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Potential ecological consequences of invasion of thequagga mussel (Dreissena bugensis) into Lake Mead,Nevada–ArizonaWai Hing Wong a , Todd Tietjen b , Shawn Gerstenberger a , G. Chris Holdren c , Sara Muetinga , Eric Loomis a , Peggy Roefer b , Bryan Moore d , Kent Turner d & Imad Hannoun ea Department of Environmental and Occupational Health , University of Nevada Las Vegas ,4505 Maryland Parkway, Box 453064, Las Vegas , NV , 89154-3064b Southern Nevada Water Authority , 1299 Burkholder Blvd., Henderson , NV , 89015c Bureau of Reclamation, U.S. Department of the Interior , Denver Federal Center , PO Box25007, Denver , CO , 80225-0007d Lake Mead National Recreational Area, National Park Service , 601 Nevada Highway,Boulder City , NV , 89005e Flow Science Incorporated , 420 Neff Avenue, Suite 230, Harrisonburg , VA , 22801Published online: 19 Jan 2011.

To cite this article: Wai Hing Wong , Todd Tietjen , Shawn Gerstenberger , G. Chris Holdren , Sara Mueting , Eric Loomis ,Peggy Roefer , Bryan Moore , Kent Turner & Imad Hannoun (2010) Potential ecological consequences of invasion of thequagga mussel (Dreissena bugensis) into Lake Mead, Nevada–Arizona, Lake and Reservoir Management, 26:4, 306-315, DOI:10.1080/07438141.2010.504071

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Lake and Reservoir Management, 26:306–315, 2010C© Copyright by the North American Lake Management Society 2010ISSN: 0743-8141 print / 1040-2381 onlineDOI: 10.1080/07438141.2010.504071

Potential ecological consequences of invasion of the quaggamussel (Dreissena bugensis) into Lake Mead, Nevada–Arizona

Wai Hing Wong,1,∗ Todd Tietjen,2 Shawn Gerstenberger,1 G. Chris Holdren,3 Sara Mueting,1

Eric Loomis,1 Peggy Roefer,2 Bryan Moore,4 Kent Turner,4 and Imad Hannoun5

1Department of Environmental and Occupational Health, University of Nevada Las Vegas,4505 Maryland Parkway, Box 453064, Las Vegas, NV 89154-3064

2Southern Nevada Water Authority, 1299 Burkholder Blvd., Henderson, NV 890153Bureau of Reclamation, U.S. Department of the Interior, Denver Federal Center,

PO Box 25007, Denver, CO 80225-00074Lake Mead National Recreational Area, National Park Service, 601 Nevada Highway, Boulder

City, NV 890055Flow Science Incorporated, 420 Neff Avenue, Suite 230, Harrisonburg, VA 22801

Abstract

Wong WH, Tietjen T, Gerstenberger S, Holdren GC, Mueting S, Loomis E, Roefer P, Moore B, Turner K, HannounI. 2010. Potential ecological consequences of invasion of the quagga mussel (Dreissena bugensis) into Lake Mead,Nevada–Arizona. Lake Reserv Manage. 26:306–315.

The recent invasion of the quagga mussel (Dreissena bugensis) in Lake Mead, Nevada–Arizona, USA has thepotential to alter biological relationships in this western reservoir. We evaluated the potential impacts by examiningseveral measurements in the Boulder Basin of Lake Mead after the introduction of quagga mussel (2007–2008).Analysis of variance did not reveal any basin-wide changes in chlorophyll a concentrations or water clarity (Secchidisk depth). Although significantly lower chlorophyll a concentrations were found in the outer basin, this reductionwas likely related to the decline of dissolved phosphorus concentrations. The abundance of cladocerans, copepodsor rotifers has not changed since 2007. Overall, the results suggest that there are no statistically significant changesto many of the standard water quality indices routinely measured in the Boulder Basin of Lake Mead; however,given the complexity of biological, chemical and physical processes driving this ecosystem, the long-term impactsof quagga mussels remain uncertain. This manuscript identifies impacts known to be altered by quagga and zebramussels in other systems and aims to help lake managers develop experimental and monitoring programs that willaccurately address the impacts of quagga mussels.

Key words: chlorophyll a, invasive species, Lake Mead, quagga mussel, water quality

Two dreissenid mussels have been introduced into NorthAmerica: (1) zebra mussels (Dreissena polymorpha), whoseoriginal range stretched across Europe prior to the lastglaciation event, after which they were restricted in theBlack, Caspian and Azov seas (Starobogatov and Andreeva1994); and (2) quagga mussels (Dreissena bugensis), ini-tially identified in the Bug portion of the Dnieper-Bug Estu-ary near Nikolaev of Ukraine (Andrusov 1890). The biologyand ecology of these dreissenid mussels have been studiedmore extensively in North America since their discovery in

∗Corresponding author: david.wong@unlv.edu

the Laurentian Great Lakes in the late 1980s (Hebert et al.1989, May and Marsden 1992, Mills et al. 1993, Nalepaand Schloesser 1993). Because of these mussels’ high fe-cundity, their passively dispersed planktonic veliger larvalstage and their ability to attach to submerged objects withproteinaceous, byssal threads, they rapidly spread to othermajor basins, such as the Illinois River, Mississippi Riverand Hudson River (Ram and McMahon 1996). Before itsdiscovery in the Boulder Basin of Lake Mead (Nevada)on 6 January 2007, the quagga mussel was primarily re-stricted to the Great Lakes region and the Mississippi Rivernear St. Louis, Missouri. The presence of quagga musselsin Lake Mead in 2007 was the first confirmed introduction

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of a dreissenid species in the western United States, and itwas also the first time that a large ecosystem was infestedby quagga mussels without previous infestation by zebramussels (LaBounty and Roefer 2007). Following their in-troduction to Lake Mead, quagga mussels have colonizedthe lower Colorado River system as well as lakes and reser-voirs in Arizona, California, Colorado and Utah. Zebra andquagga mussels have become arguably the most serious non-indigenous biofouling pest introduced into North Americanfreshwater systems (LaBounty and Roefer 2007). Dreissenidmussels have ecological, recreational and economic impactson systems they invade because they can filter large quan-tities of water, impact other organisms, affect water qualityand clog infrastructure. Although these invasive dreissenidspecies may impact ecosystems in different ways, they gen-erally increase water clarity by removing suspended parti-cles (e.g., phytoplankton, debris, silt and microzooplankton)in the water column (Griffiths 1993, Leach 1993, MacIsaac1996, Wong et al. 2003). The increased water clarity canthen affect other ecosystem components; decreased densi-ties of microalgae result in lower uptake rates for dissolvednutrients from the water column (Nicholls et al. 1999), andbenthic algae and plants benefit from increased water clarity(Hecky et al. 2004). Mussels can alter the morphologicaland physical properties of their habitat areas, thereby affect-ing the availability of resources such as competition on foodand space in the food web (Karatayev et al. 1997, Vander-ploeg et al. 2002, Hecky et al. 2004). Overall, these invasivedreissenid mussels are efficient ecosystem engineers thatcan alter the ecology of a system directly or indirectly.

Lake Mead, the largest reservoir in the United States interms of volume (36.7 × 109 m3), is a 66,000 ha, deepsubtropical reservoir on the lower Colorado River, Nevada–Arizona (Fig. 1) (LaBounty and Burns 2005). The limnol-ogy and ecology of Lake Mead is complicated by 4 inflows(Colorado River, Virgin River, Muddy River and Las Ve-

Figure 1.-Lake Mead and the 3 sampling stations in Boulder Basin.

gas Wash), 4 basins (Boulder, Virgin, Temple and Gregg)and variable seasonal and annual operation patterns. LakeMead provides favorable environmental conditions, such aswarm water, high calcium concentrations, rocky substrates,suitable pH and sufficient dissolved oxygen (LaBounty andBurns 2005, LaBounty and Roefer 2007), for quagga mus-sel growth, recruitment and reproduction. Some subsurfaceareas of Lake Mead have been almost completely coveredby quagga mussels, especially those rocky subsurface areas(Moore B, National Park Service, Dec. 2008, pers. observ.).Past studies have identified a limited benthic communitydominated by chironomids, oligochaetes and Asian clamsin Lake Mead (Melancon 1977, Peck et al. 1987). A recentbenthic survey conducted in 2008 identified quagga musselsas one of the most dominant species in the soft sediment(S. Chandra, University of Nevada, Apr 2009, pers. comm.).The purpose of this study was to evaluate whether quaggamussels have had impacts on chlorophyll a (Chl-a), waterclarity and zooplankton by analyzing data collected fromthe outer, middle and inner regions of Boulder Basin priorto and following invasion.

Materials and methodsBoulder Basin, the most downstream basin of Lake Mead,is where quagga mussels were originally identified and isthought to be the location where the introduction of themussels occurred. It is the most intensively monitored basindue to its importance as a source of drinking water for theLas Vegas Valley, the downstream transport of water to Cal-ifornia and Arizona, and its recreational value. Additionally,Boulder Basin receives all drainage from the Las Vegas Val-ley via the Las Vegas Wash that flows into Las Vegas Bay.The distance from Boulder Canyon to Hoover Dam (BlackCanyon) is about 15 km, and the distance from the conflu-ence of Las Vegas Wash to Hoover Dam is about 16 km(Fig. 1), with 3 descriptive regions, inner, middle andouter basins, based on their proximity to the Las VegasWash (LaBounty and Burns 2005). The influx of nutri-ents from the Las Vegas Wash contributes to the higherproductivity of the inner basin, while the deepest outerbasin is more oligotrophic (LaBounty and Burns 2005).The spatial heterogeneity of this basin can lead to sig-nificant ecological differences regarding the invasion ofquagga mussels; therefore, 3 sampling stations, CR346.4(36◦03′42′′ N, 114◦44′21′′W), LVB 7.3 (36◦05′29′′ N,114◦47′18′′W) and LVB4.15 (36◦07′01′′ N, 114◦49′50′′W)representing the outer, middle and inner basins, respec-tively, were used to evaluate the potential impact of quaggamussels in Boulder Basin (Fig. 1). In recent years, LakeMead’s ecosystem has experienced significant natural andhuman-caused changes in addition to the quagga musselinvasion. For example, lake levels fluctuate yearly, andwe are in the midst of a severe drought that began in

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Figure 2.-Lake Mead surface elevation (solid line) and totalphosphorus (TP) discharge (triangle connected with dashed line)from Las Vegas Wash to Las Vegas Bay (triangle) from 1990 to2009. Data for water surface elevation is from www.usgs.gov; datafor TP loading was provided by Dr. Douglas Drury.

2000 (Fig. 2). Anthropogenic nutrient loading from LasVegas Wash (Fig. 2) to Las Vegas Bay and Boulder Basinby wastewater treatment plants has affected the chlorophyllconcentrations in Las Vegas Bay and Boulder Basin. There isa long-term trend of decreasing total phosphorus (TP) load-ing from Las Vegas Wash to Las Vegas Bay, with the lowestamount recorded in 2005 (Fig. 2). Analysis of variance onmonthly TP loading from 2000 to 2008 demonstrated that thedifference from 2002 to 2008, however, was not significant(P > 0.05). From 2002 to 2008 the lake level was relativelylower due to the recent drought (Fig. 2). To minimize theimpacts from TP loading and lake level on Chl-a concen-tration and other parameters, the data collected from 2002to 2008 were used in the present investigation. Usually, thefirst year when invasive dreissenid mussels were found is setas the start of the post invasive period, such as zebra musselsin Lake Erie (Leach 1993) and the Hudson River (Strayeret al. 1999). Although the first Lake Mead-born cohort ofquagga mussels might have settled in Boulder Basin of LakeMead as early as August 2005, their exponential growth andspread started in 2007 (B. Moore, National Park Service, andW.H. Wong, University of Nevada Las Vegas, Sept. 2008,unpubl.). The impact from quagga mussels before 2007 isassumed to be negligible due to their lower abundance. As aresult, pre- and post-quagga periods for the purpose of thisstudy were set from 2002 to 2006 and 2007 to 2008, respec-tively. Overall, data collected from these stations from 2002to 2008 should provide valuable insight on the potentialecological impacts of quagga mussels in Boulder Basin.

Chlorophyll a (Chl-a)

Chl-a data from the 3 sampling stations were collectedweekly to monthly by Southern Nevada Water Authority(SNWA) and US Bureau of Reclamation (USBR) from 2002

to 2008. Integrated surface water samples (from surface to 5or 6 m) were collected using a flexible hose. The sample wasdecanted into a sample bottle and filtered on-board using aMillipore Corporation manifold and vacuum pump appara-tus. In most cases, 650 mL of water was filtered through aWhatman GF/C fiberglass filter. The filter was frozen un-til further processing in the laboratory could take place.Acetone was used in the extraction method for Chl-a fol-lowing standard procedures established by American PublicHealth Association (APHA 2005). A spectrophotometer wasused to measure the absorbance of Chl-a, and concentrationwas calculated based on Method 10200 H (2) described byAPHA (2005). The average concentration of Chl-a by monthwas calculated and the annual means of monthly Chl-a con-centrations were compared.

Water clarity

Secchi disk data collected monthly by USBR and SNWAat the 3 sampling stations from 2002 to 2008 were used toevaluate the effect of quagga mussels on water transparency.Using a standard black and white Secchi disk and a plasticor metal cable marked with centimeters and meters, read-ings were taken by lowering the disk until it was no longervisible. All Secchi readings were made using a 1 m-longviewscope, black inside with a tilted clear plastic base. Useof the viewscope greatly reduces the influences of wind (andresulting wave action) and variable light, thus equalizingreadings from time to time (LaBounty 2008). The averageSecchi reading for each month was calculated and used torepresent the transparency of that month; annual averageswere calculated for comparison.

Zooplankton

Monthly zooplankton samples were collected from 2002to 2008 with a Wisconsin plankton net (with 64 µmmesh) towed from 0 to 30 m of the water column.The collected samples were placed in bottles with Lu-gol’s solution. Identification and enumeration of thesesamples were conducted by Dr. John Beaver, BSA En-vironmental Services, Beachwood, Ohio. More informa-tion on the protocol can be found at the following web-site: http://www.bsaenv.com/aquatics/zoops/index.html. Allsamples were identified to species or genus level., For pre-sentation purposes, however they were grouped into the fol-lowing major zooplankton taxa: copepods, cladocerans, ro-tifers and quagga mussel veligers. Abundance of each groupwas calculated for each month and further compared to seewhether any significant difference existed among years.

Statistics

One-way analysis of variance (ANOVA) was used tocompare Chl-a concentration, Secchi disk depth and

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zooplankton abundance to determine whether there was anysignificant change from 2002 to 2008. If a significant dif-ference was found, posthoc multiple comparisons were per-formed with Fisher’s least significant difference (LSD). A T-test was used to compare the annual and seasonal (monthly)difference before (prior to 2007) and after (2007 to 2008)quagga mussel invasion into Lake Mead (prior to the T-test, the data were first tested using equality of variances todecide whether the T-test was to be carried with equal vari-ances or unequal variances). Simple linear regression wasused to analyze the temporal trend of Chl-a concentrations,the temporal trend of orthophosphate (OP) concentrations(T Tietjen, Southern Nevada Water Authority, unpubl.) andthe relationship between Secchi disk depth and Chl-a. Step-wise multiple regression was used to examine the potentialfactors affecting Chl-a by using annual Chl-a concentra-tion as the dependent variable and corresponding annual OPconcentration, TP loading from Las Vegas Wash, and waterlevel as independent variables. All variables left in the modelare significant at the 0.15 level. Basin-wide difference be-fore and after quagga mussel invasion was tested by 2-wayANOVA with station (CR346.4, LVB7.3 and LVB4.15) andperiod (prior to 2007 [2002–2006] and after [2007–2008])as 2 independent variables. We used SAS 9.1 (SAS InstituteInc. Cary, NC) to perform all the statistical analysis. All testsused the significance level of α = 0.05 and high significancelevel of α = 0.01.

ResultsChl-a

Monthly Chl-aconcentrations at the 3 sampling stations(Fig. 3) show that at LVB4.15 and LVB7.3, no differencein any month between pre- and post quagga periods wasfound (T-test, P > 0.05). At CR346.4, significantly lower

Figure 3.-Chl-a concentrations at the 3 sampling stations inBoulder Basin of Lake Mead from 2002 to 2008.

Chl-a concentration was found in the post-quagga period inJanuary, May, July, August and September, respectively (T-test, P < 0.05). At CR346.4, the lowest mean annual concen-trations of Chl-a occurred in 2008 and 2007 and had a signif-icant difference from those observed in 2002 to 2005 but nodifference from those in 2006 (Table 1, supplemental infor-mation). At LVB7.3, Chl-a concentrations in 2006 to 2008were lower than those observed in 2002 and 2003 at LVB7.3.No difference was found among years from 2002 to 2008 atLVB4.15 (Table 1, supplemental information). Chl-a in thepost-quagga period (2007–2008) at CR346.4 was only 57%of that during the pre-quagga period (2002–2006), and T-testshows a significant difference between these 2 periods (P <

0.05, Fig. 4). However, further analysis shows that the con-centration of Chl-a decreased linearly with year at CR346.4,and OP concentration at this station also decreased linearlywith year (Fig. 5). Stepwise multiple regression betweenChl-a, OP, TP loading from Las Vegas Wash and water levelreveals that OP concentration was the only variable affect-ing Chl-a concentration at this station: 82.9% of variation ofChl-a was explained by OP (Chl-a = 0.628 + 0.395 OP,

Figure 4.-Chl-a concentrations within Boulder Basin before(2002–2006) and after (2007–2008) quagga mussel invasion(different letters on top of the bar in the same station showsignificant differences).

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Figure 5.-Temporal trend of Chl-a concentration (triangle anddashed line, Y = 364.1 − 0.181 X (R2 = 0.85)) and OPconcentration (dark square and solid line, Y = 832.2 − 0.414 X(R2 = 0.84)) at CR346.4 of Boulder Basin. Primary Y-axisrepresents Chl-a, secondary Y-axis represent OP concentrationand X-axis represents year.

R2 = 0.829, P < 0.01). Although Chl-a concentrations alsodeclined in the post-quagga period at LVB7.3 and LVB4.15,the difference was not significant (P > 0.05; Fig. 4). Two-way ANOVA shows significant differences in Chl-a amongthe 3 stations (LVB4.15 > LVB7.3 = CR346.4, P < 0.01),while no significant difference was found before and after thequagga mussel invasion (P > 0.05). The difference amongstations is thought to be caused by nutrients discharged fromthe Las Vegas Wash, which led to a more eutrophic innerbasin (LaBounty and Burns 2005). The annual TP loadingfrom Las Vegas Wash from 2002 to 2006 and 2007 to 2008did not differ significantly (T-test, P = 0.97). This is onepossible reason that no significant difference was found be-tween the pre- and post-invasion periods.

Water clarity

The monthly Secchi depths in CR346.4, LVB7.3 andLVB4.15 (Fig. 6) show that no difference was found inany month at any of the 3 sampling stations between pre-and post quagga periods (T-test, P > 0.05). Secchi depthsdid not differ significantly among years in each of the 3sampling stations (One-way ANOVA, P > 0.05), althoughthe highest values were recorded in 2008 at CR346.4 andLVB7.3 (Table 2, supplemental information). The annualSecchi disk depth was either significantly correlated withthe annual Chl-a concentration at CR346.4 (R2 = 0.62, P <

0.05) or marginally related with the annual Chl-a at LVB7.3(R2 = 0.56, P = 0.05) and LVB4.15 (R2 = 0.53, P = 0.06).The annual mean Secchi values for before and after quaggamussel invasion in each of the 3 stations did not show anysignificant difference (T-test, P > 0.05). Two-way ANOVAshows that no basin-wide impact from quagga mussels onwater clarity was detected before and after the arrival of

Figure 6.-Secchi depths at the 3 sampling stations in BoulderBasin of Lake Mead from 2002 to 2008.

quagga mussels (P > 0.05), but LVB4.15 had lower Secchidisk depth than the other 2 stations. Our result is consistentwith prior observation of an increasing trend in water clarityfrom the inner basin to the open water in Boulder Basin(LaBounty 2008).

Zooplankton

The monthly abundance of cladocerans, copepods, rotifersand quagga mussel veligers was calculated (Fig. 7). Forcladocerans, the monthly abundance in CR346.4, LVB7.3and LVB4.15 did not differ before and after quagga musselinvasion (T-test, P > 0.05) except in July when abundanceof cladocerans was lower in the post-quagga period (T-test,P < 0.05). For copepods, no difference was identified inany month at any station between pre- and post quagga pe-riods (T-test, P > 0.05). No difference was found in anymonth from January to December in any of the 3 sam-pling stations between pre- and post quagga periods (T-test,P > 0.05). For rotifers, the abundance was not lower in thepost-quagga period than pre-quagga period in any month ofany of the 3 stations; the abundance of rotifers was evenhigher in September at CR346.4 or LVB7.3, or in June orSeptember at LVB4.15 (T-test, P < 0.05). Quagga musselveligers were first detected in March 2007 in the 3 stations.Veligers had one peak in early summer of 2007 and 2 peaksin 2008, with one in the summer and the other in the fall(Fig. 7 D).

Based on the annual abundance of different groups of zoo-plankton (Table 3, supplemental information), copepods,cladocerans and rotifers did not show significant differ-ence among years (one-way ANOVA, P > 0.05), and T-test did not detect any significant difference before andafter quagga mussel invasion (T-test, P > 0.05). Because

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Figure 7.-Zooplankton abundance (#/L) at the 3 sampling stations in Boulder Basin of Lake Mead from 2002 to 2008. A: cladocerans; B:copepods; C: rotifers; and D: quaggga mussel veligers.

no veligers were found before 2007, the concentrations ofveligers were significantly higher in 2007 and 2008 thanin other years (ANOVA, P < 0.01). However, the post-quagga-period veliger concentrations were only marginallyhigher than those of the pre-quagga period (T-test, P = 0.08).For abundance of zooplankton, 2-way ANOVA shows nobasin-wide difference before and after the quagga musselinvasion in cladocerans, copepods and rotifers (P > 0.05),but the difference among the 3 stations for each group wassignificant (LVB4.15 > LVB7.3 = CR346.4, P < 0.01).Two-way ANOVA also shows the abundance of veligers ofpost-quagga mussel period was significantly higher than be-fore the 2007 period (P < 0.01), but no difference was foundamong stations (P > 0.05).

DiscussionIn Boulder Basin of Lake Mead, no basin-wide impact onChl-a concentrations by quagga mussels was detected. Al-though Chl-adecreased significantly in the open waters ofBoulder Basin (CR346.4) following the arrival of quaggamussels, the reduction was likely caused by a concurrentdecline in dissolved phosphorus because the biological pro-

duction in Lake Mead is primarily limited by phosphorus(LaBounty 2008 and references therein). It is impossible toquantify the degree to which quagga mussels can impactChl-a or water clarity because we have neither sufficientrecords of quagga mussel abundance nor do we know how ef-ficiently these mussels can filter water in this system. Never-theless, it is useful to make some calculations to estimate themaximum possible effects. Based on the maximum recordeddensity of quagga mussels and shell length frequency fordifferent substrates (e.g., silt [184 individuals/m3] vs. rock[2505 individuals/m3]) in 2007 (W.H. Wong, University ofNevada Las Vegas, and B. Moore, National Park Service,Sept 2008, unpubl.), and assuming that (1) the filtrationrates with different sizes are similar to those recorded inLake Erie (Zhang et al. 2008 and references therein) and(2) mussels are actively feeding 100% of the time, we esti-mated that it takes these mussels 169 days to filter the waterof the entire Boulder Basin. Based on the monthly record ofquagga mussel veligers in the Boulder Basin and assumingthat their filtration rates are similar to those recorded for ze-bra mussels in the western Lake Erie (MacIsaac et al. 1992),we estimated that it takes these veligers 264 days to filterthe entire basin. By combining the filtration rates of veligers,

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juveniles and adults, it takes at least 103 days for these mus-sels to filter the entire basin. The density of Dreissena onHen Island Reef west of the Pelee Island in western LakeErie was about 25,000 individuals/m3 in October of 1990and 1991, and the filtration rate of these mussels was capa-ble of filtering a 7 m water column between 3.5 and 18.8time per day (MacIsaac et al. 1992). Compared to other sys-tems such as western Lake Erie and the Hudson River whereit only took dreissenid mussels 1–3 days to filter the entirewater body (Bunt et al. 1993, Roditi et al. 1996), the filtra-tion capacity of these invasive quagga mussels in BoulderBasin of Lake Mead is very low. Lower filtration capacityis likely the main reason we have not seen any significantbasin-wide change in chlorophyll; or if there is any changein Chl-a at a specific site such as CR346.4, the impact fromquagga mussels must be very limited as the key driver of bio-logical production is phosphorus (Fig. 5). Another potentialfactor that could constrain the impact of quagga mussels onchlorophyll is the “benthic-pelagic” boundary layer abovethe mussel bed (Ackerman et al. 2001, Zhang et al. 2008).Because Lake Mead is a deep reservoir, quagga mussels liv-ing at the bottom of the lake may indeed be subject to theboundary layer effect. However, the boundary layer effect inLake Mead should not be as significant as it is in Lake Erieor other systems where the lake bottom is a large portion ofthe subsurface area. Lake Mead formed when impoundedColorado River water filled deep canyons, and, furthermore,there are several “islands” located in the middle of BoulderBasin such as Sentinel Island and Boulder Island. Hence, alarge portion of the basin’s rocky subsurface area is verticalfrom the bottom to the water surface.

Top-down control on primary production by dreissenid mus-sels is mainly due to their efficient filtering behavior. Sincezebra mussels were first discovered in Lake Erie in 1988,Chl-a concentration decreased by 43% in the western basinand 27% in the west-central basin between 1988 and 1989(Leach 1993). A 20-year (1983–2002) dataset shows adownward trend of −0.07µg/L/yr in the central basin ofLake Erie, but no trend was found before the arrival ofzebra mussel (Rockwell et al. 2005). In the Hudson River,New York, zebra mussels were found to reduce chloro-phyll concentrations by 90% of the pre-zebra mussel level(Caraco et al. 2006). In some areas, such as in GreenBay of Lake Michigan, Chl-a decreased immediately fol-lowing dreissenid invasion but increased afterward (Quallset al. 2007, De Stasio et al. 2008). A recent model of LakeErie suggests that zebra mussels decreased algal biomasswhen they first invaded and their population was small(6000 individuals/m2). However, when the population grewlarger (120,000 individuals/m2), dreissenid mussels in-creased nondiatom inedible algae by excreting a largeamount of ammonia and phosphate (Zhang et al. 2008).The density of quagga mussels collected from 138 loca-

tions in Lake Mead in 2007 was 505 ± 667 mussel/m2 (B.Moore, National Park Service, and W.H. Wong, Universityof Nevada Las Vegas, unpubl.), which was lower comparedto other ecosystems such as western Lake Erie (MacIsaacet al. 1992). Therefore, the top-down control function fromthese invasive mussels in the deep Lake Mead may not be assignificant as those found in the relatively shallow westernLake Erie.

At each of the 3 sampling stations, variations of Chl-aconcentrations explained more than 50% of the variationof Secchi depth readings, which is similar to the resultsobtained by LaBounty and Burns (2005). The key reasonwhy there is no detectable impact from quagga mussels onwater clarity is there has been no significant basin-widereduction on Chl-a following the arrival of quagga mussels.Apart from Chl-a, water clarity decreased with droppingsurface elevation in Boulder Basin (LaBounty and Burns2005) due to increased exposure of lake bottom, resultingin resuspension of fine former lake-bottom sediment. Watersurface elevation needs to be considered when evaluatingSecchi depth in Boulder Basin. The water surface elevationbefore quagga mussel invasion was about 9 m higher thanin the post-quagga period (Fig. 3). The immediate effect ofdreissenid mussels on water clarity is obvious in a short timefor shallow lakes and basins, such as the western basin andwest-central basin of Lake Erie; between 1988 and 1989, theSecchi disk transparencies increased by 1.24 m (85%) and1.75 m (52%), respectively (Leach 1993). During the first4–5 years of dreissenid invasion, the deeper eastern basin ofLake Erie did not demonstrate any immediate improvementin water clarity (Makarewicz et al. 1999) until more quaggasestablished in this basin for almost 10 years (Barbiero andTuchman 2004). Because Lake Mead is a deep and stratifiedreservoir with relatively low densities of quagga musselsin 2007 and 2008, a significant increase in water claritywas not observed following quagga mussel introduction.Similarly, water clarity enhancement after dreissenidinvasion was not observed in several systems, such as thewestern basin of Lake Erie (Barbiero and Tuchman 2004),part of Green Bay of Lake Michigan (Qualls et al. 2007)and the Hudson River (Strayer et al. 1999). This is partiallydue to the large inputs of suspended solids into these lakesystems and the Hudson River. For instance, spring waterclarity in Lake Erie’s western and west-central basinshas declined significantly in the post-dreissenid period,although chlorophyll did decline by about 50% during bothspring and summer following Dreissena invasion (Barbieroand Tuchman 2004). This discrepancy in the responses ofwater clarity and chlorophyll is probably a consequence ofboth the large sediment loads entering the western basin andresuspension of unassimilated nonalgal particulates. But itis not clear whether the impact from sediment loading andresuspension is an issue in the Boulder Basin of Lake Mead.

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No major group of zooplankton showed any significantchange with the exception of the presence of the planktonicquagga mussel veliger. The higher abundance of zooplank-ton in LVB4.15 should be associated with the higher chloro-phyll concentration in the inner basin. It has been speculatedthat zooplankton food resources were depleted by the filter-ing activity of dreissenid mussels when they established inthe Great Lakes (Mills et al. 1993). Also, like zebra mussels(MacIsaac et al. 1995, Strayer et al. 1996, Jack and Thorp2000, Wong et al. 2003), quagga mussels can consume zoo-plankton directly, especially microzooplankton such as ro-tifers. Field sampling and in situ enclosure experiments haveshown that dreissenid mussels can impart a strong top-downcontrol on zooplankton by direct grazing on zooplanktonor through indirect competition for food with zooplankton.In the western basin of Lake Erie, rotifer abundance de-clined by 74% between 1988 and the 1989–1993 period,which occurred coincident with the establishment of dreis-senid mussels (MacIsaac et al. 1995). But cladoceran den-sity did not change, although copepod and copepod naupliideclined by 39–69% between 1988 and 1993. In the Hud-son River, microzooplankton such as rotifers, tintinnids andcopepod nauplii dropped to 10–20% of their pre-invasionlevels (Pace et al. 1998). Based on the present investigation,the major food resources (phytoplankton and chlorophyll) ofzooplankton have not yet experienced significant depletionand, therefore, their populations are still healthy in BoulderBasin.

Overall, there has been no significant basin-wide impact onChl-a, water clarity or zooplankton abundance in BoulderBasin due to quagga mussels 2 years after they were foundin Lake Mead. At 8 stations in Boulder Basin, it seemsthere is an increase in veliger concentrations (#/L), but thatthe values do not differ statistically from April 2007 (0.4),April 2008 (2.0) to April 2009 (3.8; C. Holdren, US Bureauof Reclamation, unpubl. data). Quagga and zebra musselveligers prefer hard substrates such as rocks on which tosettle and grow. Lake Mead was created by impoundingthe Colorado River to fill canyons; 49% of its subsurfacearea is hard substrate, which is much higher than othersystems such as the Hudson River that are composed ofonly 7% hard substrate (Strayer et al. 1996). Populations ofthese invasive mussels will likely continue to grow becauseof the large proportion of hard subsurface area, an almostunlimited calcium supply from the Colorado River anda year-round favorable water temperature. Accordingly,if the invasive quagga mussel population continues tothrive, the potential ecological consequences should not beoverlooked. The loss of chlorophyll peaks and the deepestSecchi depths recorded in the open water of BoulderBasin in recent years may be a warning of changes thatmay still come (LaBounty 2008; T. Tietjen, SouthernNevada Water Authority, pers. observ.). Although it has

been reported that quagga mussels can maintain positivegrowth at very low Chl-a concentration (0.05 mg/m3;Baldwin et al. 2002), food could be a likely potentiallimiting factor for quagga mussels because Lake Mead isa less productive system, especially in the outer portionof Boulder Basin where Chl-a is <1 mg/m3 (Fig. 5). Ifthe mussel population reaches its carrying capacity in thenear future, there may not be any detectable impact fromquagga mussels in this system; or if there is any impact, itmay not be as significant as in other systems such as LakeErie or the Hudson River. To gain a better understandingof the consequences and potential consequences of quaggamussels in Lake Mead, a long-term standardized monitoringplan, Interagency Monitoring Action Plan (I-MAP) forQuagga Mussels in Lake Mead, has been developed totrack mussel size, abundance and distribution at morethan 50 sampling sites throughout Lake Mead (Wongand Gerstenberger 2008). The plan was implemented bylake managers and participating agencies in late summer2009. Strategic monitoring will help lake managers betterunderstand and accurately quantify quagga mussel impactin this large reservoir in the Western United States.

SummaryTwo years after the arrival of quagga mussels at BoulderBasin of Lake Mead, there has been no basin-wide sig-nificant impact on Chl-a or water clarity to date. Majorzooplankton groups have not shown adverse responses toquagga mussel invasion. This investigation suggested littleecological impact of quagga mussels following the first 2years of colonization in the Boulder Basin of Lake Mead.This is likely due to the low density of quagga mussels inthe Boulder Basin of Lake Mead in the early stage of theirintroduction. Given the well established ecological conse-quences of quagga mussels on other systems, and coupledwith lower chlorophyll and higher water clarity in the openwater of Boulder Basin in recent years, Lake Mead needsto be monitored strategically to mitigate any potential ad-verse impacts in the future. Scientifically based mathemati-cal models will also be helpful for better estimating quaggamussel populations and their impacts on this ecosystem.

AcknowledgmentsWe thank Dr. Douglas Drury for nutrient loading data fromLas Vegas Wash. Assistance from Mark Sappington, Dr.Jennell M. Miller, Dr. Craig J. Palmer and Warren Turkettare appreciated. We also thank Dr. Chad Cross and Dr.Sheniz Moonie for their support in statistical analysis.Review by Alan Sims, Warren Turkett, Scott Schiefer andLawrence Bazel improved the quality of this manuscript.This study was carried out through a Great Basin

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Cooperative Ecosystem Studies Unit agreement betweenthe National Park Service and the University of Nevada,Las Vegas. It was supported by Southern Nevada PublicLand Management Act funding awarded to the NationalPark Service, Lake Mead National Recreation Area. Theconstructive comments from 3 anonymous reviewers werehelpful in improving the quality of this manuscript.

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