Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 108

communities in outdoor mesocosms over a 9-month period. They found that Agral
90 did not inluence the effect of fomesafen on phytoplankton. Fomesafen inhibited
Chlorophyceae but produced abundance and biovolume increases of Cyanobacteria,
Cryptophycea, Dinophycea, and Bacillariophycea, thus enhancing taxonomic phy-
toplankton diversity. Phytoplankton community composition was also affected by
the herbicide methabenzthiazuron, during a 5-month microcosm study (Wellmann
et al. 1998 ). Population dynamics were dependent on herbicide concentrations as
well as light intensity, temperature, and nutrient concentrations. Primary production
was temporarily inhibited at metabenzthiazuron concentrations ranging from 89 to
3,371 mg/L, while lower concentrations (10, 21 and 43 mg/L) induced no or only
transient weak responses in the phytoplankton. The most sensitive algae belonged
to Chlorophyceae, whereas Cryptophycea exhibited strong recovery at the higher
concentrations during the study period. In ield-paddy mesocosms, Sánchez et al.
( 2006 ) observed no signiicant effect of the rice herbicide profoxydim (at rates from
45 to 377 g/ha) on phytoplankton density and composition, within 9 exposure days.
Using in situ enclosures, Faber et al. ( 1998 ) tested the effect of the herbicides glu-
fosinate-ammonium and bialaphos on phytoplankton communities in a eutrophic
lake. At the highest treatment levels (10 mg/L), both herbicides caused a signiicant
decrease in small phytoplankton cell species (1-3 mm). The effects were transient,
and recovery was observed earlier for bialaphos (14 days post application) than for
glufosinate-ammonium (49 day). Larger phytoplankton was generally not adversely
impacted by these herbicides.

2.2 Effects of Herbicide Mixtures

2.2.1 Mixtures of Similarly Acting Herbicides

The effects of mixtures of PS-II inhibitors have been investigated on freshwater
microbial communities in several studies; the tested mixtures were of various triaz-
ine (Pollehne et al. 1999 ) and/or phenylurea herbicides (Knauert et al. 2008, 2009 ;
Knauer et al. 2010 , Pesce et al. 2010b ). Using a 10-day mesocosm approach,
Pollehne et al. ( 1999 ) found no herbicide-speciic effects on estuarine phytoplankton
communities exposed to the combined triazines simazine and atrazine, at concentra-
tions of 0.04-6 mg/L each. Knauert et al. ( 2008, 2009 ) performed a 5-week outdoor
mesocosm study to evaluate the effects of a mixture of equitoxic concentrations of
atrazine, isoproturon and diuron on phytoplankton photosynthesis and community
succession. The herbicide mixture adversely affected photosynthetic activity and
signiicantly inluenced community structure in terms of abundance, diversity, and
species composition. The results demonstrated that the combined effects of the
three PS-II inhibitors herbicides could be predicted, based on the concept of
concentration addition (Faust et al. 1994 ). This outcome was in line with previous
indings from phytoplankton bioassays, in which the combined effects of similarly
acting herbicides were assessed (e.g., Faust et al. 2001 ; Junghans et al. 2003 ; Chèvre