RESULTS
Hydrologic setting
California is characterized as having a Mediterranean
climate with distinct wet and dry seasons. These seasons
result in large fluctuations in freshwater inflow to
the northern reach of San Francisco Bay. The magnitude
of these fluctuations can vary greatly from year-to-year.
The study described here occurred
over a period of extreme year-to-year differences in
hydrodynamics (hydrograph).
Drought conditions persisted from 1989 through 1992.
In 1993 there was an end to a 7-year drought but in
1994 the drought conditions returned for one year. The
years following 1994 were "wet" with several
of the years being considered extreme.
The seasonal and annual hydrologic
variability captured during this study exposes aquatic
organisms living in the estuary to highly variable environmental
conditions. These include fluctuations in salinity,
contaminant inputs from both the rivers and the bay,
both dissolved and sediment bound, suspended sediment
loads, and carbon load to the bay. Understanding the
seasonal and annual variability is critical to deciphering
natural and anthropogenic effects.
Ag
Concentrations
For the overall study period,
the grand mean Ag concentrations in clam tissues were
highest at the two mid estuary sites, CNWS and CS ,
and lowest at the two end-estuary sites. Annually, Ag
concentrations were observed to decrease over the study
period with the two mid estuary sites, CNWS and CS showing
the most dramatic decrease (annual
means). Variability showed was highest at the two
most contaminated sites, CNWS and CS.
The monthly patterns of tissue Ag
concentrations showed that variability in annual mean
silver concentrations was related to inflow from the
Sacramento and San Joaquin rivers. Silver was seen to
accumulate in clam tissues during extended periods of
low flow. Periods of increased discharge were associated
with decreases in Ag concentrations in clam tissues
(monthly
means at Carquinez Strait ).
Reproductive
Activity
With a few exceptions, the majority of clams at the
estuary end-member sites were observed to be reproductive,
81% and 80% for MI and SPB, respectively. In contrast,
the populations at the CNWS and CS were only reproductive
67% and 61% of the months. For these two mid-estuary
sites, the proportion of clams reproductively active
was seen to increase over throughout the study period.
Comparison of reproductive activity with Ag concentrations
showed that the increase was coincident with a decrease
in metal concentration (reproductive
activity and Ag concentration). When Ag concentrations
were highest (annual mean >1ppb), clams were reproductively
active only 20 to 60% of the year. In contrast, when
Ag concentrations were lowest (annual mean <1ppb),
clams were reproductively active 70 to 100% of the year.
A significant correlation was observed when the reproduction
data was combined with the metal data (Reproductive
activity vs Ag).
Discussion
The most likely source of Ag
contamination is anthropogenic input. The high concentrations
observed at the two mid-estuary sites during the early
part of this study suggest local sources. However, it
is difficult to determine the exact source of contamination.
In 1993, new restrictions were imposed to reduce the
amount of Ag in industrial wastewater. For example,
at the Contra Costa County sewage treatment plant, the
concentration of Ag in effluent was seen to drop from
11ppb to <1ppb. The decrease in inputs coupled with
the changing hydrologic environment is most likely responsible
for the drop in Ag contamination.
A number of factors may affect reproductive
activity in clams. A short list includes changes in
salinity, organo-metal complexes, specifically organotin,
and/or pesticides. The evidence indicates that these
influences can be discounted. Discharge patterns which
govern salinity gradients do not entirely match the
observed trend of increasing reproductive activity.
Contaminants, such as organotin complexes or pesticides
which have sources in the Napa River and Central Valley
respectively, may be discounted because their sources
are external to the estuary and the contamination is
clearly from internal sources. The most compelling evidence
of the role of Ag is the correlation between a drop
in reproductive activity and increasing Ag concentration.
Determining the effects of anthropogenic
influences on ecological systems is difficult due to
natural environmental variability. The case study sumarized
above looked at hydrology, tissue concentrations of
silver and reproductive variability of the clam Potamocorbula
amurensis. Through the use of high intensity monthly
sampling over a long time-period, patterns in environmental
variability can be identified and cause-and-effect relationships
evaluated.
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