Chapter 27

The Aquatic Ecotoxicology of Triazine Herbicides

Jeffrey M. Giddings1 and Lenwood W. Hall, Jr.2

1Springborn Laboratories, Inc., 790 Main Street, Wareham, MA 02571 2Wye Research and Education Center, University of Maryland,

Queenstown, MD 21658

The effects of triazine herbicides on aquatic species and ecosystems are reviewed. Effects on aquatic plants are reversible; photosynthesis resumes when the herbicide disappears from the water, and sometimes even while it is still present. Effects on aquatic plant communities are further ameliorated by species replacements, so the communities as a whole are less sensitive than their most sensitive species. Atrazine, a representative triazine herbicide, is acutely toxic to aquatic plants (algae and macrophytes) at concentrations in the range of 20 to 200 µg/L. Chronic toxicity to plants occurs at concentrations ten times lower than acute toxicity. Aquatic invertebrates and fish are much less sensitive than plants, with acute toxicity occurring at 1000 to 200,000 µg/L. Ecologically significant effects in aquatic ecosystems are likely only if plant communities are severely damaged by prolonged exposure to high atrazine concentrations.

The objective of this paper is to review the data on triazine herbicide toxicity to aquatic organisms and ecosystems. The focus is on atrazine because atrazine is the most widely used and widely studied triazine, and because a detailed ecological risk assessment of atrazine has recently been completed (/). Besides reviewing the laboratory data on triazine toxicity, we will discuss the ecological implications of triazine effects in real ecosystems, drawing particularly on evidence from microcosm and mesocosm studies.

Triazine herbicides are photosynthetic inhibitors. Their primary physiological effect is to block electron transport in Photosystem II (2). The effect is reversible: when the herbicide is removed from the plant cell, photosynthesis resumes, and the plant's

©1998 American Chemical Society 347

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normal biochemistry is quickly restored. The effect is non-lethal to aquatic plants unless exposure continues for a very long time—weeks or months, long enough for the plant literally to starve. In fact, the effect of triazines on aquatic plants is similar to the effect of reduced light, such as occurs when suspended solids shade the plants in a muddy stream after a rainstorm. Because triazines act by blocking a specific photosynthetic mechanism, they are not highly toxic to animals.

Acute Toxicity Distributions

The acute toxicity of a substance to aquatic organisms is generally expressed as the LC50 or EC50 concentration, which is the concentration that kills half of the test population or reduces plant growth by 50%. A lower LC50 concentration implies a more sensitive species. Based on studies submitted to EPA for product registrations (J), atrazine is generally intermediate in toxicity compared with other triazines (Table I). As expected, aquatic plants (algae and duckweeds) are considerably more sensitive to atrazine than animals, with values in the range of 20 to 500 pg/L. Invertebrates are less sensitive than plants, with LC50 values from 1000 to 7,000 pg/L (and, in one extreme case, a crab, nearly 200,000 pg/L). Fish are less sensitive still, with most values between 10,000 and 100,000 pg/L, roughly two to four orders of magnitude higher than for aquatic plants. The same trends are evident in the data for other triazines as well. The acute toxicity values for even the most sensitive aquatic animals are always greater than 1,000 pg/L, whereas some plants are sensitive at concentrations less than 100 pg/L. In a risk assessment of triazine herbicides, we are therefore concerned with (a) the potential for reduced productivity of the aquatic plant community due to direct toxic effects, and (b) indirect effects on aquatic invertebrates and fish due to loss of food supply, alteration of habitat, or changes in water quality caused by reduced photosynthesis.

In our risk assessment of atrazine (7), we used a probabilistic approach (4) to characterize the sensitivity of aquatic species. Acute toxicity data for 52 species were compiled from several sources, sorted in order of sensitivity, and plotted as a cumulative log-normal distribution (Figure 1). The horizontal axis represents the LC50 or EC50 (in pg/L, on a log scale), and the vertical axis represents the ranking of species sensitivity, expressed on a probability scale. The line through the points is a least-squares regression, assuming a log-normal distribution of species sensitivity. In our risk assessment, we used the regression line to estimate the LC50 of the tenth percentile of species sensitivity—that is, the concentration that would be expected to cause acute effects to one-tenth of the species for which we have data. For atrazine, the concentration that would protect 90% of the aquatic species from acute toxic effects was estimated to be 37 pg/L. Of course, the affected species would all be plants; it would take much higher concentrations to cause effects on animals. This approach assumes that protecting ten percent of the species will also protect the ecosystem as a whole, an assumption that turns out to be conservative, as will be discussed below.

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LC50 or EC50 (pg/L)

Figure 1. Log-normal distribution of acute toxicity values for atrazine. See text for explanation. Adapted from réf. 1.

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Table I. Summary of acute toxicity data (LC50 and EC50 concentrations, in pg/L) for five triazine herbicides to aquatic species. Data from Ref. 3.

Species Atrazine Cyanazine Simazine Ametryn Prometryn

Plants

Isochrysis galbana 22 500 10

Skeletonema costatum 24 18 600 8

Selenastrum capricornutum 53 6 100 4 12

Chlamydomonas sp. 60

Monochrysis lutheri 77 14

Ne oc hi or is sp. 82 36

Platymonas sp. 100 24

Chlorococcum sp. 100 2000 10

Thallassiosira fluviatilis 110 58

Microcystis aeruginosa 129

Chlorella sp. 140 320

Lemna gibba 170 64 140 12

Phaeodactylum tricomutum 200 500 20

Chlorella pyrenoidosa 282

Nitzschia clusterium 290 62

Porphyridium cruentum 308 36

Dunaliella ter Hole eta 431 5000 20

Navicula inserta 460 97

Navicula pelliculosa 5 90 1

A nabaena fl o s-aquae 24 36 40

Achnanthese brevipes 19

S tau rone is amphoroides 26

Cyclotella nana 55

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Table I (continued). Acute toxicity data for five triazine herbicides to aquatic species.

Species Atrazine Cyanazine Simazine Ametryn Prometryn

Invertebrates

Chironomus tentans 1000

Penaeus aztecus 1000

Mysidopsis bahia 5000 2300 1700

Gammarus fasciatus 6000 2000

Daphnia magna 7000 45500 1100 28000 18590

Uca pugilator 198000

Crassostrea virginica 20000 1000

Palaemonetes pugio 56000

Pteronarcys calif or nica 1900

Cypridopsis vidua 3700

Gammarus lacustris 13000

Penaeus duorarum 113000 1000

Mercenaria mercenaria

Fish

11000 21000

Salve l i nus fontinalis 5000

Cyprinodon variegatus 13000 18000 5800 5100

Oncorhynchus my kiss 14667 9000 53900 3200 7200

Pimephales promelas 15000 18500 5700 5700

Notropis atherinoides 16000

Lepomis macrochirus 39400 23000 50333 5433 10000

Carassius auratus 60000 14000 4000

Ictalurus punctatus 12667 85000

Leiostomus xanthurus 1000 1000

Lepomis gibbosus 27000

Micr opter us sal mo ides 46000

Lepomis macrolopus 54000

Pimephales notatus 66000

Ictalurus natalis 110000

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Chronic Toxicity Distributions

The chronic toxicity of a substance to aquatic organisms is typically expressed as a No Observed Effect Concentration (NOEC). Because NOECs apply to longer exposure times and more sensitive toxicity endpoints, the concentrations are lower than for acute toxicity. The tenth percentile of the chronic toxicity distribution for atrazine is 3.7 pg/L (Figure 2), exactly one-tenth the tenth percentile for acute toxicity.

Model Ecosystem Studies

The results reviewed so far are based on standard laboratory toxicity tests with single species. Numerous studies have also been conducted to measure atrazine effects on model aquatic ecosystems—microcosms and mesocosms. These studies help us to understand and evaluate the significance of the laboratory toxicological data, because the microcosm and mesocosm studies address aggregate responses of multiple species in intact communities. They also allow observation of indirect effects and ecological recovery. Most of these studies involved continuous doses or repeated pulses of atrazine, or took place in static systems in which concentrations remained at fairly steady levels for weeks at a time; thus, they represent the effects of chronic exposure.

Based on results of more than 20 microcosm and mesocosm studies, atrazine exposures below 20 pg/L generally cause no effect on aquatic plants, and where an effect occurs there is always recovery (Figure 3). Between 10 and 100 pg/L there is sometimes an effect but still always a recovery. For example, atrazine at 10 pg/L reduced macrophyte productivity in wetland microcosms, but productivity returned to pretreatment levels within 7 days (while atrazine was still present); macrophyte biomass was unaffected (5). In a study with laboratory streams, periphyton productivity—again, not biomass—was affected at 10 pg/L and recovered within 3 weeks (6,7). The productivity of pond phytoplankton in microcosms exposed to 15 pg/L recovered within 2 weeks (8). Wetland macrophytes exposed to 20 pg/L showed reduced productivity but no effect on biomass, and recovered in 6 weeks (9). Stream periphyton exposed to 24 pg/L recovered after 12 days (10). Pond periphyton exposed to 32 pg/L recovered after 3 weeks (//). Pond phytoplankton exposed to continuous input of 50 pg/L recovered within one day after atrazine input ceased (12). Benjamin et al. (The Institute of Wildlife and Environmental Toxicology and Clemson University, unpublished data) showed the same phenomenon in a species of green algae: growth of Selenastrum capricornutum was severely inhibited during 32 days of exposure to 10 pg/L atrazine, but when the cells were transferred to clean medium their growth resumed at a normal rate, equal to controls.

Based on these results, we conclude that atrazine exposure of 20 pg/L or less, even for extended periods of time (one of these studies continued for three years), causes no lasting harm to aquatic plant communities. Fifty pg/L is taken, conservatively, as the lowest effect concentration, even though recovery still occurs. Above 100 pg/L there is always an effect and often no recovery.

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Figure 2. Log-normal distribution of chronic toxicity values for atrazine. Adapted from réf. 1.

No Recovery

Recovery

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Figure 3. Summary of effects of atrazine on plants in mesocosm and microcosm studies. Each point represents an observed response at one exposure level in one study. Circles: Phytoplankton. Squares: Periphyton. Triangles: Macrophytes.

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Thus we have 3 ecotoxicological endpoints for atrazine: 37 pg/L as the tenth percentile of acute toxicity, 3.7 pg/L as the tenth percentile of chronic toxicity, and 20 pg/L as a conservative no effect level for the plant community as a whole.

Recovery, Resistance, and Replacement

Why are plant communities unaffected by prolonged exposure to 20 pg/L atrazine, even though a substantial fraction of plant species are affected by atrazine concentrations as low as 3.7 pg/L? In terms of total biomass and productivity (and these are the ecological endpoints we are usually interested in protecting, rather than the success of particular species of algae) an aquatic plant community is less sensitive to atrazine than its most sensitive species. Three mechanisms contribute to this.

The first is RECOVERY. As demonstrated above, even while exposure continues, and immediately after it ceases, aquatic plants are typically able to return to normal levels of productivity. The exact physiological mechanisms that allow this aren't clear, but it is sometimes observed that atrazine-exposed plants produce increased amounts of chlorophyll to compensate for the reduced efficiency of that chlorophyll in photosynthesis. A similar phenomenon is observed when plants respond to shading.

A second mechanism that may be at work is RESISTANCE. deNoyelles et al. (13) observed that algal communities in mesocosms treated with atrazine developed a physiological tolerance, such that a greater concentration of atrazine is needed to cause photosynthetic inhibition. Other investigators have been unable to reproduce this effect in laboratory studies (6,7,12,14), but resistance was demonstrated in field studies by Fromm (75).

A third mechanism is REPLACEMENT. Aquatic plants vary greatly in their sensitivity to atrazine, and the more resistant species are generally able to replace those that are affected. The overall structure and function of the plant community is unchanged, even though the proportions of the species may shift (13-16). Unless there is concern about a particular species of plant, these changes would not generally be considered significant to the ecosystem.

Indirect Effects

Even if atrazine causes no direct toxic effects on fish and invertebrates, and only temporary inhibition of plants, the possibility of indirect effects must be considered. One type of indirect effect that could occur is a change in water quality due to reduced photosynthesis: reduced dissolved oxygen and pH, and increased alkalinity, conductivity, and nutrient levels, all due to lower rates of C 0 2 uptake, nutrient uptake, and oxygen production. These effects have been observed in microcosm and mesocosm studies. For example, dissolved oxygen in pond mesocosms was 1 mg/L less than controls for 7 days following treatment with 20 pg/L atrazine (75). At higher treatment levels (500 pg/L), dissolved oxygen was reduced by 1 to 3 mg/L for up to 22 days, and pH fell by 0.3 units (13); total alkalinity increased by 5 to 10 mg/L (77). In non-flowing laboratory systems, dissolved oxygen decreased after exposure to 100 pg/L atrazine and higher, but not after exposure to 20 pg/L (18). Several investigators have reported

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increases in inorganic nitrogen, phosphorus, conductivity and alkalinity (5,8,12,19) at atrazine concentrations high enough to cause severe effects on the plant community (100 pg/L and greater). Such changes would be most significant in static macrophyte-dominated systems, in which water chemistry is strongly influenced by biological activity. In flowing water, especially small streams, water chemistry is generally controlled by physical processes (advection, diffusion), and changes in rates of photosynthesis and nutrient uptake would be expected to have little or no impact on water quality.

A second type of potential indirect effect would be reduced growth or survival of fish caused by changes in their food supply, especially small invertebrates—assuming that the invertebrates were themselves reduced by a reduction in plant productivity. Indirect effects on fish production have been observed in some studies (8,13,20), but only at atrazine exposure levels that cause major impacts on the plant community. They do not occur at levels that cause only subtle effects on plants.

Summary

In interpreting the ecological significance of the toxicity data for atrazine and other triazines, several factors must be taken into account: (1) Effects of triazines on aquatic plants are transient and reversible. (2) Aquatic plant communities are less sensitive than individual species due to the potential for recovery, resistance, and replacement. (3) Indirect effects occur only at high levels of exposure (high enough to cause major damage to the plant community). (4) Other stressors (such as nutrients, and shading caused by suspended solids) often accompany triazine exposure and could be of greater ecological significance than triazines.

This review has been essentially qualitative. Our published risk assessment on atrazine (7) put this information into a quantitative framework, and determined the probability that atrazine concentrations measured in US surface waters actually cause significant ecological effects. The conclusion of the probabilistic risk assessment was that atrazine residues in surface waters do not present a significant risk to the aquatic environment, though risk is higher in some small watersheds with extensive pesticide use, and in reservoirs which receive drainage from those watersheds. Site-specific risk assessments were recommended for the ecosystems at highest risk.

Acknowledgments

This review was adapted from the report of the Atrazine Ecological Risk Assessment Panel (Keith Solomon, David Baker, Peter Richards, Kenneth Dixon, Stephen Klaine, Thomas La Point, Ronald Kendall, Carol Weisskopf, Jeffrey Giddings, John Giesy, Lenwood Hall, and Marty Williams). The Panel was supported by Ciba Crop Protection and coordinated by Richard Balcomb.

Literature Cited

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