The Abundance and Distribution Pattern of Quagga Mussels in the Las Vegas Bay of Lake Mead, Nevada and Potential Correlation with Environmental Parameters

UNLV Theses, Dissertations, Professional Papers, and Capstones

12-1-2012

The abundance and distribution pattern of quagga mussels in the Las Vegas Bay of Lake Mead, Nevada and potential correlation with environmental parameters

Scott Rainville University of Nevada, Las Vegas

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This Thesis has been accepted for inclusion in UNLV Theses, Dissertations, Professional Papers, and Capstones by an authorized administrator of Digital Scholarship@UNLV. For more information, please contact [email protected]. THE ABUNDANCE AND DISTRIBUTION PATTERN OF QUAGGA MUSSELS IN

THE LAS VEGAS BAY OF LAKE MEAD, NEVADA AND POTENTIAL

CORRELATION WITH ENVIRONMENTAL PARAMETERS

Scott Allen Rainville

Bachelor of Science California State University, Long Beach 1982

A thesis submitted in partial fulfillment of the requirements for the

Master of Public Health Department of Environmental and Occupational Health School of Community Health Sciences Division of Health Sciences

The Graduate College University of Nevada, Las Vegas December 2012

THE GRADUATE COLLEGE

We recommend the thesis prepared under our supervision by

Scott Rainville

entitled

The Abundance and Distribution Pattern of Quagga Mussels in the Las Vegas Bay of Lake Mead, Nevada and Potential Correlation with Environmental Parameters

be accepted in partial fulfillment of the requirements for the degree of

Master of Public Health Department of Environmental and Occupational Health

Shawn Gerstenberger, Ph.D., Committee Co-Chair

David Wong, Ph.D., Committee Co-Chair

Mark Buttner, Ph.D., Committee Member

Jacimaria Batista, Ph.D., Graduate College Representative

Tom Piechota, Ph.D., Interim Vice President for Research & Dean of the Graduate College

December 2012

ABSTRACT

The abundance and distribution pattern of quagga mussels in the Las Vegas Bay of Lake Mead, Nevada and potential correlation with environmental parameters

Scott Allen Rainville

Dr. Shawn Gerstenberger, Examination Committee Co-Chairperson Executive Associate Dean of School of Community Health Sciences

Dr. David Wong, Examination Committee Co-Chairperson Associate Research Professor School of Community Health Sciences University of Nevada, Las Vegas

The non-native quagga mussel [Dreissena bugensis (Andrusov, 1897)] was initially discovered in Boulder Basin of Lake Mead, Nevada-Arizona on January 6, 2007. This occurrence marked the first biological invasion of dreissenid mussels in the Western

U.S., following a significant North American range expansion of invasive quagga mussels from populations that invaded and colonized the Great Lakes Region in the

Northeastern U.S. during 1991. This nonindigenous mollusk species quickly spread from

Boulder Basin and became established throughout the entire reservoir by the end of 2007, with the exception of the inner Las Vegas Bay. There was an apparent absence of settled juvenile and adult life forms, in spite of the fact that embryonic veligers were abundant in

Las Vegas Bay in early 2009. It is theorized that this phenomenon may be attributable to the elevated inorganic nutrient loading to Las Vegas Bay from treated wastewater effluent released to Las Vegas Wash by the municipal wastewater treatment plants, and the pollutants that are carried in stormwater runoff from the Las Vegas Valley to the

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inner bay. The Southern Nevada Water Authority routinely collects and analyzes samples from Lake Mead in order to make biological and chemical assessments of water quality. However, this is the first limnological study since the survey conducted by

Prentki, Paulson & Baker (1981) to analyze the chemical, biological, and physical parameters of benthic sediment samples collected from Las Vegas Bay. The goal of this limnological study was to determine the environmental factors that may be related to the density of settled quagga mussels inhabiting Las Vegas Bay of Lake Mead. This study reports that adult and juvenile quagga mussels have inhabited LVB since the third quarter of 2009. No significant differences in pH or electrical conductivity in the benthic sediments were found in Las Vegas Bay; therefore, these parameters could not be statistically compared to the density of settled quagga mussels. Similarly, the composition of benthic sediment (i.e., % clay, % sand, and % silt) was found to be statistically similar, so a comparison to quagga mussel density could not be made. This study found a positive correlation between water depth and the distance of the station from Las Vegas Wash, and quagga mussel density.

TABLE OF CONTENTS ABSTRACT ...... iii

LIST OF TABLES ...... vii

LIST OF FIGURES ...... viii

CHAPTER 1 INTRODUCTION Background ...... 1 Significance of Study ...... 2 Hypotheses ...... 9

CHAPTER 2 REVIEW OF RELATED LITERATURE Inflow Volume to Lake Mead ...... 14 Limnological Characteristics of Lake Mead ...... 18 Mollusca Phylum ...... 29 Spread and Introduction of the Quagga Mussel to the U.S...... 31 Spread and Introduction of the Asian Clam to the U.S...... 37 Reproductive Patterns of the Bivalvia Class ...... 38 Lifecycle and Reproductive Stages of Dreissena bugensis...... 39 Lifecycle and Reproduction of Corbicula fluminea...... 45 Mollusca Phylum, Gastropoda Class of Invertebrates ...... 46 Spread and Introduction of the New Zealand mud snail to the U.S...... 48 Lifecycle and Reproduction of Potamopyrgus antipodarum...... 49 1987 Lake Mead Limnological Research Report on Benthic Invertebrates ...... 51

CHAPTER 3 METHODOLOGY Site Selection ...... 55 Study Design ...... 55 Method of Veliger Sample Collection and Analytical Techniques ...... 59 Benthic Sediment Collection Field Methods and Analytical Techniques ...... 63 Chemical Analyses of Benthic Sediment ...... 65 Statistical Analysis Methods ...... 66

CHAPTER 4 RESULTS ...... 73

CHAPTER 5 DISCUSSION ...... 85

APPENDIX 1 DISTRIBUTION OF INVASIVE MOLLUSKS IN LVB, 2011 ..... 94

APPENDIX 2 SCIENTIFIC RESEARCH AND COLLECTION PERMIT ...... 95

APPENDIX 3 LVB VELIGER DATA, SAMPLING LOCATION #11 ...... 98

REFERENCES ...... 103

VITA ...... 119

LIST OF TABLES

Table 1 Current Wasteload and Load Allocations on Las Vegas Wash TMDLs ...... 17 Table 2 Nutrient losses from Hoover Dam from an Epilimnion and Hypolimnion Discharge during July 1979 through July, 1980 ...... 20 Table 3 Environmental Parameter Requirements for Quagga Mussels…… ...... 27 Table 4 Benthic Density of Asian clams in Lake Mead, NV, 2009-2010 ...... 52 Table 5 Benthic Densities of Invertebrates by Depth, 1987 Limnological Study ...... 54 Table 6 Veliger and Settled Quagga Mussel Sample Collection Sites in Las Vegas Bay, Lake Mead ...... 57 Table 7 Total Volume of Water Filtered at Predetermined Tow Depths ...... 60 Table 8 Sample Stations Included in Data Analyses ...... 67 Table 9 Analyses Results of Benthic Sediment – Chemical and Physical Variables Used in Statistical Calculations ...... 72 Table 10 Descriptive Statistics for Settled QM Populations in LVB ...... 74 Table 11 Variable Tabulations, Sediment pH ...... 81 Table 12 Variable Tabulations, Sediment Electrical Conductivity ...... 81 Table 13 Variable Tabulations, Sediment Composition (% Clay, % Silt, % Sand) ...... 82 Table 14 Veliger Data for Samples Collected in Las Vegas Bay ...... 83 Table 15 Veliger and QM Abundance, LVB ...... 84

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LIST OF FIGURES

Figure 1 Inflow to Lake Mead ...... 14 Figure 2 Point Source Inputs to LVW by Municipal Treatment Plants ...... 16 Figure 3 Map Showing Dam Locations of Colorado River System...... 19 -3 Figure 4 Inorganic Phosphate (PO4 ) Concentration Gradient in LVB, Lake Mead .... 21 Figure 5 Biologically Mediated Reduction Processes in the Oxidation of Organic Matter and Oxidation of Reduced Elements in Lake Mead ...... 25 Figure 6 Limnological Monitoring Program Sampling Stations in Las Vegas Bay ..... 26 Figure 7 Mean Substrate Size Preference for Quagga Mussels ...... 29 Figure 8 Anatomy of Generalized Mollusk ...... 30 Figure 9 Location Where Quagga Mussels Were First Discovered in the Bug-Dnieper Estuary of the Black Sea ...... 31 Figure 10 Spread of Quagga Mussels in the Don River and Volga River Basin in 1981 ...... 32 Figure 11 Transatlantic Spread of Quagga Mussels to the Lower Great Lakes of the Central U.S ...... 33 Figure 12 Discovery of Quagga mussels in the Upper Mississippi River and Ohio River ...... 34 Figure 13 Spread of Asian clam from China, Philippines and Australia to the Columbia River Basin, in the State of Washington ...... 37 Figure 14 Annual Abundance of Quagga Mussel Veligers in Boulder Basin of Lake Mead ...... 40 Figure 15 Life Cycle of Dreissena bugensis ...... 41 Figure 16 Dreissena bugensis,Veliger Larva ...... 42 Figure 17 Byssal Attachment Mechanism...... 43 Figure 18 Gastropoda Species found in Lake Mead, IMAP Project, 2009-2010 ...... 47 Figure 19 Parthenogenic Female, Mud Snail Releasing Embryos ...... 50 Figure 20 IMAP Sampling Stations – LVB 7.3, Sentinel Island CR 346.4 ...... 52 Figure 21 Sampling Zones, 1987 Limnological Study of Lake Mead ...... 53 Figure 22 Location of Sampling Sites in Las Vegas Bay, Lake Mead ...... 58 Figure 23 Standard Plankton Tow Net ...... 60 Figure 24 Equation for Determining the Concentration of Veligers in 1L Sample ...... 63 Figure 25 Quagga Mussel Data for Organisms Collected from LVB, 2011 ...... 73 Figure 26 Histograms – Cumulative Distributions of Shell Length Measurements ...... 76 Figure 27 Box and Whisker Plots, QM Shell Lengths – Inner, Mid, and Outer LVB Population data...... 78 Figure 28 Mann Whitney U-Tests and Kolmogorov-Smirnov Test Data, QM Populations Inhabiting LVB ...... 79 Figure 29 Zooplankton Data for LVB 6.7 Site ...... 86 Figure 30 Mean Veliger Abundance for LVB, 2009-2010 ...... 87 Figure 31 Abundance of Veliger and Settled QM Populations in LVB ...... 88 Figure 32 Mean Secchi Depth Values in Las Vegas Bay, 2009-2010 ...... 89 Figure 33 Mean Water Temperature Values in Las Vegas Bay, 2009-2010

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CHAPTER 1

INTRODUCTION

Historically, the possible causes of the water quality problems in Lake Mead have been extensively studied (Hoffman, Tramutt, & Heller, 1967; Evans & Paulson, 1983;

City of Las Vegas, Nevada, 1987; LaBounty & Horn, 1996; LaBounty & Horn, 1997;

Bureau of Water Quality Planning [BWQP], 2003; Algae Task Force [ATF], 2004), mostly because of objectionable algal blooms and declines in fisheries; however, little research has focused on the benthic invertebrate communities, especially the nonindigenous organisms inhabiting the bottom sediments of this manmade reservoir.

Only after 2007, subsequent to the discovery of destructive quagga mussels in Boulder

Basin of Lake Mead were several federal research grants awarded to study the environmental impact and possible methods for controlling the spread of this aquatic biofouling pest. Up until then, the only limnological studies of invertebrate organisms in

Lake Mead include: (1) a master’s thesis (Melancon, 1977) whose focus was limited to the macrobenthos inhabiting Las Vegas Bay, (2) an analysis report of the chemical and biological composition of sediments collected by Prentki, Paulson & Baker (1981), and

(3) a benthos survey conducted to establish baseline densities of organisms (Peck, Pratt,

Pollard, Paulson, & Baepler, 1987). This research project is the first limnological study since 1981 that involves collecting benthic sediment samples from the deeper hypolimnion layer in Las Vegas Bay, Lake Mead (LVB) and analyzing these for their chemical, biological and physical characteristics. The purpose of this investigation is to determine if a correlation exists between the abundance of quagga mussels, and the chemical and physical characteristics of the benthic sediment they inhabit in LVB, from

the terminus of Las Vegas Wash (LVW) to the mouth of Boulder Basin, Lake Mead,

Nevada.

Significance of the Study

Several freshwater, non-native mollusks have successfully invaded and established themselves in Lake Mead over the past thirty-five years. These include the

New Zealand mudsnail [Potamopyrgus antipodarum (Gray, 1843)], a gastropod that was discovered in LVB during 2008 by a research team from the University of Nevada Reno

(Wong, Rainville, Austin, & Gerstenberger, 2010); and two bivalve species: the Asian clam [Corbicula fluminea (Mueller, 1774)], discovered in the Overton Arm at the north end of Lake Mead in 1976 (U.S. Geological Survey [USGS], 1991), and the quagga mussel [Dreissena bugensis (Andrusov, 1897)]that was first detected in Boulder Basin during 2007 (Wong & Gerstenberger, 2011). All of the aforementioned biofouling pests pose serious economic consequences related to impeding water flow through filtering screens, and the clogging of water intake pipes supplying municipal water treatment systems, and water-cooled turbines used for hydroelectric power generation. Every biological invasion by a nonindigenous species carries with it the potential for impacting the entire lake ecosystem ecology (i.e., species abundance and distribution, nutrient cycling, food web effect). This study reports the distribution and abundance of all three invasive mollusks that compose the benthic community in LVB (see Appendix 1,

Distribution of Invasive Mollusks in LVB, 2011) as an estimation of their competitive success for survival. However, particular attention is focused on the quagga mussel, because of this species’ unparalleled potential for harming the aquatic environment, the

threat to public health, and the role this mollusk may have in the water quality

deterioration of Lake Mead that potentially impacts the potable drinking water supply of the entire Colorado River.

The most obvious public health implication caused by quagga mussels are possible lacerations to the hands and feet of lake visitors, with the risk of acquiring a bacterial infection from handling or stepping on broken quagga mussel shells that are characteristically sharp. Swimmers, windsurfers, and jet skiers who come in contact with lake water polluted with high concentrations of decomposing quagga mussel soft tissue, which harbors and promotes Escherichia coli bacterial growth, may be at increased risk of becoming ill. Moreover, it has been demonstrated that freshwater mollusks are able to assimilate heavy metals (i.e., lead, mercury, and cadmium), polychlorinated biphenyls

(PCBs), and polycyclic aromatic hydrocarbon congeners (PAHs) from the suspended particles in the water column and those trapped in the bottom sediments, and bioaccumulate these contaminants in their soft tissue and shell (Lau, Mohamed, Yen, &

Su’ut, 1998; Gossiaux, Landrum, & Fisher, 1998). The risk is that the concentration of these contaminants could theoretically be magnified up the aquatic food web as they are transferred to higher trophic level organisms and potentially pose harm to human beings, especially those who consume ducks and fish.

Quagga mussels may indirectly contribute to the proliferation of the cyanobacteria genera of blue-green algae that oftentimes dominates the algal population in freshwater lakes, especially during the summer months, and cause cyanobacterial harmful algal blooms (CHAB) (World Health Organization [WHO], 1999). It is largely accepted that excessive aquatic plant growth, especially green filamentous algae on lake bottoms, and

sporadic outbreaks of surface blooms or surface scums of blue-green algae occurs as a consequence of the nutrient overload (eutrophication) and the deeper penetration of solar radiation from an increase in the water column clarity (Colorado River Regional Sewer

Coalition [CRRSCO], 2008). The high densities of quagga mussels in Lake Mead may contribute to this process by effectively removing phytoplankton, zooplankton and other organics from the water column, due to their voracious filter-feeding habits, which results in greater penetration of the sunlight into the water column. Additionally, dressenids contribute to the nutrient enrichment of the profunal sediments by excreting large quantities of phosphate and ammonia that are the metabolic waste products of the phytoplankton they consume. The public health consequence is that a number of toxigenic species of cyanobacteria are capable of producing hepatoxins, endotoxins, neurotoxins and acute dermatoxic compounds. However, the most often cited illnesses reported in the literature are swimmers itch, skin rashes, nausea, and gastrointestinal disorders from coming into direct contact with or ingesting algal toxins that are released in the water (Colorado River Regional Sewer Coalition [CRRSCO], 2008; U.S.

Environmental Protection Agency [EPA], 2008, Kalff, 2002). Mounting evidence suggests that in lower-nutrient lake basins with low to moderate total phosphorus concentrations in the 10-25µg range, with established dressenid populations, the

Microcystis aeruginosa algal biomass is greater, and the microscystin toxin levels are 3.3 times higher than in comparable lakes without dressenids (Knoll et al., 2008). The blue-

green algae phylum of cyanobacteria has been the most plentiful group of phytoplankton

in Lake Mead since 1952. A long history exists of the lake and its shores being blanketed

with visible blooms of filamentous algae with concomitant fish kills in the 1950’s, mid-

1960’s and throughout the mid-to-late 1970’s (The City of Las Vegas, 1987). The most troubling event in Lake Mead to date is the detection of Cylindrospermopsis.

Concentrations of this cyanobacteria grew to concentrations approximating 27,000 cells/mL in LVB during 2003 (Algae Task Force [ATF], 2004). Certain species of this genera are able to produce saxitoxins, which are also known as paralytic shellfish poisons

(PSPs) that target the nerve axons, in addition to hepatotoxins that can cause widespread tissue damage to the kidneys and lymphoid tissue in mammals (World Health

Organization [WHO], 1999). The Lake Mead, Algal Task Force reported that one of the water samples collected from LVB yielded a positive result of 0.054 µg/L of toxin; however, the task force did not publically divulge the species of Cylindrospermopsis or the type of toxin. No other noteworthy outbreaks of nuisance or harmful algal blooms have occurred in Boulder Basin since the time of the quagga mussel invasion in 2007.

Therefore, it remains to be determined if the presence of quagga mussels is associated with an increase in the frequency of algal blooms.

An unforeseen environmental consequence of the quagga mussel infestation at

Lake Mead was the increase in the concentration of trihalomethane disinfection byproducts in the drinking water supply. This occurrence may have human health relevance for the 1.95 million residents in Clark County, Nevada (U.S. Census Bureau,

2010) and approximately 38.9 million people who visit Las Vegas, Nevada annually (Las

Vegas Convention and Visitors Authority [LVCVA], 2012). The increase occurred as a result of the Southern Nevada Water Authority (SNWA) implementing a new, chlorine pre-treatment disinfection regimen in July 2007 for the raw water that is withdrawn from

Lake Mead. The SNWA began directly injecting chlorine into both intake tunnels

located off of Saddle Island, Boulder Basin in response to the quagga mussel infestation in Lake Mead. This was done to prevent live mussels from attaching to the trash racks of the drinking water intake pipes and to reduce the likelihood of free-floating veligers from being transported through their water treatment plants (Roefer, Johnson, LaBounty, &

Zegers, 2007). In addition to the pre-treatment regimen, chlorine is also added to the water as it leaves the Alfred Merritt Smith and River Mountains Water Treatment facilities, prior to being distributed to the end-users. Unfortunately, an undesirable side effect of treating drinking water with chlorine based disinfectants is the formation of trihalomethanes (THMs) that is formed from the reaction of naturally occurring organic matter (NOM) and chlorine. Four halogenated disinfection byproducts (DBPs) are currently regulated by The Environmental Protection Agency (EPA), because they present a potential risk to public health; these include trichloromethane (chloroform), dibromochloromethane, dichlorobromomethane and tribromomethane. There is strong evidence that the metabolism of bromoform and dibromchloromethane in humans is hepatotoxic. A meta-analysis of findings from several epidemiological and toxicological studies on human exposure to chlorination by-products in drinking water suggests that a correlation exists between bladder and rectal cancers; an overall relative risk estimate was reported of 1.21 (95% CI: 1.09, 1.34) and 1.38 (95% CI: 1.01, 1.87), respectively

(Morris, Audet, Angelillo, Chalmers, & Mosteller, 1992). The EPA lists bromoform as a probable human carcinogen, and dibromochloromethane as a possible human carcinogen.

Furthermore, the available toxicity data on adverse developmental or reproductive outcomes in laboratory animals suggests that bromoform and dibromochloromethane may be fetotoxic; however, the scientific findings are inconclusive to date (Agency for

Toxic Substances and Disease Registry [ATSDR], 2005). Based on this information, the

National Primary Drinking Water Standard sets the Maximum Contaminant Level of total

THMs (TTHMs) in drinking water at 0.08 mg/L or 80 ppb (United States Environmental

Protection Agency [US EPA], 2012). The concentration of TTHMs has increased by approximately 15 ppb, to an average value of 63 ppb or 0.063 mg/L (annual average) since the SNWA changed their chlorination disinfection regimen in 2007 (Southern

Nevada Water Authority [SNWA], 2011). The SNWA Limnology Project Manager reported a decrease in concentration levels of TTHMs in drinking, in the fall of 2011.

This was accomplished by the chemical substitution of chlorine with chloramine disinfectant used at their raw water intake tunnels located in Boulder Basin, Lake Mead

(T. Tietjen, personal communication, October 18, 2011).

The successful biological invasion, settlement and spread of nonindigenous aquatic species are the least controlled and least reversible on freshwater ecosystems (Strayer,

2010) that threaten the sustainability of the world’s limited freshwater resources, especially the drinking water supply that is critical to public health and the sustenance of life. A case in point is the invasion and infestation of Boulder Basin by quagga mussels.

This invasive mollusk has greatly impacted the biological diversity in the Lake Mead reservoir ecosystem by substantially changing the species richness and abundance

(diversity) of the biota, in addition to attaching themselves to piers, docks and boat hulls.

Furthermore, these biofouling pests obstruct raw water intake systems that are critically important for cooling hydroelectric power generating plant turbines, and crucial for public water supplies. The economic impacts are substantial in terms of higher than expected equipment maintenance costs from downtime required for disinfecting and

removing attached mussels from substrate surfaces, and revenue loss from the

interruption of hydroelectric power generation at Hoover Dam. Up until the time of the

quagga mussel invasion, the limnological research in Lake Mead centered around

problems affecting the recreational uses of lake, specifically the water quality issues

responsible for the fisheries decline (i.e., striped bass, rainbow trout) and nuisance algal

blooms that negatively impact Lake Mead National Recreation Area tourism. This

research is the first limnological investigation of the LVB benthos, since the 1981 study

that analyzes the biological, chemical and physical characteristics of the deep

hypolimnetic sediments. LVB has unique characteristics compared to the other five

-3 - basins that compose Lake Mead. The phosphate (PO4 ) and nitrate (NO3 ) enrichment caused from treated sewage effluent being released to LVB makes this body of water slightly eutrophic, compared to the rest of Lake Mead that is oligotrophic. Furthermore, the pollutants in urban and stormwater runoff are transported from the Las Vegas Valley through channels to LVW untreated, where they enter the LVB and disperse in the water column. The unique physical characteristics of LVB include that the water depth ranges

from only 7.5 meters to approximately 66.75 meters (25-225 feet), and that large

quantities of sediment are transported from LVW into the bay as a result of flash floods.

Lake Mead provides an unparalleled ecosystem to study the relationships among

freshwater invertebrates, because it is the largest man-made, freshwater reservoir in water

volume and surface area in the United States. The goal of this research is to develop

information related to the possible relationships between the physical and chemical

parameters of the benthic sediments, and abundance of quagga mussels inhabiting LVB.

Hypotheses

Research Question I – Is the distance of the habitat from LVW a physical factor that affects the density of settled quagga mussels in LVB?

Hypothesis I

The cause of the eutrophication in LVB is mostly attributed to the anthropogenic phosphorus and nitrate inputs from highly-treated wastewater effluent released to the

LVW [LaBounty, J. F., & Horn, M. J. (1997)]. One consequence of an enriched supply of nutrients on freshwater ecosystems is an increase in the phytoplankton growth that results in periods of hypolimnetic oxygen depletion (Smith, Tilman, & Nekola, 1999).

The high nutrient input from LVW creates a concentration gradient, with the highest concentration of contaminants located in the inner LVB that decreases towards the middle and outer bay area (City of Las Vegas, Nevada, 1987). Although the tolerance limits for enrichment are unknown for quagga mussels, it is hypothesized that the population density of mussels inhabiting the soft bottom sediments in the inner bay area will be lower, compared to those inhabiting the outer bay area because of the degradation of the habitat quality.

Hypothesis Testing

H01: There is no significant difference in the density of the quagga mussels inhabiting the inner, middle and outer areas of the Las Vegas Bay.

HA1: The mean density of the quagga mussels inhabiting LVB is the lowest in inner bay compared to the middle and outer bay area.

Research Question II – The relationship of quagga mussel density to habitat depth.

Is water depth an environmental factor that affects the density of settled quagga mussels

in LVB?

Hypothesis II

Quagga mussels exhibit a low tolerance to elevated water temperatures and have evolved

traits that allow the species to colonize the deeper, colder hypolimnion layer (Cohen &

Weinstein, 2001). These traits include a low respiration rate under different seasonal

temperatures (Nalepa, Fanslow, & Pothoven, 2010); and higher assimilation efficiency

(Baldwin et al., 2002) so at low food levels the organism maintains higher growth and

fecundity rates (Zhulidov et al., 2004). It is hypothesized that the density of mussels will

increase as the habitat depth increases.

Hypothesis Testing

H02: Depth is not a significant physical environmental factor associated with the density

of settled, adult and juvenile quagga mussels in LVB.

HA2: Habitat depth is a significant physical factor that is positively correlated with

density of settled quagga mussels inhabiting LVB.

Research Question III – The density of settled quagga mussels with respect to benthic

sediment chemistry.

Are there significant differences in benthic sediment, pH and specific conductance (Eh)

in LVB that affects the density of quagga mussels?

Hypothesis III

Quagga mussels are sedentary organisms that inhabit and depend on the sediment

environment to sustain its life functions. Quagga mussels are sensitive to both short and

long-term changes in sediment and water quality, because they are predominately

epibenthic and live on the sediment (Interstate Technology Regulatory Council [ITRC],

2011). Electrical conductivity that is used to measure total dissolved solids (TDS), and

pH are two parameters used to assess sediment quality (Singara, Mishra, Trivedi, 2011),

and predict the success or failure of an invasion (Renata Claudi, Tom Prescott, 2009).

Quagga mussels were shown to have a narrow range of pH tolerance of values between

7.5 and 9.3 that may affect the settlement, growth, and survival of dreissenid mussels

(Claudi et al., 2012). Field experiments indicate that a pH value of 7.1 will prevent

dreissenid settlement from occurring, and that the shells of adult mussels experience a

significant amount of loss of calcium at a pH value of 7.1 (Renata Claudi et al., 2012).

In freshwater ecosystems, certain cations (i.e., Ca+2, Mg+2, Na+, and K+) and

- -2 -2 - anions (i.e., HCO3 , CO3 , SO4 , Cl ) comprise the majority of TDS (Goodfellow et al.,

1999). While aquatic organisms require a relatively constant level of these ions for their survival, concentrations can increase to toxic levels that affect the growth and reproduction of organisms (Goodfellow et al., 1999). The inflowing water from LVW

+2 -3 - enters LVB as a density current, high in total dissolved solids (i.e., Ca , PO4 , NO3 ,

+ + - Na , K and Cl ) and undissociated ammonia (NH3), which can cause an increase in water

pH values. The pollutant plume flows beneath the euphotic zone, at or close to the

bottom of the bay (City of Las Vegas, Nevada, 1987). Increasing concentrations of TDS

may be a predictor of toxicity and sediment conductivity (µS/cm) that are possibly

correlated with the abundance and distribution of benthic invertebrates (George, Abowei,

& Daka, 2009).

It is hypothesized that the benthic sediment in the inner, middle and outer portions of LVB will exhibit significantly different EC and pH values, and that the density of quagga mussels will increase as a function of (1) ↑ pH, and (2) ↓ specific conductance.

Hypothesis Testing

H03: No significant differences exist in benthic sediment pH or specific conductance in

LVB.

HA3: There are significant differences in benthic sediment pH and specific conductance in the inner, middle and outer portions of LVB, and sediment pH and electrical conductivity is correlated with the density of settled quagga mussels.

Research Question IV

Are there differences in the benthic sediment composition that affect population density of quagga mussels in LVB?

Hypothesis

Settlement is an active process in which quagga mussel pediveligers crawl onto and select the site and substrate that they eventually attach to, and live out a mostly sedentary lifestyle (J. Ackerman, B. Sim, 1994). Quagga mussels are capable of discriminating between different substrata at settlement and exhibit a preference for specific substrates

(Bayne, 1976). If a negative stimuli is received, the pediveliger withdraws its foot and swims away from the substrate. A predictive model to determine the occurrence of quagga mussels was developed by Jones and Ricciardi (2005) that determined that mean

substrate size is a significant factor that accounts for 38% of the variation in population density.

It is hypothesized that there will be a significant difference in the benthic sediment composition (%sand, %silt, %clay) in the inner, middle and outer portions of

LVB, due to the sediment loading of eroded silts and clays, and minor amounts of sand and gravel from LVW that are transported towards Boulder Basin. It is hypothesized that quagga mussels will show a preference for substrates comprised of a higher percentage of larger sand, because of the larger particle sizes compared to clay and silt.

Hypothesis Testing

H04: There is no significant difference in the composition of benthic sediment

(percentages of sand, silt, and clay sized particles) in the inner, middle and outer portions of LVB.

HA4: Significant differences exist in the benthic substrate composition in LVB, and the areas of the bay with the highest percentage of sand have the largest density of settled quagga mussels.

CHAPTER 2

REVIEW OF RELATED LITERATURE

Lake Mead is the largest man-made, freshwater reservoir in the United States that was formed as a result of the completion of Hoover Dam in 1936, which restrains the flow of the Colorado River in the Mojave Desert between Nevada and Arizona. At a maximum water surface elevation of 1,229 feet, the surface area of Lake Mead spans

162,916 acres (65,929 hectare) and has an available capacity of 27.6 million acre-feet

(ACF) or approximately 9 trillion gallons of water (U.S. Department of Interior [DOI],

2011). While the Hoover Dam was built for the purposes of flood control and generating hydroelectric power for distribution to the states of Arizona, California and Nevada, the

Lake Mead reservoir provides domestic drinking, irrigation and industrial water for over

22 million users (United States Geological Survey [USGS], 2011), in addition to providing a wide array of recreational outdoor activities (i.e. watercraft, water skiing, swimming, scuba, sport fishery) for the nearly eight million annual visitors to the Lake

Mead National Recreation Area (NRA) (Dickinson, 2010).

Figure 1. Inflow to Lake Mead (source: Bureau of Water Quality Planning [BWQP], 2003).

The lower Colorado River discharge through Lake Powell accounts for 97% of the inflow volume to Lake Mead, while small tributaries of the Muddy River and Virgin

River that feed into the Overton Arm, account for .10% and 1.38% respectively (Fig. 1)

(Las Vegas Valley Watershed Advisory Committee [LVVWAC], 2009). Additionally, the LVW, an open natural drainage channel, provides the only outlet for urban and stormwater runoff, and shallow groundwater seepage for the entire Las Vegas Valley hydrographic basin that encompasses nearly 1600 square miles ([LVVWAC], 2009). The

Las Vegas Wash Organization estimates that the LVW carries on average over 150 million gallons per day (MGD) of the Las Vegas Valley’s excess water from tributaries, including channels, creeks and floodways (Las Vegas Wash Coordination Committee

[LVWCC], 2012). While the flow from the LVW only accounts for about 1.5% of the total input to Lake Mead, it is consequential because urban runoff carries street surface pollutants (i.e. bacteria, oil, grease, pesticides, fertilizer residue) into the storm drainage system that eventually make their way to Las Vegas Bay. The pollutant plume disperses in the LVB water column and may affect the distribution and abundance of benthic organisms, and threaten the water quality in the Lower Basin of Lake Mead.

Point source water pollution discharges to LVW by the local reclamation facilities include approximately 214 MGD of highly treated sewage and wastewater effluent (Fig.

2). In addition to high levels of nitrogen and phosphorus, the pollutant stream may

+ + contain ammonium ions (NH4 ), high salt concentrations from sodium (Na ), chloride

- - (Cl ) and bicarbonate (HCO3 ) ions, and undissociated ammonia (NH3) that can cause an increase in water pH values. In Hoffman’s report, “The Effect of Las Vegas Wash

Effluent upon the Water Quality in Lake Mead,” he indicates that the salt load from LVW

is a significant contributor to the salinity in Lake Mead (Hoffman, Tramutt, & Heller,

1971).

Water Reclamation Facility Volume (MGD) of treated effluent released to LVW

Clark County WRD (CCWRD) 94

City of Henderson WTP 32

City of Las Vegas WPCF 63

City of North Las Vegas WPCF 25

Figure 2. Point Source Inputs to LVW by Municipal Treatment Plants (Source: Southern Nevada Water Authority [SNWA], 2009).

While nitrogen, phosphorus and potassium provide the necessary growth nutrients for algae that zooplankton eat that forage fish (i.e. threadfin shad) eat, which striped bass and other game fish prey on (City of Las Vegas, Nevada, 1987), an excessive amount of nutrients (nutrient enrichment) cause large visible algal blooms. These were first noticeable in Lake Mead during the early 1950’s (City of Las Vegas, Nevada, 1987). The deteriorating water quality in Las Vegas Bay was attributed to the Las Vegas Valley sewage treatment plants discharging effluent containing high concentrations of phosphorus to LVW beginning in 1956; however, there were previous occurrences when the lake was “carpeted with blue-green algae,” even before Clark Country and Las Vegas began releasing poor-quality water into the wash (City of Las Vegas, Nevada, 1987; Las

Vegas Wash Coordination Committee, 2009). The highest peak loading of phosphorus to

The Lake occurred during the 1972-1981 period, when quantities reached 1,200 - 1,500 lbs/day, and as a result the Nevada Division of Environmental Protection (NDEP)

developed water quality standards for both Lake Mead and the LVW that established

Total Maximum Daily Loads (TMDL) for both phosphorus and ammonia flows to The

Wash (List et al., 2006). In July of 1981, The City of Las Vegas and Clark County

Sanitation District WWTPs introduced a tertiary treatment for phosphorus removal that reduced total phosphorus (TP) loading to LVB to only 400 lbs/day, although the loads steadily increased until they reached 620 lbs/day by 1986 (Hannoun et al., 2005). This was followed by the addition of a Biological Nutrient Removal (BNR) activated sludge and filtration process at the City of Las Vegas WWTP in 2003 that was engineered to

-3 - remove PO4 , NH3, NO3, and organic matter from the effluent before being discharged to the LVW. The current total phosphorus and ammonia TMDLs on the LVW were established in 1989 by the Bureau of Water Quality Planning, NDEP that became effective in 1994 and 1995, respectively (Table 1).

Table 1. Current Wasteload and Load Allocations (TMDL) Established for Las Vegas Wash (Source: BWQP, 2003)

While it is established that there are increased loads on inorganic nitrogen and phosphorus to the Las Vegas Basin, the tolerance limits for enrichment are unknown for

quagga mussels. Hypothesis I tests whether the abundance (density) of quagga mussel populations inhabiting the inner, middle and outer areas of LVB significantly differs.

Limnological Characteristics of Lake Mead

There are two reasons why most of the Lake Mead reservoir is classified as an oligotrophic (nutrient poor) to mildly mesotrophic (moderately nourished) aquatic ecosystem (LaBounty & Horn, 1997; Paulson, Baker, & Deacon, 1980). The first stems from the completion of construction of the Glen Canyon Dam in 1963 (Fig. 3) and impounding of Lake Powell that markedly altered the chemical characteristics of the

Colorado River water inflow to Lake Mead (Peck et al., 1987). The second reason for the poor nutrient status of Lake Mead is that Hoover Dam is operated from a deep-water discharge (100 meter depth), which strips inorganic phosphorus along with other elements from the nutrient-rich hypolimnion layer of the reservoir when it is thermally stratified (Table 2) (Prentki, Paulson, & Baker, 1981).

Figure 3. Map Showing Dam Locations of Colorado River System (from the SNWA water resource plan, 2009).

While the Glen Canyon controlled the stormwater levels and eliminated the recurring flooding events in the lower basin of the Colorado River that typically occurred as a result of the snowmelt between May and July, it also resulted in a 93.5% reduction of the phosphorus loads to Lake Mead with an accompanying 80% decrease in phytoplankton productivity (Evans & Paulson, 1983, Peck et al., 1987). To provide scale, up until the time of the post-impoundment period, as much as 140 million tons annually of suspended sediment containing loosely bound inorganic phosphate particles flowed into Lake Mead during the spring flooding and high runoff (Evans & Paulson, 1983). The impoundment of Lake Powell resulted in a 70-80% reduction of suspended sediment load to Lake Mead

(Paulson & Baker, 1980).

Table 2. Nutrient losses from Hoover Dam from an Epilimnion and Hypolimnion Discharge during July 1979 through July, 1980 (from Prentki, Paulson, & Baker, 1981).

Lake Mead is composed of an Upper Basin, the area that lies above The Narrows, which consists of Virgin Basin, Overton Arm and the Upper Arm; and the Lower Basin that includes Boulder Basin and Las Vegas Bay (Fig. 1). Paulson contributes that the,

“disproportional loading of nitrogen and phosphorus at distant ends of Lake Mead, causes each basin to operate like different lakes;” while the Colorado River contributes 85% of the nitrogen input to the Lake (5 µg/L), LVW accounts for nearly 60% of the phosphorus input (Paulson & Baker, 1980). This dynamic has resulted in the Upper Basin becoming more phosphorus limited, and the Lower Basin being more nitrogen limited since the impoundment of water in Lake Powell (Prentki & Paulson, 1983). Another important distinction is that Las Vegas Bay is the only portion of Lake Mead that is slightly eutrophic (nutrient rich) that impacts the ecosystem, including the organisms inhabiting the basin (Paulson, Baker, & Deacon, 1980). The cause of the eutrophication in Las

Vegas Bay, especially the inner bay, is mostly attributed to the anthropogenic phosphorus inputs of highly-treated wastewater effluent to LVW (Fig.4); however, researchers also surmise that the erosion from LVW may contribute to the heavy phosphorus loading problem (Goldman, 1976).

-3 Figure 4. Inorganic Phosphate (PO4 ) Concentration Gradient in LVB, Lake Mead (Baker & Paulson, 1980).

In a report that analyzes water-quality standards proposed by the NDEP, The City of Las

Vegas states that, “The Inner Las Vegas Bay is a small productive area in a huge unproductive lake” (City of Las Vegas, Nevada, 1987).

Another important determinant of the vertical chemical profile of the reservoir, in addition to the loss of nutrients directly discharged from the penstocks at Hoover Dam to the river below the dam, is the restrictive effect that seasonal thermal stratification has on the mixing or circulation of nutrients in the lake (Kalff, 2002). Lake Mead is a warm monomictic lake (single mixing period, annually) that becomes stratified during the summer into three distinct zones, the epilimnion, metalimnion, and hypolimnion; and undergoes turnover in the fall and continuously circulates throughout the winter

(Hoffman, Tramutt, & Heller, 1967). During stratification, a thermocline forms that acts like a barrier between the upper warmer layer of the epilimnion and the colder hypolimnion below (Hoffman et al., 1967). Fischer reports that during July through

August, the basin has a well-mixed surface layer, a main thermocline, and continuous density stratification below the level of the thermocline (Fischer & Smith, 1983). Also

according to Fischer, the main thermocline nearly always occurs at a depth of approximately 15 meters (49 feet). During the summer stratification period, no additional oxygen (O2) is supplied to the hypolimnion and the carbon dioxide concentration [CO2] increases from the utilization of O2 by benthic microorganisms; primarily bacteria, although algae, protozoa and fungi may also participate in the oxidation of organic material as it settles out of the upper water column and decomposes (Hoffman, Tramutt,

& Heller, 1967; Hargrave,1972). While the normal pattern of the dissolved oxygen (DO) concentration in lakes is a gradient that decreases from surface to bottom, Lake Mead has a negative heterograde profile that develops during thermal stratification. This means that lower DO concentrations (< 3 mg/L) are present in the thermocline than in the epilimnion or hypolimnion (Hoffman et al., 1967). Kalff relates that this phenomenon is linked with the respiration of large populations of zooplankton, and/or high numbers of heterotrophic bacteria oxidizing organic matter that settles out of the epilimnion layer above to the thermocline (Kalff, 2002; Hoffman et al., 1967). Shad and zooplankton respiration, in addition to the bacterial decomposition (oxidation) of shad excrement and other detritus, have also been suggested as causes of the metalimnetic oxygen deficiency in Lake Mead (Goldman, 1976).

During the mixing period, Lake Mead is resupplied with organic material and nutrients that are carried-up off the bottom sediments, in addition to the mixing area being re-oxygenated (Hoffman, Tramutt, & Heller, 1967). The fall overturn is induced by wind currents, in addition to convection currents that are created from the surface water cooling, becoming denser and sinking into the underlying warmer water (Hoffman,

Tramutt, & Heller, 1967). Complete vertical mixing of Lake Mead only occurs at the end

of winter, usually around February, when the Lake becomes completely destratified.

During the remainder of the year, strong vertical mixing is limited to the epilimnion,

which results from wind and heat convection currents, in contrast to the hypolimnion

layer that remains relatively undisturbed (Hannoun et al., 2005). LaBounty contributes

that a complete turnover does not occur in Lake Mead every year. Only areas of the

reservoir shallower than 60-70 meters (200-230 feet), including Las Vegas Bay, experience an annual turnover from November through early January (LaBounty & Horn,

1997). The negative effects related to the spring overturn are that (1) carbon dioxide is brought up from the lower zones that can cause fish kills, and (2) the phosphorus that is brought up from the bottom sediments may provide a nutrient source for algal blooms

(Hoffman et al., 1967).

The predominant biological process in the hypolimnion is decomposition of

planktonic organisms and detritus sedimenting from overlying strata that are oxidized via

microbial respiration. As the DO concentration becomes depleted and CO2 increases in

the benthic zone as a result of oxidation of organic material, one of the effects is the

+2 +2 -3 release of the soluble forms of iron (Fe ), manganese (Mn ) and phosphorus (PO4 )

into solution from the benthic mud, as these chemical species gain an electron (reduction)

+3 +2 +4 +2 under anaerobic conditions (i.e., Fe → Fe , Mn → Mn ) (Fig. 4) (Hoffman et al.,

-1 1967). This is in contrast to well oxygenated zones (> 4.0 mg O2 l ) where nitrification occurs

+ - and microorganisms are able to convert (oxidize) ammonium (NH4 ) to nitrate (NO3 ) in order to obtain energy for their metabolism (Kalff 2002). Settled quagga mussels are exposed to anaerobic as well as aerobic conditions, because they are mostly sessile in their benthic habitat.

Although dressenids are facultative anaerobes, they prefer to aerobic respiration due to its higher efficiency in producing energy (De Zwaan & Mathieu, 1992).

Kalff relates that the dynamics of phosphorus cycling in lake sediments are

-3 complex. He says that much of the PO4 ion concentration entering lakes is effectively removed from the water column under aerobic conditions by being precipitated as iron

(III) phosphate (Fe+3 PO4-3), and subsequent to sorption to highly insoluble iron oxide aggregates and sedimenting organic particles prevent its return to the water column even under anaerobic conditions (Kalff, 2002). In addition, between 10-75% of the potentially soluble sediment phosphorus is unavailable, because it is stored in the cells of microbes as polyphosphate granules accumulated under aerobic conditions (Kalff, 2002).

Notwithstanding, an increase in the rate of release of phosphorus from the benthic sediments has been documented in the presence of an abundance of benthic invertebrates, particularly due to their burrowing and filter feeding habits that are characteristic of bivalves (Kalff 2002).

Figure 5. Biologically Mediated Reduction Processes in the Oxidation of Organic Matter, and Oxidation of Reduced Elements (from Kalff, 2002).

The inflowing water from LVW enters LVB as a density current, high in total

+2 -3 - + + - dissolved solids (i.e., Ca , PO4 , NO3 , Na , K and Cl ) and undissociated ammonia that flows beneath the euphotic zone, at or close to the bottom of the bay, until it reaches water with a similar density (City of Las Vegas, Nevada, 1987). During the summer months when the lake is stratified, the plume lies slightly below the thermocline (Fischer

& Smith, 1983). Fisher says the reason for the “plunging inflow” is that the stream water is warmer and denser than the lake water, because of its high salinity value.

Experimental data collected during the winter suggests that the plume becomes entrained in bottom currents at a depth between 40-60 m (130-196 ft) supported by the hypolimnion, with the exception of about 10% of the volume that becomes available for

uptake in the surface waters from the upwelling caused by wave motion (Fischer &

Smith, 1983; LaBounty & Horn, 1997).

Figure 6. Limnological Monitoring Program Sampling Stations in Las Vegas Bay (source: Fischer & Smith, 1983).

The City of Las Vegas reports in their study that a “great deal of lake water” mixes into the plume as it becomes increasingly dilute from the point of the Las Vegas Wash inflow until it finally disperses, almost to the point of disappearing, between sample station 5

(middle bay area) and station 8 that is located adjacent to the SNWA intake at Saddle

Island in Boulder Basin (Fig. 6) (City of Las Vegas, Nevada, 1987). LaBounty adds that the inflow from LVW recovers dramatically from nutrient enrichment within the first 4 kilometers (2 ½ miles) it travels through LVB until it reaches approximately where sample station 5 is located (Fig. 6); however, depending on environmental conditions it can travel 8-10 kilometers (5-6 miles) and reach Boulder Basin, even traveling as far as

Hoover Dam (LaBounty & Horn, 1997). Increasing total dissolved solids may be a predictor of toxicity and sediment conductivity that is possibly associated with the abundance and distribution of benthic invertebrates (George, Abowei, Daka, 2009).

Because quagga mussels are organisms that predominately live on the sediment

(epibenthic), they are sensitive to changes in sediment and water quality and have known tolerances and preferences for environmental parameters (i.e., temperature, salinity, calcium, pH, DO, specific conductance) (Table 3).

Table 3. Environmental Parameter Requirements for Quagga

Mussels.

A study of thirty-six, freshwater lakes located in northeastern Poland (Stanczykowska,

1964) found there was no correlation between the density of D. polymorpha and the trophic status of the reservoir. However, Mackie relates the more dense populations have been reported in lakes that are less eutrophic (oligotrophic), deeper, greater surface area

(> 1,000 ha), and higher calcium and lime concentrations than in highly eutrophic bodies of water (Mackie, Gibbons, Muncaster, & Gray, 1990). Robert McMahon underscores this fact, as a result of his literature review of the physiological ecology of mussels, when

-3 - he concludes that the density of D. polymorpha declines with PO4 and NO3 enrichment

(McMahon, 1996). However, he notes that the environmental tolerance limits for D. bugensis for organic enrichment are unknown and that more research is needed.

In a study (Jones & Ricciardi, 2005) where multiple stepwise regression was used to analyze the abundance and distribution of quagga mussels collected from the St.

Lawrence River, the only environmental predictor variables kept in the model that were found to significantly affect their abundance and distribution was depth, and size of substrate (Fig. 7) that accounted for 32% and 26% of the variability, respectively (Jones

& Ricciardi, 2005). However, Jones warns that while quagga mussels show a preference for deeper depths, there is no simple relationship between depth and substrate size as a predictor of abundance. Lastly, Jones found that while the [calcium] predictor variable was not statistically significant, (1) quagga mussels failed to inhabit sites below 12 mg/L,

(2) calcium concentrations > 25 mg/L increased adult mortality, and (3) calcium concentrations > 32 mg/L reduced juvenile growth rates (Jones & Ricciardi, 2005).

Figure 7. Mean Substrate Size Preference for Quagga Mussels (from Jones & Ricciardi, 2005).

Mollusca Phylum

The Mollusca phylum is made up of an extraordinarily large group of invertebrate organisms, including clams, oysters, mussels and snails; estimates range between 50,000–

150,000 species comprise this group, making it second in size only to the Arthropoda phylum (Ruppert, Fox, & Barnes, 2004). The gastropods are the largest class of mollusks, followed by the bivalvia class. The aforementioned are the only two classes out of the seven taxa of the mollusca phylum that have evolved features different from their marine ancestors (i.e. physiological mechanism for cellular salt retention) that allows some of these exotic bivalves and gastropods to successfully invade freshwater habitats, including the zebra and quagga mussel, Asian calm and New Zealand mud snail

(Thorp & Rogers, 2011). Ruppert reports that of the nearly 8,000 species of clams, oysters, scallops and mussels in the bivalve taxonomic class of marine and freshwater mollusks, only about 15% inhabit fresh water habitats (Ruppert et al., 2004).

Figure 8. Anatomy of Generalized Mollusk (from Ruppert et al., 2004)

An important adaptation of modern mollusks is the evolution of a bivalve filter-feeding

mechanism, where the filtering gill(s) selectively removes suspended phytoplankton cells

and detritus particles from the water column, which differs from the respiratory gills of their marine ancestors that were used solely for gas exchange (Ruppert et al., 2004).

During feeding, mollusks extend their inhalant siphon, through which mineral and organic particles are brought inside the mantle cavity, to ciliary sorting fields where the organic matter is separated from indigestible mineral particles on the gills and labial palps, and in the stomach if it is ingested (Ruppert et. al., 2004). Phytoplankton and microzooplankton (i.e., ciliates, rotifers) food particles, mostly ranging in the size from 4-

45 µm, are wiped off the bivalve gill filaments by the palps and moved to the mouth,

while the indigestible particles and filtered water are discharged through the exhalent

siphon and returned to the water column as pseudofaeces (Fig. 8) (Kissman, Knoll, &

Sarnelle, 2010). The maximum capacity for filtering water is adapted to the

concentrations of phytoplankton that are present during productive times of the year,

according to Jorgensen (1996). In addition, bivalves are able to physiologically regulate

the gape of the valve, such as retracting their siphon and mantle edges, to reduce water

pumping during unfavorable environmental conditions (i.e., low DO, low/high

concentration of suspended particles) (Jorgensen, 1996). In fact, Thorp says that quagga

mussels are able to survive inhospitable environmental conditions by closing their shell,

slowing their metabolic rate and extracting ions from their shell that mostly consists of

calcium carbonate (Thorp & Rogers, 2011).

Spread and Introduction of the Quagga Mussel to the U.S.

The dreissenid mussel, Dreissena bugensis was discovered in 1890 by Nicolai

Ivanovich Andrusov, a Russian geologist, in the Bug portion of the confluence where the

Dnieper and Southern Bug Rivers flow into the Black Sea, in the Ukraine (Fig. 9)

(Karatayev, Padilla, Minchin, Boltovskoy, & Burlakova, 2007).

Figure 9. Location where Quagga Mussels Were First Discovered in the Bug-Dnieper Estuary of the Black Sea (from J. Hoyby, Water Quality Modeling). Prior to the 1940’s, this dreissenid species remained largely confined the lower portions of the Bug and Ingulets Rivers where the water temperature is higher than in the north, and in the Black Sea’s Dnieper-Bug channel where the average temperature ranges from

75.2 – 77 oF during the summer months (Mills et al., 1996). This invasive mollusk

subsequently spread throughout the entire Ukrainian portion of the Dnieper River (Fig. 9) as the construction of reservoirs occurred during the 1950’s and 1960’s, including the

Zaporozhye and Kakhovka reservoirs (Mills et al., 1996). Nalepa indicates that the confinement of bodies of water leads to the degradation of environmental conditions (i.e., reduced water velocity, stable water temperature) that are more prone to the establishment and spread of the invasive species (Nalepa, 2010).

Figure 10. Spread of Quagga Mussel in the Don River and Volga River Basin in 1981 (Source: http://www.theeasternfront.co.uk/russiantopography.htm) The first record of quagga mussels spreading from the Ukraine to Russia was in 1980.

This aquatic invader was discovered in the lower and middle portions of the Don River that is situated between the Dnieper River and Volga River (Fig. 10) (Son, 2007) which transverses the central portion of Russia and drains into the Caspian Sea. It was subsequently found in the Volga River Basin during 1981. The lifting of the grain

embargo against the former Soviet Union by the U.S. in the late 1980’s may be indirectly responsible for the introduction of this non-indigenous dreissenid to the U.S., as a result of this organism being released into the Laurentian Great Lakes when contaminated ballast water was discharged into the waterways from ocean-going barges transporting cargo (Karatayev et al., 2007).

Figure 11. Transatlantic Spread of Quagga Mussels to the Lower Great Lakes of the Central U.S. (Source: Environment Canada, http://www.ec.gc.ca).

Following the initial discovery of quagga mussels in the Lower Great Lakes of the

Central U.S. in September 1989, in the eastern basin of Lake Erie, this non-indigenous species not only became widely dispersed throughout Lake Erie, but in Lake Ontario and the St. Lawrence River, as well, by 1993 (Fig.11) (Mills et al., 1996).

Figure 12. Discovery of Quagga Mussels in the Upper Mississippi River and Ohio River (Source: Grigorovich et al., 2008).

The discovery of quagga mussels outside of the Great Lakes Region was made for the

first time during 1991, in the Illinois River (Fig. 12). This event was followed by

subsequent invasions of the Ohio River in 1994 and the Upper Mississippi River system

in 1995 (Fig. 12), albeit in reportedly low population densities ranging from 4.6 to 117.7

individuals/m2 in the Mississippi River and 1.1 to 9.1 individuals/m2 in the lower Ohio

River (Grigorovich et al., 2008). Grigorovich professes that the utilization of river systems for the transportation of goods was responsible for the infestations of quagga mussels in the Mississippi, Illinois and Ohio Rivers. By 1997, quagga mussels had spread from the southern basin in Lake Michigan into the northern part of the reservoir, and into Lake Huron; first colonizing the shallow regions, less than 150 feet in depth, and

then spreading to the colder depths as deep as 300 feet (Nalepa, Fanslow, & Pothoven,

2010). This stands in stark contrast to the European variety of quagga mussels where this

species’ habitat is limited to relatively shallow areas no deeper than 90 feet (Mills et al.,

1996). In 2005, the quagga mussel invaded and colonized Lake Superior (Nalepa, 2010).

The high concentration of suspended sediment, concentrations reported in excess of 100 milligrams per liter (mg/L) that are found in the Missouri River Basin, may be the reason

why quagga mussels have not invaded this river system (Nalepa, 2010). Excessive

suspended sediment can clog the bivalve gill filaments and interfere with the

physiological mechanisms responsible for feeding and respiration (Thorp & Rogers,

2011).

While transport by cargo barge is the ostensible vector thought to be responsible

for the spread of dressenids from Eastern Europe to the Great Lakes Region and Upper

Mississippi River system in the U.S., overland transport of a mussel infested, trailered

watercraft that was launched into Lake Mead, Nevada is the most likely mechanism

responsible for the introduction of D. bugensis into Boulder Basin sometime prior to

January 6, 2007 when quagga mussels were first discovered by scuba divers with the

National Park Service, U.S. Bureau of Reclamation (NPS USBR) (McMahon, 201). This

non-native, invasive species quickly spread to all four of Lake Mead’s basins by the end

of 2007 and its population exponential grew from its initially reported mean density of

624 + 707 mussels m-2 in Boulder Basin during 2007 (Wong & Gerstenberger, 2011).

During the 3rd quarter 2009, The Interagency Monitoring Action Plan (I-MAP) was developed by numerous governmental agencies and research institutions, including the

University of Nevada at Las Vegas (UNLV) and the National Park Service, following the

discovery of quagga mussels in Lake Mead. This comprehensive monitoring program

was implemented to evaluate and develop information related to the infestations of D.

bugensis in Lake Mead and Lake Mojave, and to spearhead an effort to mitigate the

potential impact of quagga mussels to their facilities (i.e., Southern Nevada Water

Authority, Hoover Dam) and lake ecology. An overall analysis of the IMAP data for

Boulder Basin for hard substrate samples that were collected by the NPS USBR, scuba

divers ranging from depths of 10’ to 100’, on a quarterly basis from September 2009

through 3rd quarter 2010, showed that the mean hard substrate density had increased to

8,653 mussels m-2 during 2009 (N=22), and population density slightly decreased to

7,726 mussels/m2 during 2010 (N=45) (Rainville, Moore, Wong, Gerstenberger,

unpublished data). The collection site located at Sentinel Island (CR 346.4), during the

same study period, had the highest density of quagga mussels on a hard substrate sample;

20,576 individuals/m2, with the largest mussel shell length measuring 22.8 mm (SNT-40

on 2/23/10, 40’ depth). The highest density of quagga mussels found in a benthic

sediment sample was also collected from the Sentinel Island collection site; 116,615

individuals/m2 (SNT-100 122’ depth on 2/24/2010) with the largest shell length

measuring 12.4 mm (Rainville, Wong, Gerstenberger, unpublished data). The discovery

of quagga mussels in Lake Mead, Nevada-Arizona in 2007 was consequential because for the first time it extended the U.S. range of this indigenous species west of the Mississippi

River, more than 1,750 statute miles from its first known occurrence in the U.S., in Lake

Erie during 1989.

Spread and Introduction of the Asian Clam to the U.S.

The Asian Clam, Corbicula fluminea (Müller, 1774) is another exotic species of freshwater mollusks that has invaded Lake Mead. It is indigenous to China, The

Philippines, Australia and Africa (Fig. 13) (McMahon, 2000).

Figure 13. Spread of Asian clam from China, Philippines and Australia to the Columbia River Basin, in the State of Washington (Source: McMahon, 2000).

This burrowing bivalve was first found outside its native geographical range in 1924,

when it was discovered along the coast of the Pacific Ocean in the Pacific Northwest

(Sousa, Antunes, & Guilherino, 2008). However, the first living organisms were

discovered in the Columbia River Basin, in the State of Washington during 1938 (Simard

et al., 2012). The most likely mode of their introduction to the U.S. from Southeast Asia

is by Asians who immigrated to the U.S., because Asian clams are commercially

harvested for human consumption in Japan and Taiwan (Karatayev et al., 2007). Since

its introduction to North America, the Asian clam has expanded its geographical range throughout 36 states in the Continental U.S., in addition to Hawaii (McMahon, 2000).

After its introduction in the U.S., the dispersal method was likely juvenile organisms attached to barge hulls, in the bilge water of trailered leisure boats, in dredged sand and gravel from lakes and rivers, or as fish bait (McMahon, 2000).

In the State of Nevada there was a confirmed sighting of an Asian clam at

Overton Arm, Overbrook Beach, located at the north end of Lake Mead, on July 31,

1976. The organism was reported to the central repository of the Nonindigenous Aquatic

Species (NAS) information resource for the United States Geological Survey (U.S.

Geological Survey [USGS], 1991). The Lake Mead Limnological Research Center at

UNLV conducted a benthos study during 1986-1987 that reported densities up to 1,175 individuals/m2 in Lake Mead (Peck et al., 1987). During the period, September 2009

through December 2010, the total mean density of Asian clams in 106 benthic samples

was 136 individuals/m2; the highest density found was 3,076 individuals/m2 at the LVB

7.3 sampling site (Rainville, Wong, Gerstenberger unpublished IMAP project data).

Reproductive Patterns of the Bivalvia Class

An adaptation in the bivalves that has resulted in the successful spread of a number of species far beyond their native ranges and may explain their success as an invader, is the physiological evolution of different reproductive strategies. Bivalves characteristically

had separate sexes and a planktonic (free-swimming) larval stage of development (Thorp

& Rogers, 2011). North American freshwater bivalves, with the exception of the non-

indigenous dreissenid species (D. bugensis, D. polymorpha) have largely abandoned the

planktonic life stage of their marine ancestors that made the larvae more susceptible to

predators. Additional traits that the dressenids possess that make them a successful

invader are early sexual maturation and high fecundity. Adult Dreissena are generally

able to reproduce at 6-12 months of age, when their shell length approximates 8-10 mm

(Claudi and Mackie, 1994). Over the course of a 1 ½ to 5 year lifespan, it is reported that

a single female quagga mussel is capable of producing a million or more ova during a

spawning season (Hopkins, 1990; Nichols, 1996; Karatayev, Burlakova, & Padilla,

2006). Other bivalve organisms, such as the Asian clam reproduce hermaphroditically

and brood their offspring to a fairly large juvenile stage (Thorp & Rogers, 2011).

Lifecycle and Reproductive Stages of Dreissena bugensis

D. bugensis is a gonochroic species that broadcasts their gametes directly into the water column where the ova are externally fertilized by the spermatozoa (Mackie,

Gibbons, Muncaster, & Gray, 1990). Reproduction occurs at the end of the first year or

during the second year of life, when the organism becomes a sexually mature adult (shell

length range 5 to12 millimeters) (Mackie et al., 1990; Nichols, 1996). The Dreissenid species is a sequential≃ spawner that means the sperm is released subsequent to female releasing ova into the water column, usually when the mean water temperature ranges between 53.6 – 75.2 °F (12 – 24 °C), the range necessary for moderate to good growth of the embryonic veliger to occur (Nichols, 1996; Claudi and Mackie, 1994). The first gametes are released in the spring beginning in early to mid-May and lasting until early to mid-August, and a second spawning event may occur during the summer, in August or

September (Mackie, 2000; Nichols, 1996). It is generally reported in the literature that the planktonic larvae are first observed during the month of May, while peak densities are not reached until the fall (Nichols, 1996).

Figure 14. Annual Abundance of Quagga Mussel Veligers in Boulder Basin of Lake Mead (Source: Wong, Gerstenberger, Baldwin, & Moore, 2012).

The quagga mussel population inhabiting Lake Mead is unique in that the embryonic veligers are present year-round, albeit in low densities < 1.1 veligers/L when temperatures are below 55.4 ºF (13 °C) that can occur between January and March (Fig.

14) (Wong, Gerstenberger, Baldwin, & Moore, 2012).

Figure 15. Life Cycle of Dreissena bugensis (Source: http://el.erdc.usace.army.mil/zebra).

After fertilization, the free-floating larvae live a planktonic lifestyle similar to their marine ancestors that lasts an average of 8–10 days, but up to 16-24 days post fertilization (dpf) (Mackie, Gibbons, Muncaster, & Gray, 1990; Thorp & Rogers, 2011), during which several stages of embryonic development occurs outside of the female (Fig.

15).

Figure 16. Dreissena bugensis, Veliger Larva (Source: http://el.erdc.usace.army.mil/zebra).

The metamorphosis of a trocophore (shell length 57–100 µm, 6–96 h post fertilization)

(Fig. 15) into a veliger is marked by the development≃ of a ciliated velum (Fig. 16), an

organelle that aids the organism in its locomotion and feeding activities, which is

reabsorbed at a later time in development and replaced with a siphon. The term,

“veliger” applies to all larval stages of development when the velum is present (AMEC

Earth & Environmental, 2009). Mackie shares that in the event of any disturbance, even

the slightest water turbulence, the velum can be retracted causing the organism to cease

swimming and sink. This fact is substantiated in a study by Kachalova and Sloka (1964),

who reported changes in veliger density over the course of a day; maximum densities of

veligers were collected in the morning near the surface of the lake, and subsequently at a

depth of 8-10 meters (26-32 feet) between 3:30 pm until midnight (Mackie et al., 1990).

Next, the developing embryo undergoes a straight-hinged stage (shell length 97–112

µm, 2-9 dpf) (Fig. 15) during which a D-shaped shell is secreted from its shell≃ glands

(Mackie, 2000; Ackerman, Sim, Nichols, & Claudi, 1994, Ackerman et al., 1994, U.S.

Army Corps of Engineers [USACE], 2002). The umbonal stage is the last free-

swimming larval form (shell length 112–228 µm, 2-24 dpf), before the umboned veliger metamorphosizes into a pediveliger≃ (shell length 231– 462 µm, > 10 dpf) (Fig.

15), the life stage during which a byssal apparatus (Fig. 17)≃ and accompanying muscular organ, called a “functional foot” is formed (U.S. Army Corps of Engineers [USACE],

2002; Thorp & Rogers, 2011). The foot can be extended or retracted from the bivalves

(shell valves), and is used by the organism to crawl and test hard and soft substrates in advance of settling and to sweep small particles (3-8 µm) to its gills (Mackie, 2000;

Ackerman, Sim, Nichols, & Claudi, 1994, Ackerman et al., 1994).

Figure 17. Byssal Attachment Mechanism [Source: 17(a) Farsad & Sone, 2012; 17(b) UNLV Environmental Laboratory, 2011].

During the plantigrade) stage (shell length 340–500 µm, 18-90 dpf), the successful pediveliger “settles” onto a substrate surface≃ and attaches itself via a proteinaceous adhesive apparatus (byssus) composed of individual plaques and a stem that are secreted by byssal glands in the foot of the mussel (Fig. 17) (Farsad & Sone, 2012). The juvenile and adult dressenids live a sedentary, benthic existence for the remainder their life; however, post-settlement translocation by juvenile and adult mussels over short distances to a more sustainable habitat can occur (Ackerman et al., 1994). The byssal gland at the base of the foot that forms the byssus threads used for attachment can also produce an enzyme that is capable of dissolving the byssal threads (Nichols, 1996). The post-veliger

(plantigrade) time of development is the most critical for survival; mortality rates can exceed 99% in the event that temperature shock, anoxic conditions, or inability to find a suitable substrate to settle occurs, in addition to being preyed upon by crustacean zooplankton (i.e., cyclops) or larval fish (Mackie, Gibbons, Muncaster, & Gray, 1990;

Mackie et al., 1990). While temperature, habitat depth, quantity and quality of food, and water chemistry are all factors that can affect the growth rate of invertebrates (Garton &

Johnson, 2000), G. L. Mackie contributes that for the Dreissena polymorpha species, a close relative of Dreissena bugensis, growth rates can exceed 15 mm annually to a maximum shell length that approximates 40 mm (Mackie, Gibbons, Muncaster, & Gray,

1990). The largest quagga mussel found in the samples collected for the IMAP project during the period September 2009 – December 2010, is an organism with a shell length of 28.8 mm (SNT-60 on 2.23.10) at a depth of 60 feet, where the mean population density was 8,128 individuals/m2 at this location (Rainville, Wong, Gerstenberger, unpublished

data).

Lifecycle and Reproduction of Corbicula fluminea

The Asian clam is a burrowing bivalve that has planktonic larvae that swim in the water

column. It has evolved a hermaphroditic sexual trait where the organism is capable of

both self- and cross-fertilization at sexual maturity is dissimilar from quagga mussels.

Reproduction usually occurs between 3-12 months of age when the shell length approximates 6–10 mm (McMahon, 2000). While the female gonads produce mature ova throughout the year, sperm is only produced during reproductive periods, during the spring and fall, when the mean water temperature is above 61°F (McMahon, 2000);

reproduction and shell growth is suppressed at temperature above 86 °F (Aldridge &

McMahon, 1978). Another difference is that the Asian clam broods its fertilized eggs for

approximately five days, until the zygote reaches the pediveliger stage (shell length ~ 200

µm); the embryo is released through its exhalent siphon into the water column where it

immediately settles and metamorphosizes into a juvenile (shell length ~ 1 mm)

(McMahon, 2000). Aldrich and McMahon report in their study that adult clams released

on average 350 pediveligers day-1 during an approximate one-hundred day reproductive

period during the spring, yielding a mean annual fecundity of nearly 70,000 pediveligers

adult-1 yr-1 (Aldridge & McMahon, 1978). Asian clams burrow in the benthic sediments,

unlike quagga mussels that are epibenthic, and are proficient at both pedal and filter

feeding (Thorp & Rogers, 2011). Similar to the dressenids, C. fluminea is able to

produce a byssus, usually a single byssal thread from its exhalent siphon. However, uses

this as a long mucous dragline to re-suspend itself in the water column where the

organism can be passively carried downstream to a more hospitable environment (i.e.,

more favorable nutritional conditions) (McMahon, 2000). McMahon shares that adult

clams also are able to utilize this same technique to disperse themselves from areas where there are high clam densities, although this is an infrequent occurrence (McMahon,

2000).

Mollusca Phylum, Gastropoda Class of Invertebrates

The North American, class of Gastropoda is divided into two phylogenic groups of organisms that are predominately benthic: the caenogastropod snails, which are distinguished by the presence of an internal gill, are the largest and most diverse group and occupy a wide range of habitats (i.e., benthic epifunal or burrowers, pelagic drifters, active swimmers) (Colgan, Ponder, Beacham, & Macaranas, 2007); and the pulmonate snails, whose ancestral gills have been replaced by a highly vascularized saclike modification of their mantle cavity (Thorp & Rogers, 2011; Ruppert et al., 2004). Their habitat is limited to shallow water, because they respire air. In the 1987 Research Report on Benthic Invertebrates (Peck et al., 1987), six different genera of Gastropoda were identified in Lake Mead: These include Ferrissia, Fossaria parva, Gyraulus parvus,

Physella gyrina, Physella virgata, Hydrobiidae, Pyrgulopsis and Tryonia clathra (spring snail in the hydrobiidae family). The 1987 benthic survey (Peck et al., 1987) was conducted almost a decade before the invasion of the nonindigenous New Zealand mud snail, a member belonging to the hydrobiidae family that was discovered in the Colorado

River, below the Glen Canyon Dam in 1995 (Cross et al., 2010). According to Thorpe, the mudsnail is a grazer that feeds on detritus and organisms (i.e., algae attached to rocks and other substrates). Its densities in the Snake River of Idaho have reduced primary productivity by 90%, in addition to altering the flow of inorganic nutrients in the

reservoir (Thorp & Rogers, 2011). Thorp says that the ecological functioning of aquatic

communities is threatened by the New Zealand mud snails that he characterizes as one of

the “worst offenders” of all the invasive species.

Figure 18. Gastropoda Species found in Lake Mead, IMAP Project, 2009-2010 (UNLV Environmental Sciences Laboratory).

Benthic samples collected for the IMAP project during the period September 2009 –

December 2010, revealed that while P. antipodarum (Fig. 18B) was found in all of Lake

Mead’s Basins, Overton Arm and LVB, the only other species of gastropods found were

two species of Planorbidae (figs. 18c and 18d above) and a single species belonging to the

Physidae family (Fig. 18A). Only low numbers of both species were found. This is far

from the species richness reported in Lake Mead just a decade earlier (Peck et al., 1987)

and is illustrative of the threat a biotic invasion by nonindigenous organisms pose to

native populations.

Spread and Introduction of the New Zealand mud snail to the U.S.

The New Zealand mud snail, Potamopyrgus antipodarum (Gray, 1843), is a mollusk that is native to New Zealand. It belongs to the Gastropoda class that is composed of small aquatic snails and slugs. Transoceanic transport of veligers in ships ballast water is commonly cited as the dispersal mechanism (Alonso & Castro-Diez, 2008). This organism can be can also be transported by attaching itself to the extremities on birds

(i.e., feathers), as well as traveling through the digestive tract of its predators (i.e., fish, birds) and passing through their feces unharmed (Mackie, 2000). It was first found in the

U.S. in the Snake River, State of Idaho during 1987, where densities ranging from 17,550

– 500,000 snails/m2 were reported (Alonso & Castro-Diez, 2008). This was followed by its discovery in Lake Ontario in 1991, followed by the Columbia River basin in Oregon during 1997 (Alonso & Castro-Diez, 2008). The mud snail has successfully invaded most of the streams and rivers in the Western U.S., with the exception of New Mexico

(Cross et al., 2010). Since the mid-1980’s, densities in some reservoirs have reached ¾ million individuals/m2 (Engineer Research and Development Center, U.S. Army [ERDC

- U.S. Army], 2007).

The New Zealand mud snail was first detected in the Colorado River in benthic samples collected from the Colorado River, below the Glen Canyon Dam in 1995. At first in low densities that grew to a mean of 59,900 individuals/m2 between 1997-2006

(Cross et al., 2010).

It was discovered in Las Vegas Bay of Lake Mead (AZ-NV) during 2008 by a research

team from the University of Nevada Reno (Wong, Rainville, Austin, & Gerstenberger,

2010). In Lake Mead, the mean population density was determined to be 950

individuals/m2 from an analysis of 106 benthic sediment samples collected between

December 2009 – September 2010 (Rainville, Wong, Gerstenberger, unpublished IMAP

Project data). The largest density was 24,980 individuals/m2 found at the Calville Bay

CR 351.7 sampling station on March 17, 2010 (CLV-300, 307’ actual depth).

Lifecycle and Reproduction of Potamopyrgus antipodarum

Potamopyrgus antipodarum reaches a maximum shell length of 6-7 mm in indigenous habitats, compared to 12mm length in native habitats (Alonso & Castro-Diez,

2012) over its 4-5 year lifecycle (Thorp & Rogers, 2011). The average shell length for

snails found in the Western U.S. ranges from 4-5 mm (U.S. Army Corps of Engineers,

2007). The results of an analysis of 106 benthic sediment samples collected from Lake

Mead during December 2009 – September 2010 yielded similar findings. The shell

lengths ranged from 410 µm – 6.84 mm and the mean shell length was 4.03 mm

(Rainville, Wong, Gerstenberger, unpublished IMAP Project data). The U.S. Army

Corps reports that the adult female reaches sexual maturity when its shell length

approximates 3 mm, which occurs between 3-6 months of age (U.S. Army Corps of

Engineers, 2007). Dioecious, sexual male and female populations are very rare in the

Western U.S.(U.S. Army Corps of Engineers, 2007). Every reported population in the

U.S. consists of parthenogenic females (undergoes asexual reproduction without

fertilization) that are born with developing embryos in their reproductive systems

(ovoviviparous), while males are rare (Thorp & Rogers, 2011; U.S. Army Corps of

Engineers, 2007).

Figure 19. Parthenogenic Female, Mud Snail Releasing Embryos (U.S. Army Corps of Engineers, 2007).

An individual female is able to brood between 10-230 embryos (Fig. 19) and offspring can be produced year round with up to six generations annually (U.S. Army Corps of

Engineers, 2007; Alonso & Castro-Diez, 2012).

1987 Lake Mead Research Report on Benthic Invertebrates

The 1987 study conducted by the UNLV Lake Mead Limnological Center is important, because it is the first research that attempted to determine the baseline densities and distribution of benthic organisms inhabiting Lake Mead, Nevada. Whole lake sampling

(Fig. 21) using a standard ponar dredge was conducted on a quarterly basis, from October

1986 through July 1987. The objectives of this study were to (1) establish baseline densities of benthic invertebrates inhabiting Lake Mead, and (2) evaluate the distributions of benthic organisms in relation to existing habitat conditions and limnological characteristics of Lake Mead (Peck et al., 1987). For analysis the organisms were grouped as follows: Chironomidae family (Insecta class), Oligochaeta subclass (Annelida phylum), Corbicula (Mollusca, Bivalvia class), attached fauna, and miscellaneous invertebrates (Peck et al., 1987). The 1987 study was performed two decades before the invasion of Lake Mead by the New Zealand mudsnail and the quagga mussel, but ten years after the introduction of the Asian clam.

One of the noteworthy findings reported in this study was that Asian clams were found to a depth of 90 meters (295 feet) in the outer LVB, which according to Peck is

“extremely deep and the greatest depth every reported for a gastropod” (Peck et al.,

1987). Peck notes that there was a steep decline in the Asian clam population density between April and July, from 925 to 125 individuals/m2. She attributes this to the heavy predation of the young clams following the summer spawning season. Another study finding was if the total Asian Calm biomass was removed from the calculations, the densities for the remainder of benthic invertebrate organisms collected remained fairly constant throughout the year (Peck et al., 1987).

Table 4. Benthic density of Asian Clams, Lake Mead, NV, 2009-2010 (Rainville, Wong, Gerstenberger, IMAP Project, Unpublished data).

During the IMAP Study of benthic sediments during 2009-2010, the largest Asian Clam

density was 3,075 individuals/m2 in LVB at the mouth of Boulder Basin, at a depth of

71.7 feet (LVB 7.3D sample on 5.25.2010) (Fig. 20). However, Asian Clams were

collected up to 382 feet deep at Sentinel Island (Sampling Station CR 346.4) (Table 4)

(Rainville, Wong, Gerstenberger, unpublished IMAP Project data).

Figure 20. IMAP Sampling Stations – LVB 7.3, Sentinel Island CR 346.4 (from Fish.n.Map Co.)

Figure 21. Sampling zones, 1987 limnological study of Lake Mead (Peck et al., 1987).

Peck reports that the highest benthic densities of invertebrate organisms were found at

Zone 9 (Fig. 21) near the input from the Colorado River, and remained high in Zone 8

(Fig. 21). The densities drastically dropped in the Virgin Basin and Overton Arm (Zones

4 and 5) (Fig. 21). The population density increased again in Boulder Basin (Zone 3)

(Fig. 21), but only to about 60% of that reported for Zone 9 (Fig. 21) (Peck et al., 1987).

In Peck’s study, the numbers reported for the narrow canyons area (located between Zone

3 and 4) (Fig. 21) were considered outliers due to the high densities that were found at these locations, and were omitted.

Table 5. Benthic Density of Invertebrates by depth, 1987 Limnological Study (Peck et al., 1987).

An important finding of Peck’s study is the ecological distribution of invertebrate

organisms. She found that the mean population density of benthos averaged 1200/m2

between a 5-40 m depth, increased to a maximum of 2,300/m2 at 70 meters, and

decreased to 1,700 m2 at a depth of 120 meters (Table 5) (Peck et al., 1987). However,

an analysis of the overall results showed a lack of any clear trend in benthic densities compared to depth.

CHAPTER 3

METHODOLOGY

Institutional review board (IRB) approval was unnecessary; however, a scientific

research and collecting permit needed to be obtained from the U.S Department of

Interior, National Park Service (see Appendix 2, Scientific Research and Collection

Permit). Dr. David WH Wong was the principal investigator for this project.

Site Selection

LVB was specifically chosen as the study site, because only embryonic veligers

were found in the water column after the quagga mussel invasion. No juvenile stage or

adult organisms were found inhabiting the bottom sediments. A possible cause suggested

for this was the disproportionate phosphorus and nitrate loading to LVB. While organic

phosphorus and nitrate are assimilated by plants for the purposes of energy transfer and

are essential in the synthesis of nucleic acids and proteins, enriched nutrient loading on

freshwater ecosystems increases phytoplankton growth. The increase in primary production and high respiration rates results in periods of hypolimnetic oxygen depletion

(Beeby & Brennan, 2008, Smith, Tilman, & Nekola, 1999). It is theorized that the elevated levels of inorganic compounds in LVB may be attributable to the lack of settled populations of juvenile and adult quagga mussels.

Study Design

Settled quagga mussels were collected from the benthic sediment and embryonic veligers from the water column during the late spring mixing period when the water temperature

in the epilimnion layer should be reaching 18°C (Fig. 14). This is an optimal time to sample embryonic veligers at the beginning of their spring spawning, before the commencement of the summer stratification of Lake Mead. The mean water temperature ranged between 14.7-15.2°C on the actual sample collection dates listed in Table 6.

Veligers were collected using a plankton tow net (Wildco® veliger net, Wildlife Supply

Company, Yulee, FL), and settled organisms from the benthic sediment using a standard

9” x 9” ponar grab (WILDCO® Ponar® Grabs, Wildlife Supply Company, Yulee, FL).

See Benthic Sediment Collection Field Methods, page 61; and Method of Veliger Sample

Collection, page 57 for standard operating procedures that were followed). The samples were collected to determine the population density of quagga mussels at each site in

LVB. In addition, separate sediment samples were also collected at each sampling location so the chemical and physical properties could be analyzed.

Table 6. Veliger and Settled Quagga Mussel Sample Collection Sites in LVB, Lake Mead

Transect sampling was the method selected to study the environmental gradient of

chemicals in LVB. Three vectors were established to identify sampling locations; one

along the deeper mid channel deeper mid-channel where the water depth ranges from 25 feet to nearly 220 feet at the mouth of boulder basin, and two lateral transects that run parallel to the shoreline (Fig. 22).

Figure 22. Location Sampling Sites in Las Vegas Bay, Lake Mead

A total of twelve-(12) sampling locations on the transect lines using a topographic map, an area covering approximately 5 2/3 miles in length by 2 2/3 miles wide (Fig. 22). In

the field, a topographical map was used to locate landmarks in and around LVB for

determining the locations along each vector where the sample should be collected. The

actual latitude and longitude coordinates, and depth of each sample point were

determined using a Garmin-GPSmap 4208 Chart plotter (Table 6). Lastly, the actual

coordinates were plotted on a map using Arc View Version 10 – ArcGIS software to

identify the exact map locations where the samples were collected and to verify that the

points chosen actually fell along each of the predetermined transect lines (Fig. 22). The

sampling locations were also plotted to compare the spatial relationship of the

ponar/veliger sampling sites chosen for this project to the permanent sample locations

that the SNWA and government agencies collect their water quality samples at (i.e., LVB

3.5, LVB 7.5). A ponar grab sample using a standard dredge was used to collect the

benthic sediment samples at the same time as the veliger tow.

Method of Veliger Sample Collection and Analytical Techniques

The EPA Standard Procedure for Zooplankton Sample Collection and Preservation Field

Procedure (SOP LG402) was followed for collecting dreissenid veliger samples

(Environmental Protection Agency [EPA], 2005). A plankton tow net (Fig. 22) was used to collect veliger samples at 5m, 30m and 50m depths. However, it was not possible to perform all of the tows at each of the sampling sites, because the actual depths in the

LVB range from 26.4’ to 219.8’. While the 5m (16’ depth) tow could be performed at all twelve locations, the 30m (98’ depth) tow could only be conducted at sites #4, #7, #11, and the 50m (164’ depth) tow at sites #7 and #11 that are located on the middle transect where the water is deeper at the mouth of Boulder Basin.

Figure 23. Standard Plankton Tow Net (from Environmental Protection Agency [EPA]).

For each sample site, a vertical plankton tow (mesh size 63 µm) with a 0.0314m2 net area opening (Wildco® veliger net, model 33-E28, Wildlife Supply Company, Yulee, FL) was

2 used to filter a maximum volume of water (VM) determined by the formula, VM = π · r

· Hi · # tow (where r = the radius of the net orifice; and Hi = depth of the tow).

Table 7. Total Volume of Water Filtered (VL) at Predetermined Tow Depths.

Table 7 lists the values that were used to calculate the maximum amount of water volume

filtered at each of the respective tow depths. Prior to collecting each sample, a Nalgene®

specimen bottle was pre-labeled with the pertinent site information, including the depth

of tow, GPS coordinates, date and collection time. In addition, the information was

entered on a field data sheet. The samples were collected from a boat by carefully

lowering the plankton net into the water and allowing it sink to a depth of 5m, 30m, or

50m, based on the depth of the water at the sampling site, and then manually retrieving

the veliger net using a hand-over-hand technique at a rate of approximately 0.5 m/s (1.5 ft/s). Each time the net was retrieved, any organisms adhering to the sides of the net were washed into the cod end by lowering the net back into the water while keeping its opening above the water surface, and gently lifting the net from the water several times.

After each tow was completed, the collection cup was removed from the cod end of the apparatus and the contents of the sample transferred to a 1 L Nalgene® specimen bottle.

Any residue from the sides of the collection cup was rinsed using distilled water

dispensed from a squirt bottle. Each sample was preserved by adding approximately 250

mL of 95% ethanol to the collection bottle to achieve a final concentration of ~ 25%.

The samples were placed in a cooler on ice and following the field excursion,

immediately transported to the UNLV Environmental Health Laboratory where they were

stored at 4°C for sample preparation and analysis.

Each veliger sample was processed and organisms enumerated according to the

guidelines set forth in Appendix V, Detailed Protocols for Quagga Mussel Monitoring by

the Interagency Monitoring Action Plan (I-MAP) (Wong, Turner, Gerstenberger, &

Miller, 2011).

The suspended contents in the 1000 mL specimen bottle, in addition to the deionized (DI)

water used to rinse the container, were transferred to an Imhoff Settling Cone (Cole-

Parmer, Vernon Hills, IL) and the sample allowed to settle for 45 minutes. A stirring rod

was used along the edges of the settling cone to loosen any material that might be present

on the sides of the cone and the contents allowed to settle for an additional 15 minutes.

Next, a 50 mL aliquot of the settled sample was drained from the bottom of the Imhoff

Cone into a centrifuge tube. Laboratory counting was performed by mixing the contents

in the centrifuge tube and pipetting a 1 mL aliquot into a Sedgwick-Rafter counting cell.

A cover slip was place onto the counting cell and examined under a Carl Zeiss SteREO

Discovery.V8 microscope (Carl Zeiss Microscopy, LLC, Thornwood, NY) under cross polarized light. The integrated camera controls allowed digital images of the organisms in the sample to be photographed and the image analyzed (i.e., precise measurements, determination of embryonic life stage) using Zeiss AxioVision 4 image analysis software

(Carl Zeiss Microscopy, LLC, Thornwood, NY). Two replicates were performed for each sample and the concentration of each subsample was calculated (veliger/mL).

The equation shown in Figure 24, listed in Appendix V, Detailed Protocols for

Quagga Mussel Monitoring in the Interagency Monitoring Action Plan (I-MAP) (Wong,

Turner, Gerstenberger, & Miller, 2011), was used to calculate the density of veligers

(Ci:veliger/L). The 50 mL volume subsample was divided by the total volume of water filtered during the plankton tow and multiplied by 1000 [50 mL (centrifuge tube)/volume of filtered water (L) ·1000 (1 mL aliquot → 1 L)] to calculate the concentration of veligers in a one liter from the subsample in the centrifuge tube,.

Figure 24. Equation for determining the concentration of veligers in one liter sample.

Benthic Sediment Collection Field Methods and Analytical Techniques

The EPA Field Sampling Guidance Document EPA 905-R-03-002 (LG406; Revised

2007, March 2002) was followed for the collection of the surface sediment samples in the

Las Vegas Bay, Lake Mead. The sampling was conducted from a boat, using a full size,

9’ x 9” standard ponar dredge sampler (0.052 m2) (WILDCO® Ponar® Grabs, model

1728-G40,Wildlife Supply Company, Yulee, FL Model 1728-G40) to collect the lake

bottom sediments, to determine the horizontal distribution of variables for the

invertebrate population of organisms inhabiting the benthic sediments. Prior to deploying

the sampler, pertinent information including the water depth, temperature and GPS

coordinates for the location being sampled was recorded on a field data sheet. The ponar

grab was released and allowed to sink under its own weight until it struck and penetrated

the soft sediment surface, causing the trip bar to release and the sampler to close. If the

jaws of the dredge failed to fully close or engage the bottom surface, the sample was

discarded and the process was repeated. Upon recovery, the dredge was opened and the

sediment emptied into an EPA-recommended, U.S. No. 30, 500 µm mesh wash frame

(Wildco® model 188-E50, Wildlife Supply Company, Yulee, FL) where it was treated as

a bulk sample. The fine sand and sediment were washed out using very low pressure

water spray and the remainder of the sample was carefully washed into a pre-labeled

sample container. Preservation of the biological samples was accomplished by adding

enough aqueous solution (mixture of 95% ethanol and Rose Bengal dye) to cover the

contents in the collection jar. If more than one container was used, each container label

was accordingly numbered (i.e., 1 of 2, 2 of 2). Rose Bengal dye was chosen as a stain to

increase the efficiency of the laboratory sorting process, because it stains organic material

light pink while leaving the mineral particles (i.e., sand, silt, gravel) uncolored. Each

sample container was placed in cooler containing ice packs and transferred to the UNLV

Environmental Health Laboratory following the survey excursion. No preservative was added to a separate set of benthic soil sediment samples that were collected for chemical analysis. These were temporarily stored in an ice chest while on the survey excursion and immediately transferred to a secure freezer, where the samples were stored in a freezer at -20 °C at the UNLV Environmental Laboratory.

Each soft substrate sample was initially processed by removing all macroscopic

organisms from the specimen container with forceps and placing these in a petri dish

where they could be taxonomically identified and their shell length measured with a

caliper. The samples contained large quantities of sand and gravel, so a sieve

fractionation process was employed using a Tyler screen scale (16 stainless steel mesh

screen with 1 mm openings; Wildlife Supply Company, Yulee, FL) that was placed

directly on top of a 32 mesh sieve (32 stainless steel mesh screen with 500 μm openings;

Wildlife Supply Company, Yulee, FL) so any smaller organisms that washed through the

larger sieve would be trapped in the 32 mesh screen sieve. Small portions of the

sediment were removed from the sample jar using a spatula and scooped into the screen

scale and the material was carefully washed. The material that remained in both sieves

was gently removed with a spatula and transferred to the counting chamber of an

invertebrate counting tray and a teasing needle used to gently sift through the contents

under a dissecting microscope. The majority of organisms were able to be removed with

forceps and transferred to a petri dish. However, organisms smaller than 5 mm that were

unable to be removed from the counting chamber without crushing the shell, mostly the

Asian clams, were photographed in place and the shell measurements determined using

Zeiss AxioVision 4 image analysis software.

100% of the invertebrate organisms in the samples were taxonomically identified,

enumerated and measured, with the exception of a large sample collected from sites #5.

Separating large numbers of invertebrate organisms from the sand and gravel in each

sample is a very time consuming process, so a wet-weight subsampling protocol was used

for the sample collected at site #5 to reduce processing time that still provided a reliable

estimate of the whole sample density of organisms. Finally, only organisms with intact

bivalves were counted and included in the density calculations.

Chemical Analysis of Benthic Sediment

The benthic sediment samples collected from LVB were transferred to the UNLV

Environmental Soil Analytical Laboratory (ESAL), Department of Geoscience under the direction of Dr. Yuanxin Teng, Laboratory Manager. The following physical and chemical analyses were completed for each soil sediment sample:

A1: pH

A2: Soils salinity (electrical conductivity) ECe assessment

A26b: Determination of soil texture (laser)

The procedures in the Soil Survey Laboratory Methods Manual (2004) were followed by

the ESAL for the analyses of the sediment samples (Burt, 2004).

Statistical Analysis Methods

The majority of samples contained a large number of organisms (sample size >

30) (Fig. 25); however, the following was unknown: (1) whether the data are drawn from

populations with equal variances, or (2) whether the sampled population was normally distributed. Settled quagga mussels are sessile organisms and each sample was independently drawn; therefore, each group was treated as a simple random sample collected from their respective populations. The sample sizes for the inner, middle and outer bay populations studied were 190, 326, and 1,424 organisms, respectively (Fig. 27).

The data for sites #4, #5, and #9 were eliminated from the analyses to lessen the

chance of skewed results. These observations are considered data outliers, because (1) no

organisms were collected from site #4, (2) the density calculated for site #5 is extremely

large (401,826 individuals/m2, N = 20,895), and (3) the location of sampling site #9 is not

closely approximated to the northernmost lateral transect. The high population density at

site #5 is most likely an anomaly, because this site is located directly below a steep cliff

face where the quagga mussels may fall from above. For the statistical analyses, three

groups (inner, middle, outer bay) were defined containing three samples each, based on

the distance of the sampling station from LVW (Table 8).

Table 8. Sample Stations Included in Data Analyses

Station Distance (Km) Group Location QM Density from LVB (individuals/m2) Station_01 5.6315 Inner Bay 76.9

Station_02 7.1616 Inner Bay 1096.2

Station_03 7.9662 Inner Bay 250

Station_06 9.5756 Middle Bay 942.3

Station_07 9.5756 Middle Bay 1711.5

Station_08 9.5756 Middle Bay 3615.4

Station_10 11.7457 Outer Bay 1019.2

Station_11 11.7457 Outer Bay 5557.7

Station_12 11.7457 Outer Bay 15307.7

The Two-Sample Kolmogorov-Smirnov (K-S) test was performed using SPSS version 20 statistical software to assess the normality of the populations collected from three areas in LVB. This normality test was used to compare the cumulative distributions of the quagga mussel (QM) shell lengths between two groups. The results of the test indicate that the underlying structure of the three population distributions is not uniform

(Fig. 25). The normality of each sample collected from LVB was also assessed by using the K-S One-Sample Test. The cumulative distributions of shell length measurements for the individual samples, with the exception of those collected from sites #5 and #6, were also non-normal distributions (Fig. 26). Due to the non-normally distributed data sets

and small number of LVB collection sites, non-parametric statistical methods were used

to evaluate each hypothesis. With normally distributed data, the mean is equal to the median

and the mean is used as the measure of center. However, if our data is skewed, then the

median is a much better measure of center.

Non-parametric tests are rank tests where each observation from the data set are

placed in order or ranked. This procedure was used for investigating the distribution of

values for the variables. The PROCRANK procedure was the nonparametric method

used to analyze the ranks of the independent variables. A two-sample t-test is applied to the ranks, is the equivalent of a Wilcoxin rank sum test that uses the t-approximate to determine the significance level. Next, a MULTTEST procedure was performed that applies t-tests to the median scores to contrast the two groups, which is the equivalent of a median test

(http://support.sas.com/documentation/cdl/en/proc/65145/HTML/default/viewer.htm#n1p e8iekjrnnfin1hdw34ik02tcw.htm). Ranked data was used instead of the raw data, due to the small number of samples and to correct for outliers in the data.

The CORR procedure was used to compute the Spearman’s rho correlation

coefficient to determine the association between the independent variables and the

outcome variable (quagga mussel density). The Spearman rank-order correlation was the

nonparametric measure chosen to determine the association and probability associated

with these statistics. This Spearman procedure is the non-parametric equivalent of the

Pearson product-moment correlation coefficient, and requests a table of Spearman correlation coefficients based on the ranks of the variables. The correlations measure the strength of a linear association and range from -1 to 1

(http://support.sas.com/documentation/cdl/en/procstat/63104/HTML/default/viewer.htm#

procstat_corr_sect004.htm).

The following statistical methods were followed to analyze the research questions:

Research Question I – Is the distance of the habitat from LVW a physical factor that

affects the density of quagga mussels in LVB?

Hypothesis Testing

H01: There is no significant difference in the density of the quagga mussels inhabiting the

inner, middle and outer areas of the Las Vegas Bay.

HA1: The mean density of the quagga mussels inhabiting LVB is the lowest in inner bay

compared to the middle and outer bay area.

Statistical Tests

(1) A Spearman’s rho correlation coefficient (rs) will be calculated using paired

observations (Xi, Yi) to test for correlation and slope, where Xi = distance of the population from LVW and Yi = QM density.

Research Question II – The relationship of invertebrate density to habitat depth.

Is water depth an environmental factor that affects the density of settled quagga mussels

in LVB?

Hypothesis Testing

H02: Depth is not a significant physical environmental factor associated with the density

of settled, adult and juvenile quagga mussels in LVB.

HA2: Habitat depth is a significant physical factor that is positively correlated with

density of settled quagga mussels inhabiting LVB.

Statistical Tests

(1) A Spearman’s rho correlation coefficient (rs)’s will be calculated using paired

observations (Xi, Yi) to test for correlation and slope, where Xi = habitat depth and Yi =

QM density.

Research Question III – The density of settled quagga mussels with respect to benthic

sediment chemistry.

Are there significant differences in benthic sediment, pH and specific conductance (Eh)

in LVB that affects the density of quagga mussels?

Hypothesis Testing

H03: No significant differences exist in benthic sediment pH or specific conductance in

LVB.

HA3: There are significant differences in benthic sediment pH and specific conductance in

the inner, middle and outer portions of LVB, and sediment pH and electrical conductivity

is correlated with the density of settled quagga mussels.

Statistical Tests

(1) A PROC MULTTEST procedure using ranked data for pH and Eh values at each site

will be used in the calculation to determine if significant differences exist in the sediment

samples collected from the inner, middle and outer bay areas.

(2) If significant differences exist, a Spearman’s rho (rs) statistic will be calculated to test

for the correlation of quagga mussel density with pH and Eh values.

Research Question 4 – Density of Settled quagga mussels and substrate selection

Are there differences in the benthic sediment composition that affect the density of

quagga mussels in LVB?

Hypothesis Testing

H04: No significant differences in benthic sediment (percentages of sand, silt, and clay

sized particles) exist in the inner, middle and outer areas of LVB.

HA4: Significant differences in the composition of benthic substrate exist in LVB. The

area with the highest percentage of sand positively correlated with a highest density of

settled quagga mussels.

Statistical Tests

(1) A PROC MULTTEST procedure using ranked data for percentages of clay, silt, and

sand collected at each sampling site will be used to determine if there is a significant

differences in the composition of sediment composition in the inner, middle and outer

bay.

(2) If significant differences exist, a Spearman’s rho (rs) statistic will be calculated to test

for the correlation of quagga mussel density with benthic sediment composition.

The data listed in Table 9 was used in the statistical analyses of the hypotheses. A

standard error of mean is not reported, because only a single sample from each location

was analyzed.

Table 9. Analyses Results of Benthic Sediment - Chemical and Physical Variables Used in Statistical Calculations

CHAPTER 4

RESULTS

Quagga mussels (QM) were found throughout LVB. The mean density of organisms calculated, inhabiting the inner, middle and outer bay, was 474.4, 2,089.7 and

7,294.9 individuals/m2, respectively (Table 10). A standard error of mean is not reported

for population density, because only one sample was collected from each location. The

population density was the smallest in the inner bay and largest in the outer bay area.

The sample size at each sampling site, population density, and range of shell lengths is

reported in Figure 25.

Figure 25. Quagga Mussel Data for Organisms Collected from LVB, 2011

Table 10. Descriptive Statistics for Settled QM Populations in LVB

The mean shell lengths of QM found in the inner, middle and outer bay samples were

4939.8 µm + 243.6 µm, 6873 µm +285.7 µm, and 3612 µm + 85.2 µm, respectively

(Table 10). The median shell length was the smallest in the outer bay area that is related to the large number of juvenile mussels found at this location. The cumulative distributions of shell length measurements for each sampling location are shown in

Figure 26.

Figure 26. Histograms - Cumulative Distributions of Shell Length Measurements, LVB Collection Sites 1-12

The sample analyses also show that the only veligers found in LVB were in the outer bay, with the exception of one organism found at sampling site #5, which may be associated with a high density of juvenile staged organisms in the benthos near the mouth of

Boulder Basin. The Box and Whisker Plots for the QM shell lengths are depicted in Fig.

27 that show smallest and largest values, first quartile (median length) and data outliers.

Figure 27. Box and Whisker Plots, QM Shell Lengths – Inner, Mid, and Outer LVB Population data.

The Mann Whitney U-Test procedure was used to test for differences between the three

groups of QM populations, and indicates that the populations collected from the inner,

middle and outer bay area significantly differ from one another with respect to the

measurements of their shell lengths (Figure 28). The Box and Whisker plots shown in

Figure 27 show that the quagga mussel population inhabiting the mid bay area has the

largest median shell length (5811.1 µm) and the population of mussels inhabiting the

outer bay have the smallest median shell length (2219.3 µm).

3612 µm

Populations Mann-Whitney U- Asymp. Sig. K-S Asymp. Sig. Statistic

Inner vs. Middle Bay 3264.0 < 0.001 < 0.001

Inner vs. Outer Bay 31085.0 < 0.001 < 0.001

Middle vs. Outer 135121.0 < 0.001 < 0.001 Bay

Figure 28. Mann Whitney U-Tests and Kolmogorov-Smirnov Test Data, QM Populations Inhabiting LVB.

Research Question 1

Is the distance of the habitat from LVW a physical factor that affects the density of

quagga mussels in LVB?

A Spearman’s rho correlation coefficient (rs) was calculated using paired observations

(Xi, Yi) to test for correlation and slope, where Xi = distance of the population from LVW

and Yi = QM density. The (rs) value was 0.6901. The z-score is equal to 0.6901 √ 9-1 =

1.951897 that yields a right-tailed, p-value equal to 0.0255. This analysis indicates that

there is a significant association between the density of quagga mussels inhabiting LVB

and distance the population is located from LVW towards Boulder Basin. The reliability

2 2 factor (rs) is equal to (0.6901) that indicates that distance accounts for 47.6% of the

variability in the population samples.

Research Question 2

Is depth an environmental factor that affects the density of settled quagga mussels in

LVB?

A Spearman’s rho correlation coefficient (rs) was calculated using paired observations

(Xi, Yi) to test for correlation and slope, where Xi = habitat depth and Yi = QM density.

The (rs) value was 0.8667. The z-score is equal to 0.8667 √ 9-1 = that yields a one-sided, p-value equal to 0.0142. This analysis indicates that there is a significant association between the density of quagga mussels inhabiting LVB and water depth. The reliability

2 2 factor (rs) is equal to (0.8667) that indicates that distance accounts for 75.1% of the

variability in the population samples.

Research Question 3

Are there significant differences in benthic sediment, pH and specific conductance (Eh)

in LVB that affects the density of settled quagga mussels?

A PROC MULTTEST procedure using ranked data for pH and Eh values calculated for

each site was used to determine if significant differences exist in the sediment samples

collected from the inner, middle and outer bay area. An overall significance level of α =

.05 was set for hypothesis testing. The p-values calculated that contrast the inner, middle

and outer bay data for sediment_pH indicate that the pH values of the samples collected

from LVB are not statistically different (Table 11). Similarly, the p-values that contrast

the inner, middle and outer bay data for sediment_EC indicate that the values do not

significantly differ throughout LVB (Table 12). Because the pH and EC values were not

statistically different from each other in the samples collected from LVB, it could be

determined whether there variables of interest are related to QM density.

Table 11. Variable Tabulations, Sediment pH

Location NumObs Mean Contrast p-Values

Inner Bay 3 7.94 Inner vs. Middle 0.4647

Middle Bay 3 7.52 Inner vs. Outer 0.1579

Outer Bay 3 7.31 Middle vs. Outer 0.9823

Table 12. Variable Tabulations, Sediment Electrical Conductivity

Location NumObs Mean Contrast p-Values

Inner Bay 3 1.3343 µS/cm Inner vs. Middle 0.8017

Middle Bay 3 1.0067 µS/cm Inner vs. Outer 0.3226

Outer Bay 3 2.6790 µS/cm* Middle vs. Outer 0.2287

* Note the Sediment_EC value for the sample collected at Station_12 was 5.628 µS/cm, which skewed the raw data for the outer bay area mean. The non-parametric statistic procedure using ranked data corrected for this outlying value.

Research Question 4

Are there differences in the benthic sediment composition that affect the settlement and density of quagga mussels?

The PROCRANK procedure was performed to analyze the ranks of an independent variable. to determine if there is a significant statistical difference in the composition of benthic sediment collected from the LVB sampling sites. The results indicated that there no significant differences in the percentage of clay, sand and silt that make compose the sediment in the inner, mid and outer LVB (Table 13). Because the composition of the benthic sediment was not statistically different throughout LVB, it could be determined whether this variable of interest is related to QM density.

Table 13. Variable Tabulations, Sediment Composition (% Clay, % Silt, % Sand)

Variable NumObs Mean Contrast p-Values

Clay 3 3.64% Inner vs. Middle 0.935

3 2.92% Inner vs. Outer 0.972

3 3.05% Middle vs. Outer 1.0

Sand 3 53.68% Inner vs. Middle 0.617

3 39.98% Inner vs. Outer 0.764

3 43.28% Middle vs. Outer 0.993

Silt 3 42.68% Inner vs. Middle 0.642

3 57.10% Inner vs. Outer 0.802

3 53.67% Middle vs. Outer 0.993

Lastly, the dreissenid veliger samples that were collected from LVB were analyzed.

While veligers could be collected from all twelve sites at the 5m (16 ft. depth), the 30m

(98 ft. depth) and 50m (164 ft. depth) tows could not be performed at the shallower

sampling sites. The 30m tow was only conducted at sites #4, #7, and #11, and the 50m

tow at sites #7 and #11. The analysis yielded the following results:

Table 14. Veliger Data for Samples Collected in Las Vegas Bay

No veligers were found in the samples collected from the depth 5m depth, with the

exception of one organism that was collected from sampling site #5 (Table 14). The

only other veligers collected from LVB were at the 30m and 50m depths at site #11,

which is located at the mouth of Boulder Basin along the mid-channel transect.

Embryonic veliger stages of development identified at this site include trocophore, D- shaped and umbonal life stages. At the 30m depth the sample composition was 18% trocophore, 24% D-shaped and 58% umbonal stages, and 29% trocophore, 30% D-shaped and 41% umbonal stages at the 50m depth (see Appendix 3, Las Vegas Bay Veliger Data,

Sampling Location #11).

A statistical procedure was performed using the CORR procedure that yielded a

Spearman’s rho correlation coefficient. The p-values listed in Table 14 indicate a lack of association between the independent variables (veliger density) and the outcome variable

(settled quagga mussel density).

Table 15. Veliger and QM Abundance, LVB

Life Form Contrast p-

Value

Veliger Inner vs. Middle Bay 0.544

Veliger Inner vs. Outer Bay 0.302

Veliger Middle vs. Outer Bay 0.569

Settled Inner vs. Middle Bay 0.564

Settled Inner vs. Outer Bay 1.0

Settled Middle vs. Outer Bay 0.564

CHAPTER 5

DISCUSSION

Despite the fact that no settled juvenile or adult quagga mussels were found in

LVB during early 2009, samples collected from sites LVB 3.5 and LVB 7.3 in the fall of

2009, and March, May and September of 2010 show that quagga mussels had spread to the bay. The mean density of quagga mussel populations at the LVB 3.5 and LVB 7.3 sampling stations were 138.5 + 2,040.9 and 1179.5 + 184,445.5 individuals/m2

respectively. In the current study, quagga mussels were also found throughout LVB.

The mean density of organisms calculated inhabiting the inner, middle and outer bay is

474.4, 2,089.7 and 7,294.9 individuals/m2, respectively. The population density has

increased by 342% at the LVB 3.5 and 618% at the LVB 7.3 collection site, since the

period 2009-2010. While quagga mussels are the most abundant, New Zealand mud

snails and Asian clams were also collected throughout LVB, indicating that all three

invasive mollusks are able to coexist (see Appendix I, Distribution of Invasive Mollusks

in LVB, 2011).

In the water column, veligers were only found at the 30m and 50m depths at collection station #11, located in the deepest portion of LVB (219.8 ft.) at the mouth of

Boulder Basin (Table 14). No veligers were collected from the middle or inner bay.

Warren Turkett, Environmental Biologist at the SNWA, was contacted regarding information related to veliger sampling in LVB by the U.S. Bureau of Reclamation during spring of 2011 in order to validate the data that was collected during March 26 –

May 1, 2011 for this study. Mr. Turkett provided data from the Lower Colorado River

Water Quality (LCRWQ) Database for the veliger collection 10m and 40m depth tows,

conducted at sampling sites LVB, LVB 4.15 and LVB 6.7 for March through April, 2011

(Fig. 29). For the period between March 22 and April 19, 2011, the mean number of veligers/L was 2.76 and 1.43 for the LVB 4.15 and LVB 6.7 collection sites, respectively.

The veliger abundance plot in Fig. 29 indicates that the LVB veliger density was low at the time the samples were collected for this study, and peaked during June and July,

2011. The mean veliger density in LVB for the period 2009-2010, a subset of the settled

QM population in 2011, is listed in Figure 30. A comparison was made between the veligers in the water column and settled population in the benthos using this data.

Figure 29. Zooplankton Data for LVB 6.7 Site (source: Southern Nevada Water Authority)

Distance of Collection Site from LVW Mean Veliger Density (#/L)

(Km)

0 1.2

5.6315 3.9

7.9662 7.2

10.7826 4.4

11.7457 5.1

Figure 30. Mean Veliger Abundance for LVB, 2009-2010 (source: Kent Turner, NPS)

The veliger abundance data for LVB (Fig. 30) was compared with the settled QM population density gathered in the current study. The scatter plots depicted in Fig. 31 show that the veliger and settled quagga mussel densities may be related. However, no significant statistical correlation was found between veliger density and the density of settled juvenile and adult organisms at a particular location in LVB (Table 14).

Figure 31. Abundance of Veliger and Settled QM Populations in LVB

Also, a statistically significant correlation was found when comparing the density

of settled juvenile and adult quagga mussels inhabiting LVB, with distance of the

population from LVW (Hypothesis I). The lowest density of quagga mussels was found

at collection station #1 (76.9 individuals/m2) nearest to LVW, and the highest density was

at station #12 (15,307.7 individuals/m2) located at the mouth of Boulder Basin. The analysis determined that 47.6% of the variability in the samples collected was due to this

parameter. Factors other than water quality may be responsible for the low density of quagga mussels in the inner bay, and larger density of mussels found closer to the mouth of Boulder Basin. For instance, the abundance and quality of available resources (i.e. phytoplankton) may be associated with the density of the population. While no data was available in the SNWA Lower Colorado River Database for algal chlorophyll values that provide a reasonable estimate of algal biomass, data was available for Secchi depth disk that has a positive relationship with total nitrogen and algal chlorophyll (Bachmann,

Horsburgh, Hoyer, Mataraza, & Canfield, 2002). The water turbidity is mostly attributable to phytoplankton (Kalff, 2002). While the focus of the current study was on the potential chemical and physical parameters of the benthic sediment, water secchi disk depth may be also a factor that impacts the abundance of quagga mussels at a particular site. The water becomes more transparent with increased distance from LVW towards

Boulder Basin, using Secchi depth data collected from LVB sites during 2009-1010 (Fig.

32) (Spearman Correlation, p-value = 0.008).

Distance_Km from LVW x̅ Secchi Depth

0 0.47

5.6315 2.32

7.9662 5.89

10.7826 8.83

11.7457 9.23

Figure 32. Mean Secchi Depth Values in Las Vegas Bay, 2009-2010

Water depth was also found to be a statistically significant variable correlated with the density of settled juvenile and adult quagga mussels in LVB. Quagga mussels first colonized the shallower habitats less than 90 feet deep, and then adapted to colder and deeper depths to 300 feet, after colonizing the Great Lakes Region in the U.S. The highest density of mussels were found at station #11 (219.8 feet deep; 5,557.7 individuals/m2) and at station #12 (120 feet deep; 15,307 individuals/m2) where there may be less competition for resources by other species, due to the water temperature and periodic hypolimnetic oxygen depletion at deeper water depths. The analysis determined that 75.1 % of the variability in the samples collected in this study was due to water depth. However, Jones warns in her study (Jones & Ricciardi, 2005) that while quagga mussels show a preference for deeper depths, there is no simple relationship of depth as a predictor of abundance. The water column temperature that generally becomes colder at deeper depths, may be may be an environmental parameter associated with quagga mussel density. While the current study focus was on variable in the benthic sediment, water temperature data for LVB was analyzed from the SNWA Lower Colorado River

Database, for the period 2009-2010. While a horizontal water temperature gradients exists in LVB, where the water column is cools the greater the distance from LVW towards Boulder Basin (Spearman Correlation, p-value = 0.0417), no correlation was found with the density of quagga mussels collected in this study (Spearman Correlation, p-value = 0.3095) (Fig. 33).

Distance_Km from LVW x̅ Water_Temp (°C)

0 22.26

5.6315 19.91

7.9662 17.54

10.7826 15.62

11.7457 16.7

Figure 33. Mean Water Temperature Values in Las Vegas Bay, 2009-2010

It was also hypothesized that pH and electrical conductivity (EC) of the benthic sediment would be correlated with the abundance and distribution of quagga mussels, because pediveligers could selectively show a preference for sediments with a higher pH and lower EC (dissolved solids) values when settling. While this may be the case, the pH and Eh values of the benthic sediment samples collected from LVB revealed that the differences in pH and Eh values were statistically insignificant.

Lastly, it was hypothesized that the composition of the substrate (% clay, % sand,

% silt) would affect the abundance and distribution of quagga mussels inhabiting LVB.

A predictive model for size of substrate and QM settlement was developed by Jones

(Jones, Ricciardi, 2005) that shows quagga mussels prefer larger sized substrates (i.e., gravel vs. sand, sand vs. silt, and silt vs. mud). Although, the IMAP Study that was conducted from 2009-2010, indicates that the highest density of quagga mussels were found in the soft benthic sediments (116,615 individuals/m2) at the Sentinel Island (CR

364.4) station, compared to the population density of mussels attached to the surfaces of larger rocks (20,576 individuals/m2) at the same site. While quagga mussels may preferentially select substrates when settling with a higher percentage of sand particles that are larger than those in silt and clay, no statistical relevant differences in the benthic sediment composition was found in the inner, middle and outer LVB. Therefore, no conclusion could be drawn between quagga mussel abundance and composition of the benthic sediment.

Limitations of this study include the small number of samples collected, the sample collection taking place over a period of a month, that replicates were not collected, and the time span over which the study was conducted. Kalff (Kalff, 2002) states that while the vast majority of limnological studies are carried out over a period of less than a year. The intent was to complete the sample collection over one day.

However, the ponar dredge became lodged on the cliff face at sampling site #5 and had to be cut loose and later retrieved by divers. Inclement weather also hampered planned excursions when high winds prevented boating on Lake Mead. Finally, time was a factor that accounted for replicate samples not being collected. Collecting samples with the ponar dredge proved arduous, because the dredge frequently closed prematurely when it struck the bottom sediments. Each sample took numerous attempts using the dredge before an adequate sample was collected.

The study findings include the existence of three invasive mollusks population in

LVB that are able to coexist with one another. This focus of the study was on the benthic sediment characteristics in LVB that may be associated with the density of quagga mussels. Only water depth and the distance from LVW towards Boulder Basin were the

only factors found to be positively correlated with population density. The sediment pH and electrical conductivity was found to be similar throughout LVB, so no statistical comparisons could be made related to QM density regarding these parameters.

Furthermore, there may be numerous water column variables (i.e., nutrients, temperature, and transparency) that affect the variables that were studied. For instance, water column temperature could be a factor that is associated with veliger abundance and the successful settlement of juvenile mussels in the benthos. This study attempted to use a field setting to study the environmental factors that affect the density and distribution of quagga mussels, as opposed to an experiment conducted in a controlled laboratory setting. While sediment quality is believed to impact sessile organisms that are epibenthic, a flaw in the study was to make the assumption that the physical and chemical characteristics of the benthic sediment significantly differ in LVB. Future studies should include sediment samples collected from the other basins in Lake Mead. Finally, the quagga mussel, New

Zealand mud snail and Asian clam are mollusks that have successfully invaded and spread throughout Lake Mead. It was outside the scope of this study; however, the interaction between all three invasive species should be studied to determine the factors that may limit the survival and reproduction of each competitor species.

APPENDIX 1

DISTRIBUTION OF INVASIVE MOLLUSKS IN LVB, 2011

Station Distance QM_Density AC_Density NZMS Density (Km) (Individuals/m2) (Individuals/m2) (Individuals/m2) Station_01 5.6315 76.9 0 0

Station_02 7.1616 1096.2 365.4 38.5

Station_03 7.9662 250 596.2 38.5

Station_04 7.9662 0 0 0

Station_05 7.9662 401826.9 0 115.4

Station_06 9.5756 942.3 115.4 0

Station_07 9.5756 1711.5 19.2 615.4

Station_08 9.5756 3615.4 1057.7 19.2

Station_09 10.7826 1461.5 57.7 96.2

Station_10 11.7457 1019.2 461.5 788.5

Station_11 11.7457 5557.7 57.7 1038.5

Station_12 11.7457 15307.7 230.8 3038.5

QM = quagga mussel, AC = Asian clam, NZMS = New Zealand mud snail.

APPENDIX 2

SCIENTIFIC RESEARCH AND COLLECTION PERMIT

APPENDIX 3

LVB VELIGER DATA, SAMPLING SITE #11

30M Depth Replicate #1 Replicate #2

Length (um) Developmental Stage Length (um) Developmental Stage 140.99 Umbonal 109.7 D-shape 123.38 Umbonal 84.4 Trocophore 122.76 Umbonal 126.77 Umbonal 143.52 Umbonal 72.61 Trocophore 109.7 D-shape 101.04 D-shape 129.29 Umbonal 109.23 D-shape 163.53 Umbonal 113.96 D-shape 157.53 Umbonal 81.77 Trocophore 147.06 Umbonal 80.18 Trocophore 111.22 D-shape 109.7 D-shape 179.29 Umbonal 88.71 Trocophore 176.39 Umbonal 64.94 Trocophore 161.08 Umbonal 93.09 Trocophore 213.22 Umbonal 114.97 Umbonal 149.14 Umbonal 168.87 Umbonal 227.91 Umbonal 136.54 Umbonal 204.42 Umbonal 148.1 Umbonal 120.96 Umbonal 126.77 Umbonal 135.03 Umbonal 218.11 Umbonal 143.61 Umbonal 116.74 Umbonal 126.77 Umbonal 117.95 Umbonal 86.36 Trocophore 147.41 Umbonal 107.81 D-shape 93.23 Trocophore 109.7 D-shape 79.21 Trocophore 111.68 D-shape 93.23 Trocophore 96.83 D-shape 87.24 Trocophore 216.63 Umbonal 106.49 D-shape 184.59 Umbonal 104.05 D-shape 204.67 Umbonal 140.62 Umbonal 122.65 Umbonal 109.23 D-shape 122.65 Umbonal 113.96 D-shape 88.56 Trocophore 195.74 Umbonal 177.48 Umbonal 125.96 Umbonal 109.23 D-shape 89.71 Trocophore

142.08 Umbonal 115.64 D-shape 129.58 Umbonal 112.59 Umbonal 118.71 Umbonal 175.15 Umbonal

190.55 Umbonal 114.97 D-shape 109.47 D-shape 121.81 Umbonal 111.22 D-shape 108.52 D-shape 143.52 Umbonal 123.8 Umbonal 190.89 Umbonal 123.8 Umbonal 89.71 Trocophore 131.85 Umbonal 194.82 Umbonal

97.41 Trocophore

LVB Veliger Data, Sampling Location #11

50M Depth Replicate #1 Replicate #2

Length (um) Developmental Length (um) Developmental Stage Stage 158.83 Umbonal 220.17 Umbonal 104.54 D-shape 107.81 Umbonal 76.23 Trocophore 83.17 Trocophore 80.18 Trocophore 122.65 Umbonal 111.22 D-shape 87.24 Trocophore 83.94 Trocophore 106.61 D-shape 91.42 Trocophore 200.61 Umbonal 97.94 Trocophore 89.71 Trocophore 213.22 Umbonal 104.05 D-shape 111.22 Umbonal 112.59 D-shape 116.74 Umbonal 111.33 D-shape 73.13 Trocophore 77.41 Trocophore 101.93 D-shape 162.03 Umbonal 78.89 Trocophore 157.53 Umbonal 107.57 D-shape 100.4 Trocophore 79.21 Trocophore 89.86 Trocophore

125.45 Umbonal 71.17 Trocophore 123.49 Umbonal 93.5 Trocophore 142.08 Umbonal 184.59 Umbonal 100.66 D-shape 117.07 Umbonal 109.23 D-shape 104.42 D-shape 81.29 Trocophore 220.17 Umbonal 124.73 Umbonal 152.21 Umbonal 144.23 Umbonal 101.42 D-shape 82.55 Trocophore 158.75 Umbonal 84.4 Trocophore 133.11 Umbonal 101.42 D-shape 118.38 Umbonal 205.92 Umbonal 170.09 Umbonal 212.98 Umbonal 188.72 Umbonal 104.54 D-shape 189.87 Umbonal 80.18 Trocophore 89.71 Trocophore 98.46 Trocophore 96.88 D-shape 76.74 Trocophore 86.21 Trocophore 100.02 D-shape 96.35 D-shape 115.86 D-shape 68.97 Trocophore 105.52 D-shape 68.22 Trocophore 108.52 D-shape 110.75 D-shape 137.95 Umbonal 93.23 Trocophore 104.05 D-shape 87.24 Trocophore 136.26 Umbonal 82.55 Trocophore 64.14 Trocophore 82.59 Trocophore 102.68 D-shape 187.01 Umbonal 81.77 Trocophore 104.05 D-shape 157.53 Umbonal 180.57 Umbonal 113.39 D-shape 129.09 Umbonal 84.4 Trocophore 101.42 D-shape 108.88 D-shape 140.99 Umbonal 108.41 D-shape 102.05 D-shape 93.23 Trocophore 154.81 Umbonal 106.49 D-shape 152.55 Umbonal 96.35 Trocophore 106.61 D-shape 166.34 Umbonal 197.25 Umbonal 81.14 Trocophore 167.42 Umbonal 97.41 D-shape 118.38 D-shape 109.23 D-shape 165.57 Umbonal 96.88 D-shape 101.42 D-shape 117.07 Umbonal 104.42 D-shape 172.12 Umbonal 106.97 D-shape 136.07 Umbonal 101.42 D-shape

100

125.24 Umbonal 133.88 Umbonal 83.17 Trocophore 90.29 D-shape 101.42 D-shape 218.11 Umbonal

206.92 Umbonal 229.15 Umbonal 127.08 Umbonal 172.71 Umbonal 201.95 Umbonal 136.54 Umbonal 115.64 Umbonal 111.68 D-shape 243.88 Umbonal 131.85 Umbonal 122.76 Umbonal 198.81 Umbonal 101.42 Trocophore 115.75 D-shape 86.36 Trocophore 100.4 Trocophore 114.97 Umbonal 82.55 Trocophore 96.88 Trocophore 154.23 Umbonal 209.94 Umbonal 193.1 Umbonal 108.88 D-shape 71.71 Trocophore 104.05 D-shape 97.41 Trocophore 111.68 D-shape 118.27 Umbonal 209.08 Umbonal 114.97 Umbonal 129.29 Umbonal 100.66 D-shape 84.4 Trocophore 76.06 Trocophore 140.99 Umbonal 215.26 Umbonal 97.54 Trocophore 89.71 Trocophore 179.43 Umbonal 68.97 Trocophore 105.52 D-shape

101

108.88 D-shape 66.12 Trocophore 115.64 D-shape 118.38 Umbonal 179.86 Umbonal

102.05 D-shape

102

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Mead, Nevada-Arizona US [Journal Article]. Aquatic Invasions, 7(1), 7-19.

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VITA

The Graduate College University of Nevada, Las Vegas

Scott Allen Rainville

Degrees:

Bachelor of Science, Zoology, 1982 California State University, Long Beach

Publications:

Comeau S, Rainville S, Baldwin W, Austin E, Gerstenberger S, Cross C, Wong WH. 2011. Susceptibility of quagga mussels (Dreissena rostriformis bugensis) to hot-water sprays as a means of watercraft decontamination. Biofouling 27(3):267-274.

Thesis Title: The Abundance and Distribution Pattern of Quagga Mussels in the Las Vegas Bay of Lake Mead, Nevada and Potential Correlation with Environmental Parameters

Thesis Examination Committee:

Co-Chairperson, Shawn Gerstenberger, Ph.D. Co-Chairperson, David Wong, Ph.D. Committee Member, Mark Buttner, Ph.D Graduate Faculty Representative, Jacimaria Batista, Ph.D.

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