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Widespread use of environmentally persistent herbicides has led to increasing concern over their impact on aquatic ecosystems and nontarget organisms. The triazine herbicide atrazine (6-chloro-N-ethyl-N 11-[1-methylethyl]-1,3,5-triazine-2,4- diamine) remains one of the most heavily used pesticides worldwide, and was the second most commonly used conventional pesticide in the U.S. agricultural market in 2001. Atrazine is an S-triazine herbicide that targets the photosynthetic process to control broadleaf weeds. Due to the moderate solubility (33 mg/L at 22°C) and relative persistence of atrazine in water (Solomon et al. 1996), contamination of surface waters as a result of non-point source inputs is a threat to lakes and streams. Atrazine contamination has been documented year-round in the Platte River in Nebraska (Nelson et al. 1999), and concentrations as great as 691 μg/L have been observed in first-order stream samples collected during post-planting storm events (Langan et al. 1993). The effect of surface-water contamination on nontarget organisms is especially important for freshwater algal communities, given their role as the predominant primary producers in most lotic environments (Herman et al. 1986). Moreover, freshwater algae form the base of aquatic food webs and changes in algal community structure and composition may have cascading effects on other components of the community due to changes in food availability and important community-level interactions. A number of laboratory, microcosm and fi eld studies have examined atrazine toxicity in freshwater algae (DeNoyelles et al. 1982; Herman et al. 1986; Solomon et al. 1996; Tang et al. 1997; Fairchild et al. 1998). From these studies, it is generally recognized that certain algal species are more sensitive to atrazine (DeNoyelles et al. 1982; Tang et al. 1997; Fairchild et al. 1998). Green algae generally are more susceptible to atrazine than diatoms (Tang et al. 1997). However, relatively few algal divisions and species have been tested for their response to atrazine exposure. The overall objective of this research was to determine the differential sensitivity of atrazine to algal species common in freshwater ecosystems in the Midwest. Several of the previously described studies examined a small number of algal species and divisions, focusing on green algae, cyanobacteria (also known as blue-green algae), and diatoms. We used a greater number and variety of species to define tolerance to atrazine over a broader range of freshwater algae. Additionally, cell size was examined as a possible determinant of differential toxicity to atrazine among algal species.