SummaryLaboratory leaching tests conducted on Lake

Mendota sediments have shown that under oxic andanoxic conditions, phosphorus is released from thesediments to the water. The initial rate of releaseunder anoxic conditions is much more rapid thanunder oxic conditions; however, over extendedperiods of time, oxic release approximately equaledthat obtained under anoxic conditions. It is believedthat oxic release of phosphorus is primarily related tomineralization reactions and that this mode of releaseplays a much more important role in lakeself-fertilization than generally thought.

Another approach used to estimate relativesignificance of oxic vs anoxic release is one in whichan overall mass balance of the total phosphoruswithin the lake is assessed at intervals of every fewweeks over the annual cycle. From this, a total massof phosphorus present in the water body or partsthereof can be assessed. From estimates of theamounts of phosphorus derived from externalsources, it is possible to obtain an estimate of the netflux of phosphorus to and from the sediments. Thisapproach has been utilized in a three-year study ofLake Mendota located near Madison, Wisconsin.This study has shown that there can be substantialphosphorus released from the sediments under oxicconditions which can play a major role in stimulatinglate spring-early summer blue green algal blooms.

Substantial amounts of phosphorus are released fromthe sediments during the summer under anoxicconditions. Little of this phosphorus reaches theeuphotic zone during the summer recreational periodexcept during periods of thermocline migrationcaused by high winds. The results of both thelaboratory and field studies indicate that ferric ironprobably plays a relatively minor role in controllingphosphorus release from Lake Mendota sediments.

Introduction

Increasing emphasis is being placed today oncontrolling excessive fertilization of lakes andimpoundments. One of the primary concerns in anycontrol program is that of the role of lake sedimentsin recycling nutrients. The basic issue is one ofwhether the sediments of the lake are sinks forphosphorus or do they act as buffers maintaining acertain phosphorus level in the overlying waters. In order to gain some insight into the potentialsignificance of lake sediments in controllingoverlying water phosphorus concentrations, a seriesof studies has been conducted on Lake Mendota nearMadison, Wisconsin. Two experimental approacheshave been used. The first involved taking sedimentsamples into the laboratory and incubating them fora period of time during which phosphorusmeasurements were made. The other approachinvolved examining overall phosphorus dynamics inthe lake where the total mass of phosphorus present

Reference as, Lee, G. F., Sonzogni, W. C. and Spear, R. D., "Significance of Oxic versus Anoxic Conditionsfor Lake Mendota Sediment Phosphorus Release," Proc. International Symposium on Interactions betweenSediments and Fresh Water, Amsterdam, 1976, W. Junk, Purdoc, The Hague, pp 294-306 (1977).

SIGNIFICANCE OF OXIC VS ANOXIC CONDITIONS FOR LAKE MENDOTA SEDIMENTPHOSPHORUS RELEASE

G. Fred Lee1

Center for Environmental Studies, University of Texas-Dallas, Richardson, Texas, USA

W. C. SonzogniGreat Lakes Basin Commission, Ann Arbor, Michigan, USA

R. D. SpearU.S. Environmental Protection Agency, Edison, New Jersey, USA

1 Current affiliation: G. Fred Lee & Associates, El Macero, California, gfredlee@aol.com, http://www.gfredlee.com

in the lake was determined in relation to the annualphosphorus input and removal by the outlet. From thenet phosphorus input and the total content of the lake,it is possible to discern the flux of phosphorus to andfrom the sediments. This paper summarizes the resultsof a series of studies designed to elucidate the overallrole of Lake Mendota sediments in controlling theoverlying phosphorus concentrations.

Literature review

The classical limnological model for phosphorusdynamics in lakes and impoundments is one involvingthe iron cycle where, under oxidizing conditions, ferriciron combines with orthophosphate and is transportedto the sediments. Under reducing conditions, ferric ironis reduced to ferrous with liberation of phosphate to theoverlying waters. In addition, there is phosphatetransport from the water column to the sedimentsthrough the sedimentation of planktonic organisms,detrital minerals and precipitates. This model is basedprimarily on the classical work of Einsele (1936, 1937,1938a, 1938b, 1941) and Mortimer (1942), conductedin the 1930's and 1940's. A number of other studiesconducted since that time (using radioisotopes),principally those of Hayes (1963), have shown thatthere is very rapid exchange of phosphate amongvarious phosphorus pools in lakes.

Lee (1970) presented a review of the variousreactions governing the exchange of materials betweenwater and sediments. He pointed to the importance ofmixing processes within sediments, the overlyingwaters and between the sediments and water incontrolling the release of phosphorus and othercompounds from natural water sediments. Thisconclusion was based to a considerable extent on thefact that most lake sediment interstitial waterphosphorus concentrations are considerably higher thanthe overlying water, indicating that the chemical andbiological processes associated with the release ofphosphorus to the interstitial water were proceeding ata greater rate than the mixing of the interstitial waterwith the overlying water.

Lee (1970) also reported that the classicalpicture of well-defined laminae or varves representingannual deposition layers in lake sediments may be a rareoccurrence rather than the general situation. It

appears that most lake sediments are mixed from a fewcm to tens of cm in depth each year as a result ofphysical, chemical and biological processes. Based onthese observations, it was felt that the classical work ofMortimer (1942) on Lake Windermere sediments maynot present an accurate representation of the overallphosphorus release from lake sediments because of theexperimental approach used which did not provide foran accurate simulation of the mixing processes thatoccur within the sediments and between the sedimentsand the overlying waters. It was therefore decided toconduct a series of studies on the release of phosphorusfrom Lake Mendota sediments under situations wheremixing would not be the rate controlling step. Acomplete description of these studies and acomprehensive review of the literature on the exchangeof phosphorus in water and sediments is presented inthe Ph.D. dissertation of Spear (1970). Only an overallsummary of the approach and results are presented here.

Laboratory studies

Experimental procedure

The experimental procedure for these studiesconsisted of taking samples of sediments from severallocations in Lake Mendota using an Ekman sedimentgrab sampler. The sediments were placed in a reactionflask into which could be passed nitrogen gas, carbondioxide, various mixtures of these two gases or air. Theanoxic experiments involving N2 or N2-CO2 mixtureswere purified to remove traces of residual oxygen. Thesystem was continuously stirred with magnetic stirrerand teflon-coated stirring bar. Distilled water was usedas the leaching solution for the majority of the tests;however, after contact with the sediments, the waterquickly dissolved some of the calcium carbonatepresent in the sediments. Therefore, it is bestcharacterized as a calcium-magnesium carbonateleaching solution. The pH of the leaching solution wasadjusted by the partial pressure of CO2 in the gas whichpassed over the surface of the water-sediment slurry.

Two lakes were studied in this investigation–Mendota, a hard water eutrophic lake located in southcentral Wisconsin, and Trout Lake, a moderately softwater, mesotrophic lake located in north centralWisconsin. Part of the Lake Mendota data will bepresented in this paper. This lake has a surface area of38 square kilometers and a maximum depth of 24meters. Thermal stratification becomes well

First, as expected, anoxic release took place muchmore rapidly, reaching a plateau within about 200hours after initiation of the leaching experiment.Oxic release, however, while initially slower, did,over the 1200 to 1400 hour incubation period, releasesoluble orthophosphate which appeared to plateau atapproximately one half the concentrations reached inthe anoxic tests. It is somewhat surprising to see therelatively large amount of oxic release that took placein these tests since Lake Mendota sediments fromthese locations typically contain from 10,000 to20,000 ppm iron (Lee, 1962). It is evident that theferric iron formed upon aeration of these sedimentswas not controlling phosphorus release. A number ofother similar paired experiments were run by Spear(1970) utilizing sediments from these locations.Examination of the effect of temperature on the rateof phosphorous release under oxic and anoxicconditions showed that the rate of release wasmarkedly affected by temperature under bothconditions. Under anoxic conditions, temperature affected notonly the rate of release but also the apparent plateau,with approximately half as much phosphorus beingreleased at 11EC as 24EC. Under oxic conditions theeffect of temperature was substantially greater withan eight fold reduction in the rate of release duringthe first 800 hours. It does appear, however, thatover extended periods of time the lower temperatureruns could reach the same plateau as the 24EC run. In order to investigate the potential impact of pHand addition of air on the release of phosphorus fromLake Mendota sediments under anoxic conditions, anexperiment was conducted in which the sedimentswere kept anoxic for approximately 480 hours bybubbling purified nitrogen gas through the solution.At this time the gas used to maintain anoxicconditions was changed to a mixture of N2-CO2,which resulted in the pH decreasing from about 8 toapproximately 7. Examination of Figure 4 shows thatthe typical anoxic release pattern was

established by late May-early June and persiststhrough September. The hypolimnion becomesanoxic usually about mid-July.

For the anoxic studies it was found that thefiltration of the samples of the slurry had to beconducted under anoxic conditions in order toprevent loss of phosphorus in the ferric hydroxideprecipitate which rapidly formed upon exposure toair. This was done by passing purified nitrogen gasover the top of a covered funnel. Solubleorthophosphate was measured on all samples as afunction of time using 0.45 micron pore sizemembrane filters to distinguish between soluble andparticulate phosphorus. The phosphomolybdatemethod with stannous chloride as a reducing agentwas used for phosphorus measurements in accordwith Standard Methods, 12th Ed. (APHA, et al.,1965). Additional details on the experimentalprocedures are available in the dissertation by Spear(1970).

Results

Sediments were taken from three differentlocations within Lake Mendota - from the deep holearea (Station 1), from the slope of the deep hole(Station 2), and from University Bay (Station 3).University Bay is separated from the deep hole areaby a sill. The sediments from all three areas weretaken from depths which are normally below thethermocline during the summer stratification period.These sediments had 62 to 66 percent water, 31 to 36percent carbonates, 12 to 15 percent organic matterand 51 to 52 percent clastics (defined as thedifference between the organic content of thesediments and the carbonate content).

Figures 1, 2 and 3 show the rates of solubleorthophosphate release from these three locationsunder oxic and anoxic conditions at a temperature of24EC. Several things are evident from these figures.

significance, the conclusion is drawn that the reason forthis position is primarily based on the fact that thethermocline acts as a trap for anoxic release. On theother hand, under oxic conditions, the lake is normallywell mixed with the result that any phosphorus releasedfrom the sediments due to mineralization processes iscirculated throughout the water column and thereforenot readily detected because of the much greaterdilution and uptake by algae.

Interestingly, the rates of phosphorus release underoxic conditions, while slower than anoxic conditions,are still sufficiently rapid to be of significance ininternal cycling of nutrients. Also the rates of releaseunder both oxic and anoxic conditions from thesesediments are much more rapid than would be expectedunder quiescent conditions such as those frequentlyused by various investigators (a sample of sedimentplaced in a carboy and the phosphorus released to theoverlying waters measured as a function of time). Thelake probably behaves somewhere between thequiescent conditions and the completely slurriedconditions used in these studies. During periods ofstorms shallow water sediments often are readilysuspended and thereby provide the opportunity for oxicrelease of phosphorus associated with the particulatematerial as well as from the interstitial waters.

Since it is impossible to simulate the hydrodynamicconditions that exist in lakes in the laboratory with aknown degree of reliability, it was felt that furtherlaboratory studies on the potential significance of theoverlying waters would be of limited value. Therefore,the whole lake mass balance approach was used, theresults of which follow.

Field studies-mass balance It was felt that the best approach to take for assessingthe actual role of lake sediments in controllingphosphorus in the overlying waters is the mass balanceapproach where the total phosphorus present in thewater column can be examined as a function of time.By correcting the total water column phosphorus for thenet phosphorus input, it should be possible to determine

obtained during the first 480 hours. Changing the pHfrom 8 to 7 had no effect on the orthophosphate contentof the solution. It did, however, markedly affect theamount of iron present in solution. At about 620 hoursair was introduced into the test system. This resulted ina marked decrease in the orthophosphate and iron. Thissystem was again made anoxic and the pH againadjusted to 7. This time, as in the past, there was noeffect on the release of phosphorus. This experiment demonstrates the classicallimnological model for iron and phosphate behaviorwhere under anoxic conditions phosphorus is releasedwith the amount of release independent of pH in theneutral to slightly alkaline region. The introduction ofair into the system caused the precipitation of ferric ironwith the concomitant removal of the solubleorthophosphate. Based on these experiments, it is clearthat the classical limnological model for iron control ofphosphate holds for short periods of time. However,with extended periods of time, i.e., hundreds of hours,phosphorus is released under oxic conditions. Thisrelease appears to be related primarily to mineralizationreactions of organic phosphorus. Measurements of thecalcium content of the oxic test showed that it increasedthroughout the incubation period. This is believed to bedue to a solubilization of calcium carbonate present inthe sediments. Mineralization reactions result in theproduction of CO2 which would interact with the solidcalcium carbonate present in the sediments to formcalcium bicarbonate. As noted in Figures 1 through 3,the pH during the course of the various tests did notchange significantly. The sediments containedapproximately 30 percent calcium carbonate andtherefore represent a well buffered pH system.

Discussion

The implications of these results to the lake aresignificant. They indicate that potentially large amountsof phosphorus release could be occurring under oxicconditions. This is somewhat contrary to what isgenerally believed. If one examines the basis of thegeneral position that anoxic release is of major

the net fluxes of phosphorus to and from the sediments.This was the approach used by Sonzogni (1974). Atperiodic intervals over a three year period measurementswere made of the total phosphorus content of LakeMendota waters. Also, soluble orthophosphate wasmeasured as well as various nitrogen species. Thispaper will present a summary of the phosphorus data.The complete data are available in Sonzogni (1974).

Coincident with measurement of the totalphosphorus content of the lake, estimates were made ofthe phosphorus input to the lake. Table 1 presents asummary of these estimates. According to theseestimates Lake Mendota receives approximately 60,000kg/yr of total phosphorus of which slightly over half isin the dissolved reactive form. The principal source ofphosphorus for this lake is rural runoff, with asubstantial part of this runoff derived from dairy manurespread on frozen land. The amount of phosphoruscontributed to the lake each year via this source ishighly variable, dependent primarily on theclimatological characteristics of the spring snow meltperiod. If the spring rains and snow melt occur at a timewhen the subsoil is frozen, then most of this phosphorusis transported over the surface of the soil to nearbytributaries which eventually reach Lake Mendota.However, under conditions where the subsoils are notfrozen at the time of snow melt and spring rains, asubstantial part of the phosphorus will go into the soiland become fixed on soil particles. In order to accountfor the yearly variability of phosphorus loading, a rangeof possible values is given in Table 1 for totalphosphorus loading. Table 1 is based on prediversion wastewater figures.

In 1971 the wastewaters from several small townslocated in the Lake Mendota watershed were divertedaround the lake. Following diversion, wastewater

discharges of total P decreased from 24 percent of thetotal (Table 1) to approximately 2 percent. The ruralrunoff post-diversion figures increased to 63 percent ofthe total P input to the lake.

Sonzogni (1974) has estimated that approximately15,000 kg of total P leaves Lake Mendota each year viaits outlet. Of this, approximately 11,000 kg is in adissolved reactive (DRP) form. Therefore, the netphosphorus input to Lake Mendota each year is on theorder of 46,000 kg/yr total P (TP).

Procedure

Water samples were taken in the region of the deephole at 1 m intervals during the spring, summer and falland at 2 m intervals during the winter. Measurementswere made of Secchi depth, dissolved oxygen andtemperature. The lake was sampled approximatelyweekly, except during the winter, when sampling wasconducted at bi-weekly intervals. The samplingprogram was initiated in the summer of 1970 andcontinued until July, 1973. Dissolved reactivephosphorus was measured by filtration through 0.45 µmpore size membrane filters followed by the Murphy andRiley (1952) procedure. Total phosphorus was initiallydigested with persulfate in accord with the procedurespresented in Standard Methods, 13th Ed. (APHA et al.,1971).

When a thermocline was present, the depth of thethermocline was determined by examining thetemperature and chemical profile. In subsequentcomputations of hypolimnion volume, the metalimnionwas considered to be part of the hypolimnion. Duringperiods of ice cover the lake was arbitrarily divided intotwo layers consisting of the top 17 m and bottom 5 m. Figures 5 through 8 present the results of the total anddissolved reactive phosphorus content of Lake Mendotafrom August, 1970 through November, 1973. Inaddition, where appropriate, the two forms ofphosphorus present in the hypolimnion and epilimnionare also presented. Table 2 presents the average contentper month for the study period for each of the two

forms of phosphorus measured. Examination of thesefigures and this table shows that the total phosphorus inAugust 1970 and 1971 was .approximately 7 x 104 kg.Examination of the three years of record shows thatapproximately 80 percent of the phosphorus present inthe lake at any time is in the dissolved reactive form.

The greatest year-to-year variability of thephosphorus at any time of the year occurred shortlyafter ice-out. In the spring of 1971 a massive diatom

bloom occurred in Lake Mendota which caused the totalphosphorus content of the lake to decrease from slightlyover 5 x 104 kg to about 1.7 x 104 kg in association withthe settling of this bloom. Following the bloom, Figure6 shows a more or less linear increase in phosphorusthrough September. In the period mid-April throughearly July the bottom waters of the lake were oxic. Thisyear was a particularly dry year in which there wasbelow normal precipitation in the spring and early

summer. Lake Mendota had the best water qualityduring that period that it has had in many years. Thisappears to be the result of the diatom bloom tying up thephosphorus in the cells and transporting it to thesediments. It is evident from Figure 6 that there wassignificant oxic phosphorus release arising frommineralization reactions of the phosphorus present inthe sediments in the form of algal cells.

Examination of the spring 1972 and 1973 shows thatthe 1971 pattern was not repeated. In 1972 ice-out wasaccompanied by an increase in the phosphorus content,probably due to transport from the lake's watershed. In1973 the total phosphorus content of the lake was thegreatest of any of the four spring seasons monitored.This spring was one of the wettest springs in many yearsand the high phosphorus represents transport from thewatershed. During periods of summer stratification, usuallybeginning in late August, there is an inverse relationshipbetween the total mass of phosphorus in thehypolimnion compared to the epilimnion. This is theresult of thermocline migration, where during this timerapid downward movement of the thermocline results inthe transport of phosphorus from the hypolimnion to theepilimnion. This downward movement wasaccompanied by a decreased hypolimnetic volumeaccounting for the decrease in the total mass of total andsoluble orthophosphate present in the hypolimnionduring this time.

The year-to-year variability in the phosphorus contentof the lake as shown in Table 2 is strongly influenced byalgal population dynamics and input from thewatershed. The primary factor controlling thephosphorus input to Lake Mendota is the precipitationand snow melt with the associated runoff to tributariesof this lake. It is important to emphasize that during thisperiod Lake Mendota received in excess of 98 percentof the total phosphorus input from diffuse sources suchas urban stormwater drainage, rural runoff, atmosphericprecipitation and dustfall.

Interestingly, the annual average total phosphoruscontent of Lake Mendota, 4 to 6 x 104 kg, isapproximately equal to the normal annual input. Thisresults in phosphorus residence time for Lake Mendotaof about one year (Sonzogni et al., 1976). During the summer stratification period the primarycontrolling factor for algal blooms has been found byStauffer and Lee (1973) to be the passage of cold fronts,in which high intensity winds tend to sharpen and lowerthe thermocline. Each time the weather frontal systempassed, there was generally a period of high intensitywind which sharpened the thermocline and moved itdownward, 0.5 to 1 m. This migration downwardresults in the transfer of sufficient amounts of availablephosphorus from the hypolimnion to the epilimnion tocause a phytoplankton bloom about a week afterpassage of the front. Some of the variability noted in Figures 5 through8 during the summer reflects these algal blooms.Following the bloom, Secchi depth increases and totalphosphorus content decreases due to the uptake ofphosphorus by the algae and their subsequent settling tothe bottom of the lake. Table 3 presents an estimate of the net increase ofthe total phosphorus content of Lake Mendota duringsummer stratification for 1971, 72 and 73. In 1971 theaverage daily total phosphorus increase was about 187kg/day; for 1973 it was approximately half this amount(92 kg/day). These differences probably reflect the factthat 1971 was a somewhat peculiar year in terms oftypical phosphorus input from the watershed and themassive diatom bloom that occurred at ice-out. Thebloom probably removed the phosphorus entering thelake in the spring providing the opportunity for anoverall greater increase in phosphorus during summerstratification than was possible in the other years. Also,the overturn, which usually occurs in October, did notresult in a significant change in the total phosphoruscontent of the lake. This is significant since classicalmodels of phosphorus dynamics indicate that

phosphorus tends to be removed during fall overturnin association with the oxidation and precipitation ofiron.

Discussion

The mass balance approach for Lake Mendota andthe laboratory results of Spear (1970) both indicatethat under certain conditions, mineralizationprocesses occurring under oxic conditions can resultin significant release of phosphorus from lakesediments. As discussed earlier, in both thelaboratory and in the lake, the ferric iron present inthe surface sediments does not appear to becontrolling the release of soluble orthophosphate. Itfurther appears that at fall overturn the mixing of theferrous iron-rich hypolimnetic waters in LakeMendota does not have a major influence on the totalphosphorus present in the water column.

The phosphate scavenging power of hydrous ferricoxide appears to be dependent on a number offactors. It has been found (Lee, 1975) that theformation of the ferric hydroxide floc in the presenceof the phosphate was much more effective than apreform floc. This phenomenon likely accounts forthe fact that the ferric hydroxide floc is formedrelatively rapidly upon aeration of the sediments andtherefore the phosphorus that is released underperiods of days to weeks later comes in contact onlywith an aged floc. The same kind of phenomenonwould be occurring in the surface sediments wherethe overlying waters are oxic.

Conclusions It is concluded from these studies that at leastfor Lake Mendota sediments and quite possibly for

many other lake sediments, the classical limnologicalmodel of ferric iron controlling phosphorus releasefrom sediments and phosphorus content of theoverlying waters is inappropriate to explain majorphosphorus fluxes within the water body. It isfurther evident that while the sediments of lakesrepresent potentially significant sources ofphosphorus over limited periods of the year, on anannual cycle the sediments of most lakes are sinks forphosphorus. Therefore, significant reductions of theinput phosphorus from external sources will likelyresult in significant improvement in water qualityarising from a decrease in the frequency and severityof noxious algal blooms in those water bodies wherephosphorus is the key limiting nutrient or can bemade so by input reduction.

Acknowledgment

The support for the studies which serve as a basisfor this paper was derived from several grants fromthe US Environmental Protection Agency. Inaddition, support was given these studies by theUniversity of Wisconsin Department of Civil andEnvironmental Engineering where the studies werecarried out. Further support was given by the Centerfor Environmental Studies at the University of Texasat Dallas and EnviroQual Consultants & Laboratoriesof Plano, Texas. We wish to acknowledge theassistance of Gerald Jayne and M. Torrey for theirhelp in connection with the studies which serve as abasis for this paper.

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