Environmental Engineering Reference
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possible nutrient leakage from the macrophyte leaves following herbicide exposure.
Changes were also detected on the species composition of periphytic algal commu-
nities in enclosures exposed to 20 mg/L, while metsulfuron-methyl did not alter
phytoplankton community biomass or composition. This suggests that the toxicity
of sulfonylurea herbicides on phytoplankton is limited, as asserted by Seguin et al.
(
2001
) and Leboulanger et al. (
2001
), who showed that nicosulfuron was far less
toxic to phytoplankton than was atrazine. Nevertheless, they emphasized that nico-
sulfuron (10 mg/L) can affect phytoplankton species composition by inhibiting more
diatoms than chlorophytes. Abdel-Hamid et al. (
1996
) also observed community
composition effects on lake phytoplankton from exposure (1, 10, and 100 mg/L) to
another sulfonylurea herbicide, chlorosulfuron.
2.1.5
Glyphosate
Effects of High Glyphosate Concentrations
Vera et al. (
2010
) showed that a high concentration of the commercial formulation
of Roundup
®
(8 mg/L of the active molecule glyphosate) produced a clear delay in
periphytic colonization and reduced periphytic dry weight, as well as chl
a
, in com-
parison with control mesocosms. This occurred despite a signiicant increase in
total phosphorus concentrations in the treated mesocosms. Pérez et al. (
2007
) also
observed a signiicant phosphorus release in mesocosms after the addition of
Roundup
®
(6 and 12 mg/L of the active ingredient glyphosate), which was associ-
ated with structural changes in the planktonic and periphytic microbial assem-
blages, within a few days. In treated mesocosms, total phytoplankton abundance
decreased, whereas primary production and picocyanobacteria abundance increased.
Similar patterns have been observed in periphyton, which showed increasing abun-
dance of cyanobacteria following glyphosate exposure (Vera et al.
2010
). Schaffer
and Sebetich (
2004
) also found that Rodeo
®
treatments (0.125 and 12.5 mg/L of the
active ingredient glyphosate) led to signiicant stimulation of primary productivity
of a lake phytoplankton community during a 7-h incubation period. They hypothe-
sized that this effect could have resulted from the use of the nitrogen and phospho-
rus released through the glyphosate degradation process. Similarly, Relyea (
2005
)
observed an increase in periphytic algal biomass after a 2-week Roundup
®
exposure
(3.8 mg glyphosate/L), which, he suggested, could result from a decrease in grazing
pressure caused by Roundup
®
effects on herbivorous organisms.
Chronic Effects of Environmentally Relevant Glyphosate Concentrations
Using a more environmentally relevant glyphosate concentration (i.e., 6.9 mg/L),
Relyea (
2009
) showed that Roundup
®
caused insigniicant effects on an aquatic
food web comprising periphyton, phytoplankton, and other higher trophic level
organisms. This was consistent with results from Pesce et al. (
2009
), who demon-
strated that any effects of glyphosate (10 mg/L; 14 days) on riverine algal communi-
ties was limited to ones of community composition (assessed by microscopic
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