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There is evidence that an algalspecies shift may occur as a result of tebuthiuronconcentrations approximating 200 #g 1- (Priceet al., 1989). A shift to unfavorable algal speciesmay explain the chironomid density depression at200 g 1- '. It is possible that algal species shiftsin response to tebuthiuron concentrations at orbelow 200 jg 1-' cause chironomid densitydepressions, whereas decreased primary produc-tion and/or algal species shifts are responsible fordensity depressions at tebuthiuron concentra-tions greater than 200 g 1-'. Reductions inchironomid density at 200 /g 1-' may have sig-nificant negative consequences for insectivorouspredators (e.g. fish) in sandy west Texas streamsystems.Fish biomass was not affected by the range oftebuthiuron doses used in this study (Table 3, 5,and 7). Moreover, fish biomass was not correlatedwith primary production (r = 0.38, p < 0.283),chironomid density (r = 0.37, p < 0.293), or chi-ronomid biomass (r = 0.12, p < 0.优尔). Thesefindings may be due largely to two factors. First,the fathead minnow populations in each meso-cosm may not have been close to system carryingcapacity. Any reductions in food sources wouldbe less likely to affect a population below equi-librium. All other investigated biotic components presumably had sufficient opportunity to closelyapproach carrying capacity. Secondly, the feedingstrategy of fathead minnows may have bufferedthem from herbicide effects to the system.Fathead minnow diets in our systems includedchironomid larvae, aquatic beetles, and algae.Natural populations of fathead minnows havebeen found to be omnivorous as well (Pflieger,1975), which is a common feeding strategy ofmany endemic fish species in the western U.S.(Lee et al., 1980). Omnivory is advantageous influctuating or perturbed environments because itallows the animal a much larger food base ascompared to more specific feeders (Dill, 1983),resulting in greater population stability (Murdochet al., 1975).In contrast to our results, others (Goodyearet al., 1972; McConnell, 1965) have found a sig-nificant relationship between primary production(gross photosynthesis) and production of herbi-vorous and omnivorous fish species. Mesocosmsused in those studies did not contain a sedimentcomponent, which may have precluded signifi-cant benthic macroinvertebrate production;therefore, fish may have been forced to narrowtheir food base. Any changes in phytoplanktonproduction in such systems would essentially bea change in the overall food base; hence, there isa greater chance for alterations in primary pro-duction to be reflected in the fish population.Hall et al. (1970) studied mesocosms con-taining both water and sediment (more similar tomesocosms used in this investigation). They alsonoted no relationship of fish production to benthicproduction despite the fact that the omnivoroustest fish fed largely on benthic macroinverte-brates. The three-phase mesocosm's (i.e. con-taining primary, secondary, and tertiary produ-cers) broader food base, which is a more-accuratereflection of natural systems than the two-phasesystems discussed above, might be responsible forthis lack of correlation. Likewise, the broad foodbase available in the mesocosms used in our studymay have lessened the dependence of fatheadminnows on any one component, thus partiallyisolating the fish population from perturbations toa lower trophic level (i.e. chironomid density).Accordingly, omnivorous fish species in the wildmay not be affected by tebuthiuron concentra-tions ranging up to 200 g 1- 1. Tebuthiuroneffects on insectivorous fish species should beinvestigated since depressions in chironomid den-sity, an important macroinvertebrate group,occurred at 200 g 1- 'AcknowledgementsFunding for this research was provided by theNoxious Brush and Weed Control program atTexas Tech University. Appreciation is extendedto T. Berry, T. Lawson, T. Schlagenhaft and D.Terre for their assistance in field and laboratorywork. R. Meyerhoff and Lilly Research Labora-tories are recognized for assistance in design andresidue analysis. The senior author gratefully ack-nowledges the Virginia Cooperative Fish andWildlife Research Unit, Virginia PolytechnicInstitute and State University, for financial sup-port during data analyses and manuscript revi-sion.ReferencesBowling, J. W., J. P. Giesy, H. J. Kania & R. L. Knight, 1980.Large-scale microcosms for asessing fates and effects oftrace contaminants. In J. P. 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