Dentrification as a nitrogen sink in Lake Mendota, Wisconsin

Abstract

4 The significance of denitrification as a sink in the nitrogen budget has been determined for Lake Mendota, Wis. About 28,100 kg. of nitrogen were lost from the lake hypolimnion during the summer of 1966. However, this relatively large amount represented only about 11% of the estimated total annual nitrogen input. Denitrification rates ranged from 8 to 26 pg. of N per liter per day, while rates of nitrate reduction (to ammonia and organic nitrogen) were found by 15N tracer techniques to range from 1.4 to 13.4 pg. of N per liter per day in the hypolimnion of the lake. Nitrate depletion is more than an order of magnitude slower than oxygen depletion in the hypolimnion of Lake Mendota, and denitrification is probably not significant with respect to respiration and catabolic processes in the lake hypolimnion. There is some evidence to indicate that dissolved nitrogen gas concentrations increase above those expected on the basis of solubility as the result of denitrification. Evaluation of various nitrogen sinks for Lake Mendota has revealed that only about one third of the estimated annual nitrogen input can be accounted for by currently evaluated sinks. Sediment deposition probably accounts for most of the remaining two thirds. enitrification probably occurs in stratified lakes which D lose their hypolimnetic oxygen, but the significance of this has not been properly assessed. Hutchinson (1957) has reviewed many examples of lakes in which the late summer nitrate distribution is dichotomic as the result of depletion by nitrate assimilation in the surface waters and by denitrification and nitrate reduction in the anoxic bottom waters. Only recently Dugdale, 1966a, 1966b) have estimates been made of in situ denitrification rates in natural waters. However, these rates do not necessarily define the quantitative significance of denitrification as a sink in the nitrogen budget of a water body. A recent investigation of nitrogen cycle dynamics in natural waters (Brezonik, 1967) permits an estimate of this sort for Lake Mendota, Wis.Lake Mendota has been one of the most frequently studied lakes in the world, and descriptions of its characteristics are numerous in the limnological literature. A few facts germane to the following study should be mentioned. The lake covers 3938 hectares and has a maximum depth of about 24 meters. The lake is moderately eutrophic and loses its dissolved oxygen below about 10 meters during summer stratification. A heavy influx of nutrient compounds results from the lake's Present address, Department of Bioenvironmental Engineering, University of Florida, Gainesville, Fla. 32601 location in a basin of fertile crop and pasture land and partially from the urban influence of the city of Madison, which now nearly surrounds the lake. Lee et al. (1966) have recently estimated nitrogen and phosphorus sources for the lake which show an annual input of nitrogen equivalent to 0.50 mg. of N per liter of lake water and an annual phosphorus input equivalent to 0.047 mg. of P per liter of the lake. This ra...

Nitrogen fixation in Lake Mendota, Madison, Wisconsin1

“…Valucs were maxima1 each summer in July, followed by a minimum in mid-August and a final spurt of activity at the end of August. These July and August patterns were also observed in 1968and 1969(Rusncss and Burris 1970Stewart et al, 1971.). Aphanixommon was usually the predomi- nant Na-fixer, if there w&re any, in the samples collected.…”

“…(For example, NII:,-N and N08--N each run GO.06 mg liter-' in the summer cpilimn-ion. ) In addition, denitrification occurs contemporaneously in the anaerobic waters (Brezonik and Lee 1968) and the sediments ( Keency ct al. 1971) of the hypolimnion.…”

Aspects of the limnology of Lake Rotongaio

“…The accumulation of nitrate under these conditions suggests a slow rate of nitrification (1.9 mg m 3 d ') for June-July compared with > 12.3 mgrrr 3 d~' at 7.5 m in October-November, and compared with reported nitrification rates from other lakes such as Blelham Tarn, 22.4-55.4 mg nr 3 d 1 (Christofi et al 1981), Lake Mendota, 0.7-14.8 mgm ^d 1 (Brezonik & Lee 1968), and Lake Taupo 0.5-4.1 mgirr3d ' (Vincent & Downes 1981).…”

Microbial Nitrate Reduction in Freshwater Sediments

“…The methods used to study denitrification in the natural environment have included measurements of changes in nitrate concentration, 15N-labelling (Brezonik & Lee, 1968), 13N-labelling (Gersberg et al, 1976), gas chromatographic measurement of N,O reduction (Garcia, 1974) and acetylene inhibition . Measurement of nitrate reductase has been used in marine systems to study denitrification (Packard et al, 1977) and the nitrogen nutrition of phytoplankton (Eppley et al, 1969;Collos & Slawyk, 1977).…”