Hypolimnetic redox and phosphorus cycling in hypereutrophic Lake Sebasticook, Maine

Water Res. Vol. 16. pp. 1189 to 1196, 1982 Printed in Great Britain. All rights reser',ed

0043-1354/82,071189-08503.00/0 Copyright © 1982 Pergamon Press Ltd

HYPOLIMNETIC REDOX AND PHOSPHORUS CYCLING IN HYPEREUTROPHIC LAKE SEBASTICOOK, MAINE LAWRENCE M. MAYER1, FREDERICK P. LIOq"rA2 and STEPHEN A. NORTON3 tDepartment of Geological Sciences, Ira C. Darling Center, University of Maine at Orono, Walpole, ME 04573. 2Amax Exploration. Inc., Denver, CO 80228 and ~Department of Geological Sciences. University of Maine at Orono, Orono. ME 04469, U.S.A. (Received October 1981)

Abstract--The vertical distribution of reduced species and phosphate was monitored throughout a summer stagnation period in hypereutrophic Lake Sebasticook, Maine. The oxygen demand of reduced species released from hypolimnetic sediments was dominated by methane, followed by ammonium, and included minor contributions from Fe 2+, Mn 2 + and sulfide. Release of iron and manganese from the sediments was apparently controlled by dissolution of siderite and rhodochrosite, with the requisite acidity for the dissolution provided by methanogenesis. Redox recycling of these two metals occurs in two vertically displaced "wheels", which enhance oxidant delivery to the hypolimnion beyond that provided by oxygen diffusion alone. Phosphorus is tightly coupled to the "ferrous wheel", in the absence of wind events, and is scavenged with a stoichiometry consistent with Tessenow's (Tessenow U., Arch. Hydrobiol. Suppl. 47, 1-79, 1974) laboratory studies.

INTRODUCTION

cycling of these species and their effect on phosphate delivery to the epilimnion. Lake Sebasticook, the location of these studies (Fig. 1), is a hypereutrophic lake which has been the object of various studies (Mackenthun et al., 1968; Nolan & Johnson, 1975). Most recently Dubiel (1977) described the phosphorus reservoir in the sediments. It was clear from several studies that internal cycling of phosphorus was largely responsible for the present trophic status of the lake. Hannula (1978) modeled the phosphorus cycling within the lake. The close linkage between anoxia and phosphorus release led to our studies of oxygen-controlling mechanisms in the water column.

The onset of anoxia in the hypolimnia of stratified lakes can bring about dramatic changes in water column chemistry, notably the disappearance of oxidizing substances and the appearance of reducing species and nutrients. The chemical gradients established between the hypolimnion and epilimnion result in exchange across this interface with important consequences for the lirnnetic oxygen and nutrient status. Consumption of oxygen at the metalimnetic aerobic-anaerobic interface is due to a combination of biological and inorganic oxidation reactions. Various bacteria in this zone use the proximal availability of oxidants and reductants to metabolic advantage by SAMPLING AND ANALYTICAL METHODS catalyzing their combination and extracting the resulting energy change. Numerous attempts have Water samples were collected at various depths above been made to discriminate between biological and ' the profundal zone of the lake, using a PVC van Dorn sampler. The temperature and dissolved oxygen were dechemical oxygen demand by poisoning the biological termined in situ with a Yellow Springs Model 54 D.O. reactions and measuring the resultant chemical oxyMeter. Immediately after collection, the pH was measured gen demand (e.g. Brewer et al., 1977; Wang, 1980). with a combination electrode and an Orion Model 404 Specific Ion Meter and alkalinity determined with a sulHowever, some oxidation steps can occur with or furic acid titration to a fixed end point (pH = 4.2) {Rainwithout biological mediation; poisoning therefore can water & Thatcher, 1960). The remaining water was then only provide potential fractionations and will not divided into as many as four aliquots. Aliquot 1 consisted necessarily represent the actual in situ partitioning of of 135 ml of water added to 5 ml of 6 N HCI in an acidbiological and chemical oxygen demand. Another use- washed polypropylene (PP) bottle. Aliquot 2 consisted of 145 + 10ml, filtered through a 0.45,am Millipore filter ful approach to the fractionation of oxygen demand would be by investigating the types of reduced species (Type HA) into a glass filter flask containing 5 ml of 6 N HCI, after which 14Oral were transferred into an acidavailable for oxidation. washed PP bottle. Aliquot 3 involved pouring 50ml of This paper examines the types of reduced species unfiltered lake water into an acid-washed 125 ml Edendelivered to the metalimnion of a hypereutrophic meyer flask which was then covered with Parafilm. Aliquot lake. We consider the dynamics of hypolimnetic water 4, drawn predominantly from anoxic and near-anoxic waters, consisted of 90 ml of unfiltered lake water drawn chemistry through the summer stratification period directly into an acid-washed PP bottle containing 50 ml of with emphasis on the factors controlling the introducsulfide antioxidant buffer (Frant & Ross. 1970), and then tion of reduced species to the water column, internal placed in an ice bath until analysis. 1189

1190

LAWRENCE M. ~I.-I.YERer (21.

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Fig. 1. Map of Lake Sebasticook, showing sampling site (S).

Samples for methane analysis were collected at prescribed depths through a 0.95cm i.d. PVC flexible tube, drawn by a plastic, hand-operated bilge pump in the boat; samples were transferred into a 150 ml ground glass-stoppered reagent bottle already containing 1 ml of 0.4 M NaOH. The bottle was stoppered without trapping any air bubbles, inverted and placed into an ice bath to minimize degassing. Water from aliquots 1 and 2 were analyzed for Mn, Fe, Mg, Ca, Na and K on a Perkin-Elmer 303 Atomic Absorption Spectrophotometer. Results from the analyses of Aliquot 2 waters are termed "'dissolved" and the concentration differences between Aliquots 1 and 2 are termed "particulate". Aliquot 3 was analyzed for total phosphorus using a Technicon Autoanalyzer (EPA, 1971) by the Maine Department of Environmental Protection. Samples from Aliquot 4 were analyzed for dissolved sulfide by the method of Green & Schnitker (1974). Methane was measured on a F & M-Hewlett-Packard Model 402 gas chromatograph by the method of Rudd & Flett (1975). RESULTS The establishment of winter and summer anoxic hypolimnia in Lake Sebasticook follows a pattern typical of a eutrophic dimictic lake (Liotta, 1978; Hutchinson, 1975). Figure 2 shows isopleths of temperature and dissolved oxygen during the summer study period, to provide a context for the discussion that follows. Superposition of the two sets of isopleths demonstrates that the summer fluctuations of the oxycline represent intrusions of hypoxia into the

metalimnion and lower epilimnion rather than massive reoxygenation of the hypolimnion. The response of iron and manganese to this anoxic period is shown in the isopleths of Fig. 3. Both metals were released from the sediments immediately after the onset of hypolimnetic anoxia with manganese apparently preceding iron, as found by Mortimer (1941). Both metals increased in concentration in the hypolimnion throughout the summer, but in different patterns. Dissolved manganese developed an upper hypolimnetic maximum in the later summer-early

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1191

Hypolimnetic redox and phosphorus cycling

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Fig. 3. Isopleths of (a) dissolved manganese (,aM); (b) dissolved iron (,uM): (c) particulate manganese (,aM): and (d) particulate iron (,aM). fall, while dissolved iron retained a lower hypolimnetic maximum indicative of release from the sediments. Particulate manganese showed high concentrations in the lower epilimnion, above the pycnocline, which were most evident during times of maximum depression of the oxycline. This pattern implies that the particulate manganese accumulated by in situ oxidation of upward diffusing, divalent manganese, derived from the hypolimnion below• Were the particulate manganese to have been allochthonous, as suggested by Davison (1981) for Esthwaite Water, maximal accumulation would be expected in the pycnocline rather than above it. Particulate iron built up in the hypolimnion, particularly the lower part, rather than the metalimnion and epilimnion. The iron was probably in the form of sulfide, as evidenced by the black appearance of filters from this zone which rapidly turned brown upon exposure to air.

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Total phosphorus (Fig. 4) showed a pattern similar to the metals, building up intensively in the hypolimnion and to a much lesser extent in the epilimnion. Measurement of dissolved sulfide did not commence until mid-summer; this species (Fig. 5a) was high in the hypolimnion and showed a marked decrease in mid-August, much earlier than seen with dissolved manganese and iron• Sulfide levels decreased in the lower hypolimnion along with the development of the particulate iron maximum, leaving an upper hypolimnetic maximum for this species. Sulfate levels were below detection (< 5 ppm) in this lake with the barium precipitation technique used. The pH levels in the epilimnion were in the range 7-9; values in the hypolimnion were lower, 6.7-7.1 and were well-buffered by the elevated bicarbonate and phosphate levels. Potassium was also apparently released from the sediments during the summer, as indicated by increasing potassium concentrations toward the sediment-water interface; this release may have resulted from an ion exchange reaction in which potassium was displaced by ammonium derived from deamination of organic material. Considerable alkalinity was also released from the sediments (Fig. 5b); at the hypolimnetic pH this alkalinity is representative of the total inorganic carbon. Methane was measured only during the early fall months (Fig. 6); concentrations were as high as

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1192

LAWRENCEM. MAYERet at.

770/.tin 1-~ just above the sediment-water interface and fell to zero at the metalimnion.

mined using the extended Debye-Htickel expression. - l o g 7 = Az2,/l/(1 + Bh,/l). Rapid attainment and maintenance of saturation levels for these two phases is evident throughout the summer. Hoffman & EisenDISCUSSION reich (1981) have argued for a similar control mechanThe reducing conditions in the hypolimnion result ism for dissolved Mn in the summer hypolimnion of from the delivery of reduced substances to various Lake Mendota. We speculate that this carbonate conzones of hypolimnetic oxidation activity. These trol results from the conversion of higher valence oxyreduced substances include both primary organic par- hydroxides in the sediments to the reduced carbonate ticulate matter (algae, fecal matter) derived from the phases upon the onset of summer anoxia; Huber & epilimnion and various dissolved reducing species de- Garrels (1953) have demonstrated such a conversion rived from metabolic reactions in the sediments or the in the laboratory. We have observed in the laboratory water column. These dissolved reducing agents in- (unpublished observation) that a pinkish rind, preclude CH,~, H2S, HS-, Fe 2÷, Mn 2÷ and N H 2 ; their sumably MnCO3, can be formed around manganese main source appears to be the sediments rather than dioxide grains which can protect the enclosed MnO2 the hypolimnetic water column. from dissolution by a hydroxylamine hydrochloride solution, a mild reducing agent often used to dissolve Redox cyclin 9 MnOz. That the maintenance of equilibrium soluThe release of dissolved iron and manganese from bility levels reflects dissolution and not precipitation sediments in response to anoxia in the hypolimnion is control is corroborated by the common occurrence of a well-documented phenomenon. The ratio of iron to high supersaturation levels for siderite (cf. Postma, manganese released has been found to range consider- 1981). ably among lakes (Delfino & Lee, 1971; Campbell & Dissolution of these carbonate phases requires Torgersen, 1980; Kjensmo, 1967). In Lake Sebasti- acidity. Methanogenesis provides the most plausible cook, the concentrations of iron and manganese are source for this acidity, providing carbon dioxide simapproximately equal, particularly in the early sum- ultaneously with methane. The coupling of these two mer. The cause of this equality ca,n be either fortui- processes, along with the deamination reactions certously equal concentrations and kinetics of dissolving tainly accompanying them, can be schematized by the solid metal phases, or solubility control by solid following overall reaction: phases which provide the same equilibrium concen2CsH7NOz + 2MeCO 3 + I 1 H z O ~ - 5 C H 4 + tration of iron and manganese. We hypothesize that the latter control was operative in this lake, and H2CO3 + 6HCO3 + 2Me 2+ + 2NH,~. (I) suggest that carbonate phases, which have very similar solubility products for these two metals, were In this equation C~H7NO2 represents an average dissolving to provide similar iron and manganese composition for decomposing algal matter (Goiterconcentrations. Figure7 shows a plot of the ion man, 1976) and MeCO3 represents a mixture of iron activity products for siderite and rhodochrosite and manganese carbonate phases. The predictions by throughout the summer for the lowermost hypolim- this reaction for the relative concentrations of various netic samples, along with the range of Ksp values species in the lower hypolimnion, i.e. released from available in the literature (Murray et al., 1978; Aller, the sediment but not yet removed by oxidation or 1978; Emerson, 1976; Thornber & Nickel, 1976). Ac- uptake reactions, are compared in Table 1 with the tivity coefficients for these calculations were deter- actual ratios found. The excess alkalinity values in 9.5

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Hypolimnetic redox and phosphorus cycling Table 1l Comparison of predicted and observed bottom water composition, using equation (I) in text

Predicted 8/25/77 9/6/77 9/11/77 9/17/77

Methane Fe + Mn

Methane Alkalinity

Alkalinity Fe + Mn

pH

2.5 nm 2.8 2.3 2.2

0.83 nm 0.68 nm 0.57

3 3.8 4.2 nm 3.8

7.2 6.9 6.8 nm 6.8

Predicted pH calculated using [ - H z C O a ] / [ ' H C O j ' ] = 0.167 and assuming pK1(H2CO3) = 6.4. nm--Not measured. this table are the measured alkalinities minus an average springtime value. The predictions generally matched the observed values, except for pH, to within _+300~o; the discrepancies are likely due to simultaneous reactions in the sulfur and phosphorus cycles and production of organic compounds other than methane. Assuming the titration alkalinity to represent ~CO2, our CH~:excess 3zCO2 concentration ratios are < 1, considerably less than the > 1 flux ratios found by Kelly & Chynoweth (1980) and calculated by Hesslein (1980) but similar to the bottom water measured values presented in Hesslein (1980). Deamination reactions and metal carbonate dissolution can account for most of the alkalinity production; Fig. 8 shows a plot of bottom water iron, manganese and ammonium concentrations vs alkalinity throughout the summer. The alkalinity vs ammonium line is the regression line of these two parameters from Hall's (1974) data. The sum of these three parameters equals ~ 3/4 of the excess alkalinity, shown by the 1:1 line. The shortfall of these parameters results from small contributions of excess potassium and calcium observed in the bottom waters as well as probable errors in application of Hall's (1974) data to the same lake several years later. It is interesting to note that the late summer, dissolved manganese peak in the upper hypolimnion was accompanied by an increase in alkalinity in this zone; in this case the alkalinity production was presumably due to dissolution of the setting hydroxide rather than the carbonate phase as in the lower part. The separation between iron and manganese after their release from the sediments results from the different redox potentials controlling conversion between oxidized, insoluble forms and reduced, dissolved species (Krauskopf, 1957). This separation is seen both in the zones of oxidation of the dissolved species and the zones of hypolimnetic reduction of settling oxyhydroxides. Iron, with its relatively low Fe2+-Fe 3+ redox potential, was largely oxidized within the upper hypolimnion while manganese penetrated to a much greater extent into the epilimnion (Fig. 3a and b). The upper hypolimnetic maximum of dissolved manganese demonstrates a very rapid reduction of settling manganese oxyhydroxides. Similar concentration profiles have been observed in tropical lakes (Ruttner, 1953) and anoxic marine basins

1193

(Spencer & Brewer, 1971). Iron oxyhydroxides, on the other hand, appear to have sunk to the lower hypolimnion or the sediment-water interface before reduction. While this settling behavior may have resulted simply from slower kinetics of reductive solubilization, due to the lower Fe2+-Fe a~ redox potential, it may also have been aided by the formation of very insoluble sulfide coatings around the iron colloids, preventing rapid solubilization. Ferrous sulfide (troilite) was oversaturated in these hypolimnetic waters while manganous sulfide (alabandite) was undersaturated. Such an oxidation-reduction cycle has been termed the "ferrous wheel" by Campbell & Torgersen (1980); to this lexicon we add the "manganous wheel". These two "'wheels" should have the effect of enhancing lake respiratory metabolism. First, the buildup of a high manganous ion level in the upper hypolimnion will enhance the concentration gradient between this zone and the epilimnion, thereby enhancing the contribution of manganese to the dissolved oxygen demand diffusing upward. Second, the gravitational settling of the particulate oxyhydroxides serves as an additional mechanism for delivery of oxid a n t s - t h e "manganous wheel" to the upper hypolimnion and "ferrous wheel" to the sediment-water interface. These two oxidants will provide higher caloric gain for bacteria using them to respire organic carbon than would be derived by the more dominant meth-

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1194

LAWRENCEM. MAYERet al.

anogens likely comprising the bulk of metabolic activity in these zones (Froelich et al., 1979) and will therefore increase the proportion of sedimented organic carbon regenerated as COz. Insofar as these wheels are closed and efficient systems, the gravitational return of tetravalent manganese to the upper hypolimnion will be limited by the upward diffusion of dissolved divalent manganese, which is in turn limited by the same physical factors limiting delivery of oxygen downwards. The relative delivery of electron-accepting capacity, in the forms of dissolved oxygen and particulate manganese dioxide, to the upper hypolimnion is then equal to the relative molar gradients of oxygen and dissolved manganese across the metalimnion multiplied by the electron-accepting capacities of their respective oxidized forms. Toward the end of the summer stratification period, when manganese buildup in the upper hypolimnion was considerable, as much as 20~o of the oxidant (electron-accepting) delivery to this zone could have been due to settling manganese oxides that were in turn derived from hypolimnetic reduced manganese. Allochthonous manganese dioxide (Davison, 1981) would of course further enhance oxidant delivery. Assuming that diffusive transport dominates the delivery of all of these reduced substances to the principal zone of oxygen consumption, the thermocline, we can calculate the relative importance of each of these dissolved reductants to the dissolved oxygen demand (OD) from the concentrations of these species in the mid to lower hypolimnion. This calculation can be performed only for the summer-fall transitiOn period when methane analyses were made. We have assumed that only ~ of the methane delivered was available for oxidation, presumably by bacteria (Harrits & Hanson, 1980), while the remaining ½ was converted directly to biomass via methanotrophy (Ruddet al., 1974). Ammonium concentrations were not determined in this study; however, a previous study of this lake (Hall, 1974) did measure this species as well as several others we measured. Assuming a similar hypolimnetic stoichiometry from year to year we have estimated ammonium levels by regression analyses of both data sets. We have assumed that methanotrophy results in a 6:1 molar incorporation of ammonium and subtracted this ammonium from the pool available for oxidation. The oxidation reactions used to calculate OD and typical resultant percentages of OD are given in

Table 2. These calculations show that the bulk of the hypolimnetic dissolved OD was provided by methane. Anagnostidis & Overbeck (1966, cited in Golterman. 1976) found methane to cause 70% of the oxygen consumption in the Pliiss See during certain seasons. Efficiencies of methane oxidation lower than ~ due to increased methanotrophy, such as found by Rudd & Hamilton (1978) and Brown et al. (1964), will of course lower the OD contribution by this species. Ammonium may have provided a significant secondary source of OD--nitrite maxima have been found coincident with aerobic methanotrophic bacterial populations (Harrits & Hanson, 1980)--but delivery to the epilimnion and subsequent uptake by phytoplankton may well be a more important pathway for this species. Dissolved metals provided a minor portion of the OD and the contribution from sulfide was negligible. We emphasize that these calculations are only approximate. For example, divalent manganese likely oxidizes to a form less oxidized than MnO,, such as Mn304 or MnOOH, which would considerably lower its OD. Phosphorus cycling

Phosphorus removal by iron phosphate precipitation (Einsele, 1938) is clearly seen in Fig. 9, which compares the percentages of sediment-derived iron and phosphorus which precipitated in the upper hypolimnion. These precipitation percentages were derived by comparing the ratio of each element to an element also released from the sediment but conservatively mixed into the upper hypolimnion. We chose excess potassium (potassium concentration above that of spring lake water) as the conservative third element. The calculations assume that the Fe:excess K and P:excess K ratios of the mid or upper hypolimnetic waters were originally equal to those of the bottom waters sampled at the same time; the deficits from these original ratios, then, presumably represent precipitation. The mechanism of this scavenging process has been discussed by Einsele (1938), Hutchison (1941), Stumm & Morgan (1970) and Tessenow (1974). Upon exposure to low levels of oxygen in the upper hypolimk nion, iron precipitates as a partially hydrolysed ferric phosphate until the phosphate is exhausted, whereupon ferric hydroxide precipitation takes over. This ferric phosphate precipitation is contrasted with adsorption of phosphate by ferric hydroxide colloids, which is likely an unimportant process. Tessenow

Table 2. Reduced species, oxidation reaction and percentage of resultant OD, using field data from the bottom waters of the 9/6/77 sampling Species " CH4 Fez + Mn 2~NH~

Oxidation reaction CH4 + 202 ~-CO2 + 2H20 2Fe 2. + ½02 + 5H:O ~- 2Fe(OH)~ + 4H * Mn z÷ + ½Oz + H 2 0 ~ M n O z + 2H + NH~ + 202 ,-~--~NO~ + 2H ÷ + H20

)~ of OD 65.0 2.2 4.3 28.5

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Fig. 9. Precipitation extent of iron vs phosphorus in upper hypolimnetic waters. (1974) found, in experimental investigations, that the molar Fe:P ratio in the resultant precipitates never fell below 1.8-2.0. Figure 10 presents the molar Fe:P ratio in the precipitates formed in the mid to upper hypolimnion, from the calculated deficits explained above, along with the Fe:P ratios of the regenerated iron and phosphorus in the lower hypolimnion. The calculated Fe:P ratios of the precipitates generally decreased throughout the summer, except for the 25 August sampling which occurred shortly after a high wind event. The ratio never fell below 2.2, providing field confirmation of Tessenow's (1974) findings. Of particular interest is the congruence between the ratio found for the precipitates and those of the regenerated iron and phosphate in the lower hypolimnion. This congruence implies a remarkably efficient coupling of phosphorus to the hypolimnetic "ferrous wheel" in that the regeneration stoichiometry responds to the precipitate composition with a time constant of less than 2 weeks, the sampling period. The high Fe:P ratio of the 25 August sampling reflects the probable "leakage" of phosphate to the epilimnion as a result of the wind-induced rapid injection of oxygen into the hypolimnion (Stauffer & Lee, 1973). The decrease in phosphorus scavenging efficiency resulting from this mixing event may be due to a rise in pH that would accompany the injection of epilimnetic water into the hypolimnion; higher pH results in increased hydrolysis resulting in a higher OH:P ratio in the precipitate (Stumm & Morgan, 1970). SUMMARY Dissolved oxygen demand generated from the hypolimnetic sediments of Lake Sebasticook was

o- Regeneroted A - Precipitated I June

I duIy

I Aug

Sept

Fig. 10. Molar iron to phosphorus ratios in upper hypolimnetic precipitates (A) and lower hypolimnetic regeneration zone (O).

1195

dominated in the late summer by methane, with subsidiary contributions from ammonia, iron, and manganese. Sulfide contributions were negligible, being largely trapped in the lower hypolimnion by precipitation with iron. The manner in which these reductants consume oxygen derived from the epilimnion was related to the propensity for bacteria to mediate the oxidation reaction or incorporate the reductants into biomass, as well as the intensity of mixing between epilimnetic and hypolimnetic waters. Release of iron, manganese and perhaps phosphate from the hypolimnetic sediments seems to have been controlled by the dissolution of siderite and rhodochrosite. The dissolution rate of these carbonates was controlled in turn by the rate of acidity production by methanogenesis. Iron and manganese showed closed cycles in which the reduced ions diffused to a zone of oxidation in the upper hypolimnion or lower epilimnion, respectively. The newly-formed oxyhydroxides then settled gravitationally to become reduced again in the upper and lower hypolimnion, respectively. These particulates provided a source of electron-accepting capacity which enhanced the otherwise diffusion-controlled downward delivery of oxidants. Phosphate showed tight coupling to the iron cycle in the absence of strong wind events.

Acknowledgements--This research was originally supported

by a grant to Norton from U.S. Department of the Interior, Office of Water Research and Technology. The State of Maine, especially Jeff Dennis and Gardiner Hunt, provided logistical support and financial support for phosphorus analyses. Professor Thomas Hannula, University of Maine at Orono, provided logistical support and advice on the dynamics of the hydrologic behavior of Lake Sebasticook. C. Mayer-Bohne provided translations of the German literature. Liotta was supported by a grant to Hannula, also from the Department of the Interior. Parts of this report are taken from Liotta's M.S. Thesis (1978).

REVERENCES

Aller R. C. (1978) Experimental studies of changes prodficed by deposit feeders on pore water, sediment and overlying water chemistry. Am. J. Sci. 278, 1185-1234. Brewer W. S., Abernathy A. R. & Paynter M. J. B. (1977) Oxygen consumption by freshwater sediments. Water Res. 11,471-473. Brown L. R., Strawinski R. J. & McCleskey (1964) The isolation and characterization of Methanomonas methanooxidans. Brown and Strawinski. Can J. Microbiol. 10, 791-799. Campbell P. & Torgersen T. (1980) Maintenance of iron meromixis by iron redeposition in a rapidly flushed monimolimnion. Can. J. Fish. Aqat. Sci. 37, 1303-1313. Davison W. (1981) Supply of iron and manganese to an anoxic lake basin. Nature 290, 241-243. Delfino J. J. & Lee G. F. (1971) Variation of manganese, dissolved oxygen and related chemical parameters in the bottom waters of Lake Mendota, Wisconsin. Water Res. 5, 1207-1217. Dubiel R. F. (1977) Unpublished M.S. Thesis, University of Maine. Orono.

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