Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 107

identiication and PCR-DGGE on 18S rDNA gene fragment). Moreover, effects
were only perceptible on communities sampled during the summer, whereas spring
communities remained insensitive, revealing that response of natural algae to herbi-
cide exposure can be seasonally dependent. Glyphosate-induced effects (at 1, 10,
and 100 mg/L) on phytoplankton diversity were also reported by Abdel-Hamid et al.
( 1996 ) in a 13-day outdoor enclosure experiment.

2.1.6 2,4-Dichlorophenoxyacetic Acid (2,4-D)

Using bioassays on mesotrophic lake phytoplankton communities exposed to differ-
ent herbicides, Brown and Lean ( 1995 ) showed that 2,4-D exhibited the least toxic
effect on photosynthetic activity and phosphate and ammonium uptake rates. This
herbicide had EC 50 values (3 h) higher than 33 mg/L for these three biological end
points. Using an 8-day microcosm study, Kobraei and White ( 1996 ) observed a
stimulation of primary production in phytoplankton communities exposed to 2 mg/L
of 2,4-D. No stimulation was detected at 10 mg/L, yet higher concentrations nega-
tively affected phytoplankton. Gross primary production, community respiration,
chl a concentrations, algal density and biovolumes signiicantly decreased in the
100 and 1,000 mg/L microcosms. They found heterotrophic algal taxa to be the least
affected by 2,4-D at highest concentrations. Using lower concentrations in large
outdoor mesocosms, Relyea ( 2005, 2009 ) showed that levels of neither 16 mg/L nor
120 mg/L of 2,4-D produced signiicant direct or indirect effects on periphyton and
phytoplankton biomass.

2.1.7 Other Herbicides

Chronic Effects on Biomass and Primary Production

Testing the effects of ten herbicides commonly applied in rice cultivation (cloma-
zone, thiobencarb, pendimethalin, propanil, quinclorac, halosulfuron, bensulfuron-
methyl, triclopyr, 2,4-D-amine, and molinate) in aquaculture ponds at rates equivalent
to direct application, Perschbacher et al. ( 2002 ) found that, with the exception of
propanil, none of the herbicides had any measurable effect on phytoplankton pri-
mary production or chl a levels, within 9 exposure days. Propanil stimulated chl a
(Perschbacher et al. 1997, 2002 ). A 3-day microcosm study by Waiser and Robarts
( 1997 ) showed that the carbamate herbicide triallate exhibited limited effects on
lake phytoplankton biomass, since substantial declines in chl a concentrations only
occurred at a triallate concentration of 1 mg/L (but not 10 and 100 mg/L).

Chronic Effects on Community Composition

Caquet et al. ( 2005 ) investigated the effects of the herbicide fomesafen (40 mg/L),
alone and in combination with an adjuvant (Agral 90, 90 mg/L), on plankton