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Alan J. Temple', Brian R. Murphy 2 & Edward F. Cheslak3Department of Range and Wildlife Management, Texas Tech University, Lubbock, TX 79409, USA;'present address: Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and StateUniversity, Blacksburg, VA 24061, USA; 2present address: Department of Wildlife and FisheriesSciences, Texas A&M University, College Station, TX 77843, USA (author for correspondence); 3presentaddress: EA Engineering, Science and Technology, Inc.39436
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41A Lafayette Circle, Lafayette, CA 94549,USAReceived 6 February 1990; in revised form 20 December 1990; accepted 28 January 1991Key words: tebuthiuron, aquatic toxicology, mesocosmsAbstractTebuthiuron (N-[5-(1, 1-dimethylethyl)- 1,3,4-thiadiazol-2-y 1]-N,N' -dimethylurea) appears to control theriparian shrub saltcedar (Tamarix spp.); however, its use is restricted since the fate and effects of thisherbicide in aquatic systems are unknown. Possible tebuthiuron impacts on aquatic production wereexamined in ten 2846-1 three-phase, open-system mesocosms. Each mesocosm contained sediment,water, algae, micro- and macroinvertebrates and fish, and was open to the atmosphere for gas exchangeand colonization by indigenous macroinvertebrates and algae. The following nominal doses of tebuthiuronwere used: 0 (control), 10, 70, 200, 500, and 1000 g 1- '. The 200 #g 1- ' dose approximated the highestconcentration of tebuthiuron detected in a water body after experimental application to a watershed. Datagenerated from all treatment levels were used in tebuthiuron fate analysis and in correlation analysisbetween the mesocosm variables. The control and the 200 Mg 1- ' treatment level were replicated (n = 3)to allow for additional statistical analyses of treatment effects at the 200 Mg 1- 1' level. The adsorptionof tebuthiuron to sediments contained in ten mesocosms was described by the Freundlich equation,x/m = 3.24c° 0 68.
Phytoplankton primary production, chironomid density, and chironomid biomass werenegatively correlated with tebuthiuron concentration during peak system productivity. Conversely, notrends were observed at any sample date between an omnivorous fish species and herbicide concentration.At the 200 g 1- ' dose level, only chironomid density was reduced. Factors responsible for reductionsin chironomid density may include 1) a species shift in the 200 ,ug 1 - 1 treatment algal assemblages towarda greater percentage of 'unpalatable' biomass, and 2) decreased algal productivity and/or an algal speciesshift in mesocosms receiving dose levels greater than 200 Mg 1- '. Chironomid density reduction at the200 Mg 1- dose level suggests that deleterious effects may occur in some stream systems exposed to a200 Mg 1- ' tebuthiuron concentration. 118IntroductionThere are many water allocation and supplyproblems that western land resource managersmust address (Hyra, 1978; Johnson, 1978). Anissue of particular importance is that of saltcedar(Tamarix spp.) control. Saltcedar was introducedinto the southwestern United States from Europe,Africa, the Middle East, and the Far East in theearly 1800s for use as windbreak, shade, andornamental trees (Smith & Rechenthin, 1964). Ithas spread profusely, currently occupying over400000 hectares of western floodplain lands(Larner et al., 1974).Saltcedar has a substantial water consumptionrate, estimated at 28042 m3 per hectare perannum (USDI, 1959). Busby & Schuster (1973)studied a 112 km section of the Brazos River inTexas and estimated that this species complexconsumes 54279235 m3 of water annually. Con-sidering concomitant usage of nutrients andspace, saltcedar can significantly impact forageproduction, water supplies, and populations ofterrestrial and aquatic organisms.Although no satisfactory method of saltcedarcontrol is in use at present, tebuthiuron(N- [5-(1,1 -dimethylethyl)- 1,3,4-thiadiazol-2-y 1 ]-N,N'-dimethylurea) has been shown to be apotentially viable saltcedar regulator (Jones et al.,1978). Tebuthiuron is a substituted urea herbicidethat has widespread application for the control ofmany weed and brush species in the UnitedStates. It has not yet been approved or registeredby the U.S. Environmental Protection Agency foruse near streams and lakes, effectively precludingits use on saltcedar. Part of the reason for thisstatus is the lack of information on the fate andeffects of tebuthiuron in aquatic systems that mayreceive runoff from treated saltcedar-dominatedriparian zones.Research to date has determined acute andchronic tebuthiuron toxicity levels for selectedfish, invertebrate, and aquatic plant species(Elanco Products Company 1983).