Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 109

et al. 2006 ). Similarly, Pesce et al. ( 2010b ), using a PICT approach and photosyn-
thesis bioassays, demonstrated that mixtures of diuron and its metabolite N-(3,4-
dichlorophenyl)- N -methylurea (DCPMU) produced additive effects on natural
phototrophic bioilm photosynthesis.
Knauer et al. ( 2010 ) showed that long-term exposure to PS-II inhibitor mixtures
can also induce cotolerance within phytoplankton communities. However, they also
demonstrated that even if cotolerance is expected for compounds having similar
biochemical modes of action, this cotolerance may vary among molecules (in their
study, atrazine, isoproturon, and diuron).

2.2.2 Mixtures of Dissimilarly Acting Herbicides

Carder and Hoagland ( 1998 ) and Hartgers et al. ( 1998 ) assessed the effects of mix-
tures of PS-II inhibitors (triazine and/or phenylurea) and the chloroacetanilides,
which are known to affect fatty acid metabolism (Couderchet et al. 1998 ). The
effects of a combination of atrazine (12 and 150 mg/L) and alachlor (5 and 90 mg/L)
on benthic algal communities in artiicial streams appeared to be additive rather
than synergistic and led to a signiicant decrease in cell biovolumes throughout the
4-week experiment (Carder and Hoagland 1998 ). Hartgers et al. ( 1998 ) assessed the
response of phytoplankton communities to a mixture of atrazine, diuron, and
metolachlor in 28-day freshwater microcosms (600 L); concentrations were 0.01-
1.0-fold the EC 50 (72 h) values. These EC 50 values were obtained in standard algal
tests using Selenastrum capricornutum and were performed to comply with OECD
guidelines. Direct effects were detected at 0.3-fold the EC 50 treatment level and
higher. Effects included a drop in photosynthetic eficiency and a decrease in the
abundance of several phytoplankton taxa, especially the cholorophyceae
Monoraphidium sp., whereas other species such as Cyclotella or Chlamydomonas
sp. showed a marked increase in abundance as doses increased. However, it was not
possible for the authors to determine how the three tested compounds interacted.
Relyea ( 2009 ) tested a mixture of low levels (2-16 mg/L) of ive herbicides (atra-
zine, acetolachlor, metolachlor, glyphosate, and 2,4-D) in a 36-day mesocosm
experiment; results showed that phytoplankton chl a effects occurred, but, except
for acetochlor, differed from those of metolachlor, glyphosate, 2,4-D, and atrazine
alone. The ive-herbicide mixture had similar effects on periphyton abundance as
did exposure to each of the ive herbicides alone.

2.2.3 Mixtures of Herbicides with Other Organic Pesticides

Herbicides and Insecticides

Wendt-Rasch et al. ( 2003 ) investigated the effects of the sulfonylurea herbicide
metsulfuron-methyl (1, 5 and 20 mg/L) alone and in combination with the pyrethroid