1 

## Corbula amurensis Conceptual Model

Janet K. Thompson ( jthompso@usgs.gov ) and Francis Parchaso

U.S. Geological Survey

Peer Reviewed

October 20102 PREFACE

This Conceptual Model is part of a suite of conceptual models that collectively articulate the current scien tific understanding of important aspects of the SacramentoSan Joaquin River Delta ecosystem. The concep tual models are designed to aid in the identification and evaluation of ecosystem re storation actions in the Delta. These models are designed to structur e scientific information such that it can be used to inform sound public policy.

The Delta Conceptual Models include bot h ecosystem element models (including process, habitat, and stressor models) and species life history models. The models were prepared by teams of experts using common guidance documents developed to promote consistency in the format and terminology of the models http://www.delta.dfg.ca.gov/erpdelta plan/science_process.asp .

The Delta Conceptual Models are qualitative models that describe current understanding of how the system works. Th ey are designed and intended to be used by experts to identify and evaluate pote ntial restoration actio ns. They are not quantitative, numeric computer models that can be “run” to determine the effects of actions. Rather they are designed to f acilitate informed discussions regarding expected outcomes resulting from restorati on actions and the scien tific basis for those expectations. The structure of many of the Delta Conceptu al Models can serve as the basis for future development of quantitative models.

Each of the Delta Conceptual Models has been, or is currently being subject to a rigorous scientific peer review process. The peer review status of each model is indicated on the title page of the model.

The Delta Conceptual models will be updated and refined over time as new information is developed, and/or as the models are used and the need for further refinements or clarific ations are identified.3 CONTENTS

I. Introduction .................................................................................................................. ....... 4

A. A note on Corbula amurensis ta xonomy and nomen clature ........................................ 5 B. Status and invasion of Corbula amuren sis in San Franci sco Estuary.......................... 5 II. Biology ....................................................................................................................... ... 6 III. Distribution .................................................................................................................. .. 7 IV. Ecology ....................................................................................................................... .. 8

A. Ecological Influences on Corbula’s Life Cycle .............................................................. 8 1. Gametogenesis ........................................................................................................ 8 2. Spawning and Fe rtilization ....................................................................................... 9 3. Pelagic Larvae .......................................................................................................... 9 4. Recruitment ............................................................................................................ 10 5. Adult Corbula .......................................................................................................... 12 B. Corbula’s Control of Ph ytoplankton Biomass ............................................................. 14 C. Negative Influence of the Trophic Transfer of C ontaminants ..................................... 16 D. Constraints on Restoration ......................................................................................... 17 V. Future rese arch .......................................................................................................... 18 VI. References ................................................................................................................. 20 VII. Tables ........................................................................................................................ . 25 VIII. Figures ....................................................................................................................... . 284 

## I. Introduction

C. amurensis is a pest in the San Francisco Estuary (hereafter SFE) due to its ability to heavily graze pelagic food resources and it s accumulation of toxic substances that are then moved into the food web through its predators. We begin by describing what we know of its life history and establish its life cycle w ithin the framework of the ecological setting of the SFE. This model should be used within the context of a thought experiment, as there is not e nough known about the species to design a strategy to control it in the estuary or De lta. The ultimate purpose of this conceptual

model of Corbula amurensis is to explore its weaknesses.

The geographic focus of the DRERIP models is the Delta. However we examine the life history patterns of Corbula amurensis as far down bay as San Pablo Bay for two reasons. First, because some of the la rgest ecosystem effects attributed to C.

amurensis are its consumption of pelagic carbon resources, it is necessary to include areas adjacent to the Delta because of the effects of tidal disp ersion. Second, many of the life cycle patterns that are visible in San Pablo Bay are transferrable to upstream locations during drought periods.

Finally, we expect this m odel to evolve as a result of the many ongoing studies on C.

amurensis . Some of the data shown in this di scussion are the direct result of current CALFED studies being join tly done by California Depart ment of Water Resources (EMP Program D. Messer and K. Gerhts) and the USGS (Analysis of Archived Samples to Assess Patterns of Historic Invasive Bivalve Biom ass). Our coinvestigators on that study ha ve been gracious enough to al low us to show some of that data here, before it has been published.

We start the model discussion with a brief description of the taxonomic status of this

bivalve because the util ity of the literature av ailable to us is based on assumptions of the correct identification. We also very briefly describe its introduction into the estuary as a reminder of th e facts about its invasion.5 A. A note on Corbula amurensi s taxonomy and nomenclature Corbula amurensis (Schrenck, 1861) was first identified as Potamocorbula amurensis by Carlton et al. (1990) with the aid of A. Matsukuma (National Science Museum, Tokyo) who was starting a revision of the ge nus at that time. Coan (2002) in his analysis of Eastern Pacific Corbu lidae revised the identification to Corbula amurensis but he warned that it may be changed again once the Asian Corbula group is revised.

The confusion in the taxonomy of this gr oup makes it difficult for us to establish a definitive world-wide geographic distribution and for us to confidently reference

literature from areas outside SFE. Sa to and Azuma (2002) have examined the C.

amurensis from the SFE and believe it to be more similar to a newly invasive

Potamocorbula species observed in the Ariake Sea than to the native Potamocorbula

amurensis seen in northeast Japan. The species described by Sato and Azuma (2002)

as Potamocorbula sp. was not seen in Japan befo re 1990 and probably arrived from China or Korea with shipments of Corbicula . If it is the same sp ecies as found in SFE,

it would be the second invasion location for the species.

B. Status and invasion of Corbula am urensis in San Francisco Estuary Carlton et al. (1990) hypothesize that C. amurensis (hereafter Corbula ) was introduced into the SFE in ballast water, most likely originating from an Asian port. The first specimen was discovered in late 1986, when juveniles were collected in the northern estuary. Within a year the speci es had spread thro ughout the northern estuary, and within 18 months had become the dominant bivalve in the northern estuary (Nichols et al., 1990). It spread to the southern estuary within two years where it remains a common and frequently dominant bivalve (Thompson, et al. 2008) during years when bivalves occur in South Bay (Cloer n et al. 2007). Corbula is on the alert list for ISSG (Invasi ve Species Specialist Group) where it is listed as among the 100 World’s Worst Invaders, is listed as a Pest by NIMBUS (National Introduced Marine Pest Information System) (Hewitt et al., 2002), and has classified as one of “six exotic high impact species” included in an early detection surveillance system in New Zealand (Global Invasive Species Databa se, 2005). It’s presence on these lists is due in part to its ecosystem effects in the SFE.6 

## II. Biology

Corbula amurensis is a dioecious (sexes are sepa rate), broadcast spawning bivalve with external fertilization (Figure 1). The fall and summer periods of gametogenesis usually last 1-2 months although Parchaso (1993) reported animals in San Pablo and South Bays to be almost continuously reproductively active in 1989-1990 during the 1987-1992 drought.

Females produce 45,000-220,000 oocytes. The oo cytes (Table 1) are not buoyant and

are fertilized near the be d (Nicolini and Penry 2000). Males produce sperm that are active for up to 12 hours in 15oC water. Non-mobile, non-feedi ng trochophore larvae develop within 24 hours of fertilization. Su spension feeding veliger larvae develop 24 hours later and swim actively to day 7; thereafter they swim intermittently until they settle at day 17-19.

Juvenile clams become reproductively activ e at about 5mm in length (Parchaso and Thompson 2002) which can occur with in 2 months of recruitment. Corbula lives 2- 2.5 years at most locations and comm only reproduce twice a year but can continuously reproduce if conditions permit. Recruitment variability is a function of the availability of adults to produce larvae and site specific envi ronmental conditions. Thus recruitment timing varies within the es tuary. Recruitment usually occurs in the western Delta (Chipps Island) in fall, in th e northern estuary (Grizzly bay) in early spring through fall, and in San Pablo Bay in late spring/early summer (Figures 2a, b, and c). Recruitment occurs over several m onths (Figure 2d) and a second recruitment period may occur at all locations although it is most common in the Grizzly Bay/Suisun Bay populations (Parchaso and Thompson 2002).

Corbula populations have wide ranges in abunda nce within years and between years (Table 2). The maximum Corbula abundance, observed thus far was in San Pablo Bay, and the maximum biomass was seen in Carquinez Strait. The examples shown in Figure 3 of Corbula abundance and biomass at two locations in SFE are representative of three types of variability observed in the estuary: (1) The lack of a predictive relationship between biomass a nd abundance is shown at the freshest channel station (near Chipps Island) in Figure 3a; note the similarity in biomass in the7 drought years (1987-1992) and the most re cent years (1999-2006) despite much higher abundance in recent years. The re lative increase in biom ass during the drought when phytoplankton biomass was uniformly lo w (Alpine and Cloern 1992) is driven by different year class struct ure (older year classes duri ng the drought) and changes in clam condition (weight for a specific lengt h of clam was also greater during the drought). (2) Corbula populations in the shallow Gri zzly Bay water (Figure 3b) have the most consistent seasonal patterns with biomass peaking in late fall/winter and abundance peaking in summer/early fall. (3 ) The relatively steady biomass in Grizzly Bay during the drought years (1987-1992) occurred because of the near continuous

recruitment of Corbula during this period.



## III. Distribution

A genetic study of Corbula in SFE (Duda 1994) found differentiation among populations to be low with all populations s howing a high degree of variability. This is a genetic characterizati on of a species equipped to adapt to a large number of

environments which we have found is true . Based on the few physiological studies that have been completed, the primary limiting environmental factor on Corbula’s distribution is likely to be sa linity. Neither distribution nor reproduction appears to be temperature limited within the range of temperatures commonly found in the estuary. If the species identification is accurate, Corbula’s native range is from 22o to 55o N

latitude in the western Pacific and thus it is unlikely that the temperature regime in SFE would be limiting. The salinity range for adult animals, 0.1-32 (Werner and Hinton 2000 and Carlton et al. 1990), and re cruits, 2-30 (Nicolini and Penry 2000), is sufficiently broad to encompass most of the northern estuary in most seasons and years (Figure 4a and 4b).

Corbula’s distribution in the estuary has changed very little since its initial invasion with the exception of a broader distributi on in the southern estuary (Thompson et al. 2008). The similarity in Corbula’s distribution in the spri ng of drought (1988) and wet (1995) years (Figure 4a) is indicative of the intrusion of some salt beyond the confluence of the rivers most years. Corbula’s distribution in fall is more variable and shows saltwater intrusion up the rivers and into the western Delta with the reduced flow during the drought (1990) and during the latter part of 2006. The latter8 observation shows the variabil ity of salinity between the seasons as 2006 was a wet year but with very low freshwater flow in late summer and fall.

Corbula is capable of living in brackish mars hes as shown by its presence in Suisun Marsh in 1990 (Figure 4b). Its distributi on in the marsh declines with increased

freshwater flow as seen in the fa ll 1995 distribution. It is unknown if Corbula can live within Egeria densa beds; limited sampling within the beds by the U.S.G.S. in 2001 and 2003 revealed only Corbicula fluminea (U.S.G.S. unpublished data). However the salinity tolerances of Corbula and Egeria densa overlap, so it is possible that they could co-occur.

Corbula has been observed in all habitats ex cept epifaunal (i.e. attached to hard substrate) habitats in the SFE. It prefers mid-intertidal to subtidal ranges but large populations can be found in the hi gh intertidal. Individuals ha ve been collected in silt, clay, hard-pack clay, sand, gr avel, peaty mud, and shell hash. They maintain their position in hard-pack clay or high velocity areas with a single byssal thread that passes through the anterior end of the shell and attaches to a piece of debris in the

sediment. Animals live with one-half to tw o-thirds of their shell exposed which is verified by the presence of live barnacles on the posterior end of many shells.



## IV. Ecology

Corbula spends most of its 2-3 year life in the sediment and spends about 3 weeks as a larva in the water column (Figures 1 a nd 5) when it can disperse and expand the population distribution. Thus environmenta l conditions, predators, and food quantity and quality vary throughout th e life cycle of the bivalve.

A. Ecological Influences on Corbula’s Life Cycle 1. Gametogenesis The salinity range needed for the initiation of gametogenesis (0.1- 28) is quite wide (Parchaso and Thompson 2002) and thus most of the adult bivalves present in the northern estuary are reproductively active. Al most 100% of the adult bivalves in a population are reproductive at the initia tion of gametogenesis in each year. Gametogenic activity has been linked to f ood availability by Parchaso and Thompson (2002). Although chlorophyll a concentrations (used as a measure of phytoplankton9 biomass) were quite low during the Parc haso and Thompson (2002) study, they did not observe non-reproductive popula tions; this fact is most likely a reflection of the lack of data on other food sources used by Corbula such as bacteria (Werner and Hollibaugh 1993).

2. Spawning and Fertilization

Corbula can be induced to spawn in the labo ratory by withholding food and stressing the animals (Nicolini and Penry 2000) but it is unknown if these tri ggers apply to field

animals. One of the few identified environmental limits on Corbula’s reproduction and thus potentially on its dist ribution, is the salinity limit (5-25) at which spawning and fertilization can occur (F igures 5 and 6). It s hould be noted that although Nicolini and Penry (2000) found animals did not spawn at 2 and 32, they did not test salinities >2 and <5, and salin ities >25 and <32. Within the 5-25 salinity range, both eggs and sperm are quite resilient and can tolerate a 10 step salinity change. Similarly, two hour old embryos that develop in a salinity of 15, tolerate salinities ranging from 10-30 in laboratory tests.

Like all broadcast spawning animals w ith external fertilization, successful reproduction is a result of a favorable chem ical and physical environment at the time of spawning and fertilization (Figure 6). Nicolini and Penry (2000) found that the eggs settled to the bottom on release and th at the sperm were free swimming, so it is unclear what percentage of fertilization o ccurs at the bed and what percentage of

fertilization occurs in the water column w ith resuspended eggs. In either case, the near-bed hydrodynamics and currents will be a major determinant of reproductive success and the geographic positi on of the embryo as it deve lops into a larva.

3. Pelagic Larvae

Corbula has two pelagic larval stages with broa d salinity tolerances (Figure 5, 2-30). The short lived trochophore larvae are non-swimming and no n-feeding and are thus vulnerable to transport and mixing into non- hospitable water (Figure 6). The veliger larva is not a dynamic swimmer but can move vertically in the wa ter column and thus may be able to control some of its environmen t. Veliger larvae of this size mostly eat cyanobacteria and small autotrophic flagellates (Raby et al. 1997). Predators on both larval forms are likely to be the same as those on other zoopla nkton in the estuary.10 Transport of embryos and larvae with the currents is a factor in the successful maintenance and spread of this species’ popul ations and is most likely a factor in the timing of recruitment in the northernmost es tuary (Figures 2a and 3). Even if the salinity is acceptable in sp ring, the residual flow during the spring outflow period is down-estuary throughout spring of most normal freshwater outflow years. Thus the largest number of larvae settles into this reach of the estuary during fall when both currents and salinity are favorable. In cont rast to this pattern, Thompson et al. (2008)

show that the 2-3 weeks that Corbula larvae are pelagic are sufficient for larvae produced in the northern estuary to trans it the 40+km and settle in the southern estuary. Thus, it is probab le that populations throughout the estuary co-mingle and populations that are eliminated by mortality and inhospitable water quality conditions during one season are re-estab lished by larvae from non-lo cal populations. Because of the hydrologic variability in the estuary, the elimination of populat ions in the shallow water in most years (Thompson 2008), and the species short life span, the repopulation of an area by non-local larvae is a critical component of the success of this species in the estuary.

4. Recruitment As the veliger larva begins its transforma tion to become a benthic organism it swims less frequently and depends on the turbulence to transport it to the bed. Once there it has a short period of time to attach and bu rrow or to remain passive and be moved back into the water column (Crimaldi et al. 2002). There are several factors that determine the larva’s transition from pelagic to benthic organism. First the water quality has to be acceptable and the salinity must fall in the 2-30 range. For this

reason the abundance of recruits at a location can be relate d to the position of X2 in this estuary, as it is the bottom salinity th at determines the success of the settling larvae (Figures 6 and 7). Second the physical environment may be such that the time

to attach to the bottom is too short before the turbulence re-suspends the larvae. One

factor shown to affect the time for attachment is the density of adult Corbula at a location (Crimaldi et al. 2002). In ad dition, adult Corbula may limit their own success by filtering their larvae out of the water column in a manner similar to that described for copepod nauplii (Kimmerer et al. 1994). The combination of these

factors results in a threshold of adult Corbula beyond which Corbula recruits are less likely to settle. Our data show that at one location (shallow water Grizzly Bay) adult11 densities above about 2000 m -2 greatly limit the success of recruits (Figure 8a). If conditions allow the estuarine amphipod Ampelisca abdita or some other surface tube-dweller to invade, we see a threshold effect on the Corbula recruits as was seen

with adult Corbula . Nichols and Thompson (1985) reported a similar relationship

between the bivalve Macoma balthica (now recognized as M

. petalum ) and A. abidita in the southern estuary. A lthough high abundances of adult Corbula can co-occur with A. abdita , recruits appear to be limited either by the flow disturbance of the tubes (Friedrichs et al. 2000) or the feeding ac tivities of the suspension-feeding amphipod (Figure 8b).

The number of recruits is highly variab le and, in addition to the intra- and interspecific competition noted above, pred ation and environmen tal variability are likely responsible for much of the extreme variability (Figure 6). We have observed

that the number of recruits in the shallows of North Bay is only reduced by freshwater outflow when we have prolonged, high outflow events as were seen in 1995 and 1998 (Figures 3a and 3c). When a high ou tflow event is followed by extremely low freshwater flow in summer and fall (e.g. 2006) we have observed near record number of recruits (reflected in the abundance in Fi gure 3). Such patterns may be related to the removal of most benthic organism s during these flood events, leaving a depauparate benthic community that is th erefore available to an opportunist like

Corbula (Nichols et al.1990)

.

Although there have been few studi es on the predation on juvenile Corbula , we observed in the 1980’s that a remarkable number of clams 10+mm in length from Carquinez Straits showed signs of bei ng handled but not successfully eaten by a crustacean (Carlton et al 1990). Smaller clam s did not show the chips along the edge of the valve, and we interpreted that as a sign that if they were picked up by the predator they were successfully cons umed. We have since learned that Cancer magister , the Dungeness crab, occu rs in this part of the estuary and preys on Corbula (Stewart et al. 2004). Other likely predators on juvenile Corbula include demersal fish, diving ducks in the shallow water, shrimp, and the exotic opisthobranch Philine auriformis in the more saline portion of the estuary (e.g. San Pablo Bay, Gosliner 1995).12 Once settled to the bottom recruits conti nue to feed on smaller sized pelagic food sources due to their small size (Table 1). We have observed growth to be more than 5mm in length within two mont hs of recruitment in the southern estuary (Thompson et al. 2008), so the recruits likely graze on an increasing variety and size of algae and bacteria within a month of settling. Ac hieving reproductive size (Figure 9) is dependent on growth rate that varies between years and is likely a result of environmental stressors and food quantity and quality. Preliminary estimates of growth rate of Grizzly Bay Corbula range from a peak of 3-4 mm/month in spring to <1-2 mm/month in fall (analyses by J. Thompson).

Juvenile and adult Corbula are capable of being transpor ted as bedload and within the

water column. Small clams 5-8mm have b een “sampled” using zooplankton nets in the deep water of the estuary (William Bennett pers. Comm., University of California Davis). These individuals had a byssal thread still attached and we believe that like other bivalves they use the byssal thread for lift and subsequent transport (Beukema and Vlas 1989). Transport can result in individuals being both removed and deposited at a location (Figure 9).

5. Adult Corbula

Adult Corbula (>5mm in length) are remarkably to lerant of salinity changes and their wide distribution in the Estuary reflects that tolerance (Tab le 1, Figure 10). Corbula is an osmo-conformer that can adapt to ch anges in salinity within 48 hours. Adults can tolerate very low sali nities (0.1) in the laborat ory but die with prolonged freshwater exposure (W erner et al. 2003). Corbula pays a metabolic price for this tolerance. Pagannini et al . (in press) observed that Corbula had 40% lower oxygen consumption rates (VO2) at salinities fl uctuating between 4 and 14 than the VO2 observed at high (28) and low (2) salinities. Glycogen (Wer ner et al. 2003) and lipid stores (Richman and Lovvorn 2004) are generally low in Corbula relative to many bivalves. Therefore it appears Corbula can not depend on en ergy stores during periods of stress. Pagannini et al. (in pre ss) find that Corbula’s metabolic rate and filtration rate is higher at higher sa linities (28). They suggest that Corbula’s high malate dehydrogenase activity, in combinati on with a high respiration rate, allows them to rapidly oxidize food when it becomes available. Canuel et al (1995) in examining the fatty acids in Corbula also suggested that they are “capable of rapid13 assimilation and incorporation of algal-deri ved organic matter into their tissues during periods of high phytoplankton abundance.”

Although the data are not shown in their re port, McEnnulty et al. (2001) report that

Corbula “has a high tolerance to low oxygen and is found in polluted or eutrophic

areas”.

Corbula is a suspension feeder that can filte r and assimilate both phytoplankton and bacteria (<1.2 μm) from th e water column (Werner and Hollibaugh 1993). Their high secondary production in areas with lo w phytoplankton biomass (Thompson 2005) suggests it depends on carbon sources other than phytoplankton. Retention efficiency for natural bacteria is lower (28% and 13 % for 1 and 2 cm long clams) than for phytoplankton (100%). Bacterial gross assimilation rates (73-75%: Decho and Luoma 1991, Werner and Hollibaugh 1993) are somewhat lower than measured for phytoplankton (78-90% for a ra nge of phytoplankton species; Schlekat et al. 2002). Bacterial carbon also appears to be mo re quickly metaboli zed than phytoplankton carbon (Werner and Hollibaugh 1993). Net assi milation rates have been estimated at

45% for bacteria and ≈ 85% for phytoplankton (Werner and Hollibaugh 1993, Schlekat et al. 2002).

Known aquatic consumers of adult Corbula in the SFE include the Dungeness crab ( Cancer magister , Carlton et al. 1990, Stewart et al. 2004), the Sacramento Splittail ( Pogonichthys macrolepidotus , Deng et al. 2007), and the White Sturgeon ( Acipenser

transmontanus , Urquhart and Regalado, 1991, Kogut in press). It is likely that any bottom feeding fish with a sufficiently larg e mouth that feeds in areas occupied by C.

amurensis consume some quantity of the biva lves due to their large population

numbers. Birds known to consume adult Corbula include the Greater and Lesser Scaup ( Aythya marila and A. affinis , Poulton et al. 2002) and the Surf Scoter ( Melanitta perspicillata , Hunt et al. 2003). Conchiolin layers, organic laminae in the

shells that occur in bivalves in the Cor bulidae family, act both to increase the strength of the shell against predation from mechanical crushi ng and to deter gastropods from drilling through the biva lves shell (Kardon 1998).14 Populations of Corbula are reduced and in some cases locally decimated by migratory birds in fall and winter in all but the deep channel areas of the SFE (Poulton et al. 2002, Thompson et al. 2008). Therefore the shoal areas are mostly dependent on the adjacent deep water populations to supply recruits each year.

The combination of Corbula’s ability to respond to food quickly to initiate gamete production and to produce gametes and embryos that are tolerant of large changes in salinity is likely to be a good strategy fo r this species to maintain its present distribution, as well as to e xpand its distribution during se asonal salinity changes in the estuary. Individuals become reproductively active within their fi rst year of life and in many cases, spring recruits are capabl e of reproducing by summer and fall, so adults are present during most months of the year to respond to a cue to begin gametogenesis.

B. Corbula’s Control of Phytoplankton Biomass The SFE is a high nutrient, high turbidity estuary that has low primary production due to a combination of light limitation and bivalve grazing (Cloern 2001, Thompson et al. 2008). The northern estuary has always had low primary production, but declines in primary production following the invasion of Corbula lead many researchers to conclude that suspension feeding by Corbula resulted in the decline in phytoplankton

biomass and the elimination of the annual phytoplankton bloom in the estuary (Alpine and Cloern 1992, Kimmerer 2002). The annua l primary production has been reduced to <20 g C m -2 yr -1 from ≈ 100 C m -2 yr -1 (Alpine and Cloern 1992, Ja ssby et al. 2002).

Two copepods ( Acartia spp, Eurytemora affinis ), a rotifer ( Synchaeta bicornis ), and a mysid shrimp ( Neomysis mercedis ) concurrently declined with the phytoplankton, presumably due to food limitation (Kimmerer 2002, Feyrer et al. 2003). Some of the declines in copepods may also be due to direct filtrati on of juveniles by Corbula (Kimmerer et al. 1994). De spite what appeared to be a collapsing foodweb within 5 years of the invasion of Corbula , most of the species involved remain in the estuary today. The robustness of the foodweb may due to (1) its complexity and therefore weak links (Kimmerer 2002), (2) the im portance of microzooplankton and small phytoplankton that are not presently accounted fo r in the data, or (3) the migration of one important, competing filter feeder, the Northern Anchovy ( Engraulis mordax ),15 out of the northern estuary following the reduction in phytoplankton biomass (Kimmerer 2006). Although the cause of a r ecent decline in pelagic fish in the ecosystem (Sommer et al. 2007) is probably at least partially due to changes in the pelagic foodweb, there is no conclusive evidence that food is the primary or only cause for the decline of the fish species.

Corbula’s pumping rate (a physiologically contro lled rate), estimated in a flume with varying velocities, was found to range from 100-575 L (g dry weight) -1 d -1 with a mean of ~400 L (g dry weight) -1 d -1 or 1-5 L d -1 clam -1 for the phytoplankton Chroomonas salinay (Cole et al. 1992). Clearance ra tes (a physiologically controlled rate that varies with filtrate character) estim ated in beaker experiments are similar: 4

L d -1 for a 1 cm clam { filtration rate = -40+199 x shell length (cm)} for phytoplankton ( Isochrysis glabana ) and 45 ml hr -1 clam -1 (independent of shell size) for bacteria (Werner and Hollibaugh 1993). We can estimate the effect of Corbula’s grazing rate (our estimate of a field ra te) in two ways. First, Corbula’s maximum effect on the phytoplankton biomass can be estimated by using pumping rates and assuming a vertically well mixed water column. The second method estimates a minimum effect of clam grazing on the phytoplankton by (a ) assuming that a concentration boundary layer (CBL) forms within a bed of clams and (b) that not all clams feed at once (66% active clams at a time). In this case th e phytoplankton removed by the bivalves is not replaced by vertical mixing which results in a vertical gradient in phytoplankton concentration (i. e.. the clams downstream of other clams have less food). The CBL method and a more thorough explanation of the estimation technique can be found in Thompson et al. (2008). Corbula’s consumption of phytoplankton is a function of environmental factors such as temperature and suspended sediment in addition to the biomass of bivalves and vertical mixing ra tes (Figure 11). The e ffect of many of the

environmental factors is not yet well defined. Table 3 shows grazing rates using the two methods. Note that the median grazi ng rate varies by a f actor of 4 between stations for both methods. This rang e reflects the larger biomass of Corbula observed in the deep water (Table 2). Vertical turnover time, or the time that the clams need to filter the water column over them, assuming the water column is vertically well mixed, is a reflection of the potential impact that Corbula has on the phytoplankton standing stock and its capacity to achieve a bloom biomass. Turnover time using the median grazing rate is about 2-4 days for the shallow water population in Grizzly Bay16 and 4-8 days for the channel populations. Th ese turnover times are shortest in fall when biomass and grazing rate peak; Corbula could theoretically clear the water column once every 8-16 hours in Grizzly Bay and every 8-24 hours in the channel during this period. Biomass and therefore minimum grazing rates have consistently occurred in spring and early summer at al l shallow water locations and most channel stations; the biomass time seri es shown in Figures 3b and 3d are representative of this seasonality.

The rapid turnover of the wate r column by the shallow water bivalves, in conjunction with the relatively slow phyt oplankton growth rate (0.25 d -1 in the shallow water, Cloern et al. 1985) highlight th e importance of shallow water Corbula as one of the factors determining system-wide phytoplankt on biomass. The high grazing rates in the channel may also influence phytoplankton biomass, but the negative growth rate of phytoplankton in the channel due to the poor light climate (i.e. limited illumination in the water as determined by incident light, absorption, scat tering, and turbidity) (Cloern et al. 1985) is likely to be th e primary factor controlling phytoplankton biomass in the deep water. Therefore th e only locale in the northern estuary where the light climate allows phytoplankton to grow is in the shallows. Historically, a slow growing, 4-5 month long phytopl ankton bloom began in the shallows and spread to

the channels in the northern estu ary (Cloern et al . 1985). Today, high Corbula grazing rates in conjunction with relatively high water co lumn vertical mixing rates continually challenge the potential for phytoplankton biomass increase in these shallow systems. The phytoplankton consumption rates reported here for Corbula in the northern estuary will aff ect adjoining areas through tid al dispersion. Therefore the

Corbula filtration in the northern estuary may have a significant effect on the phytoplankton in the western Delta as reported by Jassby et al. (2002).

C. Negative Influence of the Trop hic Transfer of Contaminants The near-surface growth position of Corbula makes them more available to predators than the deep-burrowing bivalve that pr eviously dominated the northern estuary ( Macoma petalum , previously known as Macoma balthica ). The caloric content of the two bivalves is similar (Richman and Lovvorn 2004). Thus the invasion of

Corbula might be an advantage to bottom feed ing predators if not for the propensity

of Corbula to accumulate selenium at near toxic levels for consumers (Stewart et al.17 2004). Since its arrival in the estuary, predators on Corbula that now have selenium

concentrations in their liver in excess of the toxicity thre shold include the Sacramento Splittail ( Pogonichthys macrolepidotus ), the Dungeness crab ( Cancer magister ), the White Sturgeon ( Acipenser transmontanus ) (Stewart et al. 2004) and diving ducks (Scoter: Melanitta perspicillata and Scaup: Aythya spp) (White et al. 1987, 1988, 1989, Urquhart and Rigelado 1991, Linvi lle et al., 2002). Although Corbula also bioaccumulates heavy metals (Brown and Luoma 1995), hydrocarbons (Pereira et al. 1992), butyltins (Pereira et al . 1999), and pesticides (Gal e et al. 2003), there is no

literature on the effects of these toxins on Corbula’s predators.

D. Constraints on Restoration Corbula invaded the SFE after a 100 yr flood even t that greatly reduced the diversity and abundance in the benthic community at the invasion locati on (Nichols et al. 1990). Although Corbula then invaded other parts of the estuary in the following two years, and did so within a normally dive rse community (Thompson 2005), its ability to monopolize the depauperate benthic community in the northern estuary signifies its likely response to newly restored habitat. This opportunistic ab ility, in combination

with Corbula’s prodigious grazing rates and abil ity to accumulate and transfer contaminants may limit our restorati on options in the northern estuary.

The decline of fish that prey on zoopl ankton and shrimp, which depend on primary producers, has resulted in a primary restorat ion goal of increasing the production of phytoplankton. Because Corbula grazing can restrict the net growth rate of phytoplankton, it has been suggested that control of Corbula biomass in some select areas might allow phytoplankton to grow and be transported out of these areas. As the major control on Corbula distribution is salinity, it has been discussed if increasing freshwater outflow during peak reproductive periods would limit their distribution. The low salinity stress on the eggs, embryos and pelagic larvae in

combination with their trans port out of the area with hi gh flows (large flood events like 1995 and 1998) might reduce the popul ation size and distribution of Corbula in localized areas for short time periods. It ta kes very large, prolonged outflow events to reduce the estuary-wide populat ion (Figure 3) over an ex tended time period, as local and non-local adults can respond quickly a nd produce new larvae as long as food is

available. Increased outflow periods would need to be maintained for this to be a18 long term solution as depauparate periods such as was seen in 2006, can be followed by an increase in th e population size of Corbula during subsequent years with normal salinity distributions

. Therefore, sustained reduction in grazing would require the

water for controlled floods most if not all years.

Corbula’s ability to accumulate toxins and its place in the food web as prey is also an important consideration in restoration plans. Without the elimination of Corbula from the estuary, the best that can be done for pr otecting predators is to identify sources of contaminants and try to elim inate their availability to Corbula . Any restoration action

that increases the concentr ation of contaminants in Corbula may result in an increase

in the contaminant concentration in their predators although the predictability of the

amount transferred is contamin ant and species specific.

Climate change may make Corbula population biomass distribution less consistent between years as the frequency of high and low freshwater outflow events are predicted to increase. In addition to this “natural” population variab ility is that caused by declining freshwater resources. If ther e is an overall decline in freshwater available to the estuary as the need for fres hwater is increased in California for human consumption, Corbula will expand into the western Delt a as the salinity increases. This will further complicate restoration in the western Delta.



## V. Future research

The immediate research needs include (1) re solution of the taxono mic questions in the family, (2) incorporation of biomass estim ates in the DWR monitoring program, and (3) examination of the physiological characteristics of the species.

C. amurensis has apparently not spread to in tra-coastal ports nor has it been

introduced to other Eastern Pacific lo cations. The wide distribution of Corbula

amurensis in the Western Pacific leads us to question if the species is correctly identified. It seems unlikely that adults and larvae have not been included in ballast water that has been released in other ports. The only explanations we have at present for the lack of spread is th at (1) there is something in its physiology that limits its transport and or settlement in most situa tions, (2) the balance of trade today reduces19 the number of ships arriving fully in ballast from Asia and thus there has been less opportunity for release of ballast water, or (3) it has invaded elsewhere but in

localities without monitori ng programs. All of thes e explanations would be influenced by finding the taxonomy is incorrect.

The second research need is to incorporate estimates of biomass of the major species as part of the routine anal ysis of DWR’s Environmental Monitoring Benthic Program. As seen in Figure 3, our understanding of ecosystem effects cannot be facilitated using abundance alone. The inclusion of biomass in th e analyses would allow the benthos to be defined as carbon, the common cu rrency used in food web analyses. In addition, an estimate of biomass would give us the data needed to estimate grazing rates on phytoplankton and zooplankton, growth rates (i.e. how mu ch carbon are they consuming), and the mass and contaminant content available to predators.

We cannot confirm many of our assessments of possible controls on the temporal and spatial distribution described in this docum ent until basic physiological measurements

of Corbula are done in the laboratory. To thoroughly understand the distribution potential of Corbula in a restored area we need to know how salinity and temperature co-vary, i.e. when does temperature narrow Corbula’s salinity tolerance and when does salinity limit Corbula’s ability to withstand high temp eratures that may occur in high-residence time restored embayments. Understanding Corbula’s low oxygen tolerance will help explain distribution anomalies in Suisun Marsh where low oxygen has been concurrently observed with dead Corbula and help us understand the length of time necessary for Corbula to die in low oxygen water. Finally, grazing rates are very important if we are to understand Corbula’s effects on the estuary, but we know nothing about how its pumping rates are relate d to suspended sediment concentration, temperature, and food concentrations.

Acknowledgements Development of this model was supported the U.S. Geological Survey’s Priority Ecosystem Science and Toxic Substances Hydrology programs, CALFED Bay/Delta Program, and the CASCADE project (CAL FED Project #SCI-050C01-84). The model benefitted by the comments of two anonymous reviewers.20 

## VI. References

Alpine AE, Cloern JE, 1992. Trophic interac tions and direct physical effects control phytoplankton biomass and production in an estuary. Limnology and Oceanography, 37 (5):946-955.

Beukema JJ, Vlas J de. 1989. Tidal-current transport of thread-drifting postlarval juveniles of the bivalve Macoma balth ica from the Wadden Sea to the North Sea. Marine Ecology Pr ogress Series 52:193-200.

Brown CL, Parchaso F, Thompson JK, Lu oma SN, 2003. Assessing toxicant effects in a complex estuary: A case study of eff ects of silver on reproduction in the bivalve, Potamocorbula amurensis, in San Francisco Bay. Human and Ecological Risk Assessment, 9(1):95-119.

Brown CL, Luoma, SN. 1995. Use of the euryhaline bivalve Potamocorbula amurensis as a biosentinel species to assess trace metal contamination in San Francisco Bay. Marine Ecol ogy Progress Series, 124:129-142.

Canuel EA, Cloern JE, Ringelberg DB, Gu ckert JB. 1995. Molecular and isotopic tracers used to examine sources of orga nic matter and its incorporation into the food webs of San Francisco Bay. Limnology and Oceanography. 40(11):67-81

Carlton JT, Thompson JK, Schemel LE, Ni chols FH, 1990. Remarkable invasion of San Francisco Bay (California, USA) by the Asian clam Potamocorbula amurensis. I. Introduction and dispersa l, Marine Ecology Pr ogress Series, 66:81- 94.

Cloern JE, 2001. Our evolving conceptual model of coastal the eutrophication problem. Marine Ecology Progress Series, 210:223-253.

Cloern, JE, Cole, BE, Wong, RLJ, Alpine, AE, 1985. Temporal dynamics of estuarine phytoplankton: a case study of San Fr ancisco Bay. Hydrobiologia 129:153-176.

Cloern, JE, Jassby, AD, Thompson, JK, Hieb, K. 2007. A cold phase of the East Pacific triggers new phytoplankton blooms in San Francisco Bay. Proceedings of the National Academy of Science, 104(47):18561-18565.

Coan EV, 2002. The eastern Pacific recen t species of Corbulidae (Bivalvia), Malacologia, 44(1):47-105.

Cole BE, Thompson JK, Cloern JE, 1992 M easurement of filtrat ion rates by infaunal bivalves in a recirculating fl ume. Marine Biology, 113: 219-225.

Crimaldi, JP, Thompson, JK, Rosman, JH, Lowe, RJ, Koseff, JR. 2002. Hydrodynamcis of larval settlement: The in fluence of turbulent stress events at potential recruitment sites. Li mnology and Oceanography 47(4):1137-1151.21 Decho AW, Luoma SN, 1991, Time-courses in th e retention of food material in the bivalves Potamocorbula amurensis and Macoma balthica significance to the absorption of carbon and chromium: Ma rine Ecology Progress Series, 78:303- 314.

Deng DF, Hung SSO, The SJ, 2007. Selenium de puration: residual e ffects of dietary selenium on Sacramento sp littail (Pogonichtys macrolep idotus), Science of the Total Environment, 377:224-332.

Duda TF, 1994. Genetic populat ion structure of the recently introduced Asian clam, Potamocorbula amurensis, in San Francisco Bay. Marine Biology, 119:235-241.

Dugdale, RC, Wilderson, P, Hogue, VE, Marc hi, A. 2007 The role of ammonium and nitrate in spring bloom development in San Francisco Bay . Estuarine, Coastal and Shelf Science, 73(1-2): 17-29

Feyrer F, Herbold B, Matern SA, Moyle PB , 2003. Dietary shifts in a stressed fish assemblage: Consequences of a bivalve invasion in the San Francisco Estuary. Environmental Biology of Fishes, 67:277-288.

Friederichs, M., Graf, G., Springer, B. 2000. Skimming flow induced over a simulated polychaete tub lawn at low population densities. Marine Ecology Progress Series. 192:219-228.

Gale, R, Orazio, C, Meadows, J, Kuivila, K. 2003. Bioaccumulation of current-use pesticides by the invasive Asian cl am (Potamocorbula amurensis) of San Francisco Bay. Abstract from the 24th annual meeting of the Society of Environmental Toxicology and Chemistry.

Global Invasive Species Database, 2005. http://interface.creative.au ckland.ac.nz/database/s pecies/ecology.asp?si=136&fr=1&sts = Accessed 10 January 2008

Gosliner, T.M. 1995. Introduction and Spread of Philine auriformis (Gastropoda: Opisthobranchia) from New Zealand to Sa n Francisco Bay Bodega Harbor. Mar. Biol. (Berlin) 122(2):249-255.

Hewitt CL, Martin RB, Sliwa C, McEnnultyFR, Murphy N.E, Jones T, Cooper S, 2002. Editors. National Introduced Marine Pest Information System. Web publication <http://crimp.marine.csir o.au/nimpis>, Accessed: 19-Jan-2008

Hunt J, Ross J, Davis J, Lowe S, Lovvorn J, Crane D, Burkholder B, 2003. Selenium Concentrations in Surf Scoter and Great er Scaup from the San Francisco Estuary http://www.sfei.org/reports/SOE_2003/SO E%20poster-SE_DUCKS_JH_FINAL.pdf Accessed 15 January 2008

Jassby AD, Cloern JE, Cole BE, 2002. Annual primary production: patterns and mechanisms of change in a nutrient- rich tidal ecosystem. Limnology and Oceanography, 47(3):698-712.22 Kardon, B, 1998. Evidence from the fossil record of an antipredatory exaptation: conchiolin layers in corbulid bivalves. Evolution, 52(1):68-79.

Kimmerer WJ, 2002. Effects of freshw ater flow on abundance of estuarine organisms: physical effects or trophic linkages. Mari ne Ecology Progress Series, 243:39-55.

Kimmerer WJ, 2006. Response of anchovies da mpens effects of the invasive bivalve Corbula amurensis on the San Francisc o Estuary foodweb. Marine Ecology Progress Series, 113:81-93.

Kimmerer WJ, Gartside E, Orsi JJ, 1994. Predation by an introduced clam as the likely casuse of substantial declines in zooplankton of San Francisco Bay. Marine Ecology Progress Series, 324:207-218.

Kogut, NJ, In press, Overbite clams, Co rbula amurensis, defecated alive by white sturgeon, Acipenser transmontanus . California Fish and Game

Linville RG, Luoma SN, Cutter L, Cutter GA, 2002. Increased selenium threat as a result of invasion of the exotic bivalv e Potamocorbula amurensis into the San Francisco Bay-Delta. AquaticToxicology 57:51-64.

Luoma, S.N. and Rainbow, P.S. 2005 W hy is metal bioaccumu lation so variable? Biodynamics as a unifying concept. Environmental Science & Technology 39(7):1921-1931.

McEnnulty FR, Jones TE, Bax NJ, 2001. The Web-Based Rapid Response Toolbox. <http://crimp.marine.csiro.au/NIMP IS/controls.htm>. Accessed on 30 December 2008.

Nichols, FH, Thompson, JK. 1985. Persiste nce of an introduced mudflat community in South Francisco Bay, California: Marine Ecology Progress Series v. 24, p. 83-97.

Nichols FH, Thompson JK, Schemel LE, 1990. Remarkable invasion of San Francisco Bay (California, USA) by the Asian clam Potamocorbula amurensis. 2. Displacement of a fo rmer community. Marine Ecology Progress Series, 66:95-101.

Nicolini MH, Penry DL, 2000. Spawning, fert ilization, and larval development of Potamocorbula amurensis (Mollusca: Bivalvia) from San Francisco Bay, California. Pacific Science, 54:377-388.

Paganini A, Kimmerer WJ, Stillman JH. (in press) Metabolic responses to environmental salinity in the invasive clam Corbula amurensis . Aquatic Biology

Parchaso F, 1993. Seasonal reproduction of Potamocorbula amurensis in San Francisco Bay, California. M.S. Thesis, San Francisco, CA, US: California State University23 Parchaso F, Thompson JK, 2002. The in fluence of hydrologic processes on reproduction of the introduced bivalve Potamocorbula amurensis in northern San Francisco Bay, California. Pacific Science, 56(3):329-345.

Pereira WE, Hostettler FD, Rapp JB. 1992. Bioaccumulation of hydrocarbons derived from terrestrial and anthro pogenic sources in the Asian Clam, Potamocorbula amurensis, in San Fran cisco Bay Estuary. Marine Pollution Bulletin, 24(2):103-109.

Pereira WE, Wade TL, Hostettler FD, Pa rchaso F. 1999. Accumulation of Butyltins in sediments and lipid tissues of the As ian Clam, Potamocorbula amurensis, near Mare Island Shipyard, San Francisc o Bay. Marine Pollution Bulletin, 38(11):1005-1010.

Poulton VK, Lovvorn JR, Takekawa JY, 2002. Clam density and scaup feeding behavior in San Pablo Bay, California. Condor, 104:518-527.

Raby D, Mingelbier M, Dodson JJ, Klein B. 1997. Food-particle size and selection by bivalve larvae in a temperate emba yment. Marine Biology. 127:665-672.

Richman SE, Lovvorn JR, 2004. Relative foragi ng value to lesser scaup ducks of native and extoic clams from San Fran cisco Bay, Ecologi cal Applications, 14(4):1217-1231.

Sato S, Azuma M, 2002. Ecological and pa leoecological implications of the rapid increase and decrease of an introduced bivalve Potamocorbula sp, after the construction of a reclamation dike in Isahaya Bay, western Kyushu, Japan. Palaeogreography, Palaeoclimat ology, Palaeoecology, 185:369-378.

Schlekat DW, Lee BG, Luoma SN, 2002. Assimilation of selenium from phytoplankton by three benthic invertebrate s: effect of phytoplankton species. Marine Ecology Progress Series, 237:79-85.

Schrenck LI, 1861. Mollusken des Amur-L andes und des Nordjapanischen Meeres. Reisen and Forschungen im Amur-Lande in den jahren 1854-1856. Kaiserliche Akademie der Wissenschafgen, St. Petersburg, 2:259-974.

Sommer T, Armor C, Baxter R, Breuer R, Brown L, Chotkowski M, Culberson S, Feyrer, F, Gingras M, Herbold B, Kimme rer W, Mueller-Solger A, Nobriega M, Souza K, 2007. The collapse of pelagi c fishes in the upper San Francisco Estuary. Fisheries, 32(6):270-277.

Stewart, AR, Luoma, SN, Schlekat, CE, Dob lin, MA, Hieb, KA, 2004. Food web pathway determines how selenium affects aquatic ecosystems: A San Francisco Bay case study. Environmental Science & Technology, 38(17):4519-4526.

Thompson, JK, 2005. One estuary, one inva sion, two responses: phytoplankton and benthic community dynamics determine th e effect of an estuarine invasive suspension feeder. In: Olenin S, Dame R, eds. The Comparative Roles of24 Suspension Feeders in Ecosystems. Am sterdam, Netherlands: Springer, 291- 316.

Thompson JK, Koseff JR, Monismith SG, Lucas LV, 2008 (in press). Shallow water processes govern system-wide phytop lankton bloom dynamics: A field study. Journal of Marine Systems. doi:10.1016/j.jmarsys.2007.12.006

Urquhart KAF, Regalado K, 1991. Seleni um verification study. 1988-1990. Report 91-2-WQ, California Water Re sources Control Board, S acramento, California.

Werner I, Hinton DE, 2000. Spatial profile s of hsp70 proteins in Asian clam ( Potamocorbula amurensis ) in northern San Francisco Bay may be linked to natural rather than anth ropogenic stressors. Marine Environmental research, 50(1-5):379-384

Werner I, Clark SL, Hinton DE, 2003. Bi omarkers aid understanding of aquatic organism responses to environmenta l stressors. California Agriculture, 57(4):110-115

Werner I, Hollibaugh, JT, 1993. Potamocorbula amurensis : comparison of clearance rates and assimilation efficiencies for phytoplankton and bacterioplankton. Limnology and Oceanography, 38:949-964.

White JR, Hofmann PS, Hammond D, Baumga rtner S, 1987. Selenium verification study, 1986. Water Resources Control Board Report, Sacramento, CA

White JR, Hofmann PS, Hammond D, Baumga rtner S, 1988. Selenium verification study, 1986-1987. Water Resources Control Board Report, Sacramento, CA

White JR, Hofmann PS, Urquhart KA F, Hammond D, Baumgartner S, 1989. Selenium verification study, 1987-1988. Water Resources Control Board Report, Sacramento, CA25VII. TablesTable 1.CorbulaLife-Stage by Biological Measures.A B C D E F G H J K L M NHabitat(life stage)DatesAge Weight LengthOptimumTemp(Celsius)MaximumCriticalTemp(Celsius)MinSalinityMaxSalinityMinDOLimitsWaterVelocitypH Turbidity References(Dayspost hatch)1Benthic(Eggs/Sperm)Jan-Dec-1 UNK UNK UNK 5 25 UNK UNK UNK UNKNicolini &Penry 20002Benthic(Fertilizationof Egg)JanDec-1 UNK 60+ μm UNK UNK 5 25 UNK UNK UNK UNKNicolini &Penry 20003Pelagic(TrochophoreLarvae)JanDec0-1UNK70 μm UNK UNK 2 30 UNK UNK UNK UNKNicolini &Penry 20004Pelagic(VeligerLarvaeJanDec1-19 UNK75-135μmUNK UNK 2 30 UNK UNK UNK UNKNicolini &Penry 20005Benthic(Recruit)Jan-Dec .19~120300 μm-5 mmUNK UNK 2 32+ UNK UNK UNK UNKNicolini &Penry 2000Parchaso &Thompson20026Benthic(ReproductiveAdult)JanDecMaximum>3 years<4 years>5mm UNK UNK .1 32+ UNK UNK UNK UNKParchaso &Thompson200226Table 2.CorbulaPopulation Characteristics.* Data for D4, D7 and D41A Courtesy of DWR CALFED Biomass StudyA B C D E F G H JK L M N NGeographicLocationStationIDSourceStationdepthatMLLW(m)MaximumBiomassMaximumAbundanceAbundance Range(#/m2)Biomass Range(gC/m2)YearsExaminedgC/m2mo/yr #/m2mo/yr max yr min yr max yr min yr1ColinsvilleD4*DWR1154/91 3000 10/000-30002000 0 20020-51991020021987-1988,1991,1998, 2000,2002, 20043Chipps Island4.1USGS92210/07 12,000 11/00400-12,000200010-50019962-2220070-0.719991988-20075CarquinezStrait8.1USGS17 80 5/88 30,000 4/071600-26,000200610-800199810-8019880.2-119981988-20076Honker Bay433USGS2 4 7/90 5700 10/06100-5700200610-30019950.2-319932007<0.1-0.319961988-20032006-20077Grizzly BayD7*DWR2512/05 17,000 6/87300-16,0002006100-60019950.2-52005<0.1-0.619951987-20022006-20078Grizzly Bay417USGS<1 3 8/07 15,000 9/06 not enough data2006-20079San Pablo BayD41A*DWR2 16 9/89 39,000 5/00200-39,0002000400-100019940.3-1619890.2-119941987-20022006-200727Table 3.CorbulaGrazing Rates*using median grazing rate** Data for D7 Courtesy of DWR CALFED Biomass StudyA B C D E F GH I J K LHabitat(life stage)DatesWaterColumnDepth(m)CBL Corrected, 67% Feeding(m3m-2d-1)Pumping Rate(m3m-2d-1)mean median max minTurnoverTime(day)*mean median max minTurnoverTime(day)*1BenthicShallowD7 DWR**1988-200621 0.5 3 0 4116022BenthicDeep4.1 USGS1988-20068 1 1 10 0 82120083BenthicDeep8.1 USGS)1988-200617 3 2 15 0 674500428 

## VIII. Figures

Figures 6, 9, 10 and 12 are matrices showin g transitions from one life stage to the next. Negative and positive effects of processes on transition survival are designated by (-) and (+). Levels of importance, understanding and pr edictability of each process are given in the key below.

Low Medium Importance Understanding Predictability

Positive response (+) Negative response (-)29 Figure 1. Life cycle of Corbula amurensis in San Francisco Estuary. (Adapted from Nicolini, MH and DL Penry. 2000. Spawning, fertilization, and larval development of Potamocorbula amurensis (Mollusca: Bivalvia) from San Francisco Bay, Californi a. Pacific Science: 54(4):377-388 NO PERMISSION FOR PUB.)30 Figure 2. Distribution of months with peak Corbula recruitment in (a) the western Delta, (b) Grizzly Bay, (c) San Pablo Bay. Average recruits occurring each month in the wester n Delta over the period of 1988 through 2006 (d). Grizzly Bay data courte sy of DWR CALFED Biomass Study.31 Figure 3. Density and biomass of Corbula at the western Delta (a, b) and in Grizzly Bay (c,d). Grizzly Bay data courte sy of DWR CALFED Biomass Study. 

### b a

c

d32 Figure 4. Distribution of Corbula in SFE during (a) spring and summer months and (b) fall months. 1990 data from Hymanson (1991). 1995 data cour tesy of DWR.33 Figure 5. Life Cycle and habitat conceptual model for Corbula

.34 Figure 6. Transition probability of Corbula gametogenesis to recruit.35 Figure 7. Corbula recruit density in cores collect ed from station located at X2 km 64 (line on graph). Recruits are limited when the maximum excursion of X2 is downstream of the station, i.e. when salinities are less than 2 at the station the previous month prior to the sampling.36 Figure 8. Corbula recruit density as a function of (a) adult Corbula density and (b) Ampelisca abdita density.37 Figure 9. Transition probability of Corbula recruit to reproductive adult.38 Figure 10. Transition probability of Corbula one year old to a two year old or greater.39 Figure 11. Transition probability of phytoplankton to bloom status

.