The impact of the widely used herbicide glyphosate has been mainly studied in terrestrial weed control, laboratory bioassays, and field studies focusing on invertebrates, amphibians, and fishes. Despite the importance of phytoplankton and periphyton communities at the base of the aquatic food webs, fewer studies have investigated the effects of glyphosate on freshwater microbial assemblages. We assessed the effect of the commercial formulation Roundup using artificial earthen mesocosms. The herbicide was added at three doses: a control (without Roundup) and two treatments of 6 and 12 mg/L of the active ingredient (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 showing statistically significant differences between treatments and controls were the downward vertical spectral attenuation coefficient kd(lambda), where lambda is wavelength, and total phosphorus concentration (TP). At the end of the experiment, the treated mesocosms showed a significant increase in the ratio kd(490 nm)/k(d)(550 nm) and an eightfold increase in TP. Roundup affected the structure of phytoplankton and periphyton assemblages. Total micro- and nano-phytoplankton decreased in abundance in treated mesocosms. In contrast, the abundance of picocyanobacteria increased by a factor of about 40. Primary production also increased in treated mesocosms (roughly by a factor of two). Similar patterns were observed in the periphytic assemblages, which showed an increased proportion of dead: live individuals and increased abundances of cyanobacteria (about 4.5-fold). Interestingly, the observed changes in the microbial assemblages were captured by the analysis of the pigment composition of the phytoplankton, the phytoplankton absorption spectra, and the analysis of the optical properties of the water. The observed changes in the structure of the microbial assemblages are more consistent with a direct toxicological effect of glyphosate rather than an indirect effect mediated by phosphorus enrichment.

Shallow lakes often alternate between two possible states: one clear with submerged macrophytes, and another one turbid, dominated by phytoplankton. A third type of shallow lakes, the inorganic turbid, result from high contents of suspended inorganic material, and is characterized by low phytoplankton biomass and macrophytes absence. In our survey, the structure and photosynthetic properties (based on 14 C method) of phytoplankton were related to environmental conditions in these three types of lakes in the Pampa Plain. The underwater light climate was characterized. Clear-vegetated lakes were more transparent (K d 4.5-7.7 m -1 ), had high DOC concentrations ([45 mg l -1 ), low phytoplankton Chl a (1.6-2.7 lg l -1 ) dominated by nanoflagellates. Phytoplankton productivity and photosynthetic efficiency (a * 0.03 mgC mgChla -1 h -1 W -1 m 2 ) were relatively low. Inorganic-turbid lakes showed highest K d values (59.8-61.4 m -1 ), lowest phytoplankton densities (dominated by Bacillariophyta), and Chl a ranged from 14.6 to 18.3 lg l -1 . Phytoplankton-turbid lakes showed, in general, high K d (4.9-58.5 m -1 ) due to their high phytoplankton abundances. These lakes exhibited the highest Chl a values (14.2-125.7 lg l -1 ), and the highest productivities and efficiencies (maximum 0.56 mgC mgChla -1 h -1 W -1 m 2 ). Autotrophic picoplankton abundance, dominated by ficocianine-rich picocyanobacteria, differed among the shallow lakes independently of their type (0.086 9 10 5 -41.7 9 10 5 cells ml -1 ). This article provides a complete characterization of phytoplankton structure (all size fractions), and primary production of the three types of lakes from the Pampa Plain, one of the richest areas in shallow lakes from South America.