U.S. Water News Online
LINCOLN, Neb. -- A few years ago, after a major national review involving a panel of more than 10 researchers, a report in a leading environmental journal concluded that the most widely applied weed killer in Nebraska "Does not pose a significant risk to the aquatic environment."
A University of Nebraska-Lincoln (UNL) aquatic ecologist thinks this may be premature, however.
Kyle Hoagland and his colleagues have conducted a number of studies on the effects of atrazine and Alachlor on various forms of single-celled organisms that are fundamental to the aquatic food web in North America. While not intending to fuel alarmist fires, the UNL School of Natural Resource Sciences (SNRS) researcher believes that more needs to be done to determine the overall and selective effects of these widely applied, water-soluble herbicides.
Atrazine is the most commonly applied herbicide in the nation and alachlor is the second most widely used pesticide (a designation that includes herbicides, or weed killers).
About 15 million pounds of atrazine and four million pounds of alachlor are applied each year in Nebraska.
One of the results is that benthic diatoms, or bottom-dwelling one-celled organisms surrounded by a glass shell, subject to chronic exposure of low levels of atrazine did not show significant effects, but experienced a serious growth reduction when later exposed to an acute dose of a higher level of the herbicide.
This experiment was designed to replicate the effects of atrazine on diatoms in streams in agricultural regions that already contain chronic low levels of the herbicide and then experience "spikes" of atrazine when spring rains flush fields soon after application.
Impacts determined through a range of studies on green and blue-green algae and diatoms in streams were that even at fairly low levels, the herbicides produce differential toxicity, selectively damaging cells, blocking photosynthesis, and stunting growth in ways that vary by species, as well as by length and intensity of exposure.
The differential effects are important because they can have significant effects on community structure, which could have ripple effects up through the food web, said Hoagland.
"Ecologically, they act like a selective grazer," he said. "Some are very much affected, some not at all."
Another result was that the combined effects of atrazine combined with insecticides may be toxic to the larvae of aquatic insects such as midges, which birds and fish feed upon.
Another looked at the effect of alachlor on common wetland vegetation, such as bulrushes and cattails, in doses ranging from 10 to 1,500 parts per billion (ppb). The chemical significantly inhibited growth in hardstem bulrush at an exposure of 500 ppb and above, even when the concentration of the parent compound had decreased to 1.55 ppb by the 16th and last week of the study.
Chlorosis, a yellowing indicating lack of chlorophyll, also was present at all but the lowest concentration.
Another study, the first comparing the effects of these common herbicides on green algae relative to diatoms, found that green algae tend to concentrate more of the active ingredient than diatoms, Hoagland said.
However, this may be related to the greater ability of diatoms to use more diverse forms of nutrition, he added.
Most of the studies on one-celled life forms systematically applied increasing doses of the herbicides, ranging from 1 ppb to 1,000 ppb over periods of up to 67 days. Most were done under conditions called "microcosms" that attempt to replicate real streams.
In midwestern streams, alachlor has been found in concentrations as high as 105 ppb in one study and in 98 percent of the 149 sampled in another. Spring flush spikes in headwater streams have run as high as almost 700 ppb for both atrazine and alachlor, Hoagland said.
Regulations set by the U.S. Environmental Protection Agency put the maximum contaminant level for atrazine in drinking water at 3 ppb.
Hoagland has worked with M.M. Langan, Rebecca Spawn and Justin Carder of UNL's SNRS, Jixin Tang and Blair Siegfried of UNL's Department of Entomology and with Karen Nelson of the U.S. Fish and Wildlife Service on one or more of the studies mentioned.
Further work needs to be done, he said, on metabolites (breakdown products) for herbicides other than atrazine and on total herbicide load, as well as on synergistic and chronic effects.
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