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Effects of pulsed atrazine exposures on autotrophic community structure, biomass, and production in field‐based stream mesocosms

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We performed a multiple‐pulsed atrazine experiment to measure responses of autotrophic endpoints in outdoor stream mesocosms. The experiment was designed to synthetically simulate worst‐case atrazine chemographs from streams in agricultural catchments to achieve 60‐day mean concentrations of 0 (control), 10, 20, and 30 µg/L, respectively. We dosed triplicate streams with pulses of 0, 50, 100, and 150 µg/L atrazine, respectively, for 4 days followed by 7 days without dosing. This 11‐day cycle occurred three times, followed by a recovery (untreated) period from day 34‐60. Mean ± SE 60‐day atrazine concentrations were 0.07 ± 0.03, 10.7 ± 0.05, 20.9 ± 0.24, and 31.0 ± 0.17 µg/L for control, 10, 20, and 30 µg/L treatments, respectively. Multivariate analyses revealed that periphyton and phytoplankton community structure did not differ among treatments on any day of experiment, including during the atrazine pulses. Control periphyton biomass in riffles was higher immediately following the peak of the first atrazine pulse, and remained slightly higher than some of the atrazine treatments on most days through the peak of the last pulse. However, periphyton biomass was not different among treatments at the end of the study. Phytoplankton biomass was not affected by atrazine. Metaphyton biomass in pools was higher in the controls near the mid‐point of the study and remained higher on most days for the remainder of the study. Ceratophyllum demersum, a submersed macrophyte, biomass was higher in controls than 20 and 30 µg/L treatments before pulse 3 but was not different subsequent to pulse 3 through the end of the study. Maximum daily dissolved O2 (DO, % saturation) declined during each pulse in approximate proportion to magnitude of dose but rapidly converged among treatments after the 3rd pulse. However, DO increased in controls relative to all atrazine treatments during the last 17 days of the experiment, likely a result of metaphyton cover in the pools. Finally, atrazine significantly limited uptake of PO43‐ and uptake and/or denitrification of NO3‐ but only during pulses; percent of dose removed from the water column was > 85% for P and >95% for N after pulse 3 through the end of the study. Collectively, only DO and metaphyton biomass differed at the end of the study, and only slightly. Some other endpoints were affected but only during pulses, if at all. The high levels of primary production and accumulation of algal biomass in all streams suggest that effects of pulses of atrazine at the concentrations used in this study appear transient and likely do not represent ecologically significant adverse outcomes to periphyton, phytoplankton, and aquatic macrophytes, particularly in agricultural streams subjected to high nutrient loads. This article is protected by copyright. All rights reserved

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