Recent paleolimnology of three lakes in Northwestern Minnesota

QUATERNARY

RESEARCH

6, 249-272

(1976)

Recent Paleolimnology in Northwestern

of Three Lakes Minnesota’

HILARY H. BIRKS,’ M. C. WHITESIDE, DONNA M. STARK,4 AND R. C. BRIGHT5 Limnological Research Center, University of Minnesota, Minneapolis, Minnesota 55455 Received

April

8, 1975

A paleolimnological study was undertaken to investigate changes in three Minnesota lakes over the last 100 years and to demonstrate the hit- -i---a+‘cs&... effects,.-_of;~.L..-“cultural . ‘f--‘~-‘-in two bf. them. The-studycombined eutrophrcatron t s of the lake sediment fromYIY%. corGwii;“stratigraphic analyses of pollen, plant macrofossils, mollusks, diatoms and certain other algae, chydorid Cladocera, and Daphnia ephippia. The rise of Ambrosia type pollen (ragweed) marks the onset of interference with the landscape by European man, which can be closely dated. Calculations of sedimentation rates from this base gave reasonable correlations of other stratigraphic events with historical events. Elk La-ke is considered “unpolluted” today and was chosen as a control. Man’s effe&&e limited to logging some of the surrounding forest and to the construction of a dam. Small changes in the lake’s fauna and flora are demonstrated, showing the sensitivity of the lake to changes in its catchment area. I,a&S”$lie and St.--‘--IClair Lake, in the same watershed as the city of Detroit Lakes, have been affected not only by logging but also by addition of nutrients from agricultural runoff and sewage effluent. Considerable responses by the lake organisms are apparent. In Lake Sallie the changes were gradual, but in St. Clair Lake they were very abrupt because the lake was partially drained at the same time and the water volume was thereby reduced. The merits of such an integrated study, the types of information gained from the analyses of the various fossils, and the wider application of the results are discussed.

INTRODUCTION Cultural eutrophication of lakes, principally by the addition of sewage effluent and agriculturally enriched runoff, is widespread today throughout the world. Where the addition of nutrients is intense, great changes have taken place in the ecosystems. If a lake has amenity ‘Contribution 132, Limnological Research Center, University of Minnesota, Minneapolis. 2Present address: Botany School, Cambridge University, England. 3Rresent address: Zoology Department, University of Tennessee, Knoxville, Tennessee. 4Present address: 1000 Longfellow Boulevard, Lakeland, Florida. 5Present address: Bell Museum of Natural History, University of Minnesota, Minneapolis, Minnesota.

249 Conyright

0 1976 by the University

of Washington

value, such changes are regarded as deleterious, and efforts are often made to reduce algal blooms and macrophyte growth and to replace rough fish. These efforts frequently have no permanent success. Paleolimnological methods can be used to trace changes that have occurred during eutrophication. The remains of organisms preserved in the sediments can be used to reconstruct the lake biota, and changes can be detected in both the composition and the relative abundance of the fauna and flora. If the historical events that may have affected the lake catchment area are reasonably well known, as well as the rate of sediment then the limnological accumulation, changes can be correlated and dated, and

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perhaps the cause of a change can be identified. The nature of the change may be inferred from the nature of the historical event, in which case ecological information about the organisms affected will be gained. Alternatively, the nature, and hence the cause, of the change may be inferred from the already known tolerances of the organisms involved. In either case, the cause of simultaneous but inexplicable changes in another group of organisms may be suggested. Such information is valuable to the whole study of paleolimnology, and it also provides a basis for further studies on the ecology of the living organisms. A lake is strongly influenced by external factors, such as climatic conditions, the geology, soils, and vegetation of the catchment area, and the input and output of both solid and dissolved materials. A change in lake conditions due to a change in any environmental factor may cause a change in the populations of the lake organisms. However, if one organism is eliminated or its abundance is reduced at the expense of another, this does not necessarily mean that its environmental tolerances have been exceeded because it may have been affected by competition from other organisms that were favored more by the change. Thus, the concept of “indicator species” must be used cautiously. If possible, it is best to have several independent lines of evidence to document a change, and this will also provide greater perspective on the nature of the change. In the case of enriched lakes, it is desirable to know the reactions of the lake ecosystem to an increased input of nutrients, so that the effects of incipiently undesirable eutrophication may be noticed, and perhaps prevented, before they become serious. In badly affected lakes, the causes of the eutrophication may be predicted, and then possibly reversed, in the hope that the lake may return to a more desirable state.

ET

AL.

This paper considers the recent history (last 100 years) of three lakes in northwestern Minnesota. Lake Sallie is a recreational lake that has become intolerably eutrophic. Nearby is St. Clair Lake, a eutrophic but much smaller nonrecreational lake that takes the sewage effluent from the city of Detroit Lakes. Elk Lake is considered to be unaffected today by cultural eutrophication. Several fossil groups and sediment features were studied for each lake. Pollen analysis provided information on the vegetation and land use of the area. Plant macrofossils, cladocerans, and mollusks recorded primarily the nature of the shallow-water vegetation and its associated fauna. Diatoms yielded data on water chemistry. Various other sediment characteristics added information on water level, water chemistry, and the progression of cultural disturbance in the lake and the watershed. All sediment features were not measured for every lake, but in the present study the stratigraphic sequence for each lake is more important than the comparison among lakes. The three lakes differ somewhat in morphometry, water chemistry, and environmental setting, and the study was designed to show how cultural disturbance can produce a sedimentary record in three different situations. Other features could be investigated in the lake sediments: Ostracodes, chironomid larvae, and algae other than diatoms may leave a sedimentary record in some lakes, and the stratigraphy of major ions, pigments, and other chemical components can provide still more information on lake history in certain circumstances. We believe that the major events for these three lakes, however, are sufficiently well recorded to demonstrate the synergistic value of using several paleolimnological criteria. Elk Lake will be discussed first to provide a picture of background changes reflecting low intensity of human activity

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in the catchment area of the lake, which then may be compared with events in Lake Sallie and St. Clair Lake. H. H. Birks is responsible for the sediment descriptions except where indicated on the diagrams (Figs. 4, 6, and 7) and for the pollen and macrofossil analyses. M. C. Whiteside is responsible for chydorids, D. Stark for the diatoms, and R. C. Bright for the mollusks. The project was supported by grants to H. E. Wright at the University of Minnesota by the National Science Foundation (GB-7163), U.S. Environmental Protection Agency (Contract 16010 DXG), U.S. Atomic Energy Commission (Contract AT( ll-l)2046), and U.S. Office of Water Resources Research (B-181-Minn.).

FIG. 1. Map of Minnesota tions in Minnesota.

showing

location

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IN MINNESOTA

SITES Elk Lake is in Itasca State Park in Clearwater County (Fig. 1). It is more or less oval, about 1.6 km long, south of Lake Itasca at the headwaters of the Mississippi River. It is largely surrounded by mixed coniferous-deciduous forest and the secondary forest that developed after selective logging of white pine. The lake has a complex morphometry. It is located in the Itasca moraine of sandy calcareous drift. Bicarbonate is presumably the dominant anion, for sulfate is virtually absent (Table 1). The lake would not be regarded as polluted at the present day, and there are no dwellings around the shores. Lake Sallie is in the Pelican River

of lakes with respect

to major

vegetational

forma-

252 Physical

BIRKS TABLE 1 and Chemical Characteristics Minnesota Lake Sites’

Location Area (ha) Max. depth (m) Mean depth(m) Volume (m3X 106) Alkalinity $&yl) Mg++ Na+ K+ z4-

of the

Elk

St. Clair

Sallie

47’12’N, 95O12’w 101 30 11.0

46’47’N, 95O55’w 64.8 ca. 1.5 -

46’46’N, 95°55’w 528 16.5 6.35

11.2

ca. 0.7

33.7

3.00 38.4 11.7 7.7 1.8 0.8

5.54 66.5 30.0 34.0 6.7 12.8 13.8

3.49 32.8 27.5 10.8 4.4 15.4 33.7

aData from Lake Sallie after Peterson, 1971; St. Clair lake data from water samples processed at the University of Minnesota, St. Paul; Elk Lake data from Megard, 1968. Ionic concentrations in meq/l.

ET AL.

METHODS

Cores Short cores were obtained from the sediments under relatively shallow water. The core from St. Clair Lake was obtained in the winter of 1970 with a 5-cmdiameter plastic tube fitted with a rubber piston. The cores from Elk Lake and Lake Sallie were taken with the aid of SCUBA in summer 1970 with a 15-cm plastic tube 1.5 m long. The tube was capped at the top and bottom while still in the mud. The cores were extruded vertically to avoid disturbance of unconsolidated sediment, and contiguous segments were taken off the top. The sampling interval for the St. Clair Lake core was 2 cm and for the Elk Lake and Lake Sallie cores, they were 1 cm. The paleolimnological analyses are presented in Figs. 4, 6, and 7.

Description of Sediment watershed in Becker County (Fig. 2). It is an ice-block hollow within the outwash gravels of the Alexandria moraine complex. The core studied was taken from the north basin, which has a complex morphometry. The lake is surrounded by a narrow fringe of deciduous forest, beyond which is farmland. There are many dwellings around the lake, which has a high recreational use. It indirectly receives treated sewage from the city of Detroit Lakes (population 6000) by way of St. Clair Lake, and the water is regarded as highly polluted, with severe blue-green algal blooms in the summer and a dense growth of aquatic plants in the shallow water of the north basin. Further limnological details are given by Neel et al. (1973). St. Clair Lake (Fig. 2), just north of Lake Sallie, is much smaller and shallower. It has little recreational value. It presently receives effluent from a secondary-sewage treatment plant at Detroit Lakes.

The sediments were described with the Troels-Smith (1955) system, and the estimated proportions of each major component are shown in the diagrams. At Elk Lake and Lake Sallie a small raw sample for each level was examined microscopically, and the estimated abundance of certain fossils relative to other samples was made on a 5-point scale. For St. Clair Lake, similar estimates were made from the macrofossil samples rather than from raw samples. To estimate total organic content, the sediment was dried at 105°C and its weight loss was determined after ignition at 550°C. At Elk Lake and Lake Sallie the carbonate percentage was estimated by determining the further weight loss of samples ignited to 900°C. At Lake Sallie, the sediment was tested with dilute HCl, and the evolution of H2 S was noted by smell, indicating the presence of sulfides. At Lake Sallie, the percentage by weight of phosphorus was determined for the upper 32 cm by per-

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FIG. 2. Map of Detroit Lakes area to show locations features mentioned in the text.

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of Lake Sallie and St. Clair Lake and other

254 sulfate digestion, followed by the standard ammonium stannous chloride method.

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by titration molybdate-

Pollen Analysis Samples were prepared for pollen analysis by Erdtman’s acetolysis method, preceded by treatment with HF if necessary to remove silicates (Faegri and Iversen, 1964). Residues were mounted in silicone oil (2000 centistokes) for counting. The pollen was counted over whole slides, usually yielding about 300 grains, and was identified with the aid of the reference collection at the Limnological Research Center. Pollen percentages were calculated with a sum of total dry-land pollen and spores. The various subsums are indicated in the pollen diagrams. Pollen concentration was estimated at St. Clair Lake by addition of a known concentration of exotic pollen (Eucalyptus) to volumetric samples. Macrofossils Known volumes of sediment were washed with water through a 140-mesh screen (openings, 0.1 mm), and the macrofossils, including Daphnia ephippia and Crista tella mucedo statoblasts, were picked out. Plant macrofossils were identified with the aid of the reference collection at the Limnological Research Center. They are presented in the diagrams as number per 100 cm3 of sediment. Mollusks Mollusks were investigated only at St. Clair Lake. They were picked out along with the macrofossils from the screen residues and then identified and counted. They are presented in the diagrams as number per 100 cm3 of sediment. The material is preserved in the James Ford Bell Museum of Natural History, University of Minnesota. Chydorids Subsamples of mud for chydorid analysis were taken from selected levels of

ET AL.

the three cores. Processing and counting of the subsamples followed the procedures described by Whiteside (1970), except that treatment by HF to remove Minimum counts of silt was omitted. 150 remains are desirable, but at some levels this was not possible because of excessive dilution by sand or clay. The results are calculated as percentages of total chydorids, but only the taxa considered to show significant stratigraphic changes are presented in the diagrams. Diatoms Diatoms were analyzed in the Lake Sallie and St. Clair Lake cores. In the Elk Lake core, diatoms are not well preserved in the lower part. The processing with hot nitric acid and preparation of slides with Hyrax mounting medium follows the procedures described by Stark (1971). Counts of 500 frustules were made at each level, but the diagrams include only those taxa that are represented by more than 1% in at least one sample, or more than 0.6% in one sample if the taxon was considered to show significant stratigraphy. ELK

LAKE

History Elk Lake is situated just south of the western arm of Lake Itasca. The Indian name “Peke-gu-mog,” meaning “a water jutting off from another water” (Brower, X393), along with Brower’s interpretation of Elk Lake’s configuration as “an estuary, cast asunder by the gradual lowering of the surface of the present lake,” suggest a more intimate connection with Lake Itasca than at present. In his day there was little elevational difference between the lakes, and the direction of flow of the connecting Chambers Creek changes with variations in summer rainfall (Brower, 1893). An early photograph (Brower, 1893, p. 259) of Elk Lake clearly shows a lower water level than at present, with a zone loo-150 m wide of emergent vegetation at the north

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end around the Chambers Creek outflow. Brower’s description “Elk Lake being closed up with rushes . . . a dense growth extending far out into the lake” (Brower, 1893, p. 261) confirms the presence of a lake level lower than at present. Prior to 1900, Elk Lake was little visited. The drought recorded in the mid-nineteenth century in the Dakotas (Will, 1946) probably affected lake levels here as it did in the Detroit Lakes area. The fire history around Elk Lake was worked out by Frissell (1973). A fire in 1864 affected all but a small section of the northeast shore of Elk Lake (Fig. 3), but this area was also burned later, in 1875. The south and west shores were again burned in 1885, but since then the immediate surroundings of Elk Lake appear to have been unaffected by fires. Although the land was not cleared for farming and settlement, lumber companies logged extensively in the Itasca region at the turn of the century, and the Elk Lake basin itself was partially logged in 1917-1919. At this time a dam was built at the outlet of Elk Lake at Chambers Creek to facilitate the floating of logs from Elk Lake to Lake Itasca. During the depression years, the Federal

255

IN MINNESOTA

government maintained work camps in Itasca State Park, and at this time (1935) the original dam was replaced by a new dam, which presently maintains the water level of Elk Lake about a meter above that of Lake Itasca. The dam was maintained to prevent exposure of the shore area drowned as a result of the original dam. Sedimentation Rate A core 49 cm long was obtained under 6 m of water, near the base of a prominent underwater shelf (Fig. 4). The rise of Ambrosia pollen (ragweed) at 32 cm (Fig. 5)6 is considered to be a reliable stratigraphic marker for agricultural land clearance by European man, and thus it can be used to establish the sedimentation rate. The Itasca area itself has never been extensively farmed, so most of the Ambrosia pollen probably originates in the forest border area in northwestern Minnesota. The date of the Ambrosia pollen rise at Elk Lake is placed at 1890, according to findings at Lower LaSalle Lake, 16 km to the north, where unpublished pollen counts in annually lam6Figures section.

5-7

appear

at end

of

reference

FIG. 3. Map of Itasca State Park showing (in gray) the areas covered by major forest fires since 1820. Black areas indicate location of forest stands originating in the year of the fire. From Frissell (1973).

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FIG. 4. Section on the north side of Elk Lake showing the location of core with respect to the terrace. Generalized from Stark (1971).

inated sediments by David Foster indicate that the increase in Ambrosia pollen occurs at about this time. The sediments above the Ambrosia pollen rise at 32 cm are fairly uniform. If the sedimentation rate from 32 cm to the surface is assumed to be constant, it is thus 0.4 cm/year. Levels above 32 cm Below 32 cm are dated accordingly. there is a band of mollusk shells, and below that the sediments are much more minerogenic. The sedimentation rate of 0.4 below 32 cm therefore may not be uniform, but no other dated horizon is available. Fossil Zone a (49-46 cm) The gravel in the sediment may have been deposited at a time of low lake levels occasioned by the mid-nineteenthcentury drought referred to by Will (1946). The drought seems to have had little effect on the upland pollen rain, which is dominated by Pinus (pine), Bet&u (birch), and Quercus (oak), with some lesser taxa, both coniferous and deciduous. The macrofossils provide little evidence for limnological conditions, as all types are sparse. The lake may have been rather barren of macrophytes around the sampling point. Perhaps the gravel and coarse sand, released by a lowered lake level, was being washed over the shelf, together with a few conifer needles, and macrophyte growth was discouraged.

ET AL.

Fossil Zone b (46-3.2 cm) By inference, this sediment was deposited between the end of the midnineteenth century drought (about 1850) and the Ambrosia rise (about 1890). Gravel deposition ceased and coarse sand became rare at 46 cm. Large numbers of Najas flexilis seeds and Characeae oospores were deposited, together with Ceratophyllum spines, indicating that these taxa were growing near the sampling site, forming a kind of vegetation common in shallower parts of Elk Lake at present. Seeds of Nuphar show that Nuphar was also a member of the local aquatic community. The development of a nearby fringe of marsh vegetation is suggested by the occurrence of seeds of Scirpus acutus/S. validus, accompanied by pollen of Cyperaceae, Typha latifolia, Sparganium type, and Sagi t taria. The sediment is rich in mollusk shells. They may have originated from a large local population of mollusks living in the NajasEhara vegetation, or they may have been concentrated by current action at the base of the slope leading down from the shelf. Mollusk profiles are not plotted in Fig. 5, but the most abundant species are Gyraulus parvus, Marstonia decepta, and Valvata tricarinata, which are all typical of calcareous but not saline lakes in Minnesota today. A similar shell-rich band was found by Stark (1971) in a core taken in the same vicinity from under 7.75 m of water. However, it occurred above the level of the Ambrosia pollen rise, suggesting that the site of concentration of mollusk shells shifted with time, presumably as local physical conditions in the lake changed. Chydorid assemblages also undergo change in zone b, with Alona barbulata and A. quadrangular-is decreasing and Chydorus sphaericus increasing. The chydorid assemblage subsequently has changed very little.

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Fossil Zone c (32-14 cm) The sediments of this zone were formed between the time of the Ambrosia pollen rise (1890) and about 1935, when the permanent dam across the Chambers Creek outflow to Lake Itasca was constructed. Marked changes occur at the Ambrosia pollen rise. The increase of Ambrosia pollen, together with that of Salsola kali (Russian thistle, an introduced European weed) and Chenopodium type (goosefoot), reflect the beginning of extensive farming by European man in areas to the west. The proportion of tree pollen falls correspondingly. Pinus pollen, particularly Pinus Haploxylon (white pine type) and Pinus undiff. is affected, probably as a result of logging in the broad region of northwestern Minnesota. Macrofossils of tree taxa increase, particularly Larix laricina (larch) needles. These can be well represented in lake sediments even in deep water remote from the shore when Larix is growing on a lake margin (Birks, 1973). Thus Larix may have been locally present in a swampy community around the Chambers Creek outflow, perhaps with Sphagnum, whose spores are also recorded. There is an overall increase in pollen and seeds of marsh plants in this zone, forming an assemblage resembling that which occurs around Elk Lake today. Scirpus became less common, perhaps due to increased competition from Typha, which would be favored by the addition of nutrients, particularly phosphorus (Boyd and Hess, 1970). This development of reedswamp and marsh probably reflects the reed growth noted by Brower (1893). Changes within the lake itself are reflected in both the aquatic plant and animal assemblages. Characeae oospores are abundant, but Najas flexilis seeds decrease in numbers. Najas may have declined near the sampling site but remained common enough nearby to allow appreciable seeds transport to the site

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(Birks, 1973), perhaps from the shelf above. Other aquatic plants were Potamogeton and Ceratophyllum. Nuphar seeds ceased to reach the sampling site. It may have continued to be present, but it has a low seed representation and poor dispersal (Birks, 1973). Cristatella mucedo and Plumatella statoblasts increase in numbers, along with tests of rhizopods, probably reflecting an increase in their habitat, namely, aquatic vegetation. Among the Cladocera, ephippia of Daphniu are present in greatly increased concentration. This may reflect either an increase in the Duphniu population or increased ephippial production due to unfavorable conditions, or a combination of both. The overall change in Elk Lake at the Ambrosia rise seems to be an increase in organic productivity, reflected in a slow rise in the loss-on-ignition curve. At a depth of 26 cm, or 6 cm above the Ambrosia pollen rise, there is a further decline in Pinus pollen (especially white pine). If the proposed sedimentation rate is applied, this is dated to 1905, corresponding to the major logging of the Itasca area. Some evidence for inwash of organic soil detritus is implied by the rise of indeterminable pollen, due to increased corrosion of pollen grains. It is also seen in the presence of conifer needles and Betula fruits, as well as in the rise of fungal spores, which probably originated from soil. Wood chips of a shape that could have been produced by an axe were found in the sediment at 21 and 17 cm. Fossil Zone d (14-O cm) This zone includes the sediments above the decrease in silt and clay. With the proposed sedimentation rate, the depth of 14 cm is dated to 1935, the time of the construction of the permanent dam at the outlet of Elk Lake. Clay and silt are virtually absent from the sediment above 14 cm. The dam stabilized the water level for the first

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time in recent history, resulting in decreased inwash of mineral materials. The proportion of gyttja increases at the expense of marl, reflected by the crossing of the two loss-on-ignition curves. Najas flexilis seeds become rare, sugfesting that N. flexilis virtually disappeared from the aquatic vegetation near the core site. The slight decline in seeds and pollen of marsh plants may reflect the slight rise of the water level and reduction of the reedswamp to its relatively small extent. Diatoms become abundant in the sediment above 9 cm. No counts were made, however, because of the poor preservation at lower levels. LAKE

SALLIE

History Figure 5 shows the relationship between the city of Detroit Lakes, St. Clair Lake, and Lake Sallie. Settlement of this area and landclearing by European man began about 1870. The dam and lock between Lake Sallie and Detroit Lake was built in 1888, and the channel was deepened to allow the passage of small steamers. By 1909 a septic tank and limited sewage system was in use, and the effluent entered St. Clair Lake. In 1915 the construction of ditches led water from St. Clair Lake via Muskrat Lake into Lake Sallie (Fig. 2). Scattered cottages and resorts such as Shoreham have existed round Lake Sallie since 1900, increasing mainly since 1940. The odor from the lake became so offensive in recent years that further building has been discouraged (F. J. Long, letter 1970). In 1929 a two-stage sewage-treatment plant was constructed at Detroit Lakes and later enlarged in 1941. Even though the effluent was held and then filtered through a marshy area that was formerly part of St. Clair Lake, nutrients still reach Lake Sallie, and its fertilization continues. By 1938, residents com-

ET AL.

plained about the effects of water enrichment. During the summer of 1947 a severe algal bloom and an associated fish kill occurred in Lake Sallie. Blooms of blue-green algae and diatoms, together with extensive growth of macrophytes, have been a nuisance ever since, and bullheads have become the predominant fish, replacing walleye pike and other fish more desired by anglers. The land around Lake Sallie was cleared for farming, but an extensive belt of trees was left around the lake. The severe drought between 1934 and 1940 (Will, 1946) caused a noticeable drop in lake level (Larson, 1971). The more severe droughts between 1836 and 1851 probably had an even greater effect. Sedimentation Rate A core 75 cm long was taken below about 6 m of water at the foot of a distinct littoral shelf, as at Elk Lake. The Ambrosia pollen rise at 48 cm is taken to date from 1870 (Fig. 6). If a constant sedimentation rate is assumed above this, the sediment accumulated at 0.5 cm/ year. This may not be realistic, for the sediment has a varied composition. The sandy layer at 40-48 cm may have accumulated more rapidly by inwash from dam building and ditching in 1888 and 1889. Above 40 cm, there are indications of eutrophication in the profile corresponding to the building of cottages and resorts around Lake Sallie since 1900, and the input of sewage effluent from the town. If the rate is assumed to be 0.5 cm/year, this level at 40 cm is dated to 1890, somewhat earlier than expected, perhaps indicating that the sandy layer was more rapidly deposited. With this rate, the changes at about 25 cm to blacker, sulfide-rich sediment and other biological changes are dated to 1920. But if the 40-cm depth is taken to mark 1900, then the subsequent rate is approximately 0.6 cm/year. At this

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259

rate, the 25-cm level is dated to 1928, Pediastrum was very abundant, and about the time when the sewage-treatGloeotrichia and Botryococcus were also ment plant was built and increased present. amounts of effluent reached Lake Sallie. Within the chydorid assemblage, ChyTherefore, although there is considerable dorus sphaericus is the most common doubt as to the accuracy of the sedimenspecies, as it is throughout the profile. tation rate, in the absence of any other The most equitable distribution of chyindependent markers the latter rate is dorids occurred during the drought peprobably more nearly correct above 40 riod, when there was extensive Charal cm. Najas vegetation and alkaline or slightly Below the Ambrosia pollen rise, the saline water conditions. sediments vary in composition, and it is felt unwise to use the above sedimentation rate to estimate the age of the Fossil Zone b (70-48 cm) This section corresponds to the period horizons. from the end of the drought in the Fossil Zone a (75-70 cm) 1850’s to the Ambrosia pollen rise in 1970. The high pollen values for Quercus, The end of the drought was probably a Artemisia (sage), and Gramineae (grasses) suggest that Lake Sallie was surrounded gradual process, as there seems to have by a type of oak savanna at this time been little marked change either in the (Mc Andrews, 1966). Marsh vegetation, upland, marsh, or aquatic vegetation, or represented by seeds of Scirpus, Typha, in the animal populations. The upland and Juncus spp., was more extensive vegetation continued as a type of oak than later, perhaps because of lower savanna, but other more mesic species water levels associated with the midwere present. Seeds and pollen of marsh nineteenth-century drought (Will, 1946). taxa (Scirpus, Typha, Juncus) decline at Seeds of Chenopodium rubrum may re- the end of zone a, suggesting that the flect the occurrence of mud habitats marsh communities were reduced in temporarily extent as the water level rose and filled exposed by fluctuating water the basin. Najas marina ceases to be levels (drawdown conditions; at 57 cm. Perhaps the inWatts and Winter, 1966). All these seed represented crease in lake volume and reactivation of taxa are found together today around the outlet reduced the sulfate content of prairie and savanna lakes in Minnesota the lake below the minimum required by and North Dakota containing relatively Najas marina, namely, 50 ppm (Moyle, high amounts of dissolved ions (Birks, Seeds of Zannichellia palustris 1945). 1973). cease just below the Ambrosia rise. Among the fossils of aquatic plants, Perhaps by this time the lake was sufCharaceae oospores are exceptionally ficiently dilute to pass the tolerance of abundant, together with seeds of Najas Zannichellia also. However, the general marina, N. flexilis, and Zannichellia decline in aquatic plants may reflect not palustris. These four taxa occur together the water chemistry but the water depth, today in lakes in the deciduous forest and savanna of Minnesota, which have a for the sampling site may have become too deep for successful macrophyte relatively high concentration of carbongrowth, and seeds and oospores had to ate and sulfate in their water but which be transported from the shallower water are not truly saline (Moyle, 1945; Birks, of the shelf above. Values of Pediastrum 1973). No diatoms were recovered from zone a, but among the other algae also decline, presumably because one of

BIRKS

its main habitats, aquatic vegetation, was reduced near the sampling site. In the upper part of the zone, mollusk shells are common in the sediment, associated with increased proportions of clay and silt. These shells may have been deposited from the shelf above the sampling site, as the littoral sediments were disturbed by the rising water level. If the end of zone a is taken to represent about 1850, the average sediment accumulation rate between 70 and 48 cm was 0.9 cm/year, about twice the rate above the Ambrosia rise. Such a relatively high rate, in association with the high minerogenic and shell content, suggest that these sediments were redeposited after wave erosion. Further evidence of a rising water level is obtained from the diatom stratigraphy. Diatoms are very scarce and generally poorly preserved below 60 cm, possibly due to oxidation of the sediments and mechanical breakage at the time of low water level. Between 60 cm and the Ambrosia rise at 48 cm, the diatom flora is dominated by benthic and epiphytic species. Amphora ovalis var. pediculus declines from high percentages near the top of the zone, with a corresponding increase in other species. This change in the mainly benthic flora (Stephanodiscus cf. S. dubius is the only planktonic diatom represented by an appreciable percentage) may reflect changes in the substrates on which these species lived. These may be related to the change in the aquatic macrophyte flora discussed above or to the increase in clay and mollusk shells in the sediments. Another factor in the decline in Amphora ovalis var. pediculus may have been a rise in water level during the time represented by the sediments of zone b. This form is a benthic diatom most commonly found in wave-washed littoral environments, to which it is adapted by its small size and round dorsal contour (Bradbury, 1975). Its gradual decline through the

ET AL.

zone may reflect the replacement of shallow, turbulent conditions by deeper, quieter water at the core site. The chydorid populations show some changes in this zone. A large peak of Monspilus dispar correlates with the abundance of silt and shells in the sediments. This species is characteristic of well-oxygenated water above minerogenie sediments (Fryer, 1968). The sum of Alonella species is greatest in this interval, and their presence supports the hypothesis of lower salinity than before (Whiteside, 1969). Fossil Zone c (48-25 cm) The Ambrosia rise is delimited in the pollen diagram at the marked increase of Ambrosia type pollen, accompanied by the consistent occurrence of pollen of Salsola kali and Cerealia undiff. (cereal grains). At this horizon, there are corresponding falls in values of Artemisia, Gramineae, and Cyperaceae (sedges) pollen, taxa that include many plants of the unbroken prairie. Increasing pollen percentages of deciduous trees imply that deciduous forest developed locally around Lake Sallie, as there is no similar change at nearby St. Clair Lake. Presumably the trees were encouraged as houses were built around the lake. Farming activities became extensive but were beyond the fringe of trees. At 33 cm, pollen of Brussica kaber type (mustard) and Secale (rye) provide evidence of crops grown in the area. Mollusk shells are less common, and the proportion of sand and terrestrially derived coarse material, such as moss stems and Betula fruits, increases. This influx may originate from inflowing streams, particularly the Pelican River, receiving a larger load due to agricultural land clearance and later dam building and channel widening. Fossils of aquatic plants remain sparse, being represented by a few pollen grains of Potamogeton and leaf spines of Ceratophyllum. This

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is in contrast to the large changes in the diatom and chydorid stratigraphy. The earlier dominance of benthic and epiphytic diatom taxa is replaced by dominantly planktonic and meroplanktonic taxa which continue to the top of the core. However, the percentage decrease in benthic and epiphytic species may not represent an actual decrease in their populations, because the total diatom population may have greatly increased. Such a total increase at this time is indicated by the column for diatom abundance in the sediment section of Fig. 6. The first diatom species that exhibit marked increases in percentages above the Ambrosia rise are Cyclotella comta, Melosira ambigua, M. granulata, M. granulata var. angustissima, and Fragilaria pinna ta. Other Fragilaria spp. that become abundant early are characteristic of quiet littoral environments, where they are benthic or epiphytic. Thus their increase, associated with the decline of Amphora ovalis var. pediculus, provides evidence for deeper and less turbulent water as the lake basin gradually filled. Cycle tella corn ta is a planktonic species characteristic of oligotrophic and mesotrophic waters, indicating that Lake Sallie was not yet eutrophic in the early part of zone c. Among the chydorids, the dominance of C. sphaericus may reflect expansion of its main habitat of macrophytes, although the macrofossil and pollen records give no evidence of this. Alternatively, it may indicate blooms of blue-green algae, which can provide floating platforms for this chydorid in open water-an explanation introduced for a similar change in Shagawa Lake, northeastern Minnesota (Bradbury and Megard, 1972). However, there is no historical record of visible algal blooms in Lake Sallie before 1938, and evidence from pigment analysis was not obtained. At 39 cm (shortly after 1900) several

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changes occur that may be related to increased eutrophication of Lake Sallie, as effluent from the city of Detroit Lakes affected it. There is a marked rise in the loss-on-ignition curves for organic matter and carbonate, which may indicate an increase in productivity. However, it may just reflect the end of sand deposition with the stabilization of the water course. Fossils of aquatic macrophytes become more abundant, particularly Characeae, Myriophyllum, Ceratophyllum, and Najas flexilis. Such an assemblage is characteristic of lakes in central Minnesota today with high carbonate, nitrate, and phosphate concentrations. Cristatella mucedo statoblasts, Daphnia ephippia, and Pediastrum also increase above 39 cm, probably resulting from the increase of aquatic vegetation, which provides their habitat. Chydorus sphaericus reaches its maximum, and there is little change in the chydorid assemblage from this time to the present day. Among the diatoms, Cyclotella comta and Melosira ambigua percentages decline while several eutrophic species such as Fragilaria crotonensis, F. brevistrata, F. vaucheriae, F. capucina v. mesolepta, Asterionella formosa, Stephanodiscus astraea var. minutula, and S. niagarae increase. The rise in Fragilaria species may also reflect the increase in aquatic macrophytes, which provide substrate for many of the meroplanktonic forms. The changes at 39 cm point to increased eutrophication, presumably related to further human disturbance. No unambiguous time markers are available above the level of the Ambrosia pollen rise, and the uncertainty in the estimation of the sedimentation rate prevents reliable interpolation of dates between 1970 and the present. With the construction of sanitary sewers for the city of Detroit Lakes early in the 1900’s, the effluent was directed via St. Clair Lake to Lake Sallie, and the level of pollution

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during subsequent decades probably increased steadily as the city population grew and as the resorts and cottages were built. Fossil Zone d (25-O cm) It is likely that the sediment deposited at 25 cm dates from about the time of the construction of the sewage-treatment plant at the city of Detroit Lakes. The sewage-treatment plant removed organic carbon by biological oxidation but let through ever-greater quantities of nitrates and phosphates, which increased the organic productivity of the lake. The incorporation of increasing amounts of phosphate and organic matter in the sediments is shown in the phosphorus and loss-on-ignition curves. Because of the high organic productivity of the lake, the sediment surface became anaerobic, and sulfides were deposited rather than sulfates. This gives the sediments a blackish color when fresh and results in the evolution of HzS on addition of dilute HCl. Aquatic macrophytes probably became very dense. Pollen taxa with relatively large values include Potamogeton and Ruppia. The latter flourished in quite deep water in lakes with relatively high ionic content (Moyle, 1945). The ionic concentration of Lake Sallie water may have been increased by the influx of sewage effluent and runoff from agricultural land containing sulfate residues from fertilizers. The macrophyte growth in shallow water is a problem today, fouling boats and fishing lines. The most abundant species growing in the lake in 1949 were Potamogeton gramineus, P. pet tina tus, P. zosteriformis, P. richardsonii, P. praelongus, Najas flexilis, Ceratophyllum demersum, Chara aspera, Nymphaea odorata, Nuphar variegatum, and Vallisneria americana (Lake Survey Report on Lake Sallie, 1949). Ruppia was not recorded, but it was noted as a prominent macrophyte in 1969, 1970,

ET AL.

and 1971 ‘in water up to 3 m depth by Neel et al. (1973). No fossils of Vallisneria were recovered, and Potamogeton spp. are generally poorly represented by fruits (Birks, 1973). The dense aquatic vegetation provided habitats for an abundance of diatoms and other algae, such as Pediastrum and Gloeo trichia. The latter is planktonic during the summers (Roelofs and Oglesby, 1970) and can reach high densities. It spends the winter as spores or colonies on the sediment surface. Neel et al. (1973) report that it first appeared as an epiphyte but soon became planktonic in Lake Sallie during the summers of their study. The blooms of blue-green algae observed since 1938 are not recorded by fossils in the sediments, and algal pigments were not analyzed. However, decreasing percentages of summerblooming diatoms such as Melosira ambigua and M. granulata (and var. angustissima) may reflect the abundance and competition from blue-green algae. On the other hand, Stephanodiscus astraea var. minutula blooms in the late winter or early spring and is therefore not affected by the competition (Bradbury, 1975). The large increase in Fragilaria construens var. binodis at about 20 cm may reflect the increasing macrophyte growth. Fragilaria capucina mesolepta, associated with severe ViSl-. pollution in Lake Michigan (Stoermer and Yang, 1970), also indicates increasing eutrophication in Lake Sallie in recent years. Animal life also flourished in the lake. High values are recorded for Daphnia ephippia, Crista tella mucedo, Pluma tella, and Chydorus sphaericus. Fish fossils were not recovered, but recently there has been a marked shift from a population of game fish, particularly walleye pike, to a population of coarse fish, in which bullheads perdominate (Moyle, 1970). The pollution of Lake Sallie continues

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to the present day. Studies have been initiated to investigate the control of plant growth by cutting and removing the weeds from shallow water (Neel et al., 1973) and the control of fish populations by mass removal of bullheads (Olson, 1970). Suggestions have been made by Moyle (1970) on ways to control the flow of sewage nutrients into the lake, and this is probably the only way of effecting a permanent improvement. It is considered important to prevent nutrient-rich water from Lake Sallie flowing farther down the Pelican River to other high-amenity lakes such as Lake Melissa and Pelican Lake, which are already beginning to show signs of increased fertilization. ST. CLAIR

LAKE

History St. Clair Lake is about 2 km north of Lake Sallie. It experienced the same general historical events. Sewage treatment in the early 1900’s was by septic tank, and the effluent may have seeped into St. Clair Lake. At this time, the lake had a larger volume of 243 ha (Fig. 2). It had no outlets, and it received water only from some manmade culverts at the north end. In 1915 County Ditch 14 and St. Clair Ditch were constructed, resulting in partial drainage of St. Clair Lake, with reduction in depth and size to its present dimension (Table 1). The water entered the Pelican River, flowed into Muskrat Lake, and then passed to Lake Sallie. There have never been any cottages around St. Clair Lake. The timber was cleared to the shore and the land was used for farming. Sedimentation Rate St. Clair Lake, unlike Elk Lake and Lake Sallie, is shallow and its morphometry is simple. The core was taken in the northeast part of the present lake, where the water depth is 2 m. The Ambrosia

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pollen rise is placed at 47 cm (Fig. 7). The Ambrosia type pollen curve rises only slightly at this level, but the first pollen of Cerealia undiff. and Sulsola kali are taken as the horizon of agricultural disturbance. If the Ambrosia rise dates to 1870, the average sedimentation rate from here to the sediment surface is 0.47 cm/year. The horizon of marked changes in all groups of fossils at 33 cm is dated, on this rate, to about 1900. Such a marked set of changes probably occurred as a result of a large change in lake conditions. This could be due to the addition of relatively small amounts of sewage effluent since 1909, or more likely, to the major drainage of the lake in 1915. If this is the case, the rate of sediment accumulation between 47 and 33 is considerably slower than that above 33 cm. This may be due to a high lake productivity and a more rapid organic deposition above 33 cm. On the other hand, sediment disturbance and removal at the time of drainage may have produced a hiatus. The sedimentation rate below 47 cm is uncertain, but the base of the zone of mollusk abundance at 70 cm may correspond to the end of the mid-nineteenth century drought. Fossil Zone a (80-47 cm) This zone is the sediment from the base of the core up to the Ambrosia pollen rise of about 1870. The forest surrounding St. Clair Lake was dominated by oak, but, unlike that around Lake Sallie, it contained considerable amounts of other deciduous trees. The shores of St. Clair Lake are not steep, and the soils may have been considerably moister than those around Lake Sallie, supporting a fringe of more mesic forest. The relatively high percentages of Artemisiu and Gramineae pollen indicate that prairie communities were also widespread near the lake, producing pollen of plants typical of

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such habitats, e.g., Petalostemum purpureum and Amorpha. The occurrence of Typha latifolia pollen and Typha seeds indicates the development of a cattail marsh around the lake. Other plants in the marsh are represented by pollen of Sparganium type (possibly Typha angustifolia) and Menyanthes trifoliata and seeds of Mentha arvensis and Urtica gracilis type. The aquatic vegetation within the lake is represented by fossils of Najas flexilis, Ceratophyllum, Potamogeton, Ranunculus trichophyllus type, Nuphar, and Myriophyllum exalbescens type. Similar assemblages are found today in fertile lakes rich in carbonates but low in sulfates. Presumably the source of sulfates in Lake Sallie was local and did not affect the water entering St. Clair Lake. In this shallow fertile lake with abundant aquatic vegetation lived a large number of diatom species, mainly littoral types with benthic or epiphytic habitats. Motile benthic diatoms such as Navicula spp. presumably lived where macrophytes were sparse or absent. The considerable percentages of eutrophic planktonic and meroplanktonic species such as Stephanodiscus astraea var. minutula, Melosira granulata var. angustissima, and Fragilaria brevistriata indicate that St. Clair Lake was more eutrophic than Lake Sallie before disturbance by man, probably because of its smaller size and depth. Pediastrum colonies are abundant in this zone, and the presence of hystrichosphaerids indicates that dinoflagellates flourished. The highest values of hystrichosphaerids occur at the base of the zone, when there was perhaps a high rate of encystment during the more unfavorable conditions of the drought. The molluskan assemblage is one typical of perennial bodies of water with moderate to low salinity, low amounts of suspended solids, summer temperature max-

ET AL.

ima below about 20°C, and abundant The abundance of rooted vegetation. Pisidium spp. indicates that the cover of macrophytes was not complete (as is also suggested by the abundance of Navicula spp. among the diatoms), for these clams live in mud where water movements ensure an adequate supply of oxygen. Fossil Zone b (47-33 cm) This zone occurs between the Ambrosia pollen rise and the level taken to represent the partial drainage of the lake in 1915. The pollen diagram shows little marked change at the first rise of Ambrosia pollen, apart from the start of curves of Salsola kali and Cerealia undiff. pollen. Tree pollen declines slightly to a minimum at 33 cm, particularly Quercus, reflecting the clearing of the forest for settlement and farming. The majority of trees were probably cleared from around St. Clair Lake. The aquatic macrophytes and aquatic animals changed only slightly. The richness of the diatom assemblage is reduced above the Ambrosia rise, mainly due to the disappearance of many benthic species, perhaps because the lake became too overgrown with weeds. However, the Pisidium population, which also needs open mud, was unaffected. Perhaps the input of organic matter made the sediments relatively anaerobic, thus eliminating less tolerant benthic species. Planktonic and meroplanktonic diatoms increase and Stephanodiscus astraea var. minutula and Fragilaria brevistriata become dominant. In general, the changes in St. Clair Lake soon after the Ambrosia pollen rise were slight. The lake was already moderately eutrophic, perhaps because of its shallowness and relatively small size, and the addition of nutrients from agricultural practices or sewage effluent did not cross any significant thresholds

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tolerances

PALEOLIMNOLOGY

of the lake’s

Fossil Zone c (33-O cm) It is proposed that the large changes in the fauna and flora at 33 cm are a result of the considerable drainage of St. Clair Lake in 1915. The types of changes are consistently explicable in terms of this proposal. The exposed lake mud to the north developed into a large marshy area. An increase in Fraxinus pennsylvanical Fraxinus americana pollen suggests that green ash may have found a suitable habitat here, perhaps associated with Acer negundo (box elder), whose pollen values also increase. There is a marked rise in the percentages of Ambrosia type pollen. Perhaps Ambrosia was able to take rapid advantage of the new open habitats created. The rises in Chenopodium type and Salsola kali pollen may be for the same reason. Chenopodium species are typical colonizers of exposed mud in lakes with fluctuating water levels. The increased area of damp shore favored the expansion of Typha. Typha seeds and Typha latifolia pollen may originate from Typha angustifolia. Other taxa typical of disturbed shorelines are recorded throughout this sediment section: macrofossils of Ranunculus sceleratus, Polygonum lapathifolium type, Chenopodium rubrum, Rumex maritimus var. fueginus, Bidens cernua, and pollen of Urtica type, Bidens type, and Impa tiens. The composition of the aquatic vegetation changed markedly. Najas flexilis declined near the sampling site. Ceratophyllum demersum probably became the dominant plant. The numbers of Pediastrum colonies decline with the increased density of vegetation and shallower water. However, the numbers of hystrichosphaerides rise to a peak before declining to zero. Perhaps as conditions became unfavor-

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able most of the dinoflagellates encysted and failed to germinate again. The composition of the diatom flora changes at 33 cm, most notable being the large decrease in Fragilaria brevistriata percentages. The change in species composition resulted from increased concentration of nutrients as the lake was reduced in size, as well as from additional sewage inflow. The planktonic species Stephanodiscus astraea var. minutula and Melosira granulata var. angustissima dominate the assemblage. Presumably, Melosira had to compete with summer blooms of blue-green algae, although no direct or historical evidence is available for such blooms. Perhaps turbulence in such a shallow unstratified lake was sufficient to recirculate living Melosira continuously from the bottom (Bradbury, 1975). Stephanodiscus would escape the cyanophyte competition, as it blooms in late winter or early spring. The decline of Fragilaria brevistriata, a common epiphyte on aquatic macrophytes in quiet water, may result from competition from epiphytic diatoms tolerant of more eutrophic conditions, such as Fragilaria capucina var. mesolepta and Nitzschia frustulum. However, the percentage decline may be partly the result of the great population increase of planktonic species. In the molluskan fauna, prosobranch snails are replaced by pulmonate snails, which are characteristic of shallow, intermittent ponds, marshes, and ditches. Also, previously common fingernail clams (Pisidium spp.) became rare. This marked shift in mollusk populations is difficult to evaluate in detail because of the imprecise knowledge of mollusk ecology. But on the basis of empirical data several hypotheses seem plausible: (1) The lowered water level of the lake, runoff from cropland in the watershed, and addition of sewage effluents could have combined to increase the suspended Prosobranchs are solids in the lake.

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generally less tolerant of high loads of suspended solids than are pulmonates. (2) Lowering of the lake level probably also produced higher summer maximum temperatures of the water, thereby favoring survival of pulmonates. (3) Addition of sewage effluent to the lake via Ditch 14 might have increased the ionic concentration and lowered the oxygen concentrations that would lead to the demise of fingernail clams and prosobranch snails. Also, there might be something in the effluent that is directly toxic to both prosobranch snails and fingernail clams. (4) Increased production of plant detritus at the site, especially from Ceratophyllum, and its subsequent decay could have added further to deoxygenation of water at the site, especially in the fall, and could have provided added stress to all but the pulmonates. All the foregoing factors no doubt combined to some inseparable degree to produce a changed environment that was, at least locally, fatal to all but the most adaptable mollusks. The population of Daphnia as measured by the concentration of ephippia increased enormously, presumably thriving on the increased numbers of planktonic algae. Chydorid assemblages show a dramatic response to the changed lake level. The large Alona affinis and A. quadrangularis become rare, while Chydorus sphaericus, Alona circumfibriata, and Pleuroxus spp. increase. This latter response is typical of enrichment conditions (Whiteside, 1970). The conditions in St. Clair Lake differed from those in Lake Sallie before 1970 in that the water was moderately eutrophic, as indicated by the diatoms, chydorids, and Daphniu. The start of European man’s activities at the time of the Ambrosia pollen rise had little impact on the lake ecosystem, in contrast to both Sallie and Elk Lakes, where the addition of a relatively small amount of nutrients as a result of logging and land

ET AL.

disturbance was sufficient to cause substantial changes in the biota of those lakes. Conditions in St. Clair Lake remained in equilibrium until its partial drainage in 1915 and the subsequent ever-increasing input of nutrients. DISCUSSION General Of the three lakes studied, Elk Lake acts as a control because it has received relatively little disturbance from Euroactivities pean man, whose principal there were logging and the construction of a small dam at the outflow of the lake. It has never received sewage effluent or agricultural runoff. By contrast, Lake Sallie and St. Clair Lake have been considerably affected. The surrounding land was largely cleared for agriculture. Alterations were made in the upper catchment area of Lake Sallie, which includes St. Clair Lake, and these resulted in partial drainage of the latter. Nutrients, particularly nitrogen and phosphorus, were added in ever-increasing quantities to the lakes’ water, either directly from the surrounding dwellings or indirectly from sewage effluent from the city of Detroit Lakes. The arrival of European man is documented in the biostratigraphy by the Ambrosia pollen rise. Historical evidence indicates that this occurred at about 1890 at Elk Lake, and at about 1870 at Detroit Lakes. In the absence of other good time-markers, the average sedimentation rate was calculated above the Ambrosia rise, and, with some defendable exceptions, this gives acceptable dates for other changes in the biostratigraphy. All the cores were obtained from shallow water. Sediments in such situations are susceptible to erosion and redistribution, and thus the biostratigraphic record is unlikely to be annually precise. Ideally, annually laminated sediments would be preferred, but these

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seem to be deposited only in very deep similar changes in other sites of unknown water, and indeed they were formed in history, either undocumented or prethe deepest part of Elk Lake (30 m) and historic, can perhaps be interpreted more analyzed for diatoms (Stark, 1971). fully. (5) The first indications of the reHowever, profundal sediments alone are actions of the organisms to eutrophicafelt not to be entirely suitable for the tion have been identified, and the results study of a lake ecosystem, because large of further eutrophication documented. fossils (e.g., shells, seeds) and locally If similar changes are detected in nonproduced microfossils (e.g., aquatic polluted lakes, perhaps the signs can be plant pollen, littoral diatoms, and other recognized before it is too late to prevent algae) are not sufficiently well repreundesirable effects. sented to allow a reliable reconstruction The reactions of the different comof past lake conditions. In addition, a ponents of the lake ecosystem we studied single core, whether littoral or profundal, are evaluated as follows. may contain an inadequate representaSediments tion of events within the whole lake, especially a large lake with complex The complexity of each lake’s sedimorphometry. Cores from other parts ments makes it difficult to interpret of Elk Lake and Lake Sallie were studied sediment changes. If the bases of the by Stark (1971) and Bradbury (1975), profiles were deposited during the midand it is reassuring that they show the nineteenth century drought, the sedibroad events recorded from our cores. ments were all more minerogenic under Therefore, although only a local picture these conditions than those deposited may emerge from a littoral core, it later. At Elk Lake and Lake Sallie, the probably reflects events throughout the increased steadily organic component lake sufficiently well to make the study above the Ambrosia pollen rise, perhaps of further cores not worthwhile, unless as a result of increased productivity special facets are to be investigated. arising from the input of nutrients. AlSeveral benefits have been gained by though the sediment was considered to be more organic at St. Clair Lake above combining a study of different organisms and the sediments in an integrated way: the Ambrosia rise, the curve for loss-onignition at 550°C declined, perhaps be(1) Changes in the biostratigraphy have cause it was affected by the large number been attributed to environmental events of known nature, and therefore informaof mollusk shells present. Therefore, although the sediments can give contion has been gained about the reactions of these organisms to these conditions. firmatory evidence of processes occur(2) Such information has posed ques- ring in the lake, they cannot alone indicate a great deal about the progress tions as to the mechanisms of population change in groups of organisms for which of eutrophication. More positive evirelatively little ecological knowledge is dence may be obtained by further chemcurrently available. (3) Changes in one ical analyses, for example, of nitrogen, and plant pigments (see group have been interpreted in a certain phosphorus, Changes in another group have Shapiro et al., 1971; Sanger and Gorham, way. been used to confirm the hypothesis or 1970). cast doubt upon it, and therefore a more complete picture of events in the eco- Chydorids For all three lakes, the response to system can be built up with confidence. human disturbance in the watershed was (4) Because certain changes have been an increase in Chydorus sphaericus and a interpreted in terms of known events,

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decrease in Alona affinis and A. quadrangubaris. At Elk Lake and Lake Sallie this change in composition corresponded to the Ambrosia pollen rise; at St. Clair Lake the increase began at the Am brosiu rise and was intensified at the change in water level. Kerfoot (1974) argues that a similar shift of larger to smaller Cladocera at Frains Lake, Michigan, may have been caused by increased predation. His model is based on an apparent increase in aquatic vegetation with European man’s arrival, leading to more cover for smaller fish, which preferentially ate the larger Cladocera. We do not believe this to be the case in Minnesota lakes. Aquatic vegetation does not increase significantly at the time of chydorid change. In addition, other chydorids (Acroperus harpae, Eurycercus, and Leydigia Zeydigi) that are approximately the same size as the large Alonas do not decrease in their percentages. We attribute the change in chydorid assemblages instead to enrichment of the lakes. Enriched lakes have high percentages of C. sphaericus and lower species diversity (Whiteside and Harmsworth, 1967). At Elk Lake the shift is not so apparent, suggesting a lesser degree of enrichment, whereas both Lake Sallie and St. Clair Lake have dramatic changes that correspond to enrichment. At St. Clair Lake increased percentages of Alona circumfimbriata and Pleuroxus spp. accompany the increased pollution.

Daphnia It would appear that Daphnia ephippia are deposited in the sediments of eutrophic lakes but not mesotrophic alkaline lakes, such as Lake Sallie and Elk Lake before the Ambrosia pollen rise. Ephippia were present in the sediments of St. Clair Lake at this time, which from other lines of evidence is considered to have been moderately eutrophic already. The numbers of ephippia may be correlated

ET

AL.

with the degree of eutrophication. Their concentrations are greatest at St. Clair Lake, somewhat less at Lake Sallie, but considerably less at Elk Lake. The deposition of ephippia starts and increases in a previously unenriched lake as a response to slight enrichment, for example by logging in the catchment, but the increase is much greater as a result of nutrient addition from sewage effluent. Perhaps there are substances in the effluent apart from nitrates and phosphates that encourage the increase of Daphnia populations or cause a greater Little rate of ephippial production. appears to be known about the ecological or physiological factors affecting this process at the present time. Analyses of other Daphnia exoskeletons in the sediments might help to resolve this problem. Mollusks At all three lakes there is a zone of sediment rich in mollusk shells. At Lake Sallie and Elk Lake, the top is just above the Ambrosia pollen rise, and the zone is rather narrow. However, at St. Clair Lake the top of the zone is just above the time of maximum disturbance of the lake in 1915. At this level, there was also a dramatic change in the species composition of the assemblage, clams and prosobranch snails being totally replaced by pulmonate snails. This is interpreted as a result of overgrowth and anoxia following eutrophication. A decline of gastropods between 1915 and 1935 in Lake East Okoboji in Iowa is attributed to cultural eutrophication (Volker and Smith, 1965). The molluskan species were not identified or counted at Lake Sallie and Elk Lake, so the reaction to eutrophication there cannot be confirmed. In these lakes, the cores were taken from the base of a littoral shelf, and shell deposition may have ceased because of a change in sedimentation conditions, for example by the shelf becoming fully vegetated and the sediment there stabilized.

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Aquatic and Marsh Vascular Plants The most noticeable response of marsh vegetation to eutrophication seems to be the increased abundance of Typha latifolia, at the expense of other emergents such as Scirpus. This change is most marked at St. Clair Lake, where shallow marshy shores provided a large area of suitable habitats for Typha, especially after the partial drainage of the lake. A similar change is described by Janssen (1967) at Stevens Pond, a small shallow lake in the Itasca region, and by McAndrews (1966) from several small ponds in the same area. The increase of Typha latifolia is noticeable in roadside ditches in northern Minnesota today, in regions where Typha is otherwise rare. Recent investigations using historical records on several lakes in the northern hemisphere have shown that Typha spp. increased in response to cultural eutrophication, mainly at the expense of Scirpus spp. (e.g., Volker and Smith, 1965; Olsen, 1964). The increased nutrient supply appears to involve phosphate (Boyd and Hess, 1970). It is not always possible to distinguish an increase in marsh vegetation caused by eutrophication from one resulting from lowered water level, so interpretations need to be combined with a knowledge of other factors connected with changing lake levels, such as sediment changes, presence of “drawdown” species, etc. Before eutrophication, the submerged aquatic vegetation of all three lakes was composed principally of Najas flexilis, associated with Characeae at Lake Sallie and Elk Lake. Knowledge of the chemical ranges of the species concerned (Moyle, 1945) permits a reconstruction of the chemistry of the lake water at that time, and knowledge of the representation of aquatic plant macrofossils (Birks, 1973) allows some reconstruction of the type of community. In all three lakes, Najas flexilis suffered from eutrophication, although the decline in its seed

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numbers at Elk Lake should be interpreted with caution, for the plant is still abundant in shallow parts of the lake. It is difficult, in the absence of several cores, to distinguish the decline of a plant in a whole lake from its decline locally near the sampling site. In all three lakes, aquatic vegetation seems to have become more abundant with eutrophication, presumably in direct response to the addition of nutrients previously in short supply. At Elk Lake, the flora remained generally similar in composition, and eutrophication was insufficient to cause a change in the balance of species. In the other two lakes, the submerged vegetation responded to eutrophication by becoming dense and by changing dominance. At St. Clair Lake, Ceratophyllum demersum became most abundant, associated with mainly Po tamogeton spp. and perhaps also Lemna trisulca. At Lake Sallie, although Ceratophyllum was present, it increased only slightly in abundance, and the dominant species became Potamogeton spp. and Ruppia, associated with a variety of other taxa (see Neel et al., 1973). Aquatic macrophytes seem to be good indicators of eutrophication when taken as a group, but more information is needed about the floras of eutrophic lakes, the changes induced by cultural eutrophication, and the physiological and ecological response of different species to addition of nutrients (see, for example, Volker and Smith, 1965; Olsen, 1964; Lind and Cottam, 1969; Suominen, 1968; Uotila, 1971). In all cases, a similar group of species increases, but little is known about why one or two become dominant in some situations but not in others. The most commonly dominating plants are Ceratophyllum demersum, Lemna minor, Potamogeton spp. (particularly P. pectinatus, P. richardsonii, P. crispus), Myriophyllum exalbescens, and Vallisneria americana. All these, together with many other taxa

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favored by eutrophication, tend to be poorly represented by seeds or pollen or both in sediments (Birks, 1973). A fair representation of Ceratophyllum can be estimated from counts of leaf spines on pollen slides, but, for the rest, care has to be taken in the interpretation of their remains, as a large community of the plant may be represented by only a few macrofossils or by none at all. More information is required on the flowering and fruiting behavior of most aquatic plants, and on the production, dispersal, sedimentation, and preservation of their pollen and seeds, before detailed reconstructions can confidently be made. Diatoms Diatoms were poorly preserved in the core from Elk Lake, so we cannot assess the reaction of the diatom population near the coring site to the relatively slight input of nutrients. For the other two lakes, large reactions to eutrophication are recorded, and diatoms are sensitive indicators. In general terms, benthic and epiphytic taxa were replaced by planktonic taxa, especially species adapted to eutrophic conditions. Stephanodiscus astraea var. minutula is the most abundant species near the top of both the Lake Sallie and St. Clair Lake cores, presumably because it blooms at a different time from blue-green algae and thus avoids competition. Benthic diatoms may have been excluded by lack of space and light as macrophyte growth became anaerobic. However, the percentage decline in benthic and epiphytic species may be at least partly the result of the great increase in total diatoms, which largely reflects the increase in eutrophic planktonic species. CONCLUSIONS In general, the groups of organisms we have studied show responses to eutrophication. A knowledge of the lake vegetation provides a background to the study of the other organisms, because

ET AL.

plants form such an important proportion of their habitats, and the interpretation of the other groups of fossils has been enhanced by the information on the vegetational conditions. Because of their abundance and seemingly precise habitat conditions, diatoms are useful indicators of paleolimnological conditions. Less is known of the ecology of chydorids, and changes in their populations cannot at present be interpreted in detail, although overall increases in the abundance of Chydorus sphaericus axe becoming well documented as a response to eutrophication. The response of Daphnia also has been documented. Eutrophication results in a marked increase in numbers of ephippia sedimented, the precise reasons for which are not known. Little is known of the ecology of mollusks and their response An independent to eutrophication. knowledge of the history and conditions in St. Clair Lake was helpful in interpreting the great changes that took place in the mollusk populations there. This study was largely exploratory in investigating the effects of the same change within a lake ecosystem upon several groups of organisms, and, although only three rather different lakes were studied, the results have allowed some generalizations to be made. These results should apply to areas beyond northwestern Minnesota and should be of value to palaeolimnologists elsewhere. REFERENCES Birks, H. H. (1973). Modern macrofossil assemblages in lake sediments in Minnesota. In “Quaternary Plant Ecology” (H. J. B. Birks and R. G. West, Eds.), pp. 173-190. Blackwell, Oxford. Boyd, C. E., and Hess, L. W. (1970). Factors influencing shoot production and mineral nutrient levels in Typha latifolia, Ecology 51,269-300. Bradbury, J. P. (1975). Diatom stratigraphy and human settlement in Minnesota. Geological Society of America Special Paper 171. Bradbury, J. P., and Megard, R. 0. (1972). Stratigraphic record of pollution in Shagawa

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northeastern Minnesota. Geological of America Bulletin 83, 2639-2648. Brower, J. V. (1893). The Mississippi River and its source. Minnesota Historical Collection, Vol. VII. State Printers, Minneapolis, Minn. Faegri, K., and Iversen, J. (1964). “Textbook of Pollen Analysis.” Munksgaard, Copenhagen. Frissell, S. S. (1973). The importance of fire as a natural ecological factor in Itasca State Park, Minnesota. Quaternary Research 3, Society

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Fryer, G. (1968). Evolution and adaptive radiation in the Chydoridae (Crustacea: Cladocera): a study in comparative functional morphology and ecology. Royal Society of London Philosophical Transactions B 254, 221-385. Janssen, C. R. (1967). A postglacial pollen diagram from a small ‘I’ypha swamp in northwestern Minnesota, interpreted from pollen Ecological indicators and surface samples. Monographs

37,145-172.

Kerfoot, W. C. (1974). Net accumulation rates and the history of cladoceran communities. Ecology

55,51-61.

Larson, W. C. (1971). “Environmental Assessment Report.” U.S. Environmental Protection Agency Application No. 17010 H 11, Pelican River Watershed District and City of Detroit Lakes, Detroit Lakes, Minnesota. Lind, C. T., and Cottam, G. (1969). The submerged aquatics of University Bay: a study in eutrophication. American Midland Naturalist 81, 353-369. McAndrews, J. H. (1966). Postglacial history of prairie, savanna, and forest in northwestern Minnesota. Torrey Botanical Club Memoir 22(2),

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Moyle, J. B. (1945). Some chemical factors influencing the distribution of aquatic plants in Minnesota. American Midland Naturalist 34,402-420.

“Proposed Plan for Moyle, J. B. (1970). Demonstration Lake Improvement Project by Eutrophication Control-Lake Sallie and Connected Waters, Becker County, Minnesota.” Unpublished report. Neel, J. K., Peterson, S. A., and Smith, W. L. (1973). “Weed Harvest and Lake Nutrient Dynamics.” U.S. Environmental Protection Agency, Ecological Research Series EPA660/3-73-001. Olsen, D. (1964). Changes in the vegetation of Lake Lyngby SQ. A contribution to the knowledge of the influences of civilisation on Botamiska aquatic and swamp vegetation. Tidsskrift

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Roelofs, T. Ecological cyanophyte

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Bullheads in Lake Sallie, the latest word. News and Minnesota

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D., and Oglesby, R. T. (1970). observations on the planktonic

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15,224-229. Sanger, J. E., and Gorham, E. (1970). The diversity of pigments in lake sediments and its ecological significance. Limnology and Oceanography 15,59-69. Shapiro, J., Edmondson, W. T., and Allison, D. E. (1971). Changes in the chemical composition of sediments of Lake Washington, 1958-1970. Limnology and Oceanography 16,437-452. Stark, D. M. (1971). “Paleolimnology of Elk Lake, Itasca State Park, Minnesota.” Ph.D. Thesis, University of Minnesota. Stoermer, E. F., and Yang, J. J. (1970). “Distribution and Relative Abundance of Dominant Plankton Diatoms in Lake Michigan.” University of Michigan, Great Lakes Research Division, Publication 16. Suominen, J. (1968). Changes in the aquatic macroflora of the polluted Lake Rautavesi, S. W. Finland. Annales Botanici Fennici 5, 68-81. Troels-Smith, J. (1955). Karkakterising af I&e jordarter. Danmarks Geologiske Undersogelse and

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Uotila, P. (1971). Distribution and ecological features of hydrophytes in the polluted Lake Vanajavesi, S. Finland. Annales Botanica Fennici

8,257-295.

Volker, R., and Smith, S. G. (1965). Changes in the aquatic vascular flora of Lake East Okoboji in historic times. Iowa Academy of Science

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72,65-72

Watts, W. A., and Winter, T. C. (1966). Plant macrofossils from Kirchner Marsh, MinneGeological sota-a paleoecological study. Society

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77,1339-1360.

Whiteside, M. C. (1969). Chydorid (Cladocera) remains in surficial sediments of Danish Lakes and their significance to paleolimnological interpretations. Mitteilungen Internationale Vereinigung Limnologie

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17,193-201. Whiteside, M. C. (1970). Cladocera: modern ecology Ecological

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Whiteside, M. C., and Harmsworth, R. V. (1967). Species diversity in chydorid (Cladocera) communities. Ecology 48, 664-667. Will, G. F. (1946). “Tree Ring Studies in North North Dakota Agricultural ExperiDakota.” ment Station Bulletin 338.

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FIG. 5. Stratigraphy FIG. 6. Stratigraphy FIG. 7. Stratigraphy

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for Elk Lake core. for Lake Sallie core. for St. Clair Lake core.