Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 105

decreased following alachlor exposure, but effects persisted only for G. exinium at
the highest concentration. Similarly, Spawn et al. ( 1997 ) did not observe any signii-
cant alachlor effect within 3 weeks of exposure at 1 mg/L, although algal biomass
and cell densities were inhibited at all other concentrations (i.e., 10, 30, 100, and
1,000 mg/L). There was a shift in the dominant algae at concentrations of 30 mg/L
and higher. The centric diatom Melosira varians was the most affected by alachlor,
but other centric diatoms were not affected, demonstrating that similar taxa may
exhibit different responses to this herbicide. By contrast, Debenest et al. ( 2009 )
showed that 30 mg/L of the chloroacetamide herbicide s-metolachlor had no effect
on the abundance of M. varians, following a 3-day exposure. However, other diatom
species such as Eolimna minima and Navicula reichardtiana proved sensitive to this
herbicide, and a signiicant decrease in chl c concentrations and live-cell density
was recorded in periphtyon exposed for 3 days to levels of 5 and 30 mg/L. Using
complex outdoor microcosms, Relyea ( 2009 ) observed no signiicant effect of
metolachlor (7.4 mg/L) on phytoplankton chl a or periphyton biomass within 16 or
35 days of exposure.

Other Chloroacetamides

Mohr et al. ( 2008b ) studied the effects of metazachlor on plankton communities in
pond and stream mesocosms over a monitoring period of 140 days. Metazachlor
strongly affected both pond and stream communities at concentrations higher than
5 mg/L (i.e., 20-500 mg/L). Direct negative effects were most prominent for chloro-
phytes, whereas diatoms and cryptophytes seemed insensitive. Moreover, the herbicide
remained highly persistent in the mesocosms ( t 1/2 = 27-48 days), and chlorophytes did
not recover in the more strongly contaminated stream mesocosms, suggesting potential
long-lasting effects of metazachlor on phytoplankton in exposed aquatic ecosystems.
This contrasted with the results of Noack et al. ( 2003 ), who found only slight effects of
metazachlor on phytoplankton densities at very high concentrations (10,000 mg/L), fol-
lowed by a recovery after 30-35 days. However, these authors advised that their con-
clusions be accepted with caution, because lack of replicated mesocosms prevented
statistical evaluation of results. In model streams, the only effects recorded by Takahashi
et al. ( 2007 ) on periphyton exposed to pretilachlor (26-382 mg/L) for 7-28 days were
a slight increase of Navicula pupula and a slight decrease of Anabaena sp . Using com-
plex outdoor microcosms, Relyea ( 2009 ) observed a decrease in phytoplankton chl a
after a 16-day exposure to 10 mg/L acetochlor, whereas periphyton biomass remained
unaffected throughout the entire duration of the study (35 days).

2.1.4 Sulfonylureas

Using four levels of the sulfonylurea herbicide metsulfuron-methyl (0, 1, 5, and
20 mg/L) in freshwater enclosures, Wendt-Rasch et al. ( 2003 ) showed an increase in
the biomass of periphytic algae growing on the leaves of the macrophyte
Myriophyllum spicatum, after a 2-week exposure. They attributed this increase to a