Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 104

1999 ) to 500 mg/L (Daam et al. 2009a ). Except for Van Geest et al. ( 1999 ), who
found that three repeated linuron treatments (0.5-50 mg/L at 4-week intervals)
caused only negligible changes in algal communities; all studies produced direct
and indirect effects on phytoplankton and periphyton (Van den Brink et al. 1997 ;
Slijkerman et al. 2005 ; Daam et al. 2007, 2009a ). In tropical freshwater micro-
cosms, linuron (15-500 mg/L) inhibited photosynthesis and affected both phyto-
plankton and periphytic communities (Daam et al. 2009a ). The most sensitive
species were chlorophytes belonging to the genera Scenedesmus , Coelastrum , and
Pediastrum (phytoplankton) and the cyanobacterium Chamaesiphon sp. (periphy-
ton). However, given the development of tolerant taxa (mainly belonging to diatom
and cryptophyte classes) and functional redundancy, the authors emphasized that
chl a concentration was not a sensitive indicator of linuron exposure, especially for
the effects on phytoplankton. Daam et al. ( 2009b ) compared the effects of linuron,
in a microcosm study carried out in Thailand, with the effects reported in temperate
model ecosystem studies. The authors concluded that the sensitivity of primary
producers to the effects of linuron was similar among different climatic regions.
Such similarity supports the use of toxicity data in tropical regions that were gener-
ated in temperate ones.

Chronic Indirect Effects

Van den Brink et al. ( 1997 ), using macrophyte dominated microcosms, demon-
strated that linuron can also indirectly stimulate more tolerant phytoplankton spe-
cies such as Chlamydomonas sp . Linuron exposure decreased macrophyte biomass,
thereby increasing nitrate levels, which, in turn, produced an increase in total phy-
toplankton chl a levels. Such ecological cascading effects were conirmed by
Slijkerman et al. ( 2005 ) and Daam and Van den Brink ( 2007 ), who showed that
linuron-induced primary inhibition of the photosynthetic eficiency of primary pro-
ducers (including macrophytes) resulted in a signiicant release of nutrients in the
water, which consequently stimulated less-sensitive or fast-adapting phytoplankton
species. Van den Brink et al. ( 1997 ) and Slijkerman et al. ( 2005 ) recorded an increase
in lagellates subjected to a linuron treatment regime, whereas Daam and Van den
Brink ( 2007 ) identiied the algal genera Ephitema , Navicula , and Closterium as
being favored by changes resulting from linuron exposure.

2.1.3 Chloroacetamides

Alachlor and Metolachlor

The effects of alachlor on periphytic (Spawn et al. 1997 ) and epipelic (Carder and
Hoagland 1998 ) algal communities have been investigated in stream microcosms.
Carder and Hoagland ( 1998 ) recorded a decrease in algal biovolumes after a 4-week
exposure to 90 mg/L of alachlor, but no effects were detected at 5 mg/L. The relative
abundance of the diatoms Navicula sp. and Gyrosigma exinium signiicantly