Studies Relating Pesticide Concentrations to Potential
Effects on Aquatic Organisms in the San Francisco
by Kathryn M. Kuivila
A variety of pesticides are applied in large quantities to agricultural and urban areas in the Central
Valley of California and are transported into the San Francisco Bay-Estuary dissolved in water and
associated with suspended sediments.
These pesticides can have deleterious effects on aquatic organisms.
Three studies that relate pesticide concentrations to potential effects on aquatic organisms are currently
underway by the U.S. Geological Survey’s San Francisco Bay-Estuary Toxic Substances Hydrology
These studies are (1) measuring the impacts of herbicides on phytoplankton primary production,
(2) determining the exposure of Delta smelt to dissolved pesticides, and (3) assessing the effects of
pesticides on the Asian clam,
Large quantities of various pesticides are
applied to agricultural and urban areas in the
Central Valley of California that drain into the
Sacramento-San Joaquin Delta (Delta) and then
San Francisco Bay.
Together, the Delta and San
Francisco Bay are known as the San Francisco
studies have measured elevated concentrations of
dissolved pesticides in the Sacramento and San
Joaquin Rivers, upstream of the Estuary (fig. 1)
(MacCoy and others, 1995; Domagalski, 1996;
Panshin and others, 1998).
of dissolved pesticides have been made in the
Sacramento-San Joaquin Delta or farther
downstream in the Estuary (fig. 1) (Kuivila and
Foe, 1995; Kuivila and others, 1999).
also are transported into the rivers and Estuary
associated with suspended sediments, but little is
known about the concentrations and residence
times of these pesticides (Domagalski and
Kuivila, 1993; Bergamaschi and others, 1999;
Bergamaschi and others, in press).
Once in the aquatic environment, pesticides
can have deleterious effects on aquatic organisms;
controlling factors include the concentration,
exposure time, and bioavailability of the pesticide
When assessing a biological effect, it
is important to use the appropriate endpoint or
Endpoints reflect responses at various
physiological, whole organism,
population, and community.
Field and laboratory
studies can be used to assess effects of pesticides
on aquatic organisms.
Field studies take into
account the complexity of the ecosystem, but that
complexity makes it difficult to assign a single
cause to an observed effect.
In contrast, direct
cause and effect are more easily shown in
laboratory studies, but at the expense of
oversimplifying the interrelations in the
A combination of field and laboratory
studies usually provides the most powerful
The purpose of this report is to summarize