Hypolimnetic withdrawal in two north american lakes with anoxic phosphorus release from the sediment

#‘or. Res. Vol. 21. No. 8, pp. 923-928. 1987 Printed in Great Britain. All rights reserved

0043- I354/87

S3.00 + 0.00

Copyright Q 1987 Pergamon Journals Ltd

HYPOLIMNETIC WITHDRAWAL IN TWO NORTH AMERICAN LAKES WITH ANOXIC PHOSPHORUS RELEASE FROM THE SEDIMENT GERTRUD K. NORNBERG'*, ROSEMARY HARTLEY’ and EDWARD DAVIS~ ‘Faculty of Science, York University, Downsview, Ontario, Canada M3J IP3, *P.O. Box 341, Dorset, Ontario, Canada POA IEO and ‘The Hotchkiss School, Lakeville, CT 06039, U.S.A. (Received

Ju1.v 1986)

Abstract-Hypolimnetic withdrawal has been used to decrease eutrophication in two Connecticut lakes. This restoration technique is based on the forced discharge of nutrient-rich bottom waters in lakes with anoxic hypolimnia. while surface outflows are dammed. Internal phosphorus load amounted to 600 mg m-2 summer-’ before withdrawal. Most of this load probably originated from the sediment since the experimentally determined sediment phosphorus release rates of 2-12 mg m-* day-’ can account for the internal load. The smaller lake responded to hypolimnetic withdrawal with decreasing epilimnetic and hypolimnetic phosphorus concentrations, decreased anoxia and internal loads. The larger lake also showed a tendency towards improvement which is, however, not statistically significant. Key words-phosphorus, withdrawal

sediment release, internal load, anoxic hypolimnion,

INTRODUCTION

Lakes with anoxic hypolimnia often accumulate high phosphorus concentrations in their hypohmnia (Numberg, 1984a, b). This highly available phosphorus, which is released from the anoxic sediment surface, contributes to the lake’s eutrophication (Ntimberg and Peters, 1984b). Several restoration techniques have been developed to combat the effect of this internal phosphorus load from the sediments. Some techniques are based on the prevention of anoxia, as in hypolimnetic aeration (Taggart and McQueen, 1982), which then also prevents anoxic phosphorus releasefrom the sediment. Another technique involves the addition of chemicals (e.g. alum) which precipitate phosphorus and form phosphorus adsorbing layers on the surface of the sediment (Kennedy and Cooke, 1982). Hypolimnetic withdrawal is the discharge of hypolimnetic water, instead of epilimnetic water, when the surficial outflow is partially or completely dammed (Pechlaner, 1975; Gachter, 1976). This technique is very cost efficient, since it requires only the installation of a pipe into the deep hypolimnion and a simple pumping mechanism to pump the water downstream. When the outlet is situated below lake level, initial priming is enough to start and maintain withdrawal of the hypolimnetic water and no active pumping is required (e.g. Pechlaner, 1979). This paper discusses the effects of hypolimnetic withdrawal, which has been conducted on two eu*Address for correspondence: c/o Ministry of the Environment, P.O. Box 39, Dorset, Ontario, Canada POA IEO. 92 3

restoration, hypolimnetic

tropic lakes in Connecticut, U.S.A. for several years. The lakes are introduced and characterized and the importance of sediment phosphorus release in these lakes is evaluated. Next the size and development of the important variables like internal load, anoxia and phosphorus concentration are shown and the dependence of their changes on parameters of the restoration technique examined. Finally the most obvious and important ChangAecreased epilimnetic total phosphorus (TP) concentrations-is compared to the size of hypohmnetic TP export for additional lakes from the literature. MATERIALS Study

AND

METHODS

lakes

Hypolimnetic withdrawal was started in Lake Wononscopomuc (1980/81) and in Lake Waramaug (1983) since internal phosphorus inputs seemed to offset the efforts to decrease anoxia, TP concentrations and algae biomass by reducing external phosphorus load (Kortmann ef al., 1982). Morphometry, hydrology, water and sediment chemistry and average pre-withdrawal nutrient concentrations are presented in Table I. Since the hypolimnia of the two basins of Lake Wononscopomuc are separated by a ridge (at I1 m depth), data are listed separately for the two basins.The withdrawal pipe is located in the shallower of the two basins of Lake Wononscopomuc and in the deep SE comer in Lake Waramaug (Fig. 1). The withdrawal water is aerated and mechanically cleaned before discharge in downstream creeks. In Lake Waramaug the depth of the intake pipe has to be raised as summer stratification proceeds since the discharge concentrations of nutrients, iron and hydrogen sulfide would otherwise become too high to comply with the state regulations. An additional treatment system withdraws water from the hypolimnion at the NW basin of Lake Waramaug but discharges it back into the lake after short aeration. This system does not appear to have any

924

Phosphorus release from sediment

925

Table I. Lake morphometry. hydrology and summer averages of data related to lake trophy oi3 years before treatment (SE in paren!heses. when no SE is given, only 2 years are available) Wononscopomuc Characteristics

Waramaun

Watershed area (km*) Lake area (ha) Mean depth (m) Maximum depth (m) Water residence time (yr) Epilimnetic TP (pg I ‘) Hypolimnetic TP’ @g I-‘) Secchi disk depthf (m) Anoxic period (days) Depth of oxycline (m)

Deco

37 272 7 13 0.83 35 (0.8) 381 (67.3) I .6 (0.03) 146 (4.3) 4.6 (0.12)

5.99t 70 12.5 33 4t 27 450 5.6 (0.29) 157 8.7 (0.88)

Shallow 24 8.5 15 21 409 5.6 (0.31) 184 8.7 (0.88)

‘Maximum concentration. tData for both basins combined. tLake Wononscopomuc: n = 12.

effect on the extent of anoxia or TP concentrations of the lake (Niimberg, unpublished data).

significant

Experiments

Table 3. Comparison of internal TP loads (mg m -’ summer‘) derived from increases of hypolimnetic TP mass (in situ) and from incubation experiments (cores)

and analysis

To determine whether the high TP concentration in the anoxic hypolimnia originates in the sediment, phosphorus release rates (mg mm2day-‘) were measured in undisturbed and sealed core tubes containing sediment and water from several anoxic sites in Lake Waramaug and Lake Wononscopomuc in the summer of 1985 (open circles in Fig. 1). The procedure is described in Niirnberg ef al. (1986). except that in this study the cores were incubated in the anoxic hypolimnion of the lake rather than in the temperature controlled incubator in the laboratory. TP, soluble reactive phosphorus (SRP) and iron in the release rate experiments were determined according to Niimberg (1984b). Oxygen profiles were determined by the standard Winkler technique or polarographically and TP concentrations by standard methods. A measure of anoxia was calculated from weekly or biweekly oxygen profiles on five sites in Lake Waramaug and the two basins in Lake Wononscopomuc. We called this measure anoxic factor; it was computed from area and extent of anoxia according to equation (I): Anoxic factor = (duration x area)/lake area. (1) This factor represents the number of days when a sediment area, equal to the whole lake surface area, is overlain by anoxic water (< 1 mg 0, I-’ at 1 m above the sediment surface). The multiplicatl’on of this anoxic factor with the phosphorus release rate yields internal phosphorus load (mgmm2 lake surface area summer-‘). Internal TP loads were also calculated from the maximum increase of hypolimnetic TP mass in the late summer after correction for a background TP concentration (i.e. mean epilimnetic TP concentration of June and July) and division by lake area (Niimberg er al., 1986). RESULTS

AND

DISCUSSION

The release rates differed from site to site in Lake Waramaug and in the two basins of Lake WonTable 2. Average

iron,

Cores

1980 1981 1982 1983’ 1984. 1985’

719 703 614 614 606 622

Wononscopomuc, 1980 1981 I982

large

641 690 454

Wononrcopomuc,

401 348 378

small

1980 1981’ 1982.

347 II3 74

140 II9 109

*Years of hypolimnetic reduced by exported

withdrawal. load (Table

except

9.00(1.355) 6.32 (0.864) ll.89(1.966) 7.30 (1.983) 2.10(1.153) Waramaug

III situ load is 5, last column).

onscopomuc, though the cores were all highly anoxic (redox potential < 100 mV), (Table 2). SRP was released at a lower rate than TP, but its potential influence on the biota is still substantial considering its highly available form (Niimberg and Peters, 1984a). Iron was also releasedfrom sediments of both lakes, though at a lower rate in the hydrogen sulfide rich sediments of Lake Wononscopomuc. Internal load determined in situ and from multiplication of experimental releaserates with the anoxic factor can differ as much as 70%. However as a group they are not significantly different for 6 years of Lake Waramaug and 3 years each of Lake Wononscopomuc’s two basins (Paired Wilcoxon signed ranks test, n = 12, Table 3). Hence the approximate estimate of internal TP load can be determined from

TP 7.5 m 9.5 m I2 m

Wononscopomuc, 28 m WononscoDomuc. I5 m Fe = total

In situ Waramoug 657 496 219 315 246 390

release rates (mg m-‘d-l) determined in core tubes in July and August 1985. SE is given in parentheses

Location Waramaug, Waramaug, Waramaug,

Year

9.5 m: ferrous

SRP

Fe

n

3.38(1.170) 3.83 (I ,280) 7.69(1.560) 6.51 (1.315) 2.21 (1.469)

105.8(11.23) 23.3 (5.73) 80. I (20.80) 10.3 (4.13) 5.0 (5.03)

6 6 7 4 3

iron:

n = number

of cores.

GERTRUD K.

926

N~~RNBERG

et ol.

700 L i z N k

500

-

F 0 B 300E

I

I.7 -

600

I

I

1983

1985

I 0 ::

: :

-

‘E

:: :

::

‘.

E

k .o_ 5 .-E G 0. I” I 1979

I 1981

1979

1983

1981

1985

Year

Fig. 2. Change of trophy dependentvariablesin Lake Wononscopomucwith time. The arrow indicates start of hypolimneticwithdrawal; +, shallow basin; 0, deep basin. the in situ increases of hypolimnetic TP mass during anoxic stratification. Furthermore the release experiments indicate that the accumulation of phosphorus in the hypolimnion is mainly due to sediment release and not settling plankton from the epilimnion. In order to evaluate the effect of hypolimnetic withdrawal on lake trophy, pre- and post-withdrawal annual summer averages of nutrient, oxygen and phytoplankton data were compared. In both basins of Lake Wononscopomuc epi- and hypolimnetic Table 4. Lake Waramaug, Year 1980 1981 1982 1983 I984 1985

pre- and post-withdrawal

epi-TP hypo-TP On8 m-3 37 35 34 23 42 33

500 375 261 570 600 617

Int. load Cm3 m-3 657 496 219 315 246 383

phosphorus concentrations, internal P load (deep basin lagging behind) and anoxia clearly decreased with the years of treatment (Fig. 2). Metalimnetic blooms of the bluegreen algae Oscillutoria rubescens completely vanished in both the basins after withdrawal was commenced in the small basin (Kortmann et al., 1983), though secchi disc depth as a measure of phytoplankton biomass, does not show any consistent trend. The averages of summer secchidisc depths (m) for pre- and post-withdrawal years for both basins are shown in the following table (SE in parentheses):

characteristics

Anoxia (4 89 87 76 76 75 ;s

Secchi (4 1.59 I so 1.58 I .84 I 71 iii

Withdrawal began in May 1983. epi-TP = average epilimnetic TP concentrations in August and September; hypo-TP = maximum hypolimnetic TP concentrations; Int. load = seasonal internal load, determined from hypolimnetic TP increases; Anoxia = anoxic factor; Secchi = secchi disk depth.

Before After

Deep

Shallow

5.6 (0.29) 5.4(0.18)

5.6 (0.31) 5.2(0.18)

Years I2 5

In Lake Waramaug no clear trend with withdrawal duration could be observed (Table 4) and therefore a non parametric test (Mann-Whitney) on pooled years before and after withdrawal start was used to decide if any significant changes took place. Because of the small number of data, all changes at the 10% level are

Phosphorus release from sediment Table

5. Specifics

of hypolimnetic

withdrawal

.c‘-

TP export Year

Period Luke

1983 1984 1985

13 May-7 I7 Apr.-IO IO May-27

1981 1982 1983 1984 1985

Lake I9 Jun.-28 Oct. 3 May-II Oct. I3 May-22 Oct. 2 May-20 Oct. I May-4 Nov.

‘Based

Oct. Aug. Sep.

on the shallow NA = not available.

Volume (In’ x IO’)

(kg)

(mg m-9

Waramoug 1153 1514 I323

137 160 99

48 56 34

o-

:

Wonon.wopomuc 140 212 201 287 286

22 21 NA

NA 13

23 23 NA NA I3

E -20 .u f .-E

-

‘$ -&Jo89, 88.

.E

8 s -60

\

= ” 52.

basin only.

31

I 100 Areot

considered to be significant. Anoxia, expressed as the depth of the oxycline, number of anoxic days and anoxic factor, a measure of overall anoxia, decreased after withdrawal began. Epilimnetic TP concentration, internal load and secchi disc depth did not decrease significantly, while maximum hypolimnetic TP concentrations increased. Therefore the effect of hypolimnetic withdrawal is not as clear in Lake Waramaug, as it is in Lake Wononscopomuc. Lake Waramaug has a much larger watershed to lake area ratio (12.6) than Lake Wononscopomuc (5.4) and changes in external TP load to Lake Waramaug during the period of investigation might be responsible for the apparent lack of beneficial withdrawal effects (R. Kortmann, Ecosystem Consulting, Conn., pers. comm.). In addition, TP export in Lake Waramaug may be too small to have a major effect on the lake. To examine this possibility, TP export due to withdrawal was calculated from withdrawal rate, period of operation and TP concentration of the withdrawn water or the water at the depth of the withdrawal pipe (Table 5). The TP export via withdrawal decreased internal load only by lO-20% which should result in an average annual TP decrease (- TP) of 4 pg 1-i in Lake Waramaug (8 pg 1-l in Lake Wononscopomuc) at equilibrium conditions, according to the model (Niimberg, 1985): -TP = -internal

921

load/qs,

where qs = annual waterload (m yr-‘). This small expected decrease might not be detectable and maximizing TP export with higher pumping rates and positioning the intake pipe at the depth of maximum TP concentration (i.e. closer to the bottom) could produce a better response. It also might take longer than 3 years to establish an equilibrium so that the effects of withdrawal become apparent. The hypothesis that a decrease of epilimnetic TP depends on the amount of exported TP via withdrawal and time of operation is supported by several European withdrawal operations. The regression of the decrease of epilimnetic TP concentration on logarithmic transformed area1 TP export is significant at the 10% level (P < 0.089, r2 = 0.556, n = 6; Fig. 3).

I 316 TP export

5*

,6* 1000

(mg m-2yr-‘)

Fig. 3. Comparison of areal TP export via hypolimnetic withdrawal and changes in epilimnetic TP concentration in six lakes from the literature (Niimberg, 1987). Lake Waramaug (3) and Lake Wononscopomuc’s shallow basin

(5) are shown for comparison (open circles)but are not included in the regression. The numbers indicate years of operation.

In these lakes visible changes in TP concentrations occurred only after at least 5 years of operation (Fig. 3). Data from the literature and the lakes examined in this study suggest that the beneficial effect of hypolimnetic withdrawal is based on TP export. Another possible help in restoring a lake is the export of anoxic water (reduced substances) which could effect a 10% decrease in internal load due to decreased anoxic sediment area, since the anoxic factors decreased by 10 units in the study lakes. The conclusion, that the direct export of TP appears to be the main factor of improvement was also drawn from the literature evaluation of this restoration technique in European lakes (Niimberg, 1987). SUMMARY

The discharge of anoxic hypolimnetic water instead of surface water can decrease lake trophy. Especially the smaller lake with a small watershed shows decreased TP concentrations and internal load after 5 years of hypolimnetic withdrawal. The larger lake, with a larger watershed and small area1TP export via withdrawal also shows a tendency towards improvement, which is, however, not significant for the 3 years of withdrawal. Hypolimnetic withdrawal as a restoration technique appears to decrease epilimnetic TP concentrations in reducing internal P load primarily via TP export. Therefore maximizing TP export via positioning of the intake pipe at a deep depth and operation during the time of maximum TP concentrations might enhance ohgotrophication. Acknowledgemenu--Interest by and discussions with D. Henry and the members of the Lake WaramaugTaskForce, Connecticut (LWTF) are gratefully acknowledged. The principal author was supported by a Jessie Smith Noyes Foundation grant to the LWTF during experimental studies and data analysis.

GERTR~IJ K. N~~RNBERGef al.

928 REFERENCES

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