EFFECTS OF THE HERBICIDE ROUNDUP ON FRESHWATER MICROBIAL COMMUNITIES: A MESOCOSM STUDY

PEREZ,G. L.; TORREMORELL; H. MUGNI; P. RODRIGUEZ; M. SOLANGE VERA,; M. DO NASCIMENTO; L. ALLENDE; J. BUSTINGORRY; R. ESCARAY; M. FERRARO; I. IZAGUIRRE; H. PIZARRO,; C. BONETTO,; DONALD P. MORRIS; H. ZAGARESE

ECOLOGICAL APPLICATIONS

ECOLOGICAL SOC AMER

Año: 2007 p. 2310 - 2322

1051-0761

The impact of the widely used herbicide glyphosate has been mainly studied interrestrial weed control, laboratory bioassays, and field studies focusing on invertebrates,amphibians, and fishes. Despite the importance of phytoplankton and periphytoncommunities at the base of the aquatic food webs, fewer studies have investigated the effectsof glyphosate on freshwater microbial assemblages. We assessed the effect of the commercialformulation Roundup using artificial earthen mesocosms. The herbicide was added at threedoses: a control (without Roundup) and two treatments of 6 and 12 mg/L of the activeingredient (glyphosate). Estimates of the dissipation rate (k) were similar in the two treatments(half-lives of 5.77 and 7.37 d, respectively). The only two physicochemical parameters showingstatistically significant differences between treatments and controls were the downwardvertical spectral attenuation coefficient kd(k), where k is wavelength, and total phosphorusconcentration (TP). At the end of the experiment, the treated mesocosms showed a significantincrease in the ratio kd(490 nm)/kd(550 nm) and an eightfold increase in TP. Roundup affectedthe structure of phytoplankton and periphyton assemblages. Total micro- and nanophytoplanktondecreased in abundance in treated mesocosms. In contrast, the abundance ofpicocyanobacteria increased by a factor of about 40. Primary production also increased intreated mesocosms (roughly by a factor of two). Similar patterns were observed in theperiphytic assemblages, which showed an increased proportion of dead : live individuals andincreased abundances of cyanobacteria (about 4.5-fold). Interestingly, the observed changes inthe microbial assemblages were captured by the analysis of the pigment composition of thephytoplankton, the phytoplankton absorption spectra, and the analysis of the opticalproperties of the water. The observed changes in the structure of the microbial assemblages aremore consistent with a direct toxicological effect of glyphosate rather than an indirect effectmediated by phosphorus enrichment.