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Sep 11, 2016 - ABSTRACT: Fecal indicator bacteria (FIB) are leading causes of impaired surface waters. Innovative and environmentally appropriate best...
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Escherichia coli Reduction by Bivalves in an Impaired River Impacted by Agricultural Land Use Niveen S. Ismail,†,‡,§ Jake P. Tommerdahl,†,‡ Alexandria B. Boehm,*,†,‡ and Richard G. Luthy*,†,‡ †

Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States ReNUWIt Engineering Research Center, Stanford University, 473 Via Ortega, Room 117, Yang & Yamazaki Environment & Energy Building, Standford, California 94305, United States

‡

S Supporting Information *

ABSTRACT: Fecal indicator bacteria (FIB) are leading causes of impaired surface waters. Innovative and environmentally appropriate best management practices are needed to reduce FIB concentrations and associated risk. This study examines the ability of the native freshwater mussel Anodonta californiensis and an invasive freshwater clam Corbicula f luminea to reduce concentrations of the FIB Escherichia coli in natural waters. Laboratory batch experiments were used to show bivalve species-specific E. coli removal capabilities and to develop a relationship between bivalve size and clearance rates. A field survey within an impaired coastal river containing both species of bivalves in an agricultural- and grazing-dominated area of the central coast of California showed a significant inverse correlation between E. coli concentration and bivalve density. An in situ field spiking and sampling study showed filtration by freshwater bivalves resulting in 1−1.5 log10 reduction of E. coli over 24 h, and calculated clearance rates ranged from 1.2 to 7.4 L hr−1 bivalve−1. Results of this study show the importance of freshwater bivalves for improving water quality through the removal of E. coli. While both native and invasive bivalves can reduce E. coli levels, the use of native bivalves through integration into best management practices is recommended as a way to improve water quality and protect and encourage re-establishment of native bivalve species that are in decline.

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INTRODUCTION Fecal contamination is a major water quality concern worldwide.1 The presence of fecal indicator bacteria (FIB) such as Escherichia coli in surface waters can indicate the presence of waterborne pathogens2 and can correlate to adverse health outcomes during contact recreation3,4 as well as limitedcontact recreation such as boating and fishing.5 Both point and nonpoint sources can contribute FIB to surface waters. In rural environments, grazing and agricultural activities can contribute fecal material to runoff, which can adversely impact receiving water quality.6 In the United States (US), the Clean Water Act (CWA) Section 303(d) requires states to designate water bodies as impaired if their quality does not conform to water quality criteria.1 The United States Environmental Protection Agency (USEPA) promulgated water quality criteria for FIB after epidemiology studies documented a correlation between their concentrations and health risks in swimmers.7 For fresh waters, E. coli concentrations should be less than 126 colony forming units (CFU)/100 mL.7 Water not conforming to this standard may be unsafe for many activities, including fishing and irrigation. Similar water quality rules exist for countries around the world.8 In the US, total maximum daily loads (TMDLs) aim to rehabilitate impaired water bodies by designating maximum pollutant discharges or inputs beyond which water quality standards cannot be met. For point sources, permitting procedures and zero discharge requirements are sufficient © 2016 American Chemical Society

control measures to reduce inputs; however, nonpoint sources can be more difficult to control. Best management practices (BMPs) are intended to reduce pollutant loads, particularly from runoff. Examples of structural BMPs include bioswales, bioretention ponds, and sand filters. A recent meta analysis indicates that structural BMPs are generally not reliable for reducing FIB concentrations in runoff.9 Development of innovative and cost-efficient BMPs that are effective for removing FIB from natural waters is needed. Reduction of waterborne FIB can occur through physical mechanisms of attachment and sedimentation, sunlight inactivation, and predation by other microorganisms and invertebrates.10 Bivalves have been shown to be effective in reducing FIB in natural surface waters.11−13 Studies have also demonstrated that bivalves can use bacteria as a food source and can inactivate E. coli upon ingestion.12,14−17 Further study under both field and laboratory conditions is needed for a more complete understanding of how effective bivalves can be in removing E. coli from natural waters. The Pajaro River, our field site location, is an impaired coastal river in California with high levels of E. coli. The Pajaro River is home to two different freshwater bivalve species, the native mussel Anodonta californiensis (order Unionida) and the Received: Revised: Accepted: Published: 11025

June 17, 2016 September 3, 2016 September 11, 2016 September 11, 2016 DOI: 10.1021/acs.est.6b03043 Environ. Sci. Technol. 2016, 50, 11025−11033

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Environmental Science & Technology

Bivalve Collection and Preparation for Laboratory Experiments. A. californiensis and C. f luminea were collected from the Pajaro River (Permit SCP-12858) for laboratory batch experiments. Prior to experimental use, the bivalves were depurated for a minimum of 48 h in Pajaro River water filtered through a tangential flow filtration system with a 30 kDa poresize membrane (Pall Corporation, Port Washington, NY). Laboratory Batch Experiment System Setup. Laboratory studies were completed to determine the impact of bivalve species type and size on clearance rate of E. coli. Batch experiments were performed by placing individual bivalves for 12 h in 400 mL beakers containing 200 mL of Pajaro River water spiked with E. coli. The volume of the water used in the experiments fully covered all sizes of bivalves. All experiments were conducted with water at 15 °C which was collected from the Pajaro River within 24 h of the start of the experiment. Pajaro River water was first filtered using a tangential flow filtration system as described above and then spiked with indigenous bacteria, including E. coli, collected from the Pajaro River using a modified EPA method.44,45 Briefly, ∼750 L of Pajaro River water was passed through presterilized positively charged NanoCeram VS2.5−5 cartridge filters (Argonide Corporation, Sanford, FL) and then eluted from the filter under positive pressure with N2 gas using a solution of 3% beef extract (BD and Co, Franklin Lakes, NJ) and 3% Tween80 (Fisher Scientific, Chicago, IL). Collection of indigenous bacteria was completed within 6 h of the start of each laboratory experiment, and water was collected from the same location for each experiment, as shown in Figure S4. The collected Pajaro River water was filtered prior to spiking with indigenous bacteria to limit variability in experimental results that could result from heterogeneity of different types of particulate matter in the water collected on different days. Experimental beakers contained a single clam (C. f luminea) or mussel (A. californiensis), and a beaker containing spiked water with an empty bivalve shell served as the negative control to test for settling or die-off of E. coli during the experiment. These single bivalve experiments were replicated 4 times to obtain a total of 30 data points. Results from a total of 15 A. californiensis and 15 C. f luminea of varying sizes were included in this study. A. californiensis lengths ranged from 13 to 73 mm, and C. f luminea lengths ranged from 8 to 24 mm. Water samples (1 mL) were extracted from each beaker every hour for 12 h, and E. coli was enumerated using EPA method 1603.46 Prior to membrane filtration, samples were serially diluted (10−5 to 10−3 depending on sample time point) as necessary using a phosphate buffer to achieve plate counts in the 10−100 CFU range. Bivalve shell length, measured as the greatest anterior to posterior dimension, was correlated to soft tissue dry weight (excluding shell) at the end of each experiment. Bivalve soft tissue was removed from the shell and placed in a 50 °C oven for a minimum of 48 h to obtain dry weight. General Field Study Site Characteristics. The field study was completed in a 1.34 km reach of the Pajaro River, 14.4 km from where the river drains into Monterey Bay (36°54′49″N, 121°42′28″W) (Figure S1). The study was conducted during the dry season from September through November 2014, and minimal rainfall occurred for the study duration (less than 1.5 in.).47 Due to the lack of rainfall, this section of the river consisted of distinct pools of water separated by portions of a dry riverbed with no surface flow of water between pools. Due to this separation and lack of surface flow between the pools in this portion of the river, we treated the 17 pools as replicate

invasive clam Corbicula f luminea (order Veneroida). Like many other native freshwater mussels, A. californiensis is in decline and considered to be of conservation concern.18−20 In North America, approximately 70% of native freshwater mussels are at risk of extinction.21−23 C. f luminea is an invasive Asian species that is widely introduced and often abundant.24 The ecological impact of C. f luminea is unclear, although some studies suggest that it may negatively affect native bivalves.25−29 In the portion of the Pajaro River studied, A. californiensis remains the dominant bivalve species, but colonization of C. f luminea has commenced in parts of the river. In some parts of the world, colonization and subsequent domination of freshwater systems by C. f luminea has already resulted in disruption of ecosystem processes and caused negative economic impacts through macrofouling of raw water systems.24,30−34 While both species of bivalves have the potential to improve water quality through filter feeding, A. californiensis can provide this ecosystem service without posing potential negative environmental impacts associated with invasive species. Thus, quantifying the ecosystem service of FIB reduction provided by A. californiensis could provide incentive to protect this declining freshwater mussel species as well as other threatened and endangered freshwater mussel species. In this study, we couple laboratory and field experiments to examine the potential of bivalves A. californiensis and C. f luminea to remove waterborne E. coli. This study evaluated the effects of bivalve species, size, and density on indigenous E. coli concentrations under environmentally relevant conditions through kinetic uptake studies. Both laboratory and field experiments used a batch system approach with natural river water and field-collected indigenous bacteria. The laboratory batch system experiments were used to calculate clearance rates of E. coli by individual C. f luminea and A. californiensis of varying sizes. Field sampling data and experiments were used to determine the correlation between bivalve density and E. coli concentration. This is the first study to quantify E. coli removal and calculate uptake rates of E. coli by undisturbed bivalves under field conditions. The data from this study inform the use of bivalves as an innovative BMP to meet TMDLs.

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EXPERIMENTAL SECTION Study Site Description. The Pajaro River, located in the central coast of California, drains an approximately 3400 km2 watershed and empties into Monterey Bay35 (Figure S1). Land use in the watershed is dominated by agriculture and grazing, with 76% of the watershed used as rangeland or for irrigated agriculture.36−38 In the lower portion of the Pajaro Valley watershed, which is the focus of this study, 47% of land surface is covered by irrigated agriculture.36 In the entire Pajaro Valley, 84% of freshwater resources are used for irrigation.39,40 Land runoff from agriculture and grazing land represents a pollution source to the river41 along with untreated urban stormwater runoff.36 Precipitation in the region is typically limited to the rainy season during winter and spring, and there is virtually no precipitation during summer and fall.41 The Pajaro River was listed as America’s most endangered river in 2006 due to water quality and quantity issues42 and is currently listed as an impaired water body pursuant to Section 303(d) of the CWA because of high levels of a number of pollutants. TMDLs for the Pajaro River have been established by the California State Water Resource Control Board for several parameters, including fecal coliforms/E. coli.43 11026

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Environmental Science & Technology Table 1. Summary of Pool Characteristics and E. coli Clearance Rates from the 24 h Sampling Experimenta

percent of bivalve density (%) pool

volume (L)

hypothetical clearance rate (L hr−1 bivalve−1)

bivalve density (no. bivalves m−2)

A. californiensis

C. f luminea

A B C

1930 1230 3400

1.2 ± 0.6 4.7 ± 1.9 7.4 ± 2.1

8.7 2.8 4.1

91.7 90.7 91.3

8.3 9.3 8.7

a

The percent contribution values refer to the number of bivalves of each species counted during surveys. The error associated with the clearance rates is one standard deviation of the clearance rates as determined from the standard deviation obtained from the first-order decay model curve fit.

tests (Figure S6) was utilized to report an estimated bivalve density on the basis of soft tissue biomass (g DW bivalves m−2). 24 h Field Study. Four pools (three treatment and one control) in close proximity to each other (less than 275 m apart) with similar tree cover and variable bivalve densities were chosen for a 24 h spiking study conducted between November 8 and 9, 2014 (Figure S4) to further examine the relationship between bivalve presence and E. coli removal. Each pool was spiked with indigenous bacteria to simulate elevated levels experienced after a runoff event. Indigenous bacteria were collected from the same pool location within the field site, as previously described above for the laboratory studies (Figure S4). The concentrated indigenous bacteria were mixed in the pool using gentle wave action created by repeatedly pushing an inflatable ball in a vertical motion from opposing ends of the pool. Pools were remixed every 3 h using this method. After the addition of the concentrated indigenous bacteria, the concentration of E. coli in the pools increased approximately an order of magnitude over background to between 103 to 104 CFU/100 mL. The control pool had a very low number of bivalves ( 0.05, data not shown), suggesting pools did not exhibit vertical stratification at the time of sampling. The mean (± SD) of E. coli concentrations in wet sediment was 4.9 ± 2.3 CFU g−1 and did not correlate to bivalve density (Spearman’s, p > 0.05, Figure S8). The concentration of E. coli in the analyzed sediment is orders of magnitude lower than levels found in other surveyed freshwater systems67−69 and levels found in the water column in our system. Hence, sediment is not considered a reservoir of E. coli that contributes to the total concentration of E. coli in each of the pools. Mean (± SD) water temperature (14.9 ± 0.64 °C), DO (3.4 ± 1.1 mg L−1), pH (7.6 ± 0.2), and salinity (0.8 ± 0.05 ppt) were consistent among the pools and were not correlated to bivalve density or E. coli concentration (Spearman’s and ANOVA, p > 0.05, Figure S9). Correlation of Bivalve Density to E. coli from Field Surveys. Spatial surface water samples were collected from all 17 pools on 3 separate days followed by bivalve surveys. The

(1)

where Co is the E. coli concentration at t = 0, Ct is the E. coli concentration at a given time point, t is time in hours, k is the uptake rate in hr−1, and Cres is the residual E. coli concentration as represented in the tail portion of the model; when tailing was not observed, Cres = 0. Also, k-values (kother) were obtained for the control beakers to account for changes in E. coli concentration due to processes other than bivalve uptake such as E. coli growth and death as well as adsorption to bivalve shells. Control beakers showed a slight increase in E. coli concentration over the experimental time course (Figure S7), suggesting growth in the presence of only the bivalve shell. kbivalve was calculated as k − kother to account for the apparent growth observed in the controls. The clearance rate of E. coli was defined as the volume from which E. coli was cleared (removed) per unit time. Clearance rate was determined by multiplying kbivalve value by the volume of the system (Figure 1)54−56 (see Supporting Information text for more details, Figure S7). The clearance rate is a value often used to quantify bivalve filtration rates.55,57,58

Figure 1. Bivalve clearance rate as a function of soft tissue dry weight. The solid line represents the power law fit to the data, and the dashed lines represent 95% confidence bands of the fit. The error bars represent one standard deviation of the clearance rates as propagated from the standard deviation obtained from the first-order decay model curve fit. The listed fit equations include the standard deviation of the fit reported as the ± values in the equation.

Allometric equations relating clearance rate to soft tissue dry weight were developed for the two bivalve species (Figure 1). The clearance rate was significantly correlated to the soft tissue dry weight for both species (A. californiensis: Spearman’s ρ = 0.9, p < 0.05; C. f luminea: Spearman’s ρ = 0.7, p < 0.05). As shown in Figure 1, on a dry weight basis, C. f luminea had significantly higher (p < 0.05) clearance rates than A. californiensis. This result agrees with previous studies that 11028

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Figure 2. E. coli concentration as a function of bivalve densities for 17 distinct pools sampled 3 times. The error bars represent one standard deviation for five to seven spatial samples on September 30, 2014 and three spatial samples on October 7 and 9, 2014.

concentration of E. coli was significantly negatively correlated to the bivalve density (number of bivalves m−2 and g DW bivalves m−2) (Figure 2, Spearman’s ρ = −0.6, p < 0.05). The correlation suggests that the presence of bivalves leads to reduced E. coli concentrations in the pools. However, other sources and sinks may also affect E. coli concentrations, including inputs and losses from unquantified subsurface flows70 and growth due to high dissolved organic carbon and phosphorus nutrient concentrations71,72 and sinks from sunlight73,74 and predation.71,72 In contrast to the overall result, two pools showed a low bivalve density and low E. coli concentration, which may be related to the above listed factors. A significant negative correlation between bivalve density and E. coli was observed for the remaining 15 pools (Figure 2). To better understand the relationship between E. coli concentration and bivalve density, a field experiment was conducted to further test the relationship between bivalve presence and E. coli concentration. Indigenous E. coli Uptake by Bivalves in the Pajaro River Field Test. Concentrated, indigenous bacteria (including E. coli) were inoculated into four pools to test for a relationship between the presence of bivalves and E. coli concentrations. After 24 h, the concentration of E. coli in pools containing bivalves was reduced between 1 to 1.5 log10 (Figure 3). In contrast, removal of E. coli in the control pool showed less than 0.3 log10 reduction (Figure 3). The temporal decrease in concentration of E. coli in the treatment pools followed first order kinetics, and eq 1 was used to calculate the k-value for each pool and kbivalve = k − kother, where kother is obtained from the control pool (Figure 3). The kother (± SD) value obtained was 0.03 ± 0.02 h−1. The volume of each pool was estimated and used to calculate the bivalve clearance rate for each pool. The estimated bivalve densities were used to calculate the clearance rate on an individual bivalve basis (Table 1). Because the pools in this in situ experiment were dominated by A. californiensis, the observed clearance rates (Table 1) can be attributed to A. californiensis filtration. However, given the low

Figure 3. Removal of E. coli from water in three distinct pools containing bivalves in the Pajaro River over a 24 h sampling period. Co is the initial concentration of E. coli in CFU/100 mL, and Ct is the concentration of E. coli at time t in hours. The error bars represent one standard deviation for three samples collected from each pool at various time points. E. coli concentrations were not available for hour 13 from the control pool and hour 15 from pool C.

density of C. fluminea, we were not able to directly determine the impacts of C. f luminea on A. californiensis filtration. We also did not determine the impacts of other factors removing E. coli that might have differed among the pools. Comparison of Laboratory and Field Clearance Rates. Field clearance rates were much higher than expected based on bivalve populations and lab filtration rates. If the field clearance rates are attributed solely to A. californiensis, the estimated rates per mussel were about 5−30 times higher than the maximum lab-measured clearance rates for A. californiensis (0.23 L hr−1 bivalve−1). This disparity could be due to (1) underestimation 11029

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In the pools studied in the in situ experiment, the dominant bivalve species is the native A. californiensis; hence, excluding other filter feeding organisms, the observed uptake of E. coli can likely be attributed to the activity of A. californiensis rather than that of C. f luminea. While the in situ field experiments were conducted in pools without surface flow, when mean flows in the river are considered, A. californiensis would still be capable of making a significant impact on water quality by reducing E. coli. For example, using the average of the calculated hypothetical clearance rates and bivalve density from the field experiments, we can estimate 1-log reduction of the E. coli in the flowing Pajaro River can be achieved within a length of 8 km of the river (see Supporting Information text). Because studies have shown that bivalve filtration activity is physiologically regulated and dependent on a variety of environmental factors such as water temperature, flow conditions, and the availability and composition of suspended particulate matter quantity,26,60,84−87 clearance rate values need to be adjusted for these system-specific parameters prior to application to other natural systems. The results presented in this paper demonstrate that native bivalves are capable of making quantifiable improvements in water quality of natural systems through the removal of E. coli. Results from these laboratory and field studies provide incentive to protect native freshwater species that are in decline because native bivalves can provide water quality benefits without the risk of negatively altering an ecosystem, as observed with invasive species.

of mussel clearance rates in the lab or (2) the action of other factors removing E. coli in the field. Laboratory experiments likely underestimated clearance rates. Previous studies have shown that underestimation of clearance rates occur when laboratory setups deviate from environmental conditions. Laboratory experiments often alter concentrations and types of particulate matter and do not provide sediment for bivalves to burrow in.30,60,61,75 One study found that undisturbed freshwater bivalves that burrowed in sediment and were fed high organic content algae had significantly higher (at least 4×) clearance rates than those previously reported for the same species.61 Our lab measurements of A. californiensis did not provide natural conditions, so higher clearance rates might pertain in nature. At the same time, the clearance rates estimated from the field data may overestimate clearance rates due to uncertainties associated with complex environmental systems. The control pool was used to account for abiotic factors that contribute to decreases in E. coli but does not account for activities of varying densities of other grazing organisms in the pools. Previous studies have shown that the bacterial clearance rate, which is the removal rate of bacteria, of protists and rotifers can be 10− 20 × 10−6 L hr−1 grazer−1,71,76 and host-specific virus predation on bacteria can be 1 × 10−7 L hr−1 virus−1.77 These rates are orders of magnitude less than clearance rates reported from bivalves, but the densities of protists, rotifers, and viruses are unknown in the pools studied here, so their overall contributions to E. coli reduction cannot be properly scaled. Grazing and viral predation may have a greater impact on E. coli clearance rates within pools with lower bivalve densities.78,79 While in pools with higher bivalve densities, the densities of rotifers, protists, and viruses may be low due to bivalve predation;80 hence, the overall contribution of these organisms to E. coli uptake is expected to be small. Another source of uncertainty is the reported bivalve densities calculated from systematic survey results. These densities provide only estimates of abundances, and previous studies have shown that over- or underestimation of densities can occur when extrapolating survey counts from patchy or clustered distributions.50,81 Despite these uncertainties that may have led to an overestimation of our reported field-derived clearance rates, numerous studies have shown nonetheless that in natural systems containing bivalves, these organisms are the dominant filter feeders.59,82,83 Implications of Laboratory and Field Data. The laboratory data quantify the relationship between body size and weight and clearance rate and thus provide a relative comparison of C. f luminea and A. californiensis clearance rates of E. coli. The laboratory results show that bivalves are responsible for removal of E. coli in controlled conditions in which confounding environmental factors are excluded. Although A. californiensis has a clearance rate on a dry weight basis lower than that of C. f luminea, the laboratory experiments demonstrate that, based on the sizes of the bivalve species sampled in the river, a single A. californiensis is capable of filtering an equivalent amount of E. coli as a single C. f luminea. The field survey results and in situ spiking field experiment provide evidence that bivalve filtration plays a dominant role in the decrease of E. coli in these pools, and E. coli concentration negatively correlates with bivalve density. The laboratory data and field data support the hypothesis that bivalves help to improve water quality through the reduction of E. coli concentrations.

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ENVIRONMENTAL SIGNIFICANCE We demonstrated the ability of the native freshwater species A. californiensis to remove E. coli in an impaired water body where FIB exceed levels for beneficial use and TMDLs are in place to improve water quality in this system. Surface water contamination by FIB has implications for human health beyond illness from direct or indirect contact. For example, the 2006 pathogenic E. coli O157:H7 outbreak associated with spinach produced in the central coastal region of California had nationwide health and economic repercussions.41,88 Studies identified this pathogen within the Pajaro Valley watershed, which provides irrigation water for one of the farms producing spinach affiliated with the outbreak.41 Thus, initiatives to maintain and protect populations of native freshwater bivalves in decline can result in bivalves providing the ecosystem service of reducing FIB concentrations while benefiting the ecosystem. Implementation of native bivalves as natural filters with other BMPs can provide a novel, cost-effective method to meet TMDLs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03043. Additional information on calculations and figures referenced in text (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 650-724-9128; fax: 650-723-7058; e-mail: aboehm@ stanford.edu. *Phone: 650-721-2615; fax: 650-725-9720; e-mail: luthy@ stanford.edu. 11030

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Environmental Science & Technology Present Address

by removal of Escherichia coli through the action of the bivalve Anodonta californiensis. Environ. Sci. Technol. 2015, 49 (3), 1664−1672. (13) Proakis, E. Pathogen removal in constructed wetlands focusing on biological predation and marine recreational water quality; WEFTEC: Los Angeles, CA, 2003. (14) Perkins, F. O.; Haven, D. S.; Reinaldo, M. A.; Rhodes, M. W. Uptake and elimination of bacteria in shellfish. J. Food. Prot. 1980, 43, 124−126. (15) Prieur, D.; Mevel, G.; Nicolas, J.; Plusquellec, A.; Vigneulle, M. Interactions between bivalve molluscs and bacteria in the marine environment. Oceanogr. Mar. Biol. Annu. Rev. 1990, 28, 277−352. (16) Vaughn, C. C.; Nichols, S. J.; Spooner, D. E. Community and foodweb ecology of freshwater mussels. J. N. Am. Benthol. Soc. 2008, 27 (2), 409−423. (17) ZoBell, C. E.; Feltham, C. B. Bacteria as food for certain marine invertebrates. J. Mar. Res. 1938, 1, 312−327. (18) Frest, T. J.; Johannes, E. J. Interior Columbia basin mollusk species of special concern: Final Report; Deixis Consultants: Seattle, WA, 1995; http://www.icbemp.gov/science/frest_1.pdf. (19) Mock, K. E.; Brim-Box, J.; Miller, M.; Downing, M.; Hoeh, W. Genetic diversity and divergence among freshwater mussel (Anodonta) populations in the Bonneville Basin of Utah. Mol. Ecol. 2004, 13 (5), 1085−1098. (20) Nedeau, E.; Smith, A. K.; Stone, J. Freshwater mussels of the Pacific Northwest; Pacific Northwest Native Freshwater Mussel Workgroup: Vancouver, WA, 2005; http://www.xerces.org/wpcontent/uploads/2009/06/pnw_mussel_guide_2nd_edition.pdf. (21) Lydeard, C.; Cowie, R. H.; Ponder, W. F.; Bogan, A. E.; Bouchet, P.; Clark, S. A.; Cummings, K. S.; Frest, T. J.; Gargominy, O.; Herbert, D. G. The global decline of nonmarine mollusks. BioScience 2004, 54 (4), 321−330. (22) Vaughn, C. C.; Taylor, C. M. Impoundments and the decline of freshwater mussels: a case study of an extinction gradient. Conserv. Biol. 1999, 13 (4), 912−920. (23) Williams, J. D.; Warren, M. L., Jr; Cummings, K. S.; Harris, J. L.; Neves, R. J. Conservation status of freshwater mussels of the United States and Canada. Fisheries 1993, 18 (9), 6−22. (24) Sousa, R.; Antunes, C.; Guilhermino, L. Ecology of the invasive Asian clam Corbicula f luminea (Müller, 1774) in aquatic ecosystems: an overview. Ann. Limnol. 2008, 44 (02), 85−94. (25) Atkinson, C. L.; Opsahl, S. P.; Covich, A. P.; Golladay, S. W.; Conner, L. M. Stable isotopic signatures, tissue stoichiometry, and nutrient cycling (C and N) of native and invasive freshwater bivalves. J. N. Am. Benthol. Soc. 2010, 29 (2), 496−505. (26) Atkinson, C. L.; First, M. R.; Covich, A. P.; Opsahl, S. P.; Golladay, S. W. Suspended material availability and filtrationbiodeposition processes performed by a native and invasive bivalve species in streams. Hydrobiologia 2011, 667 (1), 191−204. (27) Cherry, D. S.; Scheller, J. L.; Cooper, N. L.; Bidwell, J. R. Potential effects of Asian clam (Corbicula fluminea) die-offs on native freshwater mussels (Unionidae) I: water-column ammonia levels and ammonia toxicity. J. N. Am. Benthol. Soc. 2005, 24 (2), 369−380. (28) Strayer, D. L. Effects of Alien Species on Freshwater Mollusks in North America. J. N. Am. Benthol. Soc. 1999, 18 (1), 74−98. (29) Vaughn, C. C.; Spooner, D. E. Scale-dependent associations between native freshwater mussels and invasive Corbicula. Hydrobiologia 2006, 568 (1), 331−339. (30) Hakenkamp, C. C.; Ribblett, S. G.; Palmer, M. A.; Swan, C. M.; Reid, J. W.; Goodison, M. R. The impact of an introduced bivalve (Corbicula f luminea) on the benthos of a sandy stream. Freshwater Biol. 2001, 46 (4), 491−501. (31) Lucy, F.; Karatayev, A.; Burlakova, L. Predictions for the spread, population density and impacts of Corbicula f luminea in Ireland. Aquatic invasions 2012, 7 (4), 465−474. (32) Sousa, R.; Nogueira, A. J. A.; Gaspar, M. B.; Antunes, C.; Guilhermino, L. Growth and extremely high production of the nonindigenous invasive species Corbicula f luminea (Müller, 1774): Possible implications for ecosystem functioning. Estuarine, Coastal Shelf Sci. 2008, 80 (2), 289−295.

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N.S.I.: Picker Engineering Program, Smith College, Northampton, Massachusetts 01063, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) Engineering Research Center for Reinventing the Nation’s Urban Water Infrastructure (ReNUWIt) EEC1028968 and a NSF Graduate Research Fellowship. We thank Michael Sapunor and the Santa Cruz County Department of Public Works for granting us permission to access and conduct the field work, Mel Todd from Pace Engineering for escorting us on site, and Gary Kittleson from Kittleson Environmental Consulting for helping us identify our study site. We thank researchers at Moss Landing Marine Lab, in particular Dr. John Oliver, for providing extensive field support. We thank Dr. Chris Barnhart for reviewing and providing feedback on the manuscript. We thank staff at Watsonville Wetlands Watch for providing access to their facility during field work. Finally, we would like to thank the many Stanford students who helped with field sampling.

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