Experiment One
To examine top-down and bottom-up effects in the presence of snail predators, we established experimental freshwater ponds in 75 800L mesocosms filled with 500L of water (mean pH = 9.09) in the summer of 2011 at a facility approximately 20 miles southeast of Tampa, FL, USA. Tap water in mesocosms was allowed to age for 48 hours before being seeded with algae (periphyton and phytoplankton) and zooplankton collected from local ponds. Algal and zooplankton communities were allowed to establish over a four-week period and water was mixed among tanks weekly to attempt to homogenize initial communities before application of agrochemical treatments. Plastic containers with sediment (1 L play sand and 1 L organic topsoil (The Scotts Company, Marysville, OH, USA)) and five rooted shoots of H. verticillata were added to each tank on 5 July 2011. Immediately before application of agrochemical treatments on 11 July 2011 (Week 0), we added snails (21 Bi .glabrata [NMRI strain] and 12 Bu. truncatus[Egyptian strain], provided by NIAID Schistosomiasis Resource Center) and snail predators (2 juvenile Procambarus allenicrayfish, 7 Belostoma flumineum water bugs, and 3Lethocerus spp. water bugs collected from local ponds) to each tank.
Tanks were randomly assigned to one of fifteen treatments (5 tanks per treatment) including twelve pesticides at their respective EEC (estimated environmental concentration), fertilizer (440 µg/L Phosphorus; 4400 µg/L Nitrogen, observed levels from the field; (Chase, 2003)), solvent control (0.0625 mL/L acetone), and water control in five replicated spatial blocks. Water and solvent controls were used to ensure that any observed effects of pesticides could not be attributed to the presence of solvent. All pesticides were dissolved in acetone and applied at their respective estimated peak environmental concentrations (Tables S1-S2). EEC values were determined using the USEPA’s GENEEC (v2.0, USEPA, Washington, D.C.) software, manufacturers’ label application recommendations, and the physicochemical properties of each pesticide (Table S1-S2).