e:> Pergamon

pn: 50273-1223(98)00038-9

Waf. Set T«1t. Vol. 37, No.2, pp. 31»-316,1998.C 1998 IAwQ. Published by Elsevier Science LId

Printed In 0""'1 Britain.0273-1223198 $19'00 +0-00

ALGAL CONTROL ANDDESTRATIFICATION ATHANNINGFIELD RESERVOIR

Jeremy Simmons

Essex & Suffolk Water. Hall Strut. Chelmsford. Essex CM2 OHH, UK

ABSTRACf

In May 1994 an anificial destratification system was installed at tho eutrophic 27.200ML HanOlngfield rawwater reservoir in Essex U.K. The main objective of this installation was to prevent poor raw water quality,which can result from hypolimnetic isolation in the Summer month. when thermal stratification often occurs.By adopting an Intennittent destratiJicatioD litrateIY to constantly change reservoir conditions thuspromoting competition amongst algae, it is hoped that an overall decrease in annual phytoplankton biomassat the reservoir will result. This in tum will reduce the pressure on the raw waler treatment processesrequired to produce a potable water supply. This paper evaluates algal, meteorological, and other monitoredvariables to assess the effect of destratification on Hanningfield Reservoirs phytoplankton community from1994 to 1996. Although noting the limited data period the results show little dominant phytoplankton typechanges, but notes a fall of 66% in mean tolal biomass in 1996 compared to 1994 values. <Cl 1998 IAWQ.Published by Elsevier Science Ltd

KEYWORDS

Biomass; destratification; eutrophic; meteorological; phytoplankton; stratification; thermal.

INTRODUCfION

Hanningfield Reservoir is a large raw water reservoir which ultimately supplies drinking water to over half amillion customers in the Essex region of the United Kingdom. With a total capacity of 27200ML, when fullthe reservoir has a mean depth of 7m, and a maximum depth of 17m (Essex Water Co., 1982). Being highlyeutrophic the phosphate and nitrate levels rarely fall below lmgll as P04, and l.5mgll as NO) respectively.As a result of this nutrient status the reservoir often suffers from phytoplankton blooms which can developunder thermally stratified reservoir conditions. Such algal growth places extra pressure on the treatmentprocesses needed to supply a potable water product from the reservoir. In May 1994 a destratification systemwas installed at Hanningfield Reservoir with the main aim of reducing poor raw water conditions in theSummer resulting from thermally induced hypolimnetic isolation.

Using effective on-line water profile and meteorological monitoring the destratifier system is run by anintermittent policy . This allows for minimal stratification to develop (up to a 213 degree C. thermalgradient), before quickly breaking the gradient down to isothermal conditions within 48 hrs beforedetrimental water chemistry occurs. The objective of this strategy is to influence phytoplankton growth. It ishoped that by constantly deselecting anyone type of environmental condition phytoplankton competitive

309

310 J.SIMMONS

stress is promoted. thus leading to a lower overall annual algal biomass yield from the reservoir. It is the aimof this report to comment on the effect of the destratification system on the phytoplankton community atHanningfield Reservoir since the installation. See Simmons (1995) for further information concerningHanningfield Reservoir and thermal stratification events. and for general destratification system needs seeCooke and Carlson (1989).

METIlODS

General manual water profile sampling for phytoplankton and chemical analysis was taken at 4 sites acrossthe reservoir. The result tables which fOllow this section are based on mean values for all sites and at alldepths with a weekly or bi-weekly sampling frequency. All phytocount values are given in cell nolml unlessstated otherwise (i.e. filament no.lml or colony no.lml)

All phytoplankton analysis was based on the sedimentation microscoP1 technique (see Lund 1951. andUtermohl 1931), with biomass estimations resulting from the computation of mean biovolumes.

All the meteorological information was supplied by 24hr on-line monitors placed on the Valve Towerassociated to the reservoir itself.

(Further information on sampling methods. monitoring sites. and the destratification system itself can befound in Simmons. 1995)

Table 1. Showing monthly phytoplankton dominants and mean countslbiomass at Hanningfield Reservoirin 1994

MONTH DOMINANT MEAN MEAN COMMENTS(by Biomass) PHYTOCOUNT BIOMASS

cell nolml ug/lJanuary Aphanizomenon sp. (filaments) 380 49February Rhodomonas sp. 748 63March Diatoms (notably Navicula sp.) 1282 149April Diatoms (notably Cye/otella 2078 209

sp.)May Aphanizomenon sp. (filaments) 1136 487 Sub. dominant Anabaena

sp.June Anabaena sp. (filaments) 1513 227July Anabaena sp. (filaments) 1598 869August Melosira sp. (filaments) 1239 1032September Aphanizomenon sp. (filaments) 880 667October Cryptomonas sp. 818 227November Cryptomonas sp. 458 83December Cryptomonas sp. 587 68TOTAL 12717 4130

Algal control and dcstratification 311

Table 2. Showing mean monthly values for parameters measured at Hanningfield Reservoir during 1994

Month P04 N03 N02 NH4 SI02 Dissolved Water pH Zooplankton Secchiug/l mgll as mg/l as mgll mgll as Oxy. cone. Temp. noll discasP N03 N02 as SI02 (%sat) °C exline.

NH4 (m)Jan 579 24 0.21 0.2 105 100 4.3 8.3 12 3.1Feb 535 25.4 0.171 0.164 10.2 95 3.8 8.3 9 2.8Mar 511 27.4 0.135 0.1 9.3 98 6.3 8.4 13 4.1Apr 489 26.8 0.123 0.019 5.1 103 8.3 8.6 30 3.1May 478 25 0.205 0.086 4.5 96 13.2 8.6 43 4.2Jun 510 22.2 0.213 0.042 5 100 16.4 8.7 90 3.4Jul 549 18.1 0.246 0.097 5.7 111 20.7 8.9 93 3.5Aug 637 15.1 0.38 0.108 1.0 90 19.8 8.9 66 4.1Sep 718 12.5 0.318 0.062 6.0 99 15.2 8.8 III 4.1Oct 793 13 0.192 0.08 7.7 95 12.3 8.7 55 4.4Nov 815 16.3 0.169 0.157 8.1 89 10.7 8.4 31 4.2Dec 824 19.5 0.225 0.153 9.7 76 8.6 8.3 20 3.1

Table 3. Showing monthly phytoplankton dominants and mean countslbiomass at Hanningfield Reservoir- -

MONTH DOMINANT MEAN MEAN COMMENTSPHYTOCOUNT BIOMASScell no.lml ug/l

January Rhodomonas sp. 523 56February Stephanodiscus astraea 522 70March Stephanodiscus astraea 1454 884April Aphanizomenon sp. (filaments) 525 552May Aphanizomenon sp. (filaments) 1448 382June Aphanizomenon sp. (filaments) 1207 165July Microcystis sp. (colonies) 1296 683 Limited Oocystis sp.

growthAugust Aphanizomenon sp. (filaments) 1032 436 Microcystis sp.

(colonies) sub dominantSeptember Aphanizomenon sp. (filaments) 539 314 Microcystis sp.

(colonies) sub dominantOctober Aphanizomenon sp. (filaments) 993 820

November Rhodomonas sp. 619 89December Rhodomonas sp. 299 53TOTAL 10457 4504

312 J.SIMMONS

Table 4. Showing mean monthly values for parameters measured at Hanningfield Reservoir during 1995

Month P04 N03 N02 NH4 SI02 Dissolved Water pH Zooplankton Seeehiug/l mg/l as mg/l as mg/l as mg/l as Oxy. cone. Temp. noll discasP N03 N02 NH4 SI02 (%sat) °C extine.

(m)Jan 749 26.3 0.215 0.128 9.9 79 4.3 8.3 13 2.1Feb 605 30.1 0.182 0.096 9.5 81 6.1 8.4 14 2Mar 487 3l.S 0.142 0.021 6.4 92 6 8.5 12 1.9Apr 457 31.3 0.178 0.078 0.8 103 9.9 8.7 49 4.9May 434 29.6 0.251 0.043 2.1 90 13.1 8.7 87 4.2Jun 517 25.9 0.251 0.066 2.7 83 15.8 8.6 71 4.8Jul 568 21.4 0.3 0.069 2.8 93 21 8.9 81 3.6Aug 391 15.1 0.346 0.075 6 114 22.2 8.9 65 3.1Sep 528 11.3 0.268 0.143 5.8 100 16.9 8.7 79 3.5Oct 685 12.8 0.199 0.09 5.6 79 14.6 8.7 64 3.2Nov 823 15.4 0.169 0.307 8.9 93 9 8.4 33 3.4Dec 827 18.2 0.151 0.327 9.7 107 4.6 8.4 24 3

Table 5. Showing monthly phytoplankton dominants and mean eountslbiomass at Hanningfield Reservoirin 1996

MONTH DOMINANT MEAN MEAN COMMENTSPHYTOCOUNT BIOMASScell noiml ugll

January Rhodomonas sp. 276 18February Rhodomonas sp. 611 17March eye/Ofelia sp. 9546 37April Cye/ofella sp. 1755 298 Sfephanodiscus astraea

was the sub dominantMay Sfephanodiscus asfraea 193 271June Rhodomonas sp. 2107 102July Cryptomonas sp. 1862 155 Anabaena sp. was sub

dominantAugust Anabaena sp. 1022 114 Microcystis sp. colonies

were sub dominantSeptember Aphanizomenon sp. (filaments) 492 199 Microcyst/s sp. colonies

were sub dominantOctober Aphanizomenon sp. (filaments) 642 140November Rhodomonas sp. 314 43December Rhodomonas sp. 154 20TOTAL 18974 1414

Algal control and desualificalion 313

Table 6. Showing mean monthly values for parameters mesured at Hanningfield Reservoir during 1996

Month P04 N03 N02 NH4 SI02 Dissolved Water pH Zooplankton Secchiug/I mg/I as mgll as mg/I as mgll as Oxy. conc. Temp. noll discasP N03 N02 NH4 SI02 (%sat) °c extinc.

(m)Jan 736 27.3 0.171 0.328 10.2 95 4 8.4 18 2.7Feb 718 32.6 0.174 0.265 9.6 114 2.3 8.6 17 2Mar 591 35.9 0.159 0.037 5.1 109 4.7 8.6 37 1.6Apr 555 33.8 0.181 0.035 1.5 85 8.5 8.8 298 2.3May 576 28.9 0.239 0.23 1.5 84 10.7 8.6 271 4.7Jun 603 25.2 0.275 0.165 2.7 79 17.9 8.6 102 4.6Jul 673 18.5 0.276 0.1 4.5 86 18.7 8.7 155 3.9Aug 708 13.8 0.32 0.091 4.9 94 20.1 8.8 114 4.5Sep 739 9.9 0.297 0.118 6.3 96 13.4 8.9 199 3.9Oct 833 10.9 0.166 0.071 7.5 91 12.7 8.7 140 4.2Nov 759 17.3 0.148 0.183 8.1 103 4.7 8.5 43 2.8Dec 741 25.1 0.225 0.233 9.5 90 4 8.3 20 3

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Graph I. Hanningfield Reservoir mean weather data for 1994.

800 18_Sol. radiation •700 - .. Air Temperature I, 16

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14.:1.5 600 I

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Graph 2. Hanningfield Reservoir mean weather data for 1995.

_Sol. Radiation- .. Air Temperature •~ndSpeed " ,... ,.I r" •,.

~ ,... L::' I~

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314

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Graph 3. Hanningfield Reservoir mean weather data for 1996.

RESULTS AND DISCUSSION

Tables I. 3. and 5 show the phytoplankton succession at Hanningfield Reservoir over the past three years. Itis clear that Cryptophytes such as Rhodomo1UlS sp. and CryptOrrw1UlS sp. act as the background group at thereservoir often becoming dominant in the late AutumnlWinter period. Diatom blooms. notably the centricdiatoms such as Stephanodiscus QStraea are dominant in the Spring period. with the Cyanobacteria such asAphanizomenon sp. peaking in the Summer/early Autumn seasons.

It is interesting to note the absence of Chlorophyte dominance in the population. At Hanningfield ReservoirOocystis sp. have in previous occasions (notably 1990 and 1992 ), in the absence of early Summer strongCyanobacteria competition risen dramatically from 10 or 20 cells/ml to 50,000 cells/ml in a matter of days.In 1995 it appears that Microcystis sp. restricted an Oocystis sp. bloom developing.

Based on the evidence of the last three years there seems little change in the type of phytoplankton found atHanningfield Reservoir since the destratifier installation. Pre 1994 algal type data seem to indicate thatphytoplankton dominants and the seasonal succession follow a similar pattern to the findings between 1994and 1996.

Evaluation of the phytoplankton biomass differences between 1994 and 1996 does indicate changes.Cumulative values for 1994 and 1995 are similar with 1995 showing a 9% increase on the previous year.However. 1996 biomass total was 66% lower than the 1994 value (see tables 1,3, and 5). Evaluation of thedata in tables 2,4, and 6 shows that nutrients remained relatively high throughout the study period thusnutrient limitation was unlikely to be the cause of the large decrease in biomass.

Could the phytoplankton biomasS decrease be a result of controlled Summer thermal stratification atHaoninweld Reseryoir?

Previous experience of Hanningfield reservoir indicates that under conditions of wind speeds < 4 m1sec.,with> 600 W/m2 incoming solar radiation, and air temperatures of> 16°C thermal gradients detrimental towater quality can develop within hours if destratification capabilities are not applied (Simmons 1995).Graphs I to 3 indicate that such weather conditions were likely to occur during the 1994 to 1996 studyperiod. However, the maintenance of high dissolved oxygen, and low ammonia/nitrite values indicated inTables 2, 4, and 6 shows that the intermittent destratification strategy was successful in sustaining goodwater chemistry throughout the study.

Althouah Cyanobacteria can dominate in Winter (see Table I), optimal growth. (especially Microcystis sp.)is generally associated with stable. thermally stratified midllate Summer conditions (see Reynolds 1984 for

Algal control and destratification 31'

phytoplankton ecology). It is possible that by limiting thermal stratification periods in the Summer atHanningfield Reservoir, this has restricted Cyanobacteria growth and is part of the reason for the decline inoverall biomass. Evidence from 1992 when monitoring was in progress at Hanningfield Reservoir butartificial destratification was not, indicates that under uncontrolled thermally stratified conditionsCyanobacteria growth was up to twice as much compared to when similar weather conditions prevailed andthermal gradient control was possible (e.g. in 1994, see Simmons 1995).

Further evidence to support these study results in showing a reduction in algal biomass using intermittentdestratification is given by Steinburg and Gruhl 1992 who reflect on the inability for gas vacuoledCyanobacteria to adapt the size quickly enough to the light changes caused by intermittent mixing, thusrestricting optimal development of these types, (also see Oskam (1978) and Reynolds (1986). Ward andWertzel 1980 give further examples of bloom forming Cyanobacteria which are unable to adjust to varyinglight climates in the water column within a 48 hour change period.

In many respects the results of apparent destratifier effect in biomass reduction are encouraging. However, itmust be realised that drawing firm conclusions from just three years data is difficult, and informationgathered over the next five years is important to confirm any sustainable effects of artificial destratificationat Hanningfield Reservoir on the phytoplankton population.

CONCLUSIONS

The following conclusions can be drawn from this study.

I. Mean annual phytoplankton biomass at Hanningfield Reservoir was 66% less in 1996 compared to 1994.With nutrients non limiting at any time, and climatic trends similar between the two years it is possible thatthe decrease in overall biomass may in part be as a result of the adopted intermittent destratification strategy.

2. The evidence of this study suggests that the type or succession of phytoplankton of HanningfieldReservoir has not so far significantly changed as a result of intermittent destratification.

3. With only three years of post destratification installation data available only indicative evidence ofartificial destratification affects on the Hanningfield Reservoir phytoplankton population is possible. Thus, itis vital that future monitoring is sustained to assess whether the current indications of phytoplanktonbiomass reduction and succession are maintained in the future.

ACKNOWLEDGEMENT

My sincere thanks go to Matthew Sellick without whom this Paper would not have been possible.

REFERENCES

Cooke and Carlson (1989). ReservOir Management For Water Quality and THM Precursor Control. American Water WorksResearch Foundation.

Essex water Company (Circa. 1982). "Hanningfield". (Promotional materiDl).Lund, J. W. G. (t95 t). A Sedimentation Technique For Counting Algae and Other Organisms. Hydrobiologia, 3, 143-170.Oskam, G. (1978). Die Vorausberechnung Der Algenbiomasse In Den Biesborsch Speicherbecken. Theorie Und Praxis. In

Vermindderung Der ....genentwicklung In Speicherbecken Und Talspe"en (cds G. Oskam and H bernhardt). DVGW·Schriftenreihe Wasser, Ie>, 90-107.

Reynolds, C. S. (1984). Cambridge University Press. The Ecology Of Phytoplankton.Reynolds, C. S. (1986). Experimental Manipulations Of The Phytoplankton Periodicity In Large Limnetic Enclosures In Blelham

Tam, English Lake District. Hydrobiologia, 138, 43-64.Simmons, J. (1995). The Effects Of Artificial Destratification At Hanningfield Reservoir During The Summer of 1994. In Water

and Waste Water Treatment· Towards Cleaner Leaner Operation, (ed. M. White). Mechanical Engineering PublicationsLId,67-96.

Steinberg, C. E. W. and Gruhl, E. (1992). Physical Measures To Inhibit Planktonic Cyanobacteria. In Eutrophication: Researchand Application To Water Supply. Freshwater Biological Association, pp. 163- 184

316 I. SIMMONS

Utennohl, H. (1931). Neue Wege In Der Quantitativen Erfassung Des Planktons. Verhanlungen Der Internationale VereinigungFur Theoretische Und Angewandte Limnologie, 5, 567-96.

Ward, A. K. and Wetzel, R. G. (1980). Photosynthetic Responses Of Blue - Green Algal Populations To Variable LightIntensities. Archiv Fur Hydrobiologie, 90,129-138.