current research
Recent Sedimentary History of Lake Mendota, Wis. Gilbert C. Bortleson and G. Fred Lee' Water Chemistry Program, University of Wisconsin, Madison, Wis. 53706
The chemical composition of several sediment cores taken from Lake Mendota, Madison, Wis., has been investigated to determine changes in the flux of various chemicals to this lake. The uppermost sediments consist of approximately meter of black gyttja. The chemical stratigraphy of a 9.9 meter-long core indicates that stable conditions existed in Lake Mendota and its watershed prior to the settlement of the area surrounding the lake by white man in the mid to late 1800s. Since that time, there has been an appreciable increase in the amounts of phosphorus, iron, manganese, aluminum, and potassium in the uppermost sediments. The organic carbon content of the sediments has fluctuated several times in the past with the most recent sediments showing slightly higher values than the older sediments. This study has demonstrated the feasibility of using the chemical composition of lake cores to estimate the influence of cultural activities of man on the rate of eutrophication of a lake.
V
arious human activities contribute to accelerated enrichment (cultural eutrophication) of waters. Among the symptoms of cultural eutrophication are nuisance blooms of algae, increased nutrient levels, depletion of hypolimnetic oxygen, increased turbidity, and changes in the speciescomposition of phytoplankton, invertebrates, and fishes. In most instances cultural eutrophication is an accomplished fact because there are few data t o document changes that have occurred in precultural times. The information needed t o trace changing limnological and watershed conditions must come from a record preserved in lake sediments. Although several chemical studies have been performed on postglacial sediments and interpreted on the basis of longterm historical trends (Brown, 1969; Gorham, 1961; Horie, 1966; Hutchinson and Wollack, 1940; Kendall, 1969; Livingstone and Boykin, 1962; Mackereth, 1966; Pennak, 1963), few investigators (Stockner and Benson, 1967; Shapiro et al., 1971) have examined the recent historical changes of lakes through the interpretation of chemical profiles of lake sediment cores. Lake Mendota sediment cores were examined in this study. This lake is a hard-water eutrophic lake formed by morainic damming of the preglacial Yahara River Valley near Madison,
To whom correspondence should be addressed.
Wis. (Twenhofel, 1933). The lake currently receives domestic drainage from agricultural lands, urban runoff, and some municipal and industrial waste effluents contained in entering streams. For a survey of limnological information on the lake the reader should see Frey (1963). Experitnentul Procedures
In October 1966, cores WC-84 and WC-89 were taken from Lake Mendota in 11.2, 18.3, and 23.2 meters of water, respectively, in line from University Bay to the center of the lake as shown in Figure 1 . The main tributary to the lake, the Yahara River, enters from the north. The only surface outlet from the lake is located on the eastern shoreline. The outlet is actually a continuation of the Yahara River, which flows from Lake Mendota into Lake Monona and subsequently into two other lakes located toward the southeast. University Bay has two inlets; one coming from a pumping station of lowland drainage, and a small creek, University Creek, which serves as a storm sewer for urban Madison. The sample drive through 1 meter of sediment was accomplished by a coring device consisting of a circular cutting head on the lower end, a clear acrylic plastic core barrel (3.5 in. in diam), a piston assembly, and a set of weights a t the upper end. The design and operation of the piston corer was described in detail by Wentz (1967) and Bortleson (1968). A sampling platform and a winch were provided by a converted Army amphibious Dukw to facilitate raising the corer. The sample material after inspection for laminations was extruded and fractionated in the field into 5-cm intervals.
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Figure 1. Bathmetry and coring locations for Lake Mendota Volume 6, Number 9, September 1972 799
Table I. Precision of Analytical Methods
No. of
Element Iron Phosphorus Manganese Magnesium Potassium Calcium Aluminum Organic nitrogen Total carbon
Av value of
repli- N replicates, cates, N mg/g 7 23.6 5 1.37 5 0.622 5 13.1 5 12.6 5 110.0 5 31.3 5 7.08 5 128.0
Std dev,
mg/g 0.6 0.02 0.012 0.4 0.1 2.0 0.28 0.36 7.0
The samples brought to the laboratory were stored frozen at -2OOC until commencement of analysis. After thawing, the wet sediments were homogenized in a blender and air-dried. Prior to acid digestion, sediment sample aliquots were ground, passed through a 100-mesh screen, and heated to constant weight at 105OC. Acid digestion consisted of adding 5 ml of 48% H F to a 0,500-gram sample in a 50-ml polypropylene beaker. The sample was heated to about 100°C for 8 to 12 hr, after which only a dry
residue remained. The solid residue was removed from the beaker, transferred to a 100-ml Kjeldahl flask, and further digested for two hours to decompose the organic matter in the presence of 5 ml of concentrated " 0 3 and 60 HC104. After this digestion, the sample was cooled, passed through a prerinsed Whatman No. 2 filter, and drained directly into a 100-ml volumetric flask. This solution, or an appropriate aliquot thereof, was used for total P, Fe, Mg, K , Ca, and A1 determinations. Iron was determined by the orthophenanthroline method (Olson, 1965) and P by the vanadomolybdophosphoric (VM) yellow colorimetric procedure (Jackson, 1958). Wentz and Lee (1969a) have discussed the sensitivity, minimum detectable concentration, precision, and working range for the VM yellow color procedure. Analyses for Mn, Al, Mg, K, and Ca were made by direct aspiration of the digestion solution, or diluted aliquots, into a Perkin-Elmer Model 303 atomic absorption spectrophotometer. Organic nitrogen was determined by semimicro Kjeldahl technique outlined by Bremner (1965). Total carbon was determined by a dry combustion technique using a LECO (Laboratory Equipment Corp.) lowcarbon analyzer (Model 589-400) and LECO induction furnace. The COS released by combustion of carbon compounds and decomposition of carbonate was measured by a thermal conductivity cell. With calcareous sediments, the choice is
0
1965
IO - 1950
XI 1930 Y) Lz
33-1915
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40-1950 50-1880
Figure 2. Chemical and Ambrosia pollen stratigraphy of Lake Mendota core WC-89
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= Figure 3. Chemical and Ambrosia pollen stratigraphy of Lake Mendota core WC-86
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800 Environmental Science & Technology
&o$o7b&sbob'l2'1'6';o Organic C lmg/gl P lrng/gl 6 7 8 9 1 0
Q d ; O l ; l 4 1 6 1 1 2 b 2 2 2 4 ' l l b ' 1 5 0 ' 1 ~ ' 010103J40 Fo lmg/yl Co lmg/ql Ragweed counts (i6d 01 0 3 0 5 0 7 0 9 1 1 13 15 17 0 2 4 6 8 10
between determining organic C on the sample after removal of carbonate C or computing organic C by subtracting carbonate C from total C. In this study the Ca dissolved by HF-HN03-HC104 acid system was converted to carbonate C equivalents; this value was subtracted from total C to obtain organic C. Studies by Bortleson (1970) have shown a good correlation between measured and calculated inorganic C in Lake Mendota sediments. Sediment sample aliquots (0.200-gram sample, 100-mesh, dried lOS0C) for Ambrosia pollen analysis consisted of removal of carbonates with HC1, silicates with HF, solubilization of humic material with KOI-I, and removal of cellulose with acetolysis solution (Faegri and Iversen, 1950; Andersen, 1960; and Maher, 1969). Since a total pollen assemblage was not counted and identified-e.g., oak, pine, hickory, grass pollen-to obtain a percentage of ragweed pollen, it was necessary to add an internal standard. A suspension volume of 0.20 ml of internal standard (1 gram of pure Eucalyptus pollen in 500 ml of tertiary butyl alcohol) was pipetted while well-stirred and at constant temperature (27.5-0.5OC) and transferred to the sediment aliquot. The Eucalyptus pollens, which are exotic pollens, were easily identifiable triangularshaped grains. Identification of Ambrosia-type pollen was made with high power magnification using a Bausch & Lomb microscope. N o reference was given to a particular species of ragweed. Continuous sweeps were made across the entire width of the preparation; in each sample, 100 to 200 grains were counted. The abundance of ragweed found in the sediment cores was based on the average ragweed-to-Eucalyptus pollen ratio of duplicate or triplicate samples. The results presented for the chemical analyses of each core are mean values of two or five replicate determinations. Data for the precision of the analysis are given in Table I. The relative standard error was less than 5 for 73 of the analyses performed in replicates of five. Duplicate pollen counts were made from each processed sample. In the transition zone (high or low ragweed) triplicate counts were made. Results and Discussion The sediments laid down from the precultural period to the present day can be subdivided into four zones according to changes in ragweed pollen and chemical distribution patterns as shown in Figures 2-4. The main physical and historical features of the Lake Mendota cores are shown in Table 11, and the mean concentrations of organic C, carbonate
C, P, Fe, K, and Z solids are shown in Table 111for each of the sediment zones. N o evidence of laminations or lenses, which could be used for dating purposes, was observed for any of the cores. To identify pre- and postcultural periods of deposition in the sedimentary column, Ambrosia pollen counts were performed; the appearance of these pollen grains provided a stratigraphic horizon which could be dated from historical records showing when man moved into southern Wisconsin and began modifying the environment. Ragweed pollen occurs in relatively high percentages (5-40 %) in surface sediment samples from the deciduous forest region of the northeastern and northcentral USA; the plant seems to have increased as a result of disturbance and creation of open habitats through forest clearance by European settlers (Ogden, 1967; Davis, 1967; Wright, 1968). For instance, a short core (50 cm) was taken from Frains Lake in Michigan by Davis (1968). The ragweed pollen increased from less than 1 to about 30% of the total from below to above 25 cm of sediment, respectively. According to Davis, the time of land settlement and forest clearance around Frains Lake occurred in 1830. The lowermost sediments in Zone I consist of a buff marl containing 45560% C a C 0 3 or 190-255 mg/g Ca. The sedimentary concentrations of Fe, Mn, Al, K, Mg, P, and organic C show little or no change with depth in the buff marl. Minimum ragweed pollen counts were observed in Zone I. Presumably, the buff marl was laid down prior to any major disturbance by white settlers in the Lake Mendota drainage basin. Although most of the Lake Mendota cores observed by Murray (1956) showed a simple sequence of black gyttja (called sludge by Murray) over buff marl, other sequences were found in parts of the Lake Mendota basin, usually in the shallower regions. Among the sequences found with sediment depth by Murray (1956) were a sludge-marl-blue plastic marl, sandy sludge-blue plastic marl-pink clay with sand and pebbles, and a sludge-blue plastic marl-blue plastic clay. The sediments in Zone I1 consist of gray-colored gyttjamarl which represents the transition zone between the buff marl and black gyttja. The reported (Murray, 1956) knifesharp nature of the contact between gyttja and marl was not observed in any of the cores used in this study. In all the core section$ examined, the buff marl passed gradually upward into a gyttja marked by a gradual darkening of color. Apparently, the false impression of a knife-sharp contact was
Figure 4. Chemical and Ambrosia pollen stratigraphy of Lake Mendota core
WC-84
Volume 6, Number 9, September 1972 801
Core No. WC-89 WC-86 WC-84
WC-89 WC-86 WC-84
WC-89 WC-86 WC-84
WC-89 WC-86 WC-84
Table 11. Main Features of the Lake Mendota Cores and Corresponding Historical Events Sediment depth and Type of thickness of Chemical Relevant historical events Period deposit zones, crn zone Accelerated urbanization in Madison (pop. Black Late cultural 0-1 5 IV 67,000-158,000, 1940-1965) 0-1 0 1940 to present gyttja “Detergent era” 0-1 5 Decline in cisco (Coregonus artedii) after 1940 (Frey, 1963) Large decline in wetland from 1938 to 1950’s Sewage diversion around lake December 1971 Rapid urbanization of Madison (pop. 12,00015-50 I11 Black 67,000, 1884-1940) Midcultural 10-40 gyttja Rapid settlement in L. Mendota basin 1880 to 1940 15-45 L. Mendota received sewage effluent from Madison, 1884-1899 Early urbanization in Madison (pop. 2500Early cultural 50-70 I1 Gray1820 to 1880 12,000, 1850-1884) colored 40-60 Raised lake level 5 ft by outlet dam in 1847 40-75 gyttjaEarly settlement in southern Wisconsin marl Privies, cesspools, and direct drains to lake to dispose sewage Presettlement in southern Wisconsin 70-95 I Buff marl Precultural 60-1 00 before 1820 75-100 C-14 date,
