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

Abstract

Abstract:The authors 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-d mean concentrations of 0 mg/L (control), 10 mg/L, 20 mg/L, and 30 mg/L. The authors dosed triplicate streams with pulses of 0 mg/L, 50 mg/L, 100 mg/L, and 150 mg/L atrazine for 4 d, followed by 7 d without dosing. This 11-d cycle occurred 3 times, followed by a recovery (untreated) period from day 34 to day 60. Mean AE standard error 60-d atrazine concentrations were 0.07 AE 0.03 mg/L, 10.7 AE 0.05 mg/L, 20.9 AE 0.24 mg/L, and 31.0 AE 0.17 mg/L for the control, 10-mg/L, 20-mg/L, and 30-mg/L treatments, respectively. Multivariate analyses revealed that periphyton and phytoplankton community structure did not differ among treatments on any day of the 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 present study. Phytoplankton biomass was not affected by atrazine. Metaphyton biomass in pools was higher in the controls near the midpoint of the present study and remained higher on most days for the remainder of the study. Ceratophyllum demersum, a submersed macrophyte, biomass was higher in controls than in 20-mg/L and 30-mg/L treatments before pulse 3 but was not different subsequent to pulse 3 through the end of the present study. Maximum daily dissolved oxygen (DO, percentage of saturation) declined during each pulse in approximate proportion to magnitude of dose but rapidly converged among treatments after the third pulse. However, DO increased in controls relative to all atrazine treatments during the last 17 d of the experiment, likely a result of metaphyton cover in the pools. Finally, atrazine significantly limited uptake of PO 4 3-and uptake and/or denitrification of NO 3 -but only during pulses; percentage of dose removed from the water column was >85% for P and >95% for N after pulse 3 through the end of the present study. Collectively, only DO and metaphyton biomass differed at the end of the present 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 the present 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. Environ Toxicol Chem 2016;35:660-675. #

Data quality scoring system for microcosm and mesocosm studies used to derive a level of concern for atrazine

“… The cosm evaluation system succeeded in objectively distinguishing studies of high quality such as Baxter et al () and King et al () from those of lesser quality. With appropriate minor modification, the evaluation system could be adapted for other chemical classes. When the data quality weighting results were incorporated into the PATI and CASM ATZ LoC models, the atrazine LoC increased by 8% to 12% (or more, for preliminary CE‐LOC PATI with the USEPA data set).…”

A weight‐of‐evidence approach for deriving a level of concern for atrazine that is protective of aquatic plant communities

“…To derive a highly robust and scientifically defensible LOC for atrazine, the SAP () recommended conducting a lotic cosm study that included periphyton and primary producers with a broad range of sensitivities and test concentrations bracketing the proposed LOCs. The study recommended by the SAP () has since been conducted and is described in King et al (). The results of the King et al () study were not included in any of analyses considered in this article and thus can be used as an independent test of the weighted LOC of 23.6 μg/L that we derived herein.…”

“…The study recommended by the SAP () has since been conducted and is described in King et al (). The results of the King et al () study were not included in any of analyses considered in this article and thus can be used as an independent test of the weighted LOC of 23.6 μg/L that we derived herein. King et al () evaluated the effects of pulsed atrazine exposure on a lotic test system containing riffles, pools and glides, and simulated worst‐case atrazine chemographs in streams of agricultural catchments.…”

“…The results of the King et al () study were not included in any of analyses considered in this article and thus can be used as an independent test of the weighted LOC of 23.6 μg/L that we derived herein. King et al () evaluated the effects of pulsed atrazine exposure on a lotic test system containing riffles, pools and glides, and simulated worst‐case atrazine chemographs in streams of agricultural catchments. Atrazine was applied as 3 96‐h pulses with 7 d of no application between pulses to attain 60‐d rolling average concentrations of 0, 10, 20, and 30 µg/L.…”

Leaf litter identity alters the timing of lotic nutrient dynamics

“…When appropriate, we used the varIdent function to weight variance heterogeneity and the corAR1 function to model covariance among repeated measures by stream (King et al, 2016;Zuur, Ieno, Walker, Saveliev, & Smith, 2009). Inclusion of weighting terms was determined by model comparisons based on significant reductions of AIC and better meeting the statistical assumptions of linear modelling (i.e.…”