Modern diatom assemblages from lake sediments in the boreal–arctic transition region near the Mackenzie Delta, N.W.T., Canada

Modern diatom assemblages from lake sediments in the boreal-arctic transition region near the

Mackenzie Delta, N. W .T., Canada1

L. K. Ko~vo Etlgitzeerirlg Geologv, T(it,lper~. U~~iversily of T~c/ltlology. Box 527, 33/01 Tatt~pere 10, Fit~lotzcl

AND

J . C. RITCHIE Scnrbororrgl~ College, Utli~~ersity ofToro~~to, West Hill, Ottt., Cotlcrda MIC IA4

Received October 7. 1977

Kolvo, L. K., and J . C. RITCHIE. 1978. Modern diatom assemblages from lake sediments in the boreal-arctic transition region near the Mackenzie Delta. N.W.T.. Canada. Can. J . Bot. 56: 1010-1020.

Sul-face lake mud samples from 20 lake sites across the transition from northern boreal forest to tundra, near the Mackenzie Delta, N.W.T., yielded 284 taxa of diatoms of wide-ranging, mainly subarctic-boreal-nemo~.al geogl.aphical affinity. The ionic composition of the water column shows that 17 of the lakes are oligotrophic with roughly similar diatom assemblages. Distinctive diatom assemblages were recorded only from the three chemically exceptional lakes. saline, meromictic, and slightly N enriched. respectively. The ratio of Araphidineae tocentrales proved to have little use in classifying these particular lakes, although i t has been effective in other geoglxphical regions.

Kolvo, L. K., et J . C. RITCHIE. 1978. Modern diatom assemblages from lake sediments in the boreal-arctic transition region near the Mackenzie Delta, N.W.T., Canada. Can. J . Bot. 56: 1010-1020.

Des Cchantillons de vase provenant de la couche superficielle de sediments lacustres ont etk prelevks dans 20 lacs de la region de transition entre le nord de la fortt borkaleet la toundral pres du delta du Mackenzie. Ten-itoires du Nord-Ouest; ces Cchantillons contenaient 284 taxons de diatomkes de distribution vaste et d'affinitk gkographique surtout subartico-borko-nknlorale. La composition ionique de la colonne d'eau montre que 17 des lacs sont oligotrophes avec des assemblages B peu pres semblables de diatomees. Des assemblages distincts dediatomeesont kte retrouves seulement dans les trois lacs exceptionnels du point de vue chimique, soit, respective- ment, un lac salin, un lac meromictique et un lac legkrement enrichi en azote. Le rapport Araphidinae-Cent~iles s'est avert peu utile pour classifier ces lacs, contrairement qux lacs d'aiutres regions geoglxphiques.

[T~xduit par le journal]

Introduction I t has been demonstrated that surface sediments

of lakes contain a roughly integrated sample of the various seasonal populations of diatoms. The evidence for this is (a ) that there is a close similarity between the diatom content of surface muds and the combined assemblages in live collections of planktonic and littoral forms taken throughout the open season at the same sites (e.g., Nygaard 1956; Bright 1968; Duthie and Sreenivasa 1972) and (6) that transects of surface samples show constancy in composition, indicating that diatom frustules are easily transported throughout the lake by water movement, except for the inner littoral zone, al-

IA complete list of the diatoms and their percentage frequen- cies is available, at a nominal charge. from the Depository of Unpublished Data, CISTI, National Research Council of Canada, Ottawa, Ont., Canada KIA OS2.

though exceptions to this generalization (e.g., Bradbury and Winter 1976) suggest that caution is necessary in interpretation.

Surface sediment analysis probably provides a more effective sample of total diatom composition than samples from plankton tows because the latter rarely include either epiphytic or very small (<5 pm) diatoms.

Surveys of the diatom flora of lakes in boreal and nemoral regions of Sweden (Florin 1957), Denmark (Foged 1947-1948), Finland (Molder and Tynni 1967-1975; Tynni 1975, 1976), and Minnesota (Bright 1968; Tarapchak 1973) have shown the rela- tions between diatom composition and certain as- pects of water chemistry. These surveys illustrate that there is a need for more precise, quantitative data on diatom autecology so that the commonly used schema of diatom indicators introduced by Hustedt (1939) and Kolbe (1927) can be refined.

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KOIVO AK ID RITCHIE 1011

Diatom analysis, used both in palaeoecology and limnological surveys, will become increasingly effective when these basic data are available.

The objective of this study was to determine the diatom composition of lakes in the uplands east of the Mackenzie Delta, across the sharp gradient in vegetation and bioclimate from the northern boreal forest to the low-arctic tundra (Hare and Ritchie 1972). The study was planned as an adjunct to studies of the Pleistocene ecology of northwestern Canada (Ritchie and Koivo 1975).

The lower Mackenzie River Valley has been in- tensively surveyed in recent years, chiefly as part of the preparatory documentation supporting, or otherwise relating to, various applications to con- struct natural-gas pipelines. Although these surveys included studies of freshwater lakes and streams along the proposed route (Brunskill et al. 1973), there have been no investigations of diatom assemblages in the area. This is surprising as it is familiar and unquestioned that diatoms provide a sensitive indicator of various types of an- thropogenic disturbance to aquatic systems.

Regional Ecology Detailed reports on the climate (Burns 1973,

1974), land-forms (Mackay 1963), vegetation (Reid 1974; Inglis 1975; Ritchie 1977), and freshwater environments (Brunskill et a / . 1973) are available. The following is a brief summary of relevant infor- mation abstracted from these and other sources.

The climate of the study area shows steep gra- dients of the significant thermal parameters during the growing season. Net radiation ranges from 14 kly ( I ly = 0.70 kW/m2) in the south to 10 kly at Tuktoyaktuk (Fig. 1). The growing season, defined as the number of days with mean ambient tempera- ture greater than P C , ranges from 120 to 100, and mean monthly July temperatures vary from 13°C at Inuvik to 10°C at Tuktoyaktuk. Total annual pre- cipitation decreases sharply northwards, from 25 cm at Inuvik to 13cm at Tuktoyaktuk. Small (5-10 ha) lakes are ice free by June 15th and freeze over by October 15th in the northern forest zone, and these events occur roughly 2 weeks later and earlier, respectively, in the tundra zone of the study area. Large (>50 ha) lakes become ice free roughly 2 weeks later than small ones. The vegetation of the northern boreal forest represented in this study area shows abundant evidence of the effects of natural fires, and relatively few tree stands are older than 200 years. (Black, personal communica- tion). Older forests on upland sites are dominated by spruce (usually Picea glauca) with an under-

story of fruticose lichens (Cladina spp. and Stereoca~llon spp.) and ericoid shrubs (Vaccitliutn ~lliginosum, V. vitis-idaea, Led~ltn declltnbetw, and Oxycocc~w). Poorly drained sites are occupied by Picea mariatla stands with varied ground cover dominated by Cladina spp., Sphagtz~lm spp., and Betula glandulosa. Upland sites in the tundra zone are dominated by dwarf shrubs and fruticose lichens, while poor1 y drained peaty areas show ice-wedge polygon and frost hummock development with dominance by Eriophor~rtn vagitlatutn and Carex stands. The landforms are predominantly low irregular hills of glaciofluvial silts and sands. Occasional drumlinoid features, bedrock- controlled relief, and eskers occur. Low areas have high ice content, silty soils, with abundant perma- frost features such as ice-wedge polygons, pingos, and hummocks. Surface materials are derived mainly from siltstones, sandstones, and shales of Mesozoic age, with the exception of a small area SE of Inuvik where Devonian and Ordovician limestone and dolomite outcrops occur.

Methods Luke Sampling

The lake sites were chosen (Fig. I ) according to the following criteria. (i) They should provide a reason;~bly even distribution across the three phytogeographic-bioclinlatic zones, northern boreal forest, forest-tundra, and tundra. (ii) They should be in uplands. well beyond the effects of deltaic and marine pro- cesses. (iii) They should be large enough (> 10 ha) and deep enough (>2 m) to minimize the chemical changes and bottom disruption caused by winter ice formation up to thicknesses of 1.5 m. (iv) Wherever possible. they should be closed drainage lakes.

Samples of sediment were taken from a floatplane or a boat, using a modified cup sediment sampler (Welch 1948). This sanl- pler penetrates the upper 2-5 cm and removes a sample approximately 10 ml in volume.

Water samples for chemical analysis were collected from the surface water in late July 1975 in acid-rinsed (0.1 N HCI), sample-rinsed, 500-ml polyethylene bottles. The samples were immediately vacuum filtered (Whatman Filter GF/C) and ship- ped for analysis to the laboratories of the Freshwater Institute, Environment Canada, for nutrient and major element analysis using standard procedures (Stainton et 01. 1974).

Dinton? Preparatiotl and Co~rnting M e t t ~ o d . ~ Organic matter from the samples (2 g fresh weight) for diatom

analysis was removed in 30% HzO, for 24 h at 50°C. The diatom frustules were separated from the mineral particles by several water washes and decantations. The cleaned material was sus- pended in 10 ml of distilled water. Each sample was mixed thoroughly and 0.1 ml was pipetted onto a cover slip and spread evenly. dried, and mounted in Hyrax (R.I. = 1.65). Ten such slides were counted of each sample.

A qualitative diatom analysis was first performed for each sample and then duplicate counts of 100-1000 diatoms were made under 1000 x magnification. The relative density of diatoms varied as indicated in Table 2, which also gives the totals counted for each sample and the relative frequency of the dominant taxa.

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1012 C A N . J. BOT. VOL. 56. 1978

FIG. I. A sketch map of the study area with an inset map showing its location in northwest North America. The sample sites are shown as numbered solid dots. The tundra zone Iiesnorthof the broken line withopendotsand thecontinuous forest zone lies south of the broken line with open squares. The modern Mackenzie River Delta is shown as the hatched area.

The identification of the diatoms followed Hustedt (1930. 1930-1966), Cleve-Euler (1951-1955). Patrick and Reimer (1966). Patrick and Freese (1961), Foged (1954. 1964). Lund (1962), and Ross (1947). The identification of several taxa was checked by comparison with reference specimens at the Philadelphia Academy of Natural Sciences. U.S.A.

A 'standard' sample size has nor been established in diatom analysis. An examination of numerous published studies (Pat- rick e/ 01. 1954; Patrick 1963, 1968: Williams 1964; Stockner and Benson 1967; Stockner 1971; Tarapchak 1973) reveals that an attempt to include rare species would necessitate counts of perhaps 5000- 10000 specimens per sample. Ritchie and Koivo (1975) used a mode truncated log normal curve in determining sample size for coi-e samples. Andrews (1972) recommends a semiquantitative analysisof200diatoms persample. Ritchie and Koivo (1975) explored this question in a limited way and con- cluded that while the number of new taxa added by continuing the count beyond 200 is considerable, the relative frequenciesof the ecological groups change very little. Andrews (1972) also suggests a semiquantitative method where the relative frequency of the taxa is estimated by the following parameters: rlot~lirlnrl~, abundant specimens within the field of view at all times; ~ b l t t l d ~ t 1 1 , at least one specimen in every field of view; corrlrnotl, at least one specimen in every two to five fields of view: freqltenr, several specimens observed on the entire slide;

rare, only one or two specimens observed on the entire slide. It should be noted that site 14 has been omitted because of lack of data, but the original numbering has been retained throughout.

Results Water Clzernistry

The results of ionic composition analysis (Table 1) show a dominance of Ca2+ and HC0,-, reflecting the moderately calcareous drift derived from sedimentary bedrock in the area (Mackay 1963). In general, the results are similar to those collated by Armstrong and Schindler (1971) from lakes in boreal Canada with drainage derived partially from shield and partially from areas of drift with variably soluble materials. All lakes fall in the category oligotrophic but show markedly higher ionic compositions than lakes on the Canadian shield.

Lakes 4, 11, and 21 are exceptions. Lake 4 is shallow, peaty, with many streams draining northern forest sites with recent fire disturbance, and it shows high total dissolved N (TDN) and Si values.

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TABLE 1. A summary of the locations, vegetation zone (F, forest; Ft, forest tundra; T, tundra), depths, areas, and water chemistry of the lakes

Average Conduc- Vegeta- depth, Area, HC03 , Cl, SO4, Ca, Mg, Na, K, TDN,* TDP,t Si, tivity

No. Latitude Longitude tion m kmz mg/! mg/! mg/! mg/! mg/! mg/! mg/! pH pg/! pg/! pg/! pmhos/cm

*Total dissolved N. tTotal dissolved P.

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TABLE 2. The diatom dominants of 20 N.W.T. lakes. The number of the sample, the number of specin~ens counted, and the number of taxa are given together with the percentage representation of the listed taxa

Percentage representation

Achr~anthes cotlspicua A. Mayer 1 .0 0 .1 13.7 - -

Alnphora ovalis Kiitz. 2.0 0.9 2.9 1 .4 0 .2 Cyclotella cofnta (Ehr.) Kutz. - - 0 . 6 - 0.1 C. glornerata Bachm. 14.0 10.8 1 . 0 0.9 0.6 Diplorleis oblongella (Naeg. ex

Kutz.) Ross - - - 0.1 -

Fragilaria construer~s (Ehr.) Grun. - 16.1 5 .5 0 .2 0.1 F. construens var. venter (Ehr.)

Grun. 2 .0 12.4 9 .5 1.8 4.6 F. crotonensis Kitton 21.0 2 .9 0.3 1 .2 0 .4 F. pinr~ata Ehr. 5.0 23.7 32.2 5 .3 54.4 Gyrosignla acun~inaturn (Kiitz.)

Rabh. 11.0 0.3 0.7 4.8 -

G. atterluatunr (Kutz.) Rabh. - - 0.1 - -

Melosira arnbiglra (Grun.)O. Muller - - - 27.0 -

Navicula pupula Kiitz. - 1.3 2 .6 2.5 2 .8 Z Naviculae 3 .0 6.9 8 .9 12.1 12.4 Pit~nrtlaria microstauror~ (Ehr.)Cl. - - - 2.9 2.0 Stephar~odiscus astraea var.

fnirlutltla (Kiitz.) Grun. 10.0 0.1 0.2 1 .9 -

*Number of sample. ?Number of specimens counted. $Number of taxa.

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KOIVO A N D RITCHIE

LAKE cT1.3&4=8.31 8=81.8 IM 78.61 10

FIG. 2. A bar diagram summary of the diatom ratios in each lake sediment sample, showing percentages of Centlales (C), Atxphidlneae (A), Bi~aphidineae (B), and Monol.aphld~neae (M).

Lake 1 1 is a large bay of the Eskimo (Husky) Lakes system. linked by a series of very narrow channels to the Beaufort Sea, and this is reflected in the high Na+, C1-. and conductance values. Lake 21 is a small, deep, meromictic lake (Ritchie 1977), oc- cupying a solution karst depression in limestone-dolonlite bedrock, giving high values for Ca2+, HC0,-, and Si.

. . . . . . .

. . . . . . . . . . Diat orn s Diatom Asser7zblages The small number of lakes and few samples

within lakes preclude quantitative analyses of the diatom frequency results. The following groupings are based on inspection of the data and, in particular, use the diatom dominants, that is, the most numerous species in each sample (Table 2, Fig. 2). These groups of individual lake sites will be de- scribed briefly below and any general relationship with morphometry or water chemistry will be indicated.

Luke 4-This is the only lake with abundant Melosira anzbig~rci. The lake has many small stream entrants, draining fire-disturbed forest sites. The water sample has high values of TDN and Si.

Lake 11-This distinctive assemblage includes taxa characteristic of brackish water and not recorded from any of the other lakes sampled. These

species are Gyrosigma distortiirn var. parkeri, Ncrvicrrln nliltica. Diplorleis interrrlpta, Surirella srricrrrrlcr, Nitzsclzia Ji.r[st~~lur~z, and N . .fi.[[str~lr[rn var. subsalinn. This lake is linked to the Beaufort Sea by several narrow channels over a distance of 150 km, and this accounts for the very high values of Na+ and C1- in the water sample.

Lake 21-This assemblage differs from all others in that i t is both poor in species and is dominated by Na1jicrrla spp. (46.4%). Amphorcr olwlis (19.0%), and Cyclotella cort7tcr (1 1.6%). The distinctive physical properties of the lake are referred to above and in more detail in Ritchie (1977).

Lakes 5, 7, 9, 12, 13, 16, anli20-They are dominated by Frngilaria pirzriata (rough1 y 30-60%) with roughly 5-10% of each of F. construens, F . con- srrrrens var. I3erzter, and N a v i c ~ ~ l a spp. These lakes show no particular features in common of geographical zone, morphometry, or ionic composition.

Lcrke 3-This lake is similar to the previous group in that i t is dominated by Frngilciricipirinata, but i t differs in having an abundance of Acllrzanthes conspicua, a littoral species. It has no obvious physical features distinguishing i t from the previous group.

Lakes 1, 2, 6, 8, 15, 17, and 19-These lakes show approximately similar proportions of Frag-

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1016 CAN. J. BOT. VOL. 56. 1978

FIG. 3. A set of bar diagram summaries showing the percentage frequency of the 14 diatom dominant species (+ total Naviculae). The species order is as in Table 2: the large numbers refer to each lake site. Open bars indicate percentage values x 0.1.

ilaria crotonensis, F. pinnata, Cyclotella glomel-- while lakes 1, 8, and 17 lack that species entirely. ata, Amphora ovalis, and Navic~ila spp., while lake These lakes are of variable size, depth, and geog- 19 has also 21.6% of Gyrosigma attenrratum. Lakes raphical location, but all fall within the ionic com- 2,6, 15, and 19 have 4-16% Fragilaria construens, position group designated above as oligotrophic.

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Lakes 18 and 10-Lakes 18 and 10 are dominated by Navicula spp. with Amphora ovalis, Fragilaria pinnata, and Pinnrrlaria microsta~rron.

KOlVO A N D RITCHIE

Planktonic vs. Littoral Forms The tribe Araphidineae and the order Centrales,

referred to below as A and C, respectively, are predominantly made up of planktonic species, while the Biraphidineae (B) and Monoraphidineae (M) are mainly littoral forms. A survey of the

l1 5 6 7 8 9

10 11 12 13 U 15 16

l i 5 6 7 8 9

10 11 12 13 U 15 (6

1 5 ; 5 6 7 8 9

10 11 12 13 U 15 (6 c 6

7 8 9

10 11 12 13 14 15 (6

% 5 10 15 20 25

diatom composition of the sediments in these groupings (Fig. 3 ) has some value in suggesting relationships to such physical parameters as length of shoreline, development of littoral habitat, and other factors.

Lakes 10, 18, and 20 are dominated by littoral forms (B + M values approximately 80%), and all three lakes have extensive rocky shores with local shallow bays. In addition, lake 21 shows evidence of chemical meromixis, its water has a high pH

21

6 7 i 8 9

10 11 12 13 14 15 16

5 6 7 1 8 9

10 11 17 13 l 4 15 =I

1 6 7 11 l7 10 11 12 13 U 15 (6

O1, 5 10 15 20 25 30 35 40 45

lk 10 11 12 13 U 15 16

181[ lo

10 11 12 13 U 15 16

8 i l

6

10 11 12 13 U 15 16

l9

10 11 12 13 U 15 (6

7 5 10 15 20 25

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1018 CAN. J. BOT. VOL. 5 6 , 1978

TABLE 3. Diatom ratios, their components, the number of taxa (S), sample size (N), and percentage relative abundance of Biraphidineae (B), Monoraphidineae (M), and Raphidioidineae (R)

Lake C:P, A : C CS PS A % c % 3 % M9 % , % s N

Boreal forest 1 0.17 1.40 5 29 38.0 28.0 28.0 2.0 4.0 34 100 2 0.09 3.75 7 74 63.0 16.6 15.0 4 . 2 0.6 81 1000 3 0.08 21.13 6 78 48.6 2 .3 26.7 20.0 2.4 84 1000 4 0.09 0.43 7 81 14.6 33.8 48.1 3 .O 0 .5 88 lo00 5 0.05 46.14 3 67 64.6 1 . 4 29.5 4 .4 0.1 70 1000 6 0.35 1.80 12 34 53.5 29.7 11.6 3.9 1 . 3 46 225 7 0.08 69.23 4 5 3 90.0 1.3 5.5 3.0 0 .2 57 1000 8 0.26 2.52 11 42 64.0 25.4 10.1 0 .5 0 .0 53 1000 9 0.01 217.67 1 82 65.3 0.3 21.8 11.8 0.8 83 1000

Forest tundra 10 0.02 6.38 1 57 8 .3 1.3 81.8 7 .9 0.7 58 1000 11 0.16 3.26 6 3'8 39.1 12.0 36.6 12.3 0.0 44 267 12 0.13 72.33 5 40 86.8 1 . 2 9 .6 2.4 0 .0 45 1000 13 0.03 204.25 2 62 81.7 0.4 15.1 2.5 0.3 64 1000

Tundra 15 0.18 2.77 9 48 49.9 18.0 30.5 1 . 2 0 .4 57 25 1 16 0.02 762.00 1 5 3 76.2 0.1 23.0 0 . 6 0 .1 54 lo00 17 0.18 2.57 8 44 53.3 20.7 14.2 11.8 0 .0 52 3 67 18 0.10 6.17 5 53 17.9 2 .9 78.0 1.2 0.0 58 1000 19 0.11 9.68 3 28 49.4 6 .2 44.4 0 .0 0 . 0 31 176 20 0.11 11.88 8 72 60.6 5.4 31.2 2 .8 0 .0 81 1000 21 0.06 0.05 3 47 0 .8 14.0 85.2 0.0 0 . 0 50 600

(8.2), low TDN, and no evidence of algal blooms, suggesting a poorly developed planktonic assemblage.

On the other hand, not all of those lakes with the highest proportion of planktonic forms (A + C is roughly 80%) have poorly developed littoral zones, and it is likely that firm conclusions are not possible in the absence of more detailed environmental analyses.

Diaton~ Indices of Trophic Status Diatom ratios have been used as indicators of

trophic conditions of temperate region lakes.

TABLE 4. A summary (range; mean, 2; median, tn ; standard deviations, SD) of diatom ratios, their components, the number of taxa (S), and sample size (N) and Biraphidineae (B),

Monoraphidineae (M), and Raphidioidineae (R)

Variable Range x m SD -

Nygaard (1949) used the ratio of centric to pennate species (C:P) to set limits on the categories oligotrophic, mesotrophic, and eutrophic; Stockner and Benson (1967) and Stockner (1971) used the ratio of Araphidineae to Centrales in making the same dis- tinctions. Both ratios are dependent on sample size, and Tarapchak (1973) suggests that a count of 500-600 per sample gives a reasonable estimate of the ratios.

The calculated C:P and A:C ratios for the lakes studied give C:P values between 0.01 and 0.35, and all values fall within the oligotrophic category except lake 6, which is in the mesotrophic class (Ta- bles 3, 4). On the other hand, the A:C ratios for these lakes are highly irregular, with high and low values occurring in lakes in the same biogeographic zone and with similar ionic composition and general morphometry.

Conclusions (1) From these lakes, 284 diatom taxa were re-

corded, comprising a group of wide-ranging, predominantly boreal-subarctic species. Many taxa occur also in nemoral lakes. The taxonomic analysis involved leads us to a conclusion identical with that of Foged (1964) in describing the diatoms of Spitzbergen, namely that "many variable species appear with forms differing from those in other geographical areas, but the differences are

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KOIVO AND RITCHIE 1019

often so slight that to describe them as species or nesota lakes. wlth preliminary notesondiatoms. Univ. Minn.,

varieties seems rash." A few arctic taxa reported Limnol. Res. Cent. Interim Rep. 3. BRUNSKILL. G. J . , D. M. ROSENBERG, N. B. SNOW, and R. by Patrick and from northern WAGEMANN. 1973. Ecological studies of aquatic systems in

occur frequently in some of these lakes: Surirefla the ~ackenzie-Porcupine drainages in relation to proposed terry; var. arctica Patrick and Freese, Amphora pipeline and highway developments. Environ. Can., Ottawa, ovalis var. barrowiana Patrick & Freese, A. birnir- 0"'.

kiana patrick & F ~ ~ ~ ~ ~ , and stauroneis hannae BURNS, B. M. 1973. The climate of the Mackenzie Valley. Vol.

Patrick & Freese. 1. Climatolog~cal Studles 24, Environment Canada, Ottawa, nnt -....

(2) The diatom assemblages and ionic composi- - 1974. The climate of the Mackenzie Valley. Vol. 2. tion of the water of the lakes in the uplands east of Climatological Studies 24, Environment Canada, Ottawa,

the Mackenzie Delta, consisting of variously modified glacial deposits derived from Mesozoic shales, siltstones, and sandstones, fall in the oligotrophic category. Two exceptional sites, one (lake 11) with saline influences and the other (lake 21) a small meromictic lake in the limestone doline, yielded distinctive assemblages and ionic compositions.

(3) The ratio of Araphidineae (Fragilaria, Synedra, and Asterionella) to Centrales (Cy- clotella, Melosira, and Stephanodiscus) used by Stockner (1971) to develop a classification for lakes in the central boreal forest region of northwestern Ontario was determined for our data but proved to

I have little relevance. Tarapchak (1973) found a similar lack of agreement when he used the ratio to

I analyze 68 Minnesota lakes and, in fact, proposed a I trend which is the reverse of Stockner's. It appears I that the A:C ratio has value but not in all geographi- I cal regions.

i Acknowledgments I

I This work was supported by the National Re- search Council of Canada (grant A6320) and the Finnish Academy of Sciences. We gratefully acknowledge the assistance of Dr. Ruth Patrick and Dl-. Charles Reimer in making available to one of us (L.K.) the reference collections of the Academy of Natural Sciences, Philadelphia. We thank Dr. L. R. Freese for helpful discussions on Alaskan diatoms, Dr. Veikko Lappalainen for making available facilities at Tampere University of Technology, Finland, Dr. G. J. Brunskill for arranging the chemical analyses of the water samples, and Mr. John Ostrick, Manager, Research Laboratory, Inuvik, for logistic support.

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Documents

Modern diatom assemblages from lake sediments in the boreal–arctic transition region near the Mackenzie Delta, N.W.T., Canada