Journal (~fPaleolimnology 15: 129-132, 1996. @ 1996 KluwerAcademic Publishers. Printed in Belgium.

Sediment indicators of meromixis: comparison of laminations, diatoms, and sediment chemistry in Brownie Lake, Minneapolis, USA *

B. Tracey, N. Lee & V. Card* Biology Dept., Macalester College, 1600 Grand Ave, St. Paul, MN 55105, USA * Author for correspondence

Received 3 October 1994; accepted l0 June 1995

Key words." meromixis, diatoms, laminations, varves

Abstract

Meromixis has several powerful effects on lakes, yet there is no single definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in Brownie Lake, Minneapolis, Minnesota, a small eutrophic lake that became meromictic around 1925. The results show that in Brownie the onset of laminations and changes in the iron to manganese ratio most likely occurred before the development of permanently anoxic bottom water and that changes in the diatom assemblage occurred later, most likely only when meromixis was well developed.

Introduction

A lake is meromictic in a given year if the deep- est water remains anoxic (Hutchinson, 1967; Culver, 1977). Meromixis has several effects on a lake includ- ing exclusion from the benthos of oxygen-requiring animals and protozoans. Meromixis can also sequester in the monimolimnion (the nonmixing bottom waters) nutrients such as iron and phosphorus and thereby decrease the nutrient status of a lake. The differ- ence in density between mixolimnion (upper water) and monimolimnion caused by increases in solutes in the monimolimnion creates a positive feedback effect that increases the stability of the chemical stratifica- tion. As a result of these and other related changes, meromictic lakes often preserve finely laminated, or varved, sediments. Meromixis can also be expected to cause changes in the diatom assemblage and in rela- tive concentrations in the sediment of metals such as aluminum, iron and manganese (Swain, 1984).

* This is the 1 lth in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19-22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.

Despite the number and magnitude of effects of meromixis there is no definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in a small eutrophic lake that used to be holomictic and is now meromictic. The results suggest that two of them, the onset of lamina- tions and changes in the iron to manganese ratio, occur before the development of permanently anoxic bottom water, and the third, changes in the diatom assemblage, occurs later. These results also suggest that although meromixis in this lake was triggered by a single event the development of meromixis took several years.

Site description

Brownie Lake is a small meromictic lake in the Min- neapolis Chain of Lakes in east-central Minnesota, USA (fig. 1). Brownie Lake is notable for its meromix- is and for the resultant exclusion by iron limitation of blue-green algae (Swain 1984). Onset of meromix- is in Brownie Lake dates from approximately 1925, triggered by construction of a canal in 1917 that dras- tically increased the relative depth of the lake (Swain 1984).

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Brownie Lake has a surface area of seven hectares, a maximum depth of 14.5 meters, and a catchment area of about 30 hectares. The catchment is mostly residen- tial and includes a wetland that was formerly part of the lake. Secchi disk depths ranged from 2.5 to 3 meters in the winter of 1994, and from 1 to 3 meters in the sum- mers of 1971, 1972, and 1994 (Shapiro & Pfannkuch 1973). The chemocline occurs at 5 to 6 meters depth and separates surface water with conductivity of 400 to 900 micromhos from bottom water with conduc- tivity from 1400 to 1800 micromhos. Brownie Lake has a winter ice-cover lasting from early December to early April with a maximum ice-thickness of about 50 centimeters and annually variable snow cover.

The Minneapolis lakes are eutrophic and have summer algal populations dominated by blue-greens and dinoflagellates (Shapiro & Pfannkuch 1973). The phytoplankton of Brownie in contrast is dominat- ed by diatoms and pigmented flagellates. Common planktonic diatom species include Asterionella for- mosa and Fragilaria crotonensis. Common epiphyt- ic species include Fragilaria capucina and Tabellar- ia floceulosa. Zooplankton found in Brownie include several species of copepods and cladocera and fish include sunfish (Lepomis spp.), yellow perch (Per- ca flavescens), bullheads (Ictalurus spp.), and large- mouth bass (Micropteris salmoides) (Minnesota State Department of Natural Resources unpubl, data).

Methods

Four freeze cores were taken in February 1994 from 14, 13.5, 10, and 5 meters depth. For diatom analy- sis, one cubic centimeter subsamples were taken with a modified disposable syringe and soaked in 30% hydro- gen peroxide and a trace of potassium dichromate until organic matter was removed. Samples were cen- trifuged, mounted in water, and identified at 400 • with reference to Dodd (1987). A coarse diatom profile was constructed by sampling at 6-centimeter intervals, and a fine profile by sampling at 1 centimeter intervals across the onset of laminations at 42 cm. A coarse iron and manganese profile was constructed by taking twenty-five milliliter samples from the same depths, diluting with 75 ml demineralized water, and analyzing according to Hach (1988). A fine geochemical profile was constructed by taking twenty-five milliliter sam- ples from the same depths, ashing and analyzing by the Mehlich II extract method with an ARL 3560 ICP (by University of Minnesota Research Analytical Lab).

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'-'44.6~

Fig. l. Location of study site in North America.

Results

The longest core taken was 44 centimeters and was taken from a depth of 14 meters. On this core, sed- iment laminations begin at a depth of 42 centimeters below the sediment-water interface. On cores taken from shallower depths, laminations begin closer to the sediment-water interface.

The iron to manganese ratio peaks at 42 cm with concentrations of iron five to twelve times greater than that of manganese and declines to less than one by 12 cm (Fig. 2).

The diatom assemblage below the laminations is dominated by Stephanodiscus hantzschii, with signifi- cant numbers ofAulacoseira, AsterionelIaformosa and Cocconeis placentula. Between 36 and 30 centimeters Aulacoseira and Cocconeis are lost from the assem- blage. Between 30 and 24 centimeters, S. hantzschii drops from 35% to less than lO%.Asterionellaformosa increases in abundance over time and is the dominant species in the upper 20 centimeters (Fig. 2).

Discussion

The three indicators of meromixis, onset of lamina- tions, changes in the iron to manganese ratio, and changes in the diatom assemblage, do not occur at the same depth in the sediment core. The iron to

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Iron and manganese, Brownie Lake, Minneapolis, MN

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Fig. 2. Iron and manganese in Brownie Lake, by Hach t and b y Mehlich III extraction 2. Onse t o f laminat ions occurs at 42 cm, c. 1925, by

correlation with lead-210 dated cole o f Swain (1984).

Diatom percentages, Brownie Lake, Minneapolis, MN

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Diatom percentages in Brownie Lake. Onse t o f laminat ions occm~ at 42 cm, c. 1925, by correlation with lead-2 I0 dated core of Swain

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manganese ratio peaks near the beginning of lamina- tions, and the greatest change in the diatom assemblage occurs within the laminated zone.

The occurrence of seasonal sediment laminations does not necessarily indicate meromixis. Sub-annually laminated sediments occur in several holomictic Min- nesota lakes that have periods of hypolimnetic sum- mer anoxia, including Elk Lake (Bradbury & Dean, 1993), Big Watab Lake (Card, 1994) and Lake Harriet (unpubl. data). The beginning of preservation of fine sediment laminations in Brownie Lake may indicate the reduction or elimination of benthic turbulence by water movement and burrowing bottom fauna (Davis, 1974), as a direct result of the water level lowering or as an indirect result via meromixis. Alternatively, the preservation of laminations might have begun before the lowering of water level as a result of increased bottom-water anoxia caused by cultural eutrophica- tion, similar to that experienced by neighboring Lake Harriet (Brugam & Speziale, 1978).

The rise in the iron to manganese ratio indicates the increasing duration of seasonal bottom water anoxia (Mackareth, 1966; Nuhfer et al., 1993). Manganese and iron are both soluble in their reduced form, and insoluble in their oxidized forms, forming complexes with hydroxyl, sulfate and other available ions. How- ever, manganese is quicker to dissolve under reduc- ing conditions (Mackareth, 1966) so under conditions of prolonged but not permanent bottom water anoxia the iron to manganese ratio in the sediment increases. Under conditions of permanent anoxia, however, bot- tom water becomes saturated with both metals, and the ratio drops (Swain, 1984).

Meromixis affects phytoplankton communities by reducing phosphorus resuppIy from sediments. Sedi- ment can be a major source of phosphorus for eutrophic lakes. If this mechanism were important, the effect of meromixis on the diatom community would be to shift to a lower trophic assemblage. This effect would be gradual, increasing in strength as a rising chemocline encloses increasingly larger proportions of the sedi- ment surface. In Brownie Lake the development of meromixis appears to have been gradual and the effect on the diatom assemblage may have occurred several years after the initial onset of meromixis.

Conclusions

Comparison of indicators of the onset of meromixis in Brownie Lake suggest that the iron to manganese

ratio may be the earliest indicator, followed closely by the preservation of laminations, but that both of these may have preceded the creation of permanently anoxic bottom water. The third indicator, the shift in diatom flora to an assemblage indicative of lower-trophic sta- tus, is likely to have occurred only when meromixis was sufficiently well developed to significantly reduce phosphorus flux from sediments. In a sediment core the actual date of the beginning of meromixis is therefore likely to be found below the biological indicator and above the physical and chemical indicators.

These results also suggest that meromixis in Brownie Lake, while triggered by a single well-dated event, developed over time. In the beginning the depth of the chemocline was almost certainly deeper than the present, although whether it rose gradually or abruptly is still unknown.

References

Bradbury, J.E & W. E. Dean, (eds) 1993. Elk Lake Minnesota: Evidence for Rapid Climate Change in the North-Central United States, Geological Society of America Special Paper 276.

Brugam, R. B. & B. L Spezia[e, [983. Human disturbance and the paleolimnological record of change in the zooplankton commu- nity of Lake Harriet, Minnesota. Ecology 64: 578-591.

Cm'd, V. M., 1994, lce-out, Diatom Community Ecology, and Varve Formation in Big Watab Lake, Minnesota: Phytoplank- ton Response to [nterannual Climate Variation. Ph,D. Thesis, University of Minnesota, St. Paul.

Culver, D. A., 1977. Biogenic meromixis and stability in a soil-water take. Limnol. Oceanogr. 22: 667-686,

Davis, R. B., 1974. Stratigraphic effects of tubificids in profundal lake sediments. Limnol. Oceanogr. 19: 466-488.

Dodd, J. J., 1987. The Illustrated Flora of Illinois: Diatoms. Southern Illinois University Press, Carbondale and Edwm'dsville, Illinois,

Hach, 1988. Analytical Methods, Hach Co., Colorado. Hutchinson, G. E,, 1967. Treatise on Limnology. Vol If. Wiley, New

York. Mackareth, F. J. H. 1966. Some chemical observations on post-

glacial lake sediments. Phil. Trans. r. Soc., Lond. 250: I65-213. Nuhfer, E. B,, R. Y, Anderson, J. E Bradbury & W. E. Dean, 1993.

Modern sedimentation in Etk Lake, Clearwater County, Minneso- ta. In Bradbury, J.E & W.E. Dean, (eds,), Elk Lake Minnesota: Evidence for Rapid Climate Change in the North-Central U. S. Geological Society of America Special Paper 276.

Shapiro, J. & H. O. Pfannknch, t973. The Minneapolis Chain of Lakes, a Study of Urban Drainage and its Effects 1971-1973. Interim Report Number 9, Limnological Research Center, Uni- versity of Minnesota.

Swain, E. B. 1984. The Paucity of Blue-green Algae in Meromictic Brownie Lake: Iron-limitation or Heavy-metal Toxicity. Ph.D. Thesis, University of Minnesota.