Some Considerations of the Chemical Limnology of Meromictic Lake MARY1

Some features of the chemical limnology of meromictic Lake Mary were compared to similar features of neighboring dimictic Lake Rose. This dimictic lake and the mixolimnion of Lake Mary exhibited similar seasonal variations in their chemical composition. The chemical composition of the monimolimnetic waters of Lake Mary did not vary throughout the 21-month study. Methane production in Lake Rose began soon after thermal stratification and concentrations in the anoxic hypolimnion reached about 14 mg liter-‘. Methane concentrations in the Lake Mary monimolimnion increased with depth and ranged up to 21 mg liter-l. The concentration and vertical distribution of CH,, together with an alkalinity increase of 22 mg liter-l above that in the surface waters and an NHr’ increase of 4.2 mg liter-l, indicated that biological activity helps sustain the meromixis of Lake Mary. However, the major factor allowing meromixis is the morphometry of the lake basin. The permanent stratification of meromictic lakes offers an opportunity for longterm study of the chemical processes occuring in eutrophic lakes during transitory periods of thermal stratification. With few exceptions, notably the studies of Fayetteville Green Lake, New York ( Brunskill and Ludlam 1969; Brunskill and Harriss 1969), most studies of permanently stratified water masses have measured only a few chemical or biological parameters. Our study was undertaken to evaluate the relative roles of several chemical species to determine the species of primary importance in promoting or maintaining meromixis. The meromictic lake we studied was Lake Mary, a 1.2 ha, highly colored dystrophic lake located in the northwest corner of Vilas County, Wisconsin. Lake Rose, a typical dimictic lake, and Lake Mary are the first and second in a chain of five interconnected lakes, Both are in small basins sheltered from wind action by heavily forested hihs, seated in glacial drift mal This investigation was supported by a NDEA IV predoctoral fellowship, FWPCA Training Grant 5Tl-WP-184, and by the Department of Civil and Environmental Engineering, University of Wisconsin. ’ Present address : Battelle Northwest Laboratory, Richland, Washington 99352. 3 Present address: Department of Civil Engineering, Texas AM the remainder of the materials consists of schist, quartz, sandstone, and iron formations. Bedrock in the area is also chiefly igneous ( Martin 1965). Lake Rose is fed by drainage as land runoff and possible groundwater flow. The only visible sources of water for Lake Mary are land runoff and inflow from the small stream connecting Lakes Mary and Rose; there is also a surface outlet from Lake Mary. Groundwater effects on the water budgets of Lake Mary and Lake Rose are unknown. The main objectives of this study (Weimer 1970) were to compare the chemical limnology of a meromictic lake with that of a similarly sized dimictic lake located in the same environment (see Table 1 ), to verify whether Lake Mary is, as Hutchinson ( 1957) stated, biogenically meromictic, and to assess the important chemical paramctcrs of meromictic Lake Mary. The approach involved detailed analyses of the dominant ionic species in the distinct water layers of these lakes. Dissolved nitrogen, methane, oxygen, and hydrogen mlfide were also measured. EXPERIMENTAL METHODS Water from Lakes Mary and Rose was analyzed irregularly during a 21-month period from August 1966 through May 1968. LIMNOLOGY AND OCEANOGRAPHY 414 MAY 1973, V. B(3) CHEMICAL LIMNOLOGY OF LAKE MARY 415 Table 1. IIydmgraphic clata for Lakes Mary and Rose (Juday and Birge 1941) Parameter Lake Mary 'Lake Rose Length m 125 210 Width m 120 92 Area ha 1.2 1.4 Maximum depth m 21.5 13.0 Mean depth m 7.7 5.2 The lakes were sampled at least once during each season with the majority of the sampling times from late fall through late spring. Samples were generally collected at Z-m depth intervals, and at l-m intervals in the region of the thermocline or chemocline and near the sediments. Temperature and dissolved oxygen were also measured at l-m intervals. The sampling station in Lake Mary was the ccntcr of the lake, generally at a depth of 20-21 m. Although a small area of Lake Rose has a maximum depth of 13 m, the typical depth at the sampling site was lo-11 m. Samples were transported immediately to the Kemp Biological Station, Woodruff, Wisconsin, about 80 km from the lakes, and several analyses were performed within 10 hr after sampling. Scparatc samples for later analysis for major nutrients (nitrogen and phosphorus species) were treated with preservatives in the field. Other unpreserved samples were also collected. In situ measurements included Secchi disk transparency, temperature profiles with a thermistor-type underwater thermometer, and dissolved oxygen with an oxygen meter (YSI model 54RC). Water samples were collected with a plastic sampler similar to a Van Dorn sampler, Samples were *brought to the surface and transferred through rubber tubes attached to the sampler to bottles for storage before analysis (glass or polyethylene depending on the analyses to be performed), Analyses made at the Kemp Station on unpreserved samples included true color, pH, and specific conductance. Temperatures were measured concurrently with the conductivity, and the specific conductance values were corrected to a standard Fig. 1. Lake Mary bathymetric contours. Scale: 2.54 cm = 19 m. temperature of 25°C. Total alkalinity was determined by titration with standard 0.02 N lS on preserved samples, nitrogen and phosphorus species. Analyses for suspended and dissolved solids, chemical oxygen demand, and chloride were performed as outlined by Amer. Public Health Ass. ( 1971) . An atomic absorption spectrophometer was used in the analyses for C,a2+, Mg2+, Na+, and K+ ions. The pH of the samples CHEMICAL LIMNOLOGY OF LAKE MARY CONDUCTANCE, pmhoskm CORRECTED TO 25’ C PH 0123 45 TEMP, ‘C 5 IO 15 20 25 30 ALKALINITY, mg/l as CaCO, CH, mg/l 2p5 H,S (total)mg/l