Effects of Organic Herbicides on Phototrophic Microbial Communities in Freshwater Ecosystems - Reviews of Environmental Contamination and Toxicology - page 98

photosynthetic activity, which was measured by the [ 14 C]bicarbonate assimilation rate.
Schmitt-Jansen and Altenburger ( 2005a ) conirmed that prometryn is more toxic to per-
iphytic communities than is atrazine in short-term inhibition tests of photosynthesis.
Chronic Effects and Recovery Processes
Fairchild and Sappington ( 2002 ) conducted a 6-week study in outdoor mesocosms and
observed no statistically signiicant effect of metribuzin on periphyton biomass at con-
centrations up to 75 mg/L. Similarly, Brock et al. ( 2004 ) showed that metribuzin, at
nominal concentrations less than or equal to 56 mg/L, had only mild and transient effects
on phytoplankton and periphyton, and recovery occurred within 8 weeks. Long-term
effects, lasting longer than 8 weeks, were only found in the 180 mg/L enclosures. In
these two experiments, the absence of effects at lower concentrations may have resulted
from the rapid dissipation rate of metribuzin in water (half-life of 5-9 days). Brock et al.
( 2004 ) found that another triazine herbicide (metamitron; 14-4,480 mg/L) was even less
persistent in the water column (half-life of 1-3 days). Enclosure experiments with met-
amitron revealed treatment-related effects for photoautotrophic communities only at the
two highest concentrations (i.e., 1,120 and 4,480 mg/L), followed by a fast recovery,
thought to derive from the agents short dissipation half-life in water.
When the long-term effects produced by metribuzin and metamitron were com-
pared to data from standard toxicity tests, in which an assessment factor was applied
(irst-tier approach; i.e., Lowest Observed Effect Concentrations; LOEC), and an
SSD approach was used, Brock et al. ( 2004 ) concluded that these two assessment
procedures proved highly protective, since they did not account for dissipation rate
or recovery processes in complex ecosystems.
Gustavson et al. ( 2003 ) also recommended that exposure duration be considered
when assessing herbicide effects on periphyton communities. Indeed, these authors
observed that the effect concentration of metribuzin decreased by one to two orders
of magnitude when exposure time increased from 1 to 2 to 24 h. The effect of expo-
sure duration was even more signiicant for hexazinone. Hexazinone stimulated
photosynthesis at the three lowest test concentrations (i.e., 0.4, 2, and 10 mg/L) after
a 1-h exposure, whereas the stimulation disappeared after 24 h. Moreover, Gustavson
et al. ( 2003 ) observed a recovery of photosynthetic activity within stream periphy-
ton communities that were exposed to metribuzin for a period up to 48 h, following
exposure for 48 h in herbicide-free water. Photosynthetic activity recovered even at
the highest concentration (i.e., 50 mg/L), whereas photosynthesis suffered an 80%
inhibition. Comparable recovery processes that occurred within 24 h after herbicide
addition ended were observed by Schneider et al. ( 1995 ) for stream periphytic
communities exposed to 145-432 mg/L hexazinone.
PICT Assessment
Measures of photosynthetic activity showed that an induced-tolerance existed in
communities chronically exposed to prometryn concentrations of 2.5 mg/L and
higher (Altenburger 2005a). Diatom species, especially Nitszchia sp., clearly
became predominant following long-term exposure to higher test concentrations