Agricultural herbicides such as atrazine constitute the
largest amount of all pesticides applied to agricultural land in the U.S.
Atrazine contamination of surface waters as a result of agricultural non-point
source inputs was identified as a majo r threat to aquatic ecosystems
nationwide (EPA 1994). The documented presence of atrazine in surface waters
has prompted a large number of studies on its potential adverse effects on
nontarget organisms such as freshwater algae, yet our current understand ing of
the overall impacts of atrazine is lacking in several key respects' how has
atrazine fundamentally affected algal community composition in streams
receiving inputs and how will it impact streams previously unexposed? Algae are
the most important pr imary producers in aquatic systems and account for nearly
the same percentage of total global net primary production of carbon annually
as do cultivated plants. In addition, algae represent the basal component of
aquatic food webs, since they are consumed by a variety of invertebrates or
directly by fish, both of which are consumed by other fish species. These
attributes of algae make them fundamentally important to surface water quality
and point to the importance of finding solutions to the above questi ons.

It has recently been revealed that different divisions of
freshwater algae and even clones of the same algal species may exhibit
differential responses to herbicide exposure. Toxic effects of atrazine to a
number of freshwater algal species were determ ined recently in our
laboratories based on measurements of cell density and chlorophyll a. The
inhibition percentage and EC50 values (effective concentrations causing 50%
inhibition compared with control) were calculated. It was found that there were
sign ificant differences in atrazine toxicity to different algal species, with
green algae significantly more susceptible to atrazine than diatoms (Tang et
al. in press). The mode of action of atrazine involves a competition with
plastoquinone for binding to t he Q <SUB>b </SUB> polypeptide in
the photosystem II complex of the chloroplasts (Velthuys 1981). In order to
inhibit photosynthesis in freshwater algae, atrazine must be absorbed through
the algal cell membrane. Using 14C-labeled atrazine, hi ghly significant
correlations were also observed between the uptake capacity (bioconcentration
factors) of the algal species studied and the toxicity of atrazine to those
algal taxa (Tang et al. in review). The results suggested that different
degrees of uptake might account for the differences in the toxicity of atrazine
to different algal species and that uptake might be a prerequisite for atrazine
toxicity in algae. Clearly, there is a great need to more precisely determine
what causes the differential sensitivity of algae to atrazine exposure. Because
of the relatively limited number of algal taxa (i.e. Divisions or Phyla)
examined to date, it is also necessary to determine the relationship between
atrazine bioconcentration and toxicity among a broade r range of algae
representatives from streams and rivers throughout the Great Plains, while at
the same time defining the relative atrazine tolerances among these Divisions.
Lack of such data obviates the formation of acceptable predictive models on the
r eal and potential impacts of atrazine and other common herbicides on surface
water quality in the Midwest.

The information obtained by this study will include' (1)
the relative toxicity of atrazine to five of the most common algal Divisions,
and (2) the mechanisms of these differences in atrazine toxicity. The former
data will significantly broaden our curr ent understanding of the potential for
atrazine to alter algal community composition and abundance in aquatic
ecosystems, while the latter information will provide crucial insights
regarding the mechanisms of differential toxicity in algae applicable to o ther
pesticides and pollutants. Because many aquatic systems in North America
receive inputs of atrazine and other soluble herbicides (see for example,
Richards et al. 1987, Gregor and Gummer, 1989, Glotfelty et al. 1987), this
study addresses a potential problem of significant proportion. In the
agricultural Midwest where even higher order streams such as the Platte River
have detectable levels of atrazine virtually year-round, it is clearly
important to understand how these toxicants currently affect co mmunity
structure and biomass production.

Statement of Research Benefits:

A fundamental goal in the ecotoxicology of surface waters
in the Great Plains is to predict the effects of pesticide stressors. Our lack
of critical knowledge concerning the impacts of pesticides on Midwestern
aquatic communities is attributable t o a general lack of understanding of the
physiological and biochemical processes that result in the toxicity of
herbicides to various algal species and the relative susceptibilities of the
dominant algal groups. This study is designed both to investigate the
underlying mechanisms of differential algal sensitivity to the most commonly
used herbicide in the U.S. and to elucidate the relative tolerances of a much
broader range of algae than have been previously examined. Given our initial
findings of differe ntial algal toxicities (Tang et al. in press) and the
importance of algae as primary producers at the base of aquatic food webs,
obtaining more comprehensive information on algal responses to pesticides is
clearly warranted and prerequisite to our assessm ent of pesticide impacts on
aquatic systems throughout the Region. The proposed study will provide a much
more complete understanding of the relative tolerances of the most common algal
constituents in streams, lakes, and wetlands, as well as identify the basic
underlying mechanisms which explain their differences in atrazine tolerance.
Taken together, this approach will allow us to extrapolate our findings to
other aquatic systems and to better predict the impact of atrazine on surface
water quality in g eneral and on overall ecosystem health. This information
will also provide water managers and policy makers with important information
on acceptable levels of this ubiquitous toxicant.