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Effects of whole lake mixing on water quality and phytoplankton
Water Research Vol. 15, pp. 1205 to 1210. 19Sl Printed in Great Britain. All rights reserved
0043-1354/81/101205-06102.00,~ Copyright ¢~ 1981 Pergamon Pre'~ Lid
EFFECTS OF WHOLE LAKE MIXING ON WATER QUALITY AND PHYTOPLANKTON DALE W. TOETZ School of Biological Sciences, Oklahoma State University, Stillwater, OK 74078, U.S.A. (Received March 1981)
Abstract--This paper describes the effect of artificial mixing of two Oklahoma lakes with a downflow pump on water quality and algal biomass. Artificial pumping in Arbuckle Lake (951 ha), advanced autumnal turnover, but never destratified the lake completely. Ammonia decreased in the epilimnion, while sulfide (H2S) declined and dissolved oxygen (DO) increased in the hypolimnion. Other water quality parameters did not change. Near-bottom concentrations of manganese (Mn z+) increased, indicating pumping did not affect water chemistry near deep sediments (> 16 m). Pumping did not change significantly the depth of the Secchi disc or algal biomass as measured by chlorophyll a. The algal flora was dominated by diatoms at all times, and the density of blue-green algae was always low. Pumping kept Ham's Lake (41 ha) destratified, but seldom produced completely isothermal conditions or isochemical concentrations of DO. There was no drastic change in other water quality parameters. However, artificial mixing decreased water ciarity and increased algal biomass by a factor of about 2.5, probably by reducing sinking rates of the phytoplankton. Artificial mixing apparently eliminated a fall pulse of Microcystis.
INTRODUCTION Artificial destratification of reservoirs is a promising technique to ameliorate water quality problems. Artitidal destratification usually converts an anoxic hypolimnion to an oxic state and decreases the concentration of reduced compounds, thus improving water quality and extending fish habitat (Irwin et aL, 1966; Brezonik et al., 1969; Symons et al., 1967; and others). The effect of artificial lake mixing on phytoplankton in unclear. In some cases, algal biomass declines (Weiss & Breedlove, 1973; Malueg et al., 1973) or remains the same (Fast et al., 1973; Haynes, 1973). Biomass of blue-green algae sometimes declines (Weiss & Breedlove, 1973; Malueg et al., 1973; Robinson et al., 1969) holding out the promise of eliminating these organisms. However, in some cases bluegreen algae have been observed to increase in density (Knoppert et al., 1970; Drury et al., 1975). This paper describes the effects of artificially mixing two Oklahoma Lakes for two seasons or more, Arbuckle Lake and Ham's Lake, which have been described by Garton et al. (1976). Baseline data which were obtained during years when the lakes were not mixed artificially are compared to data when the lakes were so mixed.
two composite samples of the water column at Stas 1, 3, 4 and 5 and pooling like aliquots of water taken at the surface and at depths of 1, 2, 3 and 4 m. Water for other chemical analyses was taken at the surface and at 4, 8, 12. 16, 20 and 24 m at Sta. 1. At Ham's Lake temperature and DO were measured at meter intervals at Sta. I during the summers of 1976, 1977 and 1978 (Fig. 2). Other chemical parameters were measured monthly at the surface, 4 and 6-8m (near bottom). At Stas 1-5 Secchi disc transparency was measured and two water samples were collected weekly from the top 2.5 m of the water column with a weighted plastic hose. Like aliquots from each station were pooled to form two composite samples for the lake which were used to enumerafe algal density and to measure the concentration of chlorophyll a. Ammonia analyses followed
METHODS During the summers of 1976, 1977 and 1978 water quality parameters were measured at Stas 1, 3, 4 and 5 at Arbuckle Lake (Fig. 1)~ The depth at these stations was 24, 4, 17.5, 17.5 and 14.0 m, respectively. Temperature and DO measurements were taken at each station using a YSI 51A temperature and oxygen meter and probe. Samples for algal biomass (chlorophyll a), Mn 2+ from near the bottom, and measurements of Secchi disc transparancy were taken Fig. I. Map of Arbuckle Lake, Oklahoma, showing samat each station. Pigment samples were taken by drawing piing stations. 1205
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Fig. 3. Depth of the thermocline at Sta. 1 in Arbuckle Lake, Fig. 2. Map of Ham's Lake, Oklahoma, showing sampling stations. Soloranzo (1969). Strickland & Parsons (1968) was used for the other analyses. During 1977 and 1978 a total of 16 pumps was employed in Arbuckle Lake in the configuration of a hollow square near Sta. 1 (Fig. 1). Quintero & Garter (1973) describe the construction details of the pumps. Each pump had a blade 1.96m in diameter, which was suspended 2 m below the surface, and powered by a 1.5 h.p. electric motor. One pump moved about 160 m 3 rain-1 al a daily power cost (1980) of about $1.00. Pumping took place between 4 July and September 1977, and between April and September 1978. A 1.8 m pump was operated in Ham's Lake between 3 June and 4 July; a 1.1 m pump was operated between 20 July and 9 September 1976. During 1978 a 1.1 m pump was operated during the period 19 April-6 July; 1.6 m pump was operated 11 July-19 September. The pump was located between Stas 1 and 2. RESULTS AND DISCUSSION
Arbuckle Lake During 1976 (the control year) the depth of the thermocline in Arbuckle Lake gradually increased during the course of summer. For example, at Sta. 1 the thermocline was about 7 m in late June: by early September it was about 10m (Fig. 3). The depth of the thermocline also increased during 1976 at Stas 3 and 4. Artificial Pumping accelerated the rate at which the depth of the thermocline increased during 1977 and 1978. At Sta. 1 the depth of the thermocline increased from 8 m during late June to 15 m during early September (Fig. 3). The thermocline was initially deeper (1 m) during 1977 than during 1976 before pumping began, but after pumping began the difference in the depth of the thermocline between 1976 and 1977 widened considerably. During 1978 the decline of the thermocline was even more rapid, from 5 m in late July to 18 m in September. At Stas 3 and 4 the depth of the thermocline increased in a comparable manner during August. The depth of the thermocline on 1 September 1977 was about 5.0, 4.5, and 7.0 m deeper than on 1 Sep-
tember 1976 at Stas 1.3 and 4. respectively. However, the thermocline was no deeper than about 16 m on the last sampling date, 11 September 1977, On 1 September 1978. the thermocline was 7 m deeper than on 1 September 1976, at all stations and also about a meter deeper than on the same day in 1977. Pumping advanced turnover of the lake in the autumn of 1977. as it did in 1975 (Toetz. 1977). The thermal profile of the lake in 1976 showed strong thermal stratification on 11 September and weak mixing (if any} between 10 and 16 m IFig. 4). On the same date during 1977 and 1978 the thermal profile revealed deeper stratification but also strong mixing below 10 m. The volume of the lake containing 1 mg 1-1 D O or above also increased during 1977 and 1978, and the increase was probably the result of mechanical pumping. During 1976 the depth at which 1 nag 1-1 D O occurred increased gradually during the summer from 7.5 to 8.5 m. Although the depth at which t m g l - t D O occurred was 1-2 m lower during 1976 than during 1977 at the outset of the summer, it rapidly mcreased after pumping began (Fig. 5). By 1 September 1977. the depth at which I mg 1-1 D O occurred Was 15 m at Sta. 1 : during 1976 it was only 8.5 m. During 1978, the depth at which 1 mg I- ~ D O occurred also
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Fig. 5. Depth at which at least Img dissolved oxygen 1- l occurred at Sta. 1 in Arbuckle Lake. increased, although less rapidly than in 1977. At other stations the depth at which 1 mg 1-t DO occurred also increased rapidly during 1977 and 1978 as compared to 1976. A t-test was used to compare water quality, algal biomass, and Secchi disc transparency between years at the same station. Comparisons are made for data taken in July, August and September on comparable sampling dates. Comparisons were made for 5 sampiing dates for near bottom concentrations of Mn 2+, algal biomass and Secchi disc transparency. All other comparisons were made using 3 sampling dates. In this analysis samples taken at 0, 4 and 8 m were considered to be representative of the epilimnion, while those taken at 12, 16 and 20 m were considered to be representative of the hypolimnion. Detailed comparisons can be found in Toetz (1979). The most significant effect of the pump was to decrease NH~ in the epilimnion and H2S in the hypolimnion (Table 1). The mean values for NH~ declined from 77 to 19 and 18/zg NI- 1, between 1976 and 1977 and 1978, respectively. The mean value for hypolimnetic H2S was 504 in 1976 and only 100 and 72/zg 1- t in 1977 and 1978, respectively. In all years the value of the hypolimnetic concentration of Mn 2÷ was about 0.5-1.0mg 1-t. The decrease in hypolimnetic H2S is an expected response in an artificially destratified lake. One would also expect NH~, BOD5 and Mn 2+, to decrease as well. The fact that they did not decline when the lake was mixed suggests that the Garton pump did not mix the lake well enough to improve all aspects of water quality. Epilimnetic alkalinity averaged about 130 mg I- 1 in all 3 years and mean pH was about 7.5-8.0. Phosphate appeared to be the limiting nutrient in all 3 years as its concentration was usually below 1 pg P I - t while nitrate + ammonia was usually 25--80pg Nl-1 Near bottom concentrations of Mn 2+ were significantly higher during 1977 at three of the 4 stations where measurements were made. For example, at Sta. 3, Mn 2+ increased from 0.64 to 1.17mgi -1. At Sta. 5 it was about 0.7 mg 1-1 in both years. During 1978 near bottom concentrations of Mn + were significantly higher at Sta. 3 (0.64 vs 0.98mgl-t), but unchanged at the rest. There was an overall tendency for near W.R. 1 5 1 ~ F
1207
bottom concentrations of Mn 2+ to increase rather than decrease at stations deeper than 17 m, as would be expected. Profiles of DO suggest that lake mixing was not effective below 16 m. At Sta. 5 the concentration of Mn 2+ either declined or was unchanged as would be expected, since it is only 14 m deep. Except for one case, the depth of the Secchi disc did not change significantly between years at any of the stations. Mean transparency of the lake ranged from 1.60 to 1.98 m in 1976, from 1.62 to 1.88 in 1977, and from 1.39 to 1.90 m in 1978. The concentration of chlorophyll a was significantly higher in 1977 than in 1976 at one of the 4 stations, lower at another and unchanged at the rest. The mean concentration of chlorophyll a was the same in 1978 as in 1976. Overall, there was no significant change in algal biomass as measured by chlorophyll a as a result of lake mixing. The mean values for all stations were 9.1, 10.0 and 9.5 #g I- 1 for 1976, 1977 and 1978. The diatom flora was compared between years, using an index of similarity (S):
S =
2C A+B
-
where A = number of species in 1 year, B = number of species in the other year and C = number of species common to both years. The number of species of diatoms declined from 41 in 1976 to 28 and 27 in 1977 and 1978, respectively. Between 1976 and 1977 S for this group was 0.67. However, between 1977 and 1978 S was 0.90. Thus, changes in the species composition of diatoms are probably related to mechanical mixing. In all years, the flora was dominated by diatoms, (18-1039 organisms ml-t), blue-green algae (1-496 organisms ml-1), green algae (0-16 organisms ml-i), and flagellates (0-522 organisms ml-1). Although the diatoms were numerically abundant, and undoubtedly contributed largely to the chlorophyll biomass, the
.Table 1. Comparisons of water quality parameters between 1976 and 1977 and between 1976 and 1978 at 0.4 and 8 m (epilimnion)and 12, 16 and 20 m (hypolimnion) at Arbuckle Lake
Parameter Ammonia-N Nitrate-N Nitrite-N Sulfide-S Manganese
Change Compared to 1976 Epilimnion Hypolimnion 1977 1978 1977 1978 + + ND ND
0 0 ND ND
0 0 0
0 0 0 0
BODs
0
-
-
0
pH Total alkalinity
0 +
0 0
0 0
+ -
+ = Increase over 1976; - = decrease over 1976; 0 = no change. All differences were significant at the 0.05 level. ND = not detected. Data are for 3 comparable d a t e s in each year.
1208
DALE W. TOETZ
density (loglo) of diatoms was no greater in the summers when mixing was practiced than in the control summer (t = 0,13 and 0.79, for 1976 vs 1977 and 1976 vs 1978, respectively, P = 0.05). If 1976 is accepted as typical control year for Arbuckle Lake, then the data can be interpreted as showing that mechanical pumping significantly lowered the thermocline and delivered DO to depths which normally would be anoxic, Water quality in the epilimnion was basically the same in 1977 and 1978 as it was in 1976. The concentration of NH2 was lower in both years, but NH2 concentrations depend not only on how rapidly it is produced by decay and excretion but also how rapidly it is used in microbial oxidation and algal uptake. Therefore. changes in algal productivity and/or rates of regeneration could result in dramatic changes in NH2 concentration. Water quality generally improved in the hypolimnion during 1977 when the lake was mixed, but not markedly during 1978, The mean concentration of H2S was lower in both years, when mixing was practiced, but other reduced compounds and BOD5 did not consistently decrease in concentration. These results were not unanticipated, since mixing was not vigorous enough to change rapidly the concentration of DO in the hypolimnion or to alter the chemistry of deep sediments. Compounds such as H2S, which disappear rapidly in the presence of small amounts of DO, did decrease, but other compounds such as Mn 2+ did not behave similarily. There was no evidence that algal biomass changed or that the nutrient environment of the phytoplankton was affected drastically by artificial lake mixing. The density of blue-green algae such as Anabaena and Aphanizomenon was normally low and the lake has not had a history of algal problems. Artificial lake mixing did not appreciably change the situation.
Ham's Lake The analysis of effects of artificial mixing of Ham's Lake is somewhat confounded because of water level fluctuations during 1976-1978. On 19 January 1976, the lake level was 3.45 m. By 24 December 1976, it was 2.42 m: on 13 April 1977 it was 2.12 m. A precipitious increase occurred during late May 1977, when the lake level rose from 2.19 to 3.67m, after which there was slow decline in lake level until by the end of August 1978, it was less than 2.22 m. Because lake level fluctuated so much the vertical extent of the hypolimnion (or potential hypolimnion) changed. Values for temperature and DO at 6 m were used as representative of the hypolimnion, in making comparisons between years, However, other water quality parameters were only measured at one depth in the hypolimnion, just off the bottom in the deepest part of the lake. The depth at which these samples were taken varied from 6 to 8 m. The parameters are reported as near bottom values. Pumping in 1976 and 1978 reduced overall differ-
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Fig. 6. Difference in temperature between the surface and 7 m (A'-C)at Sta. 1 in Ham's Lake. ences in the temperature between the surface and 6 m, IA~'C). During 1977 A'~C was 8-10°C for most of the summer, hut much less (0-2"-C) during the summers of 1976 and 1978. Under natural conditions duriag 1977 there was a rapid decline in A:C during early August, when the lake turned over (Fig. 6). The lake restratifled again in mid August but the stratification was weaker since the A~' was about half the value in July. This suggests that it is impossible to separate effects of artificial mixing from natural turnovers in this lake during August. The 1977 data, however, suggest that Ham's Lake is so strongly stratified during June and July that natural mixing does not occur. Thus it is reasonably safe to use the June and July data as control data. ignoring for the momem any year to year variation in the unperturbed lake. During 1976 and 1978. the values of A'C corresponded reasonably well on the same date, except for the values in early June. The lack of correspondence. is probably due to the fact that stratification was well under way m 1976 before pumping began. However. the initial values of A'-C declined rapidly after 3 June 1976, when the pumps were turned on. In contrast. pumping began much earlier during 1978 (19 April) and the lake never became stratified IA~C values were always low). The concentration of DO at 6 m rapidly declined to zero in late May 1977, (control) and remained zero until turnover in early September (Fig. 7). There was a
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Fig. 7. Concentration of dissolved oxygen (DO) at 6 m at Sta. 1 in Ham's Lake.
Effects of whole lake mixing Table 2. Comparisons of water quality parameters between 1976 and 1977 and between 1977 and 1978 at 0 and 4 m (epilimnion) and near the bottom at Sta. 2 at Ham's Lake
Parameter Ammonia-N Nitrate-N Nitrite-N SulfideS Phosphate-P
Change Compared to 1977 Epilimnion Near bottom 1976 1978 1976 1978 + 0 + ND +
0 0 ND 0
0 0 0 0 +
0 0 0 0 ND
BODs
+
0
0
0
pH Total alkalinity
+ +
0 0
+ +
+ 0
+ = Increase over 1977, - = decrease over 1977, 0 = no change. All differences were significant at the 0.05 level. ND = not detected. similar rapid decline in 1976 but the concentration of DO never reached zero, because pumping began on 3 June. During 1978 stratification never occurred since pumping began in April. However, the concentration of DO at 6 m did decline in May and June and then began a slow rise between June and September. This slow rise can also be seen in the 1976 data. During 1976 the pump was shut down from 10 to 19 July, however, there was no apparent effect on the concentration of DO. It is probably true that little or no effect on the concentration DO occurs if the pump is shut down for a few days. The persistent decline in the concentration of DO before 17 July may be interpreted as being due to the ineffectiveness of the smaller pump propeller. If 1977 is considered the control year, then it can be shown that epilimnetic concentrations of NH2, NO~, pH, total alkalinity, BODs and PO~ ÷ were higher during 1976 when mixing was practiced (Table 2). Relative to 1977, mixing did not have the same effect in 1978 as only NO~ was changed. During the period of observation epilimnetic values for BODs were about 0.8-1.0 mg 1-1 while values for NH~ were low, 1-10/~g 1-1. The ranges of epilimnetic total alkalinity and pH were 123-160mgl -~ and 7.8-8.4, respectively. The range for N O ~ - N was 7-10#g1-1. The nutrient environment for phytoplankton did not appear to change markedly as a result of artificial distratification, i.e. the concentration of N and P nutrients were always low. Further, there was little change in pH, which could have caused a shift in species composition of the phytoplankton (Shapiro, 1973). During 1976 artificial mixing apparently elevated near bottom pH, total alkalinity and phosphate, but during 1978 only pH changed slightly. It is difficult to see any improvement in water quality from these results, particularily H2S, NH2 and BODs. Typical values for these parameters in the control year were 20/~g H2S1-1, 138/zg NH,~-N1-1 and 1.95mgl - l BODs. However, pH did increase from 7.9 to 8.2
1209
when mixing occurred as expected. These statistical tests may be of limited value because the concentrations of many water quality parameters, particularly H2S, NO~, PO~ + were often near the limits of detectability on many dates. For example, during the summer of 1977, PO~ + was detected in the hypolimnion on only 3 of the 19 sampling dates and then the concentration was very low, 0.7-1.1/zg !-1. In summary, the Garton Pump improved the concentration of DO in Ham's lake but did not affect other water quality parameters. Lake water was clearer during the control year (1977) than when mixing was in progress. During 1977 the mean depth of the Secchi disc was 1.68 m during 1976 and 1978 it was 0.96 and 1.19 m, respectively. Algal biomass was significantly higher when artificial lake mixing was in progress. During 1977 the mean concentration of chlorophyll a was 4.6 #g Iduring 1976 and 1978 it was 8.5 and 13.2 /zg1-1, respectively. The algal community was dominated by diatoms in all years when mixing was practiced. Melosira, a filamentous diatom, was absent during 1977 when mixing was not practiced, but was present in the other years. This genus relies on turbulent mixing to remain suspended in the euphoric zone (Lund, 1971). During the control year, blue-green algae were observed mostly during the late summer and autumn. During the years when mixing was practiced, bluegreen algae were not observed during eary summer and did not appear to exhibit a regular periodicity. There was pulse of Microcystis during September 1977 (control year), but none during the other years, suggesting that pumping was effective in eliminating this genus. Mixing a small lake with the Garton pump increased algal biomass and decreased the visibility of Secchi disc. The reason algal biomass was lower during the control year may be because the pump did not circulate the lake completely. When artificial mixing was practiced the vertical profiles of temperature and D O showed that the lake was not stratified but neither was it completely mixed. Deep mixing well below the compensation point (about 3-4m) may have brought nutrients to the surface which ordinarily would be lost to the hypolimnion for the season and/or many have decreased sinking rates of phytop!ankters. It is impossible to uncouple these two factors, but it is likely that nutrients were adequate, given rapid rates of recycling for phosphate and the relatively high concentrations of nitrogen nutrients. Thus, artificial mixing may have reduced sinking losses of the phytoplankton and it may be that this factor alone accounted for the larger algal biomass observed. Artificial mixing with the Garton pump increased algal biomass by a factor of about 2.5. The apparent management implication is that in lakes where nutrient depletion is common, such as Ham's Lake,
1210
DALE W. TOETZ
particular emphasis on phytoplankton. Hydrobioloqia 43, 463-504. Hooper F., Ball R. & Tanner H. (1953) An experiment in the artificial circulation of a small Michigan lake. Truns. Am. Fish Soc. 83. 222-242. Irwin W.. Symons J. & Robeck G. (1966). Impoundment destratification by mechanical pumping. J. sanit. Engn~! Dil'., Proc Am. Soc. tit'. Engrs 92(SA6~, 21-40. Knoppert P., Rook J., Hofker T. & Oskam G. (1970) Destratification experiments at Rotterdam. J. Am Wat, Wks Ass. 62, 448-454. Lund J. W. G. (1971) An artificial alteration of the seasonal cycle of the diatom Melosira italica suhsp suharctica in an English lake. J. Ecol. 59, 521-533. Acknowledgement.~ The work upon which this publication Malueg K., Tilstra J., Schults D. & Powers C. (1973~ Effect is based was supported in part by funds provided by the of induced aeration upon stratification and eutrophicaOffice of Water Research and Technology Project A-078 tion processes in an Oregon farm pond. Geophysical OK. U.S. Department of the Interior, Washington, DC as Monoyraph Series, Vol, 17, pp. 578-587. American Geoauthorized by the Water Research and Development Act of physical Union, Washington, DC. 1978. ! am indebted to the following persons for the technical assistance: Hong Chau, Patrick Downey, Ronald Eby, Quintero J. E. & Garton J. E. (1973). A low energy lake destratifier. Trans. Am. Soc. agric. Enyng 16, 973-9781 Steven Halterman and May Yue. 1 thank Dr James Garton for use of his data and Dr Joseph Shapiro for reviewing the Robinson E., Irwin W. & Symons J. (1969) Influence of artificial destratification on plankton populations in immanuscript. poundments. Trans. Ky. Acad. Sci. 30, 1-18. Shapiro J. (1973) Blue-green algae: Why they become REFERENCES dominant. Science 179(4071), 382-384SolSrzano L. (1969) Determination of ammonia in natural Brezonik P., Dclfino J. & Lee G. (1969) Chemistry of N waters by the phenolhypochlorite method. Limnol. and Mn in Cox Hollow Lake, Wisconsin, following Oceanogr. 14, 799-801. destratification. J. sanit. Engng Dil'., Am. Soc. cil'. Engrs Strickland J. & Parsons T. (1968) A practical handbook of 95($A5), 929-940. seawater analysis. Fish. Res. Bd Can., Bull. 167, 311 pp. Drury D., Porcella D. & Gearheart R. 119751 The effects of artificial destratification the water quality and microbial Symons J., Irwin W. & Robeck G. (1967) Impoundment water quality changes caused by mixing. J. sanit. Engng populations of Hyrum reservoir. PRJEWOLL-I. 174 pp. Dit'., Proc., Am. Soc., cir. Engrs 93(SA2~, 1-20. Utah Water Research Laboratory. College of EngineerToetz D. (1977). Effects of lake mixing with an axial flow ing, Utah State University, Logan. pump on water chemistry and phytoplankton. HydrohioFast A.. Moss B. & Wetzel R. (1973). Effects of artificial Iogia 55: 129- t38. aeration on the chemistry and algae of two Michigan Toetz D (1979). Effects of whole lake mixing on algae, fish lakes. War. Resour. Res. 9, 624--247, and water quality. 46 pp. Technical Completion Report, Garton J.. Summerfelt R., Toetz D., Wilhm J. & Jarrell H. A-078-OKLA, Oklahoma Water Resources Research In(19761 Physicochemical and biological conditions in two stitute. Stillwater. Oklahoma reservoirs undergoing artificial destratification, 138 pp. Oklahoma Water Resources Research Insti- Weiss C. & Breedlove B. (1973) Water quality changes in an impoundment as a consequence of artificial destratifitute. Stiltwater. cation, 216 pp. Water Resources Research Institute. UniHaynes R. (1973). Some ecological effects of artificial circuversity of North Carolina. Raleigh lation on a small eutrophic New Hampshire Lake with
algal biomass might be increased by artificial mixing. However, it is not correct to conclude that this increase is undesirable, since the biomass of algae in Ham's Lake never reached bloom proportions. In fact, it may be that artificial lake mixing increased the carrying capacity of Ham's Lake. Certainly, one would not expect this to be the case for other lakes, particularly those that are already highly eutrophic. Thus, caution is warranted, before using a downflow pump on these ecosystems.
0043-1354/81/101205-06102.00,~ Copyright ¢~ 1981 Pergamon Pre'~ Lid
EFFECTS OF WHOLE LAKE MIXING ON WATER QUALITY AND PHYTOPLANKTON DALE W. TOETZ School of Biological Sciences, Oklahoma State University, Stillwater, OK 74078, U.S.A. (Received March 1981)
Abstract--This paper describes the effect of artificial mixing of two Oklahoma lakes with a downflow pump on water quality and algal biomass. Artificial pumping in Arbuckle Lake (951 ha), advanced autumnal turnover, but never destratified the lake completely. Ammonia decreased in the epilimnion, while sulfide (H2S) declined and dissolved oxygen (DO) increased in the hypolimnion. Other water quality parameters did not change. Near-bottom concentrations of manganese (Mn z+) increased, indicating pumping did not affect water chemistry near deep sediments (> 16 m). Pumping did not change significantly the depth of the Secchi disc or algal biomass as measured by chlorophyll a. The algal flora was dominated by diatoms at all times, and the density of blue-green algae was always low. Pumping kept Ham's Lake (41 ha) destratified, but seldom produced completely isothermal conditions or isochemical concentrations of DO. There was no drastic change in other water quality parameters. However, artificial mixing decreased water ciarity and increased algal biomass by a factor of about 2.5, probably by reducing sinking rates of the phytoplankton. Artificial mixing apparently eliminated a fall pulse of Microcystis.
INTRODUCTION Artificial destratification of reservoirs is a promising technique to ameliorate water quality problems. Artitidal destratification usually converts an anoxic hypolimnion to an oxic state and decreases the concentration of reduced compounds, thus improving water quality and extending fish habitat (Irwin et aL, 1966; Brezonik et al., 1969; Symons et al., 1967; and others). The effect of artificial lake mixing on phytoplankton in unclear. In some cases, algal biomass declines (Weiss & Breedlove, 1973; Malueg et al., 1973) or remains the same (Fast et al., 1973; Haynes, 1973). Biomass of blue-green algae sometimes declines (Weiss & Breedlove, 1973; Malueg et al., 1973; Robinson et al., 1969) holding out the promise of eliminating these organisms. However, in some cases bluegreen algae have been observed to increase in density (Knoppert et al., 1970; Drury et al., 1975). This paper describes the effects of artificially mixing two Oklahoma Lakes for two seasons or more, Arbuckle Lake and Ham's Lake, which have been described by Garton et al. (1976). Baseline data which were obtained during years when the lakes were not mixed artificially are compared to data when the lakes were so mixed.
two composite samples of the water column at Stas 1, 3, 4 and 5 and pooling like aliquots of water taken at the surface and at depths of 1, 2, 3 and 4 m. Water for other chemical analyses was taken at the surface and at 4, 8, 12. 16, 20 and 24 m at Sta. 1. At Ham's Lake temperature and DO were measured at meter intervals at Sta. I during the summers of 1976, 1977 and 1978 (Fig. 2). Other chemical parameters were measured monthly at the surface, 4 and 6-8m (near bottom). At Stas 1-5 Secchi disc transparency was measured and two water samples were collected weekly from the top 2.5 m of the water column with a weighted plastic hose. Like aliquots from each station were pooled to form two composite samples for the lake which were used to enumerafe algal density and to measure the concentration of chlorophyll a. Ammonia analyses followed
METHODS During the summers of 1976, 1977 and 1978 water quality parameters were measured at Stas 1, 3, 4 and 5 at Arbuckle Lake (Fig. 1)~ The depth at these stations was 24, 4, 17.5, 17.5 and 14.0 m, respectively. Temperature and DO measurements were taken at each station using a YSI 51A temperature and oxygen meter and probe. Samples for algal biomass (chlorophyll a), Mn 2+ from near the bottom, and measurements of Secchi disc transparancy were taken Fig. I. Map of Arbuckle Lake, Oklahoma, showing samat each station. Pigment samples were taken by drawing piing stations. 1205
1206
DALE W. ToErz 5
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1976
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Aug.
Sept,
Fig. 3. Depth of the thermocline at Sta. 1 in Arbuckle Lake, Fig. 2. Map of Ham's Lake, Oklahoma, showing sampling stations. Soloranzo (1969). Strickland & Parsons (1968) was used for the other analyses. During 1977 and 1978 a total of 16 pumps was employed in Arbuckle Lake in the configuration of a hollow square near Sta. 1 (Fig. 1). Quintero & Garter (1973) describe the construction details of the pumps. Each pump had a blade 1.96m in diameter, which was suspended 2 m below the surface, and powered by a 1.5 h.p. electric motor. One pump moved about 160 m 3 rain-1 al a daily power cost (1980) of about $1.00. Pumping took place between 4 July and September 1977, and between April and September 1978. A 1.8 m pump was operated in Ham's Lake between 3 June and 4 July; a 1.1 m pump was operated between 20 July and 9 September 1976. During 1978 a 1.1 m pump was operated during the period 19 April-6 July; 1.6 m pump was operated 11 July-19 September. The pump was located between Stas 1 and 2. RESULTS AND DISCUSSION
Arbuckle Lake During 1976 (the control year) the depth of the thermocline in Arbuckle Lake gradually increased during the course of summer. For example, at Sta. 1 the thermocline was about 7 m in late June: by early September it was about 10m (Fig. 3). The depth of the thermocline also increased during 1976 at Stas 3 and 4. Artificial Pumping accelerated the rate at which the depth of the thermocline increased during 1977 and 1978. At Sta. 1 the depth of the thermocline increased from 8 m during late June to 15 m during early September (Fig. 3). The thermocline was initially deeper (1 m) during 1977 than during 1976 before pumping began, but after pumping began the difference in the depth of the thermocline between 1976 and 1977 widened considerably. During 1978 the decline of the thermocline was even more rapid, from 5 m in late July to 18 m in September. At Stas 3 and 4 the depth of the thermocline increased in a comparable manner during August. The depth of the thermocline on 1 September 1977 was about 5.0, 4.5, and 7.0 m deeper than on 1 Sep-
tember 1976 at Stas 1.3 and 4. respectively. However, the thermocline was no deeper than about 16 m on the last sampling date, 11 September 1977, On 1 September 1978. the thermocline was 7 m deeper than on 1 September 1976, at all stations and also about a meter deeper than on the same day in 1977. Pumping advanced turnover of the lake in the autumn of 1977. as it did in 1975 (Toetz. 1977). The thermal profile of the lake in 1976 showed strong thermal stratification on 11 September and weak mixing (if any} between 10 and 16 m IFig. 4). On the same date during 1977 and 1978 the thermal profile revealed deeper stratification but also strong mixing below 10 m. The volume of the lake containing 1 mg 1-1 D O or above also increased during 1977 and 1978, and the increase was probably the result of mechanical pumping. During 1976 the depth at which 1 nag 1-1 D O occurred increased gradually during the summer from 7.5 to 8.5 m. Although the depth at which t m g l - t D O occurred was 1-2 m lower during 1976 than during 1977 at the outset of the summer, it rapidly mcreased after pumping began (Fig. 5). By 1 September 1977. the depth at which I mg 1-1 D O occurred Was 15 m at Sta. 1 : during 1976 it was only 8.5 m. During 1978, the depth at which 1 mg I- ~ D O occurred also
:/9-11-77
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IS
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.
i/ i
20
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I 20
00
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I 30
Fig. 4. Vertical distribution of temperature at Sta- l Arbuckel Lake on comparable dates in September.
in
Effects of whole lake mixing Q
s
2sTs
9
|
u 12 15
i
\
u
~
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IS
'
June
I
July
I
Aug.
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Fig. 5. Depth at which at least Img dissolved oxygen 1- l occurred at Sta. 1 in Arbuckle Lake. increased, although less rapidly than in 1977. At other stations the depth at which 1 mg 1-t DO occurred also increased rapidly during 1977 and 1978 as compared to 1976. A t-test was used to compare water quality, algal biomass, and Secchi disc transparency between years at the same station. Comparisons are made for data taken in July, August and September on comparable sampling dates. Comparisons were made for 5 sampiing dates for near bottom concentrations of Mn 2+, algal biomass and Secchi disc transparency. All other comparisons were made using 3 sampling dates. In this analysis samples taken at 0, 4 and 8 m were considered to be representative of the epilimnion, while those taken at 12, 16 and 20 m were considered to be representative of the hypolimnion. Detailed comparisons can be found in Toetz (1979). The most significant effect of the pump was to decrease NH~ in the epilimnion and H2S in the hypolimnion (Table 1). The mean values for NH~ declined from 77 to 19 and 18/zg NI- 1, between 1976 and 1977 and 1978, respectively. The mean value for hypolimnetic H2S was 504 in 1976 and only 100 and 72/zg 1- t in 1977 and 1978, respectively. In all years the value of the hypolimnetic concentration of Mn 2÷ was about 0.5-1.0mg 1-t. The decrease in hypolimnetic H2S is an expected response in an artificially destratified lake. One would also expect NH~, BOD5 and Mn 2+, to decrease as well. The fact that they did not decline when the lake was mixed suggests that the Garton pump did not mix the lake well enough to improve all aspects of water quality. Epilimnetic alkalinity averaged about 130 mg I- 1 in all 3 years and mean pH was about 7.5-8.0. Phosphate appeared to be the limiting nutrient in all 3 years as its concentration was usually below 1 pg P I - t while nitrate + ammonia was usually 25--80pg Nl-1 Near bottom concentrations of Mn 2+ were significantly higher during 1977 at three of the 4 stations where measurements were made. For example, at Sta. 3, Mn 2+ increased from 0.64 to 1.17mgi -1. At Sta. 5 it was about 0.7 mg 1-1 in both years. During 1978 near bottom concentrations of Mn + were significantly higher at Sta. 3 (0.64 vs 0.98mgl-t), but unchanged at the rest. There was an overall tendency for near W.R. 1 5 1 ~ F
1207
bottom concentrations of Mn 2+ to increase rather than decrease at stations deeper than 17 m, as would be expected. Profiles of DO suggest that lake mixing was not effective below 16 m. At Sta. 5 the concentration of Mn 2+ either declined or was unchanged as would be expected, since it is only 14 m deep. Except for one case, the depth of the Secchi disc did not change significantly between years at any of the stations. Mean transparency of the lake ranged from 1.60 to 1.98 m in 1976, from 1.62 to 1.88 in 1977, and from 1.39 to 1.90 m in 1978. The concentration of chlorophyll a was significantly higher in 1977 than in 1976 at one of the 4 stations, lower at another and unchanged at the rest. The mean concentration of chlorophyll a was the same in 1978 as in 1976. Overall, there was no significant change in algal biomass as measured by chlorophyll a as a result of lake mixing. The mean values for all stations were 9.1, 10.0 and 9.5 #g I- 1 for 1976, 1977 and 1978. The diatom flora was compared between years, using an index of similarity (S):
S =
2C A+B
-
where A = number of species in 1 year, B = number of species in the other year and C = number of species common to both years. The number of species of diatoms declined from 41 in 1976 to 28 and 27 in 1977 and 1978, respectively. Between 1976 and 1977 S for this group was 0.67. However, between 1977 and 1978 S was 0.90. Thus, changes in the species composition of diatoms are probably related to mechanical mixing. In all years, the flora was dominated by diatoms, (18-1039 organisms ml-t), blue-green algae (1-496 organisms ml-1), green algae (0-16 organisms ml-i), and flagellates (0-522 organisms ml-1). Although the diatoms were numerically abundant, and undoubtedly contributed largely to the chlorophyll biomass, the
.Table 1. Comparisons of water quality parameters between 1976 and 1977 and between 1976 and 1978 at 0.4 and 8 m (epilimnion)and 12, 16 and 20 m (hypolimnion) at Arbuckle Lake
Parameter Ammonia-N Nitrate-N Nitrite-N Sulfide-S Manganese
Change Compared to 1976 Epilimnion Hypolimnion 1977 1978 1977 1978 + + ND ND
0 0 ND ND
0 0 0
0 0 0 0
BODs
0
-
-
0
pH Total alkalinity
0 +
0 0
0 0
+ -
+ = Increase over 1976; - = decrease over 1976; 0 = no change. All differences were significant at the 0.05 level. ND = not detected. Data are for 3 comparable d a t e s in each year.
1208
DALE W. TOETZ
density (loglo) of diatoms was no greater in the summers when mixing was practiced than in the control summer (t = 0,13 and 0.79, for 1976 vs 1977 and 1976 vs 1978, respectively, P = 0.05). If 1976 is accepted as typical control year for Arbuckle Lake, then the data can be interpreted as showing that mechanical pumping significantly lowered the thermocline and delivered DO to depths which normally would be anoxic, Water quality in the epilimnion was basically the same in 1977 and 1978 as it was in 1976. The concentration of NH2 was lower in both years, but NH2 concentrations depend not only on how rapidly it is produced by decay and excretion but also how rapidly it is used in microbial oxidation and algal uptake. Therefore. changes in algal productivity and/or rates of regeneration could result in dramatic changes in NH2 concentration. Water quality generally improved in the hypolimnion during 1977 when the lake was mixed, but not markedly during 1978, The mean concentration of H2S was lower in both years, when mixing was practiced, but other reduced compounds and BOD5 did not consistently decrease in concentration. These results were not unanticipated, since mixing was not vigorous enough to change rapidly the concentration of DO in the hypolimnion or to alter the chemistry of deep sediments. Compounds such as H2S, which disappear rapidly in the presence of small amounts of DO, did decrease, but other compounds such as Mn 2+ did not behave similarily. There was no evidence that algal biomass changed or that the nutrient environment of the phytoplankton was affected drastically by artificial lake mixing. The density of blue-green algae such as Anabaena and Aphanizomenon was normally low and the lake has not had a history of algal problems. Artificial lake mixing did not appreciably change the situation.
Ham's Lake The analysis of effects of artificial mixing of Ham's Lake is somewhat confounded because of water level fluctuations during 1976-1978. On 19 January 1976, the lake level was 3.45 m. By 24 December 1976, it was 2.42 m: on 13 April 1977 it was 2.12 m. A precipitious increase occurred during late May 1977, when the lake level rose from 2.19 to 3.67m, after which there was slow decline in lake level until by the end of August 1978, it was less than 2.22 m. Because lake level fluctuated so much the vertical extent of the hypolimnion (or potential hypolimnion) changed. Values for temperature and DO at 6 m were used as representative of the hypolimnion, in making comparisons between years, However, other water quality parameters were only measured at one depth in the hypolimnion, just off the bottom in the deepest part of the lake. The depth at which these samples were taken varied from 6 to 8 m. The parameters are reported as near bottom values. Pumping in 1976 and 1978 reduced overall differ-
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9
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Aug.
Ju~y
I
Sept.
Fig. 6. Difference in temperature between the surface and 7 m (A'-C)at Sta. 1 in Ham's Lake. ences in the temperature between the surface and 6 m, IA~'C). During 1977 A'~C was 8-10°C for most of the summer, hut much less (0-2"-C) during the summers of 1976 and 1978. Under natural conditions duriag 1977 there was a rapid decline in A:C during early August, when the lake turned over (Fig. 6). The lake restratifled again in mid August but the stratification was weaker since the A~' was about half the value in July. This suggests that it is impossible to separate effects of artificial mixing from natural turnovers in this lake during August. The 1977 data, however, suggest that Ham's Lake is so strongly stratified during June and July that natural mixing does not occur. Thus it is reasonably safe to use the June and July data as control data. ignoring for the momem any year to year variation in the unperturbed lake. During 1976 and 1978. the values of A'C corresponded reasonably well on the same date, except for the values in early June. The lack of correspondence. is probably due to the fact that stratification was well under way m 1976 before pumping began. However. the initial values of A'-C declined rapidly after 3 June 1976, when the pumps were turned on. In contrast. pumping began much earlier during 1978 (19 April) and the lake never became stratified IA~C values were always low). The concentration of DO at 6 m rapidly declined to zero in late May 1977, (control) and remained zero until turnover in early September (Fig. 7). There was a
g
7
[5 Z
/
,
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........... I
May
n.o!./ . . . . . . . . . . . . . I
June
I July
,
., ,
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Fig. 7. Concentration of dissolved oxygen (DO) at 6 m at Sta. 1 in Ham's Lake.
Effects of whole lake mixing Table 2. Comparisons of water quality parameters between 1976 and 1977 and between 1977 and 1978 at 0 and 4 m (epilimnion) and near the bottom at Sta. 2 at Ham's Lake
Parameter Ammonia-N Nitrate-N Nitrite-N SulfideS Phosphate-P
Change Compared to 1977 Epilimnion Near bottom 1976 1978 1976 1978 + 0 + ND +
0 0 ND 0
0 0 0 0 +
0 0 0 0 ND
BODs
+
0
0
0
pH Total alkalinity
+ +
0 0
+ +
+ 0
+ = Increase over 1977, - = decrease over 1977, 0 = no change. All differences were significant at the 0.05 level. ND = not detected. similar rapid decline in 1976 but the concentration of DO never reached zero, because pumping began on 3 June. During 1978 stratification never occurred since pumping began in April. However, the concentration of DO at 6 m did decline in May and June and then began a slow rise between June and September. This slow rise can also be seen in the 1976 data. During 1976 the pump was shut down from 10 to 19 July, however, there was no apparent effect on the concentration of DO. It is probably true that little or no effect on the concentration DO occurs if the pump is shut down for a few days. The persistent decline in the concentration of DO before 17 July may be interpreted as being due to the ineffectiveness of the smaller pump propeller. If 1977 is considered the control year, then it can be shown that epilimnetic concentrations of NH2, NO~, pH, total alkalinity, BODs and PO~ ÷ were higher during 1976 when mixing was practiced (Table 2). Relative to 1977, mixing did not have the same effect in 1978 as only NO~ was changed. During the period of observation epilimnetic values for BODs were about 0.8-1.0 mg 1-1 while values for NH~ were low, 1-10/~g 1-1. The ranges of epilimnetic total alkalinity and pH were 123-160mgl -~ and 7.8-8.4, respectively. The range for N O ~ - N was 7-10#g1-1. The nutrient environment for phytoplankton did not appear to change markedly as a result of artificial distratification, i.e. the concentration of N and P nutrients were always low. Further, there was little change in pH, which could have caused a shift in species composition of the phytoplankton (Shapiro, 1973). During 1976 artificial mixing apparently elevated near bottom pH, total alkalinity and phosphate, but during 1978 only pH changed slightly. It is difficult to see any improvement in water quality from these results, particularily H2S, NH2 and BODs. Typical values for these parameters in the control year were 20/~g H2S1-1, 138/zg NH,~-N1-1 and 1.95mgl - l BODs. However, pH did increase from 7.9 to 8.2
1209
when mixing occurred as expected. These statistical tests may be of limited value because the concentrations of many water quality parameters, particularly H2S, NO~, PO~ + were often near the limits of detectability on many dates. For example, during the summer of 1977, PO~ + was detected in the hypolimnion on only 3 of the 19 sampling dates and then the concentration was very low, 0.7-1.1/zg !-1. In summary, the Garton Pump improved the concentration of DO in Ham's lake but did not affect other water quality parameters. Lake water was clearer during the control year (1977) than when mixing was in progress. During 1977 the mean depth of the Secchi disc was 1.68 m during 1976 and 1978 it was 0.96 and 1.19 m, respectively. Algal biomass was significantly higher when artificial lake mixing was in progress. During 1977 the mean concentration of chlorophyll a was 4.6 #g Iduring 1976 and 1978 it was 8.5 and 13.2 /zg1-1, respectively. The algal community was dominated by diatoms in all years when mixing was practiced. Melosira, a filamentous diatom, was absent during 1977 when mixing was not practiced, but was present in the other years. This genus relies on turbulent mixing to remain suspended in the euphoric zone (Lund, 1971). During the control year, blue-green algae were observed mostly during the late summer and autumn. During the years when mixing was practiced, bluegreen algae were not observed during eary summer and did not appear to exhibit a regular periodicity. There was pulse of Microcystis during September 1977 (control year), but none during the other years, suggesting that pumping was effective in eliminating this genus. Mixing a small lake with the Garton pump increased algal biomass and decreased the visibility of Secchi disc. The reason algal biomass was lower during the control year may be because the pump did not circulate the lake completely. When artificial mixing was practiced the vertical profiles of temperature and D O showed that the lake was not stratified but neither was it completely mixed. Deep mixing well below the compensation point (about 3-4m) may have brought nutrients to the surface which ordinarily would be lost to the hypolimnion for the season and/or many have decreased sinking rates of phytop!ankters. It is impossible to uncouple these two factors, but it is likely that nutrients were adequate, given rapid rates of recycling for phosphate and the relatively high concentrations of nitrogen nutrients. Thus, artificial mixing may have reduced sinking losses of the phytoplankton and it may be that this factor alone accounted for the larger algal biomass observed. Artificial mixing with the Garton pump increased algal biomass by a factor of about 2.5. The apparent management implication is that in lakes where nutrient depletion is common, such as Ham's Lake,
1210
DALE W. TOETZ
particular emphasis on phytoplankton. Hydrobioloqia 43, 463-504. Hooper F., Ball R. & Tanner H. (1953) An experiment in the artificial circulation of a small Michigan lake. Truns. Am. Fish Soc. 83. 222-242. Irwin W.. Symons J. & Robeck G. (1966). Impoundment destratification by mechanical pumping. J. sanit. Engn~! Dil'., Proc Am. Soc. tit'. Engrs 92(SA6~, 21-40. Knoppert P., Rook J., Hofker T. & Oskam G. (1970) Destratification experiments at Rotterdam. J. Am Wat, Wks Ass. 62, 448-454. Lund J. W. G. (1971) An artificial alteration of the seasonal cycle of the diatom Melosira italica suhsp suharctica in an English lake. J. Ecol. 59, 521-533. Acknowledgement.~ The work upon which this publication Malueg K., Tilstra J., Schults D. & Powers C. (1973~ Effect is based was supported in part by funds provided by the of induced aeration upon stratification and eutrophicaOffice of Water Research and Technology Project A-078 tion processes in an Oregon farm pond. Geophysical OK. U.S. Department of the Interior, Washington, DC as Monoyraph Series, Vol, 17, pp. 578-587. American Geoauthorized by the Water Research and Development Act of physical Union, Washington, DC. 1978. ! am indebted to the following persons for the technical assistance: Hong Chau, Patrick Downey, Ronald Eby, Quintero J. E. & Garton J. E. (1973). A low energy lake destratifier. Trans. Am. Soc. agric. Enyng 16, 973-9781 Steven Halterman and May Yue. 1 thank Dr James Garton for use of his data and Dr Joseph Shapiro for reviewing the Robinson E., Irwin W. & Symons J. (1969) Influence of artificial destratification on plankton populations in immanuscript. poundments. Trans. Ky. Acad. Sci. 30, 1-18. Shapiro J. (1973) Blue-green algae: Why they become REFERENCES dominant. Science 179(4071), 382-384SolSrzano L. (1969) Determination of ammonia in natural Brezonik P., Dclfino J. & Lee G. (1969) Chemistry of N waters by the phenolhypochlorite method. Limnol. and Mn in Cox Hollow Lake, Wisconsin, following Oceanogr. 14, 799-801. destratification. J. sanit. Engng Dil'., Am. Soc. cil'. Engrs Strickland J. & Parsons T. (1968) A practical handbook of 95($A5), 929-940. seawater analysis. Fish. Res. Bd Can., Bull. 167, 311 pp. Drury D., Porcella D. & Gearheart R. 119751 The effects of artificial destratification the water quality and microbial Symons J., Irwin W. & Robeck G. (1967) Impoundment water quality changes caused by mixing. J. sanit. Engng populations of Hyrum reservoir. PRJEWOLL-I. 174 pp. Dit'., Proc., Am. Soc., cir. Engrs 93(SA2~, 1-20. Utah Water Research Laboratory. College of EngineerToetz D. (1977). Effects of lake mixing with an axial flow ing, Utah State University, Logan. pump on water chemistry and phytoplankton. HydrohioFast A.. Moss B. & Wetzel R. (1973). Effects of artificial Iogia 55: 129- t38. aeration on the chemistry and algae of two Michigan Toetz D (1979). Effects of whole lake mixing on algae, fish lakes. War. Resour. Res. 9, 624--247, and water quality. 46 pp. Technical Completion Report, Garton J.. Summerfelt R., Toetz D., Wilhm J. & Jarrell H. A-078-OKLA, Oklahoma Water Resources Research In(19761 Physicochemical and biological conditions in two stitute. Stillwater. Oklahoma reservoirs undergoing artificial destratification, 138 pp. Oklahoma Water Resources Research Insti- Weiss C. & Breedlove B. (1973) Water quality changes in an impoundment as a consequence of artificial destratifitute. Stiltwater. cation, 216 pp. Water Resources Research Institute. UniHaynes R. (1973). Some ecological effects of artificial circuversity of North Carolina. Raleigh lation on a small eutrophic New Hampshire Lake with
algal biomass might be increased by artificial mixing. However, it is not correct to conclude that this increase is undesirable, since the biomass of algae in Ham's Lake never reached bloom proportions. In fact, it may be that artificial lake mixing increased the carrying capacity of Ham's Lake. Certainly, one would not expect this to be the case for other lakes, particularly those that are already highly eutrophic. Thus, caution is warranted, before using a downflow pump on these ecosystems.