Interstitial inorganic phosphorus concentrations in lakes Mendota and Wingra

Water Research Vol. 11, pp. 1041 to 1047. Pergamon Press 1977. Printed in Great Britain.

INTERSTITIAL INORGANIC PHOSPHORUS CONCENTRATIONS IN LAKES MENDOTA AND WlNGRA G. C. HOLDREN,JR. and D. E. ARMSTRONG Water Chemistry Laboratory, University of Wisconsin, Madison, WI 53706, U.S.A. and R. F. HARRIS Department of Soil Science, University of Wisconsin, Madison, WI 53706, U.S.A.

(Received April 1976; in revised form April 1977) Abstract--Interstitial P levels in Lake Mendota and Lake Wingra were evaluated as a function of season and water column and sediment depth. Interstitial water was obtained by the centrifugationfiltration method. Temporal variations were observed over the entire 15 cm sediment depth interval examined in all four locations evaluated. Interstitial reactive P (IRP) levels in Lake Mendota ranged from 0.014-1.67 mg 1-t at the 5-6 m water column depth and from 1.20-5.75 mg 1-1 at the 18-19.5 m depth. IRP levels in Lake Wingra ranged from 0.029-2.15 mg 1-~ at 3.5 m and from 0.191-3.96 mg 1at 2 m. Variations, in interstitial P were attributed to variations in oxidation state of Fe as influenced by oxygen transport and reduction rates.

INTRODUCTION

of ground water may also affect transport from interstitial to overlying waters (Syers et al., 1973). The purpose of this investigation was to evaluate changes in interstitial P concentrations with time and depth below the sediment-water interface for Lakes M e n d o t a and Wingra. This paper reports on the relationships between I R P concentrations and the temperature, water column depth, and oxygen status of the overlying water. Subsequent research is concerned with mechanisms controlling the observed concentrations.

Sediment interstitial inorganic P is the sediment P fraction most sensitive to environmental conditions and highest in chemical mobility (Syers et al., 1973). As a result, information on the P levels in the interstitial waters of lake sediments may be more important t h a n information o n the sediment total P for predicting the dynamics of sediment P release (Stumm & Leckie, 1971). However, only limited information is available on the levels of P in sediment interstitial waters, a n d relationships between interstitial P and EXPERIMENTAL PROCEDURES rates of P release from sediments have not been estabSediment cores were taken with either a larger diameter lished. Interstitial P concentrations often exceed P concen- (8.9 cm i.d.) piston corer (Bannerman, 1973) or a Jenkins corer (Mortimer, 1971). Initially the piston corer was used trations in the overlying water (Rittenberg et al., 1955; for all cores,, but beginning in December, 1973, the piston Sutherland et al., 1966; Sullivan, 1967; Gahler, 1969; corer was used in water less than 8 m in depth and the Brunskill et al., 1971). Although G a h l e r (1968) Jenkins corer was used in deeper water to minimize disreported interstitial P levels to be higher in eutrophic turbance at the interface during sampling. Bulk sediment lakes t h a n in oligotrophic lakes, we have observed samples were obtained with an Ekman dredge. Cores were taken from Lakes Wingra and Mendota, n u m e r o u s exceptions to this trend for Wisconsin both eutrophic lakes containing calcareous sediments and lakes. Seasonal variations in interstitial P have also located in Madison, Wisconsin. In Lake Wingra, cores been reported (Sullivan, 1967; Sanville et al., 1974; were taken from an open-water location (3.5 m) in the Serruya et al., 1974). The sharp P concentration gra- west-central part of the lake and from the littoral zone (2m) in the southwest part of the lake. Lake Mendota dients observed between sediment interstitial water cores were taken from open-water sites in both epilimnetic a n d overlying lake water indicate a potential for (5-6 m) and hypolimnetic (18-19.5 m) areas of University transfer of P from the sediments into the overlying Bay. Bulk samples were obtained from Lake Koshkonong (Jefferson County) and Lake Waubesa (Dane County) in water (Rittenburg et al., 1955; Syers et al., 1973). F o r an undisturbed interface, diffusional transport addition to Lakes Mendota and Wingra. These additional lakes are also eutrophic and contain calcareous sediments. t h r o u g h the interstitial water is likely the rate deter- Various sediment parameters for these lakes have been mining step for P transfer from the sediment to the reported elsewhere (Williams et al., 1970, 1971a, 1971b; overlying water (Stumm and Leckie, 1971). The diffu- Bortleson, 1971; Li et al., 1972; Bannerman, 1973). After the core was obtained, a water sample was sional flux of ions from the sediment would be prosiphoned from a few cm above the sediment water interface portional to the difference in concentration between for analysis, and the temperature of the sediment was the interstitial and overlying water (Lerman & Bruns- measured with a mercury thermometer. Cores were seckill, 1971). Mixing processes or the upward m o v e m e n t tioned to a depth of 15 cm by extruding three cm layers 1041

1042

G.C. HOLOREN JR., D. E. ARMSTRONGand R. F. HARRIS

Table 1. Effects of centrifugation temperature on observed IRP concentrations for sediments stored in plastic bags during transport to the laboratory. Samples were centrifuged about 30 rain (Mendota) or 2 h (Koshkonong) after sampling Lake sediment

Sediment temp. °C

Mendota

5

Koshkonong

9.5

Centrifugation temp. °C 5 10 15 20 0 5 10 15 20

Lake sediment

IRP* mg 1-1 2.59 2.33 2.07 1.89 0.209 0.207 0.191 0.167 0.148

Table 2. Effects of centrifugation temperature on observed IRP concentrations for sediments placed in polypropylene centrifuge tubes immediately after sampling. Samples were centrifuged about 30 min after sampling

+ 0.22 ___0.14 + 0.38 + 0.12 + 0.016 + 0.015 _+ 0.014 + 0.007 + 0.004

* Based on triplicate samples. of sediment into commercial plastic bags (quart-size "Hefty" bags). After briefly mixing the sediment in the bags, two sub-samples from each layer were squeezed into 50ml polypropylene centrifuge tubes. All of the above steps were conducted immediately in the field to minimize oxidation and temperature effects. Water content was determined by drying sediment samples at 105°C overnight. Interstitial water was obtained by high speed centrifugation followed by filtration through a 0.45/~m membrane filter (Millipore type HA). Centrifugation was conducted at 17,000 x g for 15min at the in situ temperature of the sediment surface in a Sorvall RC2-B automatic refrigerated centrifuge. All samples were centrifuged immediately upon return to the laboratory within one hour after sampling. Dissolved reactive P (DRP) in water samples and interstitial reactive P (IRP) in interstitial water samples were analyzed by the colorimetric method of Murphy and Riley (1962). Samples for total P (TP), total dissolved P (TDP), and interstitial total P (ITP) were digested in an autoclave using the persulfate digestion procedure (APHA et al., 1971), neutralized with NaOH, and analyzed by the Murphy & Riley method. Evaluation of experimental techniques The centrifugation-filtration method of interstitial water separation was used because larger volumes of interstitial water are more readily obtained in a relatively short time as compared to pressure-filtration methods. The separation

Sediment temp. °C

Mendota

10.5

Mendota

15

Waubesa

15

Mendota

15

Centrifugation temp. °C 0 5 10.5 10.5t 15 20 5 15 15~: 15§ 25 5 15 28 5 15 15§ 28

IRP* mg 1 3.64 3.78 3.28 2.31 3.25 3.44 3.67 3.80 3.91 2.43 3.53 0.843 0.797 0.712 0.457 0.458 0.397 0.284

+ 0.29 _+ 0.21 _ 0.29 + 0.89 _+ 0.13 _+ 0.47 _+ 0.28 + 0.38 + 0.15 + 0.27 ___0.52 ± 0.049 _ 0.032 + 0.038 ___0.061 + 0.057 + 0.055 + 0.043

* Based on triplicate samples. t Stored in polycarbonate centrifuge tubes. :~Filtered under N v § Returned to the laboratory in plastic bags and then placed in polypropylene centrifuge tubes. of interstitial water from the bulk sediment inherently poses several problems due to the effects of factors such as temperature (Bischoff et al., 1970; Sayles et al., 1973) and oxidation (Bray et al., 1973; T r o u p e t al., 1974) on the concentrations of various ions. Consequently, the effects of centrifugation temperature and various storage methods on interstitial P values were evaluated. Temperature effects were found to depend mainly upon the previous handling of the sediments. Sediments returned to the laboratory in plastic bags exhibited a marked decrease in IRP levels with increasing centrifugation temperature (Table l). Sediments placed in polypropylene centrifuge tubes immediately after sampling exhibited only small changes in IRP levels with increases or decreases in centrifugation temperature (Table 2). Other factors were also found to affect observed IRP levels. Centrifugation in polycarbonate tubes resulted in

Table 3. Effects of storage at 4°C on IRP values. Sediment samples were placed in containers in the field and centrifuged at the in situ sediment temperature Lake sediment

Sediment temp. °C

Container

Mendota

10.5

Centrifuge tubes

Mendota

15

Centrifuge tubes

Waubesa

15

Centrifuge tubes '

Wingra

24

Centrifuge tubes

Storage time days

IRP* mg 1-1

0 1 0

3.28 + 0.29 2.84 _+ 0.42 3.80 + 0.38 4.21 + 0.01 0.797 + 0.032 0.908 +__0.012 0.680 ___0.008 0.676 + 0.013 2.59 _ 0.22 0.434 + 0.025 0.627 _+ 0.123 1.90 + 0.61

1

Mendota

5

Centrifuge tubes Plastic bags Double plastic bags Glass jar

0 1 0 3 0 1 1 1 0

* Based on triplicate samples.

Interstitial inorganic phosphorus concentrations much lower IRP concentrations than centrifugation in polypropylene tubes (Table 2). Other results obtained in this laboratory indicate IRP concentrations also decrease if filtration is delayed more than a few minutes following centrifugation. Filtration under N2 did not significantly affect IRP concentrations (Table 2) for the sediments used in this investigation and results obtained by the centrifugation-filtration method compared favorably with results obtained with a membrane squeezer in an inert atmosphere. Although the use of the centrifugation-filtration method immediately following sample collection gave satisfactory results, storage of samples for more than a few hours gave inaccurate IRP values (Table 3). Both increases and decreases in IRP concentrations were observed following overnight storage. Storage in other containers always gave lower results than immediate centrifugation (Table 3). Redox changes involving oxidation by atmospheric oxygen or reduction resulting from electron acceptor demand by sediment microorganisms were likely the cause of the observed changes in IRP levels. RESULTS

Series of cores were taken from two locations in Lake Mendota and two locations in Lake Wingra to evaluate variations in interstitial "P concentrations with time. The sampling locations were chosen to provide a comparison between well- and poorlymixed areas of the lakes. Interstitial organic P (lOP) concentrations, as determined by the difference between IRP and ITP levels, were generally low and exhibited no clear trends with either depth below the interface or time of year in both Lake Mendota and Lake Wingra sediments. The IOP levels comprised a significant fraction of ITP only when IRP levels were low and l O P was apparently not appreciably influenced by environmental conditions.

1043

Cores from the 5 to 6 m depth of Lake Mendota exhibited IRP concentrations ranging from 0.014-1.67 mg 1-1 (Fig. 1). The IRP levels usually increased or remained nearly constant with depth below the interface. Notable exceptions were cores for July, 1973 (sharp maximum at 3 ~ cm) and August 1974 (minimum at 12 15 cm). Due to the large variations in IRP concentrations, the gradient between IRP concentrations in the upper three centimeters of the sediment and DRP concentrations in the overlying water exhibited large seasonal variations. The interstitial water was enriched with P by a factor of 5-20 compared to the overlying water during the summer months as a result of the high IRP concentrations and relatively low DRP concentrations in the overlying water. The fall overturn period resulted in high DRP concentrations in the overlying water and a decrease in IRP concentration. During this period IRP levels were initially slightly higher than DRP levels, but IRP concentrations in the 0-3 cm layer decreased to levels below the overlying water DRP concentrations shortly after overturn. Interstitial P concentrations continued to decrease at all levels as the sediment temperature decreased to 2-3°C under the ice cover. Throughout the winter and continuing after ice-out in the spring, the interstitial water was actually depleted in IRP with respect to DRP in the overlying water by a factor of about five. The observed gradient may explain the reduction i n the amount of P in the water column during this period reported by Sonzogni (1974). In contrast to the 545 m location, cores from the depth of 18-19.5 m in University Bay were taken from a site well below the thermocline where mixing is expected to have little influence on P transport,

2.0

1,0 -

~

Loke Mendo'lo (5-6m)

0.8

Tj

0.6

~'~ 0.4

Q..- 0.2 i .~

i

i

i

~

I

i

J

1.0

~

08

~

0.6

0.4 0.2 iMr J = j 197'5

l

~

j

iFiMrAIM, j,j,Ar 1974

Month

S,OINIDi jiFIMIAI 1975

Fig. l. Interstitial P Levels in Lake Mendota (5-6 m).

i

G.C. HOLDRENJR., D. E. ARMSTRONGand R. F. HARRIS

1044 ;

5.0

•

40

Mendot"a (18.0-19.5m)

r

T. 3 0

E~°I Loke water

Q." LO i

i i

i

~ 5.0-5~ 4.0-~ 3.0 2,0 ~,~ LO

-

--

0 MI A i M i d id i A I s i o i N =

197'3

DidiFiMijiMid

Month

i diAislo

r

1974

Fill. 2. Interstitial P Levels in Lake Mendota (18-19.5 nm).

except during the overturn periods. IRP concentrations (Fig. 2) were higher than those in the shallower area, ranging from 1.20-5.75mg 1-1. Phosphorus levels in the overlying water ranged from 0.056-0.471 mg 1-1 and were always depleted, often by a factor of 20 or more, with respect to the high IRP levels observed. IRP levels varied inconsistently with depth below the interface;.concentrations generally decreased with depth in the winter, and reached a maximum, usually at 3-6 cm, during most of the summer and fall. Seasonal changes were usually manifested first in the upper layers and were followed by similar changes at the greater depths. Sediment temperatures at the 5-6 m sampling site in Lake Mendota varied from a maximum of 22-23°C from July to September to a minimum of 2-3°C from approximately December to March when the lake was under ice cover. Significant oxygen depletion was not expected in the bottom waters at this location. Temperature and dissolved oxygen levels at the 18-19.5 m Lake Mendota sampling site differed from the other locations because this sediment was located below the thermocline. Although the minimum temperatures were the same as for the other locations, the summer maximum temperatures were only 14-15°C. In addition, oxygen depletion occurs in this area during both the summer and winter stagnation periods. Results from Sonzogni (1974) indicate the bottom waters are completely anoxic from July to the fall overturn in October, and dissolved oxygen concentrations are less than 2mg 1-1 from midJanuary to ice-out in March or April. The Lake Wingra sampling sites were also chosen to provide a comparison between areas subjected to different levels of mixing. The 3.5 m site was an openwater area where complete mixing is expected to occur throughout most of the year. In contrast, mix-

ing in the 2 m sampling location in the littoral zone was restricted by the presence of rooted aquatic macrophytes, mainly Myriophyllum spicatum. Oven-dried sediment is similar in appearance for both locations although the first few layers of cores from the littoral zone often contained large amounts of decaying plant material. Interstitial P concentrations at the 3.5 m sampling site (Fig. 3) generally increased with depth, but overlying water DRP concentrations exhibited little change throughout the year. While IRP concentrations varied from 0.029-2.15 mg 1-1, DRP concentrations ranged from 0.0024).043 mg 1-1. Although data is limited, IRP levels appeared to reach minimum values shortly after ice cover occurred in the fall and again after ice-out in the spring. Interstitial P concentrations gradually increased during the summer months and under the ice in the winter. Levels of IRP in the littoral zone cores (Fig. 4) were usually much higher than those for open-water cores taken at the same time and ranged from 0.191-3.96 mg 1-1. An increase in IRP concentrations with depth occurred, similar to that observed at the 3.5 m site. Large differences in IRP levels were not observed between the summer cores, but levels began to decrease as the sediment temperatures began to decrease and mixing increased due to secession of the weed beds. After formation of ice cover in December, IRP concentrations had decreased below the levels found at the 3.5 m site in all except the 0-3 cm layer, and the IRP concentration was low in that layer as well. An increase in IRP levels under the ice was followed by a decrease after ice-out in the spring. Lake Wingra sediment temperatures were similar to those observed for the 5-6m Lake Mendota

2.4--

Lake Wingre { 3 . 5 m ]

2.0 1.6

12

7

/

E O.8

3-6om

,,%

A

~" 0.4 . . . . .

~

Lake water

o

~ 24 ~ 2,0 ~

L6

.

9-12cm

1.2-

•

0.8 ~

12-15crn

. . . .

"f

,//

~t / / ~ .

X/L

0 d~ j , j,A,SJO~N~ D,j ~F,M~AIM, j j j ~AiSlOiNrD~d ~F,M~A~M,j ~ 1972

1973

1974

Fig. 3. Interstitial P Levels in Lake Wingra (3.5 m).

Interstitial inorganic phosphorus concentrations

i'it

Lake Wingr0(2m)

>= 4.0--

o

3.0

F-

¢n

2.0

1.0

0

jd ,A i S l O i N =

1973

DI j , F I M J A I M

Month

1974

Fig. 4. Interstitial P levels in Lake Wingra (2 m). sampling location with summer temperatures of 2324°C and winter temperatures of 2 3°C. Some oxygen depletion may occur in Lake Wingra bottom waters during the period of ice cover, but measurements by Boylen & Brock (1975) indicate that, even in the littoral zone where sediment oxygen demand is expected to be the greatest, dissolved oxygen concentrations rarely drop below 2 mg 1-1. DISCUSSION

Recent investigations have shown that a high proportion of the sediment inorganic P in Lakes Mendota and Wingra is associated with hydrous Fe oxides (Williams et al., 1971b). Consequently, interstitial P concentrations should be controlled largely by the oxidation state of sediment Fe, which is a function of the relative availability of electron donors, primarily organic C, and alternate electron acceptors, such as O2, in the sediment system. Levels of interstitial P should increase as sediment oxygen becomes depleted and Fe(III) becomes reduced. Maximum interstitial P levels under reducing conditions will be determined by the levels of potentially mobile Febound P in the sediments and competing sorption and/or solubility equilibria limiting P release. Oxidation of sediment Fe(II) will cause sediment interstitial P levels to decrease. Under completely oxidized conditions, minimum interstitial P levels will be determined by the degree of saturation of P-retaining components in the system or possibly the solubility of phosphate minerals. As a result of the above considerations, factors determining sediment 02 levels should be of major importance in determining observed interstitial P levels.

1045

Mixing and diffusion are the major factors controlling the transfer of 02 into the sediments. The relative importance of these two factors and the degree and depth to which mixing occurs are determined largely by lake morphology. Rapid equilibration of overlying water and surface sediments is expected in shallow lakes due to the relatively large sediment surface to lake volume ratio (Hayes et al., 1952). In contrast, larger lakes require greater energy inputs before turbulent mixing can occur at the interface in deeper areas. The presence of a thermocline in deep lakes restricts greatly the amount of mixing in the hypolimnion. While turbulent mixing at the sediment-water interface may occur in the epilimnion, oxygen transport into sediments below the thermocline is probably limited by diffusion. The observed gradients of IRP concentration with depth below the interface arise from a combination of the diffusion, mixing, and redox processes described above. In shallow open-water areas such as the Lake Mendota (5-6 m) and Lake Wingra (3.5 m) sites, the observed gradients of increasing IRP levels with increasing depth below the interface are indicative of P release into the overlying water and/or oxygen transfer into the sediments followed by oxidation of Fe(II) and subsequent adsorption of phosphate. Because adsorption-desorption reactions involving Fe(III) and phosphate are relatively rapid (Li et al., 1971), the lower IRP levels in the surficial sediments are more likely a result of adsorption of inorganic P by the sediments than release of inorganic P to the overlying lake water. The IRP concentration gradient at the 18 19.5 m Lake Mendota sampling site shows the maximum IRP levels occur near 3-6 cm. This reflects the lower extent of mixing at this site than at the other site investigated. However, large concentration gradients between overlying and interstitial waters indicate diffusional release of IRP to the lake water could occur throughout the year. An oxidized layer of 2-3 mm observed on the sediment surface during the overturn periods would limit interchange during these periods of greatest mixing of hypolimnetic sediments, but the combination of large concentration gradients and oxygen depletion indicates that significant diffusion of IRP into the hypolimnetic waters could occur. Increases in the amount of P in the hypolimnion have been measured using a mass balance approach for Lake Erie (Burns & Ross, 1972) and Lake Mendota (Sonzogni, 1974). In both cases, the increase was attributed to a combination of sediment P release and to the deposition and subsequent mineralization of plankton settling from the epilimnion. Estimates of diffusional release into the hypolimnion can be made from the observed concentration gradients between interstitial and overlying waters. Concentration gradients for the summer, 1973, and winter, 1973-74, periods were estimated by calculating the difference between average overlying water DRP and 0-3 cm IRP concentrations from the July

1046

G.C. I"IOLDRENJR., D. E. ARMSTRONGand R. F. HARRIS

and October (2 cores), 1973 and January and February, 1974 cores, respectively. Based on a phosphate diffusion coefficient of D = 10 -6 cm 2 s-1 (Stumm &' Leckie, 1971), the measured concentration gradients of 3.5 (summer) and 1.6 mg 1-1 (winter), and a diffusion path of 1.5 cm (the distance to the center of the 0-3 cm layer from the interface), diffusional release rates of 2.9 and 1.3 mg m -2 day -1 were calculated for the summer and winter periods, respectively. These values are slightly lower than the value of 810 mg m -2 day calculated by Sonzogni (1974) using a mass balance approach for the summer, 1973 stratification period. However, the latter value included P accumulated due to transport from the epilimnion. If this epilimnetic P accounts for 509/o of the total accumulation, as indicated by Burns & Ross (1972), the resulting values are in good agreement. The oxygen uptake by the sediments is another important factor affecting the oxidation state of sediment Fe and varies with the supply of available organic carbon, sediment microbial activity, and temperature. Decaying algae and macrophytes and possibly runoff can increase the available organic C supply and the sediment oxygen demand, while increasing temperature will cause increased microbial activity. Therefore, the rate of electron transfer from organic carbon to oxygen and other electron acceptors will increase with increasing temperature, resulting in the reduction of Fe(III) to Fe(lI) with concurrent release of P into the interstitial water. Temperature also affects sorption and solubility equilibria, but the major effect of temperature on IRP concentrations occurs through changes in biological activity. The rate and extent of oxygen uptake by the sediment is reflected by IRP concentrations. For example, high IRP levels in the July, 1973, and August, 1974, Lake Mendota (5-6 m) cores occurred when decomposing algal blooms were observed at the sediment surface. These high IRP levels were likely the result of high oxygen demand created by the algal cells leading to the reduction of Fe(III) in the sediments. Based on experiments conducted in this laboratory, some variation in IRP is expected even among cores taken at the same location on the same date, and only differences of about 40~o or more in interstitial P levels can be considered significant for cores taken on different dates from approximately the same location. These differences are expected to be especially large for the deep Lake Mendota cores, where distance from shoreline reference points led to the greatest horizontal and vertical variation in sampling sites, and in the littoral zone of Lake Wingra, where the amount of vegetation could cause large changes in P concentrations over relatively short distances. Seasonal changes in interstitial P in the cores from all four locations were observed over the entire 15 cm interval examined, indicating that temperature, mixing, and oxygen penetration from the overlying water are affecting the interstitial water chemistry at depths

of 10-15 cm below the interface. Temperature and the biological and chemical processes affected by temperature apparently have a considerable effect on IRP levels through their effects on redox at all sediment depths examined. CONCLUSIONS Interstitial P concentrations in Lakes Mendota and Wingra exhibited large temporal and spatial variations at all sediment depth intervals examined. In general, IRP levels increased with depth below the interface in epilimnetic sediments. In littoral and hypolimnetic sediments IRP levels were higher and usually increased to a maximum before decreasing with depth. The observed variations can be explained based on mixing and the effects of temperature on the rate of sediment oxygen demand exertion. At all sampling locations and depths below the interface, IRP levels increased with temperature and reached maximum concentrations during the summer months when sediment temperatures and, consequently, the rate of oxygen demand exertion and Fe(III) reduction were highest. Minimum IRP concentrations coincided with the spring and fall overturn periods when sediment temperatures were low and mixing was most intense, facilitating the oxidation of sediment Fe(II). The higher IRP levels in littoral and hypolimnetic than in epilimnetic sediments also reflect in part the restricted mixing and consequent tendency for Fe(III) reduction in these areas. Interstitial P levels may be useful for the prediction of P release from lake sediments. A simple diffusion model and the concentration gradients between the interstitial and overlying water can be used to predict P release across an undisturbed anaerobic interface. While release from a mixed or oxidized interface cannot be easily quantified, the concentration gradient provides an indication of the direction and possibly the magnitude of P exchange between sediments and overlying water. IRP levels are useful in predicting P release from lake sediments only if precautions are taken in sampling procedures. Loss of IRP occurs if improper procedures are used for the separation of interstitial water from the bulk sediment. In addition, because of variations in IRP levels, the choice of sampling time and location are of primary importance for predictions of P release. A few IRP concentration measurements during the stratification period appear to be sufficient to predict diffusional P release in hypolimnetic areas. In other locations, where greater variability in IRP levels is observed, more frequent sampling is required. In epilimnetic areas, IRP concentrations measured during the summer months, when both IRP levels and demands for P are greatest, would be the most useful for predicting the effects of sediment P release on aquatic growth. Concentration gradients between interstitial and overlying

Interstitial inorganic phosphorus concentrations water provide a basis for estimating the importance of P release by lake sediments on the P budget of a lake. However, further information is needed on P transport across the oxidized zone in Fe-containing sediments to quantify the rate of sediment P release to epilimnetic waters. Acknowledgements--This research was supported in part by US EPA Grant R-800609 and by the USDA, ARS under Cooperative Agreement No. 12-14-4001-226. REFERENCES

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