4.0 Bivalve Monitoring

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4.1 Background

The purpose of monitoring contaminant concentrations in bivalve tissue for the RMP is two-fold. First, bivalves integrate the bioavailable portion of contaminants in the water column over time, and second, for many contaminants, bivalves are good indicators of contaminant transfer from water into the food web. Bivalves will accumulate certain contaminants in concentrations much greater than those found in ambient water (Vinogradov, 1959). This phenomenon is a result of the limited ability of bivalves to regulate the concentrations of most contaminants in their tissues. This method of active biomonitoring has been widely applied by the California State Mussel Watch Program (Phillips, 1988; Rasmussen, 1994) and others (Young et al., 1976; Wu and Levings, 1980; Hummel et al., 1990; Martincic et al., 1992). For reviews of bioaccumulation monitoring, see Luoma and Linville (1996) and Gunther and Davis (1997).

Bivalves were collected from sites thought to be uncontaminated and transplanted to 15 stations in the Estuary during the wet season (April) and the dry season (September). Contaminant concentrations in tissues, survival, and biological condition were measured before deployment (referred to as time zero (T-0) or background) and at the end of the 90-100 day deployment period. Because of the variability between each individual bivalve organism, composite samples of tissue were made from T-0 organisms and from surviving organisms from each deployment site (up to 45 individuals) for analyses of trace contaminants. The Corbicula reference site for the wet season was not optimal, since initial concentrations were found to be high after changing the site from Lake Isabella and Putah Creek to Lake Chabot. For the dry season clams could no longer be found at "clean" sites and consequently additional specimen were collected from a native population in the Sacramento and the San Joaquin rivers.

The effects of high short-term flows of freshwater on the transplanted bivalves west of Carquinez Strait were minimized by deploying the bivalves near the bottom where density gradients tend to maintain higher salinities. All bivalves were kept on ice after collection and deployed within 72 hours. Multiple species were deployed at several stations due to uncertain salinity regimes and tolerances. Detailed sampling and analysis methods are included in the Description of Methods. Data are tabulated in the Data Tables.

Overall, the bivalve bioaccumulation and condition study objectives for 1998 were met, although the unusual wet season with extremely high freshwater inputs from January until March caused high mortality rates in Mytilus spp. during the winter/spring deployment.

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4.2 Accumulation Factors

In addition to using the absolute tissue concentrations at the end of each deployment period and comparing them to initial tissue concentrations prior to transplanting the bivalves to the Estuary (T-0), this report uses accumulation factors (AFs) to indicate accumulation or depuration (loss of constituents from bivalve tissue) during the 90-100 day deployment period. The accumulation factor is calculated by dividing the contaminant concentration in transplants by the initial bivalve concentration at T-0. For example, an accumulation factor of 1.0 indicates that the concentration of a specific contaminant remained the same during the deployment period compared to the initial contaminant level prior to transplanting the bivalve sample to the Estuary. An AF less than 1 indicates that the bivalves decreased in contaminant concentration during the deployment period, while an AF above 1 indicates accumulation.

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4.3 Guidelines

Fish and shellfish consumption advisories for the public are issued by the California EPA°s Office of Environmental Health Hazard Assessment (OEHHA) to protect residents from the health risks of consuming contaminated non-commercially caught fish and wildlife. These advisories inform the public that high concentrations of chemical contaminants have been found in local fish and wildlife and include recommendations to limit or avoid consumption of certain fish and wildlife species from specific waterbodies or waterbody types. The U.S. EPA has developed guidance documents for estimating risks to human health from the consumption of chemically contaminated, non-commercial fish and wildlife. Figures 4.1-4.16 used the recommended tissue screening values (SVs) for use in State fish/shellfish consumption advisory programs for the general adult population* from table 5-2 of EPA document #823-R-95-007 (Methods for Sampling and Analyzing Contaminants in Fish and Shellfish Tissue). Tissue guidelines are generally expressed in wet weight, while the RMP tissue data are reported in dry weight. A wet-to-dry weight conversion factor of 7 was applied to the guideline values for comparative purposes. This value is based on an average moisture content in bivalves of 85%. Listed in Table 4.1 are converted dry weight SVs for those parameters reported by the RMP. It should be noted that the U.S. EPA screening values only apply to human health risks associated with consuming contaminated fish and wildlife. There are published guidelines protective of wildlife (NAS, 1973). However, they are only for total DDT and total PCB, although evidence exists of adverse effects on wildlife above certain contaminant thresholds for a variety of contaminants (e.g., Young et al., 1998).

* general adult population: Risk level = 10-5 for carcinogens given an average consumption rate of 6.5 g/day for a body weight of 70 kg.

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4.4 Biological Condition and Survival

The biological condition (expressed as the ratio of dry tissue weight to shell cavity volume) and survival rates of transplanted bivalves following exposure to Estuary water are evidence that the animals were healthy and capable of bioaccumulation at most sites (Figures 4.17 and 4.18). However, the data on survival and condition of the transplants indicate that certain sites are generating physiological stress in the animals at certain times, which confounds the interpretation of bioaccumulation data and interferes with the bivalves' usefulness as biomonitors.

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4.5 Bivalve Trends

Transplanted bivalves are valuable in assessment of long-term trends because they provide an integrated measure of contamination over a three month period. This interval is more appropriate for assessment of interannual trends than the one-hour interval represented by RMP water samples or the approximate 20 year interval represented by RMP sediment samples.

This section presents plots of RMP bivalve bioaccumulation data for trace elements and trace organics from 1993 to 1998 (Figures 4.19-4.31). Concentrations in these plots are expressed as net bioaccumulation or depuration during the deployment period (initial concentrations prior to deployment have been subtracted from final concentrations measured after deployment). Presented in this manner, the plots are capable of showing the presence or absence of both trends and accumulation during deployment. In many cases (e.g., arsenic) there was either little accumulation or even net depuration during deployment. Cadmium in mussels has exhibited a consistent seasonal pattern, with higher concentrations in summer samples in all six years. The trace metals database accumulated so far is fairly noisy, and clear trends are not expected to be discernible for the near future.

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4.6 Discussion

Bivalve monitoring is a valuable tool to assess to what degree contaminants in the water column of the Estuary are bioavailable and bioaccumulate. Bivalves are also a good indicator of long-term contaminant trends. As currently designed, this program component is unable to evaluate contaminant bioavailability and accumulation in different segments of the Estuary due to the different bioaccumulation characteristics of the three species deployed in segments with different salinities (see Method Section and RMP Regional Monitoring News Vol. 4, Issue 2 ¿RMP Bivalve Study Field Methods (or how we do what we do)î, by Jordan Gold and David Bell). http://www.sfei.org/rmp/rmp_news/vol_4_issue_2_html/Volume_4_Issue_2.html

Oysters showed higher PAH concentrations and greater accumulation during the 1998 dry season compared to previous years. Consistent with previous years, the concentrations measured were higher than any measured values in mussels or clams. The highest concentration was measured in oysters at Coyote Creek - about 48 times above the pre-deployment concentration. Oysters seem to either accumulate PAHs at a higher rate or have a lower capacity for metabolism and excretion of this compound.

The lipid-normalized data indicated higher PAH concentrations in all species for the 1998 wet season compared to the previous year, and above the running mean concentration. In contrast, the 1998 dry season concentrations in mussels were below the running mean from all years combined and showed a clear decrease from 1997 data. In 1998 mussels had the highest mean accumulation factor for PCBs for the dry season, although the absolute concentrations were within the range of previous years. Consistent with the previous years° data, the 1998 PCB tissue concentrations were strongly correlated with the lipid content of the bivalves. 1998 was the first year that oysters discontinued their decline in DDT concentrations.

Although the absolute concentrations were lower during the wet season than in previous years, the lipid-normalized values were higher for all stations but Grizzly Bay and Sacramento River. Given that during high rainfall years pollutants stored in the watershed may be mobilized, a reversal in the existing trend is not too surprising. Another significant difference occurred in mussels at Pinole Point. At this station, bivalves accumulated twice the amount of DDT than the year before during the dry season. Oysters appeared to have a much higher lipid-normalized dieldrin concentration than in previous years where the tissue concentration had decreased continuously. For all chlorinated hydrocarbons, the lipid normalized data showed an increase in concentrations for the bivalves deployed during the wet season.

There is a consistent pattern of higher CHC concentrations after the wet season that could be explained by increased mobilization of deposits of these compounds due to the unusually heavy rainfall during the El Niño year of 1998. Dry-season concentrations showed a distinct decrease in dieldrin and chlordane and a slight decrease for PCBs and DDTs compared to 1997. The DDT concentrations were lower in 1995 and 1996 so that a clear trend over the years is nor yet apparent for DDTs.

Yerba Buena Island (BC10) discontinued the decline for PCBs from the previous year; concentrations were slightly higher in 1998 compared to the 1997 dry season. Due to the loss of mooring during the winter deployment period, no comparable data for April are available for this station. A higher level in this contaminant concentration could be caused by intense mixing of the sediment due to strong tidal currents or winds. Paralleling the bivalve concentrations, a much higher level of PCBs was indicated in the sediment data for this station as well.

Lipid-normalization for HCHs and PAHs revealed patterns that were not always apparent otherwise. For example, the absolute concentrations in DDTs seemed to decrease for almost all stations in 1998°s wet season, but related to the lipid content of the bivalves, the values increased compared to 1997. Higher concentrations of total PAHs, PCBs, Chlordanes, and DDTs in the water column, especially at the Estuary Interface, Southern Sloughs, and South Bay were reflected in the tissue bioaccumulation. Regarding trace metals, oysters seem to accumulate cadmium to a higher degree consistently, while the other species do not exhibit substantial bioaccumulation over the years. As in previous years all bivalve species used in the program have not been good indicators for bioaccumulation of mercury.

For the 2000 monitoring year, mercury measurements in bivalves were discontinued and replaced with triennial fish tissue measurements as a better indicator for mercury bioaccumulation. The nickel concentration in mussels and oysters went slightly lower compared to previous years. For the first time clams accumulated nickel much more than Mytilus and Crasosstrea. The measured concentration at the end of the deployment period during wet season was about nine times higher than the initial concentration prior to deployment. The silver accumulation in mussels was considerably lower in 1998 compared to the previous year, only twice the initial concentration compared to 3-10 times in 1997. For the first time, mussels showed a lower accumulation than oysters, which maintained their accumulation range for silver. Mussels continued to exhibit the highest accumulation factor of the three bivalve species, but compared to previous years, absolute concentrations were considerably lower.

During the dry season, sampling high trace element concentrations occurred over the running mean, especially noticeable in Corbicula fluminea for lead, nickel, and zinc. These metals were consistently higher through the July water sampling as well, due to high stream flow and the highest Baywide TSS concentration ever measured in the RMP. These few metals seem to be heavily influenced by extreme hydrologic conditions. They are mobilized throughout the watershed and reflected in above-average bivalve tissue concentrations. On the other hand, concentrations for arsenic, copper, and tributyltin decreased significantly compared to the mean values of the previous years.

Generally, bioaccumulation factors were consistent and comparable to previous years of RMP sampling. Almost every metal°s accumulation varied within a range of plus or minus two times the contaminant accumulation. Only accumulation of nickel and lead during the wet season was three and five times higher in Corbicula fluminea in 1998 compared to 1997. This change may have been caused by a higher transport rate of these metals into the Estuary because of El Nino°s exceptionally heavy precipitation. 1998°s mean bioaccumulation factors showed no appreciable differences in pre- and post-deployment periods for most metals. Corbicula fluminea in the wet season showed the highest accumulation rates for lead (10.4 times), and nickel (9.3 times the initial concentration). Arsenic, selenium, and mercury did not show significant bioaccumulation above background concentrations in any of the three species. Cadmium, chromium, silver, and copper showed mean accumulation factors between 1.1 and 4.2, although a slight but consistent increase in copper over the last four years of RMP sampling is evident. Condition, % lipid, and % moisture measurements were made prior to deployment and after the transplants were collected to show natural variables affecting condition, such as weight loss due to reproduction, which can also account for a decrease in contaminant accumulation.

Some water quality parameters in the Estuary were outside optimum levels for the bivalve species and therefore may have affected bioaccumulation at certain times. In mussels, for example, survival, condition, and percent lipid are significantly positively related to dissolved oxygen and salinity (Hardin & Hoenicke, 1999). The unusual wet season with extremely high freshwater inputs caused an impact and higher mortality rates in Mytilus californianus. They are deployed at sites with highest expected salinities because the tolerance of the organism to freshwater exposure is low. Their natural habitat is the ocean°s intertidal and they only survive short-term exposure to salinities as low as 5%.

Other potential effects that dissolved oxygen, salinity, temperature, total suspended solids, and chlorophyll could have on the bioaccumulation of contaminants also confound the ability to describe spatial concentration patterns throughout the Bay. The San Francisco Estuary exhibits very high spatial and temporal variations in water quality parameters. That is why trends can be compared among sites with the same species only. Corbicula fluminea are no longer transplanted from clean reference locations, but resident clams from the Sacramento and San Joaquin Rivers have been analyzed since the 1998 dry season. Due to the missing T-0 value, the bioaccumulation factor for the dry season could not be analyzed here. Trend monitoring is much more responsive in bivalves than it is in water or sediment, because accumulation or even depuration is shown over a three months deployment period and not only for a one-hour sampling interval.

Most of the metals show either little net accumulation or even net depuration (e.g. arsenic and selenium) over the deployment period. Mercury, copper, and zinc in oysters continued to show a consistent seasonal pattern with higher concentrations during the summer sampling period. Trace organics trends indicate a much higher net bioaccumulation than trace metals. Slight seasonal patterns were exhibited in mussels for PAHs and PCBs with higher concentrations during the summer.

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4.7 References

Gunther, A.J. and J.A. Davis. 1997. An evaluation of bioaccumulation monitoring with transplanted bivalves in the RMP. In 1996 Annual Report: San Francisco Estuary Regional Monitoring Program for Trace Substances. San Francisco Estuary, Oakland, CA, pp. 187-200.

Hummel, H., R.H. Bogaards, J. Nieuwenhiuze, L. DeWolf, and J.M. VanLiere. 1990. Spatial and seasonal differences in the PCB content of the mussel Mytilus edulis. Science of the Total Environment 92:155-163.

Luoma, S.N. and R. Linville. 1996. A comparison of selenium and mercury concentrations in transplanted and resident bivalves from north San Francisco Bay. In 1995 Annual Report: San Francisco Estuary Regional Monitoring Program for Trace Substances. San Francisco Estuary, Oakland, CA, pp. 160-170.

Martincic, D., Z. Kwokal, Z. Peharec, D. Margus, and M. Branica. 1992. Distribution of Zn, Pb, Cd, and Cu between seawater and transplanted mussels (Mytilus galloprovinciatis). Science of the Total Environment 119:211-230.

NAS. 1973. National Academy of Sciences and National Academy of Engineering. NAS Guidelines and FDA Action Levels for Toxic Chemicals in Shellfish.

Phillips, P.T. 1988. California State Mussel Watch ten year data summary, 1977-1987. Water Quality Monitoring Report No. 87-3, Division of Water Quality, State Water Resources Control Board.

Rasmussen, D. 1994. State Mussel Watch Program, 1987Ê1993 Data Report. State Water Resources Control Board 94-1WQ.

U.S. EPA. Methods for Sampling and Analyzing Contaminants in Fish and Shellfish Tissue. U.S. EPA document #823-R-95-007. http://www.epa.gov/OST/fishadvice/vol1/doc2ndx.html.

Vinogradov, A.P. 1959. The geochemistry of rare and dispersed chemical elements in soils. Chapman and Hall, London.

Wu, R.S.S. and C.D. Levings. 1980. Mortality, growth and fecundity of transplanted mussel and barnacle populations near a pulp mill outfall. Marine Pollution Bulletin 11:11-15.

Young, D.R., T.C. Heesen, and D.J. McDermott. 1976. An offshore biomonitoring system for chlorinated hydrocarbons. Marine Pollution Bulletin 7:156-159.

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