Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 99

(i.e., 160 and 320 mg/L), which suggests their high tolerance to prometryn. Similarly,
Chang et al. ( 2011 ) and Kasai and Hanazato ( 1995a, b ) reported high tolerance of
diatom species to another triazine herbicide, simetryn. Kasai and Hanazato ( 1995b )
isolated algal strains from nontreated and treated microcosms that had been exposed
for at least 35-days. The authors investigated genetic changes that occurred follow-
ing simetryn exposure. The most signiicant inding concerned the chlorophyta
Scenedesmus gutwinskii var. heterospina , which exhibited a tolerance level 26-57
times higher for strains preexposed to simetryn than for controls strains. Interestingly,
the authors showed that the isolated strains maintained their tolerance for nearly 2
years in the absence of simetryn, conirming the importance of genetic adaptation in
tolerance induction, within exposed photoautotrophic communities.

2.1.2 Phenylureas

Diuron

Short-Term Effects

Brown and Lean ( 1995 ) performed a short-term bioassay to test the toxicity of 16
pesticides (including 14 herbicides) to lake phytoplankton. They demonstrated that
diuron was the most toxic substance to photosynthetic activity (see Table 5.3 ). For
example, using the same biological end-point, they found that another phenylurea
herbicide, monuron, was about 20-fold less toxic than was diuron. Francoeur et al.
( 2007 ) observed drastic adverse effects of a 20 mM diuron level (i.e., 4.66 mg/L) on
periphyton photosynthesis after only 5 min of exposure.

Chronic Effects and Recovery Processes

A negative diuron exposure effect was revealed in several studies on chl a levels and
on primary production in both phytoplankton (Perschbacher and Ludwig 2004 ;
Knauert et al. 2008, 2009 ; Knauer et al. 2010 ) and periphyton (McClellan et al.
2008 ; Tlili et al. 2008, 2010 ; Ricart et al. 2009 ; López-Doval et al. 2010 ) communi-
ties (Table 5.3 ). Diuron can impact these parameters at exposure concentrations
even lower than 0.1 mg/L, within a few weeks of initial contact (McClellan et al.
2008 ; Ricart et al. 2009 ). Nevertheless, Tlili et al. ( 2010 ) demonstrated that the
effects of diuron (10 mg/L for 3 weeks) on photosynthetic activity can be inhibited
when water contains high PO 4 3− concentrations. It has also been shown that a 21-day
exposure to 10 mg/L of diuron inhibited the development of phototrophic communi-
ties, whereas they bloomed in untreated microcosms (Pesce et al. 2006 ).
Phototrophic community composition can also be affected by chronic diuron
exposure (Table 5.3 ), although results vary greatly among various studies.
Perschbacher and Ludwig ( 2004 ) showed that phytoplankton community composi-
tion was impacted by diuron (at 2 and 20 mg/L): cyanobacteria were severely
reduced, while diatom and green algae were stimulated. Conversely, McClellan
et al. ( 2008 ) and Tlili et al. ( 2010 ) observed a decrease in the relative number of