George R. Abbe

Pathways of silver uptake and accumulation by the American oyster (Crassostrea virginica) in Chesapeake Bay

Abstract Metabolic pathways of silver (Ag) accumulation were examined by exposing hatchery-reared oysters to Ag in a variety of forms including dissolved in water, associated with phytoplankton, and sorbed to sediments. Silver dissolved in water (2 and 5 μg l −1 ) was rapidly accumulated, with oysters reaching soft-tissue concentrations of 4 to 6 μg g −1 (several times that of controls) in 3–4 weeks. Uptake was negligible from suspended sediments as oysters rejected the particles as pseudofeces. Uptake of Ag associated with algal cells was negligible also: virtually all of this Ag passed through the oysters and was eliminated in the feces. Thus, the major pathway of accumulation in these studies was from dissolved Ag.

Silver and copper accumulation in two estuarine bivalves, the eastern oyster (Crassostrea virginica) and the hooked mussel (Ischadium recurvum) in the Patuxent River estuary, Maryland

In order to assess the source of trace elements and to compare uptake between different bivalve species, oysters and mussels were transplanted to five sites in the upper Patuxent River estuary during 1988. Transplant sites were located above and below the discharge of Chalk Point Steam Electric Station (CPSES)—a historic point source for copper. Organisms were sampled approximately monthly for 1 yr and analyzed for copper and silver. During spring and summer, concentrations of silver and copper increased in oysters at the upstream stations. There was very little change in silver and copper concentrations in the mussels with either time or among stations. Copper concentrations accumulated by oysters approached those found during previous periods when CPSES used Cu−Ni alloy condenser tubes (1966–1987), suggesting that corrosion from the Cu−Ni condenser tubes was only a minor contributor to the copper burdens of oysters in the river nearby. The lack of accumulation by mussels at the same sites suggests that part of the reason for the accumulation by oysters may be a taxonomically specific physiological effect caused by the salinity regime in the upper Patuxent.

Factors that influence the accumulation of copper and cadmium by transplanted eastern oysters (Crassostrea virginica) in the Patuxent River, Maryland

To investigate the continued accumulation of copper and cadmium by oysters in the Patuxent River, MD, which have been at high levels since at least the mid 1960s, hatchery-raised Eastern oysters were transplanted into trays at four sites in the upper estuary. At each site two groups of oysters were used to determine growth and mortality, and another group was sampled for meat condition, metal concentration and body burden. Copper in oysters in the discharge of a coal-fired power plant was significantly greater than at all other sites, but maximum concentration of 310 micrograms g-1 was well below the maximum of 1880 micrograms g-1 detected in 1982. Cadmium levels were also significantly greater in the discharge than at the other sites, but probably because of higher discharge temperatures. Trace metal loadings, the local salinity gradient and the higher temperatures caused by power plant operation all appear to contribute to metal accumulation by oysters in the upper Patuxent estuary.

Temporal and spatial variations of trace metal concentrations in oysters from the Patuxent River, Maryland

Concentrations of copper (Cu), silver (Ag), and cadmium (Cd) in eastern oysters (Crassostrea virginica) from the upper Patuxent River estuary analyzed since 1986 (Cd since 1991) were high relative to concentrations in other sites in the United States analyzed by the National Oceanic and Atmospheric Administration National Status and Trends program. Patuxent River oysters had above average concentrations of Ag and Cu, and unusually high concentrations of Cd. Metal concentrations were highest in summer, a period in which oyster meat condition index was relatively low. Copper values were highest in 1986–1987, likely elevated by erosion from Cu-Ni alloy condensers at a local power plant. Silver and Cd values exhibited more year-to-year variation. A number of factors were examined as candidates to explain the interannual differences, including river flow, salinity, and oyster condition, but none was able to explain the high and low years. Samples collected in spring and late summer of 1996 at a number of oyster bars located along the length of the Patuxent River showed that concentrations of all three metals increased with distance up-river. For Cd, the upstream increase was linear with distance, while for Ag and Cu, there was a secondary maximum near river km 16. A close correlation was observed between Ag and Cu for individuals at each site, with a poorer correlation between Cd and either Ag or Cu.

Condenser replacement in a coastal power plant: copper uptake and incorporation in the American oyster, Crassostrea virginica

Abstract In order to understand the effects of copper releases from power plants and to determine the impact of replacement of the cooling system condensers on such releases, three sets of genetically similar oysters were placed in the Patuxent River estuary in the vicinity of the Chalk Point Steam Electric Station from May to December, 1982 and monitored along with copper concentrations in the water. Copper discharges from the plant were approximately twice those estimated before condenser replacement, even though only one of two electrical generating units was operating during this time, and it was not in continuous operation. Copper was accumulated by oysters at all stations, but the accumulation rate in the plants Discharge Canal was much greater than that of stations located in the estuary. Final average copper concentration of Discharge Canal oysters was 1480 mg kg −1 , compared with 225–400 mg kg −1 at surrounding stations. Mean copper-zinc ratios indicated that the power plant was largely responsible for increased copper levels in oysters. Although Discharge Canal oysters had the highest copper concentrations, they also had significantly greater overall growth rates and significantly lower mortality, indicating little adverse effect of their 31-week exposure to plant effluent. However, this study did not encompass worst-case conditions; continuous operation of both units could yield higher copper concentrations over a much wider area of the estuary.

Silver uptake and subsequent effects on growth and species composition in an estuarine community.

Silver within Chesapeake Bay is rapidly taken up by phytoplankton, in accordance with geochemical controls over silver speciation. Phytoplankton accumulate large cellular burdens at non-lethal concentrations, setting up the potential for transfer of silver to herbivores that graze upon the phytoplankton community. However, the American oyster, Crassostrea virginica, does not assimilate silver from phytoplankton, but rather from that dissolved in water. Oysters can accumulate high burdens of silver via this uptake pathway, without apparent effect. There is another, perhaps more significant, mechanism by which silver can impact higher trophic levels. Continuous inputs of low concentrations of silver can cause large changes in species composition and species succession in Chesapeake Bay phytoplankton communities maintained in experimental ecosystems. In addition, oyster growth was reduced significantly relative to growth of oysters of unaltered communities. Such indirect effects of low levels of toxic substances, although difficult to detect, must be considered when assessments of pollutant impacts are made.

Decline in Size of Male Blue Crabs (Callinectes sapidus) from 1968 to 2000 near Calvert Cliffs, Maryland

Maryland commercial landings of the blue crab (Callinectes sapidus) and catch per unit effort (CPUE) have remained fairly stable over the past 33 yr despite occasional large deviations from the long-term average. During this time, however, significant declines in the percent of legal male crabs and the mean size of legal males caught in fishery-independent surveys near Calvert Cliffs, Maryland have become apparent. Sublegal females and two of the three legal female classes (152–177 and >178 mm CW) showed no significant trends over this 33-yr period when examined by linear regression. Males showed significant trends for all size classes. Sublegal males increased from 24% of the male population during the first 5 yr of the study (1968–1972) to 71% during 1996–2000. All classes of legal males, however, exhibited downward trends. Males 127–151 mm CW decreased from 45% of the male population in the earliest period to 22% during the last 5 yr. Males 152–177 mm decreased from 27% during 1968–1972 to only 6% during 1996–2000, and males > 178 mm declined from 4% in the earliest period to 0.5% in the recent period. These size decreases for the most valuable portion of the blue crab population are further evidence of over-exploitation. The declining trends in male size indicate that growth overfishing is occurring as intense fishing pressure removes so many male crabs from the population as they reach legal size that few remain to molt to larger size. A 3-yr data set from the Patuxent River, where commercial use of crab pots is restricted and fishing pressure is lower, suggests that legal male crabs are able to attain larger size compared to an area where the pot fishery is intense. A recommendation could be made for reducing effort where the pot fishery is intense by means of time, gear, catch limits, and/or by increasing the minimum size of legal crabs to allow larger crabs to enter the fishery.

The influence of oyster toadfish (Opsanus tau) and crabs (Callinectes sapidus and Xanthidae) on survival of oyster (Crassostrea virginica) spat in Chesapeake Bay: does spat protection always work?

Abstract Field experiments were conducted in central Chesapeake Bay to determine whether oyster toadfish Opsanus tau ( Linnaeus ), mud crabs ( mostly Panopeus herbstii Milne-Edwards), and blue crabs ( Callinectes sapidus Rathbun) influence the survival of juvenile oysters ( Crassostrea virginica (Gmelin ) ), and whether interactions among these species modify their individual effects. Mesh-lined steel cages were used to manipulate toadfish and blue crab access to oyster spat and mud crabs. Oyster survival in the presence of toadfish was no different than in caged or uncaged controls. Spat set on cultch tended to have slightly lower survival in cages with only blue crabs than in other treatments. Survival of both cultchless spat and spat set on shells was negatively correlated with numbers of mud crabs in some, but not all, sets of replicates. Mud crabs were more abundant in August than in September each year and were also more abundant in 1988 than in 1987. Both temporal and habitat factors were probably responsible for the patterns of mud crab abundances. Results of 2 years of experimental studies suggest that the oyster toadfish is not an effective means of oyster predator control in the subtidal area of the Chesapeake Bay studied. These results contrast with those of studies conducted on oyster spat elsewhere and with tests of the use of toadfish to protect juvenile hard clams.

The role of suspended sediments and phytoplankton in the partitioning and transport of silver in estuaries

Abstract Silver sorbs readily at low salinities to both phytoplankton and suspended sediments. As salinity increases, the degree of sorption decreases. Nearly 80% of silver sorbed to suspended sediments at low salinities will desorb at higher salinities, but desorption does not occur when silver is associated with phytoplankton. Thus, silver incorporation onto/into cellular material will increase the retention of silver within the estuary, reducing its rate of transport.

Silver Transport and Impact in Estuarine and Marine Systems

The transport of silver through estuarine and coastal marine systems is dependent upon biological uptake and incorporation. Uptake by phytoplankton is rapid, in proportion to silver concentration, and inversely proportional to salinity. In contrast to studies performed with other toxic metals, silver availability appears to be controlled by both the free silver ion concentration and the concentrations of other silver complexes, perhaps the ion pair AgCl. Silver incorporated by phytoplankton is not lost as salinity increases; as a result, silver associated with cellular material is largely retained within the estuary. Phytoplankton exhibit a variable sensitivity to silver. Sensitive species exhibit a marked delay in the onset of growth in response to silver at low concentrations, even though maximum growth rates are similar to controls. Sublethal effects, such as delayed growth, may not be observed during classical toxicity testing of a population; however, the effects to the community may be quite significant. A delay in the onset of growth reduces the ability of population to respond to short-term favorable conditions and to succeed within the community, ensuring the dominance of resistant species with high initial rates of growth. Reliable assessment of the vulnerability of dynamic ecosystems requires that studies of potential pollutant impact consider geochemical and biogeochemical transformations and sublethal pressures upon community structure as well as the toxicity of the pollutant to keystone species.