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Large Plexiglas columns were used to evaluate the changes in the forms and amounts of nitrogen in the bottom waters and sediments of three freshwater lakes during stratified (anoxic) and turnover (aerated) conditions. Sampling ports and permanently placed black Pt electrodes were installed so that water and sediment phases could be monitored. Sediment lZh declined rapidly to -150 to -250 mV and remained at this level irrespective of the oxygen concentration in the overlying water. During anoxic conditions, NHa-N was released to the water at a relatively constant rate. Aeration affected rapid nitrification. Nitrate in the overlying water decreased with time, possibly due to diffusion into the highly reduced sediment and subsequent denitrification. Lake sediments can significantly affect the nutrient concentrations of the overlying waters (Foess and Feng 1971). One factor that affects the transformation and exchange of nutrients at the sediment-water interface is the redox potential of the sediment-water system (Mortimer 1941, 1942; Hutchinson 1957; Gorham 1958; Gorham and Swaine 1965; Lee 1970a). The potential measured with an inert clcctrodc (Eh) is often used as a qualitative measure of redox conditions in natural systems even though it defies quantitative interpretation due to the complexity of these systems. Whitficld ( 1969), Bohn ( 1968, 1971), Morris and Stumm ( 1967) and Garrels and Christ (1965) have reviewed the problems and limitations of measurement and interpretation of the Eh status of natural systems. Some of these stem from the performance of the inert metal electrodes while others are associated with the complexity of natural systems. Natural systems are inherently dynamic and thus no true equilibrium potential can be measured l Supported by the College of Agricultural and Life Sciences, University of Wisconsin and by Environmental Protection Agency Grant 16010 EHR, K. L. Chen assisted with laboratory analyses. 2 Present address : Soils Department, University of Florida, Gainesville 32601. 3 Present address: Department of Soil Science, University of the Philippines, Los Banos Units, College, Laguna, The Philippines. ( Whi tficld 1969 ) , although steady states may be approximated. Also microenvironments bearing no relationship to the total environment may be measured, while in some instances highly reversible redox reactions having no general significance to the chemistry of the environment may be responsible for the Eh level (Whitfield 1969). Despite these limitations, Eh values measured with inert metal electrodes may be utilized as a qualitative measure of the intensity of redox conditions in the environment (Whitfield 1969; van DcrWeijden et al. 1970; Bohn 1971). Most of the redox reactions in soils and aquatic environs are biologically mediated. Under aerobic conditions molecular oxygen serves as the electron acceptor in microbial metabolism while the electron donor is organic C for heterotrophs or reduced inorganic compounds for autotrophs. Eh is relatively insensitive to dissolved oxygen and remains nearly constant down to values of 0.1% saturation (Greenwood 1962). When dissolved oxygen is depleted, alternate electron acceptors are utilized. This stage generally corresponds to an Eh of +300 to +350 mV (Turner and Patrick 1968) and involves, initially, facultative microorganisms. Several redox systems may bc operating in these first stages of anaerobic respiration. However, it is generally bclicvcd that, if present, NOs is the first electron acceptor following depletion of LIMNOLOGY AND OCEANOGRAPHY 908 NOVEMBER 1973, V. 18(6) ?7 L 1;fl. ANlJ NllllUbLN ’ I-m ---““‘T TRANSFORMATIONS dissolved oxygen. Nitrate stabilizes the Eh at about +200 to +lOO mV (Redman and Patrick 1965; Bell 1969; Bailey and Beauchamp 1971). Manganese and Fe follow NO3 as electron acceptors. Reduction of SO.*“to S2occurs after Fe reduction and begins at about 0 to -150 mV (Parr 1969) ; this is the start of the second stage of anaerobic digestion and involves obligate anaerobes. Below -150 mV, H+ and CO2 are reduced forming Ha and CHZi. The H2 system appears to be the terminal system in reduced soils (Ponnamperuma et al. 1966 ) . The nitrogen budget of a lake may bc significantly affected by the Eh status of the sediments and the scdimcnt-water interface. Sediment organic nitrogen can act as a source of inorganic nitrogen to the lake water through ammonification. Byrncs et al. (1972) have shown that sediment NH4 (interstitial and exchangeable) rapidly reaches equilibrium with the overlying water and that much of the NH4 mineralizcd from decaying organic matter in the sediment is soon returned to the overlying water. If the lake water contained oxygen it is likely that the NH4 would be rapidly converted to NO3 as it moved out of the anaerobic sediment ( Chen et al. 1972b), particularly in alkaline lakes where the pH is favorable for nitrification. This might then actually result in a loss of nitrogen from the lake as some of this NO3 would likely diffuse back into the anaerobic scdimcnt and bc denitrified (Chen et al. 1972c; Mortimcr 1941). It is not known how much of the NO3 might diffuse into the sediment but the fate of that which does can bc postulated from the work of Chen et al. ( 1972a), in which nitrate ( 15N labeled) added to Wisconsin lake sediment samples disappeared within 4 days: 37 s of the added NO3 was immobilized and 63% was denitrified and lost from the system, It was also shown that the newly immobilized nitrogen was subject to rapid mineralization. Detailed discussion of nitrogen transformations in the aquatic environment can bc found in Kecney ( 1973) and Brezo