Marian M. Langan and Kyle D. Hoagland, Dept. of Forestry, Fisheries, and Wildlife, University of Nebraska, Lincoln, Nebraska
INTRODUCTION
Nonpoint source pollution is the major cause of impairment of U.S. surface waters (Baker 1992). It is well known that pesticides, often an important component of NPS, can have a significant and complex impact on the structure and function of non-target food webs and on entire ecosystems (Coman and Dordea 1990). Herbicides also can percolate into subsurface flow and be carried long distances in overland flow (Wu et al. 1983).
Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), used primarily for pre- and post-emergence control of germinating weeds in corn (Hartley and Kidd 1983), is the most commonly applied herbicide in the U.S. It acts as a powerful photosynthetic inhibitor, interrupting the light-driven flow of electrons (Esser et al. 1988). In studies done in the northeast U.S., Wu et al. (1983) found that even in areas where alachlor was applied in greater quantities, atrazine was still detected in runoff waters more frequently and in greater concentration. Atrazine concentrations of 13.9 g L-1 can negatively affect stream drift populations of both phytoplankton and zooplankton (Lakshminarayana et al. 1992). Atrazine (20 gL-1) also affects nonpredatory aquatic insects in artificial pond systems, primarily through indirect effects, i.e. by reduction in food for nonpredators and reduction in habitat because of decreases in periphyton and macrophytes (Dewey 1986). Atrazine has also been shown to inhibit photosynthetic rate and plant development of aquatic macrophytes (Forney and Davis 1981, Jones and Winchell 1984, Jones et al. 1986, Christopher and Bird 1992). Communities with submerged macrophytes may experience changes in competitive interactions in response to atrazine contamination (Cunningham et al. 1984).
Wetlands are highly dynamic communities located at the interface between terrestrial and aquatic systems (Guntenspergen and Stearns 1985). On a global scale, wetlands are only one-percent or less of the biosphere, yet they serve important functions in water quality enhancement (Richardson et al. 1978, Tiner 1991) . However, species composition and primary productivity can be altered by agrichemical contamination. Wetlands exposed to pollutants behave similarly to terrestrial and other aquatic systems, e.g. species abundance and diversity can change temporarily, habitat quality at the landscape scale can deteriorate, and energy transmission through food networks can be altered (Catallo 1993).
Wetlands perform various functions such as transforming, filtering, and storing various nutrients and pesticides (Landers and Knuth 1991, Hook 1993). They receive runoff waters and are recharged through groundwater, both of which have been found to be contaminated by herbicides (Pionke et al. 1988, Exner and Spalding 1990, Baker 1992). The greatest quantity of local pesticide inputs to wetlands is probably from runoff during rain events (Clark et al. 1993). High concentrations of pesticides in wetlands can result because dilution may be minimized due to the proximity of sources, and degradation may be minimized by the short time between application and transport to the wetland. The greatest potential for pesticide inputs to wetland habitats is at the time of application or immediately afterward, before the material has been incorporated into the soil or is degraded (Clark et al. 1993). In stream water samples collected during post-planting storm events, Langan et al. (1993) found atrazine in concentrations as high as 691 gL-1, along with 635 gL-1 alachlor, 117 gL-1 cyanazine, and six other herbicides.
It has been proposed that discharging agricultural runoff into wetlands parallel to stream channels may be a method of reducing NPS herbicides and nutrients entering streams via runoff from tile drains (Osborne and Kovacic 1993). High productivity by emergent vegetation, due to highly efficient leaf display and effective resource allocation in extensive carbohydrate stores in the rhizomes (Boston et al. 1989), should further enhance their ability to treat runoff water. Wetlands have been shown to be effective in decreasing nutrient loads in wastewater (Barten 1983). Emergents reduce the flow of sediment in runoff and exchange nutrients with associated ecosystems (Catallo 1993), and keep sediment from resuspending (Dieter 1990).
The purpose of this study was to assess the ability of two wetland macrophytes, Scirpus ref. acutus Muhl. and Typha ref. latifolia L., to tolerate atrazine contamination. Experimental microcosm bioassays were conducted to determine the direct effect of atrazine on these common wetland plant representatives, by assessing its impact on plant growth.
MATERIALS AND METHODS
Typha latifolia (broad-leaved cattail) and S. acutus (hardstem bulrush) were used as representative wetland species. Rhizomes were obtained from Wildlife Nurseries Inc. (Oshkosh, WI). One rhizome and approximately 257 g of dry soil were placed in each 9x8-cm pot, and four pots were put into one plastic 12-L tub (Tucker, Arlington, TX). Distilled water contaminated with the appropriate level of atrazine was added to the tubs to keep the water level approximately 1 cm above the soil surface. Water levels were maintained throughout the experiment by adding distilled water. The combined measurements of four plants in one tub were considered one experimental unit.
Commercially available atrazine (Aatrex®, Ciba-Geigy Corp., Greensboro, NC) was used to establish a concentration gradient. The pesticide was added directly to water used to fill the experimental microcosms. No atrazine was added to the soil prior to the experiment. A randomized complete block design was used because of an east to west temperature and light gradient in the greenhouse. Thus, the experiment consisted of six blocks, two species, and seven treatments: a control with no atrazine added, a gradient of nominal atrazine concentrations including 10, 50, 100, 500, and 1500 gL-1, and a treatment with a nominal atrazine concentration of 500 gL-1, in which the contaminated water was replaced with distilled water after two weeks to simulate a wetland dilution event. At 16 wk, the atrazine levels in both a 50 and 500 g L-1 bioassay were found to be 1.27 and 1.55 g L-1, respectively.
Metalarc lamps (400 W) were used to minimize light intensity differences among the blocks and to maximize the amount of incident irradiance. Light, measured with a quantum irradiance meter and sensor (Li-Cor, Model LI-185A), ranged between 75 and 100 mol·m-2s-1 without sunlight. Lamps were kept on a 12:12 light:dark cycle and the temperature was maintained at 70 C.
The height of S. acutus and the combined length of all leaves of T. latifolia was measured at biweekly intervals during a sixteen week period. Means comparisons were made on the contaminated treatments and the control using the least significant difference procedure. Growth curves were compared using an ANOVA with repeated measures. Comparisons between the control, the 500 gL-1 treatment, and the decontaminated 500 gL-1 treatment were made using a repeated measures ANOVA with orthogonal contrasts on the same measurements.
RESULTS
Scirpus ref. acutus
Plant height was negatively affected by atrazine concentrations of 500-1500 g L-1 (p=0.0001) (Fig. 1). Plant growth in the 0, 10, 50, and 100 g L-1 treatments did not differ significantly, but were significantly different from the 500 and 1500 g L-1 levels during the entire 16-wk experiment (Table 1), despite the low parent compound concentration found in the 500 g L-1 treatment at 16 wk.
Scirpus acutus was able to recover after short-term exposure to the herbicide, as its linear growth response was not significantly different between the control and the decontaminated treatments (p=0.9070) (Fig. 2). Linear growth of the plants was significantly inhibited for the contaminated treatment (p=0.0003). Growth in both the contaminated and decontaminated treatments was inhibited at 2 wk (p=0.0007, p=0.0022, respectively). Plant growth began to increase in the decontaminated treatment, becoming significantly greater than the contaminated treatment at 10 wk (p=0.0398). Differences in growth between the control and the decontaminated treatments were no longer evident by 16 wk. (Fig. 2).
Typha ref. latifolia
Typha latifolia was also affected by high levels of atrazine (p=0.0001) (Fig. 3; Table 1). No strong differences in plant growth were apparent during the first 4 wk of the experiment. However, from 6 wk to 12 wk, the 1500 g L-1 treatment had a significant negative effect on growth. At 14 and 16 wk, the 1500 g L-1 level remained significantly inhibited. (Fig. 3; Table 1). Plant growth in the contaminated treatment was significantly inhibited at 8 and 10 wk, (p=0.0272, p=0.0426, respectively), but no other effects on total height were found (Fig. 4).
DISCUSSION
Scirpus acutus and T. latifolia grew at low levels of atrazine exposure. Growth of S. acutus was slower, reaching its greatest height at 12 wk, while T. latifolia reached full height at 8 wk. Typha latifolia has a much larger rhizome, allowing it to grow more quickly from nutrient stores. When nutrient stores were exhausted, plant growth declined for all concentrations, including the control. Plant growth was inhibited at some atrazine concentrations, but S. acutus was not completely prevented from growing except at 500 and 1500 g L-1, and T. latifolia at 1500 g L-1. Phytotoxic effects may be greater in greenhouse studies than in the field studies because controlled conditions may make growth more rapid; with moisture conditions closer to optimum and all of the roots in treated soil, the risk from herbicide residues may be exaggerated (Riley and Eagle 1990). Though not statistically significant, a trend towards slightly enhanced growth at the 10 g L-1 concentration was seen for both species. This may be a result of plant hormone metabolism being influenced by the triazine herbicides (Esser et al. 1988).
Large watersheds with heterogeneous land use have much lower herbicide levels in runoff than those reported for ditches or crop field plots (Wu et al. 1983). If wetlands near agrichemical application sites, whether natural or constructed, were receiving atrazine contaminated runoff at concentrations of 500 g L-1 or above, it is clear that negative effects at this level are direct and the species assayed in this study would not survive. Consequently, atrazine at these levels would require prior dilution. As this study demonstrates, wetland macrophytes do not tolerate herbicide contamination equally, thus species composition is an important consideration in wetland design for treatment of highly contaminated runoff.
REFERENCES
Baker, L.A. 1992. Introduction to nonpoint source pollution in the United States and prospects for wetland use. Ecol. Engin. 1:1-26.
Barten, J. 1983. Nutrient removal from urban stormwater by wetland filtration: the Clear Lake restoration project. Proceedings of the 2nd Annual Conference of the North American Lake Management Society. U.S. EPA, Washington, D.C.
Boston, H.L., M.S. Adams, and J.D. Madsen. 1989. Photosynthetic strategies and productivity in aquatic systems. Aq. Bot. 34:27-57.
Catallo, W.J. 1993. Ecotoxicology and wetland ecosystems: current understanding and future needs. Environ. Toxicol. Chem. 12:2209-2224.
Christopher, S.V. and K.T. Bird. 1992. The effects of herbicides on development of Myriophyllum spicatum L. cultured in vitro. J. Environ. Qual. 21:203-207.
Clark, J.R., M.A. Lewis, and A.S. Pait. 1993. Pesticide inputs and risks in coastal wetlands. Environ. Toxicol. Chem. 12:2225-2233.
Coman, B. and M. Dordea. 1990. Possible mutagenic effects of alachlor. Biologia. 35(1):32-36.
Cunningham, J.J., W.M. Kemp, M.R. Lewis, and J.C. Stevenson. 1984. Temporal responses of the macrophyte, Potamogeton perfoliatus L., and its associated autotrophic community to atrazine exposure in estuarine microcosms. Estuaries. 7(4B):519-530.
Dewey, S.L. 1986. Effects of the herbicide atrazine on aquatic insect community structure and emergence. Ecol. 67(1):148-162.
Dieter, C.D. 1990. The importance of emergent vegetation in reducing sediment resuspension in wetlands. J. Fresh. Ecol. 5(4):467-473.
Esser, H.O., G. Dupuis, E. Ebert, C. Vogel, and G.J. Marco. 1988. s-Triazines. In Herbicides: Chemistry, Degradation, and Mode of Action. (P.C. Kearney and D.D. Kaufman, Eds.), Dekker, New York.
Exner, M.E. and R.F. Spalding. 1990. Occurrence of pesticides and nitrate in Nebraska's ground water. Inst. Agricul. Nat. Res., Univ. Neb. 34 pp.
Forney, D.R. and D.E. Davis. 1981. Effects of low concentrations of herbicides on submersed aquatic plants. Weed Science. 29:677-685.
Guntenspergen, G.R. and F. Stearns. 1985. Ecological perspectives on wetland systems. In, Ecological Considerations in Wetlands Treatment of Municipal Wastewaters. (P.J. Godfrey, E.R. Kaynor, S. Pelczarski, and J. Benforado, Eds.). Van Nostrand Renhold Co. New York.
Hartley, D., and H. Kidd. 1983. The Agrochemicals Handbook. The Royal Society of Chemistry. Unwin Bros. Limited, Old Woking, Surrey, United Kingdom.
Hook, D.D. 1993. Wetlands: history, current status, and future. Environ. Toxicol. Chem. 12:2157-2166.
Jones, T.W., and L. Winchell. 1984. Uptake and photosynthetic inhibition by atrazine and its degradation products on four species of submerged vascular plants. J. Environ. Qual. 13(2):243-247.
Jones, W.J., W.M. Kemp, P.S. Estes, and J.C. Stevenson. 1986. Atrazine uptake, photosynthetic inhibition, and short-term recovery for the submersed vascular plant, Potamogeton perfoliatus L. Arch. Environ. Contamin. Toxicol. 15:277-283.
Lakshminarayana, J.S.S., J.J. O'Neill, S.D. Jonnavithula, K.A. Leger, and P.H. Milburn. 1992. Impact of atrazine-bearing agricultural tile drainage discharge on planktonic drift of a natural stream. Environ. Pollut. :201-210.
Landers, J.C. and B.A. Knuth. 1991. Use of wetlands for water quality improvement under the U.S. EPA, Region V, Clean Lakes Program. Env. Man. 15(2):151-162.
Langan, M.M., K.D. Hoagland, and A.R. Everson. 1993. Pesticide Levels in Storm Runoff from Agricultural Stream Sites with Different Riparian Buffer Strips. Proceedings of the Platte River Basin Ecosystem Symposium, December 7-8, 1993.
Osborne, L.L. and D.A. Kovacic. 1993. Riparian vegetated buffer strips in water-quality restoration and stream management. Fresh. Biol. 29:243-258.
Pionke, H.B., D.E. Glotfelty, A.D. Lucas, and J.B. Urban. 1988. Pesticide contamination of groundwaters in the Mahantango Creek watershed. J. Environ. Qual. 17(1):76-84.
Richardson, C.J., D.L. Tilton, J.A. Kadlec, J.P.M. Chamie, and W.A. Wentz. 1978. Nutrient dynamics of northern wetland ecosystems. In, Freshwater Wetlands, Ecological Processes and Management Potential. (Eds. R.E. Good, D.F. Whigham, and R.L. Simpson). Academic Press, New York.
Riley, D. and D. Eagle. 1990. Herbicides in soil and water. In Weed Control Handbook: Principles. R.J. Hance and K. Holly, Eds. Blackwell Scientific Publications, Oxford.
Tiner, R.W. 1991. The concept of a hydrophyte for wetland identification. BioScience. 41(4):236-247.
Wu, T.L., D.L. Correll, and H.E.H. Remenapp. 1983. Herbicide runoff from experimental watersheds. J. Environ. Qual., 12(3):330-336.
Table 1. Summary of least significant differences for mean total height of Scirpus ref. acutus and Typha ref. latifolia. Means are listed in descending order; means for underlined treatments do not significantly differ. Atrazine concentrations in g L-1. (p<0.05).
Species Least Significant Differences
S. ref. acutus 10 100 0 50 500 1500
T. ref. latifolia 10 0 50 500 100 1500
Return to 1995 Platte River Basin Ecosystem Symposium 
Last updated by Darren A. Jack on 9/16/97