MUSSELS ON WATER INSTALLATIONS IN

Paul Elliott*, PhD, MA (Cantab), David.C.Aldridge*, MA (Cantab), Ph4 Geoff D. Moggrldge* *, BSc, PhD, and Michael Chipps***, BSc, PhD.

ABSTRACT The zebra mussel, Dreissena polymorpha, is a major biofouling pest of water treatment works, irrigation systems and power stations in Europe and North America. This paper documents current problems associated with zebra mussels in English waterworks. Questionnaires and manual surveys conducted between 2001 and2003 have revealed that over 30 water treatment works in England suffer problems assoclated with zebra mussels. Hundreds of tonnes of mussels are being removed each year from raw water Infakes, pipellnes and reservoirs. Problems have increased hthe last five years, due fo a spread in the range of zebra mussels around England and the cessation of chemical treatment at the intakes of many treatment facilities during the 1990s. The importance of faking control of zebra mussels into account in planning new water supply schemes Is highllghfed.

Key words: Blofouling; Dmlssena polymorpha; Water treatment works; Zebra mussel.

*Deparitnenf of Zoologv, University of Cambridge, Downing Street, Cambridge C62 3EJ. UK.

**Department of Chemical Engineering, University of Cambridge, Pembroke !jtreet, Cambridge C62 3RA. UK.

**%ames Water Innovation and Development, Spencer House. Manor Farm Road, Reading, RG2 OJN.

INTRODUCTION The zebra mussel. Drelssena polymorpha (Fig. l), is one of the world's most invasive freshwater pests. Originally native to the Black and Caspian seas, it spread to rivers, lakes and canals of many countries in mainland Europe during the early 1800s"). The British invasion began in the Surrey Commercial docks in the 1820~'~) and over the next twenty years, zebra mussels reached most of England's major waterways. They can currently been found in many clean, weii-oxygenated lowland rivers, canals and reservoirs in central England@). The recorded range of D. polymorpha is shown in Figure 2. but it is likely that their distribution is far more widespread; most of England's waterways are broadly suitable for zebra mussel growth and reproduction on the basis of pH (>7.4), calcium concentration (>12mg.l-') and minimum summer. temperatures (>1 3"C)C4) .

There is evidence of a recent increase in the distribution and abundance of zebra mussels in some areas of England, most alarmingly in the Thames reg i~n~~,~) . These increases may be due to recent reductions in river pollution, changes in recreational boat movement (vectors for zebra mussel

dispersal), or increasing water temperatures (which may facilitate more rapid growth)

In 1986, the first zebra mussels were discovered in the Great Lakes region of North America? In less than a decade, the mussels spread across the inland waters of eastern America, causing unprecedented environmental and economic problems(']. in 1997 zebra mussels were noted rapidly spreadlng through the River Shannon, Ireland, where they appear to have invaded via Limerick docks@). In 2001, Drelssena was reported in Spain for the first time".

Zebra mussels are particularly notorious due to their effects

367 I WEJ

Right valve view Anterior view Dorsal view

Figure 1. The external morphology of the zebra mussel, Dreissena polymorpha.

Treatment facility currently encountering problems wlth zebra mussels

Riverine zebra mussel sightings within the last thirty years (across 1Okrn' areas, data from Conchological Society records)

figure 2. Distribution of wafer freafmenf facitities affected by zebra musseis within Engtand and Scotland. Data supplied from telephone surveys and the Conchological Society of Great Britain.

on industries drawing freshwater from infested sources. Zebra mussels have planktonic larvae (see Figure 3 for D.

poiymorpha's life-cycle characteristics) which are drawn into intake pipes where they settle on the pipe walls. As they grow, and further mussels settle on top of them..the pipes can become blocked or suffer substantially reduced flow rates. Shells can be washed Into further parts of the facility where

Pbnlctonlc

sprm.WJ coo

Figure 3. Schematic of the lifecycle of D. polymorpho. showing the nomenclature and diameter of each larval stage.

they can block smaller pipes, filters. valves, screens, dosing machinery and sampling ports. It is estimated that between

1988 and 2000, D. polymorpha cost American industries (mainly power stations) around US $1 billion in removal and mitigationoo).

Zebra mussels can further affect domestic water supplies through their effects on the ecology of reservoirs and pipelines. An individual zebra mussel can filter as much as 1-2 litres of water a day "?As a result, high densities of mussels can cause major shifts in the phytoplankton communities of reservoir water. Under some conditions there is an increased risk of phytoplankton blooms of toxic Cyanobacteria ( I Z 'I).

Cyanobacteria can produce hepatotoxic microcystins which are not removed fully by filtration, coagulation, ozonatlon. or chlorination treatment processes ('') and may pose a serious health risk for human populations('51'. Also. after Several years of infestation, older individuals will start to die off, leading to Increased bacterial levels and potential odour and taste

WEJ I 368

problems if the water is supplied to domestic users (I6). 4. Larval densities were measured throughout the reservoirs and works during 2003. Each month, a 56pm sweep net The impact of zebra mussels on British industry was first

noted in the early 20th century. Stiigoe (1930)("1 documents that a water main belonging to the Metropolitan Water Board was reduced from ca.90cm to ca.3Ocm in diameter by zebra mussel encrustations. Clarke (1952)('8) mentions a water main in Suffolk that was reduced from ca.60cm to ca.30cm in diameter during the 1940s. Greenshieids and Ridley (1 957)(1p1 also found that zebra mussels in the London area reduced the diameter of a pipeline by ca.lOcm. and that a dosage of chlorine to give a residual of 0.5mgll for 7-10 days killed the 75% of mussels.

This paper reviews the current effects of zebra mussels on English industry (most notably waterworks) and show that problems with zebra mussels in England have increased since 1998.

METHODS Telephone and site surveys of waterworks and power stations A telephone survey was conducted of 23 water companies and 20 major power stations in England. Information was

requested on any documented cases of zebra mussel bio- fouling, including times and localities of each sighting and any mitigation strategies. Ten of the most heavily affected water treatment works were then visited. Site managers were interviewed regarding documented zebra mussel infestations in their treatment Drocesses.

Adult and larval zebra mussel monitoring at a Waterworks in North London At one of Thames Water's water treatment works 0 in North London, problems were monitored every month during 2003 and 2004. In addition, the densities of adult and larval zebra mussels throughout local rivers, reservoirs and filter beds

were estimated using four techniques: 1 . To estimate zebra mussel densities in the Thames and Lee

rivers (that were supplying the works), manual surveys were conducted. Five minutes were spent manually collecting zebra mussels from the submerged foundations of ten bridges along each river.

2.To monitor for larval settlement in reservoirs during 2001, steel plates (30cm x 20cm) were attached to ropes and deployed at depths of 2, 4 and 6m in each of eight

reservoirs. These were withdrawn in June 2002 and the numbers of zebra museis on each piate counted.

3. Primary filtration beds were drawn down on a monthly basis across 2003 to measure zebra mussel densities on their wails.The number of zebra mussels in three randomly placed 1 0cm2 quadrats was counted.

was dragged for 10m at a depth of two metres through each of four reservoirs.The filtrates were stored separately in 70% alcohol. The larvae in each sample were then counted using cross polarisation microscopy'm to provide an estimate of larval density in water throughout the works.

RESULTS The instances of biofouiing in English industry are far more widespread than previously documented. Ten English water companies have reported zebra mussel infestations in a total of 30 water treatment works. In addition, one cement works has encountered problems in its cooling pipelines. These facilities are distributed throughout England (Fig. 2). but all abstract water from lowland rivers. Of these 31 facilities, 18 have documented increasing problems with zebra mussels since 1998. even though they had previously been aware of their presence. A further five facilities only discovered zebra

mussels in their facility between 2000 and 2003. No power stations documented probiems with zebra mussels.

Problems associated with zebra mussels in waterworks Our surveys indicate that in every waterworks affected by zebra mussels, probiems originate from an infestation of the pipelines leading from rivers or impounding reservoirs. The effects downstream of these pipelines depend upon the particular layout of each works. Living and dead mussels are often displaced from the surface of the pipelines, particularly at times of large Row fluctuatlon. These are washed into the works where they clog pipes and processing machinery. Often, musels accumulate at the flrst obstruction to water- flow. such as filtration beds, ozone tanks and microstrainers.

The potential affects of zebra mussels within a generalised water treatment faciiity are summarised in Figure 2 and explained with examples in the following sections.

Infestation of pipelines leading from rlvers to storage reservolrs In every affected waterworks In England, the main freshwater intake pipeline has an infestation of zebra mussels. No English water company has yet made quantitative measurements of intake pipe infestation, due to the complications of draining

these pipelines and the associated reductions in water supply to the faciiity. However, works in the Essex, Veoiia, and Severn Trent. regions have examined pipelines by using rigid borescopes and revealed mussel encrustations over 20cm deep.

Zebra mussel infestations can increase turbulence and the

369 I WEJ

head needed to drive water through a pipe. In 1997. zebra mussels were the main cause of a substantial head loss In a pipe bringing water from a small lake to a cement works near Rugby. Although no loss in head has been documented elsewhere in England, should infestations worsen, the effect is inevitable.

Further problems appear when mussels become dislodged from the pipeline walls and accumulate in regions where flow is substantioily reduced such as in bends or where diameter increases. In the summer of 1999, an Anglian WTW in Llncolnshire experienced a massive blockage of mussels in the supply line to their works. An elbow in the 25km. long. 450mm diameter pipe entering the works provided a natural capture area for mussel shells. Around 5 tonnes of mussels were removed using a suction pump before the storage reservoir was drained of water. Mussel influxes continue to occur at the W, often after large changes in flow through the pipeline. The works has implemented a yearly cleaning regime, removing several tonnes of mussels each year.

Another example of shell blockage was observed within the inlet culverts at a Thames W in North London during the summer of 1997. Inlet culverts lm deep became filled with zebra mussels, necessitating an extensive removal project. A month was spent systematically digging and sucking mussels out of each culvert, removing them from the site In articulated tankers. The costs of this individual cleaning event are estimated to have been in the region of WOO.oM3.

All pipelines that draw freshwater are susceptible to infestation, not just facility intakes. During 2003, divers removed over 3 tonnes of mussels from pipelines linking reservoirs and rivers in the Thames region (Paul Fellow, Specialist Diving Services, personal.communication). On one occasion, a 120cm main on the River Lea at Ware became totally blocked by zebra mussels.The pipeline had to be cleaned out by a team of divers to keep the river flowing.

Zebra mussels are a particular problem In pipelines because they can reach vastly higher densltles than in adjacent lakes and rivers. An enormous number of iarvae can be drawn through the system and there is constant replenishment of food, an absence of predators, and continual removal of the mussels' waste products. Long or narrow pipes are particularly vulnerable to infestation and severely impended flow, provided that the flow of water Is less than 2m/s ( I h ) ,

Blockage of mlcro4llters and screens Many W s have microstrainers at the infiow to their works, In which mussels can' accumulate, clogging the machinery. At an Anglian W in Rutland, mussels from an 8km long Intake pipe main are captured in a large microstrainer faciilw, necessitating frequent removal. Between 2000 and 2003 over

20 tonnes of mussels were removed each year. Mussel influxes normally follow a brief period of pipeline chlorinatlon in November (leaving a residual of 0.1 to 0.2 mg/l). Before 1998 the mlns was chlorinated every other year, but the increasing numbers of mussels being removed necessitated a more frequent purge.

Another example of filter screen blocking occurred In 1997 at an Anglian W in Suffolk. Large numbers of mussels were discovered growing on the screens on the draw-off tower supplying the works. To prevent further occlusion, the screens had to be cleaned by using a team of divers.

Infestation of storage reservoirs Storage reservoirs provide an ideal settlement substrate and breeding refuge for zebra mussels. No waterworks has documented any problems associated with reservoir infestation, but when surveys have been conducted, they have often revealed substantial accumulations of mussels around the bottom of storage reservolrs.

In June 2002, low water levels at a Three-Valley's W in South London revealed substantial densities of mussels around the bottom of their settlement reservoirs (Figure 4). Manual sampiing reveaied shelves of musseis ca.30cm-deep with densities of 11600 i 300 SE lndivlduais/m2. At Thames Water's Lockwood reservoir, steel settlement plates became coated with mean densities of over 3098 k164 SE individuals/m2 during the summer of 2002. Larval sampllng also revealed densities of over 7% * 218 SE larvae/m3 in June 2003. Diving surveys have revealed substantial densities of mussels throughout a11 of Thames Water's reservoirs across 2003. but unfortunately no quantitative measurements were made.

Figure 4. Zebra mussel encrustatlons around the bottom of a storage reservoir at a W i n South London. The water level had been dropped Im below normal.

WEJ 1 370

Despite the absence of documented industrial problems caused by mussels living in reservoirs, such mussels are often the major source of the larvae that infest pipelines further downstream. Zebra mussel populations in reservoirs can reach densities over two orders of magnitude greater than in the rivers supplying them. For instance, manual surveys of the River Thames in 2002 revealed mussel densities of up to 41 i 8

SE musselslm2 compared to 11600 i 3M) SE individuais/mz in the reservoirs it supplies directly.

Clogging of primary filter inlets and beds The main problem encountered at Thames Water’s WTW in North London involves a continuing influx of loose mussels (both living and dead) into primary filtration inlets and beds. There were at least seven distinct shell influxes between 2M)o and 2003. A typical example occurred in October 2 0 1 when two of the primary beds had to be drained and external companies contracted to clean them. Figures 5.a., 5.b. and 6 show the extent of an influx. Large numbers of living mussels were found, indicating that the influx was not due to a natural mortality of mussels in the supply to the works; they were the result of living mussels being displaced from wails of upstream supply lines.

Figure 5. a. (Len): a primary bed inlet at Thames Wafers WTW in North london before an influx of mussels. b(righf) the same fllter bed as ln Figure 5a. after an influx of mussels..

Figure 6. Mussel shells in a primary filtrafion bed at Thames Water’s North London WfW

The flrst real problems at the North London WTW involved an increase In the depth of the fllter media in the affected beds, as mussel shells became mixed in with the sand (Figure 7). Because shells increase the particle size distribution of the media, there is a risk that affected beds may lose much of +heir ability to filter organic matter from the water. Thls was manifested as a reduction in head in the beds; Figure 8 shows the reductlon in head of one such bed following a mum1 influx. This necessitated the removal of over 30 tonnes of mussels and sand at appreciable cost. Unfortunately. when shells mix with filter media, they also prevent its cleaning and reclamation. One sand-cleaning facility in London became clogged with mussel shells in 2003, making cleaning of affected media difficult.

Figure 7. lncrease in media depth of Finer bed 5 at Tharnes Water’s North London W following an influx of mussel shelts.

:%I 1.4

1.2 1

0.8 0.6 0.4 0.2 0

Figure 8. The reduction in head in a primary finer bed in North London (RGF 4) following an influx of zebra mussel shells. (Compared fo an unaffected bed, RGF 3).

Evidence from river and reservoir surveys indicates that the mussels are entering the North London works from within a 4.8km long, 2.54m diameter spine tunnel constructed ca.1990. Anecdotal evidence indicates that influxes of zebra mussels into the primary inlet often follow large changes in flow rate through the spine tunnel. Populations in the tunnel appear to

371 I WEJ

be increasing. The number of influxes has risen from one in 2001. to two in 2002 and to four in 2003.

Blockage of ozone tanks Seven W s across the country have reported problems with dead zebra mussel shells accumulating in ozone treatment facilities. In many modern works. the weirs of ozone tanks provide the first natural capture area for displaced zebra mussel shells. In Anglian Water's W in Suffoik, over 50 tonnes of mussels were taken out of three ozone tanks in 2002. The influx followed a large increase in flow after a switch between supply lines.

Similar problems were also noted at Three-Valleys W in south London. Large numbers of zebra mussels were found in their ozonation chambers for the first time in 2001, Mussels had to be promptly removed by means of a drainage pipeline (Figure 9). imaging of the pipeline to the chambers using a Rigid Borescope. together with surveys of the facility in January 2001. showed that living mussels actually infest every stage of the works prior to the ozonation tank.

Figure 9. Mussels in the drainage chamber of an ozone tank at Veolia W in South London.

Problems associated with ozone tank blockages Involve a reduction in contact time of water and the destruction of ozone diffusers due to the sheer weight of shells. The cleaning of ozone tanks can be very problematic due to the hazardous nature of the disinfectant. Cleaners must wear full protective equipment, including breathing apparatus.

Blockage of narrow-bore sampling ports and pumps Mussel shells can block small sampling pipelines that branch from the main pipelines. Blockage of these pipelines is often the first indication of an infestation. For instance, one of Yorkshire Water's W in East Yorkshire encountered mussels in their turbidity measuring equipment for the first time In the summer of 2003. The Thames Water W in north London Is also experiencing continual problems with such sampling ports, to the extent that in the future, the ports are to be

shortened and screened. allowing removal of the musseis. In Severn Trent Water's W in Derbyshire. mussels constantly accumulate in the machinery of pumps drawing from the raw water main, necessitating frequent pump replacement at a cost of around €100.000 per year (Helen Pickett, Severn Trent Water, personal communication).

Problems down-stream of primary treatment It is very rare that zebra mussels manage to reach the later stages of water treatment within a works, let alone the treated water supply plpelines. Mussels have rarely been documented after chlorination, ozonation or flltration treatment stages. The few sltuatlons where mussels have caused problems in later stages of treatment have usually followed facility repairs. These repalrs have allowed mussels to bypass primary stages of treatment. in 1997. zebra mussels were found in the heat exchange pipes of a new ozone facllity at Thames Water's W in north London. The new ozonation system was only six months old and supplied with water from the primary sand fliters. The problem was first noticed when the ozone generator started to over-heat and trip due to a decreased efficiency. The plant was shut down far two weeks while the exchangers were removed and examined. Dead zebra mussels were found blocking the fine pipes of the heat exchangers. The problem has not been repeated since, indicating that mussels cannot usually penetrate this far Into the facility.

Zebra mussel mitigation strategies used by English water companies Most of the water companies affected by zebra mussels have used reactlve strategies to mitigate their effects. This has often involved the employment of waste removal companies to remove mussels from locations in which they have accumulated. Cleaning can be very expensive: the costs of manpower, a removal tanker, and a suction pump can reach up to €10.000 a day (Michael Chipps. Thames Water, personal communication).

Essex and Suffolk Water, Angiian Water, and Veoiia Water have also implemented brief periods of pre-chlorination to control zebra mussels in important pipelines, usually with a residual of approxlmately 0.1-0.2mg/l chlorine for two to seven days. In most cases this does appear to kill some zebra mussels: mussel shells are flushed into facilities within a few days of dosing. However, the efficiency of these techniques has never been fully quantified or optimised in England. Zebra mussels can detect oxldislng chemicals like chlorine and remain closed for up to two weeks (''! and so a brief burst of chlorine Is unlikely to be particularly efficient at killing mussels.

To these authors' knowledge, no English water company has yet attempted any form of pro-active control to prevent

WEJ I 3 7 2

zebra mussel settlement. However, many water companies

have implemented zebro mussel monitoring schemes throughout their works so that thay can rapidly respond to any

sudden and threatening increoses in zebra mussel density.

DISCUSSION Reasons for the recent zebra mussel problems The reasons for the recent increases in zebra mussel bio- fouling are two-fold.

Firstly. in some parts of the country, such as in Yorkshire and

in the Angiian regions, zebra mussels are spreading and colonising new areas. Possible reasons for the spread are reviewed in Aldridge et a1,2004~5).

Secondly, many on-site problems are likely to have increased due to the recent eiimination of pre-chlorination at many facility intakes. Pre-chlorination was first implemented at

the beginning of the Second World War, to prevent pathogenic infecti~n"~'.Typicoliy, a continuous dose of around 0.51 p.pm chlorine was applied to all piped water. After the War, chlorination was reduced due to public health fears,

prompting a spate of zebra mussel problems in the 1950s and 1 9 6 0 ~ " ~ ~ ' ~ ~ . Despite this, many water companies continued to

chlorinate at levels high enough to keep mussels under control. The situation changed when the new 1989 Water A c F placed strong limitations on permissible levels of chlorination, A growing body of evidence indicated that the by-products of disinfection could be hazardous to public health. Of particular importance were Trihalomethanes

(THMs), carcinogenic compounds produced by the reaction of chlorine with organic matter.The 1989 Water Act placed a strict limitation of lOOpg/I THMs on all supplied water

(averaged over three months). As a result of the 1989 regulations, many water companies

looked to changing their operations to reduce THM formation. Some reduced the amount of chlorine applied or shifted the point of chlorine application to later in the treatment process (reducing the amount of exposed organic material in the water). Others radically changed their treatment processes,

implementing newer ozonation. flocculation, filtration or membrane technologies on site In their facilities. Without sufficient pre-chlorination, pipelines were left unprotected from zebra mussel infestation. Over the last ten to flfteen years, generations of zebra mussels have been allowed to grow

inside these pipelines, leading to some of the effects that we are seeing today. It must be noted that pre-chlorination will become increasingly dlfficult in future years. As of 2004, THM

limits are no longer being taken as an average over three months, but as a spot ~heck(?~).This may mean that even short

periods of chlorine dosing result in excessive THM levels. It must also be noted that many new pipelines (those built

since 1989) were constructed without pre-chlorination in place, leaving them vulnerable to zebra mussel infestation. The Spine tunnel In Thames Water's North London WTW is one such tunnel, the absence of pre-chlorination undoubtedly adding to the problems that are currently being manifested.

Unlike In North America, there have been no documented infestations of power station cooling systems in England.This is because most large English power stations draw from estuarine sources that are unlikely to be infested with zebra mussels.

Future effects of zebra mussels on English industry it is clear that zebra mussels create substantial problems to fresh-water drawing industries. Without comprehensive control strategies, problems will continue, potentially getting worse as the number of zebra mussels in each facility grows. Water companies may continue to mitigate problems by physically

removing the shells, but this is often costly and requires parts of the facility to be temporarily shut down. Also, if the margins between water supply and demand decrease in future years,

shut-downs may become increasingly difficult. Eight English water companies and Water Services

Northern Ireland were so concerned with current and future bio-fouling by D. polymorpha that in 2004 they commissioned the Water Research Centre (WRC). in collaboration with Cambridge University, to produce a report specifically aimed at preventing problems with zebra mussels (2'). Also, in May 2004, a zebra mussel working group was formed containing

ecologists, chemical engineers and representatives of eight water companies. This group is currently undertaking trials to develop optimal dosage strategies for a variety of zebra mussel control chemicals (including chlorine, ozone, potassium chloride, polyDADMAC, and micro-encapsulated toxins(4)), with the aim of reducing required dosage levels to well below English discharge limits.

Zebra mussels should be an important consideration in the

construction of new underground pipelines and reservoirs if future problems are to be avoided. Many proposed water supply schemes in Britain could be particularly affected. Of particular concern are proposed national water grids that will

result in rivers in Britain being connected by huge underground pipelines@Q. Also at risk are the increasing number of new industries that draw water directly from rivers or canals(".

CONCLUSIONS 1. The zebra mussel, Dreisseno polymorpho, is currently a

major bio-fouling pest of industries drawing freshwater

373 I WEJ

2.Thirty water treatment works from ten water companies in England have experienced problems with zebra mussels. Problems have included blockage of pipelines, reductions in pipeilne water fiow, and clogging of inlets, screens, filter beds, ozone tanks, pumps. sampling ports and heat exchangers.

3. Problems have increased in the last five years because of a spread of zebra mussels around England and changes in water treatment during the 1990s that have left intake pipelines unprotected from lnfestatlon.

4. Without comprehensive new control strategies, problems caused by zebra mussels will get worse in future years as the number of mussels in each facility grows. Water companies should also consider the Impacts of zebra mussels when designing new treatment facilities.

ACKNOWLEDGEMENTS The authors are Indebted to the staff at Thames Water, Veoila Water, Anglian Water, Severn Trent Water and Yorkshire Water who generously provided access to data and advice during this study. They are particularly grateful for the help of Roy Grubb, Alf ives, Helen Plckett, Toni Hoitby, Brlan Drage, Barrie Holden, Angela Richardson, Derek Wilson, Jenny Banks, Paula Agutter, and Lindsay Neal. Work was partially funded by Thames Water and an EPSRC grant to Dr David Aldridge and Dr Geoff Moggrldge.

REFERENCES (1) KERNEY M. P AND MORTON, B. S. The distribution of

Drelsseno polymorpha in Britain. Journal of Conchology., 1970.27.97.

(2)SOWERBY J. D. C. Extracts from the minute book of the Linnean society. Trans.Llnn.Soc.Lond., 1825, 14,585.

(3) KERNEY M.P Atlas of the Land and Freshwater Molluscs of Britain and Ireland. Harley Books, Coichester, 1999.

(4) ELLIOT, P The zebra mussel in England: biology, erects and control using micrwncapsulafed foxins. PhD thesis, 2005. Department of Zoology, University of Cambridge, Cambridge, UK.

(5) ALDRIDGE, D.C., ELLIOT, P AND MOGGRIDGE. G.D. The recent and rapid spread of the zebra mussel (Dreissena polymorpha) in Great Britain. Blological conservaffon.,

(6) HEBERT, PD.. MUNCASTER. B. W.AND MACKiE. G. L. Ecological and genetic studies on Dreissena polymorpha (Paiias): a new mollusc in the great lakes. Canadlan Journal of Fisheries and Aquaffc Sciences., 1989,49,1587.

(7) KHALANSKI, M. industrial and ecological consequences of

2004. 1 19.253-261.

.

the introduction of new species in continental aquatic ecosystems: the zebra mussel and other invasive species. Bulleffn Francals de la Peche at de la Pisciculfure., 1997,

(8) McCARTHY T. K. AND FiTZGERALD, J. The occurrence of the zebra mussel, Drelssena polymorpha (Pallas. 1771), an introduced biofoullng freshwater bivalve in Ireland. lrish Naturalists lournab. 1997.25,413.

(9) ARAUJO, R., ALVAREZ R.M. El mejillocebra en ei Ebro: un grave caso de riesgo ambiental en Aragon. Naturaleza Aragonesa.. 2001.8.39 (in Spanish)

(1 0) O’NEiLL, C. Personal communication on economic impact of zebra mwels in the great lakes. 1989-2000. National Aquatic Nuisance Species Clearinghouse.. Dec. 8,2000.

(1 1) REEDERS, H. H., BIJ DE VAATE, A. AND SLIM, F. J.The Filtration- Rate of Dreissena-Poiymorpha (Bivaivia) in 3 Dutch Lakes With Reference to Biological Water-Quality Management. Freshwater Biology., 1989,22,133.

(1 2) HOLLAND, R. E. Changes in planktonic diatoms and water transparency in Hatchery Bay, Bass Island Area, western lake Erie since the establishment of the zebra mussel. Journal of Great Lakes Research., 1993.19.61 7.

422-445.385.

(13) VANDERPLOEG, H.A., LIEBIG, J. R.,CARNlCHAEL,W.W.,AGY M. A,, JOHENGEN, T. H., FAHNENTIEL, G. L. & NALEPA, T. F. Zebra mussel (Drelssena polymorpha) selective filtration promoted toxic Microcystls blooms in Saginaw Bay (Lake Huron) and Lake Erie. Canadian Journal of Fisherles and Aqudlc Sclences.. 2001,58. 1208.

(14) HOEGER, S. J., DIETRICH, D. R. & HiTZFELD, B. C. Effect of ozonatlon on the removal of cyanobacterial toxins during water treatment. Envlronmenfal Health Perspectives., 2002,110,1127.

(1 5) JOCHiMSEN, E. M., CARMiCHAEL W. W., AN, J. S., CARDO, D. M., COOKSON. ST,, HOLMES. C. E. M.. ANTUNES. M. B. D.. DE MELO, D. A., LYRA. T. M.. BARRETO, V. S. T.. AZEVEDO. S. &

JARVIS. W. R. Liver failure and death after exposure to microcystins at a hemodiaiysis center in Brazil. New England Journal of Medicine.. 1998.338.873.

(16) CLAUDi, R., AND MACKIE, G.L. (1994). Practical manual for zebra mussel monitoring and control. Lewis Publishers, Boca Raton, FL.

(17) STILGOE. H.E. Presidential address (1930).Trans. /.WE 1930., 35, 14.

(18) CLARKE, K. B. The infestation of waterworks by Dreissena polymorpha, a freshwater mussel. Journal of the lnsflfufe of Wafer Works engineerlng 1953.6,370.

(19) GREENSHIELDS, F. AND RIDLEY J. E. Some researches into the control of mussels in water plpes. Journal of the lnstlfute of Water Works englneering., 1957,11,3CQ.

(20) JOHNSON, L. E. Enhanced early detectlon and

WEJ I374

enumeration of zebra mussel (Dreissena spp) vellgers using cross-polarized light microscopy Hydrobldogla., 1995,312, 139.

(2t) CLAUDI, R. & EVANS, D. W. Chemical addition strategies for zebra mussel control in once-through service water systems. In Zebra Mussels: Biology, impacts and Control (ed. T. F. Nalepa & D. W. Schioesser),, 1993. pp, 563-574. Boca Raton, Florida: Lewis.

(22) WATER SUPPLY (WATER QUALITY) REGULATIONS 1989. Statuatory instrument, 1989 no.1147, Water, England and Wales. HMSO, London.

(23) WATER SUPPLY (WATER QUALIP/) REGULATIONS 2000. Statuatory instrument no.3184. Water, England and Wales. HSMO, London.

(24) HALL T., SHEPHERD, D., DILLON, G., HEARN. K., ALDRIDGE. D. & ELLIOTL PPreventlng problems with zebra mussels, 20%. 156pp. Swindon: Water Research Centre.

(25) LEAKE, J.Valley tipped to be biggest reservoir. The Sunday Times, May 5.2002. p16.

(26) DAVIES, R. Canals offer a reservoir of possibility. The Financial Times. August 4,2003, p8.

375 I ww