Ecological Restructuring in Experimental Aquatic Mesocosms Due to the Application of Diflubenzuron

Abstract

Abstract-Diflubenzuron is a chitin-inhibiting insecticide that is registered for use in controlling a multitude of invertebrate pests. The selective mode of action of diflubenzuron provided us with a unique tool for examining the indirect responses of an experimental aquatic ecosystem following direct impacts at the primary consumer (invertebrate) trophic level. Twelve outdoor aquatic mesocosms (0.1 ha each) were randomly assigned to three treatments of diflubenzuron (four mesocosms/treatment): control, monthly (five total 10-g/L applications), or biweekly (twice monthly; nine total 10-g/L applications). Direct impacts on invertebrates (numbers and species composition of insects and zooplankton) and indirect impacts on fish (mortality, growth, and reproduction of bluegill sunfish and largemouth bass), phytoplankton (biomass and productivity), and water quality were measured over 16 weeks posttreatment. Both monthly and biweekly treatments of diflubenzuron decreased total numbers (fivefold reduction) and species richness (twofold reduction) and increased dominance (twofold increase) of zooplankton within 4 weeks posttreatment; Cladocerans, Copepods, and Rotifers were all sensitive. Diflubenzuron reduced species richness of emergent insects, which resulted in increased dominance by a few species; however, impacts on emergent insects were confounded by a treatment ϫ time interaction. Direct reductions in invertebrate grazers caused indirect increases in algal biomass. Indirect effects on biomass (50% reduction) and individual weight (50% reduction) of juvenile bluegill occurred because of apparent decreases in invertebrate food resources. No statistically significant impacts were observed on adult bluegill or largemouth bass for the duration of the experiment. Results indicated that diflubenzuron had both direct and indirect impacts on the experimental aquatic ecosystems under the conditions tested.

Unique Information Contributed by Multispecies Systems: Examples From the Standardized Aquatic Microcosm

“…An interesting example of a counterintuitive environmental response has been shown regarding ultraviolet‐B radiation; short‐term tests demonstrated reduced photosynthesis and growth of benthic diatom communities, but long‐term, community‐level tests demonstrated an increased standing crop of diatoms because the radiation also inhibited chironomids larvae, which are algal consumers (Bothwell et al 1994). Similar increased algal standing crops have been shown in ponds when grazers were reduced by an insecticide (Hurlbert et al 1972, Boyle et al 1996) and similar results from microcosms will be described below. In real environments, pesticides often cause changes in abundance and unexpected shifts in community structure.…”

Laboratory Toxicity and Benthic Invertebrate Field Colonization of Upper Columbia River Sediments: Finding Adverse Effects Using Multiple Lines of Evidence

“…Water-quality conditions were similar to those measured in other pond studies conducted at the CERC (Boyle et al 1985(Boyle et al , 1996Fairchild et al 1992;Ingersoll et al 2005) (Electronic supplementary material Table S3). Although the pond was originally filled with CERC well water, the water quality in general was higher in pH and lower in alkalinity and hardness compared with CERC well water due to precipitation of carbonates as carbon dioxide diffused from the water.…”

“…In situ sediment-colonization tests were conducted in an outdoor experimental pond (earthen bottom; 0.1 ha, 1.5-m maximum depth) located at the CERC (Boyle et al 1985(Boyle et al , 1996Fairchild et al 1992;Ingersoll et al 2005). A total of 11 benthic treatments (sediment sources/types) were evaluated (Table 1).…”

Reduced grazing rates in Daphnia pulex caused by contaminants: Implications for trophic cascades

“…This is a concentration that has been shown to stimulate growth, photosynthesis, and chlorophyll-a synthesis in Scenedesmus quadricauda (potential bottom-up effect), probably as a result of its nitrogen or phosphorus content [29]. In comparison, xenobiotics such as DDT, diflubenzuron, and esfenvalerate, used at concentrations above the LC50 for zooplankton, decreased zooplankton populations by 70% (range 50-85%) with an increase in phytoplankton biomass of 230% (range 210-260%) [9,32,33]. D. pulex pre-exposed via water also excreted glyphosate; the body burden decreased from 50 mg/g to 10, cor-Grazing inefficiency in contaminated Daphnia Environ.…”