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The structure of momentum and concentration boundary layers developing over a bed of Potamocorbula amurensis clam mimics was studied. Laser Doppler velocimetry (LDV) and laser-induced fluorescence (LIF) probes were used to quantify velocity and concentration profiles in a laboratory flume containing 3969 model clams. Model clams incorporated passive roughness, active siphon pumping, and the ability to filter a phytoplankton surrogate from the flow. Measurements were made for two crossflow velocities, four clam pumping rates, and two siphon heights. Simultaneous use of LDV and LIF probes permited direct calculation of scalar flux of phytoplankton to the bed. Results show that clam pumping rates have a pronounced effect on a wide range of turbulent quantities in the boundary layer. In particular, the vertical turbulent flux of scalar mass to the bed was approximately proportional to the rate of clam pumping.

The San Francisco Estuary (California, USA) had abundant pelagic fish in the late 1960s, but has few pelagic fish today. A primary cause for this decline in fish is thought to be a trophic cascade, triggered by declining phytoplankton. Here, we describe the changes in pelagic community structure of the San Francisco Estuary. Then, we examine whether changes in hydrodynamics due to freshwater exports, which increased exponentially beginning in 1967, in addition to the 1986 invasion by the clam Potamocorbula amurensis, explain the phytoplankton loss. Hydrodynamic variables were reconstructed back to 1956 using statistical models fit to, and cross-validated against, output from a hydrodynamic model. Then, we regressed mean summer/fall chlorophyll a-the season with the largest phytoplankton decline-against the reconstructed hydrodynamic variables and the presence/absence of P. amurensis for 1969-2014. The regression model, which explained 78% of the interannual variation in chlorophyll a, was then used to quantify the influence of P. amurensis and exports on chlorophyll a. Based on monitoring data, chlorophyll a declined 22-fold from 1969-2014, zooplankton declined 32-fold from 1972-2014, and pelagic fish declined 92-fold from 1968-2014. Averaged over 1990-2014, the chlorophyll a model ascribed an 88% decline in chlorophyll a to P. amurensis, a 74% decline to exports (at minimum), and a 97% decline to the combined influence of P. amurensis and exports (at minimum). Thus, the decline in pelagic productivity in the San Francisco Estuary has occurred largely due to the combined impacts of the P. amurensis invasion and increased freshwater exports.

Secondary production of macrobenthic communities is evaluated for 14 different-type estuaries in Primorye on the base of 52 surveys conducted in 2007-2013 (922 stations, 2426 samples). Species composition and abundance of macrobenthos in the estuaries depend mostly on water salinity. Three types of estuaries were distinguished: i) polyhaline with prevaled salinity 18-30 ‰ (external and internal estuaries of the rivers Ryazanovka, Barabashevka, Sukhodol, Tumanovka and external estuaries of the rivers Olga and Avvakumovka); ii) mesohaline with salinity 5-18 ‰ (external and internal estuaries of the rivers Tesnaya and Gladkaya, internal estuaries of the rivers Avvakumovka, Kievka, and Partizanskaya, Lake Presnoye and external estuary of the Razdolnaya River); and iii) oligohaline with salinity 0.5-5.0 ‰ (internal estuaries of the rivers Razdolnaya, Artemovka and Lake Solenoye). Mean annual biomass of macrozoobenthos is the lowest (2.8 ± 0.4 gС/m 2 ) in the polyhaline estuaries and the highest in the meso- and oligihaline ones (6.6 ± 3.0 and 7.0 ± 2.5 gС/m 2 , respectively). Annual production of macrobenthic communities in the estuaries varies from 0.5 to 11.2 gС/m 2 , with mean values 7.5 ± 1.0 gС/m 2 in the polyhaline estuaries, 4.1 ± 1.3 gС/m 2 in the mesohaline estuaries, and 2.7 ± 1.4 gС/m 2 in the oligohaline estuaries, it is the highest in the polyhaline estuaries and the lowest in the oligohaline ones. The production is formed mainly by gastropods, bivalves, polychaetes and amphipods in the polyhaline estuaries; the same groups and also isopods and amphibiotic insects larvae form the production in the mesohaline estuaries, and bivalves and polychaetes are the main producers in the oligohaline estuaries. Daily production of macrobenthic community has strong seasonal variation with the maximum in July-September and the minimum (sometimes below zero) in May-June and October, i.e. at the beginning and the end of vegetation season. The portion of carnivore macrobenthos production varies from 0.7 to 26.5 % of the total annual production of macrobenthic community, it is the highest in the mesohaline estuaries. It increases from spring to autumn; in the autumn predators have the best feeding because of high abundance of young benthic animals at the bottom of estuaries. Mean annual P/B -ratio is 3.4, 1.1 and 0.4 for the polyhaline, mesohaline and oligohaline estuaries, respectively, that is conditioned by species composition of dominant taxonomic groups. Species composition of bivalve mollusks is the main factor of macrobenthic production variability: brackish clam Corbicula japonica with high biomass (up to 5 kg/m 2 ) and low P/B -ratio (0.4-0.7, mean value ≈ 0.5) dominates in the mesohaline and oligohaline estuaries, while euryhaline marine bivalves as Macoma spp., Potamocorbula amurensis, Laternula marilina , and others dominate in the polyhaline estuaries though their biomass is relatively low (no more than 0.36 kg/m 2 ), their P/B -ratio is 0.8-4.5. Year-to-year variability of macrozoobenthos production is supposedly significant, at least its considerable fluctuations are registered in the Sukhodol estuary (from 4.8 to 11.2 gС/m 2 in the 2009-2012). Estuarine macrobenthic communities in Primorye usually have intermediate values of production between those in freshwater and seawater biotopes.

Dynamic processes (physical, chemical and biological) challenge our ability to quan- tify and manage the ecological risk of chemical contaminants in estuarine environments. Sele- nium (Se) bioavailability (defined by bioaccumulation), stable isotopes and molar carbon-to- nitrogen ratios in the benthic clam Potamocorbula amurensis, an important food source for predators, were determined monthly for 17 yr in northern San Francisco Bay. Se concentrations in the clams ranged from a low of 2 to a high of 22 µg g �1 over space and time. Little of that variability was stochastic, however. Statistical analyses and preliminary hydrodynamic modeling showed that a constant mid-estuarine input of Se, which was dispersed up- and down-estuary by tidal cur- rents, explained the general spatial patterns in accumulated Se among stations. Regression of Se bioavailability against river inflows suggested that processes driven by inflows were the primary driver of seasonal variability. River inflow also appeared to explain interannual variability but within the range of Se enrichment established at each station by source inputs. Evaluation of risks from Se contamination in estuaries requires the consideration of spatial and temporal variability on multiple scales and of the processes that drive that variability.

https://doi.org/10.15447/sfews.2017v15iss2art6The overbite clam (Potamocorbula amurensis) is a major invasive species in the San Francisco Estuary, California, and has been implicated in the decline of pelagic productivity and native fish species. Little is known of its impact on Suisun Marsh, a large brackish tidal region of the estuary. We looked at the abundance and spatial distribution of clams in the marsh, including examining the influence of water quality, using long-term (1988–2015) otter trawl surveys. Temporal trends indicated that overbite clam abundance has been increasing, but adult clams were spatially restricted to a single large slough (Suisun). Clams were absent from most interior channels, limiting their overall effect on the marsh aquatic ecosystem. Abiotic variables, particularly salinity, proved important predictors of overbite clam abundance, although the variables examined alone could not explain overbite clam distributions. We propose that connectivity, detritus loads, and/or predation pressure may work in conjunction with abiotic variables to cause poor survival rates for recruits in interior marsh sites, keeping the distribution limited. Overall results are encouraging for restoration projects in brackish tidal marshes that need to deal with overbite clams.

New Zealand’s current surveillance programme for marine pests does not include methods for identification of organisms in the water column, therefore dispersive forms go undetected. Molecular methods provide an opportunity to detect dispersive forms thereby contributing to a more robust surveillance programme. Additionally, New Zealand has become a signatory to the IMO Ballast Water Convention and there is scope for developing enumeration methods for detection of viable organisms as well as species-specific molecular probes for indicator bacteria. We describe here the outcome of experiments testing the effects of various matrices (e.g. sediment, biofilms, benthic assemblage, seawater) against a quantitative PCR (QPCR) assay developed for Potamocorbula amurensis. The limit of detection of the assay for sediments and benthic assemblages ranged between one to five larvae per 10 g wet weight. We also report a QPCR assay for Vibrio cholerae, one of the indicator species in the Ballast Water Convention. Quantitative PCR of seawater spiked with different concentrations of culture showed that assay sensitivity was insufficient to detect the regulatory limit of 1 colony forming unit (CFU) per 100 mL. More recent work suggests that sufficient sensitivity can be achieved with cultivation based methods (most probable number-QPCR [MPN-QPCR]). We are currently appraising several multispecies detection methods including micro-arrays and next generation sequencing for their suitability in marine pest detection. The advantages and disadvantages of each are discussed herein. We highlight the need to consider issues relating to sampling design when using molecular methods for surveillance and compliance monitoring.

The ability of bivalve filter feeders to limit phytoplankton biomass in shallow waters is well-documented, but the role of bivalves in shaping phytoplankton communities is not. The coupled effect of bivalve grazing at the sediment-water interface and sinking of phytoplankton cells to that bottom filtration zone could influence the relative biomass of sinking (diatoms) and non-sinking phytoplankton. Simulations with a pseudo-2D numerical model showed that benthic filter feeding can interact with sinking to alter diatom:non-diatom ratios. Cases with the smallest proportion of diatom biomass were those with the fastest sinking speeds and strongest bivalve grazing rates. Hydrodynamics modulated the coupled sinking-grazing influence on phytoplankton communities. For example, in simulations with persistent stratification, the non-sinking forms accumulated in the surface layer away from bottom grazers while the sinking forms dropped out of the surface layer toward bottom grazers. Tidal-scale stratification also influenced vertical gradients of the two groups in opposite ways. The model was applied to Suisun Bay, a low-salinity habitat of the San Francisco Bay system that was transformed by the introduction of the exotic clam Potamocorbula amurensis. Simulation results for this Bay were similar to (but more muted than) those for generic habitats, indicating that P. amurensis grazing could have caused a disproportionate loss of diatoms after its introduction. Our model simulations suggest bivalve grazing affects both phytoplankton biomass and community composition in shallow waters. We view these results as hypotheses to be tested with experiments and more complex modeling approaches.

We monitored the current macrozoobenthic community structure after the Sea Prince oil spill around Keumo-do and Sori-do, Korea. Macrobenthic animals were collected seasonally around Sori-Keumo do using a van Veen grab () from October 1999 to April 2000. In total, 196 species were identified at 46 sites around Keumho-do. The mean density was 1,460 individuals/, and polychaetes comprised 80% of the total density of the macrozoobenthic. Dominant species were the polychaete Tharyx sp. (51%), the echinoderm Amphioplus megapomus (5%) and the polychaete Lumbrineris longifolia (3%). We identified 176 macrobenthic species at 36 sites around Sori-do. The mean density was 1,068 individuals/, and polychaetes was the most abundant faunal group, comprising 41%, followed by mollusks (24%) and echinoderms (20%). Dominant species in Sori-do were the mollusk Potamocorbula amurensis (23%), the chinoderm Amphioplus megapomus (14%), and the polychaete Tharyx sp. (10%). The overall community structures in the study area showed few seasonal changes, although there was a gradual change in the species composition within each benthic community. Tharyx sp. was the most abundant species in the study area. Because this species has not been found in the other coastal areas except during recent investigations, it is considered to have increased during the course of community succession after the oil spill. The macrozoobenthic around Keumo-do showed little change in species richness, whereas around Sori-do, it showed a few increases in species richness after the summer of 1998. The overall density showed the same trend as species richness. Based on improved community indices, we suggest that the macrozoobenthic communities around Sori-Keumo Islands are gradually recovering from the oil-spill damagecaused by the oil spill.

Invasions of exotic invertebrates have greatly altered many aquatic communities, but impacts on the foraging energetics of predators seldom have been assessed. In San Francisco Bay, California (USA), a major community change occurred with introduction of the Asian clam (Potamocorbula amurensis) in 1986. This species now greatly outnumbers the previous clam prey of a variety of sharks, rays, sturgeon, flatfish, and crabs, as well as several diving duck species for which the bay is the most important wintering area on the U.S. Pacific Coast. P. amurensis also accumulates much higher levels of some contaminants than the formerly dominant prey. Because alteration of the food base or contaminated foods on wintering areas may be factors in the population decline of scaup ducks, effects of this exotic invasion are important to assess. For Lesser Scaup (Aythya affinis), we studied effects of differences in nutrient content, digestibility, crushing resistance of shells, areal density, size, and depth in the sediments on the relative foraging value of exotic P. amurensis vs. the formerly dominant native clam Macoma balthica. P. amurensis, including shells, had higher nitrogen and energy content per clam of the same length class, and higher digestibility of energy, than M. balthica. Gut retention time did not differ between clam species, so their relative profitability for scaup was determined mainly by the intake rate of digestible nutrients during short, costly dives. For scaup foraging in an aquarium 1.8 m deep, intake rates (number of prey per second) of food items buried in sand-filled trays increased with increasing prey density up to at least 4000 prey/m2. For items buried 3 cm deep, intake rates did not differ for prey

Species composition and distribution patterns of shellfish were investigated in Eoeun and Geojeon tidal flat located in the Saemangeum area on the west coast of Korea from May to October 2000. Nineteen species of shellfish were collected in Geojeon tidal flat. The samples in number of individuals included Umbonium thomasi and Mactra veneriformis In Eoeun tidal flat, ten species of shellfish were collected and the dominant species in number of individuals were Potamocorbula amurensis and U. thomasi These results were compared with previous studies conducted before the beginning of reclamation in Saemangeum. Compared with the results from 1988, a change In species composition was observed. Laternula flexuosa and Nuttallia olivacea appeared, while Mactra chinensis and Coelomactra antiquata disappeared in the deposition area in Geojeon tidal flat. In the erosion area of Eoeun tidal flat, M. veneriformis and Meretrix lusoria appeared, while Cyclina sinensis disappeared. Based on a cluster analysis, the shellfish community in Eoeun tidal flat was classified into three station groups based on sediment types. Geojeon tidal flat was also classified into three station groups. The distribution of shellfish in the Saemangeum area was closely related to the sediment types.

We compared dietary patterns within a temperate estuarine fish assemblage (Suisun Marsh, CA, U.S.A.) during a period of high mysid shrimp abundance and after a major decline in mysid abundance caused by the invasion of the overbite clam Potamocorbula amurensis. Prior to the invasion, high dietary overlap, high stomach fullness, and low niche breadth occurred among the fishes in spring when mysid populations were high. Dietary overlaps decreased and niche breadth increased for all species but the endemic splittail Pogonichthys macrolepidotus in fall when mysid populations were low. Eight native species exhibited lower overall collective overlaps and fuller stomachs than five alien species, suggesting more efficient resource partitioning. After mysid abundance declined, only alien striped bass Morone saxatilis preyed upon mysids in greater than trace amounts. An alien mysid became an important prey for small striped bass, but striped bass also switched to piscivory at a smaller size than when mysids were abundant. Eight of 13 species exhibited significant declines in abundance during the study period, which were concordant with the original importance of mysids in their diets. Our results suggest that altered lower food web dynamics in the San Francisco Estuary caused by the invasion of the overbite clam changed fish diets and have contributed to declines in fish abundance.

This paper examines how the subcellular partitioning of Cd and Zn in the bivalves Macoma balthica and Potamocorbula amurensis may affect the trophic transfer of metal to predators. Results show that the partitioning of metals to organelles, 'enzymes' and metallothioneins (MT) com- prise a subcellular compartment containing trophically available metal (TAM; i.e. metal trophically available to predators), and that because this partitioning varies with species, animal size and metal, TAM is similarly influenced. Clams from San Francisco Bay, California, were exposed for 14 d to 3.5 µg l -1 Cd and 20.5 µg l -1 Zn, including 109 Cd and 65 Zn as radiotracers, and were used in feeding experiments with grass shrimp Palaemon macrodatylus, or used to investigate the subcellular parti- tioning of metal. Grass shrimp fed Cd-contaminated P. amurensis absorbed ~60% of ingested Cd, which was in accordance with the partitioning of Cd to the bivalve's TAM compartment (i.e. Cd asso- ciated with organelles, 'enzymes' and MT); a similar relationship was found in previous studies with grass shrimp fed Cd-contaminated oligochaetes. Thus, TAM may be used as a tool to predict the trophic transfer of at least Cd. Subcellular fractionation revealed that ~34% of both the Cd and Zn accumulated by M. balthica was associated with TAM, while partitioning to TAM in P. amurensis was metal-dependent (~60% for TAM-Cd%, ~73% for TAM-Zn%). The greater TAM-Cd% of P. amuren- sis than M. balthica is due to preferential binding of Cd to MT and 'enzymes', while enhanced TAM- Zn% of P. amurensis results from a greater binding of Zn to organelles. TAM for most species-metal combinations was size-dependent, decreasing with increased clam size. Based on field data, it is esti- mated that of the 2 bivalves, P. amurensis poses the greater threat of Cd exposure to predators because of higher tissue concentrations and greater partitioning as TAM; exposure of Zn to predators would be similar between these species.

Many aspects of metal accumulation in aquatic invertebrates (i.e. toxicity, tolerance and trophic transfer) can be understood by examining the subcellular partitioning of accumulated metal. In this paper, we use a compartmentalization approach to interpret the significance of metal, species and size dependence in the subcellular partitioning of Cd and Zn in the bivalves Macoma balthica and Potamocorbula amurensis. Of special interest is the compartmentalization of metal as metal- sensitive fractions (MSF) (i.e. organelles and heat-sensitive proteins, termed 'enzymes' hereafter) and biologically detoxified metal (BDM) (i.e. metallothioneins (MT) and metal-rich granules (MRG)). Clams from San Francisco Bay, CA, were exposed for 14 d to seawater (20 ‰ salinity) containing 3.5 µg l -1 Cd and 20.5 µg l -1 Zn, including 109 Cd and 65 Zn as radiotracers. Uptake was followed by 21 d of depuration. The subcellular partitioning of metal within clams was examined following exposure and loss. P. amurensis accumulated ~22 × more Cd and ~2 × more Zn than M. balthica. MT played an important role in the storage of Cd in P. amurensis, while organelles were the major site of Zn accu- mulation. In M. balthica, Cd and Zn partitioned similarly, although the pathway of detoxification was metal-specific (MRG for Cd; MRG and MT for Zn). Upon loss, M. balthica depurated ~40% of Cd with Zn being retained; P. amurensis retained Cd and depurated Zn (~40%). During efflux, Cd and Zn concentrations in the MSF compartment of both clams declined with metal either being lost from the animal or being transferred to the BDM compartment. Subcellular compartmentalization was also size-dependent, with the importance of BDM increasing with clam size; MSF decreased accordingly. We hypothesized that progressive retention of metal as BDM (i.e. MRG) with age may lead to size dependency of metal concentrations often observed in some populations of M. balthica.

We describe a laboratory investigation into the effect of turbulent hydrodynamic stresses on clam larvae in the settlement phase of the recruitment process. A two-component laser-Doppler anemometer (LDA) was used to measure time histories of the instantaneous turbulence structure at potential recruitment sites within reconstructed beds of the adult Asian clam, Potamocorbula amurensis.Measurements were made for two flow speeds over beds with three different clam densities and two different clam heights. We analyze the statistical effect of the turbulence on the larval flux to the bed and on the probability of successful anchoring to the substrate. It is shown that the anchoring probability depends on the nature of the instantaneous stress events rather than on mean stresses. The instantaneous turbulence structure near the bed is altered by the flow rate and the spacing and height of adult clams living in the substrate. The ability to anchor quickly is therefore extremely important, since the time sequence of episodic turbulent stress events influences larval settlement success. The probability of successful larval settlement is predicted to decrease as the spacing between adults decreases, implying that the hydrodynamics impose negative feedback on clam bed aggregation dynamics. Species success, population density, and community structure are all dependent on larval recruitment. Therefore, the processes of settlement and recruitment of benthic and epibenthic communities have been the subject of substantial research (see Olafsson et al. 1994; Wildish and Kristmanson 1997 for recent reviews). A major portion of the work has involved the transport of larvae to the bed and settling of the larvae by passive or active selection processes ( see reviews by Butman 1987; Abelson and Denny 1997). Several researchers have found that it is the combination of larval behavior (e.g., swimming, crawling, burrowing) and physical processes that deliver the larvae to the bed that is responsible for the recruitment success (Mullineaux and Butman 1990; Butman and Grassle 1992; Grassle et al. 1992; Snelgrove et al. 1993). As summarized by Underwood and Keough (2001), it is no longer a question of whether hydrodynamics or behavior are important in recruitment, rather it is a question of ‘‘under which conditions is larval behavior important?’’ We will expand this question and say that it is also important to determine the periods, during the relevant physical processes, when larval behavior is important. Related to the interest in larval recruitment is an ecological interest in what regulates temporal, spatial, and agesegregated distributions of species. Thus, the processes important in determining how aggregations of benthic 1

Although nutrient supply often underlies long-term changes in aquatic primary production, other regulatory processes can be important. The Sacramento‐San Joaquin River Delta, a complex of tidal waterways forming the landward portion of the San Francisco Estuary, has ample nutrient supplies, enabling us to examine alternate regulatory mechanisms over a 21-yr period. Delta-wide primary productivity was reconstructed from historical water quality data for 1975‐1995. Annual primary production averaged 70 g C m 22 , but it varied by over a factor of five among years. At least four processes contributed to this variability: (1) invasion of the clam Potamocorbula amurensis led to a persistent decrease in phytoplankton biomass (chlorophyll a) after 1986; (2) a long-term decline in total suspended solids—probably at least partly because of upstream dam construction—increased water transparency and phytoplankton growth rate; (3) river inflow, reflecting climate variability, affected biomass through fluctuations in flushing and growth rates through fluctuations in total suspended solids; and (4) an additional pathway manifesting as a long-term decline in winter phytoplankton biomass has been identified, but its genesis is uncertain. Overall, the Delta lost 43% in annual primary production during the period. Given the evidence for food limitation of primary consumers, these findings provide a partial explanation for widespread Delta species declines over the past few decades. Turbid nutrient-rich systems such as the Delta may be inherently more variable than other tidal systems because certain compensatory processes are absent. Comparisons among systems, however, can be tenuous because conclusions about the magnitude and mechanisms of variability are dependent on length of data record.

All ecosystems are influenced by physical forcing. Estuarine ecosystems respond most strongly on an interannual timescale to variability in freshwater flow. Several mechanisms for positive or negative flow effects on biological populations in estuaries have been proposed; however, positive effects appear to operate mainly through stimulation of primary production with effects propagating up the food web. In the northern San Francisco Estuary, abundance or survival of several common species of fish and shrimp varied positively with flow-in data through 1992. I re-examined these relationships and those of several additional taxa in an analysis of long-term (20 to 40 yr) mon- itoring data. The spread of the introduced clam Potamocorbula amurensis in 1987 provided an oppor- tunity to examine simultaneously the responses of estuarine species to flow and to changes in the food web. I separated variability into a flow response, a step change after 1987 and other sources of variability. Responses of fish and shrimp contrasted with those of lower trophic levels. All but 1 spe- cies of nekton responded positively to flow, only 2 had clear declines after 1987, and none of the rela- tionships changed in slope after 1987. In contrast with the higher trophic levels, chlorophyll a (chl a) and several species of zooplankton declined markedly after 1987, and had either weak responses to flow or responses that changed after 1987. Thus, the food web appears strongly coupled between benthos and plankton, and weakly coupled between zooplankton and fish, as has been found in other systems. More importantly, the variation with freshwater flow of abundance or survival of organisms in higher trophic levels apparently did not occur through upward trophic transfer, since a similar rela- tionship was lacking in most of the data on lower trophic levels. Rather, this variation may occur through attributes of physical habitat that vary with flow.

Phytoplankton are an important source of selenium (Se) for aquatic invertebrates, which accumulate Se primarily through dietary ingestion. The extent to which Se bioavailability varies among different phytoplankton species could help explain different bioaccumulation patterns observed for invertebrates in nature. We measured the efficiency with which 3 benthic invertebrates assimilated 75 Se from 5 phytoplankton species using standard pulse-chase techniques. The inverte- brates included the amphipod Leptocheirus plumulosus and the bivalves Macoma balthica and Pota- mocorbula amurensis. The phytoplankton species included Cryptomonas sp. (Cryptophyceae), Gymnodinium sanguinem (Dinophyceae), Phaeodactylum tricornutum (Bacillariophyceae), Syne- chococcus sp. (Cyanophyceae) and Thalassiosira pseudonana (Bacillariophyceae). The range of Se assimilation efficiency (AE) by L. plumulosus (32.1 ± 1.8 to 69.5 ± 7.1%) was the lowest of the 3 organ- isms. No relationship was observed between the proportion of Se in algal cell cytoplasm and Se AE by L. plumulosus, which is consistent with findings for assimilation of other trace elements by this organism. Se AE by M. balthica (range: 58.0 ± 3.2 to 92.3 ± 6.0%) varied according to the proportion of cytoplasmic Se in algal cells (p < 0.0001, r 2 = 0.868). P. amurensis assimilated between 78.3 ± 2.0 and 88.9 ± 3.6% of Se from algal cells, and the relationship between cytoplasmic Se and Se AE was described by the following equation: Se AE = 69.2 + 0.22 · (% cytoplasmic Se) (p = 0.003, r 2 = 0.405). This relationship suggests that P. amurensis assimilated non-cytoplasmic Se from phytoplankton, perhaps through utilization of the glandular digestive pathway. Consistently high Se assimilation from algae by P. amurensis may contribute to elevated Se concentrations observed for this organism.

Monthly censusing of reproductive condition of the Asian clam Pota- mocorbula amurensis at four sites in northern San Francisco Bay over a 9-yr pe- riod revealed year-to-year differences in local reproductive activity that are associated with patterns of hydrologic variability. Between 1989 and 1992, Northern California experienced a drought, whereas the period between 1993 and 1998 was marked by a mix of wet and dry years. We took advantage of the extreme year-to-year differences to examine reproductive responses to river in- flow patterns. Populations of P. amurensis at the upstream sites in Suisun Bay and Carquinez Strait were more reproductively active during wet years than dry years. Conversely, at the downstream site in San Pablo Bay, the population was more reproductively active during dry years than wet years. We suggest that the different reproductive patterns observed reflect the clam's response to different sources of food. During wet years, organic matter from the rivers augments food supplies in Suisun Bay. During dry years, when inflow into the San Francisco Bay Estuary from the rivers is reduced, water transported from the adjacent ocean into the estuary as far as San Pablo Bay provides a supplemental food supply for the local production. The populations take advantage of these spa- tially distinct food supplies by initiating and maintaining local reproductive activity. We conclude that the ability of P. amurensis to consume and use various types of food to regulate its reproductive activity is part of the reason for its success as an invasive species.

In biological systems energy serves as the ultimate commodity, often determining species distributions, abundances, and interactions including the potential impact of invasive species on native communities. The Asian clam Potamocorbula amurensis invaded the San Francisco Estuary (SFE) in 1986 and is implicated in the decline of native fish species through resource competition. Using a combined laboratory/field study we examined how energy expenditure in this clam is influenced by salinity, temperature and food availability. Measures of metabolism were made at whole organism (metabolic rate) and biochemical (pyruvate kinase (PK) and citrate synthase (CS) enzyme activities) levels. We found in the field, over the course of a year, the ratio of PK to CS was typically 1.0 suggesting that aerobic and fermentative metabolism were roughly equivalent, except for particular periods characterized by low salinity, higher temperatures, and intermediate food availabilities. In a 30-day laboratory acclimation experiment, however, neither metabolic rate nor PK:CS ratio was consistently influenced by the same variables, though the potential for fermentative pathways did predominate. We conclude that in field collected animals, the addition of biochemical measures of energetic state provide little additional information to the previously measured whole organism metabolic rate. In addition, much of the variation in the laboratory remained unexplained and additional variables, including reproductive stage or body condition may influence laboratory-based results. Further study of adult clams must consider the role of organismal condition, especially reproductive state, in comparisons of laboratory experiments and field observations.

Corbula gibba (Olivi, 1792) a small exotic bivalve from the eastern Atlantic and the Mediterranean has been present in Port Phillip Bay, Victoria, Australia, from as early as 1987. C. gibba is widespread, abundant, and possesses characteristics such as fast growth which may confer a competitive advantage over endemic species like the commercial scallop Pecten fumatus Reeve, 1855. Experimental manipulations in the field showed that ambient densities of C. gibba significantly affected the size and growth of juvenile P. fumatus. This effect was greater than the effect of an equivalent number of conspecific scallops. C. gibba did not, however, affect scallop condition or survival in the short term, although the power of the test to detect a 50% decrease in survival was low. The percentage of experimental scallops lost to different sources of mortality was also unaffected by the presence of C. gibba, as was the density of predators found in experimental enclosures. This study is the first to experimentally demonstrate the impact of an introduced marine mollusc on an endemic species in Australia. A possible mechanism underlying this impact is compe- tition for food, as both species are suspension feeders which live in an environment where resource limitation may occur.