late glacial/early Holocene warmth began to effect western Canada while other areas of North America were still ice-covered or effected by circulation patterns resulting from the Laurentide ice sheet (Webb et al. 1993).

This is consistent with orbital perturbations as the major climate forcing for western Canada (Ritchie and Harrison, 1993). However, the Moore Lake record demonstrates short-lived climatic changes of unknown cause superimposed on the broad trends, and the value of the palaeolimnological record which can be much more sensitive than fossil pollen vegetation reconstructions in demonstrating climate history.

4. Conclusions

Major trends are summarized in Fig. 7. The sedimentary records from Moore Lake span at least 11,830 years. Due to a possible hard water effect 14C dates and the extrapolated ages must be considered as maximum ages. ( 1) The late-glacial Holocene vegetation history of the Moore Lake region is a record of species migration following deglaciation (i.e., spruce, alder, pine) and a response to a warmer climate when summer temperatures exceeded those of today (e.g., grassland/parkland to Boreal Forest). The vegetation changes are very similar to, and possibly earlier than those at sites further northwest (i.e., Mariana Lake). (2) The diatom history of Moore Lake demonstrates, an initially shallow lake, then oscillations between planktonic freshwater and saline assemblages. The lake has remained fresh over the last 4000 yr. (3) Changes in vegetation and lake water status do not closely correlate, based upon zone boundaries. Some changes to the lake, such as the saline freshwater oscillations in diatom zone 4 are not recorded in the vegetation record. The small catchment area of the lake may make it especially sensitive to climate changes. (4) Aridity or high evaporation stress was significant during the early half of the Holocene, reaching a maximum ca. 8000-6500 yr B.P. Superimposed on the lake's response to early Holocene summer insolation values are periods of reduced evaporation or higher precipitation of shorter duration (e.g., between 9100 and 8400 yr B.P.). (5) Increased salinity reduced diatom produc-

• L Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology123 (1996) 161-178 ,0.

!

Pollen Zones Vegetation

r~

Salinity trends J,

Diatoms Flora

Diatoms Number and Accumulation Rates

Bluegreen algal Pigments

Zone 6

TAP and TC

and Myxoxanthophyll high

Little change shown by both TAP and TC high

remain high

Zone 5

3-

Boreal Forest Birch-SpruceAlder

D:C ratio

Oscillaxanthin

BorealForest Birch-PinePopulus

4-

175

seriesof .ill I Oscillaxanthin large .. i~.. and :+:+::+ peaks ....._ : Myxoxanthophyll:. :+:::X.increases.X.Li :.:.:.:displayseries.:::.

Zone ~,::

...............,............:.::.

of

:.:.::

large peaks zone 4

:::::::::::::::::::::::::::::::::::::::::::::: :+:

high

series of peaks and lows

high

small amounts

TAP and TC high

low

low

TAP and TC low

7-

Parkland/ Grassland

8-

low

Zone 3 Open Forest Birch-Spruce Zone 2 Open Forest Spruce-Birch Zone 1

9-

1011-

D

Freshwater,Planktonic Fragilariacrotonensis Stephanodiseuehantzschii

Osciiiaxafi{hiH undetectabie

. .........

:::::::::::::::::::::::::::::::::::::::::::::::::::::::: :.:.:: ~ : . : . : . :

~

Salirle Water, Planktonic ~ J PioneeringTaxa Chaetoceros Cyclotellacaspia Benthic, Epipelic

~ ~

Periodsof maximumsalinity

D

OscillatingPlanktonic Salineand Freshwater taxa

Fig. 7. A summary of the major trends.

tivity significantly, as well as species composition. The Cyanobacteria were similarly affected. Oscillatoria spp. were virtually eliminated during the saline interval but were replaced by blue-green algal species tolerant of the saline water. (6) As the saline period ended, and oscillations between fresh and saline diatom assemblages occurred, diatom and blue-green algal productivity increased dramatically with several very large maxima occurring. Peaks perhaps occurred in response to freshening of the water, coupled with internal nutrient loading in response to complete lake mixing. (7) The western Canadian palaeoclimatic record is one of warmth and aridity beginning in the late glacial and extending throught the early Holocene, and thus differs from that seen elsewhere in North America.

Acknowledgments Financial support for this study was provided through NSERC operating grants A6384 and A0823 awarded to Dr. M. Hickman and Dr. C.E. Schweger, respectively. Other financial support was provided by Alberta Environment. Thanks are extended to Culley Schweger, D.M. Klarer, R. Haag, S.E.D. Charlton and others for help in retrieving the cores.

References Antevs, E., 1955. Geologic-climate dating in the West. Am. Antiq., 20: 317-355. Archibald, R.E.M., 1971. Diatoms from the Val Dam catchment area Transvaal, South Africa. Bot. Mar., 24:17 70.

176

M. Hickman, C. E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

Bierhuizen, J.F.H. and Prepas, E.E., 1985. Relationship between nutrients, dominant ions, and phytoplankton standing crop in prairie lakes. Can. J. Fish. Aquat. Sci., 42: 1588 1594. Binford, M.W., Deevey, E.S. and Crisman, T.L., 1983. Paleolimnology: an historical perspective on lacustrine ecosystems. Ann. Rev. Ecol. Syst., 14: 255-286. Blinn, D.W., 1984. Growth responses to variations in temperature and specific conductance by Chaetoceros muelleri (Bacillariophyceae). Br. Phycol. J., 19: 31-35. Bobrowsky, P. and Rutter, N.A., 1992. The Quaternary Geological History of the Canadian Rocky Mountains. Geogr. Phys. Quat., 46:5 50. Bormann, F.H. and Likens, G.E., 1979. Pattern and Process in a Forested Ecosystem. Springer, New York, 253 pp. Bormann, F.H., Likens, G.E., Siccama, T.G., Pierce, R.S. and Eaton, J.S., 1974. The export of nutrients and recovery of stable conditions following deforestation at Hubbard Brook. Ecol. Monogr., 44: 255-277. Bradbury, J.P., 1975. Diatom stratigraphy and human settlement in Minnesota. Geol. Soc. Am. Spec. Pap., 171:1 74. Brugam, R.B., 1979. A re-evaluation of the Araphidineae/ Centrales index as an indicator of lake trophic status. Freshwater Biol., 9:451 460. Campbell, C. and Campbell, I.D., 1994. Bison extirpation may have caused aspen expansion in western Canada. Ecography, 17: 360-362. Carpenter, S.R., Elser, M.M. and Elser, J.J., 1986. Chlorophyll production, degradation, and sedimentation: implications for paleolimnology. Limnol. Oceanogr., 3l: 112 124. Cushing, J.E. and Wright, H.E., 1965. Hand-operated piston corers for lake sediments. Ecology, 46: 380-384. Deevey, E.S. and Flint, R.F., 1957. Postglacial hypsithermal interval. Science, 125:182 184. Dickman, M., 1979. A possible varying mechanism for meromictic lakes. Quat. Res., 11: 113-124. Dyke, A.S. and Prest, V.K., 1987a. Paleogeography of northern North America, 18000 5000 years ago. Geol. Surv. Can. Map 1703A, Ottawa. Dyke, A.S. and Prest, V.K., 1987b. Late Wisconsin and Holocene history of the Laurentide Ice Sheet. Geogr. Phys. Quat., 41: 237-263. Eberly, W.R., 1964. Further studies on the metalimnetic oxygen maximum, with special reference to its occurrence throughout the world. Invest. Indiana Lakes, 6:103-139. Edmondson, W.T., 1968. Water-quality management and lake eutrophication: the Lake Washington Case. In: T.H. Campbell and R.O. Sylvestor (Editors), Water Resources Management and Public Policy. Univ. Washington Press, Seattle, pp. 139 178. Faegri, K. and Iversen, J., 1975. Textbook of Pollen Analysis. Blackwells, Oxford, 265 pp. Fenton, M.M. and Andriashek, L.D., 1983. Surficial geology Sand River area, Alberta (NTS 73L). Alta. Res. Council, Alta. Geol. Surv., Edmonton. Fritz, S.C. and Battarbee, R.W., 1986. Sedimentary diatom assemblages in freshwater and saline lakes of the Northern

Great Plains, North America: preliminary results. In: Proc. 9th Diatom Symp., pp. 265-271. Fritz, S.C., Juggins, S., Battarbee, R.W. and Engstrom, D.R., 1991. Reconstruction of past changes in salinity and climate using a diatom-based transfer function. Nature, 352: 706-708. Gorham, E., Vitousek, P.M. and Reiners, W.A., 1979. The regulation of chemical budgets over the course of terrestrial succession. Ann. Rev. Ecol. Syst., 10: 53.-84. Government of Alberta and The University of Alberta, 1969. Atlas of Alberta. Univ. Alberta Press, Edmonton, Alta. Harrris, G.R, 1978. Photosynthesis, productivity and growth: the physiological ecology of phytoplankton. Arch. Hydrobiol. Beih. Ergebn. Limnol., 10:1 171. Hickman, M., 1974. Effects of the discharge of thermal effluent from a power station on Lake Wabamun, Alberta, C a n a d ~ the epipelic and epipsammic algal communities. Hydrobiologia, 45: 199-216. Hickrnan, M., 1975. Studies on the epipelic diatom flora of some lakes in the southern Yukon Territory, Canada. Arch. Hydrobiol., l l l : 121 136. Hickman, M., 1978. Ecological studies on the epipelic algal community in five prairie~parkland lakes in central Alberta. Can. J. Bot., 56:991 1009. Hickman, M., 1982. The removal of a heated water discharge from a lake and the effect upon an epiphytic algal community. Hydrobiologia, 87: 21-32. Hickman, M. and Round, F.E., 1970. Primary production and standing crops of epipsammic and epipelic algae. Br. Phycol. J., 5:247 255. Hickman, M. and Klarer, D.M., 1981. Paleolimnology of Lake Isle, Alberta, Canada (including sediment chemistry, pigments and diatoms. Arch. Hydrobiol., 91:490 508. Hickman, M., Schweger, C.E. and Habgood, T., 1984. Lake Wabamun, Alta.: a paleoenvironmental study. Can. J. Bot., 62:1438 1456. Hickman, M. and White, J.M., 1989. Late Quaternary paleoenvironment of Spring Lake, Alberta, Canada. J. Paleolimnol., 2: 305-317. Hickrnan, M., Schweger, C.E. and Klarer, D.M., 1990. Baptiste Lake, Alberta a late Holocene history of changes in a lake and its catchment in the southern Boreal Forest. J. Paleolimnol., 4: 253-267. Hickman, M. and Schweger, C.E., 1991a. Oscillaxanthin and myxoxanthophyll in two cores from Lake Wabamun, Alberta. J. Paleolimnol., 5:127 137. Hickman, M. and Schweger, C.E,, 1991b. A palaeoenvironmental study of Fairfax Lake, a small lake situated in the Rocky Mountain Foothills of west-central Alberta. J. Paleolimnol., 6:1 15. Hickman, M. and Schweger, C.E., 1993. Late glacial early Holocene palaeosalinity in Alberta, Canada climate implications. J. Paleolimnol., 8:149 161. Hickman, M. and Reasoner, M.A., 1994. Diatom responses to late Quaternary vegetation and climate change, and to the deposition of two tephras in an alpine and a sub-alpine lake

h/L Hickman, C.E. Schweger/Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161-178 in Yoho National Park, British Columbia. J. Paleolimnol., in press? Hobbie, J.E. and Lickens, G.E., 1973. Output of phosphorus, dissolved organic carbon, and fine particulate carbon from Hubbard Brook watersheds. Limnol. Oceanogr., 18: 734-742. Hornbeck, J.W., 1975. Streamflow response to forest cutting and revegetation. Water Resour. Bull., 11: 1257-1260. Husband, B.C. and Hickman, M., 1985. Growth and biomass accumulation of Ruppia occidentalis in three lakes differing in salinity. Can. J. Bot., 63: 2004-2014. Hustedt, F., 1930. Die Kieselalgen Deutschlands, Osterreichs und der Schweiz mit Beriicksichtigung der abrigen L/inder Europas sowie der angrenzenden Meeresgebiete. In: L. Rabenhorst, Kryptogamen-flora von Deutschland, Osterreich und der Schweiz. Akad. Verlagsgesell., Leipzig (reprint 1971, Johnson Reprint Corp., London). Hustedt, F., 1949. Susswasser-Diatomeen. Exploration du Parc national Alberta, Mission H. Dumais (1935-1936). Fasc. 8. Hayez, Bruxelles, pp. 1-199. Hutton, M.J., MacDonald, G.M. and Mott, R.J., 1994. Postglacial vegetation history of the Mariana Lake region, Alberta. Can. J. Earth Sci., 31: 418-425. Johansen, J.R. and Rushforth, S.R., 1985. A contribution to the taxonomy of Chaetoceros muelleri Lemmermann (Bacillariophyceae) and related taxa. Phycologia, 24: 437-447. Kaczmarska, I., Rushforth, S.R. and Johansen, J.R., 1985. Chaetoceros amanita Cleve-Euler (Bacillariophyceae) from Blue Lake Warm spring, Utah, U.S.A. Phycologia, 24: 103 109. Klassen, R.W., 1989. Quaternary Geology of the southern Canadian Interior Plains. In: R.J. Fulton (Editor), Geology of Canada and Greenland (Geol. Can., 1). Geol. Surv. Can., Ottawa, pp. 138-173. Kocaoglu, S.S., 1975. Reconnaissance soil survey of the Sand River area. Alta. Soil Surv. Rep., 34, Univ. Alta. Bull., SS-15, Alta. Inst. Pedol. Rep., S-74-34 1975. Univ. Alta., Edmonton. Leavitt, P.R., 1993. A review of factors that regulate carotenoid and chlorophyll deposition and fossil pigment abundance. J. Paleolimnol., 9: 109-128. MacDonald, G.M., 1989. Postglacial vegetation history of the subalpine forest-grassland ecotone in southwestern Alberta: new insights on vegetation and climatic changes in the Canadian Rocky Mountains and adjacent foothills. Palaeogeogr. Palaeoclimatol. Palaeoecol., 73: 155-173. MacDonald, G.M., 1993. Methodological falsification and interpretation of palaeoecological records: the cause of the early Holocene birch in western Canada. Rev. Palaeobot. Palynol., 79: 83-97. MacDonald, G.M. and Ritchie, J.C., 1986. Modern pollen spectra from the western interior of Canada and the interpreation of Late Quaternary vegetation development. New Phytol., 103: 245-268. Mitchell, P. and Prepas, E.E., 1990. Atlas of Alberta Lakes. Univ. Alta. Press, Edmonton, 675 pp.

177

North, M.E.A., 1976. Plant geography of Alberta (Stud. Geogr., 2). Dep. Geogr. Univ. Alta., Edmonton. Northcote, T.G. and Larkin, P.A., 1963. Western Canada. In: D.G. Frey (Editor), Limnology of North America. Univ. Wisconsin Press, Madison, WI, pp. 451-485. Patrick, R. and Reimer, C.W., 1966. The diatoms of the United States exclusive of Alaska and Hawaii. Philos. Acad. Nat. Sci. Monogr., 13,1: 1-866. Radle, N., Keister, C.M. and Battarbee, R.W., 1989. Diatom, pollen and geochemical evidence for the palaeosalinity of Medicine Lake, S. Dakota, during the Late Wisconsin and early Holocene. J. Paleolimnol., 2:159 172. Rawson, D.S. and Moore, J.E., 1944. The saline lakes of Saskatchewan. Can. J. Res. Sect. D., 22:141 201. Reeves, B., 1973. Concept of an altithermal cultural hiatus in northern Plains prehistory. Am. Anthropol., 75:1221 1253. Ritchie, J.C. and Harrison, S.P., 1993. Vegetation, lake levels, and climate in Western Canada during the Holocene. In: H.E. Wright Jr. et al. (Editors), Global Climate Since the Last Glacial Maximum. Univ. Minnesota Press, Minneapolis, pp. 401 414. Ritchie, J.C., Cywnar, L.C. and Spear, R.W., 1993. Evidence from north-west Canada for an early Holocene Milankovitch thermal maximum. Nature, 305:126 128. Round, F.E., 1959. A comparative survey of the epipelic diatom flora of some Irish loughs. Proc. R. Irish Acad., 60: 193-215. Round, F.E., 1965. The epipsammon; a relatively unknown freshwater algal associations. Br. Phycol. Bull., 2: 456-462. Schweger, C.E. and Hickman, M., 1989. Holocene paleohydrology of central Alberta: testing the general-circulation-model climate simulations. Can. J. Earth Sci., 26: 1826-1833. Schweger, C.E., Habgood, T. and Hickman, M., 1981. Late glacia~Holocene climatic changes of Alberta. The record from lake sediment studies. In: Impacts of Climatic Fluctuations on Alberta Resources and Environment. Environ. Can. Rep., WAES-1-81, pp. 47 56. Skullberg, O.M., 1978. Some observations on red-coloured species of Oseillatoria (Cyanophyceae) in nutrient-enriched lakes of southern Norway. Verb. Int. Verein. Limnol., 20: 776-787. Stool, J.P., 1985. The ratio of diatom frustules to chrysophyte statospores. A useful paleolimnological index. Hydrobiologia, 123: 199-208. Stool, J.P., 1988. Chrysophycean microfossils in paleolimnological studies. Palaeogeogr. Palaeoclimatol. Palaeoecol., 62: 287 297. Street-Perrot, F.A. and Roberts, N., 1983. Fluctuations in closed lakes as an indicator of past atmospheric circulation patterns. In: F.A. Street-Perrot et al. (Editors), Variations in the Global Water Budget. Reidel, Dordrecht, pp. 331-345. Strong, W.L. and Leggat, K.R., 1981. Ecoregions of Alberta. Alta. Environ. Nat. Resour., Resour. Eval. Planning Div., Edmonton. Swain, E.B., 1985. Measurement and interpretation of sedimentary pigments. Freshwater Biol., 15: 53-75. Teller, J.T., 1987. Proglacial lakes and the southern margin of

178

M. Hickrnan, C. E. Schweger/ Palaeogeography, Palaeoclimatology, Palaeoecology 123 (1996) 161 178

the laurentide Ice sheet. In: W.F. Ruddiman and H.E. Wright Jr. (Editors), North American and Adjacent Oceans during the Last Deglaciation (Geol. North Am., K-3). Geol. Soc. Am., Boulder, pp, 39 70. Teller, J.T. and Last, W.M., 1982. Pedogenic zones in postglacial sediment of Lake Manitoba, Canada. Earth Surf. Proc. Landforms, 7: 367-397. Teller, J.T. and Last, W.M., 1990. Paleohydrological indicators in playas and salt lakes, with examples from Canada, Australia, and Africa. Palaeogeogr. Palaeoclimatol. Palaeoecol., 76: 215-240. Vance, R.E., 1986. Aspects of the postglacial climate of Alberta: calibration of the pollen record. Geogr. Phys. Quat., 40: 153-160. Vance, R.E., Emerson, D. and Habgood, T., 1983. A midHolocene record of vegetative change in central Alberta. Can. J. Earth Sci., 20:364 376. Vance, R.E., Mathewes, R.W. and Clague, J.J., 1992. 7000 year record of lake-level change on the northern Great Plains: a high-resolution proxy of past climate. Geology, 20: 879-882.

Vance, R.E., Clague, J.J. and Mathewes, R.W., 1993. Holocene paleohydrology of a hypersaline lake in southeastern Alberta. J. Paleolimnol., 8:103 120. Vance, R.E., Beaudoin, A.B. and Luckman, B.H., 1995. The palaeoecological record of 6 ka climate in the Canadian prairie provinces. Geogr. Phys. Quat., 49: 81-98. Vitousek, P.M. and Reiners, W.A., 1975. Ecosystem succession and nutrient retention: a hypothesis. Bioscience, 25:376-381. Webb II1, T., Ruddiman, W.F., Street-Perrott, F.A., Margraf, V., Kutzbach, J.E., Bartlein, P.J., Wright Jr., H.E. and Prell, W.L., 1993. Climatic changes during the past 18,000 years: regional syntheses, mechanisms, and causes. In: H.E. Wright Jr. et al. (Editors), Global Climates since the Last Glacial Maximum. Univ. Minnesota Press, Minneapolis, pp. 514-535. Williams, G.D.V. and Masterton, J., 1983. An application of principle component analysis and an agroclimate resource index to ecological land classification for Alberta. Climatol. Bull., 17: 3-28.