Ecological restructuring in experimental aquatic mesocosms due to the application of diflubenzuron

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

“…As a result, Daphnia population growth was delayed, but reached greater densities in the pesticide-treated microcosms than the controls. This pattern of algal blooms as a result of insecticide treatments has been observed in many (but not all) field studies in which invertebrate grazers have been reduced by a toxic chemical (e.g., Hurlbert et al 1972, Boyle et al 1996.…”

“…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.…”

Impact of glufosinate‐ammonium and bialaphos on the zooplankton community of a small eutrophic northern lake

“…Such an effect could result from an increase in naturally resistant species, particulary if they were capable of using PTC as a food substrate as has been recently demonstrated for Chlamydomonas reinhardtii [25]. It is also possible that these observed increases in abundance were caused indirectly by herbicide application [26]. Since the zooplankton community was dominated by copepods that filter-feed on phytoplankton [14] and copepod numbers were substantially reduced by both herbicides at concentrations of 1 mg/L within a few days after treatment, grazing pressure on the phytoplankton would be reduced at this treatment level.…”

The Role of Mesocosm Studies in Ecological Risk Analysis

“…Indirect responses within ecosystems are strongly affected by variations in trophic structure (e.g., number of layers), nature of interactions, and interaction strength (Carpenter et al 1985, Vanni and Findlay 1990, Deutch et al 1992, Power and Marks 1992, Strong 1992. Indirect responses may be further exacerbated depending on the specific trophic layer that is targeted, such as the effects of herbicides on primary producers (Dewey 1986, Lampert et al 1989 or the effects of insecticides on consumers (Fairchild et al 1992, Boyle et al 1996.…”