PARENT SESSION
Organized Oral Session 8: Contaminants in aquatic systems: Individual effects and community consequences.
Organizer(s): J Kerby and R Relyea
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

Ecological significance of short-term pesticide perturbations to zooplankton communities in forest ponds.

Kreutzweiser, David^*,1, Thompson, Dean¹, Sutton, Trent^{2, 3}, Back, Richard³, ¹ Canadian Forest Service, Sault Ste. Marie, Ontario² Purdue University, West Lafayette, Indiana³ Lake Superior State University, Sault Ste. Marie, Michigan

ABSTRACT- Previous studies have shown that aerial applications of forest pesticides can result in contamination of standing water bodies. Residual pesticide concentrations in water are usually short-lived, and effects on aquatic invertebrate communities are generally short-term. We conducted a pond enclosure experiment to examine the ecological implications of these short-term pesticide perturbations. A neem (azadirachtin)-based insecticide, Neemix 4.5, was applied to forest pond enclosures at environmentally-realistic concentrations. Significant, concentration-dependent reductions in numbers of adult copepods were observed, but immature copepod and cladoceran populations were unaffected. The ecological significance of this disturbance to the zooplankton community was examined by determining biomass as a measure of food availability for higher predators, plankton community respiration, dissolved oxygen concentrations and conductivity as functional indicators of ecosystem stress, and zooplankton food web stability as a measure of effects on trophic structure. The selective removal or reduction of adult copepods was sufficient to measurably reduce total zooplankton biomass for several weeks mid season. During the period of maximal impact (about 4 to 9 weeks after the applications), total plankton community respiration was significantly reduced, and this appeared to contribute to significant, concentration-dependent increases in dissolved oxygen and decreases in conductivity among treated enclosures. The reductions in adult copepods resulted in negative effects on zooplankton food web stability through eliminations of a trophic link and reduced interactions and connectance. Results were compared to a previous enclosure study in which the pesticide was selectively toxic to cladocerans and had little effect on food web stability. This indicates that differential sensitivities among taxa of zooplankton communities can influence the ecological significance of pesticide effects in aquatic ecosystems.

Key words: pesticide, zooplankton, impact, significance